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Default motor

for revolute and prismatic joint pybullet automatically create motor that tries to hold the joint at its current position

to disabled the motor we changed to Velocity control and force=0 on request joint

Demo:

  • load robot urdf
  • disabled default motor

robot urdf

Robot without control 
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import pybullet as p
import pybullet_data
import time
import pathlib

# setup
p.connect(p.GUI)
p.resetSimulation() # type: ignore
p.setGravity(gravX=0, gravY=0, gravZ=-9.8)
p.setAdditionalSearchPath(path=pybullet_data.getDataPath())
plane = p.loadURDF("plane.urdf")

# add current file location and its 'urdf' subfolder to search paths
cwd = pathlib.Path(__file__).parent.resolve()
cwd = cwd.joinpath("urdf")
p.setAdditionalSearchPath(cwd.as_posix())

# load URDF
robot = p.loadURDF("rrbot.urdf", basePosition=[0, 0, 0.5], useFixedBase=True)

# In PyBullet, joints often have a default motor/controller active. 
# If you leave it enabled, 
# the joint may try to hold position or velocity instead of moving naturally. 
# This line switches joint 0 to velocity control but sets force=0, 
# meaning the motor is allowed to apply zero torque.
p.setJointMotorControl2(robot, 0, p.VELOCITY_CONTROL, force=0)  # free joint
p.resetJointState(robot, 0, 0.2)  # small initial angle so gravity can act

p.setRealTimeSimulation(True)
try:
    while True:
        keys = p.getKeyboardEvents()

        # 27 = ESC
        if 27 in keys and keys[27] & p.KEY_WAS_TRIGGERED:
            print("ESC pressed, exiting...")
            break

        time.sleep(1/240)

finally:
    p.disconnect()

Position control

Robot without control 
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import pybullet as p
import pybullet_data
import time
import pathlib
import math

# setup
p.connect(p.GUI)
p.resetSimulation() # type: ignore
p.setGravity(gravX=0, gravY=0, gravZ=-9.8)
p.setAdditionalSearchPath(path=pybullet_data.getDataPath())
plane = p.loadURDF("plane.urdf")
p.configureDebugVisualizer(p.COV_ENABLE_RGB_BUFFER_PREVIEW, 0)
p.configureDebugVisualizer(p.COV_ENABLE_DEPTH_BUFFER_PREVIEW, 0)
p.configureDebugVisualizer(p.COV_ENABLE_SEGMENTATION_MARK_PREVIEW, 0)
# add current file location and its 'urdf' subfolder to search paths
cwd = pathlib.Path(__file__).parent.resolve()
cwd = cwd.joinpath("urdf")
p.setAdditionalSearchPath(cwd.as_posix())

# load URDF
robot = p.loadURDF("rrbot.urdf", basePosition=[0, 0, 0.5], useFixedBase=True)

joint_id = 0
p.resetJointState(robot, joint_id, 0.0)

# Use the debug panel sliders to change the target while the simulation runs.
# PyBullet uses radians internally, but degrees are easier to adjust manually.
target_angle_param = p.addUserDebugParameter("target angle (deg)", -180, 180, 45)
max_force_param = p.addUserDebugParameter("max motor force", 0, 1000, 200)
position_gain_param = p.addUserDebugParameter("position gain", 0, 1, 0.3)
velocity_gain_param = p.addUserDebugParameter("velocity gain", 0, 1, 1.0)

p.setRealTimeSimulation(True)
try:
    while True:
        keys = p.getKeyboardEvents()

        # 27 = ESC
        if 27 in keys and keys[27] & p.KEY_WAS_TRIGGERED:
            print("ESC pressed, exiting...")
            break

        target_angle_deg = p.readUserDebugParameter(target_angle_param)
        target_angle_rad = math.radians(target_angle_deg)
        max_force = p.readUserDebugParameter(max_force_param)
        position_gain = p.readUserDebugParameter(position_gain_param)
        velocity_gain = p.readUserDebugParameter(velocity_gain_param)

        p.setJointMotorControl2(
            bodyUniqueId=robot,
            jointIndex=joint_id,
            controlMode=p.POSITION_CONTROL,
            targetPosition=target_angle_rad,
            force=max_force,
            positionGain=position_gain,
            velocityGain=velocity_gain,
        )

        time.sleep(1/240)

finally:
    p.disconnect()

alt text

TODO: explain setJointMotorControl2

Force Control with PID

Robot force control with custom PID 
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import pybullet as p
import pybullet_data
import time
import pathlib
import math


def clamp(value, min_value, max_value):
    return max(min_value, min(value, max_value))


class PidController:
    def __init__(
        self,
        kp=0.0,
        ki=0.0,
        kd=0.0,
        output_limit=0.0,
        integral_limit=0.0,
        feedforward=0.0,
    ):
        self.kp = kp
        self.ki = ki
        self.kd = kd
        self.output_limit = output_limit
        self.integral_limit = integral_limit
        self.feedforward = feedforward
        self.integral_error = 0.0
        self.previous_error = None
        self.previous_time = None

    def update(self, setpoint, current):
        current_time = time.monotonic()

