Autonomous Slot Car

Racing

Third Year Project Report

May 2013

School of Electrical and Electronic Engineering

Ganegama Vithanage Don Perera

ID number: 8091067

Supervised by: Dr. Piotr Dudek

2

Table of Contents

Table of Figures......................................................................................................................................... 4

Table of Tables .......................................................................................................................................... 5

Abstract ..................................................................................................................................................... 6

1 Introduction ....................................................................................................................................... 7

2 Aims and objectives........................................................................................................................... 8

3 Tests done on the car ........................................................................................................................ 9

4 Power Delivery to components and motor ..................................................................................... 11

5 Different configurations considered ............................................................................................... 13

5.1 1st Configuration ...................................................................................................................... 13

5.2 2nd Configuration ...................................................................................................................... 15

5.3 3rd Configuration ...................................................................................................................... 18

5.3.1 Comparator circuit ............................................................................................................ 19

6 Software .......................................................................................................................................... 20

6.1 Initial EEEPROM setup ............................................................................................................. 20

6.2 Accelerometer Test .................................................................................................................. 20

6.3 Simple sensor to PWM ............................................................................................................. 22

6.4 Testing interrupts and EEPROM............................................................................................... 24

6.5 Reading values off the EEPROM .............................................................................................. 25

6.6 Main Program .......................................................................................................................... 26

6.6.1 Power Loss ........................................................................................................................ 29

7 Total Costs ....................................................................................................................................... 30

8 Conclusion ....................................................................................................................................... 31

3

9 Bibliography ..................................................................................................................................... 32

10 Appendix ...................................................................................................................................... 34

10.1 Progress report ........................................................................................................................ 34

10.2 Project Specification ................................................................................................................ 44

10.3 Project Plan .............................................................................................................................. 45

10.4 Risk assessment ....................................................................................................................... 46

4

Table of Figures

Figure 1-1: Basic components of the slot car and track system [1] ......................................................... 7

Figure 1-2: chassis and body of car originally ........................................................................................... 8

Figure 3-1: Track to measure maximum cornering speed ..................................................................... 10

Figure 3-2: Example track using crossover ............................................................................................. 11

Figure 4-1: Power regulator and motor drive circuit ............................................................................. 11

Figure 4-2: Crossover section, shown intersection of tracks do not provide power. ............................ 12

Figure 5-1: 1st Configuration diagram ..................................................................................................... 13

Figure 5-2: Example LabVIEW front panel. ............................................................................................. 14

Figure 5-3: 2nd Configuration diagram .................................................................................................... 15

Figure 5-4: LabVIEW VI connecting from Arduino to Pc ......................................................................... 16

Figure 5-5: Horizontal axis of the car ...................................................................................................... 16

Figure 5-6: Bluetooth module test VI ..................................................................................................... 17

Figure 5-7: 3rd Configuration diagram .................................................................................................... 18

Figure 5-8: Final components ................................................................................................................. 18

Figure 5-9: Comparator circuit ............................................................................................................... 19

Figure 6-1: Accelerometer output .......................................................................................................... 21

Figure 6-2: Effect of changing scaling ..................................................................................................... 23

Figure 6-3: Optical markers .................................................................................................................... 28

Figure 8-1: Final car ................................................................................................................................ 31

5

Table of Tables

Table 3-1: Circular track speed test ........................................................................................................ 10

Table 6-1: Scaling to PWM ...................................................................................................................... 22

Table 6-2: Effect of changing scaling ...................................................................................................... 23

Table 7-1: Costs ...................................................................................................................................... 30

6

Abstract

This project involves the creation of an autonomous slot car that can run the track at a reasonable

speed. Many designs for the component configurations were considered and tested but due to some

unforeseen circumstances the most optimal was not chosen. The final car uses an accelerometer and

two infrared sensors to recognize the track. The program will save the track to its non-volatile

memory so the car can be placed back on the track if it comes off.

7

1 Introduction

Slot cars are powered miniature vehicles that run on a standard Track. They are mainly used as toys

or in the competitive hobby of slot car racing but they have also been used to model highway traffic

on scenic layouts. The cars run on a track that has a groove in it. The cars are kept on the track by a

pin or blade which extends from the car to the groove in the track, the very first slot cars ran

between two rails. The term Slot car was invented to differentiate it from older Rail cars. Power

is sent to the car from two metal rails at either side of the groove to 2 brushes on either side of the

pin or blade on the car. The power supply can provide 15 volts but a potentiometer in the controller

can alter the voltage for each of the two tracks individually, in the early days the speed control was

an optional extra.

Figure 1-1: Basic components of the slot car and track system [1]

The standard cars are quite simple as it consists of a simple chassis, a fashionable interchangeable

body, a motor, a set of gears and wires connecting the brushes to the motor with a capacitor in

parallel to the motor. In the late 1930s,some mechanics/hobbyists used miniature combustion

engines to power the cars and had no control over the cars speed, this is similar to applying a

constant voltage to a modern track and running a slot car

8

Figure 1-2: chassis and body of car originally

The main challenge of this hobby is to adjust the speed to optimize the speed around the bends or

turns so that the cars do not come off the track; full speed on the straight (straight piece of track) and

then slow down just before the corner. The more enthusiastic of the hobbyists can and will change

almost all of the components. There is a wide variety of cars with different layouts;

different tire diameters, width and material,

different number and placements of magnets under the car( magnets attract the metal rails

embedded in the tracks and so creates more down force on the car which increases the grip

and so the maximum cornering speed of the car,

the gear ratio can be changed,

Some high speed slot cars have aerodynamic shapes so as to reduce drag and also to create

down force as air passes over the car and creates different pressure levels above and under

the car.

