Talon SRX in Position Mode Won't keep zero

[ProgrammerMatt]

So i have code that runs our actuators to set our zero's for our Closed Loop PID system. using setPosition(0.0); to set the zero.

It seems to loose its zero after switching positions a few times. its off by about 0.9 rotations about 6-8 inches on our actuator. im using the SRX Mag encoder with the pulse width position feedback selected.


Thanks,

Matthew Smith

[Ty Tremblay]

We're going to need to see pertinent code to be able to help.

[ProgrammerMatt]

its really messy at the moment, just have been trying lots of things sorry.

package org.usfirst.frc.team2282.robot;


import edu.wpi.first.wpilibj.SampleRobot;
import edu.wpi.first.wpilibj.RobotDrive;
import edu.wpi.first.wpilibj.CANTalon;
import edu.wpi.first.wpilibj.CANTalon.FeedbackDevice;
import edu.wpi.first.wpilibj.CANTalon.TalonControlMode;
import edu.wpi.first.wpilibj.CameraServer;
import edu.wpi.first.wpilibj.DriverStation;
import edu.wpi.first.wpilibj.Joystick;
import edu.wpi.first.wpilibj.Timer;
import edu.wpi.first.wpilibj.Victor;
import edu.wpi.first.wpilibj.smartdashboard.SendableChooser;
import edu.wpi.first.wpilibj.smartdashboard.SmartDashboard;

/**
 * This is a demo program showing the use of the RobotDrive class.
 * The SampleRobot class is the base of a robot application that will automatically call your
 * Autonomous and OperatorControl methods at the right time as controlled by the switches on
 * the driver station or the field controls.
 *
 * The VM is configured to automatically run this class, and to call the
 * functions corresponding to each mode, as described in the SampleRobot
 * documentation. If you change the name of this class or the package after
 * creating this project, you must also update the manifest file in the resource
 * directory.
 *
 * WARNING: While it may look like a good choice to use for your code if you're inexperienced,
 * don't. Unless you know what you are doing, complex code will be much more difficult under
 * this system. Use IterativeRobot or Command-Based instead if you're new.
 */
public class Robot extends SampleRobot {
    RobotDrive myRobot;
    Joystick gamepad,coDriver;
    DriverStation DS;
    
    Victor L1,L2,L3,R1,R2,R3,intake, shooter;
    CANTalon awesomeBar, shooterFlap;
    CameraServer cam;
    
    double leftValue, rightValue;
    boolean Arcade;
    
    final String defaultAuto = "Default";
    final String customAuto = "My Auto";
    SendableChooser chooser;
    
    int loopCount;

    public Robot() {
        gamepad = new Joystick(0);
        coDriver = new Joystick(1);
        L1 = new Victor(0);
        L2 = new Victor(1);
        L3 = new Victor(2);
        R1 = new Victor(3);
        R2 = new Victor(4);
        R3 = new Victor(5);
        shooter = new Victor(7);
        awesomeBar = new CANTalon(2);
        
        //awesomeBar.configMaxOutputVoltage(12.0);
        
        
        shooterFlap = new CANTalon(1);
        shooterFlap.setFeedbackDevice(FeedbackDevice.PulseWidth);
        shooterFlap.setInverted(true);
        
        shooterFlap.reverseSensor(true);
        awesomeBar.setFeedbackDevice(FeedbackDevice.PulseWidth);
        awesomeBar.setInverted(false);
        awesomeBar.reverseSensor(true);
        
        intake = new Victor(6);
   //     cam = CameraServer.getInstance();
    //    cam.startAutomaticCapture("cam0");
        
        DS = DriverStation.getInstance();
    }
    
    public void robotInit() {
        chooser = new SendableChooser();
        chooser.addDefault("Default Auto", defaultAuto);
        chooser.addObject("My Auto", customAuto);
        SmartDashboard.putData("Auto modes", chooser);
        SmartDashboard.putBoolean("Arcade", true);
                
    }

	/**
	 * This autonomous (along with the chooser code above) shows how to select between different autonomous modes
	 * using the dashboard. The sendable chooser code works with the Java SmartDashboard. If you prefer the LabVIEW
	 * Dashboard, remove all of the chooser code and uncomment the getString line to get the auto name from the text box
	 * below the Gyro
	 *
	 * You can add additional auto modes by adding additional comparisons to the if-else structure below with additional strings.
	 * If using the SendableChooser make sure to add them to the chooser code above as well.
	 */
    public void autonomous() {
    	while(isAutonomous() && isEnabled()){
    	
