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Test bench for a Distributed Data Collector (DDC)
Master thesis, electronics
Mälardalen Univeristy
School of Inovation, Design and Engineering
Author: Mikael Bohlin
Supervisor: Daniel Kling, Addiva Consulting AB
Examiner: Mikael Ekström, Mälardalen University
Västerås, March 2010
Abstract
A system for monitoring and analysing the state of equipment and processes in i.e. the
industry will help to increase productivity and prevent unscheduled stops for maintenance.
To do so certain parameters needs to be measured and analysed. The DDC will collect this
data and pass it on to a central server for analysis.
This thesis covers the construction of a test bench for such data acquisition to get data to
send across the network and to prove that the concept works. A number of sensors have
been set up to measure four important parameters: temperature, vibrations, current
consumption and rotational velocity. Hardware and software for reading data from the
sensors has also been constructed. All hardware is built into a box where each sensor has
controllable objects to measure against, such as a motor and a heating element.
The program for data acquisition uses an interrupt-based architecture so every time a big
enough change is detected in one of the sensors the DDC-signal is updated by the program.
Each signal has a unique number for identification and is passed on across the network at set
intervals.
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Sammanfattning
Ett system som övervakar och analyserar tillståndet hos utrustning och processer i t.ex.
Industrin kan hjälpa till att öka produktiviteten och förhindra oväntade stopp för underhåll.
För att göra detta måste ett visst antal parametrar mätas och analyseras. DDC:n (Distributed
Data Collector) kommer att samla in denna data och skicka den vidare till en central server
där den loggas och analyseras.
Syftet med det arbetet som täcks av den här avhandlingen är konstruktion av en testbänk för
just sådan datainsamling för att sedan få data att skicka över nätverket och för att bevisa att
konceptet fungerar. Ett antal sensorer har satts upp för att mäta fyra viktiga parametrar:
temperatur, vibrationer, strömförbrukning och rotationshastighet. Hårdvara och mjukvara för
att läsa data från dessa sensorer har också konstruerats. All hårdvara är innesluten i en låda
där varje sensor har ett objekt att mäta på, så som en motor och ett värmeelement.
Programmet för datainsamling använder en interruptbaserad arkitektur så att varje gång en
tillräckligt stor förändring detekteras i någon av sensorerna så uppdaterar programmet DDCsignalen. Varje signal har ett unikt id-nummer och skickas vidare över nätverket med intervall
som bestäms av konfigurationen.
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Nomenclature
ADC: Analogue to Digital Converter
API: Application Programming Interface
DAQ: Data Acquisition device
DDC: Distributed Data Collector
IR: Infra Red
ISR: Interrupt Service Routine
LED: Light Emitting Diode
LSB: Least Significant Bit
MCU: MicroController Unit
PLC: Programmable Logic Controller
RPM: Revolutions Per Minute
SCADA: Supervisory Control And Data Acquisition
USB: Universal Serial Bus
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Table of Contents
1. Register ........................................................................................................................ 7
1.1 Figure register ................................................................................................................... 7
1.2 Schematic register ............................................................................................................ 7
1.3 Equation Register.............................................................................................................. 7
1.4 Table register .................................................................................................................... 7
2. Introduction .................................................................................................................. 8
2.1 Background ....................................................................................................................... 8
2.2 Purpose ............................................................................................................................. 8
2.3 Project limitations............................................................................................................. 9
2.4. The DDC ......................................................................................................................... 10
2.4.1 Overview .................................................................................................................. 11
2.4.2 The Maintenance System ......................................................................................... 11
2.4.3 The SCADA system.................................................................................................... 11
2.4.4 PLC:s ......................................................................................................................... 11
3. Problem formulation ................................................................................................... 12
4. Analysis of problems ................................................................................................... 13
4.1 Parameters of interest .................................................................................................... 13
4.1.1 Temperature ............................................................................................................ 13
4.1.2 Vibration................................................................................................................... 13
4.1.3 Current consumption................................................................................................ 14
4.1.4 Rotational speed ...................................................................................................... 14
4.2 Hardware ........................................................................................................................ 14
4.3 Software.......................................................................................................................... 15
5. Method....................................................................................................................... 16
5.1 Data acquisition device ................................................................................................... 16
5.2 Hardware for measuring ................................................................................................. 18
5.2.1 Measuring temperature ........................................................................................... 18
5.2.2 Measuring vibrations ............................................................................................... 19
5.2.3 Measuring current consumption .............................................................................. 20
5.2.4 Measuring rotational velocity .................................................................................. 21
5.2 Software for sampling and calculations ......................................................................... 22
5.2.1 Calculating temperature .......................................................................................... 23
5.2.2 Calculating vibration amplitude and frequency ....................................................... 24
5.2.3 Calculating current consumption ............................................................................. 25
5.2.4 Calculating rotational velocity ................................................................................. 25
6. Results ........................................................................................................................ 26
6.1 Test bench set up ............................................................................................................ 26
6.1.1 Analogue and digital connectors ............................................................................. 26
6.1.2 Encapsulation and operation ................................................................................... 27
6.2 Updating signals.............................................................................................................. 27
6.3 Watchdogs ...................................................................................................................... 28
