Download First Semester - Electrical Engineering and Computer Science

Square D: Improved Robotic Arm
and Turntable for Sensitivity Characterization
of
Occupancy Sensors
Prepared by:
Brian Auerbach, Sam Garza, Will Hedgecock,
Anne Killough, Bob Ramenofsky, John Sullivan,
and Havan Tucker
12/17/2007
Table of Contents
1
2
3
4
5
Introduction ................................................................................................................. 4
1.1 Team Members ..................................................................................................... 4
1.2 Project Sponsor .................................................................................................... 5
System Overview and Description.............................................................................. 6
2.1 Project Description ............................................................................................... 6
2.2 System Requirements ........................................................................................... 8
2.3 System Diagram ................................................................................................... 9
2.4 Major Component Definition ............................................................................... 9
2.5 Command and Control Assembly ........................................................................ 9
2.5.1 Interfaces ..................................................................................................... 10
2.5.2 Communication Responsibilities ................................................................ 10
2.5.3 Personal Computer ...................................................................................... 10
2.6 Operational Concept ........................................................................................... 10
2.6.1 Operational Diagram ................................................................................... 11
Robotic Arm Assembly............................................................................................. 12
3.1 Sub-Project Description ..................................................................................... 12
3.2 Robotic Arm System Requirements ................................................................... 12
3.2.1 Physical Dimensions ................................................................................... 12
3.2.2 Thermal Specifications ............................................................................... 12
3.2.3 Movement Specifications............................................................................ 13
3.2.4 Device Communications ............................................................................. 13
3.3 Robotic Arm System Diagrams.......................................................................... 13
3.4 Interface Definitions ........................................................................................... 15
3.4.1 PC to Microcontroller ................................................................................. 15
3.4.2 Microcontroller to Stepper Motors ............................................................. 16
3.4.3 Power Supply to Electrical Components .................................................... 16
3.5 Major Sub-Component Definitions .................................................................... 17
3.5.1
Microcontroller ........................................................................................... 17
3.5.2 Thermal Management ................................................................................. 17
3.5.3 Thermal (Heating) Element ........................................................................ 17
3.5.4 Motors ......................................................................................................... 18
3.5.5 Robotic Arm................................................................................................ 18
3.5.6 Power Supply .............................................................................................. 18
3.5.7 Mounting Cart ............................................................................................. 18
3.6 Operational Concept ........................................................................................... 19
Turntable Assembly .................................................................................................. 20
4.1 Sub-Project Description ..................................................................................... 20
4.2 Turntable System Requirements .........................Error! Bookmark not defined.
4.3 Turntable System Diagrams ................................Error! Bookmark not defined.
4.4 Interface Definitions ............................................Error! Bookmark not defined.
4.5 Major Sub-Component Definitions .....................Error! Bookmark not defined.
4.6 Operational Concept ............................................Error! Bookmark not defined.
Project Plan and Schedule ......................................................................................... 24
6
Appendices ................................................................................................................ 26
6.1 Appendix A: Minor Motion Coverage Pattern ................................................... 26
6.2 Appendix B: Heating Element Comparisons ..................................................... 29
6.3 Appendix C: Choice of Motors Comparison...................................................... 34
6.4 Appendix D: Power Supply Analysis ................................................................ 37
6.5 Appendix E: Gantt Chart .................................................................................... 39
1 Introduction
1.1 Team Members
1.1.1 Brian Auerbach, Biomedical/Electrical Engineer
Brian is responsible for all research and design of the thermal element required to make the
robotic arm emit infrared waves in a manner similar to the wave emission characteristics of a
human arm. Brian’s contributions include researching several heating element possibilities,
deciding upon the most suitable means of heating the robotic arm, and actually designing and
implementing the heating element in such a way that the arm emits the desired infrared
wavelengths uniformly.
1.1.2 Sam Garza, Computer Engineer
Sam is responsible for the microcontroller selection, and firmware programming for the turntable
project. Additionally, he is on the integration team with Will. This includes the design of the
software, connections, and communications between the robotic arm and turntable sub-projects.
1.1.3 Will Hedgecock, Computer Engineer
Will is responsible for the overall control and integration aspects of the robotic arm project. This
includes microcontroller selection, firmware programming of the microcontroller to control the
movements, speed, and temperature of the robotic arm, software programming of the graphical
user interface, connections and signaling to and from the robotic arm, and all integration between
the robotic arm and turntable projects.
1.1.4 Anne Killough, Electrical Engineer
Anne is responsible for the electronic systems of the turntable assembly. This includes the design
of the motor electronics, the light detection circuit, and the power systems used for supporting
the microcontroller, motors, and light detection circuit.
1.1.5 Bob Ramenofsky, Mechanical Engineer
Bob is responsible for the mechanical design of the turntable assembly. This includes deciding
upon which type of motor will best suit our needs in terms of torque, power, speed, durability,
and overall controllability. He will also be in charge of modeling the turntable project on the
computer. Additionally, he will lead the physical assembly of the turntable project.
1.1.6 John Sullian, Electrical Engineer
John will be working on the more mechanical aspects of the project, most importantly the
motors. This includes deciding upon which type of motor will best suit our needs in terms of
torque, power, speed, durability, and overall controllability. In addition to working on the
motors, John is responsible for providing appropriate power sources to all aspects of the robotic
arm, including motors, microcontroller, and thermal elements.
1.1.7 Havan Tucker, Mechanical Engineer
Havan will be aiding the robotic arm team with the mechanical aspects of their project. This
includes the design of the motor and the gear assembly. He will also aid them in the computer
modeling of their design. In addition to these responsibilities, he will also participate in the
assembly of the turntable project.
1.2 Project Sponsor
Square D: A Brand of Schneider Electric
Square D is an internationally-distributed brand of Schneider Electric, specializing in the design
and manufacturing of NEMA type electrical control products. Square D products can be found
in residential, commercial, and industrial facilities, as well as in or on the products of other
manufacturers. Their lighting control systems can be found in some of world's most prestigious
applications including the Sydney Opera House in Australia and the Imperial War Museum in
the United Kingdom.
The key design goal of Square D’s lighting products is that lighting is maintained only when a
specialized monitoring system determines that people are actually in the room, resulting in power
savings of up to 75%. Specifically, our project sponsors from Square D are Bill Stottlemyer
(Senior Electrical/Electronics Engineer), Scott Rae, and Charles Reneau. We are also receiving
assistance from a student on last year’s Square D Robotic Arm team, Kenner Warren, who
currently works for Square D in the capacity of a Testing/Electrical Engineer.
2 System Overview and Description
2.1 Project Description
The Square D occupancy sensors use both ultrasonic and passive infrared technology (PIR) to
detect occupancy in a room. This project is concerned with the sensitivity characterization of the
PIR aspect of these sensors.