2 Aims and objectives

The aims of the project is to create an autonomous slot car. It will have to:

Learn the track

Deduce the optimal speed per section of track

Run the track at a reasonably fast speed

Be of lower cost than available solution

9

The main aim of this project is to build a car that can be placed on a track with minor modifications

and to run competitively. There are other autonomous options, most notably the Scalextric Digital

series. This product requires a special controller, power unit for the track and either a digital car or a

circuit which can be bought from then to be fitted into compatible analogue cars. The basic Scalextric

Digital set for 2 cars and track will cost 175 [2] but to get the autonomous features a Scalextric

Digital Advanced 6 Car Powerbase is required which costs 150 [3]. The price point for this project is

aiming at is 40 per car. Any track is compatible with this car and will work with that standards power

base that provides at least 12V.

3 Tests done on the car

Since the limitations in speed are mainly in the bends (turns) the first tests done were to check the

maximum speed around a full circle. The circle is standard because there is only one standard section

of 45 track.

45 curved track inside lane length = 20cm full circle: 160cm Diameter: 51.5cm

45 curved track outside lane length = 26cm full circle: 208cm Diameter: 66.25cm

Full length straight track both lane = 35cm

Half-length straight track = 17.5cm

The car was first run until it reached the fastest speed without going off the track. The current drawn

was measured from the 15DC power source as this reflects the current drawn when the track has a

15VDC constant. The current measured was used to ensure a constant speed. The car was run at this

constant speed for 10 laps and then then the time are divided by ten to get a speed per lap. The 10

laps were run 5 times for each of the lanes to reduce errors due to reaction times to click stopwatch

and the accuracy of the ammeter.

10

Figure 3-1: Track to measure maximum cornering speed

The starting current of the car is 0.33A from 15.2V and the lowest current required to keep the car

moving when already moving is 0.23A.

Table 3-1: Circular track speed test

CORNER

Outside Inside

Time (s/10 laps)

Average Time (s/lap)

Time (s/10 laps)

Average Time (s/lap)

RUN 1 7.35 0.735 6.23 0.623 RUN 2 7.59 0.759 6.05 0.605 RUN 3 7.15 0.715 6.34 0.634 RUN 4 7.18 0.718 6.54 0.654 RUN 5 7.99 0.799 6.33 0.633

AVERAGE TIME 7.31 0.731 6.30 0.630

Current Drawn 0.7A = 10.71W 0.72A=11.02W

Average Speed (m/s)

0.285 0.168

This shows that there is a clear reduction in speed when travelling in the smaller radius circle but the

lap time is actually lower when the cars are in the smaller radius inner circle which may mean that

this car in these conditions is actually faster if it takes the inner lane. This reduction in time is

normally compensated by allowing cars to have a crossover which allows the car to go into the other

lane and running two laps:

Probes used to measure

current from the power

source

Connection between the

standard Power delivery

and the circular track

11

Figure 3-2: Example track using crossover

The increased power drawn from the slower car can be attributed to the increased resistance in the

variable resistor in the controller.

4 Power Delivery to components and motor

Figure 4-1: Power regulator and motor drive circuit

A Unipolar Pulse Width Modulation signal generated by the microcontroller is used to control the

speed of the motor. A unipolar pulse width modulation signal is a square wave where the equivalent

DC voltage is given by the ratio of signal ON time to the total period multiplied by the ON voltage.

12

The PWM signal is passed into the optocoupler [4] which controls a 5V regulator [5] which is used to

amplify the current from the microprocessor. The 5V regulator and the optocoupler help isolate the

sensitive Input / output microprocessor pins from the power circuit. If 10V is needed the ratio will

have to be 2/3 (2/3 * 15 = 10). The motor cannot react to the quick changes from ON to OFF so

sees it as a constant voltage. There is a considerable amount of heat generated from the transistor

which required 2 24C/W to dissipate the heat.

The microprocessor is powered with a 12V regulator [6]. The microprocessor has in built regulators

(5V and 3.3V) to power itself and the I/O. The previously mentioned 5V regulator can also be used to

power the microprocessor but the current drawn from the regulator (5V) would increase the heat

generated. Inside the confined body of the car will cause the temperature of both transistor and

regulator to increase rapidly and may damage the components. Experiments I conducted showed

that using a separate 12V regulator reduced the temperature of all the components.

Figure 4-2: Crossover section, shown intersection of tracks do not provide power.

As mentioned earlier the track provides power to the car but in some sections contain interruptions

in power like the crossover shown in Figure 4-2:

The area shown here is made of plastic so when the pickup contacts pass over this part the car will

momentarily loose power, momentum ensures the car will continue over this section but the

microprocessor will lose power. To avoid loss of power to the microprocessor there is a large

capacitor across the output of the 12V regulator. The software is also capable of continuing after a

complete power loss and this will be discussed later in the software section.

