Compendium of references for electronic motion control
This is the last online extension (4 of 4) of the main article "Electronic motion control, then and now" appearing in CE’s 60th anniversary issue, September 2014. A listing of all references cited in the main article and three related online articles is provided here. Link to all here.
This compendium of references consists largely of articles that have appeared in Control Engineering and its online publications. However, some other sources are also included. A brief abstract is included for each reference. URLs are provided for references dated after 1997; for older references not available online, other reference details are provided. The first four references are the online extensions to the main Sept 2014 article.
1) Electric motors’ influence on motion control (online extension 1)
2) Further developments in motion control (online extension 2)
3) Motion control from the archives (online extension 3)
4) Compendium of references (online extension 4) =============================
Probably the most versatile among motor control methods, ac variable-speed drives (VSDs) are available in three principal flavors. Open-loop control, sensorless vector control that eliminates the shaft encoder, and flux vector (field-oriented) control provide increasingly sophisticated command of induction motors and permanent magnet synchronous motors.
Physical size and weight provide the most visible evidence of the remarkable evolution of ac variable-speed drives in the past 50 years. However, what’s under the skin is even more dramatic for the performance, efficiency, and reliability now delivered by these controls. Making it all happen were advances in power-switching transistors, microprocessors, other hardware, plus software functions that ease users’ concerns for drive application and maintenance. . . And the evolution is accelerating.
Field-oriented control (FOC)-sometimes called flux-vector control-is a method that enables the highest performance from permanent magnet synchronous (or brushless servo) motors throughout their speed range. FOC algorithms model the torque-generating efficiency of dc motors and allow linear torque control. Better known in connection with ac induction motors, for which the technique was first developed, FOC is available for brushless servo motors from many manufacturers. FOC has different flavors and variants, much like vector control of induction motors.
Most high-performance servo systems employ an inner control loop that regulates torque. This inner torque loop will then be enclosed in outer velocity and position loops to attain the desired type of control. While the designs of the outer loops are largely independent of motor type, the design of the torque loop is inherently specific to the motor being controlled.
Comparison of the technologies is provided along with a look at selection guidelines. Tables present motor selection criteria and a comparison of three drive technologies: vector control, brush-type servo, and brushless servo.
10) Direct torque control comes to AC drives, Control Engineering, March 1995, Vol.42, No.3 (p. 9).
Direct torque control (DTC) is made possible by the combination of two factors: high-speed digital signal processing technology and an accurate motor model. The sophisticated motor model is necessary because no shaft speed or position feedback device is required.
Most variable-speed drives in the market rely on a modulator stage that conditions voltage and frequency inputs to the motor, but causes inherent time delay in processing control signals. In contrast, premium ABB drives employ innovative direct torque control (DTC)-greatly increasing motor torque response.
Many products, especially those involving complex motion, require more detailed scrutiny of how well the design will work. This is where simulation software comes into play. Simulation can provide a "virtual prototype" of the product or machine before a physical model is built. Simulation can also cut down on costly physical testing-though not entirely eliminate it.
How to apply simulation software to motion control: Computer simulation software can drastically reduce the cost and time required to design and deploy your next motion-control system, but if you aren’t careful, it can lead you down a primrose path to disaster.
MSC Software’s Adams and Easy5 Simulation software models and simulates motion control systems to improve machine performance.
Presentation includes an interesting timeline of developments for power semiconductor devices (p. 43).
The number of motion axes found in industrial automation systems has grown steadily over time because of availability of more powerful controllers and processor advancements. Yet, in many of those systems, motion axes function independently of each other. In contrast, more complex automation systems require synchronization of various motion axes. A higher breed of control is required to satisfy multi-axis motion in robotic systems, printing machines, computer numerical control (CNC) machines, and high-dynamic manufacturing lines. Fast network protocols are available to implement the data transfer and control needs.
Inside Machines: Simpler, more advanced, cost-effective motion control techniques hone a competitive advantage in new machine designs for Sunnen, a precision original equipment manufacturer.
