Differential allows two stage operation without clutch or gear change

Tom Shelley reports on a mechanism which should have a multitude of uses

A gripper design uses a differential to change from rapid, low load operation to slower, high force operation when it encounters an obstacle. Invented to improve the effectiveness of human hand prosthetics while keeping the cost down, the mechanism should be of interest to designers of robot grippers and other devices. The idea is the brainchild of student, Irfan Kausar, working under the direction of Dr Mike Dewar in the Department of Mechanical Engineering in University College London. Prosthetic hands driven by electric motors inevitably close much more slowly than real hands. Using two motors, one to make the initial fast closure, followed by another to make the firm grip adds excessive cost, weight and complexity. The differential design progressed as far as a working prototype, sufficiently advanced to allow a proper evaluation. Operation starts with the hand in the fully open position, and the thumb placed against the object. The patient's electro-myogram signal, caused by tensing the appropriate muscle is detected and initiates motor rotation at 14,000 rpm. The speed is stepped down by a double spur gear to 4,700 rpm and then, as it enters to the central input gear of the differential to 1,240 rpm. Due to friction on the thumb side, added to by use of a nylon friction screw rubbing on a brass collar, the thumb side of the differential remains stationary, and power is transmitted by the finger side output at 2,480 rpm. The drive is turned through a right angle by the use of a bevel gear, and turns the main finger drive shaft coupled to a threaded rod. As the rod turns, an acetal nut is pulled back towards the motor, closing the fingers until they make contact with the object. At this point, the fingers stall, forcing the finger output from the differential gearbox to be held stationary. The power from the differential is now channelled through the thumb side which overcomes the slight friction provided by the nylon screw and brass collar. The drive goes from the thumb side bevel gear through a shaft to a 10 tooth pinion which meshes with a 75 toothed gear. The increased torque is then fed to a short threaded rod, which pulls the thumb side acetal nut backwards. This is connected to the thumb's attachment lever by a tension spring. The thumb, which cannot go any further into the object, stops at the surface and, as the nut is still pulling the other end of the spring, starts to apply a gradually increasing force to the object until the patient stops applying the flexion signal or the motor stalls. To release the grip, the patient must apply an extension signal. The control circuit then issues an open command to the motor which turns in the reverse direction. As the force between the fingers and thumb is high at this point, the torque required to turn the finger side of the output of the differential is 12.75 times greater than that for the thumb side. The thumb opens until the finger tip force has fallen to 0.67N. At this point, the friction due to the nylon screw is dominant, and power is transferred to the fingers, which start to open until they hit their end stops. Here the power is once again transferred back to the thumb which then travels back to its open position where it trips a micro switch, causing power to the motor to be severed. Irfan Kausar has now completed his studies and moved on to other things but the idea was patented by UCL which holds the copyright. Pointers Differential allows two stage operation. First stage is fast operation with low force followed by slower motion with higher force Only one motor is required, and there is no clutch or gear change University College London