13

5 Different configurations considered

5.1 1st Configuration

All processing is done externally. This method has a small microprocessor and Bluetooth module on

board. Bluetooth is preferred because most modern laptops have embedded Bluetooth capability

and Bluetooth adapters are quite common. An ideal external processor is software on a Personal

Computer preferably LabVIEW due to its many libraries including a Bluetooth library. The

microprocessor is used to relay the values from the accelerometer and IR sensor to the external

processor via the Bluetooth connection.

If the microprocessor has no Analogue to Digital Converter (ADC) both the IR sensor and the

Accelerometer need to be digital; in this case the microprocessor will need to convert the signal it

gets from the Accelerometer to numerical values then send it to the computer while a comparator

can be used to digitize the infrared sensor

If the microprocessor has an ADC a simple analogue accelerometer can be used. While a second ADC

channel can be used for the IR sensor, using a comparator will increase the processing speed as the

processor does not have to wait for the second ADC.

The data coming from the Accelerometer and IR sensor can then be processed by the computer. The

data can then be viewed in real-time through the computer which will ease troubleshooting. The

software can also be used to create a virtual track to be viewed on the screen and track the progress

of the car. Data such as the gain and sensitivity of the infrared sensors can be changed without

Digital Accelerometer

Infrared Sensor Comparator

Microprocessor +

Wireless Module

External Processor

Motor

Figure 5-1: 1st Configuration diagram

14

needing to stop the car. The computer will then send a numerical PWM value (0-255) to the

microprocessor which will then convert it to a PWM signal to send to the motor.

Components used in this configuration do not use much power and can be powered with one 5V

regulator. The main reason this was not used is because creating the microprocessor and Bluetooth

combination would be too time consuming. Possible solutions to this would be the BLEduino [7] or

the Lightblue Bean [8]. The Lightblue Bean would be ideal for this configuration as it has 4 PWM

ports, 2 ADC channels and a built in accelerometer but it was not available during the allocated time.

Figure 5-2: Example LabVIEW front panel.

This configuration is the most optimal as the weight will be the lowest compared the other two and

the components will be comparatively small which could ease in the placement and to keep all the

weight as low as possible.

15

5.2 2nd Configuration

A microcontroller and a wireless module are on-board. The microcontroller will perform some key

functions including Using its ADC to sample analogue sensors then send digital values to the wireless

module which in turn sends this data to an external processor. This data can be saved and then

viewed in real time. The processing is done by the microprocessor i.e. the algorithm is contained in

the microprocessor. This configuration is quite flexible and can have many fail safe programs as the

processing can be done by the microprocessor or the external processor. Like in the previous

configuration some variables can be changed while the car is running and if required the

microprocessor can be reprogrammed through the Bluetooth module. This method was initially

chosen mainly due to its flexibility but the Bluetooth module purchased developed an error and

damaged the microprocessor when it failed. The Arduino UNO was used as the microprocessor [9].

Infrared Sensor

Analogue Accelerometer

Microcontroller Motor

Wireless Module

External Processor

Figure 5-3: 2nd Configuration diagram

16

Figure 5-4: LabVIEW VI connecting from Arduino to Pc

This LabVIEW VI was used to test the Arduino and to test the capabilities of the accelerometer to

check that various parts of the accelerometer and Arduino are functioning properly together. Two

analogue pins were tested for the x-axis and y-axis but final design only measures the acceleration

along the y-axis. The acceleration of the car along the x-axis was going to be used as well but the

increased processing time required outweighed any advantages of using it in the algorithm. A gain

was attached to the Y axis values to get a simple but varying PWM output that will be able to drive

the car around the track.

Figure 5-5: Horizontal axis of the car

The same VI was used to try and control the Arduino trough the Bluetooth module but it did not

seem to be able to connect to the Arduino.

17

Figure 5-6: Bluetooth module test VI

This VI was created to try and troubleshoot the problem and to figure out whether the problem was

with the Arduino or the Bluetooth module. The program looks for Bluetooth devices, choses the first

one (the only Bluetooth device connected to the computer is the Bluetooth module which is checked

via the device manager on Windows) displays the ID of the Bluetooth module and the various

services. It then selects the first service (the Bluetooth module has only one service: 1

communication port), displays its unique ID then sends some data and receives data from the

communication port.

Running the program revealed that the Bluetooth close function is not able to close the Bluetooth

connection so the Bluetooth module reports that it is still active so a physical reset is required.

An android phone with an application to read the communication from the Bluetooth module was

used to look further into the problem. Using a simple send function from the Arduino the

accelerometer data was sent over the Bluetooth module. This yielded some promising results as the

values from the accelerometer were seen on the phone but prior to being able to figure out the

reasons for this success the Bluetooth module ceased working so based on the amount of time

already invested and results obtained it was deemed that continuing this method would be a waste

of time.

18

5.3 3rd Configuration

This is similar to the previous configuration except for the lack of the Bluetooth module. A

comparator circuit is used similar to the first configuration to reduce the time taken to process the

data.

