Electric motors’ influence on motion control
Motor design and motion control: Earliest electric motor designs reach back 150 years. While motor principles have changed little over time, much has changed in how motors are manufactured and packaged. Advances in construction materials—especially magnet materials—dramatic physical size reductions, and design innovations have occurred. Some of these developments are explored in this online extension (1 of 4) of the main article “Electronic motion control, then and now,” in Control Engineering’s 60th anniversary issue, September 2014.
Among numerous electric motor types developed, several designs have special importance to motion control advancements. Brush type direct current (dc) motors found early application because of their simpler speed and torque control method. New versions of brush dc motors remain viable today, including those used in servo motion systems. Meanwhile, alternating current (ac) induction motors with simpler construction saw huge installed numbers, helped by their lower cost. They eventually came to be known as the "workhorse motor of industry." Breakthroughs in speed (and torque) control made ac induction motors still more versatile (Refs. 6, 8, 9; references are provided in a separate article, linked below), as mentioned in the main article. This motor type has also benefitted from substantial efficiency improvements prodded by enactment of minimum energy performance standard (MEPS) requirements. All references cited appear in online extension 4 (Ref. 4).
For servo motion control a major milestone has been the commercialization of brushless dc motors in the 1980s. Brushless dc offers exceptional dynamic performance when coupled with today's electronic controls. However, brushless dc motors suffer from blurred nomenclature. They're really ac synchronous machines with permanent magnets (PMs) in the rotor but provide favorable dc motor characteristics. Moreover, differences in the stator windings of brushless dc motors produce different back-EMF waveforms-resulting in either trapezoidal or sinusoidal commutation when connected to the appropriate type controller. Type of commutation presents a trade-off: higher torque density (but more torque ripple) with trapezoidal commutation versus smoother but less torque output with sinusoidal commutation (Refs. 20, 21, 22). Application specifics will dictate the choice.
Design variations of brushless servo motors include placing the magnets into the rotor structure rather than the traditional rotor surface-mount style. Internal permanent magnet (IPM) rotor design is intended for high-demand servo applications where higher torque output per motor size is an advantage. An extra torque component created by IPM design adds to torque production (Ref. 23).
Suppliers' views: servo motor advances
Yaskawa America Inc. has noted significant progress in manufacturing techniques related to brushless servo motors. Scott Carlberg, product manager at Yaskawa America, Drives and Motion Div., said, "Automated winding and segmented stator designs introduced in the past 15-20 years have enabled motor manufacturers to optimize the slot fill of the stator winding design in brushless servo motors." This has led directly to dramatic motor size reductions (see photo).
Parent company Yaskawa Electric was a pioneer in the development of IPM servo motors in the mid-1990s.
Along similar lines, Bryan Knight, automation solutions team leader at Mitsubishi Electric Automation Inc., pointed out that servo motor technology has made amazing advances over the last few years. "Improvements in magnet materials and winding techniques have yielded smaller, more efficient servo motors," Knight said. "New rotor designs including IPM [internal permanent magnet] technology have allowed manufacturers to reduce the amount of rare-earth materials used while lowering rotor inertia, torque ripple, and cost."
Knight explained that reducing rare-earth material content of motors is particularly important due to extraordinary demand for these minerals in myriad other commercial applications. "Supplies have been tight due to export restrictions imposed by the world's largest producer, China," Knight added.
Usage of rare-earth materials ranges from quite small amounts in many millions of electronic devices to huge amounts for large PM generators produced in smaller numbers. Read more about supply-demand issues of rare-earth materials and what makes them "rare" at Ref. 24.
Step motors represent a further category associated with motion control. Stepper-based systems typically perform position control in open loop—a feature unique to this technology. However, step motors sometimes operate in closed loop to meet other application requirements. Field-oriented control is an option. Stepper motion systems fill specific application needs economically and compete with servo systems where appropriate, usually at lower power and torque requirements. Servo motion systems offer a much higher power range.
Versatile step motors come with 2-, 3-, 4-, or 5-phase design and also fit into the "brushless dc" category. An even higher number of motor phases is possible, but added circuits and components and more cost diminish practicality. Controls for modern step motor systems include a microstepping feature that electronically divides the motor's relatively coarse mechanical steps (or full steps)—typically 200 steps/rev or 400 steps/rev—into much finer increments. Smoother motion and improved torque control can be obtained with microstepping techniques. Step motors have also participated in integration with controls discussed below. References 25, 26, 27, and 28 provide further information on step-motor-based motion control.
Control and motor integration
This product sector has been tagged with various names-motor-drive combination; decentralized (or distributed) drive; integrated motor drive (IMD); intelligent, integrated motor drive, etc. Whatever the naming, this product approach sought to combine motors and electronic controls (or as much of the electronics as possible) into a single package at the actuation point. IMD is the terminology used here. The first commercial products emerged in the mid-1990s, as mentioned in the main article, but initial developments go back about 20 years. "From the archives," another online extension in this article series, describes some earlier developments (Ref. 3).
IMDs promised several user benefits such as eliminating separate enclosures, reducing reflected voltage spikes and EMC issues due to long connecting cables between motor and drive, and opening many more motors to the potential of variable-speed control. Numerous manufacturers brought motor-drive combination products into the market. References 29 and 30 include tables of IMD products from some 20 manufacturers. However, IMDs have not realized the full potential initially envisioned for them—including market share and expansion of the power range. Fewer offerings exist today.
Three of the motor types discussed above have participated in integrated control designs: 1) induction motor and variable-speed drive combination, 2) PM synchronous (servo) motor and servo drive, and 3) various step motor and control packages. Some products in Categories 2 and 3 have seen particularly extensive integration, encompassing motion controller, power supply, processing intelligence, feedback device, I/O points, and communication bus, all within the motor package (Ref. 31). Future growth is more likely in product categories 2 and 3.
Current product examples in category 1 include Movimot gear motor from SEW-Eurodrive and Sinamics G110M (with optional gearbox) from Siemens. Among category 2 products are B&R Automation's Acoposmulti65m, Bosch Rexroth's IndraDrive Mi, Moog Animatics' SmartMotor line, Kinetix 6000M Integrated Drive-Motor from Rockwell Automation, and Sinamics S120M from Siemens. Step motor based IMDs (category 3) include MDrivePlus in several NEMA frame sizes from Schneider Electric Motion USA and Oriental Motor USA Corp.'s ASX Series and 5-phase PKA Series offerings.
Integrated motors and drives
Craig Nelson, product manager at Siemens Industry Inc., Drive Technologies, Motion Control, commented on the origins of motor-drive integration. First generation products were promoted for reducing long lead lengths running between motors and drives-among other benefits. In less than ideal conditions, difficulties were encountered with electronics being placed on the motor due to heating effects and vibrations, which impaired life expectancy of the early offerings. "There has been a 'rebirth of sorts' with more durable second-generation decentralized products emerging that also benefit from physically smaller drive sizes available today," Nelson said.
- Frank J. Bartos, PE, is a Control Engineering contributing content specialist. Reach him at firstname.lastname@example.org
See four related motor and motion control articles at bottom, including references.