Enhancements Help Step Motors and Controls Evolve

In their traditional open-loop operating mode, step motors and drives remain the simplest, most economical means of position and motion control for a variety of industrial applications in the power range up to one horsepower (0.75 kW). Simple control, yes, but the step-like motion can introduce resonances plus other disturbances over a motor's operating speed range.

11/01/1998


KEY WORDS
 
  • Motors & motion control

  • Step motors and controls

  • Machine control

  • Position control

  • Open/closed-loop control

Sidebars:
Torque density is key for step motors


In their traditional open-loop operating mode, step motors and drives remain the simplest, most economical means of position and motion control for a variety of industrial applications in the power range up to one horsepower (0.75 kW). Simple control, yes, but the step-like motion can introduce resonances plus other disturbances over a motor's operating speed range. The "penalty" is that users need to know quite a bit about stepper systems' intricate physics to assure full application success. Newer controls and software tools help, but can't be expected to fully replace user knowledge.

While continuing to enhance open-loop abilities, stepper systems are starting to add sensorless feedback, observer functions, and other open/closed-loop features to detect stall conditions. Such servo-like changes blur historic distinctions between the two competing technologies—steppers and servos.

Sampling the trends

Newer processor-based drives indicate a growing trend for step controls. API Motion (Amherst, N.Y.) notes that such intelligent microstepping drives eliminate the need for separate indexers or motion cards "in most applications." Ken Wyman, director of marketing, sees API's intelligent drives drawing substantial user interest. DeviceNet versions of drive products are even more popular, he states, an example of which is the DM-224i-EDN from API (see photo).

Why all the popularity? A current hot topic among machine designers is "networked, distributed intelligent drives." DeviceNet is the most popular network in the U.S. for non-contouring applications, which encompass the majority of multiaxis machines, according to Mr. Wyman. "Designers like the idea of a standards-based network with intelligent drives that allow them to cut system cost while reducing wiring complexity," he says.

Parker Hannifin Corp., Compumotor Div. (Rohnert Park, Calif.), views the arrival of fully digital drives as another significant development for step-motor control. These newer digital drives are said to extract the full performance capabilities inherent in step motors by microprocessor control of the winding current loops. Full digital control permits adding of even more sophisticated features. For example, electronic damping in Compumotor's latest digital stepper drives will enhance performance over its analog Zeta Series technology, according to the company.

"Such control will enable a variety of high performance closed-loop operating modes to be achieved with feedback and without," remarks Stuart Goodnick, manager of Power Products Group at Compumotor. He describes the mode without feedback as an "observer structure that can deduce torque magnitude on the rotor." Other attributes of digital drives include automatic drive/motor matching, simplifying parameter setups by the user, and more connectivity of stepper drives with other system components via fieldbuses and SERCOS.

Pacific Scientific, Automation Technology Group (Rockford, Ill.)—now part of Danaher Corp.—sees a growing role for stepper technology to replace older electromechanical controls. These and other wider functional needs prompt PacSci to build its stepper drives to accommodate various option cards. For example, its oscillator-controlled drives running in microstep mode let the step motor replace a clutch/brake assembly, while improving the application's positioning accuracy. Also, such an updated subsystem requires less maintenance.

Commenting on the evolving role of stepper systems, a Pacific Scientific spokesperson states, "Encoder inputs provide some stepper drives the feedback necessary to replace servo systems, in some instances, at substantially less cost."

As mentioned earlier, stepper systems' basic advantage is simple open-loop control. At the same time steppers are locked in competition with closed-loop servo systems. Dan Jones, president of Incremotion Associates (Thousand Oaks, Calif.), puts it as "increasingly competitive pressure, as never before." He mentions one goal for keeping stepper systems competitive: "They must continue to evolve toward vibration-free higher speed motion—in the 1,000 to 2,000 rpm range—while keeping a hold on lower speed motion systems up to 500 rpm."

These developments comprise several parts. Higher voltage drivers with smoother microstepped motors will achieve the desired higher speeds (up to 2,000 rpm). Higher constant current step motor drives still provide users with the highest continuous torque density positioning systems available today. "Sensorless closed-loop control techniques are also being used to smooth the step motor's speed vibrations or fluctuations," comments Mr. Jones.

