Download Paper - Colorado Space Grant Consortium

RoverSat
Prepared By:
J. Perkins and M. Albertson
University of Colorado at Colorado Springs
Mechanical and Aerospace Engineering, UH3
1420 Austin Bluffs Pkwy
P.O. Box 7150
Colorado Springs, CO 80933
In Association With:
Anna Stanko
University of Colorado at Colorado Springs
Mechanical and Aerospace Engineering, UH3
1420 Austin Bluffs Pkwy
P.O. Box 7150
Colorado Springs, CO 80933
March 28, 2005
OUTLINE
I.
II.
III.
IV.
V.
VI.
Introduction
Payload Box
a. Box
b. Internal Circuitry
c. Heater
d. Opening Mechanism
Payload Testing
Rover
a. Materials
b. Construction
c. Testing
Launch Day
Conclusion
INTRODUCTION
The goal of the RoverSat project is to design and construct a small weather balloon
satellite capable of driving autonomously and imaging a landing site. The other
requirement is that the payload could not weigh more than 1.8 kilograms. To meet these
requirements within the given timeframe, our design group was split into two teams; one
team focused their efforts on the design and testing of the payload box, while the other
concentrated on the design and construction of the rover. The goal of the payload box
design was to be able to sustain the rover through in-flight conditions and deploy the
rover upon landing. The payload box design was viewed as a critical priority based on the
failure of past RoverSat payload boxes. The goal of the rover was to deploy
autonomously and collect digital images.
PAYLOAD BOX
To accomplish the goal of sustaining and deploying the rover during and after flight the
payload box was divided into four subsystems. The box, internal circuitry, heater and
opening mechanism were the necessary components of the payload box, each was
designed and implemented separately.
BOX
The design of the box was the topmost priority of the RoverSat project. The box was
designed to be structurally sound and capable of withstanding large temperature
differentials as well as the impact of landing. The first aspect of design considered was
how the box would physically deploy the rover after landing. It was necessary to
determine the simplest way for the rover to deploy from the box. The proposed solution
was to design the box based on a four sided pyramid with the top of the pyramid removed
to form a flat top parallel to the base. Foam core was used for the box construction
because of its lightweight, thermal and structural properties. The rover would then be
mounted to the base on the inside of the box and could easily deploy after the sides had
opened. The next area of concern with the pyramid design was how the box would land.
In order for the rover to deploy properly the box would have to land on the base. This
presented a problem because since previous DemoSat projects demonstrated that the
payloads have a tendency to rotate and drag along the ground upon landing. The exact
side the payload box would land on was very difficult to determine. To solve the problem
two areas were examined. The first solution was a way for the box to right itself upon
landing. The pyramid box design had five possible sides that it could land on. The base
was the ideal landing position but the sides were also realistic possibilities. The proposed
solution to having the box land on one of the sides was to mount extension springs to the
box externally. Two springs were connected from each side to the base of the box. When
the box was in the closed position the springs would be in tension. If the box were to land
on a side the springs would act to “flip” the box to the base side down position and the
rover could deploy easily. The other area of concern in landing position was where to
connect the box to the tether. The first idea was to connect a tube to one of the joints
joining two sides. The difficulty of this design is the uncertainty that the connection
would be able to withstand the forces exerted on it by the rotation of the payload during
flight. The next idea was to mount the box to the tether via the base. Two screw eyes
were inserted into the box on one of the diagonals. The tether cord could then be easily
threaded through the two screw eyes and a tube would not need to be used. The concern
again with mounting the tether in this fashion was strength. The proposed solution was to
mount aluminum plates to the inside of the box and have the screw eyes anchor into the
plates. That would distribute the force more evenly across the base instead of having all
the force directed at one point in the foam core. In addition to the aluminum plates it was
decided that the best way to alleviate some of the stress on the screw eye joints was to
mount as many of the internal box components to the base as possible. The idea was to
keep the weight as close to the tether cord as possible to reduce the mass moment of
inertia of the payload.
