The purpose of our project was to create a variable speed throttle for a power wheels car purchased from amazon. This is an education based project for MEGR-3092 at UNC Charlotte in the department of Mechanical Engineering and Engineering Science. This device works by using a Particle Argon, a new throttle pedal sensor, and two electronic speed controllers. The controllers are programmed to allow the motors to spin at different speeds based on throttle position.
Video of the Variable Throttle System Working Properly
The general flow for this project starts at the Particle Argon. The Argon is fed a signal from the Hall Effect Variable Throttle Petal that will have a value anywhere between 1100 and 4000. This signal is then converted using the "map" command to somewhere between 0-255 based on how far the throttle is pressed. The constrain command is also utilized here to reduce the effect of the noise introduced by the petal sensor. At this point the newly mapped throttle position is transmitted, via Wi-Fi, to our Adafruit.io dashboard and plotted over time.
Picture of Our Internal Circuit Contained Under the Seat
Now that the throttle is valued somewhere between 0-255, a new variable called "servoPosition" is generated that will determine the speed at which the servo motors spin. The "servoPosition" variable takes the throttle position and again maps it somewhere between 1000-1500. This new value represents the PWM signal, in microseconds, that will then be sent to the servos through the ESCs.
A signal of 1500 microseconds written to the ESCs represents the car being in neutral and therefor there is no throttle applied. As the throttle is pressed, the signal is reduced closer to 1000 microseconds causing the servo motors to speed up. It is worth noting that normally variable speed in the forward direction would be represented by values of 1500-2000 microseconds and reverse speed represented by 1000-1500 microseconds. The reason this is switched in this specific case is that our wires were initially connected backwards and it was simpler to change the code for the signal than to redo all of the wiring. Still the same result is achieved from either action.
Block Diagram of How the Throttle Speed is Determined
In addition to the variable throttle for this car. A current sensor and voltage reading were utilized to gather real time data from the performance of our system. The current sensor was placed in-line between the batteries and ESCs (before being split in parallel) to get a reading of how much current the whole system is pulling from the battery. For the voltage reading, the positive and negative terminals of the battery were split by a voltage divider circuit with a 10:1 ratio. This ratio was required to drop the voltage of the battery to a level that could be handled by the Particle Argon pins. Once both of these values were gathered, they were then sent, via Wi-Fi, to our adafruit.io dashboard, similarly to the throttle position.
This is how each of the test variable are gathered and presented through Adafruit.io
The final component to our throttle system is a section of code implementing a slew rate traction control. This is a simple version of traction control that limits the throttle increase to a certain amount per loop iteration. In our code we have the "trottleRate" variable that sets this maximum throttle increase. Every loop, the code takes the previous throttle position and adds this throttle rate value to it. The new value is then compared to the throttle input signal and the smaller of the two values is written to the drive motors.
The idea behind this type of traction control is that is slows the acceleration of the tires instead of allowing them to go from 0-100% throttle (mapped between 0-255 in the code) in one jump. This reduces, or ideally does away with the slip between the ground and the tire. Through testing of our circuit, throttle, and cohesive vehicle system, we found that a throttle rate of 5 was ideal for our vehicle.
Video Showing Operation of Slew Rate Traction Control
This code takes the input from a variable position throttle and drives the servo motors at a rate corresponding to the petal position.
// This #include statement was automatically added by the Particle IDE.#include<OneWire.h>//Setting Variable Throttle to work on Argon without wifiSYSTEM_THREAD(ENABLED);// Define the pins for the analog inputs and servo outputconstintVOLTAGE_PIN=A3;constintCURRENT_PIN=A1;constintTHROTTLE_PIN=A2;constintSERVO_PIN=D5;// Define the ratios for the voltage divider and current sensor calibrationconstfloatVOLTAGE_RATIO=0.1;constfloatCURRENT_CALIBRATION=0.04;// Define the variables to hold the raw analog readingsintrawVoltage;intrawCurrent;intrawThrottle;// Define the variables to hold the filtered valuesfloatfilteredVoltage;floatfilteredCurrent;// Define the time interval for reading the analog inputs (in microseconds)constunsignedlongREAD_INTERVAL=20000;// Define the time interval for outputting the filtered values (in milliseconds)constunsignedlongOUTPUT_INTERVAL=10000;// Define the filter coefficients for a first-order low-pass filter with a cutoff frequency of 5 HzconstfloatFILTER_COEFFICIENT=0.0183;// Define the timer for outputting