He uses product examples from Oriental Motor USA (Torrance, Calif.) to illustrate some of these developments. Oriental's UFK(W) Series driver packages for step motors represent a higher voltage stepper system, with 160 V output as typical. Oriental's PK-M step motor with 0.9

Among products from Intelligent Motion Systems (Marlborough, Conn.) is Lynx, a stepper (and motion) controller that provides "1.5 axis control"—complete control of one axis and slaved, ratioing control of a second axis. Lynx works with full-step, half-step, and microstep drives (see products table).

Albert Leenhouts, president of Litchfield Engineering Co. (Kingman, Ariz.) and a noted, veteran consultant in stepper technology, also sees among current developments "serious efforts to put some much needed damping into motor/driver systems." He cites as examples Parker's Zeta Drive (for larger motor sizes) and the smaller 5-phase driver packages from Oriental Motor.

Commenting on the need for more education, Mr. Leenhouts thinks that today fewer engineers appear inclined to learn as much about stepper systems as five or 10 years ago. "Effective step motor control theory is well developed, but seldom taught at the academic level. This will ultimately reduce the market share of step motors," he says.

Motoring along

More performance from the same or smaller motor package remains a strong trend. Kollmorgen (Radford, Va.) points to progress especially in so-called "tin can" (low-cost) step motors. "This has been accomplished by maintaining the mechanical configuration of the motor while maximizing its internal characteristics," explains Dave Beckstoffer, director, Step Motor Group at Kollmorgen. He adds that tin can motors can reduce cost and package size in applications that once required larger motors.

A related item is availability of motion control chips, offering design engineers greater options than in the past. They simplify programming of multiaxis control that performs complex motion profiles. "All in all, the idea has been smaller, faster, and more powerful—and step motors have followed that technology trend," concludes Mr. Beckstoffer.

Jack Nordquist, principal electronics engineer at Industrial Devices Corp. (IDC, Novato, Calif.), says, "Step motors provide large amounts of continuous torque at low speeds and at very competitive cost." Mr. Nordquist believes that step-motor systems have generally higher stiffness—tendency to hold position against external disturbances—compared to similarly priced servo-motor systems, and come without an associated "hunting" disturbance.

"The motor owes its stiffness largely to the high spatial frequency of the rotor tooth structure. But this translates into a requirement to operate the fields at considerably higher frequencies, leading to serious ac losses," he continues. To compensate, designers must make flux paths within the motor more efficient. Good progress has been made here by way of new design geometries and better core materials.

Mr. Nordquist also cites effective use of three-state (or recirculating) drives to reduce switching losses at low speeds and at rest. Overall, motor heating remains a serious limitation of steppers. "Users must continue to be aware of that limitation to avoid misapplying this technology," he cautions.

Suppliers fall into step

National Instruments (Austin, Tex.) has "formally" entered the step-motor control arena with its acquisition of nuLogic in August 1997. "Motion control was a perfect addition to our extensive line of PC-based measurement and automation," notes James Truchard, president and ceo of National Instruments. Dr. Truchard told Control Engineering that NI's multiaxis motion products deliver accurate, high-performance motion for all stepper [and servo] applications, and are fully programmable from its LabView and BridgeView development software, as well as from MS-Windows NT/95.

Stepper systems—no less than other areas of automation—are embracing the latest available techniques: PC-based control, embedded real-time processors, object-oriented HMIs, and so on. Jeffrey Seiden, motion control marketing manager, describes National Instruments' direction in terms of a dual-processor architecture. "Most soft-logic solutions place a DSP chip on the board with real-time extensions for Windows NT, but NI's architecture includes a DSP chip and a second processor onboard the plug-in hardware," explains Mr. Seiden. Users can thereby off-load functions from the computer's processor that may be "too real-time" for NT extensions, he adds.

Thomson Industries Inc. (Port Washington, N.Y.) expanded its motor technology capability with the December 1997

acquisition of Airpax Mechatronics Group, a division of Philips Electronics North America Corp., (subsidiary of Philips Electronics N.V. of the Netherlands). Renamed Thomson Airpax Mechatronics llc., the entity brings long-standing step motor design and manufacturing expertise to the new group. An expanding line of 15-60 mm size permanent magnet step motors is among its pertinent products offerings.

