Thomas Industrial Library

SERVOMOTORS These are fast-response, closed-loop-controlled motors capable of providing a programmed function of acceleration or velocity, providing position control, and of holding a fixed position against a load. Closed loop means that sensors (typically shaft encoders) on the output device being moved feed back information on its position and velocity . Circuitry in the motor controller responds to the fed back information by reducing or increasing (or reversing) the current flow (and/or its frequency) to the motor. Precise positioning of the output device is then possible, as is control of the speed and shape of the motor’s response to changes in load or input commands. These are relatively expensive devices* that are commonly used in applications such as moving the flight control surfaces in aircraft and guided missiles, in numerically-controlled machining centers, and in controlling robots, for example.

Servomotors are made in both AC and DC configurations, with the AC type currently becoming more popular. These achieve speed control by the controller generating a variable frequency current that the synchronous AC motor locks onto. The controller first rectifies the AC line current to DC and then “chops” it into the desired frequency, a common method being pulse-width modification. They have high torque capability and a flat torque-speed curve similar to Figure 9-21a. Also, they will typically provide as much as three times their continuous rated torque for short periods such as intermittent overloads.

Servomotors have several advantages as drives for assembly machines. With a conventional electric motor drive, one large motor typically powers, via gearboxes or toothed-belt drives, one or more line shafts that run the length of the machine. All the

cams in the machine are mounted on these shafts. The timing of the machine is then determined by the mechanical phasing of the cams on the shafts. Torsional deflections and vibrations within the shafts and their interconnecting gearboxes and timing belts can cause dynamic phase errors in the face of the severe time-varying torque that is typical of cam-follower systems (see Figure 9-17, p. 242). With a servomotor drive system, individual motors are fitted to each station and may drive only one or a few cams for that station. This is sometimes referred to as an “electronic line shaft” since the only interconnection between the various stations of the machine now comes from the electronic coupling off all “slave” servos to the one axis chosen as “master.” The timing pulses from the shaft encoder on the master shaft are used to synchronize all the slaves dynamically. Digital shaft encoders that provide hundreds to millions of pulses per revolution are available and can be either relative or absolute.

Other advantages of servomotors include their ability to do programmed “soft starts,” hold any speed to a close tolerance in the face of variation in the load torque, and make a rapid emergency stop using dynamic braking. It is common for machines of this type to be required to come to a stop from full speed within one product cycle, which may be a tenth of a second or less in high speed machines.

Perhaps the greatest advantage of servomotors is their inherent programmability, hence flexibility. Without making any mechanical changes to the machine, it is a simple task to adjust the phasing of any cam within the machine if it is driven by its own servomotor. It is even possible to change the dynamic motion of the follower by programming the servo to rotate the cam with a nonconstant pattern of angular velocity each revolution such that the output motion becomes the combination of the mechanical program within the cam shape and the velocity pattern imposed by the servomotor.*

All this flexibility and adjustability comes at a price as servomotors and their controllers are significantly more expensive than conventional electric drives. Nevertheless, many cam-driven industrial machines are being so equipped as these expensive solutions can sometimes be cost effective when their performance advantages are considered.

* Note, however, that imposing an angular acceleration on the camshaft will change the dynamic force and torque of the system. All the analysis presented here assumes a constant shaft angular velocity, or zero angular acceleration. If a servomotor is used to provide a pattern of angular acceleration, then this must be accounted for mathematically in its dynamic analysis.