6.4 Resolution of analogue signals ....................................................................................... 28
6.4.1 Temperature resolution ........................................................................................... 29
6.4.2 Vibration amplitude resolution ................................................................................ 29
6.4.3 Current consumption resolution .............................................................................. 30
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7. Discussion ................................................................................................................... 31
7.1 Results and analysis of results ........................................................................................ 31
7.1.1 Analysis of hardware................................................................................................ 31
7.1.2 Analysis of software and interface kit...................................................................... 32
7.2 Future work .................................................................................................................... 33
8. Summary and conclusions ........................................................................................... 34
9. References .................................................................................................................. 36
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1. Register
1.1 Figure register
Figure 1 Overview of the DDC system ...................................................................................... 10
Figure 2 Phidget Interface Kit 8/8/8 ......................................................................................... 17
Figure 3 Inductive sensor ......................................................................................................... 19
Figure 4 Optical pulse method ................................................................................................. 21
Figure 5 Method for measuring current .................................................................................. 30
Figure 6 ISR issue ...................................................................................................................... 32
1.2 Schematic register
Schematic 1 Temperature measurement circuitry .................................................................. 18
Schematic 2 Vibration measurement circuitry ........................................................................ 19
Schematic 3 Current consumption measurement circuitry ..................................................... 20
Schematic 4 RPM measurement circuitry ................................................................................ 21
1.3 Equation Register
Equation 1 Temperature .......................................................................................................... 23
Equation 2 General resolution ................................................................................................. 28
Equation 4 Distance resolution ................................................................................................ 29
Equation 3 Temperature resolution ......................................................................................... 29
Equation 5 Current consumption resolution ........................................................................... 30
1.4 Table register
Table 1 Ports and connectors ................................................................................................... 26
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2. Introduction
This chapter gives a short introduction to the background, purpose and general function of
the DDC.
2.1 Background
Unscheduled stops for maintenance during production are a major issue resulting in high
costs in all industries. For example a bearing or a motor suddenly breaking down can have
repercussions throughout the entire production line bringing everything to a halt until a
service team has been assembled and been able to locate and fix the problem. A way to
predict these problems would enable far more effective planning of such maintenance work allowing it to take place during less critical stages of production and always having the
service team and spare parts ready when they are needed. Another problem is machines
appearing to work fine but that in reality are putting out faulty products due to minor errors
that in worst cases will not be spotted until the customer uses the product. These are errors
that might be hard or even impossible to find by a human operator. The sooner these
problems are attended to, the more effective the production line will be, greatly reducing
costs.
2.2 Purpose
To predict breakdowns and errors, the idea is to read data from various machines and
processes running in the industry and collect it in a central database where algorithms for
calculating the life expectancy and state of different parts are run. This is where the
Distributed Data Collector (DDC) comes in. One DDC will be placed in adjacency to every
process that needs to be monitored, collecting data that can tell if a breakdown is imminent.
Such parameters can for example be vibrations and current consumption. This can be done
either by interfacing with the sensors and systems currently in place for controlling the
process or by setting up extra sensors to fit the needs of the life length equations. The data
collected by the DDC is sent via a network of other DDC:s and stored in a central server
where all calculations and predictions are made. The system will then be able to warn the
operator that i.e. a certain part will need replacing within a week or if a machine is not
performing properly.
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2.3 Project limitations
The wide array of different systems and processes being used throughout the world makes it
virtually impossible to construct a data acquisition system that fits everywhere. Every
application will need a custom made way of interfacing with the system currently in use at
site. Therefore the hardware and data acquisition module (DAQ) constructed during the work
of this thesis can and will only be used for development and demonstration purposes and to
prove that the concept of the DDC works. The communication between the DDC:s and the
server, and the system as a whole are not covered in this thesis and will only be mentioned
briefly as an introduction to understanding the system, but not described in detail.
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2.4. The DDC
Figure 1 shows an overview of the DDC system, and this chapter briefly describes the
functions of the various components. All text and images are taken from Addiva's
documentation on the DDC.
Figure 1 Overview of the DDC system
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2.4.1 Overview
The DDC is designed to collect data from various sources on site at various production
facilities. Each of these facilities have a number of already existing systems which store and
collect data. The DDC will interface with these systems and gather any relevant information,
depending on configuration and commands from the IMOM server.
2.4.2 The Maintenance System
The Maintenance System holds information on all hardware present in the production unit. It
contains information on motors and other equipment on site, when they were acquired,
specifications, manufacturer and so on. The DDC will interface with the maintenance system
via its API which may vary between sites.
2.4.3 The SCADA system
The SCADA (Supervisory Control And Data Acquisition) system automates and supervises the
equipment on site. Many systems also provide logging of values and provide various
interfaces for monitoring the state of the production unit.
2.4.4 PLC:s
PLC:s (Programmable Logic Controllers) are used throughout the production industry to
collect data and control equipment. Interaction with the PLC from an external source (like
the DDC) is done via one of the available communication protocols.
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3. Problem formulation
The main objective of this thesis is to set up a test bench for a DDC to be used for
development and demonstration purposes. This includes:
 Investigating which parameters that are interesting to monitor
 Finding suitable sensors and ways to measure those parameters
 Constructing the hardware necessary for getting proper signals from the sensors
 Finding a cheap and simple way of interfacing the analogue and digital signals with a
PC via USB. Either building an I/O-board from scratch or ordering an existing one.
