Sensors

Switches and beam breaks that work like buttons, analog sensors, and distance and color sensors read without slowing the loop.

Switches and beam breaks

Digital sensors are on or off: limit switches, touch sensors, beam breaks. They are read at the start of every loop, with the gamepads, and work like gamepad buttons:

class Intake(parent: Component) : Component(parent) {
    val beam = DigitalInput(this, "beam", onWhenLow = true)   // most beam breaks pull their line low when broken
    var balls = 0

    suspend fun countBalls() = beam.onTurnOn { balls++ }
}

Check in the moment	Wait or react
isOn	awaitOn(), awaitOff()
turnedOn, turnedOff: in this loop	onTurnOn { }, onTurnOff { }
onFor: how long it has been on	whileOn(after = ...) { }

The handlers behave like the gamepad’s (see Gamepads).

Settings: intake.beam.onWhenLow, whether the sensor is on when its line reads low, like a REV touch sensor when pressed and most beam breaks when broken; and intake.beam.debounce, how long a change has to last to count, for a switch that bounces or a beam that flickers (20ms).

Analog sensors

Potentiometers, absolute encoders and other sensors that answer with a voltage:

val armAngle = AnalogInput(this, "armAngle")
...
val turned = armAngle.fraction * 270.degrees

voltage is in volts, from 0 to maxVoltage, 3.3 V on REV hubs, and fraction is the share of it, from 0 to 1.

Digital and analog sensors and motor encoders come with the hub’s bulk read, taken once per loop when something reads them: everything in a loop sees the same values, and a sensor nothing reads costs nothing.

Distance sensors

REV’s 2m distance sensor and others the SDK sees as a distance sensor:

class Intake(parent: Component) : Component(parent) {
    val ball = DistanceSensor(this, "ball")
    private val holding = Hysteresis(on = 13.0, off = 17.0)

    val hasBall: Boolean get() = holding.value

    override fun update() {
        ball.distance?.let { holding.update(it.inCm) }
    }
}

distance is a Distance, or null when nothing is within maxRange (200 cm), when the sensor doesn’t answer, and before its first reading. Turning distances into decisions is the part’s job: here a ball is in once closer than 13 cm, and out only once farther than 17, so the answer doesn’t flicker (see Control).

Color sensors

REV’s Color Sensor V3 and others the SDK sees as a normalized color sensor:

val sample = ColorSensor(this, "sample", readDistance = true)
...
val color = sample.color
val purple = color != null && color.value > 0.01 && color.hue in 210.0..250.0

color is an Rgb: red, green and blue from 0 to 1, hue in degrees (red 0, yellow 60, green 120, blue 240), saturation from 0 (gray) to 1, and value, the brightest channel. What a sensor sees is dim, so tell colors apart by hue and saturation, which don’t depend on the light, and check that something is there first, by value or the distance: with nothing in front of the sensor, hue and saturation are noise. On one of our robots, DECODE’s game pieces held against the sensor read:

	hue	saturation	value	distance
green artifact	162°	0.73	0.05–0.08	0.6 cm
purple artifact	230°	0.48	0.04–0.05	0.7–1 cm
nothing	~174° (noise)	~0.46 (noise)	0.001	5–7 cm (the robot’s own parts)

REV’s sensor lights what it sees with its own LED, so it sees colors well only within a centimeter or two. With readDistance, each read also takes the sensor’s distance: good for telling whether something is in front of it, not for measuring. distance is null from maxRange (15 cm), the sensor’s answer for nothing.

Slow sensors

Distance and color sensors are I2C devices, slow to read: a read takes several messages to the hub, about 9 ms for a 2m distance sensor. So they are read on a thread of their own every readInterval (50 ms), and the loop never waits for them: every loop takes the newest reading. readInterval is a setting, like intake.ball.readInterval = 60ms.

Every read takes the hubs’ connection from motors and other sensors, so turn a sensor’s reading off while nothing needs it:

shooterBall.reading = isShooting

While off, it has no reading; turned on again, it is read right away and has none until that reading. Performance explains the hubs’ connection, and how to read other slow devices this way.