Servo or Vector Control Can Handle Many Applications
Here's how to choose between two drive control types from lessons learned in printing, converting, web handling, and other applications. Criteria include available size of motors, system cost, and user comfort.
Frank J. Bartos, CONTROL ENGINEERING -- Control Engineering, 9/1/1999
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This article, Part 2 of 2, looks at applications. Part 1, in February (March International issue) provided an overview of the two control methods.
Applications in printing, converting, and web handling are among areas of competition for servo and vector control, two distinct motor-control methods. While the technologies compete, it's not always head-to-head. At times, the selection process involves replacement of older mechanical or brush dc drives—even dc servos —with newer ac drives that might go to servo or flux-vector control (VC) in the final decision.
Advances in electronics and software blur the lines between servo and flux-vector controls. Their real differentiation comes from the distinct motor types that provide the actuation. Servo systems work with permanent magnet (PM) synchronous servo motors using rotor position feedback. Flux-vector systems rely on popular, less costly asynchronous ac induction motors. However, they're physically larger and have more rotor inertia.
VC drives are adding position control capability to compete with servo drives. Still, flux-vector control has the role of the contender.
Paul Derstine, PowerMotion product manager at GE Fanuc Automation (Charlottesville, Va.) puts further perspective on these technologies. "Both technologies produce excellent holding torque at zero speed, which is required to maintain position accuracy, however servos are capable of much higher acceleration rates," he says.
It's dynamic response that favors servo systems in discrete part processes, where higher machine cycle rates are important. GE Fanuc focuses on industries and applications that can exploit servo systems' small size and cycle rate advantage.
Mr. Derstine sees vector drives at their best in applications such as continuous web material processing where the goal is to maintain tight speed regulation without the need for frequent starts and stops. (However, see the applications sidebar for a VC drive example with rapid start/stop operation.)
Servo or vectorRexroth Indramat (Hoffman Estates, Ill.) generally applies servo and flux-vector control systems interchangeably, except for attention to application power needs. However, the company considers its VC systems to be a different breed. It stresses that high performance in its vector systems comes from true servo-type drives using flux-vector algorithms and high-resolution motor feedback, not from VC derived from standard variable frequency ac drives. "Today, there's a class of vector drive that delivers servo performance—call it 'induction servo,' " explains William Erickson, staff engineer at Indramat.
It's the underlying motor type that often pushes the system choice. (See motor comparison table.) Historically, if the PM motor supplied sufficient speed, torque, and power for the application—at a comparable price—it became the system of choice. "But, this paradigm is no longer valid. Ability to implement the algorithms in low-cost electronics has made most differences moot," adds Mr. Erickson. "Most players in the high-performance arena can control permanent magnet and induction motors with the flip of a 'software switch.' There is no hardware cost to pay anymore."
Flux-vector control has penetrated the servo market in what Ron Koehler, director of product engineering at Yaskawa Electric America (Waukegan, Ill.), refers to as "low-performance" servo applications. "A low-performance servo can be defined as a vector control drive with speed response of 100 Hz and below, while a high-performance servo has speed response of greater than 200 Hz," states Mr. Koehler. He lists in the first sector applications like cut-to-length, rotary knife, printing press, machine tool change, machine tool spindle, and rough pipe cutting. The depalletizer example (see sidebar) also fits into the low-performance category.
"Other applications still well-suited for servos are ones with high technical performance such as metal cutting, die bonding, contouring, welding, and 'high performance' cut-to-length and rotary knife," adds Mr. Koehler
ABB Automation Inc. (New Berlin, Wis.) offers a special case here, since its direct torque control (DTC) method is a sensorless alternative to servo control (see CE, Feb. '99, pp. 94-95; Sept. '96, pp. 99-107). To fairly compare to servo control, this article focuses mainly on closed-loop VC.
Developed in Finland, DTC is a prime feature of ABB's ACS 600 drives. "ABB has a number of OEMs who are trying to leverage that they don't have to incur servo drive costs on applications using ACS 600," says Chuck Hollis, manager of Industrial AC Drive Sales, ABB Drives & Power Products Group. "These OEMs," he adds, "are applying our drives in material cut-off applications."
| Selection criteria | PM synchronous motor | Induction motor |
| High load inertia | Preferred | |
| Low load inertia | Preferred | |
| Feedback requirements | Absol. posit. info needed | Absol. Position info not required(1) |
| Losses | Less at low speed | Marginally less inductive losses at high speed |
| Cooling (2) | TE; Blower and fluid cooling options | Non-TE; Blower or fluid cooling for hazardous locations |
| (1) - Velocity information scalable if feedback device can't mount at back of motor (2) - Both motor types offered in explosion-proof versions TE - Totally enclosed Source: Control Engineering with input from Rexroth Indramat |
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Business issues also crucial
Business considerations along with technical ones can enter the selection process. This includes ease of maintenance and use of fewer motor types.
Quad/Graphics Inc. (Sussex, Wis.), a national printer of magazines and catalogs builds its own equipment for use in two main business areas: printing and finishing. It has replaced all dc motors and drives in its plants over the past few years with ac motors and drives of either VC or PWM type.
Ray Kolata, system installation coordinator at Quad/Graphics, explains that a typical, very long finishing line consists of three sections or machines—gathering, binding, and trimmer lines. Drive power requirements are in the 30-40 hp (22-30 kW) range. These sections have independent operating modes and are set up by different crews. "However, the sections can be run together via electronic line shaft tracking (including different speed ratios). The overall effect is that the sections become one machine," says Mr. Kolata. This is where newer digital ac drives come into play.
Mr. Kolata cites business issues such as productivity and user familiarity as further affecting the choice of control method. He thinks ac control is "far less complex," needing less troubleshooting than dc servo systems or their "special motors."
Using ac induction motors identical to ones already at work throughout its plants, Quad/Graphics obtains economy of scale. Plant employees are also familiar with these motors. Mr. Kolata is keen on VC drives from Thor Technology Corp. (Menomonee Falls, Wis.), although several other drive makers' products work at Quad.
Inventory is another factor to consider. "I have to cut down the amount of systems, parts, and costs," says Mr. Kolata. And, the use of multiple systems also affects personnel training. "It's more cost-effective for plant personnel to learn one system, plus more operating experience is gained by working with fewer (or one) systems," he adds.
Brushless servo systems were considered. Performance was not the question, but the more costly "specialty" PM servo motors and more sophisticated system maintenance requirements tilted the decision toward the VC approach, according to Mr. Kolata. However, servo drives and motors are applied on the company's other varied machinery (see sidebar).
Indramat's Mr. Erickson agrees about economics of fewer motor types. "Where cost and power advantages are marginal on a multiple axis application, using one type of motor on the machine can be a pragmatic decision, made to simplify support, purchasing, and spare inventories," he says.
With the blurring of servo and flux-vector controls due to electronic and software advances, users can look forward to more choices for their ac motor drive applications.
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| ABB Automation | Baldor Electric | Control Techniques |
| GE Fanuc | MagneTek Drives | MTS Automation |
| Pacific Scientific www.controleng.com | Rexroth Indramat | Siemens |
| Thor Technology | Yaskawa Electric | |
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