PWM Python Tutorial

The speed of the motors is controlled by a PWM (Pulse width modulation) voltage. The PWM frequency is too high for the motors to keep up with the rise and fall of the PWM voltage. Instead, the motor only respond to the effective voltage given by the fraction of the duty cycle (see figure below).

Import libraries. You will need to have Numpy, Scipy and Matplotlib installed on your system.

import numpy as np
import matplotlib.pyplot as plt
from scipy import signal
plt.ion()

Create the data.

###############   Create data  ########################
t = np.linspace(0, 1, 2000) 
sig = np.sin(2 * np.pi * t)
pwm = signal.square(2 * np.pi * 30 * t, duty=(sig + 1)/2)
pwm15 = signal.square(2 * np.pi * 30 * t, 0.15)
effectiveV15 = pwm15*0+0.15*5 
pwm85 = signal.square(2 * np.pi * 30 * t, 0.85)
effectiveV85 = pwm15*0+0.85*5

Plot the data!

plt.figure(figsize=(10, 8))
plt.subplot(3, 1, 1)
plt.plot(t, effectiveV15,'b',label = 'Effective Voltage')
plt.plot(t, pwm15*2.5+2.5,c=(255/255.,117/255.,0/255.),label = 'Voltage')
plt.ylim(-0.2, 5.2)
plt.xlim(0, 0.5)
plt.title('15% Duty Cycle')
plt.ylabel('Voltage (V)')
plt.legend(loc = 'best',prop={ 'size': 8})

plt.subplot(3, 1, 2)
plt.plot(t, effectiveV85,'b',label = 'Effective Voltage')
plt.plot(t, pwm85*2.5+2.5,c=(255/255.,117/255.,0/255.),label = 'Voltage')
plt.ylim(-0.2, 5.2)
plt.xlim(0, 0.5)
plt.title('85% Duty Cycle')
plt.ylabel('Voltage (V)')
plt.legend(loc = 'best',prop={ 'size': 8})

plt.subplot(3, 1, 3)
plt.plot(t, sig*2.5+2.5, 'b',label = 'Effective Voltage')
plt.plot(t, pwm*2.5+2.5, c=(255/255.,117/255.,0/255.),label = 'Voltage')
plt.ylim(-0.2, 5.2)
plt.xlabel('t (s)')
plt.ylabel('Voltage (V)')
plt.title('Variable Duty Cycle')
plt.legend(loc = 'best',prop={ 'size': 8})
plt.subplots_adjust(hspace=0.35)
plt.savefig('PWM.svg')