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Servo

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Rick Waldron edited this page Aug 19, 2018 · 65 revisions

The Servo class constructs objects that represent a single Servo attached to the physical board. This class is designed to work well with both Standard and Continuous rotation servos.

See also:

Parameters

  • pin A Number or String address for the Servo pin.

  • General options An object of property parameters.

    Property Type Value/Description Default Required
    pin Number, String The address of the pin the servo is attached to yes
    range Array [ lower, upper ] The range of motion in degrees. [0, 180] no
    pwmRange Array [ min, max ] The pulse width range in microseconds. [600, 2400] no
    deviceRange Array [ min, max ] The physical range of the servo in degrees. [0, 180] no
    type String “standard”, “continuous”. The type of servo being created. “standard” no
    startAt Number Any number within the deviceRange. Degrees to initialize the servo at. no
    offset Number A positive or negative value to adjust the servo. false no
    invert Boolean true or false. Optionally Invert servo movement. false no
    center Boolean true or false. Optionally center the servo on initialization. false no
    controller String DEFAULT, PCA9685. Controller interface type. DEFAULT no

  • Debug Options Although servos can run on digital pins, this can sometimes cause issues. For this reason servos are forced to use only PWM under debug mode and will emit an error if used on a digital pin.

    Property Type Value/Description Default Required
    debug Boolean true or false. Set servo to debug mode false no

  • Continuous Servo Options

    Property Type Value/Description Default Required
    deadband Array [ lower, upper ]. The deadband/stop point of the servo.* [90, 90] no

    * Some continuous servos do not have a trim pot for fine tuning the stop point, instead they have a built-in "deadband" range, which should be documented but otherwise requires manual measurement. Examples:

  • PCA9685 Options (controller: "PCA9685")

    Property Type Value/Description Default Required
    address Number I2C device address. 0x40 yes

Shape

Property Name Description Read Only
id A user definable id value. Defaults to a generated uid No
pin The pin address that the Servo is attached to No
range The range of motion in degrees. Defaults to [0, 180] No
invert A boolean, indicates whether servo values are inverted No
history An array containing records of each movement No
interval A reference to the current interval, if one exists No
isMoving A boolean flag, true when moving, false when still No
last The last movement record. Yes
position The angle the servo is at in the physical world. Yes
value The angle the servo was last set to. No
startAt The angle the servo was specified to start at upon initialization No

Component Initialization

Basic

// Create a standard servo...
// 
//   - attached to pin 10
//
new five.Servo(10);

Auto Centered

// Create a standard servo...
// 
//   - attached to pin 10
//   - centered
//
new five.Servo({
  pin: 10,
  center: true
});

Specify Range

// Create a standard servo...
// 
//   - attached to pin 10
//   - limited range of 45-135 degrees
//
new five.Servo({
  pin: 10,
  range: [45, 135]
});

Specify Starting Position

// Create a standard servo...
// 
//   - attached to pin 10
//   - starts at 120°
//
new five.Servo({
  pin: 10,
  startAt: 120
});

Specify Both Range and Starting Position

// Create a standard servo...
// 
//   - attached to pin 10
//   - limited range of 45-135 degrees
//   - starts at 120°
//
new five.Servo({
  pin: 10,
  range: [45, 135],
  startAt: 120
});

Continuous

// Direct Constructor
new five.Servo.Continuous(10);


// Options object with type property
new five.Servo({
  pin: 10,
  type: "continuous"
});

Multi-turn

// Create a multi-turn servo. 6 rotations = 2160 degrees
// Note that this is not the same as a continuous rotation servo.
// 
//   - attached to pin 10
//
new five.Servo({
  pin: 10,
  deviceRange: [0, 2160]
});

Standard or Continuous

Servo PCA9685

// Create a standard servo controlled via PCA9685...
// 
//   - attached to pin 0 (of the PCA9685)
//
new five.Servo({
  controller: "PCA9685",
  pin: 0
});

Continuous PCA9685

// Create a continuous servo controlled via PCA9685...
// 
//   - attached to pin 0 (of the PCA9685)
//
new five.Servo.Continuous({
  controller: "PCA9685",
  pin: 0
});

PCA9685

Usage

Standard Servo

var five = require("johnny-five");
var board = new five.Board();

board.on("ready", function() {

  var servo = new five.Servo(10);

  // Sweep from 0-180 and repeat.
  servo.sweep();
});

Continuous Servo

var five = require("johnny-five");
var board = new five.Board();


board.on("ready", function() {

  var servo = new five.Servo({
    pin: 10, 
    type: "continuous"
  });

