y2018/control_loops/python/arm_distal.py

#!/usr/bin/python3

# This code was used to select the gear ratio for the distal arm.
# Run it from the command line and it displays the time required
# to move the distal arm 60 degrees.
#
# Michael Schuh
# January 20, 2018

import math
import numpy
import scipy.integrate

# apt-get install python-scipy python3-scipy python-numpy python3-numpy

pi = math.pi
pi2 = 2.0 * pi
rad_to_deg = 180.0 / pi
inches_to_meters = 0.0254
lbs_to_kg = 1.0 / 2.2
newton_to_lbf = 0.224809
newton_meters_to_ft_lbs = 0.73756
run_count = 0
theta_travel = 0.0


def to_deg(angle):
    return (angle * rad_to_deg)


def to_rad(angle):
    return (angle / rad_to_deg)


def to_rotations(angle):
    return (angle / pi2)


def time_derivative(x, t, voltage, c1, c2, c3):
    global run_count
    theta, omega = x
    dxdt = [omega, -c1 * omega + c3 * math.sin(theta) + c2 * voltage]
    run_count = run_count + 1

    #print ('dxdt = ',dxdt,' repr(dxdt) = ', repr(dxdt))
    return dxdt


def get_180_degree_time(c1, c2, c3, voltage, gear_ratio, motor_free_speed):
    #print ("# step     time    theta    angular_speed   angular_acceleration  theta   angular_speed  motor_speed motor_speed_fraction")
    #print ("#          (sec)   (rad)      (rad/sec)        (rad/sec^2)      (rotations) (rotations/sec)    (rpm)   (fraction)")
    global run_count
    global theta_travel

    if (False):
        # Gravity is assisting the motion.
        theta_start = 0.0
        theta_target = pi
    elif (False):
        # Gravity is assisting the motion.
        theta_start = 0.0
        theta_target = -pi
    elif (False):
        # Gravity is slowing the motion.
        theta_start = pi
        theta_target = 0.0
    elif (True):
        # Gravity is slowing the motion.
        theta_start = -pi
        theta_target = 0.0

    theta_half = 0.5 * (theta_start + theta_target)
    if (theta_start > theta_target):
        voltage = -voltage
    theta = theta_start
    theta_travel = theta_start - theta_target
    if (run_count == 0):
        print(
            "# Theta Start = %.2f radians End = %.2f Theta travel %.2f Theta half = %.2f Voltage = %.2f"
            % (theta_start, theta_target, theta_travel, theta_half, voltage))
        print(
            "# Theta Start = %.2f degrees End = %.2f Theta travel %.2f Theta half = %.2f Voltage = %.2f"
            % (to_deg(theta_start), to_deg(theta_target), to_deg(theta_travel),
               to_deg(theta_half), voltage))
    omega = 0.0
    time = 0.0
    delta_time = 0.01  # time step in seconds
    for step in range(1, 5000):
        t = numpy.array([time, time + delta_time])
        time = time + delta_time
        x = [theta, omega]
        angular_acceleration = -c1 * omega + c2 * voltage
        x_n_plus_1 = scipy.integrate.odeint(time_derivative,
                                            x,
                                            t,
                                            args=(voltage, c1, c2, c3))
        #print ('x_n_plus_1 = ',x_n_plus_1)
        #print ('repr(x_n_plus_1) = ',repr(x_n_plus_1))
        theta, omega = x_n_plus_1[1]
        #theta= x_n_plus_1[0]
        #omega = x_n_plus_1[1]
        if (False):
            print ("%4d  %8.4f %8.2f          %8.4f          %8.4f    %8.3f    %8.3f     %8.3f      %8.3f" % \
              (step, time, theta, omega, angular_acceleration, to_rotations(theta), \
              to_rotations(omega), omega*gear_ratio*60.0/pi2, omega*gear_ratio/motor_free_speed ))
        if (theta_start < theta_target):
            # Angle is increasing through the motion.
            if (theta > theta_half):
                break
        else:
            # Angle is decreasing through the motion.
            if (theta < theta_half):
                break