The National Electric Manufacturers Association (NEMA) is the largest trade organization for
electrical manufacturers in the United States. In order to ensure the safety and reliability of
electrical products, they publish numerous standards that govern a variety of electrical products.
One of these standards is WD7-2000, which sets guidelines for the sensitivity of motion sensors
employing PIR technology. The standard is comprised of two portions: major motion and minor
motion. Our project is limited in scope to conformance with the minor motion portion of this
standard. The full specifications of this standard can be found at
http://www.nema.org/stds/wd7.cfm.
The testing of an IR-based motion sensor involves determining whether or not the sensor can
detect and be activated by several types of minor motion in different parts of its coverage area.
A coverage area is defined as the area in which the sensor should be able to detect motion,
comprising both horizontal and vertical fields of view. The coverage area is broken down into a
grid containing eighty-four 3ft by 3ft cells, which enables the tester to see precisely what
sections of the coverage area correctly detect motion and which sections have failed the motion
test. In each grid, the amount of motion that can be detected often depends on how far that grid
is from the sensor, and at what angle that grid is to the sensor. The amount of motion detected in
a grid can be classified as either major or minor motion. Minor motion is defined as the
movement of a person sitting at an office desk reaching for a telephone, turning the pages in a
book, opening a file folder, picking up a coffee cup, or other such typical desk-oriented tasks.
See Figures 2-1 and 2-2 for examples of the coverage area.
Figure 2-1: Coverage Area, Top View
Figure 2-1 shows how the grids that are close to the sensor and in direct line of sight of the
sensor can detect minor motion, while the grids that are farther back or at the edge of the
coverage area can only detect major motion.
Figure 2-2: Coverage Area, Side View
The method that Square D Lighting Control currently uses to test for conformance to the
requirements set forth by NEMA is inefficient and tedious. The current process takes around
seven hours to complete for one single sensor. It requires the use of a large meeting room that is
often monopolized by other meetings. Additionally, there is no automation in the process, and
requires the tester to make every single adjustment and record all of the data by hand.
Additionally, the robotic arm currently in use has numerous functional problems and is not a
good emitter of IR energy at the wavelength of human flesh.
2.2 System Requirements
The objective of this project is to make the testing process more efficient by reducing the space
required for testing and by speeding the process up. This project should also automate as much
testing as possible and automatically log the data obtained from the testing process. The project
should allow for the operator to test remotely (i.e. from a different room) in order to ensure that
the testing environment is not contaminated by additional PIR radiation. Additionally, this
project includes a redesign of the robotic arm in order to address the aforementioned problems.
2.3 System Diagram
Command
&
Control
Assembly
Ethernet
Network
Robotic Arm
Assembly
IR Interface
Turntable
Assembly
Figure 2-3: System Overview Block Diagram
2.4 Major Component Definition
This project can be divided into three sub projects: the robotic arm assembly, the turntable
assembly, and the command and control assembly. Each group is responsible for their respective
assembly; however, the command and control assembly will be a joint effort. Coordination of
this sub-project will be the responsibility of Will Hedgecock and Sam Garza. In addition to the
command and control assembly, these individuals will also be responsible for setting a
communications standard for the other sub-projects. The command and control assembly will be
further discussed in this section.
2.5 Command and Control Assembly
The command and control assembly will be the interface that the operator utilizes to control the
robotic arm and the turntable assemblies. This assembly will also be responsible for the
coordination between the project assemblies, as well as the data logging of test data.
Additionally, this section will prescribe which sub-project is responsible for what information
and where it should be sent.
2.5.1 Interfaces
The robotic arm, turntable, and command and control assembly will all communicate with each
other through an Ethernet network. This will allow the operator to control the system remotely.
Square D’s facility already has an existing infrastructure for this medium, making it a logical
choice.
2.5.2 Communication Responsibilities
The command and control assembly will be responsible for coordinating the movements of the
robotic arm and the turntable assemblies. It is responsible for sending information to the robotic
arm that specifies what position the robotic arm should move to. It is also responsible for
sending information to the turntable that specifies what angle it should move to.
The robotic arm assembly will be responsible for receiving movement information from the
command and control assembly. It will also be responsible for informing the command and
control assembly when it has completed its movements.
The turntable assembly will be responsible for receiving movement information from the
command and control assembly. It will also be responsible for informing the command and
control assembly when it has completed its movements. Additionally, the turntable assembly
will be responsible for transmitting information regarding the current activation status of the
occupancy sensor to the command and control assembly.
2.5.3 Personal Computer
The computer will serve as a command and control assembly. Specifically, there will be a
control GUI, running on a Windows platform, which will provide the option to either click
graphical buttons with a mouse or simply input keystrokes from a keyboard. The GUI will
include controls for the direction of movement of the robotic arm (either horizontal or vertical,
denoted by buttons with arrows pointing in the corresponding directions), and a speed control
box, allowing the user to specify whether the arm should move through a maximum of 90° of an
arc in one second or some other slower speed. Additionally, there may also be temperature
controls, allowing the user to input the desired temperature of the robotic arm remotely from the
PC, instead of directly into the thermal management device. The implementation of this aspect
of the GUI depends on what type of thermal management device we decide to use. The GUI will
also include controls for the specification of angles that the turntable assembly will need to be
moved to. Lastly, the GUI will also include data logging features. It will assimilate the
information that it receives from the robotic arm and turntable assembly and record it in a format
that is easy to understand. The PC can take the form of either a laptop or a desktop. Either will
suffice so long as it is running a version of Windows more recent than or including Windows 95
and has the proper interfacing capabilities.
2.6 Operational Concept
This section describes the overall operation of the system as a whole. The tester is defined as the
individual who is operating the system, and the test block is defined by the diagram found in
Appendix A.
2.6.1 Operational Diagram
Choose Test
Block
Tester moves
robotic arm to
proper distance
Robotic arm
iterates through
programmed
movements
Record Data: Did
the light turn on?
UI commands
turntable to proper
coordinates
NO
Legend
Movements
complete?
YES
Automated
Action/
Decision
Decision
User Action/
Decision
Action/
Process
3 Robotic Arm Assembly
3.1 Sub-Project Description
The goal of this sub-project is to improve upon an already existing version of a robotic arm, used
in the characterization and determination of the sensitivity parameters of occupancy sensors
developed by Square D Lighting Control. The robotic arm is used to simulate "minor motion",
which is defined as the characteristic motion exhibited by an individual seated at a desk while
performing normal office tasks. The arm can be positioned inside the specified coverage area for
an occupancy sensor, and provide a means by which to test whether or not a certain minor
motion will be acknowledged or ignored by the sensor circuitry. Using a robotic arm instead of a
human produces controlled, repeatable movements, assuring test objectivity and thereby lending
scientific credibility to the test results.
