"""This file implements an accurate motor model."""

import numpy as np
VOLTAGE_CLIPPING = 50
# TODO(b/73728631): Clamp the pwm signal instead of the OBSERVED_TORQUE_LIMIT.
OBSERVED_TORQUE_LIMIT = 5.7
MOTOR_VOLTAGE = 16.0
MOTOR_RESISTANCE = 0.186
MOTOR_TORQUE_CONSTANT = 0.0954
MOTOR_VISCOUS_DAMPING = 0
MOTOR_SPEED_LIMIT = MOTOR_VOLTAGE / (
    MOTOR_VISCOUS_DAMPING + MOTOR_TORQUE_CONSTANT)
NUM_MOTORS = 8


class MotorModel(object):
  """The accurate motor model, which is based on the physics of DC motors.

  The motor model support two types of control: position control and torque
  control. In position control mode, a desired motor angle is specified, and a
  torque is computed based on the internal motor model. When the torque control
  is specified, a pwm signal in the range of [-1.0, 1.0] is converted to the
  torque.

  The internal motor model takes the following factors into consideration:
  pd gains, viscous friction, back-EMF voltage and current-torque profile.
  """

  def __init__(self, torque_control_enabled=False, kp=1.2, kd=0):
    self._torque_control_enabled = torque_control_enabled
    self._kp = kp
    self._kd = kd
    self._resistance = MOTOR_RESISTANCE
    self._voltage = MOTOR_VOLTAGE
    self._torque_constant = MOTOR_TORQUE_CONSTANT
    self._viscous_damping = MOTOR_VISCOUS_DAMPING
    self._current_table = [0, 10, 20, 30, 40, 50, 60]
    self._torque_table = [0, 1, 1.9, 2.45, 3.0, 3.25, 3.5]
    self._strength_ratios = [1.0] * NUM_MOTORS

  def set_strength_ratios(self, ratios):
    """Set the strength of each motors relative to the default value.

    Args:
      ratios: The relative strength of motor output. A numpy array ranging from
        0.0 to 1.0.
    """
    self._strength_ratios = np.array(ratios)

  def set_motor_gains(self, kp, kd):
    """Set the gains of all motors.

    These gains are PD gains for motor positional control. kp is the
    proportional gain and kd is the derivative gain.

    Args:
      kp: proportional gain of the motors.
      kd: derivative gain of the motors.
    """
    self._kp = kp
    self._kd = kd

  def set_voltage(self, voltage):
    self._voltage = voltage

  def get_voltage(self):
    return self._voltage

  def set_viscous_damping(self, viscous_damping):
    self._viscous_damping = viscous_damping

  def get_viscous_dampling(self):
    return self._viscous_damping

  def convert_to_torque(self,
                        motor_commands,
                        motor_angle,
                        motor_velocity,
                        true_motor_velocity,
                        kp=None,
                        kd=None):
    """Convert the commands (position control or torque control) to torque.

    Args:
      motor_commands: The desired motor angle if the motor is in position
        control mode. The pwm signal if the motor is in torque control mode.
      motor_angle: The motor angle observed at the current time step. It is
        actually the true motor angle observed a few milliseconds ago (pd
        latency).
      motor_velocity: The motor velocity observed at the current time step, it
        is actually the true motor velocity a few milliseconds ago (pd latency).
      true_motor_velocity: The true motor velocity. The true velocity is used
        to compute back EMF voltage and viscous damping.
      kp: Proportional gains for the motors' PD controllers. If not provided, it
        uses the default kp of the minitaur for all the motors.
      kd: Derivative gains for the motors' PD controllers. If not provided, it
        uses the default kp of the minitaur for all the motors.

    Returns:
      actual_torque: The torque that needs to be applied to the motor.
      observed_torque: The torque observed by the sensor.
    """
    if self._torque_control_enabled:
      pwm = motor_commands
    else:
      if kp is None:
        kp = np.full(NUM_MOTORS, self._kp)
      if kd is None:
        kd = np.full(NUM_MOTORS, self._kd)
      pwm = -1 * kp * (motor_angle - motor_commands) - kd * motor_velocity

    pwm = np.clip(pwm, -1.0, 1.0)
    return self._convert_to_torque_from_pwm(pwm, true_motor_velocity)

  def _convert_to_torque_from_pwm(self, pwm, true_motor_velocity):
    """Convert the pwm signal to torque.

    Args:
      pwm: The pulse width modulation.
      true_motor_velocity: The true motor velocity at the current moment. It is
        used to compute the back EMF voltage and the viscous damping.
    Returns:
      actual_torque: The torque that needs to be applied to the motor.
      observed_torque: The torque observed by the sensor.
    """
    observed_torque = np.clip(
        self._torque_constant *
        (np.asarray(pwm) * self._voltage / self._resistance),
        -OBSERVED_TORQUE_LIMIT, OBSERVED_TORQUE_LIMIT)

    # Net voltage is clipped at 50V by diodes on the motor controller.
    voltage_net = np.clip(
        np.asarray(pwm) * self._voltage -
        (self._torque_constant + self._viscous_damping) *
        np.asarray(true_motor_velocity), -VOLTAGE_CLIPPING, VOLTAGE_CLIPPING)
    current = voltage_net / self._resistance
    current_sign = np.sign(current)
    current_magnitude = np.absolute(current)
    # Saturate torque based on empirical current relation.
    actual_torque = np.interp(current_magnitude, self._current_table,
                              self._torque_table)
    actual_torque = np.multiply(current_sign, actual_torque)
    actual_torque = np.multiply(self._strength_ratios, actual_torque)
    return actual_torque, observed_torque
