Machine Design

Direct-drive motors: Fast And Accurate

The fundamentals of motor design have changed little over the course of time, but the technologies used to improve tolerances, and implement new assembly techniques, materials, and stator and rotor geometries are constantly evolving.

Direct-drive motors meet the high-torque needs of large inertia rotary loads. Increasing the pole count increases the rotor diameter, total flux, and torque moment arm which produces the higher output torque. The large hole in the center of the shaft comes from the magnetic pole design and may be used for channeling wires and process fluids when necessary.

Direct-drive motors provide a large diameter mounting surface and eliminate belts, transmissions, and couplings to reduce windup and compliance. Removing the compliance lets the system remain stable under higher controller-gain settings.

Traditional high-performance rotary machines such as tables, robots, turrets, and pedestal assemblies typically run on standard servo-motors connected to rotating members through transmissions. The transmissions are usually gears or belts that convert the high-speed, low-torque of servomotors to the low-speed, high-torque needs of the rotary machine. However, in some cases the transmission can become a limiting factor or introduce errors that some applications can't tolerate. For these, direct-drive rotary motors might be the most efficient solution.

Direct-drive torque motors use unconventional magnetic-path designs to provide torque matched to the load. Their magnetic pole count is higher than standard designs, and the diameter of the rotor is larger — both of which add to the total flux of the machine as well as the torque moment arm generating higher torque output. As an added benefit, the magnetic pole design requires less space and provides a path for wires and cooling fluids to run through the center of the motor.

But magnetic-circuit designers have a particularly tough job with these motors because the steps needed to maximize torque output also produce undesired torques, such as cogging and torque ripple. Often, this can degrade system response, accuracy, and smooth operation. The goal is to produce large, undistorted torque and smooth output.

Without a transmission, the motor can reach higher accelerations with higher accuracy as well. Systems using direct-drive motors are not susceptible to the problems usually encountered by those using transmissions, such as gear chatter, belt stretching, and loss of accuracy from imperfect transmission component geometries. Furthermore, acceleration is limited only by the load and motor. Taken together, these advantages allow direct-drive systems to control motion with extraordinary speed and accuracy.

A final problem that can be solved with direct drives is resonance between motor and load. Because high-performance motion systems often rely on closed-loop control, they use high gain loops to obtain the best response possible. However, the compliant couplings usually used between motor and load cause oscillations of about 300 to 1,000 Hz. These oscillations occur at or near a frequency where the load and motor inertias resonate across the compliant coupling. Compliance in standard motors is high because the motor shaft is comparatively long and narrow.

As shown in the diagram, loads may be coupled to direct-drive motors on a large diameter, effectively eliminating resonance in servosystems. And while standard servosystems usually limit load inertia to no larger than 5 to 10 times the motor inertia, direct-drive motors have no such limit. The load inertia is often hundreds of times larger than the motor inertia with no negative effects.

Direct-drive motors have been employed in a variety of applications to improve accuracy and throughput such as wafer handling robots, and chemical-mechanical polishing machines for semiconductors, industrial rotary positioning machines, and for large-inertia rotary tables. They also improve system reliability because power transmission components are not needed and the size and weight of many other mechanisms can be reduced. Moreover, these systems eliminate belts and gearboxes, making the machines virtually maintenance free. In addition, direct drives significantly reduce audible noise, as much as 35 dB in semiconductor processing machines. In general, two major applications benefit best from direct-drive motors: those requiring high-acceleration and accuracy under high-torque, low-speed loads, and those that cannot tolerate gears and couplings that wear and need to be replaced frequently.

Information for this article was contributed by George Ellis and Tom England, Kollmorgen, Radford, VA 24141, (800) 777-3786, Fax: (540) 731-0847,, [email protected]

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