3.2 Robotic Arm System Requirements
Square D specified a minimum number of requirements which were required to be met in order
for this project to be successful. They were as follows:
3.2.1 Physical Dimensions
Arm: 18 inches long with exactly 15 inches of heated area
Cart for mounted arm: must be mounted exactly 36 inches above ground
In order to accurately represent minor motion, the arm needs to be similar in size to a human
arm. In addition, since minor motion is the motion exhibited by an individual seated at a desk,
the arm should be mounted at the same height as a person's arm would be if they were sitting at a
desk.
3.2.2 Thermal Specifications
Temperature Range: 80 to 120°F
Temperature Resolution: < 2°F
Peak Wavelength Emitted: 9.4 µm
Emitted Wavelength Range: 7 to 15 µm
One of the main improvements desired in this project is a more similar infrared emission
spectrum to the average human arm. Last year’s robotic arm failed to emit in quite the same
range as the human arm, causing the sensor not to be activated when it otherwise should have
been. Therefore, it is necessary that the thermal element be able to be heated to a very precise
temperature, with the caveat that this temperature will cause the arm to emit in the desired
infrared wave spectrum. Also, the robotic arm should emit waves uniformly in all directions and
be safe enough to touch without risk of burn.
3.2.3 Movement Specifications
Direction of Motion: 180° both vertically and horizontally, but not simultaneously
Speed of Motion: 90° of an arc per second
The coverage area of the sensors developed by Square D Lighting Control comprises both
horizontal and vertical fields of view; therefore, in order to test the full range of the sensor, the
robotic arm should be able to move a full 180° in both the vertical and horizontal directions. The
arm should also be able to move through 90° of an arc in one second, corresponding to the
amount of time it takes for an individual seated at a desk to make one swift arm movement,
whether it be typing, writing, or answering a phone.
Two additional movement requirements, based on the shortcomings of last year’s project, are
that the arm should be able to be held in any position indefinitely, and that it should be able to
make fluid, controlled movements without shaking or jerking while starting and stopping. Last
year’s arm overheated if held in a position parallel to the floor for more than a few minutes,
causing it to drop to the down position and become inoperable for an amount of time. This is
unsatisfactory as the testing requirements often mandate that the arm be held in a specific
position for upwards of five minutes. In addition, the use of servo motors and an inadequate
power source caused the noted jerkiness during the starting and stopping of the arm’s
movements. This year’s arms should overcome these inadequacies of last year’s design to aid in
more efficient testing and more accurate test results.
3.2.4 Device Communications
PC to Microcontroller Interface: Ethernet
Microcontroller to Stepper Motors: Memory Mapped I/O
Ethernet was chosen as the communication technology between the computer and the
microcontroller based on its extensive protocol documentation and packet style of data
transmission. Since there is the possibility of using either one or two microcontrollers between
the robotic arm and turntable projects, it will simplify things greatly to be able to address a
packet of data to a specific microcontroller using a static IP address. Also, Square D expressed
the need for running tests from a distance of greater than 50 feet and possibly a separate room.
This could be implemented with maximum ease for the user over wireless, and Ethernet requires
no additional data manipulation to send data over wireless instead of a wired connection.
Since stepper motors are controlled using direction pulses of varying frequency to determine the
number of steps to rotate, it would be simple to control these pulses by simply writing them to a
memory location on the microcontroller which is mapped to the correct corresponding output
port with a stepper controller attached.
3.3 Robotic Arm System Diagrams
Figure 3-1: Component Interaction Block Diagram
Figure 3-1 shows the interactions and connections between the various components of the
robotic arm. Either one variable power source or more than one separate power sources will be
required to drive the heating element, the thermal management device, and the microcontroller.
A personal computer running a control GUI program will send signals over Ethernet to a
microcontroller which will handle these signals by outputting the appropriate pulses to the motor
controller to cause the motors to rotate. The rotation of these motors, in turn, will cause the
robotic arm to move either vertically or horizontally. Some type of thermal management device
will be used, either as a completely independent device, or a PC-controlled device, to provide the
appropriate power to the heating element to reach the desired temperature. Finally, upon
completion of a desired movement by the robotic arm, the microcontroller will receive an
indication from the turntable assembly whether or not the sensor was activated and pass this
information along to the PC for logging.
Figure 3-2: Robotic Arm Operational Flowchart
Figure 3-2 shows the basic operations of the robotic arm. Every time a specific minor motion
needs to be tested, the previous procedure must be followed from start to finish. The robotic arm
ultimately receives all of its control input from the PC via the microcontroller, and all data
capturing takes place in the Turntable Assembly which then forwards the data to the PC, again
via the microcontroller.
3.4 Interface Definitions
3.4.1 PC to Microcontroller
The PC to microcontroller interface will make use of Ethernet technology over an RJ-45 cable to
transmit command and control data and also to receive sensor data for data logging. The control
signals will take the form of single ASCII characters being sent over Ethernet to control the
movement of the robotic arm. The protocol for movement control will be as follows:
Vertical Up
Vertical Down
Horizontal Right
ASCII Character
U
D
R
Decimal Value
85
68
82
Hexadecimal Value
55
44
52
Horizontal Left
Reset
New Speed
New Temperature
L
O
S
T
76
79
83
84
4C
4F
53
54
The “New Speed” and “New Temperature” characters will be followed by another octet
containing the new speed (in number of degrees traversed per second) and the new temperature
(in degrees Fahrenheit).
The reason for choosing the Ethernet architecture for transmission between the PC and the
microcontroller is due mainly to its ease of addressability, allowing transmission to multiple
destinations over a single wired connection with a minimum of effort, as well as the packet
structure of Ethernet, which works well with sending single, small-sized bursts of data to a
specified location. Since a single microcontroller will minimally be controlling the two stepper
motors, possibly the thermal management controller, as well as sending data back to the PC, it is
important to have a technology which allows for ease of addressing to avoid having to hard-wire
each of these interfaces using possibly several different transmission technologies and having to
perform data manipulation to allow for the data from one source to be sent to another source with
a different transmission format. Ethernet does just that; it allows for multiple types of data to be
addressed and sent to specific locations, regardless of the content of the data.
3.4.2 Microcontroller to Stepper Motors
The interface between the microcontroller and the stepper motors is one of the simpler interfaces
to understand. The microcontroller must simply use the control data received from the PC to
send a direction and a pulse signal to the stepper controller. This can be done using just two
output pins which can be memory mapped to a specified location in the microcontroller’s
memory. This allows the required output signals to be simply written to a memory location
which will cause them to be transmitted to the motor controllers. The stepper controllers are preconfigured to handle these direction and pulse signals and output the required voltages to the
motors to make them move the desired speed, direction, and number of steps.
