Autonomous Slot-car System
Section L01, Slot-car Team
Final Report – ECE 4007: Capstone Design Project
Author: ………………………………………………………………………
Souma Mondal
Undergraduate Student
School of Electrical and Computer Engineering
Author: ………………………………………………………………………
Dhaval Patel
Undergraduate Student
School of Electrical and Computer Engineering
School of Aerospace Engineering
Author: ………………………………………………………………………
Souma Mondal
Undergraduate Student
School of Electrical and Computer Engineering
Advisor: ……………………………………………………………………..
Aaron Lanterman, Ph.D.
Project Advisor
Associate Professor
School of Electrical and Computer Engineering
School of Electrical and Computer Engineering
College of Engineering
Georgia Institute of Technology
Atlanta, Georgia
Submitted 11 December 2008
Table of Contents
Executive Summary ........................................................................................................... i 1. Introduction ................................................................................................................... 1 1.1 Objective ................................................................................................................... 1 1.2 Motivation ................................................................................................................. 2 1.3 Background ............................................................................................................... 2 2. Project Description and Goals ..................................................................................... 3 3. Technical Specifications ............................................................................................... 7 3.1 Operational ................................................................................................................ 8 3.2 Performance .............................................................................................................. 8 3.3 Physical ................................................................................................................... 10 4. Design Approach and Details..................................................................................... 11 4.1 Design Approach .................................................................................................... 11 4.2 Codes and Standards ............................................................................................... 23 4.3 Constraints, Alternatives and Tradeoffs ................................................................. 24 5. Schedules and Milestones ........................................................................................... 25 6. Project Demonstration................................................................................................ 26 7. Marketing and Cost Analysis ..................................................................................... 27 7.1 Marketing Analysis ................................................................................................. 27 7.2 Cost Analysis .......................................................................................................... 28 8. Summary ...................................................................................................................... 29 References ........................................................................................................................ 31 Appendix A – Detailed Timeline.................................................................................... 32 Executive Summary
Slot-car racing is a popular hobby involving driving model cars around a track along
metal electrified rails. One of the biggest manufacturers of slot-car equipment is
Scalextric. A common problem for slot-car enthusiasts is the lack of human competitors.
Many of these customers lose interest over time. The autonomous slot-car, costing $230,
will provide customers with a readily available opponent, revitalizing their interest in the
hobby. The sales of other Scalextric slot-car equipment will also be bolstered by the
renewed interest in the customer base.
The autonomous slot-car project aims to build a prototype computer controlled slot-car
that can drive itself around the track. A line detector mounted on the car will detect
checkpoints taped onto the track to determine position. Another line detector mounted at
the base station will reset the position every time the car finishes the lap. Data from the
car is wirelessly transmitted to the base station then sent to the laptop. The control
algorithm determines the optimal track voltage which then amplified and put onto the
track.
Although the prototype did not meet all of the cost or performance targets, it successfully
demonstrated the validity of the concept. The data collected helped identify deficiencies
in the design approach and suggested changes be made for the future. The two major
changes would be the use of an onboard PIC and the addition of an accelerometer.
i
1. Introduction
Slot-car racing is a hobby involving racing model cars around a track on electrified rails
as shown in Figure 1. The slot-car is a 1:32 scale model with an electric motor. The
motor is driven by the voltage between the two sides of the rails. The speed of the car is
controlled by changing the voltage applied to the rails. This is accomplished by
manipulating variable resistors in the handheld controllers via the triggers.
Figure 1. Overview of a slot-car system.
1.1 Objective
The autonomous slot-car is a prototype that consists of a modified car with sensors,
reflective tape on the track, and control software on the computer. The prototype should
compete successfully against a human player.
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1.2 Motivation
A common problem for slot-car enthusiasts is the lack of human competitors. Many of
these enthusiasts lose interest over time because they are unable to play frequently. These
customers are unlikely to purchase Scalextric products because they are underutilizing
their past purchase. The autonomous slot-car provides customers with a readily available
opponent, revitalizing their interest in the hobby. The renewed interest among the
customer base should bolster the sales of other Scalextric slot-car equipment.