 Programming an application (input module) to read the sensor raw data and
converting it into the values they represent.
 Combining the input module with the main DDC program to create a DDC signal to
send across the network
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4. Analysis of problems
To reach a result and a final product the problem has been divided into several subsections
where each part is handled separately. These subsections are discussed below.
4.1 Parameters of interest
Because the main purpose of the test bench is to show that data can be sampled and sent
across the network it is basically irrelevant which parameters are read. You might as well just
write an application that simulates input from an external source. However it is favourable
from a demonstration point of view to prove that you have the necessary knowledge in the
field where the product is supposed to be used. Based on this a short investigation was
conducted to determine which parameters give away the most information on the state of
the process being monitored. Interviews with engineers working in the industry showed that
three of the most important parameters when checking for errors are temperature, vibration
and current consumption.
4.1.1 Temperature
Temperature is one of the things most commonly measured in any process. It can be used
not only to measure actual temperature, but also factors that are directly or indirectly
affected by it – or that affect the temperature themselves. Such as flow and stress on
materials due to heating. Increased temperature levels are a good indicator that something is
wrong with a process, i.e. heating due to increased friction.
4.1.2 Vibration
Imbalance, worn gear heads and old bearings can all result in unwanted vibrations which in
turn lead to even greater stress on the running machine and poor accuracy in production. If
these vibrations can be spotted at an early stage and the components causing the vibration
can be found and fixed, there are a lot of problems that can be avoided and money to be
saved.
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4.1.3 Current consumption
A motor beginning to fail tends to draw more current from its power supply. Another reason
for increased current consumption is unwanted extra external load. If the motor is working
harder than it is expected to do during its current state it is a good indicator that something
is wrong. These increased levels can either be constant or come as periodical spikes, all giving
away clues as to where and when errors occur.
4.1.4 Rotational speed
In addition to the three parameters mentioned above the rotational speed of a shaft or
motor is a relevant factor when trying to discern if something is not working properly. Since
current consumption and the frequency of vibrations can be a result of the actual speed of
an object it is relevant to know how fast it is revolving to get a ratio between speed and
other parameters of interest.
4.2 Hardware
To read analogue data from the outside world and converting it into digital values that can be
read and interpreted by a computer, specific hardware needs to be constructed. This
hardware includes sensors, amplifiers and filters and a way of interfacing the sensors with a
computer. The requirements for the interface depend on the accuracy and sampling
frequency needed and acquired from the sensors. The only demands made by the company
in charge of the project is that it uses the Universal Serial Bus (USB) of the computer and that
it does not simulate a COM port like some products do. The accuracy of the data acquisition
device is decided by the resolution of the analogue to digital converter (ADC) where the
number of bits in the conversion limits the smallest change in the signal possible to detect.
The sampling frequency is limited by several factors, but the main bottleneck is the interrupt
and response time of the USB. Construction of such a device will not only require specifying
and putting together the actual hardware, but also developing an Application Programming
Interface (API) and drivers to interact with it. All hardware will also need to be packaged to
make it portable and so that it can be presented in a way that is attractive to possible
costumers.
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4.3 Software
Protocols for network communication and the main program to be run on the DDC which
keeps track of signal subscriptions and creates the signals are provided by other developers
and are not covered in the work of this thesis. The only purpose of the module that reads the
input is to convert the signals into the values they represent and update the DDC when a
change in those signals has taken place. This is because the input module will need to be
changed and adapted for each specific system where the DDC is to be implemented. It is
therefore important to keep it as abstracted away from the main program as possible.
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5. Method
This chapter describes the methods used to solve the different problems and motivates why
they were solved in that particular manner.
5.1 Data acquisition device
The first thing that needed to be decided was how to convert the data from analogue signals
to a digital representation and reading it into the computer. This is because the specifications
of the ADC affect how the signals from the sensors need to be amplified. For example some
ADC:s have a maximum input voltage of 2.5 volts whereas others might go up to 5 volts. A
short investigation was conducted on what components and work would be required to build
an I/O-board from scratch. Apart from connectors and various passive components (such as
resistors and capacitors) the major parts needed are:
 Analogue to digital converter
 Microcontroller Unit (MCU)
 USB-interface module
The MCU would need to be programmed to handle the sampling and sending of data via the
USB-interface to the computer, and everything would need to be soldered and attached to a
circuit board. Also drivers and an API would need to be developed for the computer to
communicate with the board. All this is time consuming work that is hard to motivate at this
stage when there are relatively cheap existing models available for ordering. Therefore the
idea of constructing a board was soon abandoned and a comparison between existing ones
was initiated. These are the specifications/guidelines set up for performance of the I/Oboard, apart from the ones previously mentioned:
 8-channel, 10-bit analogue to digital converter
 At least 4 digital inputs
 Low cost
 Easily programmable and well documented API
 Supported by Linux based systems
[16]
The choice fell on the Phidget interface kit 8/8/8 (figure 2), which connects eight analogue and
eight digital inputs to the computer via USB. It also provides eight digital outputs, which are
not used in this project. The ADC has a 10-bit resolution over a span of 5 volts allowing for a
minimum detected change of 5/210 ≈ 5 mV. The sampling rate is ~65 Hz for the analogue
inputs, and ~125 Hz for the digital ones. These are relatively low figures but acceptable for
the requirements of this project. Note that this limitation is not due to the speed of the built
in ADC, but in the communication time between the board and the PC over the USB. One
major advantage with this interface kit is that all code written with the C API is platform
independent and can be moved between Windows and Linux only requiring that you include
a different library. It also draws its power from the USB-port, so no external power supply is
needed.