  // Clockwise, top speed.
  servo.cw(1);
});

I2C Servo (i.e. via Adafruit PWM controller)

var five = require("johnny-five");
var board = new five.Board();


board.on("ready", function() {

  var servo = new five.Servo({
    pin: 0, 
    controller: "PCA9685",
    address: 0x41 // Defaults to 0x40
  });

  // Sweep from 0-180.
  servo.sweep();
});

API

  • to(degrees 0-180 [, ms [, rate]]) Move a servo horn to specified position in degrees, 0-180 (or whatever the current valid range is). If ms is specified, the servo will take that amount of time to move to the position. If rate is specified, the angle change will be split into distance/rate steps for the ms option. If the specified angle is the same as the current angle, no commands are sent.

    var servo = new five.Servo(10);

// Set the horn to 90degrees
servo.to(90);

// Angle change takes 500ms to complete
servo.to(90, 500);

// Angle change takes 500ms to complete over 10 steps
servo.to(90, 500, 10);

  • min() Set Servo to minimum degrees. Defaults to 0deg, respects explicit range.

    var servo = new five.Servo(10);

// Set horn to 0degrees
servo.min();

    Or

    var servo = new five.Servo({
  pin: 10, 
  range: [ 45, 135 ]
});

// Set horn to 45°
servo.min();

  • max() Set Servo to maximum degrees. Defaults to 180deg, respects explicit range.

    var servo = new five.Servo(10);

// Set horn to 135°
servo.max();

    Or

    var servo = new five.Servo({
  pin: 10, 
  range: [ 45, 135 ]
});

// Set horn to 135°
servo.max();

  • center([ ms [, rate ]]) Set Servo to center point. Defaults to 90deg, respects explicit range. If ms is specified, the servo will take that amount of time to move to the position. If rate is specified, the angle change will be split into distance/rate steps for the ms option. If the specified angle is the same as the current angle, no commands are sent.

    var servo = new five.Servo(10);

// Set horn to 90degrees
servo.center();

    Or

    var servo = new five.Servo({
  pin: 10, 
  range: [ 40, 80 ]
});

// Set horn to 60degrees
servo.center();

  • home() Set Servo to it's startAt position.

    var servo = new five.Servo({
  pin: 10,
  startAt: 20
});

// Set horn to 90 degrees
servo.to(90);

// Return to startAt value of 20 degrees
servo.home();

  • sweep() Sweep the servo between default min and max, repeatedly.

    var servo = new five.Servo(10);

servo.sweep();

  • sweep([ low, high ]) Sweep the servo between an explicit range, repeatedly.

    var servo = new five.Servo(10);

// Repeated full range movement
servo.sweep([45, 135]);

  • sweep(options) Sweep the servo between an (optional) explicit range, within an (optional) explicit interval, and (optional) explicit steps (in degrees), repeatedly.

    • options: An object containing:
      • range: An array containing [low, high] boundaries to sweep between.
      • interval: The interval of a half sweep (eg. low -> high; a full sweep is low -> high -> low)
      • step: The size of each step in degrees.
      var servo = new five.Servo(10);
servo.sweep({
  range: [45, 135]
});

servo.sweep({
  range: [45, 135], 
  interval: 1000,
});

servo.sweep({
  range: [45, 135], 
  interval: 1000,
  step: 10
});

  • stop() Stop a moving servo.

    var servo = new five.Servo(10);

servo.stop();

  • cw(speed) (Continuous only) Move a continuous servo clockwise at speed, 0-1

    var servo = new five.Servo({
  pin: 10, 
  type: "continuous"
});
// full speed 
servo.cw(1);

  • ccw(speed) (Continuous only) Move a continuous servo counter clockwise at speed, 0-1

    var servo = new five.Servo({
  pin: 10, 
  type: "continuous"
});
// full speed 
servo.ccw(1);

Events

It's recommended to use a rotary potentiometer as the mechanism for determining servo movement.

  • move:complete This is emitted when a timed move is completed. The event won't fire unless a time argument has been passed to the servo.to() method.

Troubleshooting

If you are experiencing memory leak crashes when using your servo, make sure that you are not powering the servo directly from your board. Excessive power draw on the USB port causes such crashes. The following diagram shows an example of how to provide external power for servos:

If your continuous servo moves when it should be stopped, it likely needs to be calibrated. Find instructions here.

Examples

The JavaScript Robotics Programming Framework

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