    #print ("# step     time    theta    angular_speed   angular_acceleration  theta   angular_speed  motor_speed motor_speed_fraction")
    #print ("#          (sec)   (rad)      (rad/sec)        (rad/sec^2)      (rotations) (rotations/sec)    (rpm)   (fraction)")
    #print ("# Total time for 1/2 rotation of arm is %0.2f" % (time*2))
    return (2.0 * time)


def main():
    gravity = 9.8  # m/sec^2 Gravity Constant
    voltage_nominal = 12  # Volts

    # Vex 775 Pro motor specs from http://banebots.com/p/M2-RS550-120
    motor_name = "Vex 775 Pro motor specs from http://banebots.com/p/M2-RS550-120"
    current_stall = 134  # amps stall current
    current_no_load = 0.7  # amps no load current
    torque_stall = 710 / 1000.0  # N-m Stall Torque
    speed_no_load_rpm = 18730  # RPM no load speed

    if (False):
        # Bag motor from https://www.vexrobotics.com/217-3351.html
        motor_name = "Bag motor from https://www.vexrobotics.com/217-3351.html"
        current_stall = 53.0  # amps stall current
        current_no_load = 1.8  # amps no load current
        torque_stall = 0.4  # N-m Stall Torque
        speed_no_load_rpm = 13180.0  # RPM no load speed

    if (True):
        # Mini CIM motor from https://www.vexrobotics.com/217-3371.html
        motor_name = "Mini CIM motor from https://www.vexrobotics.com/217-3371.html"
        current_stall = 89.0  # amps stall current
        current_no_load = 3.0  # amps no load current
        torque_stall = 1.4  # N-m Stall Torque
        speed_no_load_rpm = 5840.0  # RPM no load speed

    # How many motors are we using?
    num_motors = 2

    # Motor values
    print("# Motor: %s" % (motor_name))
    print("# Number of motors: %d" % (num_motors))
    print("# Stall torque: %.1f n-m" % (torque_stall))
    print("# Stall current: %.1f amps" % (current_stall))
    print("# No load current: %.1f amps" % (current_no_load))
    print("# No load speed: %.0f rpm" % (speed_no_load_rpm))

    # Constants from motor values
    resistance_motor = voltage_nominal / current_stall
    speed_no_load_rps = speed_no_load_rpm / 60.0  # Revolutions per second no load speed
    speed_no_load = speed_no_load_rps * 2.0 * pi
    Kt = num_motors * torque_stall / current_stall  # N-m/A torque constant
    Kv_rpm = speed_no_load_rpm / (
        voltage_nominal - resistance_motor * current_no_load)  # rpm/V
    Kv = Kv_rpm * 2.0 * pi / 60.0  # rpm/V

    # Robot Geometry and physics
    length_proximal_arm = inches_to_meters * 47.34  # m Length of arm connected to the robot base
    length_distal_arm = inches_to_meters * 44.0  # m Length of arm that holds the cube
    mass_cube = 6.0 * lbs_to_kg  # Weight of the cube in Kgrams
    mass_proximal_arm = 5.5 * lbs_to_kg  # Weight of proximal arm
    mass_distal_arm = 3.5 * lbs_to_kg  # Weight of distal arm
    mass_distal = mass_cube + mass_distal_arm
    radius_to_proximal_arm_cg = 22.0 * inches_to_meters  # m Length from arm pivot point to arm CG
    radius_to_distal_arm_cg = 10.0 * inches_to_meters  # m Length from arm pivot point to arm CG

    radius_to_distal_cg = (
        length_distal_arm * mass_cube +
        radius_to_distal_arm_cg * mass_distal_arm) / mass_distal
    J_cube = length_distal_arm * length_distal_arm * mass_cube
    J_proximal_arm = radius_to_proximal_arm_cg * radius_to_proximal_arm_cg * mass_distal_arm  # Kg m^2 Moment of inertia of the proximal arm
    J_distal_arm = radius_to_distal_arm_cg * radius_to_distal_arm_cg * mass_distal_arm  # Kg m^2 Moment of inertia of the distal arm
    J = J_cube + J_distal_arm  # Moment of inertia of the arm with the cube on the end

