python example of a motor hardware interface

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To convert Twist message to PWM that's what I've done

1. I've created a test script that sends PWM duty cycle signal from the lowest to highest at specific time intervals (e.g. 1sec, 2sec and son on).
2. For each event (e.g. PWM 50 for 2sec) I've measured the distance covered by the robot.
3. With distances (x) and duty cycles (y) I've calculated the slope(m) and y-intercept(b) values of a linear equation y = mx + b
4. With the linear function I can calculate the duty cycle needed. So, for instance from the Twist linear.x velocity message value, defined as meters/second, I can derive the duty cycle. Note that when the robot goes from a stop state to a moving state some initial friction needs to be considered.

Example

def calc_vel(self, x, z): # x and z are coming from Twist 
    t = self.track #distance between wheels

    r_vel = x + ((z * t) /2)
    l_vel = x - ((z * t) /2)

    dc_r = calc_duty_cycle(r_vel)
    dc_l = calc_duty_cycle(dc_l)

    return dc_r, dc_l

def calc_duty_cycle(self, vel):
    # linear equation variables defined by experimentation
    M_SLOPE = 50
    Y_INTERCEPT = 2

    return M_SLOPE * vel + Y_INTERCEPT