Figure 5-8: Final components

Digital Accelerometer

Infrared Sensor Comparator

Microprocessor Motor

Figure 5-7: 3rd Configuration diagram

19

5.3.1 Comparator circuit

Figure 5-9: Comparator circuit

Two TCRT5000 [10] were used as the optical sensor, the TCRT5000s have an infrared emitter

alongside the sensor. If there is no light hitting the sensor the output from the sensor is pulled to 0V

as the transistor acts like an open circuit. As the amount of light increases the transistor allows more

current through which would increase the voltage across R3 or R5, this is the voltage used as the

input. The reference voltage is provided by a voltage divider shown above. To make the reference

voltage adjustable a three pole variable resistor is used. The operational Amp [11] compares the

voltage from the sensor to the reference voltage and outputs +5V if the input from the sensor is

greater than the reference voltage else it outputs 0V.

20

6 Software

6.1 Initial EEEPROM setup

#include

boolean Learning = true;

int currSection = 3;

boolean Start = false;

void setup(){ //initializes the eeprom values to the type

needed

EEPROM.write(0,Learning); //this initializes the zeroth element to boolean

EEPROM.write(1,currSection); //first element int

EEPROM.write(2,Start); //second element boolean

}

void loop(){

}

This program is needed to ensure the variables on the EEPROM (Electrically Erasable Programmable

Read-Only Memory) is in the correct format/type i.e.; int (integer), boolean.

This will ensure that there are no errors when requesting values: it will ensure the variable type

requested by the main program is the same as the value saved in the EEPROM. The programmer

cannot check for these errors when it is verifying the code.

This program is only to be run once if a new Arduino is used or the EERPROMs first three elements

have been rewritten.

6.2 Accelerometer Test

void setup(){

Serial.begin(9600); //initializes the serial communication

}

void loop(){

Serial.println(analogRead(0)); //reads the analog pin 0 then sends it over

// the serial communication port

delay(500);

}

21

This program is used to ensure the Accelerometer [12] and ADC work correctly.

Shown below is an example output from this program

Figure 6-1: Accelerometer output

During the above example, the accelerometer was given an excitation of +g and g (g = 9.80665 m/s2

acceleration due to gravity) along the Y-axis (only the y axis values are shown above). The value of

116 is obtained when there is no acceleration along the y axis. Values of 91 and 141 are obtained

with g and +g respectively giving an approximate 25 per g.

22

6.3 Simple sensor to PWM

int val = 0;

void setup(){

pinMode(6,OUTPUT);

}

void loop(){

val = analogRead(0); //read analog pin 0

val = val -116; // scaling

val = abs(val); //the analog

val = val*10; //value to

if (val = 255){val = 255;} //can be sent

val = 255- val; //to the motor

val = val*0.8; //scaling the final value down

analogWrite(6, val); //generate the PWM signal

}

This program scales the output of the ADC connected to the accelerometer (val) to control the car

such that the car goes faster on the straight and slower around the corners.

Table 6-1: Scaling to PWM

Line of code\acceleration -g 0 +g

Output of ADC 91 116 141

val = val -116; -25 0 25

val = abs(val); 25 0 25

val = val*10 250 0 250

val = 255- val; 5 255 5

As see above a lower a lower lateral acceleration results in a higher PWM value which means an

increase in speed.

23

In the piece of code (val = val*10 ) the number 10 can be changed to adjus the acceleration to spped

ratio. A Larger number such as 10 would mean a small lateral acceleration will result in a large

decrease in speed and a smaller value will result in a smaller decrease in speed.

Figure 6-2: Effect of changing scaling

Table 6-2: Effect of changing scaling

Lateral Acceleration

-9.8 0 9.8

VAL *10 5 255 5

VAL * 7.5 67.5 255 67.5

VAL * 5 130 255 130

Currently the speed of the car is limited by the weight and the fact that the centre of weight of the

car is relatively high.

0

50

100

150

200

250

-9.8 0 9.8

Lateral acceleration (m/s^2)

val *10 val * 7.5 val * 5

24

6.4 Testing interrupts and EEPROM

#include

volatile int state = LOW;

volatile int state1 = LOW;

void setup()

{

pinMode(13, OUTPUT);

pinMode(12, OUTPUT);

attachInterrupt(0, blink, RISING);

attachInterrupt(1, blink1, RISING);

}

void loop()

{

digitalWrite(13, EEPROM.read(1));

digitalWrite(12, EEPROM.read(0));

}

void blink()

{

state = !state;

EEPROM.write(1,state);

}

void blink1()

{

state1 = !state1;

EEPROM.write(0,state1);

}

The comparator circuit shown before is used to change the analogue values of the IR sensors to a

digital signal when they detect the start of the lap and the start of every section. These signals are

used to drive two interrupts. In this test the signals toggle on and off the two LEDs independently

(connected to Pin 13 and 12).

EEPROM storage is used as it is a form of non-volatile memory; this memory does not get erased

when there is a loss in power.

Saving the state of the LEDs in EEPROM means that the LEDs will still have their values after the

Arduino has been turned off.

