Lesson 11.2: Scaling Analog Voltage from Potentiometers using Range.scale

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Why Potentiometers Have an Advantage Over Encoders for Certain Mechanisms

An encoder counts rotation relative to where it was the last time it was reset. If the robot powers off with an arm partially extended, the encoder resets to zero at the next power-on and the code no longer knows where the arm actually is. Before any controlled movement can happen, the robot must perform a "homing" sequence, driving the arm to a known physical stop to re-establish a reference point.

A potentiometer bypasses this problem entirely. It is a variable resistor whose electrical resistance changes as the output shaft rotates. The Control Hub converts that resistance into a measurable voltage, and because the voltage is a direct function of the physical angle, the sensor reports the correct position the instant the robot turns on, with no homing required. For mechanisms like an arm that could be in any position when the drive team picks the robot up between matches, this reliability is highly valuable.

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From Voltage to Engineering Units

The AnalogInput class in the SDK reads the current voltage on the port and returns it as a double from getVoltage(). The maximum voltage the port can measure is available through getMaxVoltage(). On the REV Control Hub, this is typically 3.3V, though using the method instead of a hardcoded constant ensures your code works correctly on any Hub revision.

Raw voltage alone is not useful for writing control logic like "stop the arm if it exceeds 90 degrees." You need to translate the voltage into meaningful engineering units. The SDK provides a static utility method for exactly this purpose: Range.scale(input, minInput, maxInput, minOutput, maxOutput).

For a 270-degree potentiometer, the call looks like this:

double angle = Range.scale(armPot.getVoltage(), 0, armPot.getMaxVoltage(), 0, 270);

This maps the raw voltage proportionally onto the 0-to-270 degree range. When the voltage is at zero, the angle reads 0. When the voltage is at the maximum, the angle reads 270. Every voltage in between maps linearly to the corresponding degree value.

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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.AnalogInput;
import com.qualcomm.robotcore.hardware.DcMotor;
import com.qualcomm.robotcore.util.Range;

@TeleOp(name="Potentiometer_Demo")
public class PotentiometerDemo extends OpMode {

    private AnalogInput armPot;
    private DcMotor     armMotor;

    // Physical travel of this particular potentiometer in degrees
    private static final double POT_MAX_DEGREES = 270.0;

    // Software safe-zone limits in degrees
    private static final double MIN_SAFE_ANGLE  = 10.0;
    private static final double MAX_SAFE_ANGLE  = 200.0;

    @Override
    public void init() {
        // Analog sensors are functional immediately upon mapping -- no mode call needed
        armPot   = hardwareMap.get(AnalogInput.class, "arm_pot");
        armMotor = hardwareMap.get(DcMotor.class,     "arm_motor");
        armMotor.setZeroPowerBehavior(DcMotor.ZeroPowerBehavior.BRAKE);

        telemetry.addData("Status", "Potentiometer ready");
    }

    @Override
    public void loop() {
        // Convert raw voltage to degrees using the SDK's Range utility
        double armAngle = Range.scale(
            armPot.getVoltage(), 0, armPot.getMaxVoltage(), 0, POT_MAX_DEGREES
        );

        double power = -gamepad1.left_stick_y;

        // Block movement that would drive the arm outside the safe zone
        if (armAngle >= MAX_SAFE_ANGLE && power > 0) {
            power = 0.0;
            telemetry.addData("Safety", "MAX ANGLE REACHED");
        } else if (armAngle <= MIN_SAFE_ANGLE && power < 0) {
            power = 0.0;
            telemetry.addData("Safety", "MIN ANGLE REACHED");
        }

        armMotor.setPower(power);
        telemetry.addData("Arm Angle (deg)", "%.1f", armAngle);
        telemetry.addData("Raw Voltage",     "%.3f V", armPot.getVoltage());
        telemetry.update();
    }
}

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Fill-in-the-Blank Practice

1. The method used to retrieve the current voltage from an analog sensor is pot.__________().
2. The maximum voltage of a Control Hub analog port, retrieved without hardcoding a constant, is found by calling pot.__________().
3. To map an input value from one numeric range to another, the SDK provides the static utility method Range.__________().

Show answers

1. getVoltage()
2. getMaxVoltage()
3. scale()

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Template Challenge

Robot Scenario: You have a potentiometer named "wrist_pot" on a wrist mechanism with 180 degrees of travel. Read the current wrist angle and use telemetry to display it. Then return true from a method called isSafe() only if the angle is between 20.0 and 160.0 degrees.

package org.firstinspires.ftc.teamcode;

import com.qualcomm.robotcore.hardware.AnalogInput;
import com.qualcomm.robotcore.hardware.HardwareMap;
import com.qualcomm.robotcore.util.Range;

public class WristSystem {

    private AnalogInput wristPot;
    private static final double WRIST_MAX_DEGREES = 180.0;

    public void init(HardwareMap hwMap) {
        // INSERT CODE HERE: Map the "wrist_pot" AnalogInput
    }

    public double getWristAngle() {
        // INSERT CODE HERE: Return the angle in degrees using Range.scale
        return 0.0;
    }

    public boolean isSafe() {
        // INSERT CODE HERE: Return true only if angle is between 20.0 and 160.0
        return false;
    }
}

Show answer

public void init(HardwareMap hwMap) {
    wristPot = hwMap.get(AnalogInput.class, "wrist_pot");
}

public double getWristAngle() {
    return Range.scale(wristPot.getVoltage(), 0, wristPot.getMaxVoltage(), 0, WRIST_MAX_DEGREES);
}

public boolean isSafe() {
    double angle = getWristAngle();
    return angle >= 20.0 && angle <= 160.0;
}

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