Tutorial 14: Mechanism helpers

You will build: one program that runs a lift, a wrist, an intake and a spinner with the library’s ready-made helpers. The main track’s parts are made from these helpers: An intake, a lift and a claw.

You will learn: the four ready-made mechanism helpers (a motor that goes to positions, a motor that spins, a servo with positions, a continuous servo that spins round and round like a small motor), how each connects to buttons in one line, and how a Subsystems group runs them all.

You need: whatever you have of: motor lift (+ touch lift_bottom), servo wrist, motor intake, continuous servo spinner. Delete the lines for parts you do not have. Tutorials 03, 08.

Full program: Tut14Mechanisms.java Reference: mechanism.md

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1. Why helpers?

In tutorials 07, 08 and 12 you built a lift by hand: PID, limits, homing, named heights, hand control. That is about 100 lines, and every season’s robot needs it again. PositionMotor is that work, tested, in one setup chain. You keep the understanding; you skip the typing and the bugs.

2. A lift: PositionMotor

PositionMotor lift = PositionMotor.create(hardwareMap, "lift")
        .limits(0, 2600)                          // never below 0 or above 2600 ticks
        .lowerSwitch(hardwareMap, "lift_bottom")  // stops at the bottom and sets 0 there
        .slowZone(250, 0.2)                       // gentle near the ends
        .pid(0.004, 0, 0.0002, 0.05)              // from tutorial 07
        .position("DOWN", 0)
        .position("LOW", 900)
        .position("HIGH", 2400);

Each line is one decision you understand from earlier tutorials. Buttons:

.bindPress(Pad.G1, Btn.DPAD_UP, lift::stepUp)        // next position up
.bindPress(Pad.G1, Btn.Y, () -> lift.goTo("HIGH"))

Hand control, every loop: lift.manual(stick). Push the stick: you drive it. Let go: it holds where you left it (PID), instead of sagging.

3. A wrist: PositionServo

PositionServo wrist = PositionServo.create(hardwareMap, "wrist")
        .position("STOW", 0.15)
        .position("LEVEL", 0.50)
        .position("DROP", 0.85)
        .maxSpeed(1.5);                           // at most 1.5 positions per second: no swinging

wrist::next and wrist::previous on the bumpers walk through the positions in the order you added them. A servo does not move until you tell it to: here, wrist.goTo("STOW") right after START.

4. An intake: SpinMotor

SpinMotor intake = SpinMotor.create(hardwareMap, "intake").powers(1.0, -0.7);
...
.bindHold(Pad.G1, Btn.B, intake::out)
.bindRelease(Pad.G1, Btn.B, intake::stop)

in(), out() and stop() are safe to call every loop, so the trigger logic is plain:

if (!gamepad1.b) {
    if (gamepad1.right_trigger > 0.2) intake.in(); else intake.stop();
}

Add .jamDetection(80, 0.3, 0.4) (with the encoder cable) and a jam backs itself out. (A jam is when a game piece gets stuck and the intake stops turning while it has power.)

5. A spinner: SpinServo

SpinServo spinner = SpinServo.create(hardwareMap, "spinner").power(0.8);

The port must be set to Continuous Rotation Servo in the configuration.

6. One group for everything

Subsystems all = new Subsystems().add(lift, wrist, intake, spinner);
...
all.update();             // every loop: PID, limits, slow moves, jam checks
all.addTelemetry(t);      // each part adds its own lines
...
all.stop();               // in finally

Run it

1. Robot on a block, lift at the bottom. START.
2. Telemetry shows each part. If the lift says “push it DOWN to home it”, do so with the stick.
3. Y: lift to HIGH. Push the stick: you take over. Let go: it holds.
4. Bumpers: the wrist walks through its positions slowly.
5. Right trigger: intake in. B held: out.

If something goes wrong

Problem	Fix
“No DcMotorEx named …”	a part you do not have: delete its lines, and from all.add(...)
Lift only goes down, slowly	it is waiting to be homed: push it to the switch
Lift goes the wrong way	add .reversed() right after create(...)
Wrist swings too fast / too slow	change maxSpeed
Spinner does not spin	the port is configured as a normal Servo

Try this

1. Add .tunable("Lift") at the end of the lift chain and tune it with tutorial 15.
2. Make the wrist go to “DROP” automatically when the lift reaches HIGH (if (lift.isAt("HIGH"))).
3. Use PositionMotor with .units(...) for an arm in degrees.

Next

Tutorial 15: Telemetry, profiles and tuning