25

6.5 Reading values off the EEPROM

#include

int count = 0;

void setup(){

Serial.begin(9600);

}

void loop(){

if(count >= 200)count = 0;

Serial.println(EEPROM.read(count));

delay(500);

count++;

}

Example output:

The value of learning currently 0 = false as the learning phase was over

The value of the last piece of track the car was on

the value of Start 1 = true, it has started the program

This program is used to check the values written to the EEPROM

The first value is the boolean value Learning, the second is the int currSection the third is the boolean

Start.

0

13

1

183

156

116

135

145

156

161

147

137

37

73

76

100

152

159

26

6.6 Main Program

#include

volatile int val = 0;

volatile boolean Learning = EEPROM.read(0);

volatile boolean Start = EEPROM.read(2);

volatile int currSection = EEPROM.read(1);

volatile float sum = 0;

volatile float count = 0;

volatile int mtr = 100; //current motor PWM value

volatile int mtrB = 110; //previous motor PWM value

volatile float avg = 0; //average of val

volatile int state = LOW; //state of LED

void setup(){

pinMode(13, OUTPUT);

pinMode(12, INPUT);

pinMode(8, OUTPUT);

pinMode(7, OUTPUT);

pinMode(6, OUTPUT);

attachInterrupt(0, inter02, RISING); //left sensor

attachInterrupt(1, inter14, RISING); //right sensor

digitalWrite(8,HIGH);//

digitalWrite(7,LOW);

}

void loop(){

Learning = EEPROM.read(0); //refreshes Start and

Start = EEPROM.read(2); //Learning every loop

currSection = EEPROM.read(1);

if (digitalRead(12) == HIGH){ //Initializing using a pin

EEPROM.write(0,true);

EEPROM.write(1,3);

EEPROM.write(2,false);

}else{

if(Learning){

count++;

analogWrite(6,100);

delay(5);

val = analogRead(0);

//Serial.println(val);

val = val -116;

val = abs(val);

val = val*4.5;

if (val = 255){

val = 255;

}

val = (255- val) * 0.85 ;

sum = sum + val;

}else{

27

if((mtrB-mtr)>35){ //brake

analogWrite(6,0);

delay (50); //delay in milliseconds sets the

} //amount of time braking occurs

analogWrite(6,mtr); //generates a PWM signal

}

}

}

void inter14(){ //right sensor interrupt

if(Learning){

avg = sum/count; //calculates the average of the

//accelerometer values sampled

EEPROM.write(currSection,avg); //writes the average to EEPROM

sum = 0; //reset sum

count = 0; //reset count

currSection++; //increments the counter that

//indicates the current

EEPROM.write(1,currSection); //section of track then saves

//it to the permanent memory

val = 0; //reset val

}else{

mtrB = mtr; //saves the previous track's val

mtr = EEPROM.read(currSection); //reads the current track's val

currSection++; //increments current section

EEPROM.write(1,currSection);

}

}

void inter02(){ //left sensor interrupt

state = !state;//

digitalWrite(13,state); //

if(Start){

if(Learning){

avg = sum/count; //cals avg save it

EEPROM.write(currSection,avg); //like the previous interrupt

sum = 0; //reset val,count and sum

count = 0;

val = 0;

EEPROM.write(1,currSection);

}

Learning = false; //indicates learning is complete

EEPROM.write(0,false);

currSection = 4; //reinitializes the current section

EEPROM.write(1,3); // to the first track

}else{

Start = true;

EEPROM.write(2,true);

}

}

28

Figure 6-3: Optical markers

When the car is initially placed on the track the car has a constant velocity of 40%. The car will come

over the start of the lap denoted by the white strip on the left. As seen above the left sensor

interrupt will be triggered before the start section marker (white strip on the right). The left interrupt

will initially only set the Start boolean variable to true which allow the car to only start implementing

the recorded data after the next pass over the start line.

The ADC constantly samples the accelerometer and then this value is stored in val. This is then scaled

to end up with a viable PWM output which is ready to be sent to the motor as described in section

5.3. This value is then summed up over a period of time while taking count of the amount of data

being summed up. When the right sensor interrupt is triggered the average is taken. This average is

then stored in the EEPROM in the with respect to the section of the lap it is on. The sum, val, and

count are reset. The variable containing the number of the current section of track the car is currently

on (currSection) is incremented.

The incrementation of count in the main loop is placed prior to the sum being calculated. If it was

placed after sum has been calculated there is a chance of the interrupt occurring prior to count being

incremented causing a higher than expected motor speed which may cause the car to go off the

track. The interrupt will reset sum, val and count so if the incrementing if count is placed near the

end of the loop this will register an increased count causing a less than optimal average.

When the car passes over the start line again the boolean Learning will be disabled which cause the

car to implement a recall mode. This will also count as the end of the current section of track and so

will calculate and save the average into the EEPROM as in the right interrupt; this only happens when

learning is still true, only during the second pass over the start. During the first crossing of the start

29

line val is being calculated so the section of track prior to the start will have its value saved in the

third element of the EEPROM. To avoid this, the second passing of the start line will cause the

program to start reading the values from the fourth element onwards; currSection is set to 4 and will

count forward from there.

From the second lap onwards the car reads the PWM value from the memory to send to the motor.