Figure 2 Phidget Interface Kit 8/8/8
(source: www.phidgets.com, 2010-03-01)
[17]
5.2 Hardware for measuring
All hardware is driven by a twelve volt power supply. This is because the sensor for
measuring vibrations requires a minimum of ten volts and because twelve volt DC-adaptors
are commonly available. This chapter describes the different circuits and methods used to
measure the different parameters previously specified. All circuit diagrams are only
representative for the concept of measuring the respective signals and do not describe the
circuit board a whole, leaving out decoupling capacitors, LED:s and such components.
5.2.1 Measuring temperature
Schematic 1 shows the circuitry used for measuring temperature. The temperature sensor
used is a LM35DZ whose output is linearly proportional to the centigrade scale with 10
mV/°C. What this means is that at 0°C it puts out 0 V and for every degree the temperature
changes the voltage increases or decreases with 10 mV. It has a temperature span between 55°C and +155°C, however this configuration does not allow for negative voltages, meaning
that it cannot measure temperatures below zero. The supply voltage for the sensor is
brought down from 12 V to 5 V with the use of a TS7805 voltage regulator. The outgoing
signal is amplified roughly four times by the non-inverting amplifier which gives the sensor a
span from 0°C to about +120oC before the 5 V input limitation of the ADC is reached.
Temperatures above +120oC may damage the ADC and the interface kit, so it is important not
to expose the sensor to such levels. The amplification is done solely to give the readout a
higher resolution, at the cost of some accuracy due to recalibration errors. Before the signal
reaches the ADC it is passed through a standard, first order, passive low-pass filter with a low
cut off frequency of roughly 35 Hz to filter out fast fluctuations in temperature and noise
induced from other circuitry.
Schematic 1 Temperature measurement circuitry
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5.2.2 Measuring vibrations
To measure vibrations an inductive sensor (figure 3) is used which
reacts to objects with magnetic properties 3 to 8 mm away from it,
giving off a linear response relative to the distance. Note that the
sensor's sensitivity and output voltage change with the supply
voltage. It is therefore important to remember that this set up is
calibrated for a 12 V supply and that changing the supply voltage
will require recalibration of the algorithm used to calculate the
distance between the sensor and the object being monitored. A
Figure 3 Inductive sensor
(source:
www.bernstein-ag.de,
2010-03-01)
differential amplifier is used to amplify the signal around 3.2 times.
Because the sensor puts out almost 9 V when it is to far away to detect an object and
increases it's output with about 1.5 V until it reaches the minimum distance, a potentiometer
is used to get a zero reference level on the inverting input of the OP-amp. This is done to get
an output from the amplifier between 0 V and 5 V. The high output voltage from the sensor
is also the reason why an instrumentation amplifier, which has better performance (Molin,
1993, p 139-140), is not used. Since the OP-amps are only fed with a 12 V supply, the ones in
the first stage of the instrumentation amplifier will be bottomed out because of the high
input from the signals. Another
reason
amplifier
for
is
the
that
differential
it
saves
components and space on the
circuit
board.
Before
being
passed to the ADC, the signal is
low pass filtered to eliminate
frequencies above roughly 30 Hz.
Schematic 2 Vibration measurement circuitry
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5.2.3 Measuring current consumption
A small DC-motor is driven by a transistor with a potentiometer as throttle to allow for
changes in current consumption and speed without changing the load on the motor. The
principle behind measuring current is to measure the voltage drop over a small resistor
connected in series with the motor and to use that value and the known value of the resistor
to calculate the current with Ohm's law (I = U/R). Since no current is going in to the OP-amp
all current can be assumed to pass through the motor, the transistor and the resistor (R4).
The amplifier is set up to amplify the signal roughly 14 times to allow for measuring of
currents up to around 350 mA. Any currents above that limit will result in voltages in excess
of 5 V and may damage the ADC and the interface kit. No steps have been taken to limit that
voltage other than using a motor that through experimentation has never exceeded a current
of 100 mA. The outgoing filter has a low cut of frequency of around 10 Hz to eliminate as
much noise as possible from the motor.
Although the test bench only uses a small DC-motor driven by a maximum of 12 V to
measure current through, the DDC is most likely to be used in situations with industrial sized,
high power AC-motors. The choice of using a DC-motor is motivated by the need for mobility
at demonstrations and because of it being easy to manage and control.
