Servo Pi Node JS Library

Node.js Library to use with Servo Pi Raspberry PWM servo control board.

Note: Version 2.0 replaces the ServoPi class with the PWM class. The new Servo class is a helper for using the Servo Pi with analogue radio control servos.

Install

To download to your Raspberry Pi type in the terminal:

git clone https://github.com/abelectronicsuk/ABElectronics_NodeJS_Libraries.git

The Servo Pi library is located in the /lib/servopi/ directory

The example files are located in the /examples/servopi/ directory

The Servo Pi library requires the rpio and i2c-bus libraries to run.

Install rpio from npm package repository rpio with

npm install rpio

Install i2c-bus from npm package repository i2c-bus with

npm install i2c-bus

PWM Class

The PWM class contains the functions needed to control the frequency, pulse duration and other features of the PCA9685 PMW controller used on the Servo Pi.

PWM(address) 

Parameter: address - 0x40 to 0x7F. I2C address for the target device

PWM Functions

setPWMFrequency(freq, calibration) 

Set the PWM frequency
Parameter: freq - Frequency between 40 and 1000
Parameter: calibration - Oscillator calibration offset. Use this to adjust for oscillator drift. Value is normally between -10 and 10
Returns: null

setPWM(channel, on, off) 

Set the output on single channels
Parameter: channel - 1 to 16,
Parameter: on - 0 to 4095
Parameter: off - 0 to 4095
Returns: null

setPWMOnTime(channel, on) 

Set the on time on a single channel
Parameter: channel - 1 to 16,
Parameter: on - 0 to 4095
Returns: null

setPWMOffTime(channel, off) 

Set the off time on a single channel
Parameter: channel - 1 to 16,
Parameter: off - 0 to 4095
Returns: null

getPWMOffTime(channel) 

Get the off time on a single channel
Parameter: channel - 1 to 16,
Returns: 0 to 4095

getPWMOnTime(channel, off) 

Get the on time on a single channel
Parameter: channel - 1 to 16,
Returns: 0 to 4095

setAllPWM(on, off) 

Set the output on all channels
Parameter: channel - 1 to 16,
Parameter: on - 0 to 4095
Parameter: off - 0 to 4095
Returns: null

outputDisable()

Disable the output via the OE pin
Parameter: null
Returns: null

outputEnable()

Enable the output via the OE pin
Parameter: null
Returns: null

setAllCallAddress(address)

Set the I2C address for the All Call function
Parameter: address
Returns: null

enableAllCallAddress()

Enable the I2C address for the All Call function
Parameter: null
Returns: null

disableAllCallAddress()

Disable the I2C address for the All Call function
Parameter: null
Returns: null

sleep()

Put the device into a sleep state
Parameter: null
Returns: null

wake()

Wake the device from its sleep state
Parameter: null
Returns: null

isSleeping()

Check the sleep status of the device
Parameter: null
Returns: true = sleeping, false = awake

invertOutput()

Invert the PWM output on all channels
Parameter: null
Returns: null

PWM Usage

To use the Servo Pi library in your code you must first import the library:

var servopi = require('../../lib/servopi/servopi')

Next you must initialise the PWM object:

var pwm = new PWM(0x40);

Set the PWM frequency to 60 Hz with a calibration value of 0 and enable the output

pwm.setPWMFrequency(60, 0);
pwm.outputEnable();

Create an array of pulse duty cycles 25%, 50% and 75%, and a counter variable

var positions = [1024, 2048, 3072];
var count = 0;

Create a timer object and change the pulse width on pin 1 between three values

var myTimer = setInterval(clockTimer, 1000);

function clockTimer() {
    // move the
    pwm.setPWM(1, 0, positions[count]);
    count++;
    if (count > positions.length) {
        count = 0;
    }
}

Servo Class

The Servo class contains the functions for controlling analogue RC servos using the Servo Pi.

Servo(address, low_limit, high_limit, reset) 

Parameter: address - 0x40 to 0x7F. I2C address for the target device
Parameter: low_limit - Lower servo limit in milliseconds. Typically 1.0
Parameter: high_limit - Upper servo limit in milliseconds. Typically 2.0
Parameter: reset - True = All channels reset to default off state, False = All channels retain their current state

Servo Functions

move(channel, servopos, steps)

Move the servo to a new position
Parameter: channel - 1 to 16
Parameter: servopos - 0 to the number of steps
Parameter: steps - Number of steps. Typically 250 for an RC servo.
Returns: null

getPosition(channel, steps)

Get the current position of the servo. Due to rounding errors, the returned value may differ from the set value when steps is above 250.
Parameter: channel - 1 to 16
Parameter: steps - Number of steps. Typically 250 for an RC servo.
Returns: current position

setLowLimit(low_limit, channel)

Set the low limit in milliseconds
Parameter: low_limit - Typically 1.0 milliseconds. Setting the value too low may damage your servo.
Parameter: channel - 1 to 16. If the channel is omitted or set as 0 the value will be set for all channels.
Returns: null

setHighLimit(high_limit, channel)

Set the low limit in milliseconds
Parameter: high_limit - Typically 2.0 milliseconds. Setting the value too high may damage your servo.
Parameter: channel - 1 to 16. If the channel is omitted or set as 0 the value will be set for all channels.
Returns: null

setPWMFrequency(freq, calibration) 

Set the PWM frequency. For RC servos this should normally be set to 50Hz.
Parameter: freq - Frequency between 40 and 1000
Parameter: calibration - Oscillator calibration offset. Use this to adjust for oscillator drift. Value is normally between -10 and 10
Returns: null

outputDisable()

Disable the output via OE pin
Parameter: null
Returns: null

outputEnable()

Enable the output via OE pin
Parameter: null
Returns: null

offsetEnable()

Enable pulse offsets. This will set servo pulses to be staggered across the channels to reduce surges in the current draw
Parameter: null
Returns: null

offsetDisable()

Disable pulse offsets. This will set all servo pulses to start at the same time
Parameter: null
Returns: null

sleep()

Put the device into a sleep state
Parameter: null
Returns: null

wake()

Wake the device from its sleep state
Parameter: null
Returns: null

isSleeping()

Check the sleep status of the device
Parameter: null
Returns: true = sleeping, false = awake

Servo Usage

To use the Servo Pi library in your code you must first import the library:

var servopi = require('../../lib/servopi/servopi')

Next, you must initialise the Servo object. I2C channel 40, low limit of 1ms, high limit of 2ms and reset enabled.

var servo = new Servo(0x40, 1.0, 2.0, true);

Set the PWM frequency to 60 Hz with a calibration value of 0 and enable the output

servo.setPWMFrequency(60, 0);
servo.outputEnable();

Create a positions array and a counter variable

var positions = [1, 175, 250];
var count = 0;

Create a timer object and move the servo on pin 1 between three points

var myTimer = setInterval(clockTimer, 1000);

function clockTimer() {
    // move the servo channel 1 to the three positions
    servo.move(1, positions[count], 250);
    count++;
    if (count >= positions.length) {
        count = 0;
    }
}