The next area of the box design and construction was to make an inside frame for the
box. The inside frame was necessary to hold the rover in place during flight and secure
all the internal components. The frame needed to be lightweight and strong enough to
withstand in flight rotation and landing. The shape of the frame was essentially the same
as that of the box except a bit smaller. A half inch gap was placed between the frame and
the box on all sides. This was to allow for the servo arms to rotate and springs to operate.
There was also a 1/2 inch gap between the top of the frame and the box. The frame was
constructed out of 5/16 inch aluminum tubing, and the sides of the frame were
constructed out of electronic circuit board. The circuit board was chosen because it was
lightweight and easy to assemble. The base of the frame was constructed out of
aluminum sheet and positioned one inch above the base of the box. The last area of
concern was keeping the rover secure during flight. A gate was fastened to the side of the
frame where the rover would deploy from. The gate also had an extension spring
mounted similar to the springs on the outside of the box and was kept closed by a servo
arm during flight.
INTERNAL CIRCUITRY
The operations of the payload box were performed by a Basic Stamp 1. The stamp
required a 9V power supply and had eight I/O pins to connect devices to. Opening the
box was the only operation the stamp was used for. Connected to the stamp were the
servo controller and the servos. The servo controller used was the Mini SSC II Serial
Servo Controller from Scott Edwards Electronics, Inc. The servos used were HS-55
Feather servos from Hitec. The servo controller required a 9V power supply and the
servos required a 6V power supply. Initially the stamp was required to read and store
voltages coming in from a pressure transducer but due to time restrictions and the limited
electronic knowledge of the team, the pressure idea was discarded. The next idea for the
stamp was to cause the box to open based on a timer. When the timer reached a certain
value, the stamp would then activate the servo controller which would in turn activate the
servos.
HEATER
In preparation to develop a feasible rover design, past attempts at rovers, and other
experiments were investigated. Upon analyzing the past launch data, regulating the
internal temperature of the launch capsule was deemed a top priority due to the causal
effects upon electrical equipment.
In designing a heating apparatus for the capsule, several options were examined while
specifically noting their cost, size, heat output, and required power. After weighing
several different devices, only two devices successfully met the requirements. The first, a
daisy-chain linkage of ceramic resistors; designed to dissipate excess heat when subjected
to excess current. The second, a heating coil constructed from nichrome wire, designed
to rapidly heat the ambient air in the capsule. Due to weight limitations and requirements
of higher power consumption by the ceramic resistor design, the heating coil design was
chosen.
The design of the heating coil aimed at providing intermittent quick bursts of heat to the
internal chamber of the capsule. Intermittent usage of the heating mechanism was
intended to conserve battery life, and prevent any damages that may result due to
prolonged close range exposure to the device. To achieve this objective, a wall mount
thermostat was included in the circuit. This provided a constant monitoring of the
internal temperature, as well as acted as a breaker switch when the target temperature was
reached and usage of the heater was no longer required.
OPENING MECHANISM
In designing an autonomous capsule for the rover, a reliable mechanism was required to
open the casing upon landing. Due to the design of the containment unit, a mechanism
was required to secure the four sides of the box together; releasing them simultaneously
would upright the box and allow the rover to exit the capsule. To accomplish the
requirements set during the design phase, addressing the time frame of release was dealt
with first. A timing loop was created on a Basic Stamp 1, a processing chip, which upon
completion would activate a servo that initialized the release of the tethered walls.
To open the launch capsule, the opening mechanism essentially acted as a spring-loaded
lock. The four individual capsule walls were aligned such that each would overlap on the
top of the container. Through this overlap, a pin was placed from inside the casing,
protruding outward. This provided equal tension in all four walls, keeping the box from
opening. This pin was connected to a circular disk inside the capsule. Connecting from
the inner cage of the capsule to this circular “release disk” was a spring. The spring was
kept in an elongated state, prevented from contracting by the servo arm, wedged beneath
the release disk.