1.3 Background
Scalextric produced an autonomous slot-car called the Challenger in 2004 priced at $100
[1]. It has since been withdrawn from the market for unspecified reasons. The Challenger
used a micro-controller on the car to regulate the speed of the car. While there were two
speed options, they were both not challenging for experienced racers with the more
expensive and fast slot-cars [2].
The Challenger used an optical sensor to detect speed, but there was only one position
sensor to detect the beginning of the track. Therefore, in case of a derailing the
Challenger has to restart from the beginning of the lap [3]. This behavior forces the race
to end at the first derailing though traditional rules advocate simply replacing the car on
the track and continuing.
The main feature of the Challenger was its wooden consistency, delivering lap after lap
of exactly the same time. While enthusiasts liked this racing profile, the car was not fast
enough for them. However, amateurs found it intimidating to compete against such
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consistency, especially since there were only two speed options and the transition
between them was steep.
The autonomous slot-car is able to offer much more customization of the racing profile
because of the user interface.
2. Project Description and Goals
A slot-car by itself can be controlled using a hand-held controller. This controller varies
the voltage on the track which in turn controls the speed of the slot-car. The essential goal
of the project was to modify an existing slot-car with its underlying functionalities and
build a system to enable it to control itself without the need of an externally held
controller. To fulfill this functionality, the car would need to have three key subsystems:
•
Obtain data about current position and speed from its environment
•
Relay this information to the base station using wireless technology
•
Use this data through a control algorithm to determine the appropriate track
voltage to control its subsequent speed
Target Market The autonomous functionality is what would appeal to slot-car enthusiasts, which defines
the target market. Currently, slot-car racers need an opponent against whom they can
compete. It often happens that the customers buying these race-sets do not have
opponents to compete against and are unable to use the sets to their maximum potential.
3
This product aims to provide a competitive opponent to race against, which would
provide customers the flexibility in using their sets at any time.
Target Price Slot-car enthusiasts span all backgrounds and ages. The secondary goal of the project,
therefore, was to create a system that was relatively easy to use by a consumer from a
non-technical background and was cost-effective. The Scalextric Challenger (which no
longer exists) was the only similar product ever built that targeted the same market in an
attempt to provide the same features. A target price of $100 was set as a goal for the
maximum cost of the system in order to meet the cost standards set by the Scalextric
Challenger which also costs $100.
Target Characteristics Finally, it was deemed crucial that the system be flexible and reliable in terms of its
performance. The final goal of the project was to build a system that not only
autonomously controlled the car, but was also scalable. This meant that the system
controlling the slot-car should function for track layouts of any geometry. To ensure the
system was reliable, the system should also be able to recover from derailing incidents
and resume normal operation.
With these goals in mind, the following features were proposed for the autonomous
control system:
•
Autonomous control of slot-car around the track
•
Continual knowledge of car’s position on the track and instantaneous speed
•
Speed control based on position along track
4
•
Contingency feature to resume race after a derailing occurrence
•
Adaptability to different track geometries and layouts
•
Option to choose different difficulty levels
•
Ability to read voltage from the track
•
Sensor placement algorithm to place position sensors along the track
•
Ease of use and simplicity in implementation
Final Implementation The actual implementation was able to meet nearly all of the goals specified. The current
implementation of the system has enabled the electric slot-car to control itself without the
need for an external controller. Thus, the current system determines its position using the
reflective strips, relays them wirelessly to the base station, which then outputs the
appropriate voltage onto the track. This meets the primary goal of the system. The core
technical details demonstrating the working of each of the three subsystems have been
highlighted in the Design Approach section.
The only change made was with respect to the placement of the position sensors. In order
to keep the system cost as low as possible the position sensors were removed altogether.
This provided multiple advantages. It significantly reduced the cost of the system,
simplified tasks to be done at the consumer end, and removed the need to create a sensor
placement algorithm. As a substitute, strips of reflective tape were used. These are cheap
and easy to implement by the consumer.
5
The secondary goal of the system was only partially met. Due to time constraints, a proof
of concept was created. As a result, the project in its current standing is not completely
user friendly. In order to use the system, the user needs to power up the components and
enter the track layout in the computer. These steps need to be taken with some care in
order to ensure the proper functioning of the system; this makes the system more difficult
to use by younger consumers such as children.