API Motion has also been on the move. In July 1997, it acquired Portescap, the Swiss motion control company known for the " Disc Magnet" motor line, among other products. API Portescap (La Chaux-de-Fonds, Switzerland) is the new subsidiary; the company's European headquarters are located there as well.

An innovator in step motors and controls, Berger Lahr GmbH of Germany introduced both 5-phase and 3-phase step technologies to the world. For some time the company had been part of SIG Positec, a division of the large Swiss SIG (Schweitzerische Industrie-Gesellschaft) Group. In a January 1998 restructuring, the Positec Div.'s name changed to SIG Positec Automation (Lahr, Germany), with concentration on positioning drives and automation solutions for machine building. In the U.S., SIG Positec Automation (Plymouth, Mich.) replaces the former Berger Lahr Motion Technology name.

The promise of 3-phase

Step motor motion originated from so-called 2-phase (bipolar) or 4-phase (unipolar) type of control that provides basic 1.8° full step angles (200 steps/rev). Steppers grew to maturity under 2/4-phase style of control. Five-phase technology varied the theme over 20 years ago with 0.72

Berger Lahr's introduction of 3-phase steppers in 1994 was a serious challenge to existing technologies. It offered 0.36 CE , Dec. 1996, pp. 49-46 and July 1994, pp. 62-65 for more about these types of step control.

However, the challenge of 3-phase steppers has fallen short of expectations so far. Most step motor and control manufacturers agree that the giant installed base and wide availability of 2-phase products do not bode well for the few suppliers of 3-phase products (see motors table). Limited availability of 3-phase stepper drives is a further deterrent. For their part, 5-phase steppers have made gains versus the 2-phase variety, but not in a vast number of applications.

Still, the advantages of 3-phase steppers are recognized by manufacturers, including ones not supplying the technology. Here are some sample comments.

"If more companies offer 3-phase step products in the future, then over time their attractive attributes will win increasing market share. Just don't look for it to happen soon," remarks API Motion's Mr. Wyman.

"Success for this technology will require offering a range of frame sizes (NEMA 23, 34, and 42) along with compatible drives," comments IDC's Mr. Nordquist. [SIG Positec Automation is one supplier of these elements.] "The complete system should provide increased performance (more power transmission efficiency and greater smoothness) at a competitive (reduced) price before it will generate much interest."

Currently, SIG Positec Automation is the major supplier of 3-phase step motors and controls. For new applications it stresses benefits of the 3-phase approach with complete drive electronics that provide 1,000 steps/rev as a standard feature. However, its product offerings include a novel variation for "drop-in replacements" in existing systems. Here the 3-phase motor can emulate 2- and 5-phase motor operation using a DIN-format electronics board that typically resides near the existing system's PLC or other controller, according to Positec Automation applications engineer, Roger Burg. "Advantages of the 3-phase approach are more torque per size compared to 5-phase and lower resonance versus the 2-phase motor," explains Mr. Burg.

Further evolution

Stepper system evolution is bound to continue with competitive pressure from electric servos fueling further innovations. Likely approaches are along lines of closed-loop feedback methods that emulate servos, as well as ways to closely monitor motor behavior and notify the drive to minimize any disturbances before they affect a connected system.

One example of things to come, offered by Incremotion Associates' Mr. Jones, concerns active load matching. "Over the next two or three years, step-motor system manufacturers will add active load matching software to the indexer-driver package to provide users with tools for error-free step motion." For more information, visit www.controleng.com/info .

