Servo control using Raspberry Pi

Controlling servo motor using Raspberry Pi directly or with PCA9685 servo driver via I2C interface

Overview

This post covers how to control a servo motor from a Raspberry Pi.

There are two methods to control a servo motor using a Raspberry Pi: with direct PWM from Raspberry Pi, or using an additional PCA9685 servo driver via I2C interface. Using a direct PWM signal may be the easiest method, but come with more limitation: CPU usage for PWM signaling, limited number of pin for the servos, power safety risk, etc. On the other hand, controlling using an additional PCA9685 handles those limitation. The only requirement is to enable I2C interface for the communication.

This post explain how to use both methods. Please note that the Raspberry Pi is installed with Ubuntu image in this method.

1. Using PWM

To use PWM signal, wiringPi library is required.

With Ubuntu 16.04 image, installing wiringPi cannot be done directly using apt. The following steps can handle the installation [1].

Installation

Install dependency

sudo apt install build-essential
sudo apt install libi2c-dev
sudo apt install git-core

Install wiringPi
Note: cannot install wiringPi using apt install.

cd ~
git clone git://git.drogon.net/wiringPi
cd wiringPi
./build

Confirm installation
```
gpio -v
```

After the wiringPi is installed, you can try to play the gpio command to send PWM signal.

You can refer to Adafruit’s tutorial more more detail [2]. Here I only write the part after the wiringPi is installed.

Usage

Standard servo motors require PWM frequenct of 50 Hz.

\[pwmFrequency = \frac{Base Crystal Frequency}{pwmClock \times pwmRange}\]

Note that base crystal frequency for Raspberry Pi is 19.2 MHz.

Example:

Set pin 18 to PWM mode
```
 gpio -g mode 18 pwm
```

Set PWM configuration

 gpio pwm-ms
 gpio pwmc 192
 gpio pwmr 2000

Set servo to all the way to the left (pin 18)
```
 gpio -g pwm 18 100
```
Set servo to the middle (pin 18)
```
 gpio -g pwm 18 150
```
Set servo to all the way to the right (pin 18)
```
 gpio -g pwm 18 200
```

2. Using PCA9685 servo driver via I2C interface

Installation

Install raspi-config

wget https://archive.raspberrypi.org/debian/pool/main/r/raspi-config/raspi-config_20160527_all.deb -P /tmp
sudo apt-get install libnewt0.52 whiptail parted triggerhappy lua5.1 alsa-utils –y 
sudo apt-get install –fy 
sudo dpkg -i /tmp/raspi-config_20160527_all.deb 

Enable I2C
Note: depend on raspi-config menu
1. sudo mount /dev/mmcblk0p1 /boot
2. sudo raspi-config
3. Choose 9 Advanced Options > A6 I2C. Follow the rest.
Check the I2C port
1. Run ls /dev.
2. Confirm if the I2C port is set, e.g /dev/i2c-1 —> port 1.
3. Test detecting I2C on available port. E.g for port 1:
```
sudo i2cdetect -y 1 
```

Using PCA9685

(if necessary) Set locale
In case of setting locale to Japan:
```
sudo locale-gen ja_JP.utf8
```

Installing Libraries

sudo pip3 install RPI.GPIO
sudo pip3 install adafruit-blinka
sudo pip3 install adafruit-circuitpython-pca9685
sudo pip3 install adafruit-circuitpython-motor 

Refer to Documentation for Adafruit PCA9685 Library for examples and further documentation [3].

Afterword

Using PCA9685 via I2C interface is preferred.

References:

[1] https://qiita.com/frafrakki/items/3b14b0a318039e51dadd

[2] https://learn.adafruit.com/adafruits-raspberry-pi-lesson-8-using-a-servo-motor?view=all#software

[3] https://docs.circuitpython.org/projects/pca9685/en/stable/examples.html