### Author Topic: Anyone familiar with the Barrett WAM?  (Read 2002 times)

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#### deucalion

• Beginner
• Posts: 2
##### Anyone familiar with the Barrett WAM?
« on: June 26, 2010, 02:29:41 PM »
I'm working on a summer research project where we need to be able to figure out the mass of whatever object our 4 DOF WAM is holding so that it can compensate for the load it's carrying. My adviser came up with a solution that uses joint angles to estimate the mass but it only accounts for the change in torque of one of the joints, meaning that in a number of other cases it results in spectacular (not to mention dangerous) failure...at runtime I can access the gravity compensation torque applied to each joint to keep the robot in a static "position mode" and the actual applied joint torques. Subtracting the commanded torques from the gravity compensation torques yields the torque applied by the load onto each joint. I'm a computer scientist, not a mechanical engineer, so I have no idea where to go from there ( no one else here does either..). Can anyone help me figure out this problem?

#### deucalion

• Beginner
• Posts: 2
##### Re: Anyone familiar with the Barrett WAM?
« Reply #1 on: July 14, 2010, 11:58:11 AM »
bump?

• Supreme Robot
• Posts: 11,703
##### Re: Anyone familiar with the Barrett WAM?
« Reply #2 on: July 14, 2010, 01:23:58 PM »
Mass is determined using gravity, an acceleration. As such its a dynamics not a statics problem.

This is a simplified version on how to do it . . .

Mass affects inertia . . . meaning that the heavier something is, the more 'sluggish' it becomes. Have your arm perform some action and measure the joint rotational velocities. Then have your arm pick up the object, then perform the *exact same* motions again. You'll notice that the joint rotational velocities over time are different. That difference can be roughly mapped to the object mass.

Experimentally have the arm rotate with various masses and measure the dV for each case, then create a look up table, and finally create an interpolation equation with it. Depending on your kinematics and arm configuration, you might be able to map to a single joint (ideal).

note: If your arm is huge but the masses are small, you won't be able to measure any velocity differences. In this case you must add a force sensor to the end effector.

You'll also be able to find various research papers out there that have already attempted to solve this problem.

#### Soeren

• Supreme Robot
• Posts: 4,672
##### Re: Anyone familiar with the Barrett WAM?
« Reply #3 on: July 14, 2010, 09:13:42 PM »
Hi,

A couple of well placed strain gauges would be my choice.
Regards,
SÃ¸ren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?