the filtered valuesunsignedlongoutputTimer=0;// Define the servo object and the initial throttle valueServoservo;intthrottleValue=0;voidsetup(){// Initialize the serial communicationSerial.begin(9600);// Initialize the servo object and set the initial throttle valueservo.attach(SERVO_PIN);servo.writeMicroseconds(1500);}voidloop(){// Read the analog inputsrawVoltage=analogRead(VOLTAGE_PIN);rawCurrent=analogRead(CURRENT_PIN);rawThrottle=analogRead(THROTTLE_PIN);// Convert the raw values to voltages and currentfloatvoltage=rawVoltage*5.0/4095.0*VOLTAGE_RATIO;floatcurrent=(rawCurrent-2047)*5.0/4095.0/CURRENT_CALIBRATION;// Filter the valuesfilteredVoltage=FILTER_COEFFICIENT*voltage+(1-FILTER_COEFFICIENT)*filteredVoltage;filteredCurrent=FILTER_COEFFICIENT*current+(1-FILTER_COEFFICIENT)*filteredCurrent;// Convert the throttle value to a servo position between 1000 and 2000 microsecondsintthrottlePosition=map(rawThrottle,1100,4000,0,255);throttlePosition=constrain(throttlePosition,0,255);intservoPosition=map(throttlePosition,0,255,1500,1000);// Check if it's time to output the filtered valuesif(millis()-outputTimer>=OUTPUT_INTERVAL){// Output the filtered valuesSerial.print("Filtered voltage: ");Serial.print(filteredVoltage);//Serial.print(" V");Serial.print("Filtered current: ");Serial.print(filteredCurrent);//Serial.println(" A");Particle.publish("Voltage",String(filteredVoltage));Particle.publish("Current",String(filteredCurrent));Particle.publish("Throttle",String(throttlePosition));// Reset the output timeroutputTimer=millis();}// Write the throttle position to the servoservo.writeMicroseconds(servoPosition);// Wait until the next time to read the analog inputsunsignedlongstartTime=micros();while(micros()-startTime<READ_INTERVAL){// Do nothing}}
Variable Speed Throttle Code with Traction Control
C/C++
This code is the same as the variable throttle code but it will only increase at a capped rate based on the "throttleRate" variable. This is to slow the acceleration to a level that reduces the slip of the tires.
// This #include statement was automatically added by the Particle IDE.#include<OneWire.h>//SYSTEM_THREAD(ENABLED);// Define the pins for the analog inputs and servo outputconstintVOLTAGE_PIN=A3;constintCURRENT_PIN=A1;constintTHROTTLE_PIN=A2;constintSERVO_PIN=D5;//Initializing variables for traction controlintthrottleRate=5;intprevThrottle=0;// Define the ratios for the voltage divider and current sensor calibrationconstfloatVOLTAGE_RATIO=0.1;constfloatCURRENT_CALIBRATION=0.04;// Define the variables to hold the raw analog readingsintrawVoltage;intrawCurrent;intrawThrottle;// Define the variables to hold the filtered valuesfloatfilteredVoltage;floatfilteredCurrent;// Define the time interval for reading the analog inputs (in microseconds)constunsignedlongREAD_INTERVAL=20000;// Define the time interval for outputting the filtered values (in milliseconds)constunsignedlongOUTPUT_INTERVAL=10000;// Define the filter coefficients for a first-order low-pass filter with a cutoff frequency of 5 HzconstfloatFILTER_COEFFICIENT=0.0183;// Define the timer for outputting the filtered valuesunsignedlongoutputTimer=0;// Define the servo object and the initial throttle valueServoservo;intthrottleValue=0;voidsetup(){// Initialize the serial communicationSerial.begin(9600);// Initialize the servo object and set the initial throttle valueservo.attach(SERVO_PIN);servo.writeMicroseconds(1500);}voidloop(){// Read the analog inputsrawVoltage=analogRead(VOLTAGE_PIN);rawCurrent=analogRead(CURRENT_PIN);rawThrottle=analogRead(THROTTLE_PIN);// Convert the raw values to voltages and currentfloatvoltage=rawVoltage*5.0/4095.0*VOLTAGE_RATIO;floatcurrent=(rawCurrent-2047)*5.0/4095.0/CURRENT_CALIBRATION;// Filter the valuesfilteredVoltage=FILTER_COEFFICIENT*voltage+(1-FILTER_COEFFICIENT)*filteredVoltage;filteredCurrent=FILTER_COEFFICIENT*current+(1-FILTER_COEFFICIENT)*filteredCurrent;// Convert the throttle value to a servo position between 1000 and 2000 microsecondsintthrottlePosition=map(rawThrottle,1100,4000,0,255);throttlePosition=constrain(throttlePosition,0,255);// Implementing traction control and determining if the throttle petal input or// incramental throttle increase should be sent to servosintthrottleInput=throttlePosition;intthrottleOutput=min(throttleInput,prevThrottle+throttleRate);// Update the previous throttle value for the next loop prevThrottle=throttleOutput;intservoPosition=map(throttleOutput,0,255,1500,1000);// Check if it's time to output the filtered valuesif(millis()-outputTimer>=OUTPUT_INTERVAL){// Output the filtered valuesSerial.print("Filtered voltage: ");Serial.print(filteredVoltage);//Serial.print(" V");Serial.print("Filtered current: ");Serial.print(filteredCurrent);//Serial.println(" A");Particle.publish("Voltage",String(filteredVoltage));Particle.publish("Current",String(filteredCurrent));Particle.publish("Throttle",String(throttlePosition));// Reset the output timeroutputTimer=millis();}// Write the throttle position to the servoservo.writeMicroseconds(servoPosition);// Wait until the next time to read the analog inputsunsignedlongstartTime=micros();while(micros()-startTime<READ_INTERVAL){// Do nothing}}
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