    	}
    	
    }

    /**
     * Runs the motors with arcade steering.
     */
    public void operatorControl() {
    	//Calibrate Shooter Flap
    	shooterFlap.changeControlMode(TalonControlMode.PercentVbus);
    	while(!shooterFlap.isFwdLimitSwitchClosed()){
    		shooterFlap.set(-0.2);
    	}
    	shooterFlap.set(0.0);
    	shooterFlap.changeControlMode(TalonControlMode.Position);
    	Timer.delay(0.02);
    	shooterFlap.setPosition(0);
    	//Calibrate Awesome Bar
    	awesomeBar.changeControlMode(TalonControlMode.PercentVbus);
    	while(!awesomeBar.isRevLimitSwitchClosed()){
    		awesomeBar.set(-0.2);
    	}
    	awesomeBar.set(0.0);
    	awesomeBar.changeControlMode(TalonControlMode.Position);
    	Timer.delay(0.02);
    	awesomeBar.setPosition(0.0);


    	
        while (isOperatorControl() && isEnabled()) {
        	
        	shooterFlap.enable();
            Arcade = SmartDashboard.getBoolean("Arcade");
        	if(Arcade) {
        		
        		leftValue = (gamepad.getY() + -gamepad.getRawAxis(4));
        		rightValue = -(gamepad.getY() - -gamepad.getRawAxis(4));
        	}
        	else {
        		leftValue = gamepad.getY();
        		rightValue = -gamepad.getRawAxis(5);
        	}
        	
        	SmartDashboard.putNumber("Arm Position", awesomeBar.getPosition());
        	L1.set(leftValue);
        	L2.set(leftValue);
        	L3.set(leftValue);
        	
        	R1.set(rightValue);
        	R2.set(rightValue);
        	R3.set(rightValue);
        	
        	
        	awesomeBar.changeControlMode(TalonControlMode.Position);
        	SmartDashboard.putNumber("Pulse witdth pos", awesomeBar.getPulseWidthPosition());
        	awesomeBar.enable();
        	
        	
        	
        	intake.set(coDriver.getY());
        	
        	if(coDriver.getRawButton(1)){
        		shooter.set(1.0);
        		
        			shooterFlap.set(-1.2);
        			loopCount = 0;
        		
        	}
        	else if(coDriver.getRawButton(4)){
        		shooter.set(-1.0);
        	}
        	else{
        		shooter.set(0);
        		if(loopCount >= 300) {
        			//shooterFlap.set(0.0);
        		}
        	}
        	SmartDashboard.putNumber("flap pos", shooterFlap.getPosition());
        	SmartDashboard.putNumber("output voltage", shooterFlap.getOutputVoltage());
        	SmartDashboard.putString("cont mode", shooterFlap.getControlMode().name());
        	SmartDashboard.putNumber("Awesome Bar POS", awesomeBar.getPosition());
        	
        	awesomeBar.enable();
        	
               
               if(gamepad.getRawButton(6)){
        		awesomeBar.set(0.98);
        	}
        	else if (gamepad.getRawButton(5)){
        	awesomeBar.set(0.0);
        	}
        	else if (gamepad.getRawButton(2)){
        		awesomeBar.set(3.7);
        	}
        	else{
        	}
        	
        	Timer.delay(0.01);
        	loopCount++;
        	
        	//1.194 good shot position
        	
        	
       
        	/*
        	awesomeBar.changeControlMode(TalonControlMode.PercentVbus);
        	shooterFlap.changeControlMode(TalonControlMode.PercentVbus);
        	
        	
        	if(gamepad.getRawButton(5)){
        		awesomeBar.set(1.0);
        		
        	}
        	else if(gamepad.getRawButton(6)){
        		awesomeBar.set(-1.0);
        		
        	}
        	else {
        		awesomeBar.set(0.0);
        	}
        	
        	shooterFlap.set(-coDriver.getRawAxis(5));
                */
        	
        	
        }
    }

    /**
     * Runs during test mode
     */
    public void test() {
    	while(isTest() && isEnabled()){
    	
    	}
    }
}

[mikets]

Originally Posted by ProgrammerMatt

So i have code that runs our actuators to set our zero's for our Closed Loop PID system. using setPosition(0.0); to set the zero.