The direction signal takes the form of either a binary 1 or a binary 0. A binary 0 corresponds to a
low signal which informs the stepper controller that the motor will be rotating in a certain
direction (depends on the specific stepper controller). A binary 1 will cause the motor to rotate
in the opposite direction. The pulse signal is output on a separate pin and is what actually causes
the motor to rotate. Every time there is transition from low to high or high to low of the pulse
signal, the motor will rotate one “step”. The frequency of these transitions will therefore dictate
the speed at which the motor rotates.
3.4.3 Power Supply to Electrical Components
All of the electrical components in this system will require some sort of power source. These
electrical components include the microcontroller, the thermal element, and the thermal
management component. Each component will have varying power requirements, such as
differences in required voltage, maximum current, signal cleanliness, and so on. Specific
components will have to be decided upon before the details of these interfaces can be described.
3.5 Major Sub-Component Definitions
3.5.1 Microcontroller
The microcontroller represents the brains of the robotic arm. This is the component which
receives command and control signals from the PC and determines what to do with these signals.
The main function of the microcontroller for the robotic arm is to communicate with the motor
controllers to cause the arm to be set in motion, either in the vertical or horizontal direction.
The microcontroller is also responsible for accepting sensor data from the turntable assembly and
forwarding this data back to the PC for display and logging. The microcontroller is especially
important in this project because it provides the hub of integration between the robotic arm and
the turntable. As such, it is important that the microcontroller be able to perform a wide variety
of functions and have enough power and I/O pins to communicate with all of the external
components such as the motor controllers, the PC, and possibly the thermal management
component. The decision about which microcontroller to use will have to be delayed until more
specific details about the other major project components have been decided upon.
3.5.2 Thermal Management
This component is necessary to heat the infrared-emitting element on the robotic arm to a
specific temperature and, perhaps more importantly, to monitor this temperature and ensure that
it remains within 2°F of the desired temperature as specified by the project requirements. We
have not yet decided on how best to implement this component, because it depends heavily on
the specific type of heating element that we choose to use on the robotic arm.
One all-encompassing and easy-to-implement solution would be to use a PID (proportionalintegral-derivative) temperature controller which uses a feedback loop to sense when the heating
element has reached the specified temperature and ensures that it will remain at this temperature
by continually receiving feedback from the element. One concern with this solution, however, is
that we would like to potentially be able to control the temperature from the PC and we have not
yet researched this component enough to know whether or not it is possible to control such a
device remotely.
3.5.3 Thermal (Heating) Element
The heating element is one of the most critical components of the entire robotic arm project. If
this element does not emit infrared light in the correct range, then the robotic arm is nothing
more than a mechanical apparatus that can move up, down, and side to side; it is therefore
imperative that the heating element be chosen very carefully and exhibit all the required
behaviors specified in Section 3.2.2: Thermal Specifications.
Since this is such a critical component of the robotic arm, we have put much time and energy
into researching available heating elements which emit infrared radiation similar to the skin on a
human arm. Based on our research, which is described in depth in Appendix B: Heating Element
Comparisons, we have found that a form of carbon black coating would be the best solution for
the heating element on our robotic arm.
Since carbon black emits blackbody radiation very closely approximating a perfect blackbody
and has an emissivity of 0.88, it requires very low temperatures to cause the coating to emit
infrared waves in the same spectrum as a human arm. Please see Appendix B for a more detailed
discussion of the qualities of carbon black in relation to other infrared-emitting heating options.
3.5.4 Motors
The robotic arm will be made to move by employing the use of stepper motors. There will be
two stepper motors used, one for vertical motion and one for horizontal motion. Since only one
type of motion is required at a time, only one motor will have to be active at a time. This is
important because it greatly simplifies torque and strain calculations (which have yet to be
made), reduces the complexity of the code to be executed by the microcontroller, and also
reduces the total amount of power that the overall system will ever require at one time. For a
detailed analysis of our choice to use stepper motors, please refer to Appendix C: Choice of
Motors Comparison.
3.5.5 Robotic Arm
The actual physical robotic arm is required to be specifically 18 inches long with 15 of the 18
inches being heated; in other words, there must be 15 inches of cylindrical, infrared-emitting
material. Since the dimensions of the arm have been specified, the only variable aspect of this
component is the material out of which it is to be made. It is important to take the weight of the
material into account, as heavier materials will put more strain on the motors and require more
torque to allow for controlled movement of the arm and non-jerked starting and stopping.
An obvious possibility is the use of polyvinyl chloride (PVC) piping, as this is extremely
inexpensive, durable, and moderately lightweight. However, it should also be noted that we will
be required to heat 15 inches of the material. PVC cannot be directly heated, as no current can
be passed into the plastic-like material. It is possible, however, to wrap some other material that
can be heated, such as aluminum sheathing, around the PVC pipe and simply use the PVC pipe
as a support for this heating element since PVC can easily withstand the low temperatures (less
than 100°F) required to emit the necessary infrared waves. This is an area which we have yet to
thoroughly research, and plan on looking at very soon.
3.5.6 Power Supply
We have yet to decide upon an exact power supply that we will be using in this project, based on
the need to first decide upon other electrical components and assess their power needs. We have,
however, done fairly expansive research in the area of power sources, meaning that when the
time comes to decide upon a specific power source, we will be able to make an informed
decision without spending too much more time on research. For a look at our current stage of
research and analysis, please see Appendix D: Power Supply Analysis.
3.5.7 Mounting Cart
This cart provides the support for the robotic arm. Its only requirements are that it is able to be
moved easily, it is able to hold all of the required cables and wiring going to and from the robotic
arm, and it allows for the robotic arm to be mounted exactly 36 inches above the ground. This
will be the last component which we will decide upon, as it is dependent upon the design
specifications of the rest of the robotic arm and is also the easiest to procure.
3.6 Operational Concept
The robotic arm apparatus will allow for the testing of infrared-based light sensors in a consistent
and straightforward fashion. The arm will be operated from a remote location, such that testing
will not be disturbed by movements or infrared emissions being produced by the operator. The
operator will be able to move the arm in both the vertical and horizontal directions, and will be
able to choose to move the arm continuously for any number of degrees up to 180° of rotation.
Movement of the arm past 180° in either direction is not allowed.
The current method of testing already employs the use of a robotic arm, but the results produced
by this arm have proven to be unsatisfactory. Most notably, the infrared wavelength profile
emitted by this arm has shown to be incorrect with respect to the profile of an actual human arm.
The operation of the newly designed robotic arm will produce an infrared profile very similar to
that of the average human arm, causing the IR-based motion sensor to behave similarly to how it
would in a real-world situation.
