Velocity PID Tuning Tutorial

🚨 This tuning process requires the use of an external library. This means that you must be using Android Studio. This tutorial will not work with OnBot Java or Blocks 🚨

What's a velocity PID?

You'll hear the term PID Controller used a lot in robotics. It's relatively simple and pretty effective at a lot of simple tasks. A PID controller is a form of "closed loop control." This basically just means you're altering an input to some "plant" based on feedback. This concept applies to a wide range of actions but we'll take a look at velocity PID control as that is what's relevant for this year's game.

So say you have a goBILDA 3:1 1620RPM motor powering a flywheel. You want that flywheel to spin at a constant speed to ensure consistency between your shots. So you run a `motor.setPower(0.5)`` which sends 50% of 12v to the motor. The motor is getting a 6v signal (technically not true because of PWM but that's another topic). The motor should be running at 810 RPM right? That's 50% of 1620RPM. Chances are, it's not actually running at this speed. There are a number of reasons why: motors have +- 10% tolerance between them, the voltage-torque curve isn't linear, or the motor is facing some resisting force(like the inertia of the flywheel) so it takes extra power to get it up to that speed. So how do we actually ensure that our motor is running at exactly 810RPM? Most FTC motors come with an encoder built it. This allows for us to measure the velocity of the output shaft. So with the encoder all hooked up, we know that our motor isn't spinning as fast as we want it to. But how do we actually correct for this? You slap a PID Controller on it. A PID controller is a pretty basic controller (despite the daunting equation when you look it up) that basically responds to your the difference between your measured velocity and desired velocity (error) and will add more or less power based on error. You just check the velocity every loop, feed the value in the controlled, and it gives you the power you want to set it to the desired velocity.

What does it mean to "tune" your PID controller?

The Rev Hub's motor controller actually has it's own internal velocity PID! This means you don't have to program any of the fancy math for it. The FTC SDK gives you a velocity controller per motor, for free! The hub uses the internal velocity PID whenever you call motor.setMode(DcMotor.RunMode.RUN_USING_ENCODER). Whenever you call setPower() while the motor is in the RUN_USING_ENCODER, it actually pipes that into a setVelocity(ticks per second) function. However, each motor comes with a set of default PID gains set in the FTC SDK. These gains are probably not optimal. They're not tuned for your specific system in mind. They'll be okay for most people. However, if you want the best response/behavior, you're going to want to set your own PID gains. Finding the set of gains that works for you is called "tuning".

Installation

This tuning process utilizes three libraries: FTC Dashboard, Jotai, and Road Runner. FTC Dashboard allows us to live graph the target and set point, giving us a much more intuitive understanding of the tuning process. Jotai is a helper library that allows for simple Finite State Machine construction. You need not be concerned about this. We use Road Runner because it comes with a PIDF controller with the appropriate feedforward model for DC Motors. You may not wish to install the entire Road Runner library just for this controller. Feel free to replace the controller with one of equivalent functionality.

If you are starting from scratch, just download or clone this repo. You need not go through the following steps.

In your FtcRobotController/build.release.gradle file, insert the following: implementation 'com.acmerobotics.dashboard:dashboard:0.3.10'

Your file should look something like this:

/* FtcRobotController/build.release.gradle */ 

dependencies {
    implementation 'org.firstinspires.ftc:Inspection:6.1.1'
    implementation 'org.firstinspires.ftc:Blocks:6.1.1'
    implementation 'org.firstinspires.ftc:RobotCore:6.1.1'
    implementation 'org.firstinspires.ftc:RobotServer:6.1.1'
    implementation 'org.firstinspires.ftc:OnBotJava:6.1.1'
    implementation 'org.firstinspires.ftc:Hardware:6.1.1'
    implementation 'org.firstinspires.ftc:FtcCommon:6.1.1'
    implementation 'androidx.appcompat:appcompat:1.2.0'

    implementation 'com.acmerobotics.dashboard:dashboard:0.3.10'
}

Then, in your TeamCode/build.release.gradle file, insert the following:

implementation 'com.acmerobotics.roadrunner:core:0.5.3'
implementation 'com.acmerobotics.dashboard:dashboard:0.3.10'
implementation "com.noahbres.jotai:jotai:1.0.2"