AKD Basic and AKD PDMM servo drives from Kollmorgen represent the range of solutions designed from simple single-axis control up to 128 axes of complex and programmable synchronized motion. AKD Basic incorporates a programmable single-axis motion controller into the existing AKD drive footprint and eliminates the need for a separate PLC. AKD PDMM combines a high-performance multi-axis motion controller, complete IEC61131-3 soft PLC, EtherCAT master, and AKD servo drive in one compact package.
High-resolution and encoderless feedback methods, along with software tools, aid the process. More user awareness and cost are needed to expand servo markets.
Next page: See additional motor, drive, and motion control graphics, explanations, and links.
(See also, previous page.)
Shiyoung Lee, Ph.D. Pennsylvania State University; Tom Lemley, General Manager, and Gene Keohane, Director of Engineering, Moog Inc.
The three-phase permanent magnet brushless dc motor inherently needs an electronic commutation circuit to drive it because it is not a self-commutating motor. It differs from the conventional brush motor, which commutates itself. This paper presents a comparison study of three widely used different commutation methods in terms of the complexity of the commutation circuit, torque ripple, and efficiency.
Just like ac motors, brushless dc (bldc) motors eliminate brush maintenance, dust, and brush-generated electromagnetic interference. In addition, bldc motors provide a much wider speed range than inverter-driven ac motors. Brushless dc motors also run more quietly than their brush-type counterparts. Moreover, brushless construction makes the motors more thermally efficient, resulting in greater power from a smaller package.
Brushless dc motors and permanent magnet synchronous ac motors (PMSM) are both commonly referred to as brushless dc (bldc) motors, but they differ in the way their stator is wound. When rotated, the bldc’s stator is wound such as to produce a trapezoidally shaped back-emf voltage, while the PMSM produces a sinusoidally shaped voltage. Bldc motors are more costly but provide better energy efficiency and performance when controlled using advanced algorithms compared to ac induction motors; they can also scale up to serve very high-power and high-speed applications.
Alternative rotor designs exist for brushless servo motors under such names as interior permanent magnet (IPM), internal PM, embedded PM, or buried PM. Whatever the naming, the idea is to locate magnets within the rotor structure to increase motor torque-speed performance and derive other benefits. An IPM rotor’s shape and salient magnetic structure favor development of a reluctance torque component and help increase flux density. This extra torque component can be harnessed to increase output, but requires a more sophisticated servo amplifier and control algorithms.
Dramatic price increases have occurred for rare-earth magnet materials used in permanent magnet motors and large electric generators. Supply shortages may be in store for all users of permanent magnets derived from those materials. The big picture is probably not as bleak as some have suggested-due to innovative design changes, alternative magnet materials, streamlining the lengthy processing of rare-earth magnets, and developing new sources. However, this challenge has no short-term solutions.
Rare-earth materials are not actually rare in the normal sense. They’re found in various parts of the world, but less widely in the concentrated form necessary to allow processing the rare-earth elements into the strongest permanent magnets available today. Of some 17 rare-earth elements, neodymium is most used, alloyed with iron and boron to form Nd-Fe-B permanent magnets.
Motion control: Traditional stepper-motor systems represent the only motion-control technology able to operate in open loop-although the addition of position feedback to enhance performance is on the rise. Simpler controls, lower cost components, and other innovations keep stepper systems competitive with servo motion systems in numerous applications.
It’s the only motion-control method able to run in open loop, without the need for position feedback. This makes stepper-motor-based systems simpler than servo motion systems, with the lower cost of step motors adding to the attraction. Coupled with other evolving design enhancements-such as hardware miniaturization and higher torque density-stepper systems stay competitive for many motion applications that require relatively low speed and position accuracy.
Most step-motor-based motion systems operate in open loop and thereby provide a low-cost solution. However, when step motors move loads in open loop, a potential loss of synchronism between commanded steps and actual steps may occur. Closed-loop control—a subset of traditional stepper motion—offers an alternative that remains cost-effective for applications requiring added reliability, safety, or product quality assurance. A feedback device or one of various indirect parameter-sensing methods "closes the loop" in these stepper systems to verify/control "missed steps," detect motor stalling, and enable greater usable torque output.
