Servo or Vector Control Can Handle Many Applications

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.

09/01/1999


KEY WORDS

 

  • Motors, drives & motion control

  • Electric servos

  • Vector control

  • AC/DC motor drives

  • Torque & position control

Sidebars:
Mix-and-match of applications

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 vector

Rexroth 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."

Motor Comparison
While motor selection depends on specific application needs, these criteria can help.

Selection criteria

PM synchronous motor

Induction motor

(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

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


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.




Mix-and-match of applications

Paper Converting Machinery Corp. (PCMC, Green Bay, Wis.) uses Rexroth Indramat (Hoffman Estates, Ill.) servo drives and permanent magnet (PM) motors on its Vision line of common impression printing presses with up to 20 axes of motion. PCMC's newly developed ProVision product line (photo, previous page) raises the ante on performance and power. The new machines operate faster (1,200 vs. 800 ft/min; 366 vs. 244 m/min), run wider web widths (65 vs. 29 in. max.; 1.65 vs. 0.74 m), and have longer repeats (40 vs. 24 in. max.).

ProVision's larger capacity further called for larger motors (50 vs. 20 hp/37 vs.15 kW)—motors not commonly available in PM technology. "Flux vector control of induction motors was therefore selected," says Indramat staff engineer, William Erickson. "Frequency response was the same for both machines, however." To obtain PCMC's order, Rexroth Indramat demonstrated that the required dynamic response would be achieved using vector technology. PCMC uses PM servo and vector servo technologies interchangeably, based solely on power requirements, according to Indramat.

It should be noted that availability of larger PM servo motors is slowly growing. Go to Online Extra at

Delicate tension

An optical fiber Proof-Tester is one of the more sophisticated products of Watson Machinery International Inc. (Patterson, N.J.), an OEM of wire, cable and fiber-optic production machinery, in business for 153 years. The high-speed machine tests tensile strength of optical fibers prior to assembly in a cable, while running at 1,800 m/min. It consists of four motor-driven sections—payoff, input capstan, output (tension) capstan, and take-up reel. Input/output capstan drives have 2 hp rating; payoff and take-up traversing drives are rated at 1 hp. Proof-Tester can control tension up to 250 kpsi (1.72 kPa) in a 250 mm fiber, using a load cell located between the two capstans.

The machine must be able to stop very quickly (&300 msec) in case a fiber breaks during a test run. "Stopping time is a critical parameter to avoid damaging any remaining fiber on the payoff spool, as well as to avoid burying the fiber end in the take-up spool," remarks John Doherty, vp of technology at Watson Machinery. Precise winding of the fiber on the take-up spool is another vital parameter. Any overlap or build up on the spool's edges means the fiber will not pass an important optical time-domain reflectometer test—in which case the fiber must be rewound.

In upgrading this machine from brush dc, Watson looked at both closed-loop flux vector and servo control. Two varieties of MasterDrives from Siemens (Alpharetta, Ga.; Erlangen, Germany) were compared: 6SE70-VC (Vector Control) and 6SE70-MC (Motion Control-Servo). MasterDrives MC handles both induction and PM servo motors, but the latter were more costly in this application. MC drive's built-in motion controller satisfies the position-loop requirement at a lower cost than the VC drive. Also, the servo mode stops the induction motor in approximately 200 msec—a 27% improvement over the VC mode. This is due to MC drive's higher current overload capability, including "pulse-resistor braking" that allows more short-term energy feedback to the dc bus, according to Joe Zoll, manager of General Motion Control at Siemens Energy & Automation.

"After much testing and trials, we selected the servo drive [controlling an induction motor] due to the additional features and performance at a better cost," adds Watson Machinery's Mr. Doherty.

Plastics, printing, packaging

Presto Products (Appleton, Wis.), a plastic film manufacturer, chose Unidrive from Control Techniques (CT, Chanhassen, Minn.) and a PM servo motor to replace a brush dc motor and dc drive. Operating in servo mode, the Unidrive system at Presto controls such processes as blow molding, extrusion, and thermoforming. Products made range from cellophane wrap to foil packaging and disposable plastic storage containers.

