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Servo Driver

On this page we will learn

  • how to connect the Servo Driver
  • how to turn a servo to an angle
  • how to spin a continuous servo at a speed
  • how to release and calibrate a servo

Terminology

  • servo motor – a motor that can be controlled to turn to an exact angle or spin at a set speed.
  • servo driver – a module that sends the signals to control up to four servo motors.
  • horn – the plastic arm that fits onto a servo's hub.
  • channel – one of the numbered connection points on the Servo Driver where a servo plugs in.
  • continuous rotation servo – a servo that spins like a wheel at a speed from -1 (full speed backwards) to 1 (full speed forwards).
  • micro servo – a small servo that turns to an angle from 0° to 180° and holds it there.

The PiicoDev Servo Driver controls up to four servo motors, turning them to an angle or spinning them at a speed.

Possible uses:

  • opening and closing gates and doors
  • robot arms and grabbers
  • wheels for small robots
  • moving parts in models and animatronics

Connect it

  1. Connect the Servo Driver to the PiicoDev adapter with a PiicoDev cable. See Using PiicoDev.
  2. Put a horn (the plastic arm) on each servo's hub.
  3. Plug the continuous rotation servo into channel 1 (far left), with the orange wire lined up with sig.
  4. Plug the micro servo into channel 4 (far right), the same way.
  5. Plug the power supply into the driver's USB-C socket and turn it on. Servos need more power than the micro:bit can provide.
  6. Make sure the ASW switches are off.
  7. Upload these files to the micro:bit with main.py:
    • PiicoDev_Unified.py
    • PiicoDev_Servo.py

Two kinds of servo

  • A micro servo turns to an angle, from 0° to 180°, and holds it.
  • A continuous rotation servo spins like a wheel at a speed, from -1 (full speed backwards) to 1 (full speed forwards).

Set it up

Create the driver, then create a servo for each channel you use:

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from microbit import *
from PiicoDev_Servo import PiicoDev_Servo, PiicoDev_Servo_Driver

controller = PiicoDev_Servo_Driver()
servo = PiicoDev_Servo(controller, 4, min_us=625, max_us=2750, degrees=180)
motor = PiicoDev_Servo(controller, 1, midpoint_us=1500, range_us=1800)

The extra settings (min_us, max_us, midpoint_us, range_us) match the servos in our kits.

Methods

Method Parameters Returns Description
PiicoDev_Servo(controller, channel, ...) controller: the driver
channel: 1–4		 servo Creates a servo on a channel
servo.angle set to 0–180 float Turns a micro servo to an angle
motor.speed set to -1 to 1 float Spins a continuous servo at a speed. 0 is stopped.
servo.release() none none Stops sending signals, so the servo goes limp

angle and speed are properties, so you set them with = instead of brackets.

angle

Turns a micro servo to an angle from 0 to 180 degrees and holds it there.

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from microbit import *
from PiicoDev_Servo import PiicoDev_Servo, PiicoDev_Servo_Driver

# Setup
controller = PiicoDev_Servo_Driver()
servo = PiicoDev_Servo(controller, 4, min_us=625, max_us=2750, degrees=180)

# Main loop
while True:
    servo.angle = 90

PRIMM

  1. Predict what you think will happen. Be specific.
  2. Run the program.
  3. Time to investigate the code. What does each line do?
Code explanation
  • line 1 → imports all the commands from the microbit library.
  • line 2 → imports the Servo Driver classes.
  • line 5 → creates the Servo Driver and calls it controller.
  • line 6 → creates the micro servo on channel 4 and calls it servo.
  • line 9 → starts an endless loop.
  • line 10 → turns the servo to 90°.

speed

Spins a continuous servo at a speed from -1 (full speed backwards) to 1 (full speed forwards). 0 stops it.

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from microbit import *
from PiicoDev_Servo import PiicoDev_Servo, PiicoDev_Servo_Driver

# Setup
controller = PiicoDev_Servo_Driver()
motor = PiicoDev_Servo(controller, 1, midpoint_us=1500, range_us=1800)

# Main loop
while True:
    motor.speed = 0.5

PRIMM

  1. Predict what you think will happen. Be specific.
  2. Run the program.
  3. Time to investigate the code. What does each line do?
Code explanation
  • line 1 → imports all the commands from the microbit library.
  • line 2 → imports the Servo Driver classes.
  • line 5 → creates the Servo Driver and calls it controller.
  • line 6 → creates the continuous servo on channel 1 and calls it motor.
  • line 9 → starts an endless loop.
  • line 10 → spins forwards at half speed. Stop the program to stop the servo, or set speed to 0.

release()

Stops sending signals to the servo, so it goes limp and can be turned by hand.

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from microbit import *
from PiicoDev_Servo import PiicoDev_Servo, PiicoDev_Servo_Driver

# Setup
controller = PiicoDev_Servo_Driver()
servo = PiicoDev_Servo(controller, 4, min_us=625, max_us=2750, degrees=180)

# Main loop
while True:
    servo.angle = 90
    sleep(1000)
    servo.release()
    sleep(3000)

PRIMM

  1. Predict what you think will happen. Be specific.
  2. Run the program.
  3. Time to investigate the code. What does each line do?
Code explanation
  • line 1 → imports all the commands from the microbit library.
  • line 2 → imports the Servo Driver classes.
  • line 5 → creates the Servo Driver and calls it controller.
  • line 6 → creates the micro servo on channel 4 and calls it servo.
  • line 9 → starts an endless loop.
  • line 10 → turns the servo to 90°.
  • line 11 → waits 1 second.
  • line 12 → releases the servo. Try turning the horn by hand now.
  • line 13 → waits 3 seconds before the loop repeats.

Calibrating a continuous servo

If a continuous servo creeps when speed is 0, its midpoint needs adjusting.

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from microbit import *
from PiicoDev_Servo import PiicoDev_Servo, PiicoDev_Servo_Driver

# Setup
controller = PiicoDev_Servo_Driver()
motor = PiicoDev_Servo(controller, 1, midpoint_us=1500, range_us=1800)
motor.speed = 0

PRIMM

  1. Predict what you think will happen. Be specific.
  2. Run the program.
  3. Time to investigate the code. What does each line do?
Code explanation
  • line 1 → imports all the commands from the microbit library.
  • line 2 → imports the Servo Driver classes.
  • line 5 → creates the Servo Driver and calls it controller.
  • line 6 → creates the continuous servo with a midpoint of 1500.
  • line 7 → sets the speed to 0. If the servo still turns, change midpoint_us on line 6 by about 25 at a time and run again until it stops. Use the same value in your other programs.

Documentation

Documentation is the official guide written by the people who made the code library, and we can use it to look up every method and its parameters, including ones that aren't covered on this page.