Brian S. Leibowitz
The lab recently purchased a model BH8-200 3 finger hand from Barrett Technology Inc. As one of their first clients, we have been continuously testing the hand and providing feedback/suggestions to Barrett in addition to trying to do real work with it. The people at Barrett have been very helpful in this respect, and have gone to lengths to implement many of our suggestions.
The BH8-200 is a remarkably compact unit, being roughly the size of a human hand and wrist, including all of the electronics, motors, and mechanics. It contains a Motorola MC68HC11 cpu which communicates with the host computer via a serial connection. The hand CPU runs a command parser that accepts alphanumeric as well as binary commands from the host computer. There are four motors in the hand, each of which is driven by Hewlett Packard HCTL-1100 motor control IC. The motors are for the three fingers plus the palm (the pictures will make the palm motion clear). The HCTL-1100's allows PID control of absolute position of each motor or P control of motor velocity. Each finger has a strain guage that detects forces applied to the fingertips based on tension in the second joint's tendon. Each finger has two links and two joints plus an nifty automatic clutch mechanism that allows the second joint to continue to move after the first link stalls out due to object oclusion.
The psuedo tech. drawing:
The palm motion mentioned before is the movement of the two outside fingers about the circular base. The two fingers move together from a single drive and the center finger is fixed. this allows the fingers to be next to each other, spread at 120 degrees to grab a sphere, directly opposing to grab a cylinder, or anything else that might be necessary.
Inside the base joint of each finger is one of these nifty gear assemblies. The motor shaft turns two worm drives simultaneously. The faster one controls the tendon for the second joint, the slower one the base joint. This causes the finger to close with a grasping motion. The picture at the bottom shows how the base joint actually has a mechanical clutch. If the first link of the finger hits something rigid, the clutch releases and the first link stops moving but the fingertip continues to close.
Here are some of Barrett's pics of the hand in action:
We now have a MPEG frame grabber on one of the SparcStation 20s, so expect to see full motion MPEGs of the hand soon!
Barrett's logo and contact information:
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