NXT Development Tutorial Part 2: Sensor/ Motor Collaboration NTHU CS Freshman Camp Shu-Ting Wang

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Outline A Car class Ultrasonic tracker – Naive Threshold – Method based Threshold – Proportional Control Light sensor based line follower – Naïve line follower – PID controlled line follower

Outline A Car class Ultrasonic tracker – Naive Threshold – Method based Threshold – Proportional Control Light sensor based line follower – Naïve line follower – PID controlled line follower

Recall: Dog Features: – Weight – Height – Age – Is Healthy – Favorite Sports Behaviors: – Run – Bark – Bite – Eat – Sleep

Recall: Class Dog in Java public class Dog { …...Features …... public Dog(int Weight, float height, int Age) { …….How to create a dog based on these information } public void Bark(){ …….How to bark as a dog……. } public void Eat(){ …….How to eat……. }

Recall: How an Object Pops Out? Use a specific term: new We new a dog out based on our definition in class dog Dog mydog= new Dog(5,1.8,3); mydog.Eat(); Mydog.Bark();

Lab 1: Write a Car Class (1/2) public class Car { // Commands for the motors private final static int forward = 1, backward = 2, stop = 3; private static MotorPort leftMotor = MotorPort.C; private static MotorPort rightMotor= MotorPort.B; private Car(){ } }

Lab 1: Write a Car Class (2/2) We need three methods: – public static void stop() – public static void forward() – public static void backward()

Outline A Car class Ultrasonic tracker – Naive Threshold – Method based Threshold – Proportional Control Light sensor based line follower – Naïve line follower – PID controlled line follower

While() while(condition){ ….something… } Keep doing something until the condition is not true For example, keep walking until you see Delta building while (I don’t see Delta building){ dog.walk(); }

If() if(condition){ ….something A… } else ….something B… } Do something if the condition is true For example if (I am hungry){ I go to find something to eat } else{ play LOL }

Recall: Ultrasonic Sensor Get the distance from your hand public static void main(String[] args) throws Exception { UltrasonicSensor Ultrasonic = new UltrasonicSensor(SensorPort.S1); while(!Button.ESCAPE.isDown()) { LCD.clear(3,0,0); LCD.drawInt(Ultrasonic.getDistance(),3,0,0); }

Lab 2: Naïve Threshold Tracker Implement a control loop which based on the measured distance controls the movements of the car If the measured distance to obstacles/wall are over 40, the car continued straight ahead. If not, the car turning to the left Modify the run() method in Tracker.java

Lab3: isCrashed() ? Your car got stuck in chair legs during the rotation ? Made an isCrashed method whose task is to check whether the car is stuck based on repeated distance measurements similar to each other.

Proportional Control (1/2) The power to the car motors is determined by the measured error Error in this case is the difference between the measured distance to the object and the desired distance to the object: error = distance - desiredDistance;

Proportional Control (2/2) The power is proportional to the measured error by multiplying the error with a constant (gain). power = (int) (gain * error);

Lab 4 The power is always in between two extremes which are the minimum (set to 60) and maximum ( set to 100) speed Modify the run() method in Tracker.java If error>0, we move forward Otherwise, we move backward

public void run(){ while ( running ) { distance = us.getDistance(); if ( distance != 255){ error = distance -threshold; power = (int)(Pgain * error); if ( error > 0 ) { …………… } else { ……………. } Delay.msDelay(200); }

Outline A Car class Ultrasonic tracker – Naive Threshold – Method based Threshold – Proportional Control Light sensor based line follower – Naïve line follower – PID controlled line follower

Lab 5: Straight Line Follower Fix a bug that determines the light sensor input is white or black How to control the two motors?

Overview of PID Controller The Setpoint (SP) is the value that we want the process to be If the SP and the Process Variable PV are the same – then the controller is a very happy little box

Usage Scenario We want the AC in our classroom to achieve a steady temperature of as close to 22°C as possible What should we do?

Another Example How you drive a car on windy freeway ? Basically, you are a blackbox controller

The Blackbox: PID Controller

PID? Proportional: multiplies the Error by the Gain (Kp) Integral: the sum of instantaneous errors over time Derivative: a mathematical term meaning rate-of-change. It predicts future errors

Effects of increasing a parameter independently ParameterRise timeOvershootSettling time Steady-state error Stability DecreaseIncrease Small change DecreaseDegrade DecreaseIncrease EliminateDegrade Minor change Decrease No effect in theory Improve if small

Principle ? React faster: Kd Move steadily: Ki Try different parameters to make your car run faster and more steadily on the given track

How to Tune the Parameters …… PIDController pid = new PIDController (threshold, 10); pid.setPIDParam(PIDController.PID_KP, 12.0f); pid.setPIDParam(PIDController.PID_KI, 0.009f); pid.setPIDParam(PIDController.PID_KD, 300.0f); ……

PID Controller in Java (1/3) public int doPID(int processVariable){ int outputMV; this.PV = processVariable; error = setpoint - processVariable; if(Math.abs(error)<=deadband){ error=0; } if (!disableIntegral) { integral += Ki * error * dt; }

PID Controller in Java (2/3) if (integralLimited){ if (integral>integralHighLimit){ integral = integralHighLimit; } if (integral<integralLowLimit) { integral = integralLowLimit; } derivative = ((float)(error - previous_error))/dt; outputMV = (int)(Kp*error + integral + Kd*derivative);

PID Controller in Java (3/3) if (outputMV>highLimit){ outputMV=highLimit; } if (outputMV<lowLimit){ outputMV=lowLimit; } previous_error = error; outputMV=rampOut(outputMV);

Lab 6: public static void PIDControl(PIDController pid){ pid.setPIDParam(PIDController.PID_KP, 12.0f); pid.setPIDParam(PIDController.PID_KI, 0.009f); pid.setPIDParam(PIDController.PID_KD, 300.0f); pid.setPIDParam(PIDController.PID_LIMITHIGH, 200.0f); pid.setPIDParam(PIDController.PID_LIMITLOW, f); pid.setPIDParam(PIDController.PID_I_LIMITHIGH, 0.0f); pid.setPIDParam(PIDController.PID_I_LIMITLOW, 0.0f); pid.setPIDParam(PIDController.PID_DEADBAND, 1); } Insert this function inti BlakcWhiteSensor.java We want to see how these parameters work with PID controller

Reference leJOS official tutorial leJOS API documentation