Direct-drive rotary motors streamline machine design

03/01/2000


W hen electric motors are used in low-speed rotary applications, a transmission is often installed to gear down motor speed for higher output torque. This reduces cost because gearing allows small high-speed motors to produce high torque at low-speeds. However, in high-accuracy applications, such as film-coating lines and integrated circuit test machines, designers avoid gearing because it causes a host of problems like position error, lost motion (backlash), more maintenance, and audible noise.

Most designers would specify so-called direct-drive rotary (DDR) motors if they could afford them. Until recently, this option was usually reserved for high-end commercial and military applications since direct drive was too expensive for most industrial machines. Fortunately, this has changed.

Several manufacturers produce DDR motors for applications that just a few years ago could not consider this technology. Manufacturers include Kollmorgen (Radford, Va.); Compumotor (Rohnert Park, Calif.), a division of Parker Hannifin Corp.; and NSK (Ann Arbor, Mich.; Tokyo, Japan).

Direct-drive rotary motors, sometimes called torquer motors, develop high torque at relatively low speeds, usually just a few hundred rpm. Two types of DDR motors are available. In frameless motors, customers purchase components such as a rotor, stator, and feedback device. These parts are then assembled with the rest of the machine.

Housed DDR motors integrate the rotor, stator, and feedback device into one assembly. Housed torquer motors do not have an intermediate shaft coupling. Instead, the load is attached directly to the DDR rotor. The rotor has a through-hole, typically about 50-mm in diameter, which allows plumbing and wiring to pass through the center. Housed motors have independent bearings, while frameless motors rely on the bearings of the machine.

Why use DDR motors?

Increasing a machine's accuracy is the main reason to choose DDR motors. Since the load is rigidly coupled to the motor, error caused by transmission components is eliminated: there is no backlash, belt stretch, or gear-tooth error. The main limitation is the accuracy of the feedback device, but feedback devices for DDR motors are very accurate.

Also, stick-slip is usually eliminated. Stick-slip is a condition in which moving a load over very small distances cannot be done with accuracy. It often comes from transmission components that bring high-friction and high-compliance. Because DDR motors reduce friction and virtually eliminate coupling compliance, they are often not subject to stick-slip.

Another advantage is that the high stiffness between motor and load effectively removes mechanical resonance-the phenomenon in which a compliant load generates instability under high servo gains. This means the servo gains of DDR systems can be set very high, allowing faster servo response and greater resistance to torque disturbances.

Audible noise is also reduced because of fewer moving parts. Maintenance is reduced because the only wearing component in the system is the bearing. If the bearings are permanently lubricated, the assembly can achieve zero maintenance. Machines using DDR motors are often simpler and smaller because the transmission is eliminated. And DDR motors can actually reduce cost in cases where highly accurate transmission components or feedback devices would otherwise be needed.

Not right everywhere

Direct-drive motors are not right for every application. They're usually more costly than conventional rotary motors using transmissions, especially when high gear ratios (>10:1) are used to gain mechanical advantage. Feedback devices for DDR motors are usually also more costly. Low friction of DDR-based machine systems-normally an advantage-can be a problem in some designs that rely on friction to bring motion to rest when power is removed.

Finally, for engineers who are familiar with conventional rotary motor design, time is required to learn how to apply DDR technology.




George Ellis, senior scientist at Kollmorgen.

Comments? E-mail: fbartos@cahners.com





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