    error_margine = 1.0
    voltage = 10.0  # voltage for the motor.  Assuming a loaded robot so not using 12 V.
    # It might make sense to use a lower motor frees peed when the voltage is not a full 12 Volts.
    # motor_free_speed = Kv*voltage
    motor_free_speed = speed_no_load

    print("# Kt = %f N-m/A\n# Kv_rpm = %f rpm/V\n# Kv = %f radians/V" %
          (Kt, Kv_rpm, Kv))
    print("# %.2f Ohms Resistance of the motor " % (resistance_motor))
    print("# %.2f kg Cube weight" % (mass_cube))
    print("# %.2f kg Proximal Arm mass" % (mass_proximal_arm))
    print("# %.2f kg Distal Arm mass" % (mass_distal_arm))
    print("# %.2f kg Distal Arm and Cube weight" % (mass_distal))
    print("# %.2f m Length from distal arm pivot point to arm CG" %
          (radius_to_distal_arm_cg))
    print("# %.2f m Length from distal arm pivot point to arm and cube cg" %
          (radius_to_distal_cg))
    print(
        "# %.2f kg-m^2 Moment of inertia of the cube about the arm pivot point"
        % (J_cube))
    print(
        "# %.2f kg-m^2 Moment of inertia of the arm with the cube on the end" %
        (J))
    print("# %.2f m  Proximal arm length" % (length_proximal_arm))
    print("# %.2f m  Distal arm length" % (length_distal_arm))

    print(
        "# %.2f kg-m^2 Moment of inertia of the proximal arm about the arm pivot point"
        % (J_proximal_arm))
    print(
        "# %.2f kg-m^2 Moment of inertia of the distal arm about the arm pivot point"
        % (J_distal_arm))
    print(
        "# %.2f kg-m^2 Moment of inertia of the distal arm and cube about the arm pivot point"
        % (J))
    print("# %d Number of motors" % (num_motors))

    print("# %.2f V Motor voltage" % (voltage))
    for gear_ratio in range(30, 181, 10):
        c1 = Kt * gear_ratio * gear_ratio / (Kv * resistance_motor * J)
        c2 = gear_ratio * Kt / (J * resistance_motor)
        c3 = radius_to_distal_cg * mass_distal * gravity / J

        if (False):
            print("# %.8f 1/sec C1 constant" % (c1))
            print("# %.2f 1/sec C2 constant" % (c2))
            print("# %.2f 1/(V sec^2) C3 constant" % (c3))
            print("# %.2f RPM Free speed at motor voltage" %
                  (voltage * Kv_rpm))

        torque_90_degrees = radius_to_distal_cg * mass_distal * gravity
        voltage_90_degrees = resistance_motor * torque_90_degrees / (
            gear_ratio * Kt)
        torque_peak = gear_ratio * num_motors * torque_stall
        torque_peak_ft_lbs = torque_peak * newton_meters_to_ft_lbs
        normal_force = torque_peak / length_distal_arm
        normal_force_lbf = newton_to_lbf * normal_force
        time_required = get_180_degree_time(c1, c2, c3, voltage, gear_ratio,
                                            motor_free_speed)
        print ("Time for %.1f degrees for gear ratio %3.0f is %.2f seconds.  90 degree torque %.1f N-m and voltage %.1f Volts. Peak torque %3.0f nm %3.0f ft-lb Normal force at distal end %3.0f N %2.0f lbf" % \
          (to_deg(theta_travel),gear_ratio,time_required,torque_90_degrees,voltage_90_degrees,
           torque_peak,torque_peak_ft_lbs,normal_force,normal_force_lbf))


if __name__ == '__main__':
    main()