The secondary goal also aimed for the project to be cost-effective. This goal was
achieved partially. Again, due to time constraints, a proof of concept was built.
Therefore, components that would allow rapid prototyping and debugging were selected.
This resulted in the selection of an expensive data-acquisition device. However, the total
cost of all the other components combined falls within the target price of the project goal
of $100. In future developments, the data acquisition device will be substituted with the
use of an ADC (analog to digital converter), a DAC (digital to analog converter), and a
microcontroller which are a lot cheaper.
The final goal of the project was to build a system that could cater to any track layout.
This has been achieved with the use of the current algorithm. A voltage recording feature
has been implemented that allows for voltage profiles to be measured. A voltage profile
is a recording of the track voltage over time, which can be played back to use as a racereplay. A derailing feature has also been implemented in order to enable the system to
withstand any derailing incidents. The derailing feature is a display of the last triggered
checkpoint by the car. In the case of derailing, the user only needs to place the car back at
the last triggered checkpoint and the car resumes normal operation. A reset-every-lap
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feature resets the position of the car every lap in order to avoid cumulative position
errors.
Overall, the project was able to meet all the technical aspects mentioned in the feature
list. The project was unable to fully meet the ease of use and target price goals.
3. Technical Specifications
Corresponding to the goals mentioned in the previous section, technical specifications for
the project were created. These quantitatively defined the test parameters the project
would have to meet for the project to be cost-effective, user-friendly and reliable, thereby
meeting the overall goals for the system. Table 1 shows both the desired as well as the
actual achieved specifications.
As can be seen in Table 1, the technical specifications are divided into four main groups:
Operational, Performance, Interface, and Physical. All but two of the preliminary design
specifications were met, namely the position sensing error and the velocity sensing error.
Multiple specification requirements were made during the course of the semester, as the
project strategy and implementation evolved.
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Table 1. Technical Specifications for the Autonomous Slot-car System
Operational
Controller/Track Voltage
Desired
0 -16VAC (0-12VDC)
Achieved
0 – 16VAC (0-12VDC)
Performance
Position Sensing Error
Velocity Sensing Error
Data Transmission Rate
Pulse Error Rate
Data Acquisition Rate
Failures per lap
Speed range
Desired
0.3 %
0.1 %
> 15 Samples/s
< 15 %
> 30 Samples/s
< 0.2
0-950 mm/s
Achieved
0.4 %
0.6 %
56,000 Samples/s
12.5 %
10,000 Samples/s
0.1
0 – 2500 mm/s
Interface
Wireless Receiver to PC
Desired
USB
Achieved
USB
Physical
Weight of Car
Desired
< 120 % of original weight
Achieved
112.5 % of original weight
3.1 Operational
The only operational specification was to control the track voltage to the same voltage
range as the hand-held controller. The existing 0-12 VDC specification was therefore
chosen. This was achieved with the use of an additional non-inverting amplifying circuit
attached to the output of the National Instruments data acquisition device.
3.2 Performance
The performance specifications changed during the course of the project as the
techniques used to achieve the main functionality evolved. While some of the previous
specification requirements could not be met due to a change in technology used, other
specification requirements were added and met.
Originally, a chain of position sensors were implemented across the entire track. When
used with an ADC of minimum 10-bit resolution operating at 30,000 samples/s, the
8
sensors would have provided a position error of 0.3% as specified in the preliminary
design specifications. However, in order to make the system scalable, this would require
a large number of sensors for larger track layouts which would make the system
expensive. Additionally, this makes it more difficult to implement. For these reasons, the
position sensors were not used in favor of passive reflective strips.
These reflective strips would substitute for sensors on every track segment. This would
make them a much cheaper alternative and require no skill whatsoever on the user end for
implementation. However, the tradeoff that had to be made in the process was the
frequency and quality of position data. This is because position and velocity data would
now be obtained only once per segment.