The PWM value written gets updated every time the right interrupt is triggered. If the left interrupt is

triggered (car goes over start) the currSection gets initiated to 4 again and the car continues reading

from the EEPROM.

Resetting the car to learn a new track or relearn the current track can be done by applying 5V across

the pin 12, this will turn Learning true, Start false and currSection to 3.

Variables used in both the ISR function and the mail Loop function need to be declared as volatile.

This tells the compiler that the variables may change out of sync with the main code and to load the

variable from the RAM rather than the storage register.

6.6.1 Power Loss

As mentioned earlier there is a fairly large capacitor across the 12 regulator output which will keep

the Arduino running for a second and a half but what happens if the car comes off the track? It will

need to relearn the whole track.

Using the permanent values stored in the EEPROM the car will not need to relearn the track.

Currently the car cannot resume if it leaves the track during the learning phase as the values from the

accelerometer will corrupt the data; a possible solution is to check the values with the previous two

values from the same section of track and reject the value if it deviates by a large amount (as would

be expected if the car leaves the track).

Due to the fact that all the variables controlling the various phases and the final PWM values being

stored on the permanent memory the car will be able to re-join the track and continue given that the

car is placed anywhere on the section of track the car originally came off of. The first section of track

after re-joining will be done on 50% speed initially then from the next section onwards the car will

resume as normal.

30

7 Total Costs

Table 7-1: Costs

COMPONENT QUANTITY PRICE PER UNIT TOTAL PRICE

Cars 1 25 25

Track 1 27 27 Arduino Uno R3 1 15.65 15.95

solder less Breadboard 1 3.6 3.6

extractor Set 1 3.55 3.55

Analog Devices - ADXL335BCPZ - IC, Accelerometer, 3 Axis

1 3.47 3.47

VISHAY SEMICONDUCTOR - 4N25 - OPTOCOUPLER

4 0.27 1.08

ROHM - BA033T - LDO, FIXED, 3.3V, 1A, TO-220FP-3

2 1.05 2.1

JY-MCU HC-06 Wireless Bluetooth transceiver module for Arduino

1 5.89 5.89

Vishay Semiconductor - 4N25 - Optocoupler

4 0.27 1.08

Arduino Uno Atmega328 1 18.05 18.05

INTERNATIONAL RECTIFIER - IRL3803VPBF - MOSFET, N, LOGIC, TO-220

1 1.81 1.81

Aavid Thermalloy - 507302B00000G - Heatsink, TO-220, 24C/W

4 0.233 0.932

Arduino Uno Atmega328 1 18.05 18.05

Arduino Uno Atmega328 1 18.05 18.05

TOTAL 145.612

As seen above I have used four Arduinos. During assembly a 15V lead was attached to the 5V input

pin due to improper wiring where there was a confusion of colours. As mentioned earlier the second

Arduino was damaged due to fault in the Bluetooth module. The third Arduino developed a fault

which resulted in a corrupted boot loader, an attempt was made to reinstall the boot loader but

there was no success.

31

8 Conclusion

The project start was delayed due to the late arrival of the ordered cars and track pieces. Further

delays ( failing Arduinos and Bluetooth module) set back the project further causing the first

hardware prototype available mid semester two and the first fully functioning car was available in

week 10 just prior to the start of the Easter vacation.

Further developments could have been done on the Bluetooth communication to allow more

complex algorithms run by the computer.

Possibilities of using bipolar PWM to increase the braking effect to counter the increased weight of

the car were considered but were not implemented due to time constraints, inefficiency and

increased heat generated in the motor.

The car currently works as planned but the speed is less than optimal and further optimization is

needed to match the speed of a much lighter stock car.

Figure 8-1: Final car

32

9 Bibliography

[1] D. H. (. u. D.Helber), Wikipedia, 15 February 2007. [Online]. Available:

http://en.wikipedia.org/wiki/File:SlotcarElecCircuit.png.

[2] Scalextric, Scalextric Digital Racer Set, [Online]. Available:

http://www.scalextric.com/shop/sets/digital/c1327-scalextric-digital-racer-set/.

[3] Scalextric, Scalextric Digital Advanced 6 Car Powerbase, [Online]. Available:

https://www.scalextric.com/shop/pit-lane/power-and-controllers/c7042-advanced-6-car-

digital-powerbase/.

[4] Semiconductors, Vishay, 4n25, [Online]. Available:

http://www.vishay.com/docs/83725/4n25.pdf.

[5] Fairchild semiconductor, 7805, [Online]. Available:

http://www.fairchildsemi.com/ds/LM/LM7805.pdf.

[6] Texas Instruments, 7812, [Online]. Available:

http://www.ti.com.cn/cn/lit/ds/symlink/lm7805c.pdf.

[7] BLEduino, [Online]. Available: http://bleduino.cc.

[8] Punch Through Design Light blue Bean, Punch Through Design, [Online]. Available:

http://launch.punchthrough.com.

[9] Arduino, [Online]. Available: http://arduino.cc/en/Main/arduinoBoardUno.

[10] Vishay Semiconductors, TCRT5000, [Online]. Available:

http://www.vishay.com/docs/83760/tcrt5000.pdf.

