Step Motor Systems Battle for Motion Control Market Share

Satire is much in vogue in magazine ads. For example, dinosaurs have served to differentiate what's "in or out" for distributed control systems. Even step motors are not immune from satire, as in one servo system advertisement of a few years ago that referred to step motors as "great boat anchors.

06/01/2000


KEYWORDS

 

  • Drives & motion control

  • Step motors & controls

  • Open/closed-loop position control

  • Servo motors & controls

  • Machine control

Sidebars:
Integration around steppers
Hybrid step motors: an Asia/Pacific snapshot

Satire is much in vogue in magazine ads. For example, dinosaurs have served to differentiate what's "in or out" for distributed control systems. Even step motors are not immune from satire, as in one servo system advertisement of a few years ago that referred to step motors as "great boat anchors." Not to worry; step-motor-based motion control is alive and reasonably well.

Stepper technology continues to surprise users with evolutionary developments. Such hardware and software refinements are requisite in an arena of intense competition with lower power servo systems. Stepper systems' main drawing cards have been the simplicity and low cost of the original open-loop control. Also attractive are step motor characteristics of more torque per volume than other motor types and high torque at lower speeds (typically to 1,500 rpm).

Along with competition comes blending of the two technologies. Newer controllers in the market can handle both step and servo motors. And more stepper systems use (or include as an option) position feedback to monitor or control position—with or without formal loop closure.

DSPs arrive

Digital signal processors (DSPs) have enhanced the closed-loop performance of servo drives for some time. "DSPs are starting to be incorporated into step motor drives," says Ross Goluba, product marketing manager-controls at Industrial Devices Corp. (IDC, Petaluma, Calif.). In part, DSPs come to stepper drives as an "inheritance." A growing number of drives now blend control of servo and step motors into one unit, making economic use of common sections and modules along the way. As a result, DSPs serve to expand stepper motor drive capabilities as well.

DSP "brain power" that controls all motor functions and adds features previously unavailable is part of IDC's just released Impulse stepper drive. One of the drive's most notable features is Dynamic Smoothing. It allows simple full-step input commands, but synthesizes microstep output. A DSP inside the drive creates the steps. "Users can input 200 [full] steps/rev from a basic indexer or PLC and Impulse drive will microstep the motor at 25,000 step/rev," continues Mr. Goluba. The step motor benefits from performance enhancements of microstepping without a costly high-frequency-rate indexer in the system. "Upgrading of old full- or half-step systems is simplified, since only the drive has to be replaced," he adds.

DSP computational power enables other stepper system innovations as well. Jack Nordquist, manager of advanced development at IDC, mentions encoderless stall detection as one example. DSPs detect the stall and stop the drive or provide a status signal for display by an LED. Impulse drive includes an on/off stall detection feature (no user settings). "Most new stepper drives will probably use DSP technology to reduce cost and increase performance," remarks Mr. Nordquist.

API Motion (Amherst, N.Y.), now part of Danaher Corp., sees "intelligence and networkability" as two key innovation areas for its stepper drives. These capabilities are reflected in Intelligent Mini Stepper (DM-2406i), the latest of API's DM-2400 Series drives, launched in March. Intelligent features arise from the blending of step and servo controls mentioned earlier.

One graphical programming language serves either step or servo drives. Designers can switch between drive types without changing the overall control program. "Several OEMs use this feature to offer a servo and stepper version of their machine," says Gregory Woods, president of API Controls. A separate motion controller is eliminated since a common multitasking operating system handles advanced motion commands, such as electronic gearing and registration.

DM-2406i's networking features are serial, DeviceNet, or API's new Ethernet control. "OEMs are moving to network control of drives directly from the PC, eliminating the need for expensive motion control cards," adds Mr. Wood.

Twin Line from SIG Positec Automation (Lahr, Germany; Plymouth, Mich.) further illustrates the blending of drive technologies. These drives control three-phase steppers as well as servo motors—using the same mechanical design, operator interface, and many other standard components that can be applied modularly as needed. Twin Line drives include positioning controllers with integrated power electronics and a DSP. Power rating is up to 750 W for step motor control (photo). The "twin" servo drives come with higher power ratings. An integrated encoder for detecting mechanical overloads is an available option. Associated Control Tool software ensures an identical look and feel for drive configuration, parameter assignment, and error diagnostics.

A major innovator of technology, SIG Positec was the first to introduce both five-phase and three-phase step motors and controls.

