Eliminating compliance in servosystems

Oct. 5, 2000
A traditional motor-mounting arrangement consists of a housed motor coupled to a lead or ball-screw shaft by a flexible coupling and a motor mount.

A traditional motor-mounting arrangement consists of a housed motor coupled to a lead or ball-screw shaft by a flexible coupling and a motor mount.

A direct drive consists of a rotor assembly fabricated directly on the ball screw, eliminating duplicate motor parts, compliant couplings, and brackets.

The step response of a traditional motor-mounting arrangement shows an overshoot of more than 60%. The large overshoot is a result of the unwinding action of the coupling.

Eliminating the coupling improves the step response of the direct drive. The overshoot is only about 10% and the settling time is shorter.

Modern servocontrollers are packed with increasingly more computing power, so it is easy to forget the importance of the dynamic requirements of the mechanical structures. Although the servocontroller is certainly a critical component of the motion-control system, the mechanical structure and compliance in the drivetrain is just as critical.

In linear-motion applications, the drive-train typically consists of a rotary servomotor connected through a flexible coupling to a lead or ball screw. In this configuration, selecting components that can provide high dynamic stiffness and rapid servosystem response is critical. Common methods for improving stiffness include shortening the shaft, using stiffer gearboxes, and oversizing the couplings. Although the coupling is often the weakest link in the chain, selection may often be a trade off between tolerable misalignment and the amount of wind up or torsional stiffness required.

An alternative to the traditional coupling method is a direct drive. A direct drive is an integral component that reduces compliance by installing the rotor assembly and feedback device directly on to the lead or ball-screw shaft. In other words, the motor shaft becomes the ball screw. The ball-screw shaft of the linear stage is supported by two sets of duplex angular contact bearings that allow high column speed. Also, having the motor housing and bearing structure share a common reference reduces misalignment. The end result increases dynamic response, reduces overall package size, and improves reliability.

The primary improvement in dynamic response stems from eliminating wind up in the coupling. Although the torsional stiffness of couplings vary, high acceleration and deceleration rates can twist the coupling. As the servosystem tries to settle, the coupling moves back and forth as it unwinds. The direct drive, however, provides improved dynamic response by reducing settling time and increasing the ability to handle inertial loads. Increased torsional stiffness improves dynamic response. A typical servosystem bellows coupling has a torsional stiffness of 216 lb-in./° or 24.4 N-m/°, compared to a direct-drive motor mounted to a ball screw with torsional stiffness of 345 lb-in./° or 39 N-m/°. This increase in torsional stiffness amounts to a stiffer transmission which provides tighter dynamic response.

The direct drive eliminates the need for right-angle drives or wraparound timing belts, both of which introduce additional error and compliance. Eliminating parts also amounts to a smaller package size.

Information for this article was contributed by Alan Feinstein, Bayside Motion Group, 27 Seaview Blvd., Port Washington, NY 11050, (516) 484-5482, fax: (516) 625-6985, www.baysidemotion.com

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