[11] Microchip, MCP6001, [Online]. Available:

http://ww1.microchip.com/downloads/en/DeviceDoc/21733j.pdf.

33

[12] ANALOG DEVICES, ADXL335, [Online]. Available: http://www.analog.com/static/imported-

files/data_sheets/ADXL335.pdf.

34

10 Appendix

10.1 Progress report

Introduction

Slot cars are powered miniature vehicles that run on a standard Track. They are mainly used as toys

or in the competitive hobby of slot car racing but they have also been used to model highway traffic

on scenic layouts. The cars run on a track that has a groove in it. The cars are kept on the track by a

pin or blade which extends from the car to the groove in the track, the very first slot cars ran

between two rails. The term Slot car was invented to differentiate it from older Rail cars. Power

is sent to the car from two metal rails at either side of the groove to 2 brushes on either side of the

pin or blade on the car. The power supply can provide 15 volts but a potentiometer in the controller

can alter the voltage for each of the two tracks individually, in the early days the speed control was

an optional extra.

35

Figure 1: Basic components of the slot car and track system [1]

The standard cars are quite simple as it consists of a simple chassis, a fashionable interchangeable

body, a motor, a set of gears and wires connecting the brushes to the motor with a capacitor in

parallel to the motor. In the late 1930s,some mechanics/hobbyists used miniature combustion

engines to power the cars and had no control over the cars speed, this is similar to applying a

constant voltage to a modern track and running a slot car

Figure 2: chassis and body of car being worked on currently

The main challenge of this hobby is to adjust the speed to optimize the speed around the bends or

turns so that the cars do not come off the track; full speed on the straight (straight piece of track) and

then slow down just before the corner. The more enthusiastic of the hobbyists can and will change

almost all of the components. There is a wide variety of cars with different layouts;

different tire diameters, width and material,

different number and placements of magnets under the car( magnets attract the metal rails

embedded in the tracks and so creates more down force on the car which increases the grip

and so the maximum cornering speed of the car,

the gear ratio can be changed,

36

Some high speed slot cars have aerodynamic shapes so as to reduce drag and also to create

down force as air passes over the car and creates different pressure levels above and under

the car.

Aims and Objectives

The aims of this project is to create an autonomous Slot car. The car should be able to memorize the

track within one lap and then run the track as fast as or ideally faster than if a human were

controlling a similar car. This car is aimed at enthusiasts and even household consumers to test their

skills against. There are other autonomous slot car solutions, notably the Digital car and track set

from Scalextric but tis digital system requires for one to have digital enabled cars and tracks.

The major aim of this project is that the car can be placed on any track and run competitively with

little modifications; the only modification needed is small white reflective strips placed at the start of

each track section, the placement does not have to be exact.

Progress to date

Tests done on the car

Since the limitations in speed are mainly in the bends (turns) the first tests done were to check the

maximum speed around a full circle. The circle is standard because there is only one standard section

of 45 track.

45 curved track inside lane length = 20cm full circle: 160cm Diameter: 51.5cm

45 curved track outside lane length = 26cm full circle: 208cm Diameter: 66.25cm

Full length straight track both lane = 35cm

Half-length straight track = 17.5cm

The car was first run until it reached the fastest speed without going off the track. The current drawn

was measured from the 15DC power source as this reflects the current drawn when the track has a

15VDC constant. The current measured was used to ensure a constant speed. The car was run at this

constant speed for 10 laps and then then the time are divided by ten to get a speed per lap. The 10

37

laps were run 5 times for each of the lanes to reduce errors due to reaction times to click stopwatch

and the accuracy of the ammeter.

Figure 3: track to measure maximum cornering speed

Corner

Outside Inside

Time (s/10 laps)

Average Time (s/lap)

Time (s/10 laps)

Average Time (s/lap)

Run 1 7.35 0.735 6.23 0.623

Run 2 7.59 0.759 6.05 0.605

Run 3 7.15 0.715 6.34 0.634

Run 4 7.18 0.718 6.54 0.654

Run 5 7.99 0.799 6.33 0.633

Average time between the

runs

7.31 0.731 6.30 0.630

Current drawn from 15.3v

source

0.7A = 10.71W 0.72A=11.02W

Average Speed (m/s)

0.285 0.168

Table 1: Circular track speed test

Probes used

to measure

current from

the power

source

Connection

between the

standard

Power

delivery and

the circular

track

38

This shows that there is a clear reduction in speed when travelling in the smaller radius circle but the

lap time is actually lower when the cars are in the smaller radius inner circle which may mean that

this car in these conditions is actually faster if it takes the inner lane. Another surprise is the power

consumption, the slower car in the inner lane uses 0.3W more than the faster car in the outer lane;

this may be caused by over steer causing the rear wheels to slide about.

The starting current of the car is 0.33A from 15.2V and the lowest current required to keep the car

moving when already moving is 0.23A. Using:

P = I * V (1)

The power loss in the drivetrain is a maximum of 5.0W and a minimum of 3.5W, these values will

increase with the weight of components put in the slot car.

Power Delivery to components and motor

A Unipolar Pulse Width Modulation signal generated by the microcontroller is used to control the

speed of the motor. A unipolar pulse width modulation signal is a square wave where the equivalent

DC voltage is given by the ratio of signal ON time to the total period multiplied by the ON voltage.