Position & Velocity Sensing Error With the use of the control algorithm, high accuracies were still obtained for the position
and velocity determination of the car. With extensive testing, one checkpoint was lost
every 160 checkpoints (every ten laps), which constitutes an accuracy of 99.375% for
position detection. Also, the worst case error for velocity determination of the car was at
its maximum speed of 2500 mm/s with a value of 0.6%. Thus, these two operational
specifications could not be met due to the change in implementation technologies used.
Data Transmission Rate With the progression of the project, the car’s average speed around the track was also
determined. This provided the minimum rate of transmission for position data that the
wireless transmission would have to meet. With testing, it was seen that at the car’s
maximum speed of 2500 mm/s, the car would be able to cross a maximum of 15 position
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sensors (“checkpoints”). The wireless transmitter-receiver pair used is able to transfer
56,000 samples/s, thus allowing for greater position data transmission rates in the future.
Failures per Lap With testing, a reasonable performance requirement would be to have no more than one
derailing every five laps, as this was the average frequency of derailing with human
players. This would equate to 0.2 failures per lap. The system was able to achieve a better
statistic of only one derailing every 10 laps, equating to 0.1 failures per lap.
Speed Range The goal was to control the car to achieve speeds between 0-950 mm/s. This was the
average speed with which human players could guarantee a failure rate of 0.2 per lap.
However, a control speed with a range of 0-2500 mm/s was achieved with a lower failure
rate of 0.1 per lap using an optimal control algorithm.
3.3 Physical
Finally, all the above specifications could only be met if the car was light enough to
move around the track, even while bearing the additional weight of the transmitter and
position sensor. All of the components also had to fit within the width of the car so as to
ensure no interference with a car on an adjacent track. After measurements of the weights
of different components, a realistic goal would be to ensure that the weight of the car
increase by no more than 20% of its original weight. This would allow the necessary
components to be placed onto the car while ensuring it was still light enough to move
around. With the choice of the specific components, the weight of the car was increased
by 12.5% to 90g.
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Thus all the necessary technical specifications of the system were met except for
positional and velocity error rates. This was due to the tradeoff in using a relatively
inaccurate technique in order to reduce overall costs.
4. Design Approach and Details
4.1 Design Approach
The overall system consists of three main subsystems:
•
Determination of position
•
Transmission of position data wirelessly
•
Control of track voltage using the position data and a control algorithm
The three subsystems work in conjunction to allow the car to autonomously control itself.
The slot-car uses a line detector with a series of position strips called checkpoints to
determine the car’s instantaneous location on the track. The line detector senses when the
car passes over a checkpoint and relays this information wirelessly from the transmitter to
the base station. This data is then sent to the PC through a National Instruments data
acquisition device (the NI DAQ). The control algorithm in the PC determines the
appropriate voltage for the car and reproduces this voltage back onto the track through
the NI DAQ. The working of each of these subsystems will be explained in the
subsequent sections. The overall system implementation is shown in Figure 2.
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Figure 2. Overall system implementation for the autonomous slot-car system.
Position Detection The sensor shown in Figure 3 is a standard line detector used in robotic applications [4].
It is placed on the car itself and is powered indirectly by button cells on the car. It is used
to sense the position of the car along the track through the detection of reflective strips
placed on the track, also referred to as checkpoints.
Figure 3. The line detector used to sense position by detection of checkpoints.
12
As seen in Figure 4, one checkpoint is placed per track segment along the track to
facilitate position detection of the car. The reflective strips can be any reflective tape,
such as shiny duct tape. However, the line detector must be placed no more than half an
inch above the track to detect the checkpoints correctly. When line detector detects a
checkpoint, the control algorithm increments a counter that lets it know where the car is
along the track. This also allows the car to derail and still resume normal operation, as it
can be placed at the last detected checkpoint along the track.
Figure 4. An illustration showing checkpoints along the race track.
The functionality of the line detector can be understood using Figure 5. The line detector
consists of an IR LED and a phototransistor. The IR LED continuously transmits IR
radiation. However, only when the car passes over a checkpoint, the IR radiation is
reflected back from the track onto the line detector itself where it triggers the
phototransistor. This creates a 0-5V pulse on the output of the line detector. The duration
of pulse depends on the time taken for the car to pass over the entire width of the
reflective strip. This is used to determine the speed of the car, since the width of each
13
strip is a standard 50mm. Thus, a single pulse is used to increment the car position as
well as obtain its velocity.
