Vector-Controlled Motors and Servomotors Vie for Position

Sept. 11, 1998
Vector-controlled motors for motion systems can often do the same job as servomotors for less cost

Phil Strong
Director, Motion Controls
Baldor Motors and Drives
Ft. Smith, Ark.

Motion-control systems use any one of a variety of motors or combinations such as stepper, brushless dc, and induction motors. But two prime movers stand out for machines handling variable-speed loads — servomotors and vector-controlled motors.

Servomotors usually contain relatively expensive permanent-magnet rotors that provide quick, precise positioning. Vector-controlled motors, on the other hand, are a less costly type of ac-induction motor. They’re intended to produce full torque at zero speed, tighter speed control, and when used with feedback encoders, deliver superior performance to ac induction motors for positioning systems.

Although servomotors are usually PM motors with low-inertia rotors and feedback devices for sensing rotor position, the term servosystem describes all variable-speed drives comprising a motor, feedback sensor, and amplifier. Thus, closed-loop, vector-controlled motor systems fit the definition just as well and can be used as position servos in many applications. There’s one caveat: Not all vector motors come with an integrated encoder for sensing position as do servomotors. These are known as encoderless vector motors and are not suitable for position control.

Dynamic performance
The biggest difference between vector-controlled motors and servomotors is their dynamic response. PM servomotors have lower rotor inertia to torque ratios than vector motors. The key reason for the lower servomotor inertia is higher magnetic flux which leads to a longer, smaller-diameter rotor than those in vector-controlled designs. This means for equal torque output, servomotors can accelerate the same load faster than vector-controlled induction motors.

When selecting a motor for a particular application, consider whether a servomotor is required to provide the acceleration or if a vector-controlled motor will suffice. Acceleration is a function of the torque and total system inertia — the load plus the motor. For best transfer of energy between motor and load, the inertia of the rotor must roughly equal the load inertia. When the inertia of the rotor is larger than the load, the motor needs more energy to accelerate its own rotor than to accelerate the load. This means a larger, more-expensive drive is necessary to provide the required speed, torque, and acceleration.

On the other hand, when the inertia of the motor is smaller than the load, the position of the load might become difficult to control during high accelerations and decelerations. This is because most systems have a compliant coupling between the load and motor. During acceleration, the coupling winds up under torsion and the load lags the position of the motor. Conversely, during deceleration the coupling unwinds moving the load ahead of the motor. This elastic coupling can produce an unstable system that oscillates the motor or the load.

With the low inertia and high acceleration of a servomotor, matching the inertia of the load and motor becomes more critical to ensure predictable dynamic behavior. But as a rule of thumb, a mismatch of 10:1 (load to motor inertia) or less generally delivers acceptable system performance in most practical applications, where an exact match is not possible. Often, gears or belts and pulleys can match the inertias over a wide range of values. In comparison, the higher inertia of a vector motor makes controlling the dynamic response of the load easier, although at lower acceleration and deceleration rates.

The finer points
Vector-controlled motors and servomotors can deliver comparable, high-positioning accuracy because both motion systems rely on a position feedback sensor between the motor and the drive. Position accuracy is largely a function of the characteristics of this sensor. Drives typically control the current (torque) and speed of the motor, while another controller (external or embedded) regulates the motor position. Sensors are typically incremental encoders for vector-controlled motors and resolvers or incremental encoders for servomotors.

Physical size: In some installations, the motor’s physical size becomes an overriding issue. Where high torque in a relatively small package is needed, PM servomotors offer larger torque densities than vector motors. This is because their permanent magnets provide higher flux densities in a given volume. Also, with a permanent magnet rotor most heat develops in the stator and dissipates relatively easily through cooling fins and convected or forced air.

In contrast, a vector-controlled motor relies on induced currents in the rotor to generate the magnetic field. These currents develop a significant amount of rotor heat which is more difficult to remove because the air gap between the rotor and the stator is a high resistance path for heat transfer. Consequently, the torque density of a vector motor is lower than a servomotor by virtue of its construction and thermal characteristics.

Torque range: Vector motors typically come in larger torque ratings than servomotors. Baldor, for example, offers servomotors to 40 N-m of torque and vector motors to 2,000 N-m. This difference relates more to economic factors than technical ones. In theory, a permanent magnet can be designed to deliver 2,000 N-m, however its cost would far outweigh all the benefits discussed. Typically, at such high torques, the physical size of the motor becomes less significant when compared to the overall size of the machine. Also, rarely do such large machines require the high accelerations typical of a servomotor. Vector motors offer the most cost effective and practical solution for these high-torque-positioning applications.

Robots and Knife Throwing

Many applications requiring several motors, whether operating in synchronism or not, can use a mix of servomotors and vector-controlled motors. For example, in some continuous-web processing applications, vector motors with high torque control the flow of material while a permanent-magnet servomotor with faster acceleration and deceleration drives a high-speed rotating knife.

The vector-motor controller maintains a fixed relative position between the web and the rotating knife. In addition, the position controller sets the dwell and stroke of the knife, so the length of cut material can be changed on the fly. However, unlike most velocity-based systems, the positioncontrolled servosystem and vectorcontrolled motors are synchronized and position locked in step to ensure precise registration and eliminate long-term drift. Since most drives and control systems come with a signal bus capability such as Devicenet, Profibus-DP, and others, synchronizing them is a relatively easy task.

In another application, scara robots typically traverse the length of a work cell to move parts from one machining center to another. The machining cycle time is relatively long compared to the time required to move parts from one machine to the next. Here, a vector-controlled motor is satisfactory for moving the robot. But placing and removing parts from the machine is more time critical. This operation requires servomotors to deliver the shorter change cycles to ensure that the machine sees more machining time and less idle time.

© 2010 Penton Media, Inc.

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