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I’m designing a robot arm (possibly using a VLA model like OpenVLA). Two things confuse me:

  1. If an arm already has position control and encoders to confirm it reaches the commanded joint angles, why is torque control still needed? Why doesn't the normal hobby servo kind of method, ie passively controlling the torque depending on the load work here?

  2. Torque-controllable actuators are much more expensive than standard position-controlled servos. In what situations is the extra cost justified? What’s the practical advantage/necessity of torque control here in using VLA?

Apologies if I sound dumb here.

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If you're making a massage robot, and someone shaped differently than the test profile uses the robot, what happens? In a position-controlled system, either the end effector is too far from the person (if smaller than the test profile), or the robot applies peak force trying to achieve the target position and injurs a person larger than the test profile.

In applications where the goal is to control the applied force instead of the applied position, you need the ability to control and limit the torque applied by the actuators. Picking up or holding objects with shape variance, situations where you want a specific "touch" (surface grinding or other pressure-sensitive applications), etc.

This kind of force control, also called compliance control, is also useful in safety situations, where you may want to avoid injuring humans in a collaborative robotics application.

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