Control a servo using a WiPy
With Micropythons PWM module controlling a servo requires only a few lines of code. A servo's position is determined by the width of a pulse. Convention is that a 1.5 ms pulse will place the servo in its neutral (i.e. 90 degree) position. The pulse widths required to reach the left- and rightmost servo positions (0 and 180 degrees) are normally 1 and 2 ms but can differ per servo. This example shows how to determine these pulse widths and control a servo using a web-interface and Ajax.
Servo's are powered by 5V, while the WiPy's outputs operate at 3.3V. The WiPy's PWM output signal might be sufficient to drive your servo, but if it does not work then a 3.3 to 5V level converter is needed. The 5V to power the servo can be taken from pin Vin on the Expansion Board. Make sure to connect the servo's ground to the WiPy (Expansion Board pin GND). An external 5V power supply may be needed as many servo's draw more current then the Expansion Board can provide via its USB connection. For example I use a DS3115mg digital servo which can draw up to 1A when moving.
The servo I used here is connected to Expansion Board pin G15. How to set up a PWM timer and connecting it to an output pin can be found in the online Pycom Documentation. The tricky bit is choosing the right values for the PWM frequency and the duty cycle. Most often a PWM frequency of 50 Hz is used which gives a period duration of 20 ms. To place a servo in the neutral position a 1.5 ms pulse width is needed. This cannot be fed directly to the PWM timer as it expects a duty cycle. This is the high-time of the output expressed as percentage of the overall period duration. So to achieve a high-time of 1.5 ms with a period duration of 20 ms the duty cycle must be 1.5 / 20 = 0.075 = 7.5%. Luckily the WiPy's PWM function expects the duty cycle as a float between 0 and 1 so the value of 0.075 can be used directly.
from machine import Pin, PWM
pwm = PWM(0, frequency=50) # 50 Hz
servo = pwm.channel(0, pin=Pin.exp_board.G15, duty_cycle=0.075) # initial duty cycle of 7.5%
servo.duty_cycle(0.10) # change duty cycle to 10%The web interface allows you to experiment with various duty cycle values to figure out the ones for the left- and rightmost positions of your servo. Just enter a value in field Duty cycle. If the servo still moves you're not yet at the end of its reach. Once you have reached your desired end-position (note that most servos can turn over 180 degrees) then enter the current duty cycle value in field Minimum duty cycle or Maximum duty cycle. The slider allows you to move the servo between the minimum and maximum. The slider and the fields use Ajax to communicate with the WiPy.
The resulting web page looks like this.
The server code just waits for HTML requests, reads the first line and discards the rest of the request. If the requested path was "/" (which happens only when you open or refresh the webpage) an HTML page is returned with the current values for all the variables. If the requested path was "/set" then all parameters are extracted from the URL's query string and stored in a dictionary (see function url.request()). When the sliders position is changed the corresponding new value for duty cycle is sent to the client, and the inverse works for changes to duty cycle. In this way the slider and duty cycle are always in sync on the user interface.
I've used Bootstrap 4 to make the form look a little bit nicer. Communication from web interface to server is done using XMLHttpRequest. A more modern alternative is to use the fetch() interface.
If you run servo.py from Pymakr the server prints messages to the console so you can keep track of what is going on. This helps debugging. Do not forget to move file url.py to the WiPy first.
You need to assign a static IP address to your WiPy to be able to connect from your browser. You have to set this in the WLAN setup of your boot.py. Example code on how to do this can be found in the official Pycom documentation under the WLAN section.
If you want to control two servo's at the same time use servo-dual.py. It uses the same principles as servo.py. As an extra it contains a class 'servo' so you don't have to duplicate the variables and code to control for every servo. The web interface is more simple with just two sliders and for each the current position as a number.