RaspberryMotors 1.0.1

Raspberry Pi service library dedicated to motor control

RaspberryMotors

Welcome to a Raspberry Pi service library dedicated to motor control. This library has been started as an interface to control servo motors using the GPIO pins on the Raspberry pi. Other types of motors like Stepper motors and DC motors will be added as we go along.

The motivation to write this library was that using servos on Raspberry is too painful. One must set up the pulse width modulation (PWM) and calculate duty cycles in order to operate the servos! I also wanted to control several servo motors simultaneously and most example were showing only how to control one servo at a time. Mostly, I was missing the quiet elegance on the Arduino Servo library.

Getting started

There are two ways to using the library, namely:downloading from git, or using pip install.

Downloading from Git

1. Make your working directory and change directory (cd) to it
2. Issue the following command: git clone https://github.com/vikramdayal/RaspberryMotors
3. You are done! Explore and test

Using pip

Just enter the command:

pip install -i https://pypi.org/simple/ RaspberryMotors

You can always see the sample code here:

https://github.com/vikramdayal/RaspberryMotors

Exploring and testing

If you want to figure out how to use the library, you might start by using the examples in the order provided below:

1. example_simple_servo.py: simple setup and control a single servo
2. example_simple_2servos.py: simple setup and control two servos sequentially
3. example_timer_servo.py: advanced setup and control two servos simultaneously
4. example_advanced_servo.py: adanced setup using using the BCM interface and using ResetGpioAtShutdown(False)

In order to test each example, you need to be in the parent directory of the RaspberryMotors/ directory. Simply enter the commands: python3 -m RaspberryMotors.example_simple_servo python3 -m RaspberryMotors.example_simple_2servos python3 -m RaspberryMotors.example_timer_servo python3 -m RaspberryMotors.example_advanced_servo

By the time you are done with the third example, you will find that you can do pretty much anything with Raspberry Pi and the Servo library.

Please note that the servos will operate only if they are wired correctly. Simplified instructions on wiring the servos are provided below.

Wiring required to run the example

The examples provided here require two servo motors, I used Vilros SG90 (9g) and MicroServo DXW90 (9g) servos. Please feel free to use any of your own servo brands with their ratings. Keep in mind if you are using the bigger servos you would have to power them separately. The internet is full of examples of how to wire up servos and using breadboards and external power supplies, feel free to follow any of those instructions. Essentially, the main requirement for this library is to connect the control (usually the yellow) wire of the servo to the GPIO pin on the Raspberry Pi.

Please note that by default, the library uses the GPIO.BOARD settings, which means that all pin numbers are defined as physical pin numbers. All examples assume physical pin numbers.

servo-1 (referred to S1 in the example code)

---------------------------------------------
| servo wire | connected to GPIO pin on Pi  |
|------------|------------------------------|
| Brown      | 6 (GND)                      |
| Red        | 2 (5v power)                 |
| Yellow     | 11(GPIO 17 on Pi-4)          |
---------------------------------------------

servo-2 (referred to S2 in the example code)

---------------------------------------------
| servo wire | connected to GPIO pin on Pi  |
|------------|------------------------------|
| Brown      | 14(GND)                      |
| Red        | 4 (5v power)                 |
| Yellow     | 13(GPIO 27 on Pi-4)          |
---------------------------------------------

Advanced Users

The rpi.GPIO library from Raspberry Pi has been encapsulated within the servo package. For now, the GPIO interface has not been exposed to the user and the GPIO.cleanup() is performed automatically when the last servo is shutdown(). At a later stage, I might move the GPIO functions into another package when I do stepper motors and other motor control interfaces.

Setting up servo motor duty cycle to control angles

A duty cycle or power cycle is the fraction of one period in which a signal or system is active [https://en.wikipedia.org/wiki/Duty_cycle]

The angle of the servo motor is controlled by sending pulses to it. The angle of the motor is set by setting what is called the duty cycle of the PWM. In most common servo motors a duty cycle of 2.0 sets the motor to 0 degrees and a duty cycle of 12.5 sets it to 180 degrees. However, the actual numbers vary for most servo. My servos seem to operate in a range of 2.0 and 10.5 and hence this library has been coded for a default range of from 2.0 to 10.5.

If the above paragraph does not make any sense, do not worry. The process of setting up the servos has been simplified with a simple command line use. The instructions for use of a different servo motor and determining duty cycle are as follows:

1. Wire up the servo motor as per instructions shown above (let us say we wired up on GPIO pin 11, rest of instructions are for pin 11)
2. From the home directory of this project, run the following command: python3 -m motor --pin=11 --duty=2.0*
3. Observe the motor position. Vary the value of duty cycle in the above command till you observe the stepper motor is at position zero. Note that the motor will not rotate any further if you keep decreasing the value of duty cycle. Take the highest value of duty cycle when the angle is still zero.
4. Repeat the same steps in increasing values to get the corresponding value of duty cycle closest to 180 degrees
5. In the python code, instantiate the servo object as per the following code: s1 = servos.servo(pin=11, min=2.0,max=10.5,waitTime=0.5)

Rest of the code remains unchanged

Disclaimers

1. As is software. Please let me know any feature requests.
2. It's not the nest software in the world, take it with a pinch of salt.