Communication methods for Twin Line controllers include Profibus DP, Interbus, CAN (proprietary and CANopen), as well as RS-485.

Integration everywhere

Another trend in stepper technology (as in all of motion control) is borderless integration of mechanics, power electronics, controls, and communication, known as mechatronics . Katherine Voss, director of sales & marketing at Whedco Inc. (Ann Arbor, Mich.), a subsidiary of GE Fanuc, believes: "Stepping motor systems are ideal for taking advantage of these technology trends to improve productivity of both machine builders and end-users. Onboard communication, such as DeviceNet, is key to receiving the benefits of mechatronic design."

Step motors often find application in machines with numerous motion axes. An integrated motor, drive, and control—coupled with networking—can thereby greatly simplify implementation of such machine systems. Ms. Voss is keen on DeviceNet for communicating to the drive. She notes its special fit in step motor applications and cost-effectiveness for adding networks. DeviceNet also is said to:

  • Reduce system parts counts, especially wiring;

  • Allow a clean interface to the main controller, with simple loading of new position/velocity commands from the network scanner; and

  • Improve machine diagnostics by enabling system fault information; for example, undervoltage and overvoltage.

Smart stepping motors are now available from Whedco (see Stepping Motor Cube in sidebar).

Step-motor-based linear actuators form a product specialty at EADmotors (Dover, N.H.). "These threaded, hollow-shaft step motors provide linear motion as the leadscrew translates through the motor's center," explains Robert Cinq-Mars, project engineer (step motors).

Overall, this type of device isn't new. Mr. Cinq-Mars notes some recent refinements in the evolution of these linear actuators. Higher thrust forces and longer unit life result from larger diameter leadscrews—now made possible by newer high-torque step motors that have larger diameter rotors. Mating nuts made of plastic materials help improve actuator efficiency, while minimizing leadscrew lubrication. For still more efficiency and actuator life, so-called "ballscrew actuators" mate a variety of ball nuts with the linear screw.

Mr. Cinq-Mars stresses the significance of high-torque step motors now becoming available. This example of "positive downsizing" offers performance comparable to conventional design step motors that areone size larger. He questions the "future" of standard-torque step motors.

The ruggedness of step motors can be exploited for use in harsh environments. Empire Magnetics (Rohnert Park, Calif.) specializes in step motors for submersible, high-temperature, and other extreme applications (see last photo).

Step monitoring, stall detection

One drawback of step motors moving loads in open loop is potential loss of synchronism between commanded and actual position. Closed-loop feedback and other methods, new and old, exist to verify/control "missed steps" or step motor stall. Yet the bulk of stepper systems sold today remains open loop.

Step motor stall can be avoided in many cases by careful design. Proper motor sizing and "eliminating the potential of outside events [that] create an atypical problem" are key issues, according to David Coutu, president of Intelligent Motion Systems Inc. (IMS, Marlborough, Conn.). "However, for applications where detecting stall is significant, a feedback system is required."

Control of "missed steps"is considered a significant part of stepper control systems at Superior Electric (Bristol, Conn.), part of Colfax Automation. Its Slo-Syn stepper line was among the first with a feedback device that verifies the step motor's positioning of a load relative to the commanded position, according to Ray Rosati, group manager-electronics. "This standard feature allows the certainty of a closed-loop without shaft dithering often found in servos," he says.

Software enhances the stall detection, offering control choices, such as stop on error, set alarm, and correct on error. Error correction is possible during a move, as a following error, or at the end of a move. Stall control incorporates several encoder resolutions to suit different applications; an integral encoder power supply simplifies wiring.

Superior Electric stepper systems presently use RS-485 communications to peripheral devices, with daisy-chaining to form a network. Future products will include optional support in slave mode for ControlNet, DeviceNet, Interbus, Modicon Ethernet, and Profibus.

The stepper market with feedback is in a slow growth mode, according to API Motion. For many applications, an economical miniature encoder with up to 2,000 lines/rev is sufficient, according to Mr. Woods. "It adds only a small amount to total system cost," he says. Several encoder options are standard on API stepper products, with resolvers offered as a custom feature.

Closing the loop is not yet a significant part of the stepper market," says IDC's Mr. Nordquist. "Some fraction of users rely on it for safety, depending on the potential damage to their systems."

What's the cost?

Opinions vary widely about the cost of closed-loop stepper operation. Some see the cost increase as "only" the added expense of an encoder. "This is still less expensive than a comparable servo system of like power rating," states Superior Electric's Mr. Rosati, referring to other costs incurred in setting up an alternative. Rapid, correct configuration of a servo system requires expertise. "Downtime associated with lack of knowledge can be substantial," he adds.