Schematic 3 Current consumption measurement circuitry
[20]
5.2.4 Measuring rotational velocity
The method chosen to measure the rotational velocity of the outgoing shaft from the motor
is an optical one. An infra red light emitting diode (IR-LED) is placed next to a phototransistor
which responds to infra red light. The light from the LED is reflected off a piece of tape
attached to the shaft, causing the phototransistor to periodically open and close. The voltage
over the transistor is sent through a comparator, where the reference level is adjusted by a
potentiometer, putting out a logical zero when the reflexive tape passes the LED. To protect
the digital input of the interface kit from the rest of the circuit an optocoupler is used. The
optocoupler also inverts the digital signal by putting out a logical one when the tape is
passed. This is read by the interface kit as a logical zero due to its inverted inputs, which is
favourable because the interface kit responds faster to positive flanks than negative ones and
the signal stays low for a shorter period than it stays high. Another advantage that comes
with the optocoupler is that it is able represent the logical ones with 5 V whereas the OPamp used as a comparator only reaches around 3.5 V due to the 5 V power supply combined
with the limitations of the OP-amp. The pulses emitted by the circuitry are then counted and
timed and used by software to calculate the speed of the outgoing shaft of the motor.
Because of the low sampling frequency of the interface kit only one pulse is measured per
revolution. To achieve higher resolution and better accuracy more pulses should be
registered, but that would also require lowering the speed of the motor with the same factor
as the pulses have been increased with to avoid sampling errors. The motor used is fitted
with a gearbox which limits the speed of the outgoing shaft to a maximum of roughly 200
rpm.
Figure 4 Optical pulse method
Schematic 4 RPM measurement circuitry
[21]
5.2 Software for sampling and calculations
The Phidget interface kit 8/8/8 that was chosen to read the signals from the sensors uses an
event-based architecture. This means that every time a change is detected on one of the
inputs an interrupt is issued and an interrupt service routine (ISR) is called. It is possible to
ignore these interrupts and instead use an approach where the interface kit is constantly
polled for the values, however that is not recommended by the manufacturer and it would
further complicate the programming of the device. Each analogue input channel has its own
settings for sensitivity where the user can decide how big a change needs to be registered in
order for the ISR to be called. For further information on how the interface kit is being
handled refer to the documentation of the Phidget21 API (available at www.phidgets.com,
2010-03-01).
All data read from the sensors is represented by an integer value, spanning between 0 and
1024 for the analogue signals, and 0 or 1 for the digital ones. The upper limit of 1024 is due
to the 10 bit conversion in the ADC (210 = 1024). These values only represent the output
voltage of the amplifiers and need to be processed further in order to be converted into the
values of actual interest, such as temperature and distance. Formulas for interpreting the
sensor raw data are derived from measurements and calibration of the hardware. These
formulas are then used in algorithms to calculate the different parameters of interest.
Note that the performance of the interface kit is limited and that the way it is designed leads
to some problems that cannot be fully solved, only taken into consideration. Apart from the
low sampling frequency the biggest problem is that there is no way of setting the priority of
interrupts. What this means is that one ISR can be called while another one is being run
before the algorithms there have been completed leading to incorrect values. For example a
pulse from the rev counter can interrupt the function for calculating vibration frequency not allowing the vibration timer to be stopped until the rev counter ISR has returned, leading
to timing errors. The only steps taken to limit these errors is using a motor with a lower
speed and only reading one pulse per revolution. Also the low pass filter on each analogue
input prevents fluctuations to occur at to high rates.
[22]
5.2.1 Calculating temperature
The method for calculating the temperature registered by the sensor is fairly straight
forward. Since the sensor is calibrated to put out 10 mV/°C and 0 V at 0°C all you need to do
is to divide the voltage with the amplification (A) of the amplifier and multiply it with 100.
Say for example that an input voltage of 1 V is read by the ADC. Since the signal has been
amplified around four times this means that the sensor puts out
1/4 V= 0.25 V
This value is then multiplied by 1000 to convert it from V to mV and divided by 10 to see how
big a change in degrees from 0oC that represents:
0.25 V * 1000 = 250 mV
250/10 = 25oC [V/(V/°C) = °C]
The last two steps are simplified by just multiplying with 100 directly giving the final
equation:
Equation 1 Temperature
T
U in
* 100
A
Because of the use of event-driven programming the temperature value is only updated
when a big enough change has been registered. This sensitivity is set during by the program
during start up and i.e. setting it to a value of 10 in the 0 to 1024 range of the ADC will
represent roughly 50 mV. This is a method that can be used to filter out fast and small
fluctuations and excess noise coming mainly from running the motor. Note that this
limitation does not affect the resolution of the sensors; the smallest representation of the
ADC is still 5 mV.
[23]
5.2.2 Calculating vibration amplitude and frequency
Because the interface kit has such low a sampling rate of ~65 Hz on it's analogue inputs it is
in no way suitable for monitoring vibrations, which is a complex subject involving a lot of
digital signal processing and frequency analysis. So the methods used in this thesis are
extremely simplified to limit the demands on performance of the hardware and software. In
the end it is not the DDC:s job to analyse the signal, only to pass on the data acquired, and
different ways of acquiring the data will be implemented. The algorithms used here are only
designed to calculate mean values of amplitude and frequency over a set number of periods.
These values are basically only useful when measuring on objects oscillating with stable
period times and amplitudes. One feasible implementation could be to watch for
misalignment of rotating shafts and axles. However, the raw data from the sensor measuring
distance to the object in question is also passed on from the DDC to be further analysed and
processed in a later stage of the system. This is where a major issue with using interrupts
comes in; since the signal is only updated when a change has been registered there is no way
of setting the sampling frequency of the incoming signal, which needs to be known for
proper analysis. One way of getting around it is to sample the sampled signal at a known
interval, but at no higher rate than the 65 Hz limit of the interface kit to avoid aliasing and
misrepresentation.