To begin testing, the robotic arm will first be heated to a temperature of 95°F, with leeway to
increase or decrease temperature as necessary for optimal results. Once heated, the robotic arm
will positioned in such a way as to correspond to one of the cells in the coverage area (as
specified in Appendix A: Minor Motion Coverage Pattern), and will then be put through a set of
movements including a combination of both vertical and horizontal motions which will test the
sensitivity of the motion sensor at this angle and distance. Upon completion of these motions, a
log of whether or not the sensor was activated will be stored in a centralized data file in the
command and control apparatus. The robotic arm will continue to run through these motions for
each cell in the coverage area until minor motion in all of the cells has been completely tested
and logged.
4 Turntable Assembly
4.1 Sub-Project Description
The goal of this sub-project is to improve the current testing process for the sensitivity
parameters of Square D occupancy sensors. The improvements of this testing process are based
on a foundation of decreasing the testing time and minimizing the required space for the testing
process. The testing process divides the sensor occupancy coverage pattern into 84 cells. Minor
motion must be verified in each of the cells through the movement of the robotic arm assembly.
Rotation of the sensor will reduce the amount of movements required by the robotic arm. The
utilization of the turntable assembly will minimize the number of measurements taken, thereby
simplifying the occupancy testing procedure.
4.2 Operational Concept
The turntable apparatus must be able to rotate within a certain degree value to ensure that the
occupancy sensor meets the requirements set forth by NEMA. The current testing procedure
allows for the positioning of the robotic arm at a certain distance away from the sensor. In
addition, the arm is placed at a certain angle from the sensor between -90˚ and +90˚ from the
centerline. Because this method is both tedious and inefficient, a new method has been
proposed. Instead of having the robotic arm move to each designated square, the proposed
system will allow for the turntable to rotate to a specific angle made by the centerline of the
sensor and the fixed centerline of the room. In order for the proposed system to be effective, the
turntable must be capable of rotating to every angle that was measured between the robotic arm
and the fixed centerline of the current system. A table of the x-direction distance, the y-direction
distance, the resultant distance, and the equivalent turntable angle can be referenced in Section
4.3: Design Verification.
Interface components within the turntable assembly include communication between the GUI,
turntable, and sensor. The turntable assembly will receive position information from the GUI
and then rotate the sensor to the correct angle. The turntable assembly will then respond with
verification that the rotation is complete. The turntable will be idle until it receives signals from
the command and control assembly that the robotic arm has completed a movement. At this
point, the sensor activation circuit will relay the status of the sensor light bulb to the command
and control assembly.
4.3 Design Verification
All distances were measured from the sensor to the center of the indicated cell. The angle values
were rounded to the nearest tenth of a degree. Since rotating the turntable to a tenth of a degree
would be very difficult to accomplish accurately, the turntable will be rotated at 1˚ increments.
Angles with a tenth digit between 0.1 and 0.4 will be rounded to the lower whole number while
the digits between 0.5 and 0.9 will be rounded to the higher whole number. This method will
simplify the design and construction of the turntable. However, there is a possibility that the
turntable could be at most 0.5˚ off from the center of the testing square where the robotic arm
will be placed. This means that the actual turntable angle will be slightly larger or smaller than
the desired angle. It is imperative that the maximum amount of skew be smaller than a certain
value. As long as this criterion is satisfied, the rest of the angle calculations will be adequate.
The desired calculation of an arbitrary cell will be calculated to ensure that the resultant
distances are correct. A skew calculation will then be performed to ensure that it is below a
certain maximum skew value. These calculations will then be performed on the rest of the cells
in the coverage area to figure out the maximum skew distances. The sample calculation of the
arbitrary angle is shown below.
The arbitrary cell chosen to perform the desired angle calculation was cell 52, as numbered in
Appendix A: Minor Motion Coverage Pattern. The center of cell 52 was measured to be -10.5
feet from the sensor in the x-direction and 16.5 feet from the sensor in the y-direction. The angle
between the hypotenuse distance to the cell and the vertical axis at x = 0 will be calculated, as
well as the magnitude of the hypotenuse to the center of the cell:
(1)
(2)
According to the current system, the turntable assembly will be located 19.6 ft from the center of
cell 52 at an angle of -32.5˚ from the vertical axis. To accomplish this same configuration in the
proposed system, the robotic arm must be placed 19.6 ft from the turntable assembly. To
achieve the desired angle of -32.5˚ in the current system, the turntable must rotate +32.5˚ from
the vertical axis in the proposed system. This angle must now be rounded to +33˚ to satisfy the
1˚ increment criterion stated above. The robotic arm will not move from its position in cell 52,
therefore the arm will remain 19.6 ft from the turntable. Now, the new x-distance and y-distance
must be calculated as well as the difference between these new “skewed” values and the original
desired values. An angle of -33˚ will be used in the following calculation to remain consistent
with the current testing procedure. This calculation appears below:
(3)
(4)
(5)
(6)
The values in parts (4) and (6) of the calculations above are the differences between the desired
distances and the skewed distances. Both values are positive, which is equivalent to moving the
robotic arm closer to the top left of cell 52. Figure 4-1 shows a diagram of cell 52 from the
minor motion coverage pattern in Appendix A.
Cell # 52
Position 1:
Ideal robotic
arm position
Position 2:
Skewed robotic
arm position
3 feet
1.8 in.
1.2 in.
2.17 in.
3 feet
Fi
gure 4-1: Diagram of Cell 52 from Coverage Pattern Standards
Position 1 in the diagram refers to the ideal robotic arm location, while position 2 refers to the
robotic arm location once the 1˚ increment criterion has been stipulated. The total distance
between position 1 and position 2 can be calculated using the following method:
(7)
The total distance between position 1 and position 2 was calculated to be 2.17 inches, which is
the maximum skew value that the turntable assembly must account for. This skew is a
reasonable value considering the overall distance between the robotic arm assembly and the
turntable assembly. Since the skewed values do not greatly affect the occupancy sensor’s
performance, the proposed system will be effective with the 1˚ increment turntable angle.
4.4 Turntable System Requirements
Square D specified requirements for the turntable assembly that would ensure success and
qualify for the NEMA testing standards. (Refer to http://www.nema.org/stds/wd7.cfm to view
the standards.)
4.2.1 Physical Dimensions
The requirements of the turntable apparatus dictate that the sensor must be 48 inches above the
floor. This height is comparable to the height at which the sensor will actually be mounted on the
wall of a room. The physical apparatus must be portable in order to ensure storage and a
variable testing facility.
4.2.2 Rotation Specifications
The turntable must be capable of rotating ± 90 ° with a resolution of 0.5 of a degree. These
rotation specifications ensure each cell is tested to the standards of the requirements specified by
NEMA.