It seems to loose its zero after switching positions a few times. its off by about 0.9 rotations about 6-8 inches on our actuator. im using the SRX Mag encoder with the pulse width position feedback selected.


Thanks,

Matthew Smith

We have found, for various reasons, encoders don't always return the same count if the motor has spun back and forth a number of times back to the original position. We suspect in some scenarios, where optical encoders are used, there could be slight scratches on the optical wheels that a small number of pulses may be missed by the counter. It is not significant to the overall tracking of position but enough to cumulate errors that it no longer "return to zero" when the actuator is moved back to "zero position". That's why we have put code in our library that when a limit switch is used to calibrate zero position, not only we called the zeroCalibrate() method during robot initialization, we also zero the encoder whenever the lower limit switch is hit. This will prevent the "zero point drifting" problem.
Here is an example on how we did it. The code is in the method setMotorPower.
https://github.com/trc492/Frc2016Fir...cPidMotor.java

[ProgrammerMatt]

Originally Posted by mikets

We have found, for various reasons, encoders don't always return the same count if the motor has spun back and forth a number of times back to the original position. We suspect in some scenarios, where optical encoders are used, there could be slight scratches on the optical wheels that a small number of pulses may be missed by the counter. It is not significant to the overall tracking of position but enough to cumulate errors that it no longer "return to zero" when the actuator is moved back to "zero position". That's why we have put code in our library that when a limit switch is used to calibrate zero position, not only we called the zeroCalibrate() method during robot initialization, we also zero the encoder whenever the lower limit switch is hit. This will prevent the "zero point drifting" problem.
Here is an example on how we did it. The code is in the method setMotorPower.
https://github.com/trc492/Frc2016Fir...cPidMotor.java

This should not happen with the magnetic encoders though. if i watch the position of the mag encoder it will just randomly set its self to zero after switching to a position.

[mikets]

Originally Posted by ProgrammerMatt

This should not happen with the magnetic encoders though. if i watch the position of the mag encoder it will just randomly set its self to zero after switching to a position.

What do you mean by it resetting itself to zero? Is there a fixed location in the revolution at which when passing through will clear the position to zero? If so since you are using the SRX Mag encoder and I have read this sensor can be used in absolute or relative mode. In absolute mode, it will reset the reading back to zero when wrapped around. Could it be what you have observed is a behavior of absolute mode?
https://www.ctr-electronics.com/Talo...e%20Manual.pdf

[ProgrammerMatt]

Originally Posted by mikets

What do you mean by it resetting itself to zero? Is there a fixed location in the revolution at which when passing through will clear the position to zero? If so since you are using the SRX Mag encoder and I have read this sensor can be used in absolute or relative mode. In absolute mode, it will reset the reading back to zero when wrapped around. Could it be what you have observed is a behavior of absolute mode?
https://www.ctr-electronics.com/Talo...e%20Manual.pdf

I'm using the Talon in Position mode with the feedback device as pulse width, in this mode you can read positions over multiple rotations but the reference (zero) must be set, so if you look at my code the first thing we do is run the two motors down until it reaches the limit switch then sets the reference to zero using setPosition(0). during teleop i noticed after switching to a position a few times it suddenly stops going to the reference point when commanded to. I printed out the position of each talon and preformed a self test and it seems it just randomly at some point during teleop it will just change its zero (reference) to the last position it was at even know there is no setPosition being called.

Hope this clarifies anything,

[Ty Tremblay]

From what I'm reading, I don't think setPosition(0) works in absolute mode. You should instead getPulseWidthPosition() when you hit the limit switch and return to that absolute position.

If you want to use setPosition you could instead use the magnetic encoder in quadrature mode.

The whole point of absolute mode is that you don't have to calibrate on boot-up.

Also, if you're using the CTRE magnetic encoder, you should select the feedback device with one of the following options.

shooterFlap.setFeedbackDevice(FeedbackDevice.CTREMagEncoder_Absolute);

or

shooterFlap.setFeedbackDevice(FeedbackDevice.CTREMagEncoder_Quadrature);