Figure 5. The IR transmitter and phototransistor in the line detector.
A line detector was chosen due to multiple reasons:
1. Inexpensive – A line detector is relatively inexpensive as it is a commonly used
component in many robotics applications.
2. Easy to use – Line detectors are very easy to use. They only require a 5V power
supply and provide a 0-5V output pulse. Additionally, they include LEDs that
light up when a phototransistor is triggered. This is useful while trying to test the
component or while debugging a problem.
3. Digital output – The output of the line detector is either a high or a low. In this
sense, it has a purely digital output. This makes it convenient for interfacing with
other components, such as wireless transmitters.
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4. Continuous sensing – The IR LED on the line detector is continuously
transmitting IR radiation and therefore is continuously checking sensing. This
removes the need to poll the line detector for information, significantly reducing
the software needed to obtain a position reading.
A line detector has also been placed at the start point of the race track. This sensor allows
for error correction. It resets the position of the car when the car passes by every lap,
thereby avoiding position errors to accumulate from one lap to the next.
As mentioned previously, a change was made in terms of the position sensors used.
Instead of using position sensors, passive reflective strips were used to act as checkpoints
along the track. Thus, when a line detector senses a checkpoint, the position of the car is
determined with respect to the position of the sensor along the track. With this technique
used to determine position, no sensor placement algorithm was needed. Instead, one
checkpoint was setup per track segment for each track segment in the overall layout.
Wireless Transmission of Position Data A wireless transmitter is mounted on the slot-car that transmits the position data. It was
necessary to have a transmitter that was able to transmit data fast enough, with a strong
signal on a particular channel. The HP3 transmitter and receiver from Linx Technologies
[5] were used for this purpose as shown in Figure 6.
15
Figure 6. The HP3 transmitter and receivers used from Linx Technologies.
The transmitter is placed on the car itself and is also powered from the button cells
indirectly. The transmitter’s signal was strong enough for short signals, thereby
eliminating the need of antennas for the transmitter and receiver. This was a benefit as the
antennas weighed a significant amount and would have slowed down the car.
Additionally, mounting the antenna vertically on the car would have made the car
unstable by making it top-heavy.
Data Acquisition The wireless receiver is interfaced to a computer that receives the position data and
makes the necessary adjustments to the track voltage. The analog input channels of the
NI DAQ were utilized [6]. This provided two major advantages:
1. It created an integrated data collection source which allows for future
improvements easily.
16
2. It allowed the use of LabVIEW, a rapid prototyping environment. Since the goal
was to build a prototype/proof of concept, this enabled the engineers to analyze
the receive data easily and thereby make extremely quick changes to the
algorithm for further testing.
A tradeoff was made to use this data acquisition tool. It was the most expensive part in
the entire system, costing over the target price all by itself. Due to its ability to save time,
it was selected in favor of meeting time constraints and building a working proof of
concept. Its utility will be substituted by an ADC, a DAC and a microcontroller in the
future, which will allow the cost of the system to fall well below the target price. The NI
USB 6008 module used can be seen in Figure 7.
Figure 7. The National Instruments USB 6008 used as for data acquisition.
Power Supply The selection of a power supply to power the line detector and the transmitter was a
critical part of the design process. The following constraints had to be met:
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1. Weight – Since the batteries needed to be on the car, they needed to be light so as
to not slow the car down.
2. Durability – The battery needed to be long lasting in terms of its power
capabilities in order to avoid the need for continual replacement after every race.
3. Cost – The power source would need to be inexpensive so that they could be
easily replaced.
In lieu of these design constraints, the CR2450 Lithium-Manganese Dioxide button cell
shown in Figure 8 was chosen. It met all of the above mentioned constraints:
1. Weight – The CR2450 only weighs 6g. As a point of comparison, the car weighs
85gm. The cell is thereby less than 10% of the car’s body weight.
2. Durability – The CR2450 has a lifespan of 600mAh. Assuming the car was used
for a half hour continuously everyday, the battery would still last for about ten
days.