4.5 Turntable System Diagrams
TURNTABLE ASSEMBLY
Power
Supply
Sensor
on
oti n
ll M tio
Ce rifica
Ve
Sensor
Circuitry
Turntable
Microcontroller
Sensor Data
Motor Angle and Sensor Data
Command and
Control Signals
Stepper
Motors
Ro
bo
Ass tic Ar
em m
bly
PC
Figure 4-2: Turntable Component Interaction Diagram
Figure 4-2 shows the interactions and connections between the various components of the
turntable assembly. Either one variable power source or more than one separate power sources
will be required to collaborate with the turntable system and the robotic arm system. Command
control will be sent from the user/PC. This control will designate the location of the robotic arm
and the angle required of the sensor. The motor drivers will then rotate the turntable to the
desired angle, wait for the response verifying completion of the robotic arm movement, and
collect the motion data from the sensor. This data will then be sent back to the microcontroller to
store the appropriate detection response in the PC and notify the robotic arm for the next
movement.
5 Project Plan and Schedule
To see Gantt Chart corresponding to this schedule, please refer to Appendix E.
6 Appendices
6.1 Appendix A: Minor Motion Coverage Pattern and Calculations
30
83
84
79
80
81
82
27
24
71
72
73
74
75
76
77
78
61
62
63
64
65
66
67
68
69 70
51
52
53
54
55
56
57
58
59 60
41
42
43
44
45
46
47
48
49 50
31
32
33
34
35
36
37
38
39 40
21
22
23
24
25
26
27
28
29 30
11
12
13
14
15
16
17
18
19 20
1
2
3
4
5
6
7
8
9
15
12
9
6
3
3
6
9
12 15 18
21
18
15
12
9
6
3
FEET
18
Square Number
X Distance (ft.)
1
2
3
4
5
6
7
8
9
10
-13.5
-10.5
-7.5
-4.5
-1.5
1.5
4.5
7.5
10.5
13.5
Y Distance (ft.)
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
10
Distance (ft.)
13.6
10.6
7.6
4.7
2.1
2.1
4.7
7.6
10.6
13.6
Turntable Angle
Adjustment (Degrees)
83.7
81.9
78.7
71.6
45.0
-45.0
-71.6
-78.7
-81.9
-83.7
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
-13.5
-10.5
-7.5
-4.5
-1.5
1.5
4.5
7.5
10.5
13.5
-13.5
-10.5
-7.5
-4.5
-1.5
1.5
4.5
7.5
10.5
13.5
-13.5
-10.5
-7.5
-4.5
-1.5
1.5
4.5
7.5
10.5
13.5
-13.5
-10.5
-7.5
-4.5
-1.5
1.5
4.5
7.5
10.5
13.5
-13.5
-10.5
-7.5
-4.5
-1.5
1.5
4.5
7.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
7.5
7.5
7.5
7.5
7.5
7.5
7.5
7.5
7.5
7.5
10.5
10.5
10.5
10.5
10.5
10.5
10.5
10.5
10.5
10.5
13.5
13.5
13.5
13.5
13.5
13.5
13.5
13.5
13.5
13.5
16.5
16.5
16.5
16.5
16.5
16.5
16.5
16.5
14.2
11.4
8.7
6.4
4.7
4.7
6.4
8.7
11.4
14.2
15.4
12.9
10.6
8.7
7.6
7.6
8.7
10.6
12.9
15.4
17.1
14.8
12.9
11.4
10.6
10.6
11.4
12.9
14.8
17.1
19.1
17.1
15.4
14.2
13.6
13.6
14.2
15.4
17.1
19.1
21.3
19.6
18.1
17.1
16.6
16.6
17.1
18.1
71.6
66.8
59.0
45.0
18.4
-18.4
-45.0
-59.0
-66.8
-71.6
60.9
54.5
45.0
31.0
11.3
-11.3
-31.0
-45.0
-54.5
-60.9
52.1
45.0
35.5
23.2
8.1
-8.1
-23.2
-35.5
-45.0
-52.1
45.0
37.9
29.1
18.4
6.3
-6.3
-18.4
-29.1
-37.9
-45.0
39.3
32.5
24.4
15.3
5.2
-5.2
-15.3
-24.4
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
10.5
13.5
-13.5
-10.5
-7.5
-4.5
-1.5
1.5
4.5
7.5
10.5
13.5
-10.5
-7.5
-4.5
-1.5
1.5
4.5
7.5
10.5
-4.5
-1.5
1.5
4.5
-1.5
1.5
16.5
16.5
19.5
19.5
19.5
19.5
19.5
19.5
19.5
19.5
19.5
19.5
22.5
22.5
22.5
22.5
22.5
22.5
22.5
22.5
25.5
25.5
25.5
25.5
28.5
28.5
19.6
21.3
23.7
22.1
20.9
20.0
19.6
19.6
20.0
20.9
22.1
23.7
24.8
23.7
22.9
22.5
22.5
22.9
23.7
24.8
25.9
25.5
25.5
25.9
28.5
28.5
-32.5
-39.3
34.7
28.3
21.0
13.0
4.4
-4.4
-13.0
-21.0
-28.3
-34.7
25.0
18.4
11.3
3.8
-3.8
-11.3
-18.4
-25.0
10.0
3.4
-3.4
-10.0
3.0
-3.0
6.2 Appendix B: Heating Element Comparisons
Per thermal requirements, the improved robotic arm must produce a heat profile that can be
adjusted from 80 to 120 degrees Fahrenheit, with an accuracy of at least 2 degrees. However,
special consideration needs to be taken to ensure that the infrared profile being emitted by the
arm matches that of a human arm. We will first discuss this profile, and then turn our attention
to possible design implementations.
Human Infrared
Before the human emission spectrum can be discussed, we must first examine what infrared
radiation is and where it lies on the electromagnetic spectrum.
The figure above shows an electromagnetic spectrum with the infrared portions separated into
their components. The longest wavelength of light that can be perceived by our eyes is
approximately 760 nanometers, which also corresponds to the shortest wavelength categorized
by infrared radiation. Infrared is divided into three sections: shortwave, medium-wave, and
long-wave. Shortwave IR wavelengths range from about 760 nanometers to 2 microns.
Medium-wave IR falls in the range of 2 microns to 4 microns, and long-wave IR radiation, also
called IR-C, ranges from 4 microns all the way to 1 millimeter, marking the border between IR
and microwave radiation. Our bodies, with their temperatures at around 305-310 Kelvin,
produce an accurate blackbody curve with a peak wavelength of around 9.5 microns and a range
from 7 to 14 microns, as derived from the following formula:
The following figure shows a graphical representation of the human IR emission spectrum:
While the first model of the robotic arm used a simple heating pad to produce temperatures equal
to those produced by a human body, we will be considering three different approaches in order to
not only match the proper temperature of a human arm, but also to match the emission spectrum
that is produced. The next sections will discuss the positives and negatives of the following
three possibilities: infrared diodes, tungsten wire, and carbon-soot based spray paints.
