With electric drives, electrical energy is converted into mechanicalkinetic energy. Inside the electric motor, magnetic fields in the stator androtor interact; torque is created when they try to align themselves with eachother, creating movement. The "precise hands" of electric drives and motors areideal for offering high speed and precise accuracy.

In dc motors, a dc winding or apermanent magnet generates a fixed magneticfield in the stator. To delivermaximum torque, coils are wound onto the rotor and connected so that the rotorfield is perpendicular to the stator field. Speed and power in dc motors can becontrolled very effectively, but the carbon brushes that are required to switchthe current in this type of motor are subject to wear and tear.

A synchronous motor features athree-phase winding configured in a circle. Three phase-shifted currentsgenerate a rotating magnetic field. Since the rotor has a fixed magnetic field,it can only develop effective torque at a synchronous speed. With moderncurrent converters, synchronous motors can be controlled as precisely as dcmotors, but without any wearing parts. Synchronous motors have an excellentoperating efficiency, above 90 percent, but require a complex electronicregulation system and costly permanent magnets.

Asynchronous motors alsofeature stators that generate a rotating field, but with a squirrel-cagewinding. When the rotor is not following synchronously, a current is inducedthat counteracts the change in the magnetic field. Together with the magneticfield of the stator, this generates a torque that pulls the rotor along. Theadvantage is that asynchronous motors are less expensive, but because thecurrent flowing through the rotor generates heat, the motor has a lowefficiency level.

Hydraulic Drives

Withhydraulic drives, fluid is pressurized to move a piston inside a cylinder. Apump supplies the volume flow. Depending on the force required to move a load,the corresponding pressure is developed in the fluid and the pump counteractsthis pressure. In rotating drives, ahydraulic motor delivers torqueinstead of linear force.

Hydrostatic drives, whichfeature adjustable pumps that push pistons or turn hydraulic motors, areextremely efficient with years of development history. However, the speed ofthe system varies when the externalforces change and there is no easy wayto maintain a position once it has been attained. Consequently, the distancebetween the pump and the cylinder needs to be as short as possible.

In secondary control drives,the motor, not the pump, is regulated. The varied torque enables the hydraulicdrive to adapt quickly to changes in force. This highly efficient technologyallows for the regulation of speed, torque and position. However, the drivesare expensive to build and the rapid adjustability is only required by userswith special machine needs, like test bed manufacturers.

Drives with valve control arecommon in hydraulic systems for mobile equipment. Only a portion of thehydraulic energy generated is applied, via a valve. The unused portion isconverted into heat. Valve control drives are extremely accurate with excellentcontrol but lower efficiency. This can be improved with load-sensingtechnology, so that only the pressure that is needed is actually provided.

Pneumatic Drives

Inpneumatics, air is compressed and the stored energy is converted to mechanicalenergy in cylinders, motors or other units. However, using compressed air inindustrial applications is only cost-effective when low forces are required.The "nimble fingers" of pneumatics areused when small masses need to bemoved at high speeds across short distances, like in clamping, transporting,screw-tightening and in other industrial, trade or medical tasks.

Pneumatic drive systems includethree subsystems: air compression and processing; control (via valves); andoutput drive (a cylinder or motor). These components require little maintenanceduring operation and offer long service lives. Compressed air, which is readilyavailable, poses no fire or explosion hazards. However, producing and preparingcompressed air can be expensive and the noise of air exhaust may have to bemuffled. An advantage is that compressed air is insensitive to temperaturevariations. If leaks occur, they have no effect on machine safety and do notcontaminate the surroundings. The speed and force of the actuators can becontrolled simply and continuously over a broad range, but it may be difficultto achieve constant and uniform piston speeds.

AmyDeFayette, is technical marketing manager for BoschRexroth Corp.