A linear equation for calculating the current distance between the sensor and an object has
been derived from measuring and calibration of the hardware. To calculate the mean
vibration amplitude the program checks for positive and negative peaks in the signal. When a
set amount of peaks have been found a mean value of the amplitude of those peaks is
calculated and the value of the signal is updated. The method used for calculating a mean
frequency uses the same way of detecting a number of peaks, but measuring the time
between the first one and the last instead of their amplitude. This value tells how long
average time it takes between peaks and is used to calculate frequency.
[24]
5.2.3 Calculating current consumption
Current consumption is calculated simply with the use of Ohm's law and the known
amplification of the signal. Ohm's law states that I = UR/R, where UR = Uin/A is the voltage
drop over the resistor and A is the amplification of the signal. And since the resistor used to
register a voltage drop over is at R = 1 Ω, the formula can be simplified to I = Uin/A. This value
is also used to calculate the energy consumption in watts just by multiplying it with the 12 V
level of the power supply (P = I*U).
5.2.4 Calculating rotational velocity
Each time a pulse is issued by the IR-LED/phototransistor-pair a counter is incremented and
when that counter reaches a specified value the time between the first and and last pulse is
registered. This timer is used to average a value in revolutions per minute of the outgoing
shaft of the motor. The number of pulses that needs to be registered before calculating the
speed is configurable by just changing one variable in the software. If the motor rotates too
fast the interface kit will not be able to register and handle the pulses resulting in sampling
errors and incorrect values. The highest frequency of ingoing pulses possible to handle varies
with the rate the other inputs issue interrupts. That is why a motor with relatively low speed
is being used to make sure that such a frequency is never reached.
[25]
6. Results
This chapter describes the general functionality of the test bench, its performance and how it
is used.
6.1 Test bench set up
When the Phidget interface kit is attached to an USB-port of the computer being used to run
the DDC-program on the program will automatically recognize the interface kit and start
waiting for interrupts. If the interface kit is not attached when the program starts running or
is detached during runtime the program will go into a state where it waits for the kit to be
reattached. It is currently programmed to wait indefinitely, but can be set to time-out if an
attachment is not registered within a specified time frame. Note that the power supply for
the sensors and other peripheral hardware does not need to be turned on in order for the
interface kit to be recognized since it draws its power from the computer via the USB.
6.1.1 Analogue and digital connectors
Each signal has to be connected to a specific port of the interface kit, as specified in the
software. This is because there is no way for the program to know what kind of sensor is
currently connected and what it's output represents – all it reads is a voltage. Connecting the
wrong wire to the wrong input will result in the wrong algorithm being run on the wrong
signal leading to corrupt values. The inputs are currently specified as shown in table 1, but
can be changed and more can be added by editing a header in the software.
Sensor/function
Port
Colour of wire
Vibrations
A0
green/yellow
Current
A1
grey/white
Temperature
A2
blue/purple
Rev counter
D7
brown/red
Table 1 Ports and connectors
It is important that the ground pin of the interface kit is connected to ground on the circuit
board. Reversing polarity may result in damages to the computer.
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6.1.2 Encapsulation and operation
All equipment is contained in a single box with two levels. The lower level, which is concealed
from the outside, contains all electronics for amplification, controlling the motor and the
interface kit, whereas the top level, which is visible through the transparent lid of the box,
holds the sensors and motor. The only external connections needed to be done are to a 12 V
power supply and connecting the USB-cable to the computer.
To control the speed of the motor a potentiometer is mounted through the side of the box.
When the motor starts to rotate the sensor measuring vibrations will also begin detecting
changes because the outgoing shaft of the motor, which it measures against, is deliberately
mounted with a slight offset and at an angle to cause imbalance.
A light bulb, which can be turned on and of with a switch, is mounted next to the
temperature sensor. When it is turned on it generates heat and the sensor will register the
increase in temperature. This is a simple way of showing that the sensor works and responds
to changes.
6.2 Updating signals
Each signal coming from the input module is provided a unique identification number by the
DDC and this id then used and recognized by the DDC and the server logging and subscribing
to the signal. Whenever the input module registers a change in one of the values it is
monitoring it sends an update to the DDC with the new value and its corresponding id. The
DDC however does not immediately pass this update on across the network since it uses
timed updates instead of interrupts. Each signal in the DDC has an update frequency
configured by the central server and is only passed on at a certain interval. This is done to
limit unnecessary data flow across the network. Also there is a difference in what update
frequency is interesting and necessary for the different signals, i.e. the current amplitude of
vibrations changes fast and needs to be constantly sampled whereas temperature may vary
at a lower rate. It is important to remember, while using this set up for acquiring analogue
data, not to update signals faster than 65 times per second, since that is the maximum rate
of which they are sampled. Signals measured which contain frequencies higher than ~30 Hz
cannot be trusted according to the sampling theorem because they exceed the Nyquist
frequency (Harnefors et al., 2004, p 30).
