Day 8: (Mon Sep 23, Week 5) Motor Puppets; Raspberry Pi Pico; Hobby Servos

Notes for 2024-09-23.

New Assignments

1. New assignment, due Mon Sep 30: Children’s School Classroom Observation Session. Please bring your notes on paper to class.

2. Please write a variation of the servo_step.py program (below) to produce a hobby servo motion pattern of your choice to be demonstrated at the start of the next class.

Administrative

Please note: for the next class on Wed Sep 25 we will be visiting the Children’s School for the entire class period. On Wednesday please meet at the south lobby of Margaret Morrison, one floor down from the rotunda entrance. Please be prompt, we will enter the MMC-17 secure school area precisely at 10:00AM.

Agenda

1. Review results of Exercise: Motor Puppet

2. Discussion of the observation assignment.

3. Brief introduction to the Raspberry Pi Pico and CircuitPython.

4. Each person pick up a Pico and a USB cable.

5. Installing the Mu Python Editor.

6. Crash course in CircuitPython basics.

   • Introductory Examples - Raspberry Pi Pico

7. Introduction to hobby servos.

   • Hobby Servo Examples - Raspberry Pi Pico

Figures

Lecture code samples

Blink

# pico_blink.py

# Raspberry Pi Pico - Blink demo

# Blink the onboard LED and print messages to the serial console.

import board
import time
from digitalio import DigitalInOut, Direction, Pull

#---------------------------------------------------------------
# Set up the hardware: script equivalent to Arduino setup()
# Set up built-in green LED
led = DigitalInOut(board.LED)  # GP25
led.direction = Direction.OUTPUT

#---------------------------------------------------------------
# Run the main loop: script equivalent to Arduino loop()

while True:
    led.value = True
    print("On")
    time.sleep(1.0)

    led.value = False
    print("Off")
    time.sleep(1.0)

Servo Step

# servo_step.py
#
# Raspberry Pi Pico - hobby servo motion demo
#
# Demonstrates stepping a hobby servo back and forth.
#
# This assumes a tiny 9G servo has been wired up to the Pico as follows:
#   Pico pin 40 (VBUS)  -> servo red   (+5V)
#   Pico pin 38 (GND)   -> servo brown (GND)
#   Pico pin 1  (GP0)   -> servo orange (SIG)

# links to CircuitPython module documentation:
# time    https://circuitpython.readthedocs.io/en/latest/shared-bindings/time/index.html
# math    https://circuitpython.readthedocs.io/en/latest/shared-bindings/math/index.html
# board   https://circuitpython.readthedocs.io/en/latest/shared-bindings/board/index.html
# pwmio   https://circuitpython.readthedocs.io/en/latest/shared-bindings/pwmio/index.html

################################################################################
# load standard Python modules
import math, time

# load the CircuitPython hardware definition module for pin definitions
import board

# load the CircuitPython pulse-width-modulation module for driving hardware
import pwmio

#--------------------------------------------------------------------------------
# Define a function to issue a servo command by updating the PWM signal output.
# This function maps an angle specified in degrees between 0 and 180 to a servo
# command pulse width between 1 and 2 milliseconds, and then to the
# corresponding duty cycle fraction, specified as a 16-bit fixed-point integer.

def servo_write(servo, angle, debug=False):
    # calculate the desired pulse width in units of seconds
    pulse_width  = 0.001 + angle * (0.001 / 180.0)

    # fetch the current pulse repetition rate from the hardware driver
    pulse_rate = servo.frequency
    
    # calculate the duration in seconds of a single pulse cycle
    cycle_period = 1.0 / pulse_rate

    # calculate the desired ratio of pulse ON time to cycle duration
    duty_cycle   = pulse_width / cycle_period 

    # convert the ratio into a 16-bit fixed point integer
    duty_fixed   = int(2**16 * duty_cycle)

    # limit the ratio range and apply to the hardware driver
    servo.duty_cycle = min(max(duty_fixed, 0), 65535)

    # print some diagnostics to the console
    if debug:
        print(f"Driving servo to angle {angle}")
        print(f" Pulse width {pulse_width} seconds")
        print(f" Duty cycle {duty_cycle}")
        print(f" Command value {servo.duty_cycle}\n")

#--------------------------------------------------------------------------------
# Create a PWMOut object on Pin GP0 to drive the servo. The frequency argument
# specifies the pulse repetition rate in Hz (pulses per second).

servo = pwmio.PWMOut(board.GP0, duty_cycle=0, frequency=50)

# Begin the main processing loop.
while True:
    servo_write(servo, 0.0, debug=True)
    time.sleep(2.0)

    servo_write(servo, 180.0, debug=True)
    time.sleep(2.0)