Disc Motors

Nov. 15, 2002
Disc motors are permanent-magnet stepping motors that exhibit performance comparable to that of hybrid motors.

Disc motors are permanent-magnet stepping motors that exhibit performance comparable to that of hybrid motors. Rotors in disc motors are thin (typically less than 1-mm) discs, unlike the cylindrical rotors in hybrids and conventional permanent-magnet motors. Conventional permanent-magnet motors generally are limited to a minimum step angle of 30° for a maximum of 12 steps/rev.

Until recently, stepping motors provided 200 steps/rev at most, resolution previously available only through hybrid stepping motors. The latest version, which has a 0.9° step angle, provides 400 steps/rev. Due to very low distortion of the output torque sinusoid, microstepping accuracy is ±3% maximum.

The disc motors are generally a little over half as big as hybrid motors of equal power output and weigh 60% less. The rotors and magnetic circuits in the stators are quite light. And the stators are molded plastic rather than metal.

While a variety of disc motors have been available for about 20 years, they are still frequently used for their low inertia and high torque. Recent improvements include neodymium magnets and power ratings up to 250 W for industrial-grade motors.

A conventional dc servomotor uses an iron-core armature with coils of wire wound in slots. However, some servomotors contain a flat armature constructed of several layers of copper conductors bonded to a rigid insulating disc. Since no iron is needed in the disc armature, it has a very low inertia. The thin armature design provides a high torque-to-inertia ratio. High ratios produce accelerations from 0 to 3,000 rpm in only 60° of rotation.

Conventional motors also produce cogging when the permanent magnets in the stator try to line up with the iron in the rotor. This happens even if the motor is not powered and may not be tolerable in some applications. In contrast, disc armatures contain no iron and, thus, do not cog. The result is a servomotor with very smooth rotation at any speed.

Stored energy in the inductance of an iron-core rotor produces arcing at the brushes during commutation. Arcing causes more brush wear than friction. But because a disc motor contains no rotor iron, it does not arc. As a result, brushes in a disc motor may last as long as the bearings.

The lack of inductance in disc rotors also gives a low electrical time constant. Typical time constants are less than 1 msec, allowing disc motors to reach full torque and acceleration much faster than conventional motors.

Most disc motors are rated for peak currents of 10 times the continuous rating to overcome load inertias during acceleration and deceleration. Conventional motors are usually limited to peak currents of only two to three times continuous rating. Otherwise, higher currents produce a large magnetic field in the armature that can demagnetize the magnets.

Another type of flat servomotor for microstep positioning has a slightly different construction. Although it does not contain an iron-core armature, it does have a disc-magnet rotor. The motor stator has two phases that are driven with appropriate drives to obtain stepping. It has more rotor inertia than the previous disc motor, and less than constant torque over its speed range.

Disc motor makers are trying to improve their designs through development of an integral-velocity damper. The damper is a small motor attached to the servomotor shaft that is driven as a generator. It contains the same number of poles as the host servomotor (for example 1.8° steps and 50 pole pairs), but its output is amplified and fed back to the servomotor -- out of phase. Such out-of-phase feedback reduces servo error. Primarily beneficial for incremental motion control, the feedback signal eliminates overshoot or ringing.

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