gripfast

The problem of having a robot hand firmly gripping objects of varying sizes is an old one. Some common solutions to this problem include the following:

• Include a slipping 24t gear in the drive train and overdrive each hand action. The problem with this approach is maintaining the gripping force that is applied once the drive action stops.
• Use a pneumatic cylinder as the finger actuator. This is a very good solution because it easily caters for different sized objects and maintains grip force even if the control valve is turned off. The down side is the overhead of the associated pneumatic gear - pump, air storage and valve control.
• Use a rubber band to pull the fingers together and use the actuator mechanism to force them apart when release is required. I haven't thought much about this one. It could be a good solution.
• Place a touch sensor on the fingers to detect when the load has been grasped. Whether or not the gripping force is maintained when the actuator mechanism stops operating will depend on the type of mechanism.

When building my WHOBOT robot, I wanted an actuator that was electrical only and one that overcame the problems inherant in some of the other solutions.

The "gripfast" solution places a small shock absorber in the actuator drive train. Once the robot fingers have grasped the object to be picked up, the drive mechanism continues to operate and compresses the spring in the shock absorber. A touch sensor is used to switch off the drive after the spring has been compressed a small amount. Grip pressure is maintained by the compressed spring. This does assume, of course, that the pressure cannot be released back through the drive train, thus making a worm drive more or less mandatory.

	A gripfast arm viewed from the left hand side.
	The right hand side. The touch sensors act as limit switches and are wired together back to one port on the RCX.
	Top view.
	Bottom view.
	A close up of the business end of the works. This is the "hand open" position. I have removed the wire brick for the sake of clarity. You will notice that I have used "old" style Technic parts but you can do the job just as well with the new equivalent pieces. The tricky bit is to keep the touch sensor and button pressing "finger" parallel to the shock absorber. This function is carried out by the axle assembly in the lower part of the picture.
	This is the "hand closed" position. The spring is compressed, the sensor button is pushed in and the axle in the lower part of the picture has slid a small distance through the axle link.