The PWM signal is passed into the optocoupler to then control the 15V going into the motor. E.g. if

10V is needed the ratio will have to be 2/3 (2/3 * 15 = 10). The motor cannot react to the quick

changes from ON to OFF so sees it as a constant voltage.

39

Figure 4: Basic circuit layout

The power delivery circuit contains a regulator that outputs 5 volts but it is inefficient because it is

known to dissipate twice the power it provides to the 5V output. A buck regulator was considered

due to its utilization of a switching circuit which is more efficient but it is more expensive and needs

more components to run properly, the power drawn from the current setups 5V is acceptably low

(varies with the number of Input/output used on the microcontroller) so the regulator is being used.

The data connection between the microcontroller and Bluetooth module has been highly simplified

into one wire to have a neater circuit diagram.

The accelerometer outputs the acceleration through the x axis and the y axis as a voltage that can

then be read by the ADC on board the microprocessor.

Different processing methods considered

1st Processing method

All processing is done on the car. This method uses a microprocessor on board, Arduino, to sample

the accelerometer and infrared sensor then store the data, process the data into an easily

implementable PWM value to control the motors. The sensors can either be analogue or digital as

the microcontroller will have multiple ADC inputs. The advantage is that only one extra component

(microcontroller) over the standard accelerometer and infrared sensor is needed. But the

40

disadvantages are that the variable of the program can only be changed if one reprograms it. Real

time data is not available so it will be a bit harder to debug the algorithm.

Figure 5: 1st processing method

2nd Processing method

All processing is done externally. This method only has a wireless module on board. Bluetooth is

preferred because most modern laptops have embedded Bluetooth capability and Bluetooth

adapters are quite common. An ideal external processor is software on a Personal Computer

preferably LabVIEW due to its many libraries including a Bluetooth library. The wireless module is

connected to both the accelerometer and Infrared sensor, but both the sensors have to be digital

because most Bluetooth modules do not have an ADC. Accelerometers are available with digital

outputs but a comparator circuit is needed for the infrared sensor. The software on the PC will be

able to process all the data and store the data. Data can now be seen in real time easily by some

simple Graphical user interface. Without a dedicated PWM signal generator the computer software

will have to generate a PWM then send it through one of the channels of the Bluetooth module

which is then sent to the optocoupler. Similar to method one only one extra component is needed.

Variable can be changed by using a graphical user interface so the car does not need to stop to adjust

noncritical data (critical data is data that cannot be changed while the program is running such as the

Bluetooth channel being used).

INFRARED SENSOR

Analogue

ACCELEROMETER

MICROCONTROLLER MOTO

R

41

Figure 6: 2nd processing method

3rd Processing method

A microcontroller and a wireless module are on-board. The microcontroller will perform some key

functions including Using its ADC to sample analogue sensors then send digital values to the wireless

module which in turn sends this data to an external processor. This data can be saved processed and

then viewed real time. The external processor then sends a digital value to the wireless module which

then sends it to the microprocessor to then write it to the built-in PWM controller for the

optocoupler. This method is quite flexible and can have many fail safe programs as t can either be

based on this method or method one. This method needs both a microprocessor and a blue tooth

module so it may be the most expensive. Like in method two some variable can be changed while the

car is running. This method was chosen mainly due to its flexibility.

Digital ACCELEROMETER

INFRARED SENSOR Comparator

WIRELESS MODULE

EXTERNAL PROCESSOR

MOTO

R

42

Figure 7: 3rd processing method

Problems encountered

The main problem faced was the renewal of the finance system for orders so orders were placed late

and arrived late as well. Due to the unknown size and shape of the interior of the car no components

could be ordered in case the components do not fit in the car.

The next steps

Test the Arduino to make sure all the features needed work as planned.

INFRARED SENSOR

Analogue

ACCELEROMETER

MICROCONTROLLER MOTO

R

WIRELESS MODULE

EXTERNAL PROCESSOR

43

Printing out a PCB that will get screwed on between the chassis and the body.

Test and make sure the power circuit is working and does not take power away from the

motor as the power supply may have a fixed maximum current.

Run tests on the Buck regulator to see if the performance can outweigh the cost of the

component.

Conclusion

The majority of time sent on the project was spent researching the various possible methods to

control the buggy and if the components needed were too expensive or too big because the smaller

and easier a component is to install and implement the more expensive it gets. The choice was made

and this choice will bring up challenges but the outcome will be a clean and innovative design.

References

[1] Photo SlotcarElecCircuit.png from http://en.wikipedia.org/wiki/Slot_car

44

10.2 Project Specification

Project Title: Autonomous Slot Car Racing

Slot car racing is where model cars are run on a track with a slot in the track. The DC motor in the car

is controlled by varying the voltage on the metal tracks by potentiometers in a controller.

If the slot car can be automated the lap times can be reduced. A marker on each section of the track

helps in learning the track to optimise the speed into and out of corners. Processors placed on the car

or externally (example: on a computer) can either learn the track or process the track real time so

that the cars may be able to go faster than if a person were controlling it.

45

10.3 Project Plan

46

10.4 Risk assessment