Your file should look something like this:

/* TeamCode/build.release.gradle */ 

dependencies {
    implementation 'org.firstinspires.ftc:Inspection:6.1.1'
    implementation 'org.firstinspires.ftc:Blocks:6.1.1'
    implementation 'org.firstinspires.ftc:RobotCore:6.1.1'
    implementation 'org.firstinspires.ftc:RobotServer:6.1.1'
    implementation 'org.firstinspires.ftc:OnBotJava:6.1.1'
    implementation 'org.firstinspires.ftc:Hardware:6.1.1'
    implementation 'org.firstinspires.ftc:FtcCommon:6.1.1'
    implementation 'androidx.appcompat:appcompat:1.2.0'

    implementation 'org.apache.commons:commons-math3:3.6.1'
    
    implementation 'com.acmerobotics.roadrunner:core:0.5.3'
    
    implementation 'com.acmerobotics.dashboard:dashboard:0.3.10'
    
    implementation "com.noahbres.jotai:jotai:1.0.2"
}

To enable FTC Dashboard, you must copy paste the FtcRobotControllerActivity.java file from https://github.com/acmerobotics/ftc-dashboard/blob/master/FtcRobotController/src/main/java/org/firstinspires/ftc/robotcontroller/internal/FtcRobotControllerActivity.java into your own project. The file will be located under FtcRobotController/src/main/java/org/firstinspires/ftc/robotcontroller/internal/. Alternatively, follow the official FTC Dashboard instructions.

If you're using the built-in PID, copy over the following files into your own TeamCode folder:

• VeloPIDTuner.java
• TuningController.java

If you're using two mechanically linked motors, copy over the following files into your own TeamCode folder:

• LinkedMotorTuner.java
• TuningController.java

Instructions - Built In PID

1. Open up the TuningController.java file. Adjust the MOTOR_TICKS_PER_REV, MOTOR_MAX_RPM, and MOTOR_GEAR_RATIO to match your setup. Your motor manufacturer should list these specs.
2. If you would like to change the max and minimum speed, adjust the TESTING_MAX_SPEED and TESTING_MIN_SPEED accordingly.
3. Before you begin tuning, start the RC phone and open up Dashboard. Connect to the RC phone's wifi network. The password to the network is located in the Program and Manage menu.
4. In your browser, navigate to 192.168.49.1:8080/dash with a phone RC or 192.168.43.1:8080/dash with a Control Hub.
5. Run the VeloPIDTuner opmode. This will command your flywheel to ramp up its velocity, coast, ramp it back down, and suddenly change to 3 different setpoints.
6. Adjust your PIDF gains to try and match targetVelocity with velocity
   1. Raise kF until velocity reaches targetVelocity
   2. Raise kP to reduce phase lag
   3. Raise kD to dampen oscillations

Instructions - Mechanically Linked

1. Open up the TuningController.java file. Adjust the MOTOR_TICKS_PER_REV, MOTOR_MAX_RPM, and MOTOR_GEAR_RATIO to match your setup. Your motor manufacturer should list these specs.
2. If you would like to change the max and minimum speed, adjust the TESTING_MAX_SPEED and TESTING_MIN_SPEED accordingly.
3. Before you begin tuning, start the RC phone and open up Dashboard. Connect to the RC phone's wifi network. The password to the network is located in the Program and Manage menu.
4. In your browser, navigate to 192.168.49.1:8080/dash with a phone RC or 192.168.43.1:8080/dash with a Control Hub.
5. Run the LinkedMotorTuner opmode. This will command your flywheel to ramp up its velocity, coast, ramp it back down, and suddenly change to 3 different setpoints.
6. Adjust your PIDF gains to try and match targetVelocity with velocity
   1. Raise kF until velocity reaches targetVelocity
   2. Raise kA to reduce the phase lag during acceleration
   3. Raise kP to turn on error correction and reduce any further error
   4. Raise kD to dampen any oscillations

Sample Usage

If you're using the built-in PID:

• SamplePIDUse.java

If you're using mechanically linked motors:

• SampleLinkedPIDUse.java

Temporary Video Tutorials

These are informal videos. They're not edited and were improvised. Apologies for the poor quality. Hopefully, polished videos will be made in the future.

Built in PID

Linked Motor PID