Intelligent Motion Systems' Mr. Couto agrees that stepper systems with stall detection still enjoy a price advantage over servos. However, other factors influence system cost. He sees a sensorless electronic system as today's most cost-effective way to detect stall in step-motor-based applications. "However, the long-term reliability of such systems is still unproven," he says.

The more costly solution adds an actual feedback device, but derives an extra benefit: maintaining position. Software integrated into some closed-loop systems enables local control to act upon a stall condition after detecting it. A product example is IMS' MicroLynx, a bipolar microstepping drive and programmable indexer combination ( CE , April '00, p. 98). MicroLynx' 48-V dc version sells for under $300 in 100-piece lots.

Stall detection without a formal feedback device is a target for holding down costs at several manufacturers. Parker Hannifin, Compumotor Div. (Rohnert Park, Calif.) claims its Gemini GT is the first stepper drive with an encoderless technique to sense a stalled motor—as well as the industry's only fully digital stepper drive.

A software algorithm acts as the "sensor." It monitors voltage and current signals passing between the step motor and drive. By analyzing the signals, and comparing their amplitudes and frequencies with known values for normal step motor behavior, a stall condition can be recognized.The stall threshold sensitivity is user adjustable.

Compumotor sees the inherent cost difference between servo and stepper technologies residing in the motor—namely the "much more expensive to manufacture" servo motor. "Incorporating advanced techniques to drive and control a stepper motor brings its performance much closer to that of a servo at a tremendous cost advantage to the user," explains John Walewander, product manager. Compumotor's Gemini GT stepper drive eliminates the encoder for further cost savings.

Still newer products in Compumotor's pipeline include an intelligent digital stepper controller/drive with Ethernet connectivity and digital stepper drives with DeviceNet, Profibus, and SERCOS.

David Beckstoffer, director of Step Motors and Controls at Kollmorgen (Radford, Va.), thinks the ability to work with open-loop control is the chief advantage of step motors over servo motors. Yet "an increasing number of applications require some form of error detection from step motors," he says. A motor-shaft-mounted encoder embodies the most common feedback device.

Among Kollmorgen products, PCL-5022/5023 motion control chip includes "out of step" detection capability. A six-bit deviation counter compares command pulses sent to the motor with pulses returned from the encoder. "This feature is built into the chip, so no additional cost is involved for the error detection," says Mr. Beckstoffer. He still considers the closed-loop stepper solution cost-effective—even with an encoder—over the servo-motor solution.

Kollmorgen's new 2/4-axis stepper control board, which includes the motion chip, offers ISA and PC 104 bus communications. These buses will remain available, but the company sees PCI bus becoming popular in the near future.

National Instruments (Austin, Tex.) notes "significant involvement" in both open- and closed-loop stepper controllers. Mike Darden, motion control product manager, explains that NI's customers choose closed loop for two additional reasons beyond the traditional intolerance of missed steps: low-cost of obtaining the extra capability and a need for synchronized system functions.

In National Instruments' approach, closed-loop stepper control's precision comes at a "cost slightly above open loop" by eliminating PID control hardware. "The user is still able to monitor and react to following error, but the correction occurs during a special 'pull-in' phase at the end of each move," says Mr. Darden.

Besides error correction, users also like to synchronize motion with data acquisition boards or cameras. Closed-loop control affords the feedback needed to trigger on position or velocity. To help with synchronization, NI's FlexMotion boards supply a dedicated bus for sharing signals with the company's other data acquisition, image acquisition, or motion boards.

Pacific Scientific (Rockford, Ill.) sees price as one of the primary drivers for specifying stepper systems. It feels the price gap to an equivalent servo system narrows "dramatically" by adding feedback capability. Where position feedback is critical, Pac Sci offers a direct-replacement servo product line with higher performance, reported to be also comparable in price. As a result, most new product developments are dedicated to servo systems, according to a company spokesperson.

Still, Pac Sci has offered encoder feedback in its high-end stepper drives for about 10 years to track and control step motor stall. It regards that portion of the market as "insignificant." Pacific Scientific states, "Of stepper controls sold in 1999, less than 10% had feedback functionality, and of these high-end drives, we estimate under 10% are used primarily for feedback capability."

High-end stepper products from Pac Sci now offer RS-232/485 communications. SERCOS and DeviceNet are slated to follow within a year.

