shafts becomes impractical. The most common method of pressure-balancing an 

 electric drive is to place it in a fluid-filled housing with some sort of 

 flexible pressure equalizing below. All fluid-immersed parts have to be 

 compatible with the chosen fluid. The rated voltage to drive the motor must 

 be kept low to reduce electrical arc-ing of the brush contacts, which gener- 

 ates gases. Because of the higher viscosity of fluids in relation to air, all 

 revolving parts should turn as slowly as possible to reduce rotational losses 

 and hydrodynamic brush lifting, which would also cause arc-ing. 



2.2.3 Electric Motor 



An induction (or squirrel -cage) motor does not require brushes and it 

 could, therefore, be run immersed in oil or even in salt water. However, to 

 develop the high starting torques which are necessary to move the manipulator 

 joints, this motor type requires high starting currents. Thus one would need 

 relatively heavy motor controllers to invert the vehicle battery voltage, to 

 control motor speeds, and to filter the ripple voltages. The only off-the- 

 shelf motor type found to meet all requirements was a permanent-magnet, 

 armature-excited, continuous-rotation dc motor. This motor type delivers high 

 torques at low speeds and low input power. It is probably the most linear 

 kind of servoactuator ; stall torque and no-load speed are almost perfectly 

 linear functions of applied voltage. Because of its fast response and smooth 

 linear characteristics, the direct-drive dc torque motor is recommended where 

 accurate tracking over high-speed ranges are required. The ability of a 

 permanent-magnet dc torque motor to convert electrical power input to torque 

 is proportional to (1) the square root of the product of total magnetic flux 

 linking the winding from the field and (2) the magnetomotive force established 

 by the excited armature winding. This ability can be represented by 



K(M) = Torque/(Power Input) 1/2 



2.2.4 Gearing 



A gearless torque motor drive would be ideally suited to drive the manipu- 

 lator joints. The absence of gearing would eliminate errors caused by fric- 

 tion, backlash, and other gear inaccuracies, and would be free from noise 

 caused by bearing play. But a dc torque motor which would deliver the recom- 

 mended torques to move the manipulator joints would be too heavy and would 

 consume too much electrical power. An off-the-shelf unit found to both opti- 

 mally meet the requirements and mate with the pancake-shaped dc-torque motor 

 is the harmonic drive power transmission. This transmission operates on a 

 patented principle which employs a deflection wave transmitted to a nonrigid 

 flexpline member to produce a high mechanical advantage between concentric 

 parts. Harmonic drive units allow ratios from 80 to 320 in a single reduc- 

 tion, and they also allow torque outputs equal to drives twice their size and 

 three times their weight with efficiencies up to 90?',. The standard harmonic 

 drive unit has very low backlash and thereby minimizes potential stability 

 problems. Since some 10% of the teeth are in continous engagement, the effect 

 of tooth-to-tooth error is minimized and accuracy in the arc-second range is 

 obtainable with excellent repeatability. Because there are no radial loads 

 generated during the transmission of torque, the support housing can be 

 lightened. Because the harmonic drive gearing will allow a given joint to 

 back drive, it self-protects the manipulator joints in the presence of dynamic 



13 