Infrared-Emitting Diodes
Simply put, a diode is either a filament or solid-state based device that produces a small range
emission of radiation at a desired range. LEDs are a classic example of a diode used in most of
today’s electronics, producing light in the visible range of the electromagnetic spectrum. We
will be considering diodes that only output infrared radiation, which gravitates our search to
filament-based emitters, since the materials needed to output infrared radiation are not included
in a solid state device. The following graphic tabulates a list of modern infrared emitters used in
thermal imaging calibration:
The problems with this approach are immediately apparent, with the most prominent being that
all of these diodes fail to produce the required wavelengths and the intensities are several orders
of magnitude less than what is needed. Most of the applications for these diodes use
wavelengths in the near-IR range, for example, in the use of TV remote controls.
Secondary to the problem of wavelength emissions are those of radiation propagation. While the
radiation profile of a human arm appears diffuse and spread out, these diodes are designed to
emit radiation within a certain number of degrees, normally around 15°. What this means for our
design is that many diodes would need to be utilized in order to cover the entire area of emission
that is produced by a single human arm. This creates additional problems, mainly those of power
consumption and temperature. In order to provide consistent power to the fifty or more diodes
that would be needed to produce the correct emission profile, several amperes would be required.
Temperature and safety is also a prominent concern, and most of the aforementioned diodes
operate in a range centered around an average of 1073 degrees Kelvin in order to produce the
wavelengths required. Not only would we have a robotic arm that could potentially electrocute
someone, but it would also be far too hot to touch, and a PVC-based support would have to be
replaced by a heavier ceramic or metallic surface, each of which have their own unique sets of
problems. We find that this approach would be too unsafe and too costly to pursue.
Tungsten Wire
Tungsten, periodic number 74, is a corrosion-resistant metal that has been used in many
industrial applications where acid is used in production. It has the highest melting point and the
lowest vapor pressure of any metal, making it extremely useful for applications involving
infrared emission, due to the high heat required to produce such radiation. A design for the
robotic arm involving tungsten would require wire to be wrapped uniformly around the arm,
covering the entire surface so that a diffuse profile could be obtained. However, after extensive
research, it was found that this approach should be discarded due to safety concerns. In most
filament applications resulting in noticeable emission levels of radiation, the filament comprising
the tungsten wire must be heated to an excess of 1100 degrees Kelvin, far exceeding the heat that
would be produced by a human body. Similar to the diodes, covering the arm in hot metal is not
safe, and it should be noted that exposed filaments create an electrical hazard as well. Further
discussion is not necessary, as the potential safety risks greatly outweigh the benefits regarding
this approach.
Carbon Black Soot
Discussions with Dr. John Pearce at the University of Texas led to this last design approach
which could be taken to produce infrared emission similar to that of a human arm. The human
arm, under ideal conditions, produces a spectrum that nearly mimics that of a calculated
blackbody emitter. Dr. Pearce recommended simple black spray paint with a carbon black
pigment to produce nearly ideal blackbody emission. Carbon black is a common form of
amorphous carbon soot found in many commercial applications today, including black dyes and
tires. Coating an arm wrapped with a simple flexible heating element with carbon black paint
should produce the necessary characteristics with a minimum of safety concerns.
The figure above shows the emission spectrum of four different materials at 373 degrees K, with
black matte paint having the top emissivity. The most important information contained in this
graph is the working range of emissivity at the temperature measured, with highest emissivity
being in the 7-14 micron range, which is exactly what is needed for the robotic arm. Applying
carbon black paint to a flexible heater wrapped around the arm would be a cheap, and more
importantly, a safe design approach. However, validation of this approach should be carried out
next semester. We will be purchasing several different types of black paint, along with a heating
element, and using an IR- sensitive camera to measure the wavelength emissivity produced by
such an approach, thus satisfying ourselves that it is indeed equal to that of a human arm.
6.3 Appendix C: Choice of Motors Comparison
An extensive study of possible rotational devices yielded the use of either servo or stepper
motors with or without a gear reducer. The infrared-emitting, rotational portion of the arm must
satisfy each of the following requirements:
1.
2.
3.
4.
Controlled movement
Vertical and horizontal movement, but not simultaneously
Movement through 90° in one second
Remain at a stationary position indefinitely
Servo Motors
The current arm implementation uses two servo motors, one for each of the horizontal and
vertical axes. Servos are designed to operate precision controls and dials, change the position of
light loads, and rotate or otherwise manipulate miniscule to light loads. While this would
initially appear to be satisfactory for this project, the weight of the arm itself becomes a critical
factor. The servo motors in the existing design have the highest rated torque of any
commercially available model (100 ounces/inch), yet struggle to move the arm in a controlled
manner at the desired 90° per second rotational speed (Requirements 1 and 3, respectively).
As the block diagram to the right shows, servo motors
are active devices, which means that they require
constant feedback to maintain a stationary position
when external torque is applied. If a servo does not
receive feedback and the arm is in any position other
than vertically down, a loss of feedback causes the
arm to fall to the down position due to its weight.
Using a servo system therefore requires the use of a
constant active feedback loop, which places an undue
CPU burden on the microprocessor. While this
additional CPU load might not ultimately cause a
problem, it is worth pointing out that it may cause the
arm to lag in response to a control signal, or even fail
to move at all.
The problems mentioned above all contribute to
producing the operational deficiencies observed in the current design. Since the servos are both
underpowered and operate in an active, closed feedback loop, an uncontrollable oscillatory
condition exists that causes the servos to constantly reposition themselves in an attempt to hold
any non-down target location (left, right, up, out). This uncontrolled motion is due to the fact
that while the micro-controller is calculating the necessary correctional feedback to overcome the
previously measured gravitational moment torque on the servo, any additional instantaneous
torque adds a downward component that is not calculable due to the time delay introduced by the
feedback-calculate-resolve system.
What this means is that the servo motors jerkily change direction several times a second to try
and maintain a stationary position (failure of requirement 1). This overshoot of the intended
target point requires the servos to reverse rotate, only to overshoot again, which requires another
reverse rotate, etc. This unnecessary harmonic motion causes the servos to overheat after a short
time (failure of requirement 4), as they are operating under too much load. A proportionalintegral-derivative (PID) system could be used to mitigate this oscillation, but due to the
underpowered servos, the jerkiness can not be completely removed.
It is worth pointing out that gear reducers could be used to lower speed and increase torque.
Almost all commercially available servos have internal gear reducers already, so adding another
gear box serves as a supplemental reduction. The problem with reducing servo speed is that it
becomes hard to achieve the required 90 degrees of rotation per second. The reduction on the
100 ounce/inch servos is already so great that this requirement is barely met. Further reduction
would result in a failure of this requirement.
Stepper Motors
Stepper motors sequentially move a finite fraction of a 360° rotation each time the 'step' pin
receives an impulse. The direction is controlled by another pin that turns the motor one direction
when low and the other direction when high. The number of steps per revolution is determined
by both the stepper controller and the individual motor design. Specifically, the number of
separate internal coil windings in the motor combined with whether or not half steps are
supported by the controller determines the overall number of steps required for one complete
revolution.