3. Inexpensive – The CR2450 is inexpensive (< $1) and is readily available in any
convenience store. It is one of the most commonly found button cells used in a
large variety of daily applications such as watches, clocks, and displays.
18
Figure 8. CR2450 Lithium-Maganese Dioxide button cells used.
However, since the nominal voltage for this button cell is 3V, two of these are used in
series to provide 6V which is then fed into a switching voltage regulator to provide a
steady 5V power supply line that powers the line detector and the wireless transmitter on
the car itself as shown in Figure 9. Using a switching regulator ensures high efficiencies
in the voltage transformation as well as a steady 5V supply line.
Figure 9. Schematic of the voltage regulating circuit to provide the +5V supply rail.
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Amplifier Once the appropriate control voltage is determined, the voltage from the NI DAQ needs
to be applied to the track itself. However, this output voltage only ranges from 0-5V.
Since the track voltage specifications require that the track be powered from 0-12V, an
amplifier was used to boost the 0-5V control signal from the DAQ. The schematic has
been shown in Figure 10.
Figure 10. The non-inverting amplifying circuit used to amplify the NI DAQ signal.
Figure 10 shows the standard non-inverting amplifying circuit where the LM741
operational amplifier acts as the core component. Additionally, a power field-effect
transistor has been used in the feedback loop to ensure that enough current is being
supplied onto the track. Without the transistor, the motor in the car will not be able to pull
enough current from the track to be able to run, even though its voltage specifications are
met. Shown in Figure 11 is a screen capture outlining the input and output signals from
the amplifier. As can be seen, the input voltage at 5.3V gets scaled to an output voltage of
11.9V.
20
Figure 11. A screen capture showing the input 5.3V was scaled to an output 11.9V.
The amplifying circuit needs to be powered with two supply rails, well above the input
voltages into the amplifier itself. The amplifier gets saturated when the input voltage
approaches either one of the supply rails. To avoid saturation, the amplifier is powered by
+15V and -15V supply rails.
Control Algorithm The software is divided into two main loops. The read loop runs in parallel to the main
control algorithm. It gets data from the NI-DAQ and determines when the car passes over
a checkpoint. The read loop sends sensor events to the main control loop through the
sensor event queue. An overview of the main control loop is shown in Figure 12 below.
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Figure 12. Flow diagram illustrating the main control loop.
At the beginning of the race the track layout is entered by the user. Since the dimensions
of each track segment are standardized, the length and initial complexity of the segment
are known. Since the position data coming into the algorithm indicates the current track
segment the next three track segments are considered. The total complexity is determined
as shown in Equation 1 below.
.
∑ 1000
The complexity is normalized to be between 0 and 1 and labeled
(Eq. 1)
. This value is high
when the upcoming track is difficult and the car should slow down and low when the car
should speed up. An initial estimate of the desired voltage is obtained by the formula in
Equation 2.
5
5
(Eq. 2)
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The above estimated control voltage does not take into account the current speed of the
car. If the difference between the current speed and the desired speed is large then the
control voltage modified so that the car decelerates to the desired speed faster.
The car has to maintain a minimum speed to avoid stopping before the next checkpoint.
This is especially important if the estimated control voltage is lower than 2.5 volts, the
minimum voltage to start moving the car. There are therefore two thresholds, if the car is
slower than 2000 mm/s the voltage is adjusted to at least 2.5V and if the car is slower
than 1700mm/s the voltage is adjusted to at least 3.0 V.
4.2 Codes and Standards
Many of the components included in the design comply with common standards.
•
The NI-DAQ USB-6008 utilizes USB 2.0. Since the NI-DAQ links the consumers
PC to the base station it is important that the communication standard used is
widely available. The USB 2.0 standard is present on the majority of computers.
•
The Linx HP3 wireless modules use the 900MHz frequency range to
communicate. This frequency range is reserved for household electronics.
•
The output of the amplifier has to comply with the Scalextric standard track
voltage range. While this standard is not published explicitly, it is important to
ensure proper functionality and safety.
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4.3 Constraints, Alternatives and Tradeoffs
Position Sensing The first fundamental task of the system is to accurately and simply track the position of
the slot-car around the track. An accelerometer could be placed on the car and its G
forces could be measured giving information on the degree of turns the car is making.