Representative Stepper Products
Stepper Controls

Ratings Steps/rev
* - User selectable/selectable via software
**- 2X steps available in half-step mode
(†) - Supply voltage ac, unless noted
(a) - also 24-75 V dc
(b) - 2 user selectable ranges
A - Serial comm.
B - DeviceNet
Recognitions: X - CE Mark, Y - UL, Z - CSA
H - Stand-alone operation.
C - Optically isolated I/O
D - Anti-resonance feature
E - Onboard indexer
F - Onboard power supply
Company Product A/phase W peak V in (†) Axes Full ** Micro Features
Advanced Micro SystemsCMAX-810 Drive828080/160 dc120050 kSmart microstep drive; A; H; X
SAX/DAX Series340 dc1-2200-400Low-cost; H
API MotionDM-2200 Series574074 dc1200to 12.8 k*Progrmble; Networkble: A; B
Applied Motion Prods.3540i Drive/Indexerto 3.514040 dc2-50.8 k*Win GUI; I/O: 4 in/8 out; A; C; F
Continuum EngineeringMCL Series62 k90-2301-420050 kMultiaxis ctlr; Joystick input; 180+ commands; E; F
Industrial Devices Corp.Smart/Next Step0.1-7.91.3 k120/240120050.8 k *High power density; D; X; Y
S6000 Series"""1-2"25.4 kD; X; Y
Intelligent Motion SystemsLynx1.4-5.8115/230 (a)1-3400to 51.2 kEase of use, progrmg; E; F
IM483I0.4-412-48 dc1200"Opt. encoder and/or E
KollmorgenSMC-600-PSA5.5247115120010 KLine oper; Unipolar chopper; H
National InstrumentsnuDrive Step 4SX-4110.25-4640115420051.2 kRobust 4-axis unit; Pluggable screw terminals per axis.
Oriental Motor USAUFK(W) Series1.4630100-1151500125 kCompact, high-torque, 5-phase microstep drive; X; Y; Z
Pacific Scientific6400 Series0.63-5375120/240 (a)1-220050 kIdle amp. reduct; Electr. damping; E; F; Y; X (some models)
Parker/CompumotorZeta Series4-1295-26412001-50.8 k *Act. damping; Electr. viscosity
Warner ElectricWarpDrive Series1-6 (b)100-2001200to 12.8kMicrostep drive & controller; Expanded I/O count; A; X; Y
WhedcoDspMotion Step Motor Ctlr570090-130150 kPeripherals provide complete machine control system


Representative Stepper Products
Step Motors

Company Product Frame Size* Phases Step Angle (deg) Holding Torque (lb-in.)** Useful Torque, lb-in.** at (rpm) Features
* - NEMA frame size unless noted
** - Other torque units as noted
(a) - 2-ph. bipolar/4-ph. unipolar
(b) - Other sizes and torque ranges available.
A - CE Mark
B - 35, 130, 325 V motors available
C - Gearbox version avail.
D - Protected for harsh environments
E- Extra torque per size
F - 0.38 in. profile
G - Special magnet/rotor design for max. power or torque/frame size
H - Optional encoder or brake
L -. Low resonance.
API MotionTurbo Stepper Series17, 23, 34, 4221.8to 195166 (100)E; G
Eastern Air DevicesHeavy-Duty Motor34, 422/4 (a)1.8, 5, 1511.8-8831 (3,000)Torque is drive dependent; A; C
Haydon Switch & Instrument80000 Series3.15 in. OD23.7525 oz-in.12 oz-in. (250)Pancake type; F
Nyden Corp.Pulstar V Series23, 3450.72to 555 (3,000)High spd/torq vers. avail.
Oriental MotorPK-M Series17, 2320.91.4-11.81.2-11.2 (150)High torque and resolution
Pacific ScientificPowerPac34, 4221.8to 356to 94 (1,000)High accel; G
SIG PositecVRDM 36823 (b)30.3615.4 (1. 74 Nm)1.5 Nm (600)L; Options: B, C, H
Thomson Airpax MechatronicsPM Motors15-60 mm2/4 (a)3.6, 7.5, 15, 180.55-28 oz-in.9 oz-in. (1,100) [57 mm motor]C; E; G
WhedcoMTR-1000 Series23, 34, 4221.8to 85to 25 (1,200)G



Torque density is key for step motors

A major trend for step motors is continued increase of torque density—defined as rated torque per unit volume. Albert Leenhouts, president of Litchfield Engineering Co. (Kingman, Ariz.), refers to torque density as one of the outstanding features of today's hybrid step motors over competing technologies. Making it happen are "better magnet materials, increased rotor diameters, and sheer persistence in making small improvements in lamination design," he adds.

In API Motion's (Amherst, N.Y.) experience higher torque step motors—especially in the smaller NEMA 17 and 23 sizes—are very popular with designers looking for a cost-effective step solution. Rare-earth neodymium (Nd-Fe-B) magnets on these motors provide the extra torque needed without going to servos. A product example is API's Turbo Stepper Series.

Along the same lines, Nyden Corp. (San Jose, Calif.) reports use of high-energy, rare-earth magnets to double, on average, usable torque from its Pulstar V Series 5-phase step motors. These NEMA 23 and 34 size motors run with 0.72



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