Microbit Sensor Series 05 – PWM Motor

01

Introduction

The PWM motor is controlled by a PWM signal, allowing for precise control of rotation speed and high torque compared to a regular DC motor, although it does not achieve as high a speed.

02

Principle

For an introduction to the principle of PWM, please refer to my previous article on PWM servos.

Motor parameters: 500μs~CCW counterclockwise; 1500μs~stop; 2500μs~CW clockwise

The PWM motor operates based on pulse-width signals. The signal period for this PWM servo is 20ms. When the pulse width is 1500μs, the motor stops; at 500μs, the motor rotates forward at maximum speed; and at 2500μs, the motor rotates backward at maximum speed.

03

Wiring

One end of the wire is a 51065 3P connector, and the other end is a JST2.54 3P connector, with three wires:

• G: Power negative, connect to GND, brown wire
• V: Power positive, connect to 3.3V, red wire
• S: Signal wire, connect to P0, yellow wire

04

Program

Below is the implementation of speed control for a 360-degree motor:

Stop pulse width = 1500

Forward pulse width = Speed value / Maximum speed value * (Maximum forward speed pulse width – Stop pulse width) + Stop pulse width

Reverse pulse width = Stop pulse width – Speed value / Maximum speed value * (Maximum reverse speed pulse width – Stop pulse width)

Duty cycle = Pulse width / Signal period

——Signal period is 20000μs.

PWM = Maximum supported bit length * Duty cycle

——Microbit supports 10 bits, so it is 1023.

// Forward speed 50
let speed = 50
// Calculate the pulse width corresponding to this speed based on the signal period = 20ms, pulse width = 1500μs~2500μs
let us = Math.floor((2500 - 1500) * speed / 100) + 1500
// Calculate the duty cycle, then calculate the pwm value based on 10-bit resolution
let pwm = Math.floor(us / 20000 * 1023)
// Set frequency
pins.analogSetPeriod(AnalogPin.P0, 20000)
// Set analog signal value
pins.analogWritePin(AnalogPin.P0, pwm)

05

Demonstration

I have packaged this as an extension: https://github.com/tim2anna/pxt-microblue, which can be used directly after adding.

When button A is pressed, the motor speed increases; when button B is pressed, the motor speed decreases.