        # Proportional error: how far the joint is from the target right now.
        error = setpoint - current

        # The time delta since the last update is needed to calculate the integral and
        if self.previous_time is None:
            dt = 0.0
        else:
            dt = current_time - self.previous_time

        if dt > 0.0:
            # Integral term: accumulate error over time.
            # This helps remove steady-state error, but must be clamped so it
            # does not grow forever when the joint cannot reach the target.
            self.integral_error += error * dt
            self.integral_error = clamp(
                self.integral_error,
                -self.integral_limit,
                self.integral_limit,
            )

        if self.previous_error is None or dt <= 0.0:
            # No previous sample exists on the first call, so the derivative
            # cannot be calculated yet.
            derivative_error = 0.0
        else:
            # Derivative term: estimate how fast the error is changing.
            # It adds damping by resisting fast changes and reducing overshoot.
            derivative_error = (error - self.previous_error) / dt

        output = (
            self.kp * error
            + self.ki * self.integral_error
            + self.kd * derivative_error
            + self.feedforward
        )
        output = clamp(output, -self.output_limit, self.output_limit)

        self.previous_error = error
        self.previous_time = current_time

        return output


# setup
p.connect(p.GUI)
p.resetSimulation()  # type: ignore
p.setGravity(gravX=0, gravY=0, gravZ=-9.8)
p.setAdditionalSearchPath(path=pybullet_data.getDataPath())
plane = p.loadURDF("plane.urdf")
p.configureDebugVisualizer(p.COV_ENABLE_RGB_BUFFER_PREVIEW, 0)
p.configureDebugVisualizer(p.COV_ENABLE_DEPTH_BUFFER_PREVIEW, 0)
p.configureDebugVisualizer(p.COV_ENABLE_SEGMENTATION_MARK_PREVIEW, 0)

# add current file location and its 'urdf' subfolder to search paths
cwd = pathlib.Path(__file__).parent.resolve()
cwd = cwd.joinpath("urdf")
p.setAdditionalSearchPath(cwd.as_posix())

# load URDF
robot = p.loadURDF("rrbot.urdf", basePosition=[0, 0, 0.5], useFixedBase=True)

joint_id = 0
p.resetJointState(robot, joint_id, 0.0)

# Disable the default motor before applying custom torque control.
p.setJointMotorControl2(
    bodyUniqueId=robot,
    jointIndex=joint_id,
    controlMode=p.VELOCITY_CONTROL,
    force=0,
)

# PID and torque-control parameters.
target_angle_param = p.addUserDebugParameter("target angle (deg)", -180, 180, 45)
kp_param = p.addUserDebugParameter("kp", 0, 500, 80)
ki_param = p.addUserDebugParameter("ki", 0, 100, 0)
kd_param = p.addUserDebugParameter("kd", 0, 100, 8)
max_torque_param = p.addUserDebugParameter("max torque", 0, 1000, 200)
integral_limit_param = p.addUserDebugParameter("integral limit", 0, 20, 5)
feedforward_torque_param = p.addUserDebugParameter("feedforward torque", -200, 200, 0)
time_step_param = p.addUserDebugParameter("time step", 1 / 1000, 1 / 60, 1 / 240)

pid = PidController()

p.setRealTimeSimulation(False)
try:
    while True:
        keys = p.getKeyboardEvents()

        # 27 = ESC
        if 27 in keys and keys[27] & p.KEY_WAS_TRIGGERED:
            print("ESC pressed, exiting...")
            break

        target_angle = math.radians(p.readUserDebugParameter(target_angle_param))
        time_step = p.readUserDebugParameter(time_step_param)

        pid.kp = p.readUserDebugParameter(kp_param)
        pid.ki = p.readUserDebugParameter(ki_param)
        pid.kd = p.readUserDebugParameter(kd_param)
        pid.output_limit = p.readUserDebugParameter(max_torque_param)
        pid.integral_limit = p.readUserDebugParameter(integral_limit_param)
        pid.feedforward = p.readUserDebugParameter(feedforward_torque_param)

        position, _, _, _ = p.getJointState(robot, joint_id)
        torque = pid.update(target_angle, position)

        p.setJointMotorControl2(
            bodyUniqueId=robot,
            jointIndex=joint_id,
            controlMode=p.TORQUE_CONTROL,
            force=torque,
        )

        p.setTimeStep(time_step)
        p.stepSimulation()
        time.sleep(time_step)

finally:
    p.disconnect()

TODO: explain the pid part