Having more steps per revolution results in a higher number of possible angular positions and
more torque, but a slower maximum number of revolutions per minute. On the other end of the
spectrum, having fewer steps per revolution allows higher revolutions per minute to be achieved
in the motor at a cost of lost torque and angular resolution. The optimal number of steps per
revolution for this application should be very high, since high torque is needed and the arm does
not need to reach rotational speeds in excess of 90° per second or 15 revolutions per minute
(Requirement 3).
Stepper motors are designed to produce much more torque than servo motors. While servo
motors have a hard time producing more than 100 ounces per inch of torque, some of the largest
stepper motors manufactured are used to propel commercial locomotives. Thousands of
different stepper motors exist to span the gamut from grams/mm to tons/m. The amount of
torque required for this arm will be considerably more than the current value of 100 ounces per
inch, but should remain less than 400 ounces per inch. This torque increase will account for the
additional weight of any other design improvements add to the arm.
Unlike servo motors, stepper motors do not require feedback systems to determine position.
Determining angular position is simple with a stepper motor:
1. Look up the current position
2. Subtract (#1) from absolute clockwise desired position in steps
3. Move (#2) steps clockwise
4. Store (#3) as current position
In order to determine the initial position when the system is powered on, a trigger switch can be
placed in a known angular position on each stepper motor (for example in the down position).
When the system powers on, each stepper rotates until the switch trips. All subsequent steps are
performed as described above. When the system needs to move to a new location, it simply
looks up where it is, subtracts where it wants to be clockwise from where it is now, moves
however many steps required to reach this new angle, and then stores this new position. As long
as the microcontroller stores its current angular position each time it moves, no positional
feedback system is required.
The only consideration here is that if the torque limit of the stepper motor is exceeded, the
stepper will 'skip' a step and rotate to the wrong position. If this occurs, the microcontroller will
think it is at one position but will actually be in another. As long as the motor is not overtorqued, there is no need to worry about skipping. An easy way to determine if skipping has
occurred is to periodically have the microcontroller move each stepper back to where it thinks
the trigger switches are. If either switch fails to trigger on exactly the last step, skipping has
occurred.
Unlike servo motors, stepper motors do not need
constant supervision by the CPU, as shown in the
graphic to the right. Unless the CPU is sending
pulses to the stepper controller to move, no
interaction is necessary at all. When no pulses are
received, the stepper controller simply holds its
current position at maximum torque. This means
that there is never any oscillatory motion. The
worst case scenario becomes a single slip (nonoscillatory) that is discovered and corrected the
next time both trigger switches should have been
reached (in the previous example, this was at the
down position).
As the motors are designed to run at their full rated current at all times, thermal cooling is not an
issue. The stepper controller sinks most of this current and requires a large heat sink to radiate
heat. Most stepper controllers have variable current drive selectors so that the current can be
reduced until a torque value just above the maximum holding force is obtained. This helps
reduce the amount of heat generated and also conserves power.
6.4 Appendix D: Power Supply Analysis
Powering the robotic arm focuses both collectively and individually on each of the three main
electrical components: the motors, the thermal element(s), and the microcontroller. In order to
operate, each of these devices require different voltages, current ranges, and power cleanliness
factors. The electrical system breaks down to the following block diagram:
In
gen
eral
,
the
mic
roc
ontr
olle
r is
the
mos
t
sen
sitive component of the electrical system. If periodic transients enter the CPU or memory
circuits, data corruption occurs, which will cause poor system performance. For this reason, the
microcontroller requires the most voltage and current regulation. This regulation can be
achieved through an on-board, built-in power regulator, by using a separate power supply
altogether removed from the rest of the system, or by using capacitors, inductors, and/or other
components to regulate power from one centralized supply. Most microcontrollers operate
between 3.3 and 12 volts DC. A low pass filter (essentially a large capacitor in parallel with the
device) would effectively remove transients from the equation and would aid in leveling and
normalizing the voltage.
Regardless of what infrared-emitting device is used, neither a heated element nor LED emitters
require highly regulated power sources. The IR devices need roughly 2 volts DC and will
operate with AC line noise, large voltage sags, transients, etc. The heating element can operate
on either DC or AC and also requires little to no regulation aside from an approximate voltage
and current rating. While neither of these devices requires precision power control systems, it
doesn't hurt to make the signal as clean as possible.
The stepper controller is a current controlled device. The input voltage must be within a certain
wide range (for example 5-48 volts), but the important power delivery requirement is that the
maximum current necessary to hold the arm in place be deliverable by the power supply
continuously. The voltage can change as long as the current remains the same. The use of
inductors is helpful to stabilize current with a changing voltage.
Whether the robotic arm should operate off of battery power or whether it should tie into the
building mains via an outlet or other connection is an important design decision. If the device is
battery powered, it can operate tether-less, which would be useful if the arm were to be made
more mobile in the future, (Square D indicated that this feature would be a great addition in a
future project). Assuming a wireless data communication protocol was to be used in conjunction
with batteries, the arm could remain completely untethered during normal operation. Another
perk is that clean, stable DC voltages are easy to maintain when batteries are used, as the battery
acts as a reservoir that limits voltage fluctuations under normal operating conditions.
One downside is that if AC voltages are required, DC to AC converters/inverters become
necessary. Another drawback is that if tungsten heating elements are used as an infraredemitting source, this produces a large energy footprint and will require the implementation of a
large battery bank. Also, heating sources operate more efficiently if they are implemented in the
high current/low voltage AC realm, which as mentioned before will require the use of power
converters. Infrared-emitting LEDs do not individually require a lot of power to operate, but the
hundreds to thousands of diodes needed to mimic the thermal image of a human arm would
cumulatively amount to a large power demand. Under such high load, batteries would require
constant recharging. Since the test the arm performs can take the better part of a day, the use of
batteries is questionable.
While it appears that these drawbacks prohibit the use of batteries for the arm, if the carbon black
paint were used in lieu of a tungsten heating element, the power demand would be significantly
reduced. If this combination is used, a battery system would be feasible.
The other potential power source is a tethered connection to the mains of the building through a
110 volt power socket. As DC power is required for the microcontroller and motors, a rectifier
and reduction circuit would be necessary. The arm would lose its cordless status, but could run
tests indefinitely. As mentioned previously, capacitive voltage stabilizers might be necessary for
proper microprocessor operation.
A major drawback is that the use of higher voltages requires many more safety considerations in
order to lower the risk of shock or electrocution. Mistakes become more costly when using 110
volt AC power. In most cases, a person will not die if electrocuted at this voltage, but the
possibility does exist where it does not under battery conditions.
6.5 Appendix E: Gantt Chart