Step Motor Systems Have a Lot to Offer
S tep motors and controls continue to evolve and innovate. In their long-standing competition with servo-based motion control, some stepper system developments surprise the users.
Stepper drives incorporating digital signal processors (DSPs); common control units for both step motors and servo motors, selected through software; methods to monitor or control step motor stall; and a growing integration of functions around the step motor are some topics discussed in the main text article . Here is additional coverage on these and related areas.
Alignment pins and a locator dowel on the front face of Empire Magnetics' Size 11 Medical Motors help personnel wearing rubber gloves with equipment assembly in a sterilized environment.
Slow-speed disturbances have been a problem area for stepper systems in the past. Industrial Devices Corp.'s (IDC, Petaluma, Calif.) latest generation stepper drives provide substantial improvement in slow-speed smoothness through the use of precisely tailored phase-current waveforms, according to Jack Nordquist, manager of advanced development and staff scientist. 'The reduction in vibration is dramatic due to waveform tailoring,' he says.
These waveforms are derived using a software tool that IDC calls CHARM (for CHARacterized Motor). CHARM measures all necessary motor and system characteristics (cables, electrical values, etc.) for a number of step motors running with encoder feedback, then 'maps out' all variances. This motor map, together with electrical characteristics, enables fine tuning of current waveforms for motor smoothness, explains Ross Goluba, product marketing manager-controls. The actual production step motors do not need an encoder.
IDC also mentions another advantage of incorporating a DSP in the stepper drive. It allows adding a simple 'move engine' (indexer) to the drive to store several preset positional moves and jog speeds of the step motor-without requiring a full indexer board. Simple trigger commands can execute the motions. 'This functionality is a common feature in servo motor drives, but now it is crossing over to the stepper drive market,' says Mr. Goluba.
Networking and communications built into the stepper system is a prime direction for products at Whedco Inc. (Ann Arbor, Mich.). Its new smart step motor product, called Stepping Motor Cube, emphasizes DeviceNet connectivity, with 500 kbit/sec (max.) communication rate and up to 328 ft of trunk line (20 ft drop line, max.). Longer trunk lines are permitted at lower communication rates. Acquired in August 1999, Whedco is a subsidiary of GE Fanuc Automation.
Empire Magnetics (Rohnert Park, Calif.) mentions a new specialty products-NEMA Size 11 Medical Motors intended for mechanical manipulators used in neurosurgical procedures. These motors can be sterilized with alcohol and have features to simplify assembly in a sterile environment (see photo). One unnamed user reports successfully testing these motors by steam cleaning in autoclave sterilization, says Empire Magnetics. The 1.1-in. diameter, 1.8° step motor can microstep to 25,000 steps/revolution.
Running with feedback
Historically, step motor systems operated in open loop. This was a very cost-effective motion control method over servo systems that require feedback. Closing the loop intensifies the competition between the two technologies. In today's stepper systems, loop closure can be done with a formal feedback device (typically an encoder) or through newer 'encoderless' methods.
Note that position loop closure in stepper systems does not necessarily have to take place in real time, as is the case for servos. Feedback in stepper systems is often used just to verify if the commanded position has been reached. However, for 'servo systems' built around a hybrid step motor-such as Silvermax from Quick Silver Controls (Covina, Calif.) and Muscle Corp.'s (Osaka, Japan) Cool Muscle, (see text article sidebar )-the feedback is in real time.
Oriental Motor USA (Torrance, Calif.) mentions a further advantage of its AlphaStep product versus a servo-motor system when positioning loads with lower rigidity, such as a linear stage driven by timing-belts, pulleys, or ballscrew transmissions. AlphaStep reportedly completes positioning moves quicker, independent of travel distance up to 250 mm-because no adjustment is necessary with the open-loop strategy. Using a servo system with these less rigid loads require a lower gain setting for the position loop. This adjustment slows response time (and takes finite time to accomplish). AlphaStep operates in mostly in open loop ( see main article ), but goes into closed loop as required, with an encoder supplying feedback information in the latter mode.
Star Micronics' (Edison, N.J.) product offering in this arena is SAMCOP, a series of stepper motors and controllers running in closed-loop, said to provide high torque and speed in a compoact package. SAMCOP has two configurations: R Series, consisting of separate step motor and amplifier units, and RC Series that combines the motor and amplifier in one package ( see products table ). A servo version of SAMCOP (motors and drives) is also available for higher power applications.
Parker Hannifin, Compumotor Div. (Rohnert Park, Calif.) cites a user application with encoderless feedback that saves on the cost of stepper axes. Semiconductor wafer polishing machines manufactured by SpeedFam-IPEC (Vancouver, Wa.) typically have four axes of motion-two stepper and two servo. Because it's crucial for the application to detect step motor stall, the company chose Compumotor's Gemini GT digital drive that includes an encoderless stall detect feature. For SpeedFam-IPEC, Gemini GT provides motor stall information without the 'packaging difficulties' of an encoder, while saving $400 to $800 per machine. (Average cost of an encoder is taken here as $200-$400.) Gemini drives come in both stepper and servo versions, adding benefits to this application of a common programming language, footprint, and wiring for the servo as well as the stepper axes.
Asian connection, products sampler
Today, Asia is a major source of step motor manufacturing. Many step motor and control developments have come from there. Japan, in particular, has contributed numerous refinements to stepper system technology. A number of Japanese companies have active North American subsidiaries and other business arrangments. Oriental Motor (Kashiwashi) and Muscle Corp. (Osaka) are examples covered in the main article.
Sanyo Denki America Inc. (Torrance, Calif.) is an affiliate of Sanyo Denki Co., Ltd. (Tokyo), a major manufacturer of electric machinery and equipment. Stepper motors and drives as well as servo motors and controls accounted for 43% of the parent company's fiscal 1998 revenues. (Another U.S. subsidiary of Sanyo Denki is Automation Intelligence (Duluth, Ga.), whose business includes direct online sales of the parent company's motion control and servo systems at www.MotionOnline.com.)
Nyden Corp. (San Jose, Calif.) is the R&D headquarters and sales office in the U.S. for Mycom Inc. (Kyoto, Japan). Mycom is a leading supplier of motion control solutions, among them 2- and 5-phase step motors and drivers, and multiaxis programmable controllers for step motors.
Kollmorgen (Radford, Va.) and Nippon Pulse Motor Co. Ltd. (Tokyo) have had a long-standing marketing cooperation, with Kollmorgen serving as the exclusive North American distributor of Nippon's step-motor-based products for over 20 years. Nippon Pulse Motor makes a wide array of step motors and motion control/driver chips. Its U.S. branch office is also located in Radford. Some of Kollmorgen's step motors are manufactured in India and Japan.
NMB Technologies Corp. (Chatsworth, Calif.) is one of the subsidiaries of Minebea Co., Ltd. (Tokyo). The Precision Components Div.-one of two NMB Technologies Corp. divisions-makes a variety of hybrid and permanent magnet step motors for OEM customers. Both standard and made-to-order motors are manufactured.
Stepper systems represent a particularly rich product category. Two accompanying product tables-for stepper controls and motor/motor systems-provide a reasonable sampling of products, but by no means a complete picture of what's available in the market.
Performance of step-motor-based motion systems depends a great deal on the drive electronics and the control method used. It's especially true for torque output from the motor. Holding torque is the most common performance spec for step motors, but refers to standstill condition or torque at zero speed. Therefore, the motors table includes a 'useful torque' column, indicating running torque at some given speed for most of the motors listed. This is just one point on the motor's torque-speed curve, but a useful indicator of dynamic output available.
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