The downside to this setup is accelerometers are not accurate enough to reliably measure
the G forces on the slot-car. Accelerometers are also susceptible to noise and since the G
forces on the slot-car are so small in magnitude, those G forces would be
indistinguishable from noise. In addition, an accelerometer would be more expensive
than purchasing a series of sensors. The added weight of an accelerometer would hinder
the performance of the slot-car.
A better option is to have a system that knows exactly where the slot-car is on the track.
Optical or Hall sensors could be placed around the track that would send information
when the slot-car passes. However, for larger track layouts, more sensors would be
required which can become expensive. Also, the user would have to carefully calibrate
each sensor as well as make sure they are all properly powered. An optical sensor placed
on the slot-car itself is a better option. With this setup, only a strip of reflective tape
needs to be placed on each track piece. Then, the optical sensor will recognize the
reflective tape as the slot-car passes over it. This is not only easier for the end user, but
cheaper for larger track setups.
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Track Learning vs. User Input The system must know the exact track layout to track the position of the slot-car.
Enabling the user to enter track information into a software interface is preferred to
designing a system where the slot-car learns the track. A system where the slot-car learns
the track is susceptible to error. If the user enters the track layout into a PC interface, the
system can have a guaranteed error free track layout. This allows the control system to
more accurately track the position of the slot-car. Since all track pieces for slot-cars are
standardized, it is simple for the user to enter the track layout onto a PC interface.
LabVIEW vs. Microcontroller Using LabVIEW and an NI DAQ is advantageous to designing a micro-controller system.
With LabVIEW, adjustments to the control algorithm can easily and rapidly be made.
Also, test data can be displayed within LabVIEW itself making prototyping quicker. For
the end user, software is easier to use and is more interactive. Additionally, it offers
features such as allowing the user enter different difficulties for the competing slot-car
and tracking high scores. Also, software upgrades can easily be made available if future
improvements to the system are made.
5. Schedules and Milestones
There are three major milestones that need to be met to ensure that the system functions
appropriately:
•
Controlling the slot-car with the base station
•
Tracking the position of the slot-car
•
Completing the optimization of the control algorithm
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For complete scheduling details, please refer to the timeline in Appendix A.
6. Project Demonstration
The first step of the demonstration was to verify that the autonomous slot-car would
function without human intervention on different track layouts. The slot-car was tested on
two different track layouts. The short track layouts is shown in Figure 13.
Figure 13. Illustration of the short track layout.
The car adapted to the different track layouts without any problems. It accelerated and
decelerated appropriately as it went around the track. The car derailed on occasion but
continued as soon as it was put back on the track, this feature validated one of our design
objectives.
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The most important specification is that the system is competitive against a human user.
Due to vast differences between different human players it is difficult to empirically
determine success in this area. However, the car competed against Professor Tittle, an
experienced slot-car enthusiast and proposed this project.
On the short track the car was competitive and beat Dr. Tittle in a race lasting 10 laps. On
the long track Dr. Tittle was able to beat the car but acknowledged that the car performed
well. The second objective was therefore met.
The race replay feature was also be demonstrated. The race profile was successfully
recorded to a file. The data was displayed graphically to verify that it accurately reflected
the race profile. However, due to the lack of a sample clock on the output channel of the
NI-DAQ USB-6008 model, the data could not be output to the track.
7. Marketing and Cost Analysis
7.1 Marketing Analysis
The only other automated slot-car controller available is the Scalextric Challenger.
However, it is limited in terms of features and robustness. The slot-car has to perform a
“learning lap” before it can start a race. It also only has two difficulty settings: “slow”
and “fast.” Races with the Challenger are limited to 25 or 50 laps. Customer reviews state
that the Challenger was not competitive enough to address the issue of not having another
competitor to race with.
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The autonomous slot-car has a scalable interface that can be tailored to a variety of users,
ranging from seasoned slot-car enthusiasts to children. The PC interface allows the user
to set the desired difficulty of the slot-car. In addition, the autonomous slot-car will be
more accurate in tracking the position of the slot-car. This means that over many laps, the
system will still be able to reliably track the location of the car around the track. Since a
CPU will do the bulk of the calculations, the software can be upgraded via downloads
from the web. It can also log best times in addition to other statistics.
