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Lesson 9.5: Sync a Dual-Servo Gripper with Offset Tuning

Technical Context​

Mechanical misalignment or "slop" in servo splines means two physically identical servos may reach their effective grip position at slightly different software values. Applying position offsets to each servo independently compensates for this, ensuring the gripper holds a game element securely without one side stalling against its physical stop while the other is still moving.

How to Tune Two Servos So They Work Together​

This lesson combines setDirection() from Lesson 9.3 with else if chaining from Unit 5, and adds the concept of per-servo offsets. Rather than commanding both servos to the exact same value, you define named constants for each state (OPEN, CLOSED, READY) and apply a small offset to whichever servo needs mechanical compensation.

Using else if chaining instead of multiple independent if blocks is critical here: it prevents servo jitter. If two if blocks both evaluate to true in the same loop cycle (for example, if two buttons are held simultaneously), the servo would receive two conflicting position commands within a single 50Hz update, causing rapid mechanical oscillation that can strip servo gears.

A single else if chain guarantees the servo receives exactly one position command per loop cycle.

Annotated Code​

package org.firstinspires.ftc.teamcode;

import com.qualcomm.robotcore.eventloop.opmode.OpMode;
import com.qualcomm.robotcore.eventloop.opmode.TeleOp;
import com.qualcomm.robotcore.hardware.Servo;

@TeleOp(name="Dual_Servo_Demo")
public class DualServoDemo extends OpMode {

    private Servo leftClaw;
    private Servo rightClaw;

    // Named constants for each gripper state
    // Right servo has a +0.05 offset to compensate for mechanical slop
    private static final double CLOSED_LEFT  = 1.0;
    private static final double CLOSED_RIGHT = 0.95; // offset applied here
    private static final double OPEN_LEFT    = 0.0;
    private static final double OPEN_RIGHT   = 0.0;
    private static final double READY_POS    = 0.5;

    @Override
    public void init() {
        leftClaw  = hardwareMap.get(Servo.class, "left_claw");
        rightClaw = hardwareMap.get(Servo.class, "right_claw");

        rightClaw.setDirection(Servo.Direction.REVERSE);

        leftClaw.setPosition(READY_POS);
        rightClaw.setPosition(READY_POS);
        telemetry.addData("Gripper", "Ready at center");
    }

    @Override
    public void loop() {
        // Mutually exclusive states: exactly one command sent per cycle
        if (gamepad1.a) {
            leftClaw.setPosition(CLOSED_LEFT);
            rightClaw.setPosition(CLOSED_RIGHT);
            telemetry.addData("Gripper", "Closed");
        } else if (gamepad1.b) {
            leftClaw.setPosition(OPEN_LEFT);
            rightClaw.setPosition(OPEN_RIGHT);
            telemetry.addData("Gripper", "Open");
        } else {
            leftClaw.setPosition(READY_POS);
            rightClaw.setPosition(READY_POS);
            telemetry.addData("Gripper", "Ready");
        }
    }
}

Fill-in-the-Blank Practice​

  1. Using else if statements instead of multiple independent if statements ensures the servo receives only __________ position command(s) per loop cycle.
  2. Providing a __________ state in the else clause ensures the servo returns to a predictable position when no buttons are pressed.
  3. Servos may exhibit __________ if they receive multiple conflicting setPosition() commands within the same 50Hz loop update.
Show answers
  1. one
  2. default (or rest / ready)
  3. jitter (rapid mechanical oscillation)

Simulator Practice​

Use the simulator below to practice synchronizing "servoL" and "servoR" with a 0.05 mechanical offset on the right side.

Telemark Unit 9.5 Simulator
Loads the lesson-specific Telemark servo challenge with starter code for students to complete.
Includes live mechanism feedback, telemetry checks, and gamepad input for each servo-control concept.

A Team Stores Two Targets Together​

BunyipsLib's dual-servo subsystem stores a target for each side and writes both targets during its periodic update. Here is the same pattern with gripper names:

void updateGripper() {
    leftServo.setPosition(leftTarget);
    rightServo.setPosition(rightTarget);
}

Logging and framework lifecycle code were omitted. The target names were shortened so the two coordinated hardware writes are easy to compare. Adapted from BunyipsLib's DualServos.java at a pinned commit under the MIT License.

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