[27]
6.3 Watchdogs
Because the interface kit only notifies of registered changes, and updates only are made
when those changes issue a call to an ISR, some algorithms using a count of how many times
that particular ISR have been called will sometimes not be able to be fully run. Say i.e. that
the motor stops with a last known speed of 200 rpm. This means that no more pulses will be
registered and the value of 200 will not be recalculated and updated until the motor starts
running again. A similar issue is found with the function calculating vibrations. This is solved
with the use of watchdog timers which, if a change has not been registered within a specified
time frame, resets the values of the affected variables to zero.
6.4 Resolution of analogue signals
Each analogue signal is represented by a 10-bit digital value over a span of 5 V meaning that
the LSB (Least Significant Bit) of the ADC represents ~5 mV, which is the smallest change that
can be detected and the main limitation of the resolution. The different sensors also have
different built in limitations to their maximum values and in which span they work, also
restricting the resolution. This chapter describes a theoretical estimation of the resolution for
each sensor. The equation used for calculating the smallest change possible to detect is:
Equation 2 General resolution
RES X 
X
* LSB
U
Where ΔX represents the span in the parameter measured by the sensor, ΔU the span in
voltage change in the amplified signal from the sensor, and LSB is a constant (5*10-3 V)
representing the smallest change possible to detect by the ADC.
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6.4.1 Temperature resolution
Using equation 2 from above and substituting X → T, where T is temperature, the resolution
of the temperature sensor can be calculated. As mentioned in chapter 4.2.1 the sensor has a
span of 0-120°C over 5 V giving
|ΔT| = 120 – 0 = 120°C
And
|ΔU| = 5 – 0 = 5 V
Insertion into equation 2 gives:
Equation 3 Temperature resolution
RES T 
T
120
* LSB 
* 5 * 10 3  0.12C
U
5
This is the smallest change in temperature that is possible to detect with this set up of
sensor, amplifier and ADC.
6.4.2 Vibration amplitude resolution
The resolution in the current distance between the inductive sensor used for measuring
vibrations and an object is calculated in the same manner but by substituting X → d, where d
is distance. The sensor reacts to objects between 8 and 3 mm away giving
|Δd| = 8 – 3 = 5 mm.
At 3 mm it gives out ~4.7 V and at 8 mm ~0.4 V resulting in
|ΔU| = 4.7 – 0.4 = 4.3 V
And
Equation 4 Distance resolution
RES d 
d
5
* LSB 
* 5 * 10 3  6 * 10 3 mm  6 μm
U
4.3
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6.4.3 Current consumption resolution
By substituting X → I, where I is the current passing through the resistor, a resolution when
measuring current consumption can be derived. Since there is no limit to how much current
can be passed through the resistor and the motor (apart from the physical one where i.e. the
resistor overheats) ΔI needs to be calculated from the 5 V span of the ADC (ΔU = 5 V) and the
14 time amplification (A) of the signal. The maximum value I is allowed to have is one
resulting in a 5 V signal out from the amplifier. According to Ohm's law
I = Uin/R
Where in this case
R=1Ω
And
ΔUin = ΔUout/A = 5/14 = 0.35 V
See figure 5 for details. These values of Uin and R give
|ΔI| = 0.35/1 – 0 = 0.35 A = 350 mA
And
Equation 5 Current consumption resolution
RES I 
I
350
* LSB 
* 5 *10 3  0.35mA
U
5
Uin
A
Uout
I
ADC
R
Figure 5 Method for measuring current
[30]
7. Discussion
The results of the thesis, the test bench that has been deigned and its features are explained,
analysed and discussed in this chapter. Also some points on what could be improved in
future work and what problems have been encountered and solved are presented.
7.1 Results and analysis of results
The test bench constructed has relatively low performance compared to systems being used
in the industry which the DDC is supposed to interface with. The interface kit has a low
resolution in the analogue to digital conversion, and a really low sampling frequency. But
since the test bench is only designed for presentation and development purposes and not to
implemented in a live process it does not really matter. The only important thing is that
signals are being read and passed on across the network. Based on this the the sensors and
the algorithms used to analyse the values read are allowed to be poorly calibrated. Another
reason for the poor accuracy is a lack of access to equipment needed for proper calibration.
These limitations are not built into the hardware of the sensor-circuitry, so if one wishes to
improve performance one would only need to recalibrate the sensors and possibly upgrade
the interface kit. This will naturally result in having to reprogram the input module to be
compatible with the new DAQ, but this is something that will need to be done in every
unique application where the DDC is to be implemented.
7.1.1 Analysis of hardware
All hardware for the sensors, such as amplifiers and filters are soldered onto an
experimentation board, which is not an ideal solution. To further increase performance and
reduce noise it should be mounted on a proper circuit board with ground planes and
properly drawn wires. The biggest issue encountered while constructing the hardware is the
noise induced into the circuitry when running the motor. Because of the need to limit the
speed of the outgoing shaft, a motor with a gearbox is being used. This is an old and worn
out gear box which does not run smoothly resulting in large spikes of current when the gears
connect and apply load to the motor. These spikes affect the power supply of the entire
circuit and the stability of the other sensors. To limit this problem large decoupling capacitors
are used, but they do not fully eliminate the noise. The biggest problem with these spikes is
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apparent when measuring the actual current being drawn by the motor. Because the current
is constantly fluctuating with relatively high amplitude and the interface kit is not able to
handle too high data rates the signal is passed through a filter with a really low cut-off
frequency of 0.33 Hz before reaching the amplifier. If this filter is not used the signal will
greatly affect the performance of the OP-amp circuit which starts to oscillate and corrupts
the signals coming from the other sensors. Another step taken to limit the effect of these
spikes in current is that the circuitry for controlling the motor is mounted on a separate
board with its own 5 V voltage regulator. To further limit these problems a motor with a
smoother running gear box would need to be used or the DAQ could be replaced with one
with higher sampling rate to allow for the use of motor with higher speed and without gears.