7.2 Cost Analysis
The cost breakdown of the autonomous slot-car is shown in Table 2.
Table 2. Cost Breakdown for the Car and Base Station
NI USB 6009 (DAQ)
HP3 Series Wireless Receiver
Total Cost for Base Station
HP3 Series Wireless Transmitter
Line Tracker
Four 3V Coin Batteries
Total Cost of Car
Unit Total Cost
$150
$30
$180
$25
$20
$5
$50
$230
The largest part of the budget went into purchasing the NI DAQ. The cost could have
been reduced if a microcontroller was implemented instead of using the DAQ. However,
the DAQ was chosen since it is conducive to rapid prototyping.
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8. Summary
The final implementation of the design is shown below in Figure 14. The dataflow in the
system is clearly indicated. This can be considered equivalent to the design overview in
Figure 3.
Figure 14. The final design implementation diagram.
The current layout of the components on the slot-car is show in Figure 15 below. Of
particular interest is that all the circuit boards are mounted onto the car using Velcro inorder to allow the car to be restored after the project. In a permanent version the
components should be mounted more securely.
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Figure 15. Layout of the components on the slot-car.
The completed project met most of the design goals and successfully implemented the
proposed features. The slot-car was able to run on an arbitrary track and recover from
derailment. The race replay feature was successfully able to record a human player’s race
profile however the NI-DAQ USB-6008 device does not have a sample clock on the
output channel and therefore does not allow us to replay the data at 10kHz.
The slot-car, as demonstrated, shows the validity of the concept; however, it fell short on
some design goals. The target price of $100 dollars was not met, and therefore the
marketability of the product has been affected. While the NI-DAQ and the LabVIEW
environment were helpful for rapid prototyping, the added cost is too high for inclusion
in a final product.
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The recommended alternative is to use a PIC microcontroller on the car to handle the
real-time processing. The extra features such as recording statistics in addition to others
can be provided by connecting the car to custom software on the computer via USB or
similar protocol. The requirements of the control algorithm can easily be satisfied by a
mid-range microcontroller. In this approach the user cannot interact with the software in
real-time because the laptop cannot connect to the microcontroller while the car is
moving. However, the cost benefits are too significant to ignore.
References
[1] Scalextric USA Inc. (2008, Sep.) Scalextric USA. [Online]. www.scalextric-usa.com
[2] Home Racing World. Challenge Yourself! The Scalextric Challenger System.
[Online]. http://www.homeracingworld.com/challenger.htm
[3] Slot Car Garage. (2004, Jan.) Scalextric Challenger Review. [Online].
http://www.slotcargarage.com/scgarticles/radrev1042.htm
[4] Lynx Motion. (2008, Oct.) Tracker Sensor. [Online].
http://www.lynxmotion.com/Product.aspx?productID=57&CategoryID=8
[5] Linx Technologies. (2008, Oct.) HP3 Series - A low cost multi-channel RF module
available at 900MHz, capable of wireless audio and data transmission. [Online].
http://www.linxtechnologies.com/Products/RF-Modules/HP3-Series-MultipleChannel-Radio-Frequency-Module/
[6] National Instruments. (2008, Oct.) NI USB-6008. [Online].
http://sine.ni.com/nips/cds/view/p/lang/en/nid/14604
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Appendix A – Detailed Timeline
October 7
• Determined which sensors to use
October 14
• Determined which wireless module to use
October 15
• NI DAQ purchased
• HP3 Wireless transmitter/reciever purchased
• Line Detector Purchased
November 3
• Interfacing wireless transmitter with reciever
November 4
• Determining voltage control algorithm
November 5
• Amplifier Circuit for NI DAQ output built
November 7
• Milestone #1: Controllint slot‐car with PC
November 14
• Building Line detector and transmitter circuits for slot‐car
November 16
• Interfacing wireless reciever with NI DAQ
November 17
• Software development for position tracking
November 20
• Milestone #2: Position tracking of slot‐car
December 7
• Implementation of lap reset switch
December 9
• Milestone #3: Final software algorithm to control slot‐car 32