When the timing circuit initialized the opening device, the designated servo rotated 90º,
allowing the spring to contract downward towards the inner cage of the capsule. When
the spring contracted, it lowered the tethering pin. By lowering this pin inside of the
casing, the walls of the box were no longer tethered and opened outward like a blooming
flower. This simultaneous opening of the four individual casing walls would in turn
upright the capsule (if necessary) allowing the rover to dismount correctly every time.
PAYLOAD TESTING
Each component of the payload was tested separately. The box was first tested and was
simply subjected to a drop test. The box was dropped from the top of a three story
building with a simulated payload inside. The frame was not installed in the box and a
small spool of copper wire was attached to the inside of the box. The wire spool weighed
approximately 800 grams and was similar to the projected weight of the rover. Upon
completion of the test, the box was examined for failures. Overall the box held up to the
drop but was cracked in a few places on the base. The cracking was attributed to the fact
that the inside frame of the box was not yet in place. The frame would provide the
stability and strength the box lacked. The test was repeated with the interior frame and
the results were better; the box was not noticeably cracked and the payload was intact.
The only blemishes were minor indentations expected in the drop.
The internal circuitry was initially tested outside of the box. A program in Basic was
written that would activate the servos after the timing loop had run through. The times
were initially set at just a few minutes to test the servo operations. The times were then
extended to ensure that the servos would indeed activate after being dormant for a period
of hours. The anticipated flight time was estimated to approach three hours so a timer was
set accordingly. The servos worked as anticipated and were activated after the three hour
time was through.
The heater was also tested outside of the box. The main issue in testing the heater was the
required voltage to produce an effective amount of heat. The hope was that the less
voltage the heater needed the fewer batteries would need to be used and the weight could
be reduced. The heater was tested using one 9V battery all the way up to four 9V
batteries. After testing the different arrangements of batteries, three 9V batteries were
deemed adequate to power the heater.
The opening mechanism was a crucial component to test. If the latch were to fail, the
project could be lost. The mechanism was setup as described previously and the servos
were also setup via the stamp. The timer was set and tested in much the same way as the
internal circuitry was tested. The initial tests went well and the latch was released as
desired. The mechanism was then implemented in the box and tested. The next tests were
to determine if the latch could hold up to the rotation experienced during flight. Two tests
were performed in this manner and the mechanism held up each time.
ROVER
The task of the rover was to drive autonomously around and image the landing site. The
rover had a programmable stamp that allowed it “think” on its own and employed a
digital camera for imaging purposes.
MATERIALS
From a bulldozer rover kit, the gear box and motors assembly, a small portion of the
wood frame, axels, wheels, and track were used. The assembly was hooked up to a dual
H-bridge driver to control and power the motors. The dual H-bridge driver needed two 6volt batteries to operate.
For the rover’s vision, bumper switches and an infrared proximity detector were
purchased. However, only the bumper switches were used on the final rover. Each
bumper switch kit had three prongs, a switch, sleeve tube and cap, and a 10 kΩ resistor.
To image the landing site autonomously, we used an HP 435 digital camera powered by
two 1.5-volt batteries and a mini dual H-bridge driver. The mini dual H-bridge driver
replaced a timing circuit and contained a connection strip, 8 PNP transistors, 8 NPN
transistors, 4 1-kΩ resistors, and 8 diodes.
The bumper switches and mini dual H-bridge driver used the same 6-volt battery for
power.
A P-Basic computer program written to the Basic Stamp II controlled all the devices
together by way of input-output pins: four for the rover’s dual motor H-bridge driver, two
for the two bumper switches, and two for the camera’s mini dual motor H-bridge driver.
The frame’s materials were PCB board, 3-32 lock-tite® nuts, 4-inch-long 3-32 bolts, and
brackets.
