Re: Arm not affected by gravity

2019-09-23T15:34:50+00:00

Ah, I've solved this I think. It seems the tag also needs an origin. I've added one with the same values as the origin in the and tags, and it now seems to behave as I would expect.

However, I'm now unsure whether my moments of inertia are correct. The documentation at http://wiki.ros.org/urdf/XML/link discusses it, but it's pretty terse and I'm not sure if I've understood correctly what it means. Do I understand correctly that if my link is a basic shape like box, cylinder etc. whose centre of gravity is at the origin, then I can just calculate my moment of inertia around the origin, and then copy the tag from into and it should work?

Anyway, for completeness, here's the probably-corrected testbot.urdf file:

Code:

Statistics: Posted by nathaniel — Mon Sep 23, 2019 3:34 pm

Arm not affected by gravity

2019-09-23T14:23:38+00:00

Hi all, a beginner here.

I've created a basic URDF file containing a box with an "arm" attached to it by a revolute joint. The quick start guide says I can disable the default motor by setting the joint to a velocity controller with a force of 0. So my expectation is that on running the below code, the robot should fall from its starting point and its arm should flop to the ground:

Code:

import pybullet as pimport timeimport pybullet_dataphysicsClient = p.connect(p.GUI)#or p.DIRECT for non-graphical versionp.setAdditionalSearchPath(pybullet_data.getDataPath())p.setGravity(0,0,-10)planeId = p.loadURDF("plane.urdf")startPos = [0,0,1]startOrientation = p.getQuaternionFromEuler([0,0,0])robot = p.loadURDF("./testbot.urdf",startPos, startOrientation)for i in range(p.getNumJoints(robot)):p.setJointMotorControl2(robot, i, p.VELOCITY_CONTROL, force=0)for i in range (10000):p.stepSimulation()time.sleep(1./480.)p.disconnect()

However, instead the arm stays rigidly in place, not moving relative to the box when the robot hits the floor. At first I thought that disabling the motor wasn't working, but I found I can move the arm using the mouse. It behaves quite strangely, and it seems like it's not being affected by gravity - if I give it a small amount of motion upwards it will continue moving at a constant angular velocity until it hits its limits.

Am I doing something wrong here?

Here are the contents of testbot.urdf:

Code:

Statistics: Posted by nathaniel — Mon Sep 23, 2019 2:23 pm

Real-Time Physics Simulation Forum

Re: Arm not affected by gravity

Arm not affected by gravity