Stepping motor-based systems offer an economical method of motion control. Different types of drives (or controls) expand step-motor performance for added versatility. Relative performance of a typical standard hybrid step motor under various drive conditions is shown.
Combining ac induction motors and variable-speed drives into an integrated package became practical in the mid-1990s. Among benefits promised were the elimination of separate enclosures and long, costly cable runs between motor and drive.
Electric motors and their control electronics were not created as equals. So it’s not a coincidence that until recently they operated in separate locations. Electronic controls generally reside in safer, cooler, and more centralized enclosures, while motors face more severe conditions of temperature, humidity, vibration, dust, and washdown cleaning in the industrial world. Numerous manufacturers have brought motor-drive combination products into the market. A table of 20 manufacturers as of 2000 is included.
This class of motion control products combines motor, drive, controller, processing intelligence, feedback device, I/O points, communication, and more in one package. Some systems add all the elements to the motor, others only a few. Capabilities of the technology range up to multi-axis coordinated motion, CNC path profiles, and precise positioning of loads. The integrated approach promotes distributed control architecture.
32) Unidrive: Three AC drives in one, Control Engineering International, March 1995 (pp. 36-37)
A combination ac drive featuring open-loop (sensorless vector), closed-loop (flux vector), and brushless servo control in a single drive was introduced by Control Techniques Ltd. of the U.K. Type of control is selected by software.
A 4-axis servo drive provides sinusoidal commutation and resides inside Galil Motion Control’s DMC-4040 or DMC-4143 4-axis motion controller enclosure. Packaging of the multi-axis drives with the motion controller saves cost, space, and wiring. An 8-axis controller package version is also available.
34) The Time is Right for Small AC Drives, Oct 1993, Vol. 40, No. 11 (pp. 71-74)
Miniaturization is evident in several areas of control technology, but perhaps nowhere as dramatic as in ac variable-speed drives. Included here is an emerging sector of products known as "microinverters." While size reduction is important for these small drives (up to 5 hp or 3.7 kW), many of them approach the capability of their larger cousins.
Despite the temptation, size alone is not the right descriptor for ac "microdrives"-those ultracompact variable-speed drives that sprouted up during the first half of the 1990s. A products table samples the great variety of microdrives available at that time (20 models from 16 companies).
Today’s motion-control chips are a distinct breed, characterized by programmability via development tools, high data-sampling rates, and ability to incorporate more functions on one chip. Motion chips can combine motion and motor control with current sensing, gate driving, circuit protection, isolation functions, and network connectivity in one off-the-shelf package.
Motor control and motion control have similarities but also different objectives. Briefly stated, motor control typically needs less precise regulation of speed and torque. Many motor control applications work in speed-control mode alone. Motion control refers to a more complex system, with more accurate speed and torque control-and often includes position control, along with path planning, loop-closure times, and specific acceleration/deceleration requirements. Separate silicon devices apply to each type of control. Several suppliers and products offerings are discussed.
Safety functions combined with motion control allow simpler operations and cost savings. Previously, separate systems were needed for each of these functions. Integrated safety and motion promotes the approach that full shutdown of a machine can be avoided in some less than optimal situations. A comprehensive risk and reliability assessment is needed. Added safety functions prevent unwanted or accidental motion of motors and actuators.
The next edition ODVA’s CIP Safety Specification will include services for safe motion applications. Implementation of these services and conformance testing will allow application of motion systems with integrated safety features using Ethernet/IP and SERCOS III networks. ODVA is an international association that promotes interoperable information and communication technologies in industrial automation. CIP Safety is one of the specifications under ODVA’s Common Industrial Protocol (CIP).
– Compiled and edited by Frank J. Bartos, PE, a Control Engineering contributing content specialist; reach him at email@example.com.
See four related motor and motion control articles link at the bottom of this article.