CONSTRUCTION
The parts for the gear box came from a kit. Each gear had to be properly placed and
greased. Each motor controlled one side of the split axel that was put on the front of the
gear box. A track connected each front wheel with its corresponding back wheel to make
the back wheels turn the same direction as the front wheels. The back wheels were
connected by a solid axel through two holes in the rear of the gear box.
The dual H-bridge driver used to control the gear box and motors assembly required no
construction.
For each bumper switch, the 10 kΩ resistor connected the 6-volt battery (positive side) to
the input-output Basic Stamp II pin. The input-output pin then connected to the middle
prong of one bumper switch. The golden-colored (ground) prong connected to the
negative side of the 6-volt battery.
After the camera case was removed, two wires were soldered to the part of the circuitry
corresponding to the button pressed to take pictures. Also, the two power wires were
soldered together to create a short so that the camera would always be in the “on”
position. However, the two power wires kept falling off creating problems.
The components of the mini dual H-bridge driver needed to be soldered into the correct
position. The connection strip was soldered at the top of the board. One end of each
resistor was soldered to the middle pins of each side. The other end of the resistors were
soldered to the top four transistors (each PNP on the outside and two NPN transistors in
the middle). Four more PNP and NPN transistors in the same order connected the top
transistor row to the four diodes of motor A. Motor B was soldered in the same way as
motor A.
Several P-Basic DOS 2.0 programs were written and sent to the Basic Stamp II to test the
individual parts separately and together. Only one was intended to be used for the
launch. This one program for the launch used a main routine to stop the program during
flight and call two subroutines after the landing. The first subroutine was for deploying
the rover, while the second subroutine was for taking a picture autonomously.
TESTING
To test the gear box, motors, and the dual H-bridge driver, a P-Basic program was written
just to test how the dual H-bridge driver controlled the two motors. The program
included a section for driving forward, left turn, right turn, and driving in reverse. All
tests were successful.
To test the bumper switches, a variable for each switch was added to the motor test
program. All tests revealed a one-second delay from the time the switch was pressed
which closed the circuit to the time the dual H-bridge driver responded. However, none
of the tests were completely successful because the left bumper switch always had a
closed circuit whether the switch was pressed or not.
During testing, the infrared proximity detector failed every time. When it “saw”
something on either side, the detector “saw” the object as directly in front of it. This
caused the dual H-bridge driver to signal the motors to stop.
While in the hot-wired mode, the camera successfully took pictures every time.
However, the testing revealed difficulty in communication between the mini dual Hbridge driver and the camera’s two wires. When connected to the Basic Stamp II, the
camera began to take pictures irregularly instead of how the program signaled it to take
pictures.
LAUNCH DAY
Due to unexpected communication errors between our laptop and the Basic Stamps, we
were unable to load the necessary code onto the devices to launch. Due to this error, the
test program for the opening mechanism was loaded onto the Basic Stamps. Thus, any
attempt to launch the payload would have resulted in the payload opening prematurely.
Instead of launching our payload the team was there to observe the other payloads that
were launched. In particular interests to our team were the successes and failures of the
other RoverSat payloads.
CONCLUSION
The RoverSat Project was a great learning experience. The main concept that was learned
through the course of this project was to keep on task and not to try to make things more
complicated than necessary. We also learned to try to find more than one approach to
solve a problem instead of acting immediately on one idea. Not being able to launch on
the normal launch day was disappointing but not a total failure. Our team has renewed
interest this fall in continuing with our previous RoverSat project and we have some new
team members. We were also strengthened in our belief that the payload box is the most
important part of the flight. Most of the rovers were unable to deploy based on a failure in
box design. Our approach will remain the same in how we redesign and construct the
next payload but we will try to make each subsystem more efficient. Some thoughts for
the next launch are to try composite materials for the box and search for more efficient
heaters. We also plan to implement a different microcontroller that is a bit more up to
date and user friendly. Our team is dedicated to seeing this project through as long as we
have the opportunity. The project has indeed been and will continue to be rewarding.