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.. _1.3.2_py:

1.3.2 Servo
============

Introduction
--------------

In this project, we will learn how to make the servo rotate.

Required Components
------------------------------

In this project, we need the following components. 

.. image:: ../img/list_1.3.2.png

It's definitely convenient to buy a whole kit, here's the link: 

.. list-table::
    :widths: 20 20 20
    :header-rows: 1

    *   - Name	
        - ITEMS IN THIS KIT
        - LINK
    *   - Raphael Kit
        - 337
        - |link_Raphael_kit|

You can also buy them separately from the links below.

.. list-table::
    :widths: 30 20
    :header-rows: 1

    *   - COMPONENT INTRODUCTION
        - PURCHASE LINK

    *   - :ref:`cpn_gpio_extension_board`
        - |link_gpio_board_buy|
    *   - :ref:`cpn_breadboard`
        - |link_breadboard_buy|
    *   - :ref:`cpn_wires`
        - |link_wires_buy|
    *   - :ref:`cpn_servo`
        - |link_servo_buy|

Schematic Diagram
--------------------

.. image:: ../img/image337.png


Experimental Procedures
-----------------------

**Step 1:** Build the circuit.

.. image:: ../img/image125.png

**Step 2**: Go to the folder of the code.

.. raw:: html

   <run></run>

.. code-block::

    cd ~/raphael-kit/python/

**Step 3**: Run the executable file.

.. raw:: html

   <run></run>

.. code-block::

    sudo python3 1.3.2_Servo.py

After the program is executed, the servo will rotate from 0 degrees 
to 180 degrees, and then from 180 degrees to 0 degrees, circularly.

**Code**

.. note::

    You can **Modify/Reset/Copy/Run/Stop** the code below. But before that, you need to go to  source code path like ``raphael-kit/python``. After modifying the code, you can run it directly to see the effect.


.. raw:: html

    <run></run>

.. code-block:: python

    import RPi.GPIO as GPIO
    import time

    SERVO_MIN_PULSE = 500
    SERVO_MAX_PULSE = 2500
    ServoPin = 18

    def map(value, inMin, inMax, outMin, outMax):
        return (outMax - outMin) * (value - inMin) / (inMax - inMin) + outMin

    def setup():
        global p
        GPIO.setmode(GPIO.BCM)       # Numbers GPIOs by BCM
        GPIO.setup(ServoPin, GPIO.OUT)   # Set ServoPin's mode is output
        GPIO.output(ServoPin, GPIO.LOW)  # Set ServoPin to low
        p = GPIO.PWM(ServoPin, 50)     # set Frequecy to 50Hz
        p.start(0)                     # Duty Cycle = 0
        
    def setAngle(angle):      # make the servo rotate to specific angle (0-180 degrees) 
        angle = max(0, min(180, angle))
        pulse_width = map(angle, 0, 180, SERVO_MIN_PULSE, SERVO_MAX_PULSE)
        pwm = map(pulse_width, 0, 20000, 0, 100)
        p.ChangeDutyCycle(pwm)#map the angle to duty cycle and output it    
    def loop():
        while True:
            for i in range(0, 181, 5):   #make servo rotate from 0 to 180 deg
                setAngle(i)     # Write to servo
                time.sleep(0.002)
            time.sleep(1)
            for i in range(180, -1, -5): #make servo rotate from 180 to 0 deg
                setAngle(i)
                time.sleep(0.001)
            time.sleep(1)
    def destroy():
        p.stop()
        GPIO.cleanup()

    if __name__ == '__main__':     #Program start from here
        setup()
        try:
            loop()
        except KeyboardInterrupt:  # When 'Ctrl+C' is pressed, the program destroy() will be executed.
            destroy()
        

**Code Explanation**

.. code-block:: python

    p = GPIO.PWM(ServoPin, 50)     # set Frequecy to 50Hz
    p.start(0)                     # Duty Cycle = 0

Set the servoPin to PWM pin, then the frequency to 50hz, and the period to 20ms.

p.start(0): Run the PWM function，and set the initial value to 0.

.. code-block:: python

    def setAngle(angle):      # make the servo rotate to specific angle (0-180 degrees) 
        angle = max(0, min(180, angle))
        pulse_width = map(angle, 0, 180, SERVO_MIN_PULSE, SERVO_MAX_PULSE)
        pwm = map(pulse_width, 0, 20000, 0, 100)
        p.ChangeDutyCycle(pwm)#map the angle to duty cycle and output it
    
Create a function, setAngle() to write angle that ranges from 0 to 180 into the servo.

.. code-block:: python

    angle = max(0, min(180, angle))

This code is used to limit the angle within the range 0-180°.

The min() function returns the minimum of the input values. 
If 180<angle, then return 180,if not, return angle.

The max() method returns the maximum element in an iterable or largest of 
two or more parameters. If 0>angle, then return 0, if not, return angle.

.. code-block:: python

    pulse_width = map(angle, 0, 180, SERVO_MIN_PULSE, SERVO_MAX_PULSE)
    pwm = map(pulse_width, 0, 20000, 0, 100)
    p.ChangeDutyCycle(pwm)

To render a range 0 ~ 180 ° to the servo, the pulse width of the servo
is set to 0.5ms(500us)-2.5ms(2500us).

The period of PWM is 20ms(20000us), thus the duty cycle of PWM is
(500/20000)%-(2500/20000)%, and the range 0 ~ 180 is mapped to 2.5 ~
12.5.


Phenomenon Picture
------------------

.. image:: ../img/image126.jpeg