Note that the choice of properties for the amplifiers and filters are restricted by the limited
access to components, and that therefore a lot of compromises have been made when it
comes to performance.
7.1.2 Analysis of software and interface kit
The API and design of the Phidget interface kit 8/8/8 has been the biggest factor when
deciding on how to design the software for acquiring data. Because it uses interrupts to let
the user know when a change has taken place there is no need for setting timers and
sampling frequencies which can be quite complex. This has simplified programming but also
has some apparent drawbacks - the biggest one being that there is no way of setting the
priority for the interrupts. One issue encountered during development was that when several
sensors changed too rapidly, the program started to put out corrupt values. It was discovered
that this is due to one ISR being interrupted by a
change in another signal, delaying the timers used in
that particular ISR. Figure 6 shows a representation of
such an error where the timer in ISR1 is not stopped
when it should. To avoid these errors the hardware has
been given certain limitations, such as a slower motor,
and also the sensitivity of each analogue input is
ISR1
-----------<start timer>
<command>
<command>
<stop timer>
<calculate>
<rerturn>
configured not to issue interrupts from too small
changes in the signal.
Figure 6 ISR issue
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ISR2
-----------<start timer>
<command>
<command>
<stop timer>
<calculate>
<rerturn>
The configuration of sensitivity of each signal in the software also has certain effects on the
values calculated. Especially on the function calculating average frequency and amplitude of
vibrations because increasing sensitivity will result in the detection of lower peaks, which will
otherwise be filtered out. These lower peaks occur more often than the large ones and will
naturally affect the average amplitude and frequency if they are detected. The algorithms are
still somewhat useful, but they only calculate values representing an average of frequencies
and amplitudes larger than the minimum change in distance possible to detect. To get
reliable values from the algorithms calculating amplitude and frequency the incoming signal
will need to be as clean and stable as possible. Signals with high frequency content will result
in larger errors, and more misleading values. The proper analysis of vibration is not meant to
be handled by the DDC, which is why the algorithms used in this thesis are allowed to be less
accurate, as long as they result in a value of some interest. Since the raw data of the current
amplitude is constantly logged and passed on by the DDC, there is still a possibility to
implement more advanced analysis of the signal at a later stage of the system.
7.2 Future work
There is still a lot of work to be done with the DDC-system – mainly with the server where all
data is to be collected and analysed. However a method for collecting and sending data has
been developed and the concept has been proven to work, and that is the only purpose of
this thesis. Future work based on the thesis could consist of developing a product for
acquiring data from scratch to be used with a specific set of sensors that can be mounted and
used by the DDC for general data acquisition to eliminate the need to interface with existing
systems. Another thing that should be done is to properly calibrate the sensors in a more
controlled environment to get more accurate values.
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8. Summary and conclusions
The problem consisted of investigating which parameters are of interest when trying to
decide the state of a machine or process and finding sensors suitable for measuring these
parameters. Hardware and software for getting these signals amplified and read into a
computer had to be designed to be able to pass the values on across the network. Four
different parameters were found to be of most interest:
 Temperature
 Vibrations
 Current consumption
 Rotational speed
A test bench consisting of a box containing various sensors and objects to be measured
against, such as a motor and a heating element, was constructed. Temperature is measured
with a simple sensor that responds linearly to changes in temperature. To measure vibrations
an inductive sensor is used which reacts to objects between 3 and 8 mm away and puts out a
different voltage depending on the current distance to the object. This distance is used by
the software to calculate average values of amplitude and frequency of the vibrations. The
current consumption of the motor is calculated by measuring the voltage drop over a small
resistor connected in series with the motor. To measure the rotational speed of the motor an
IR-LED (Infra Red Light Emitting Diode) and a phototransistor that reacts to infra red light is
used. The LED emits light which is reflected off of a reflexive tape attached to the outgoing
shaft of the motor. This is registered by the phototransistor which periodically opens and
closes giving off one pulse per revolution of the shaft. These pulses are timed by the
software to calculate a value in revolutions per minute.
To get the analogue signals read into a computer a Phidget Inteface Kit 8/8/8 is used. This
interface kit is attached to a USB-port of a computer and supports 8 analogue and 8 digital
inputs. All analogue signals are amplified to fit within the 0-5 V span of the ADC built in to
the interface kit, and filtered to limit too high frequencies and fluctuations. Once the signals
have been sampled and their corresponding values have been calculated the module that
reads the data updates the DDC where each signal has been given a unique id number. This
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signal is then passed on by the DDC at set intervals depending on configuration and the
subscriptions made by the server where the data is to be logged and analysed.
[35]
9. References
 Harnefors, L, Holmberg, J and Lundqvist, J, (2004), Signaler och system, Liber AB,
Stockholm.
 Molin, B, (1993), Förstärkarteknik, Stiftelsen kompendieutgivningen, Stockholm
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