Looking ahead, hardware and software innovations will likely continue for stepper systems, as they pursue the parallel course of competing and merging with servo systems.




Integration around steppers

Cost-effectiveness of step motors makes them attractive to motion system integration.

Stepping Motor Cube, brand new from Whedco Inc. (Ann Arbor, Mich.)—a subsidiary of GE Fanuc—combines a NEMA 23 step motor, drive, micro-stepping control, and DeviceNet connectivity in one compact package. The unit operates in traditional open-loop mode, without a feedback device or stall detection. Features include a 3-amp (rms) drive, three stack lengths, several motor windings, and simple connection to operator interfaces. The latter can even be a low-end CE-capable operator panel. A NEMA 34 "cube" is in planning at Whedco.

Diagnostics is a key element of Stepping Motor Cube. An LED indicates device and network faults for single- or multi-axis systems. It identifies whether the Cube is online, connected to the network, timed-out, or in link failure.

Oriental Motor USA Corp.'s (Torrance, Calif.) AlphaStep motion control system integrates a sensor into the step motor to "eliminate the potential or actual problem of losing step synchronism." The operating system combines the "best of two worlds," according to Dan Jones, motion control specialist and president of Incremotion Associates (Thousand Oaks, Calif.). "It runs like a servo, but positions and stops like a stepper."

AlphaStep runs in open-loop mode whenever possible—as long as the rotor position remains within 1.8° of any winding being energized (or commutated) in sequence to execute motor control. However, when the fixed feedback loop detects a 1.8° (or larger) rotor position "error," KIS (keep in step) control switches to closed-loop mode. AlphaStep controller provides an alarm signal to the operator, in case the step motor stalls due to excessive load. No tuning is needed to apply the stall detection.

Quicksilver Controls' (Covina, Calif.) Silvermax family is billed as a fully integrated servo system in one package. Silvermax is mentioned here because it's built around a hybrid step motor and underlines the blending of the two technologies. It further illustrates a paradigm opposite to AlphaStep (see above). Silvermax operates in closed-loop virtually all the time, but switches to open mode for position holding. The step motor's holding stability provides an advantage by eliminating servo motor "dither" common at standstill.

Muscle Corp. (Osaka, Japan) also bases its integrated, closed-loop "servo" system on the lower cost step motor. Called Cool Muscle, the compact package combines a microstepping motor, 32-bit RISC CPU, built-in magnetic position sensor, amplifier, controller, and power management. Over 50% of the processor's 128-Kb memory is open to adding controller functions and custom software onboard the motor. Cool Muscle works as a stand alone or as a daisy-chained system of up to 128 units. The company's Microsoft Windows CE device or other Windows device with a serial port can control larger applications. MyoStat Motion Control (Aurora, Ontario, Canada) is Muscle Corp.'s North American subsidiary.

Another stepper integration example is SAMCOP from Star Micronics (Piscataway, N.J.). See more in Online Extra at

Hybrid step motors: an Asia/Pacific snapshot

Several Asian nations have become major manufacturing sources of step motors. The hybrid step motor, one of several motor types, is the workhorse for factory automation and industrial use.

Japanese companies control the manufacture of hybrid step motors because of the complexity of producing high-quality motors, along with having a well-established cadre of manufacturers. This existing infrastructure effectively prevents establishment of new manufacturers. Market leaders are Japan Servo (Kiryu), NMB, part of the Minebea Group (Miyota), Oriental Motor (Kashiwasi), Sanyo Denki (Tokyo), Shinano Kenshi (Maruko), and Tamagawa Seiki (Iida City).

China now claims about 10 step motor manufacturers emerging to compete with the predominant Japanese manufacturers. One notable Chinese company, Changzhou Micro & Special Motors Co. (Changzhou), was established over 40 years ago.

In Taiwan, Teco (Taipei) is the major supplier, along with three to four smaller companies. South Korea's sole hybrid step motor supplier appears to be Korea Servo Corp. (Kyunggi).

The near insurmountable market position attributed to Japanese manufacturers holds for larger hybrid step motors used in factory automation (frame size 23, high torque and larger). Standard torque, size 23 (and smaller) motors, most often used in office automation, are more open to competition.

Rivalry between stepper and smaller brushless dc servo systems is especially intense in Japan. Recent fuel on the fire is the entry of lower-cost pulse-and-direction servo drives (200 W and smaller) with special design brushless dc motors developed to compete with hybrid step motors.

For more on Asia/Pacific companies, see Online Extra at



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