Servo technology was chosen over flux vector for this recent process upgrade mainly due to size constraints. The 5-hp induction (vector) motor first considered turned out to be larger than the existing dc motor, but the servo motor from MTS Automation (New Ulm, Minn.) was both smaller and more efficient. "Rationale behind the decision was also based on the servo motor's higher torque output for its physical size and Unidrive's precise tension control capabilities," states CT application engineer, Don Blickle. CT is a division of Emerson Electric Co.

Using CT's standard speed winder DPL (Drive Programming Language) code as a base, Mr. Blickle made modifications to meet specific application requirements for using a servo motor and drive. Presto's dual-turret winder needed 2-axis control to wind plastic film in a continuous process. The film is extruded and first wound to one take-up roll. When this roll is full, DeviceNet communicates the transfer position and enables "bumpless" transfer of the film to the second roll for final winding.

The process requires precise timing on the tension dancer to ensure smooth transfer and maintain continuous extrusion processing. "Unidrive and the [MTS Automation] servo motor achieved bumpless transfer rate of 800 ft/min on a process line that previously had a maximum rate of 350 ft/min," adds Mr. Blickle. This represents more than 100% improvement in production rate.

Quad/Graphics (Sussex, Wis.), a major printer of magazines and catalogs, saw benefits in replacing dc servo drives with ac flux-vector drives in a shuttle hopper machine that stacks and binds magazines at the end of the print finishing process. A vector drive controls the machine's feed rate and start/stops occurring at nearly 7 Hz as 400 magazines or books per minute pass through the unit.

The shuttle hopper must stop at a specific position for each item being handled, then go to full speed in under 150 msec. Quad uses a 3-5 hp motor with a programmable Series 7000 Flux Vector drive from Thor Technology Corp. (Menomonee Falls, Wis.). Advanced features include encoder input, torque control, standard/dynamic braking, and position regulator software. The drive provides maximum speed of 28,000 rpm, with a positioning capability of 0.005,7 according to Thor.

"Flux vector drives allow us to achieve 'servo quality' from ac induction motors. These are basically 'no-maintenance'systems and user friendly to the maintenance staff," says Quad/Graphics' system installation coordinator, Ray Kolata.

Yet, other machines at Quad's separate printing division (same location) rely on servos. For instance, a new high-speed folder called DFPF3—developed by subsidiary company Quad/Tech Inc. —uses an automated PowerMotion servo system from GE Fanuc Automation (Charlottesville, Va.). DFPF3 improved speed, quality, and operator control over a previous model with stepper control.

Baldor Electric Co. (Fort Smith, Ark.) also mentions closed-loop VC as a competitive alternative to servo in upgrading a paper-coating machine from electromechanical controls. Lithotype Co. (South San Francisco, Calif.) produces printed labeling for flexible packaging of foods such as potato chips and dried fruits. Its Electron Beam Coater line adds the familiar shine to such bags. A Baldor 18H vector drive controls the coater's paper take-up tension rollers, using encoder signals from the paper input drive. A master pulse follower board allows wide adjustment of the follower ratio. VC was chosen because of lower cost compared to servo control the technical level of maintenance available at Lithotype, was also a factor, says Baldor.

The Busse Co. (Randolph, Wis.) originally designed its Turbo Series Depalletizers to be implemented with servo controls. But after further review, engineers at Busse found that vector control could meet or exceed the application's performance specs. This solution was also cost-effective compared to servo drives. Busse depalletizers now use VG3 drives from Yaskawa Electric America Inc. (YEA, Waukegan, Ill.). The vector drives were purchased from MagneTek Drives (Milwaukee, Wis.), one of YEA's North American distributors. Servo drives from at least two other manufacturers were initially considered for this application.

Two basic motions needed for the depalletizer are: vertical (hoist) and horizontal (sweep). Hoist motion is sized as a 10 hp vector motor/drive combination, handling up to 4,000 lb (1,818 kg) pallet loads and supplying 100% starting torque at zero speed that holds the pallet in position without using the brake. Braking requirements are thereby reduced to emergency stops, drastically cutting brake wear. Sweep motion (approx. 10 sweeps/min) comes from a separate VC drive rated at 3 hp. The motors are standard ac induction with an encoder.

Yaskawa's VG3 drive offers multistep speed selection using a PLC, with speed accuracy and regulation of 0.01% that allows the depalletizer to hold position within

A servo system alternative would use a PLC to provide commands to a motion controller that, in turn, provides the motion profile to the servo amplifier. The extra components can add cost to the system, according to Yaskawa.



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