Motion control standards have a way to go
'Lack of standards" is a complaint often heard in the motion-control arena. Given the kaleidoscope of technologies and products comprising this sector, it's not a surprising comment. However, if one includes proprietary and de facto standards, plus various communication networks used in motion control, then probably there are too many "standards" out there.
For an expanded version of this article, click here .
'Lack of standards' is a complaint often heard in the motion-control arena. Given the kaleidoscope of technologies and products comprising this sector, it's not a surprising comment. However, if one includes proprietary and de facto standards, plus various communication networks used in motion control, then probably there are too many 'standards' out there.
Standards and application guidance for motion control come from various sources. Communication methods, although vital, are not exclusive to this arena. One core area is the motion system's overall architecture, where centralized and distributed are the two main directions. Distributed architectures have drawn much attention lately, due to advances in the digital world that allow local embedding of intelligence and bring added benefits.
PLCopen (Zaltbommel, Netherlands), one of several vendor- and product-independent organizations active in standards, recently expanded its scope to motion control. Its well-known IEC 61131-3 standard for PLC programming languages also is used to program motion systems, as reported in CE 's June 2002 cover article on 'Electric Servos.' PLCopen's new activity is the specification of 'Function Blocks for Motion Control,' released as version 1.0 in November 2001. A library of function blocks based on IEC 61131-3 covers single-axis as well as coordinated multi-axis control. Function blocks work like flowcharts, helping to develop control logic. The method seeks to standardize interfaces among different motion control approaches and promotes more reuse of application software-besides specifying programming languages.
Servos, steppers, drives
Motors receive reasonable standards attention. For example NEMA Standard Publication ICS 16-2001, 'Industrial Control and Systems, Motion/Position Control Motors, Controls and Feedback Devices,' is an extensive look at these three 'interconnected components' of a motion system. In particular, NEMA (National Electrical Manufacturers Association, Rosslyn, Va.) ICS 16 addresses requirements for 'control motors' by various categories: all servo motors, all stepping motors, brush-type servo, and brushless servo. Controls and position/velocity feedback devices (rotary encoders and resolvers) are also covered.
Another NEMA source, 'Application Guide for AC Adjustable Speed Drive Systems, aids the selection of drives rated up to 600V.' Here, 'systems' refers to three-phase induction motors; voltage-source, pulse-width modulated adjustable-frequency controls; and associated components. The Guide addresses common issues to consider when selecting drive system components, and when installing or applying an entire drive system.
The International Electrotechnical Commission (IEC, Geneva, Switzerland) also issues standards on adjustable-speed electrical power drive systems and various motors, among others.
One communication standard developed specifically for motion control is SERCOS (SErial Realtime COmmunication System), which works as an interface among motion controllers, digital drives, and other system components. SERCOS claims sole international recognition via its designation as IEC Std. 6149. SERCOS is based on fiber-optic ring architecture and offers message transmission rates up to 16 Mbit/sec.
Various other communication methods see application in motion control-for example, IEEE Std. 1394, High-Speed Serial Bus (FireWire), developed by the Institute of Electrical and Electronics Engineers (New York, N.Y.). Still others fit into a de facto or proprietary 'standards' category.
'Motion control Standards,' a publication of NIST (National Institute of Standards and Technology, Gaithersburg, Md.), provides an excellent overview survey of the 'standards landscape'-along with a summary of numerous communication standards in use.
Also notable is a set of motion standards from the Advanced Photon Source (APS) research facility of Argonne National Laboratory (near Chicago, Ill.). Developed in-house, 'APS standards' provide a consistent interface to motors (typically of the stepper variety), allowing scientists to relocate their equipment from one experiment area to another; also to swap similar components, without special cables, connectors, or lengthy setups. These standards have proven useful to collaborative teams of researchers working at stations located all around a ring of approximately 1-km circumference that produces high energy x-rays.
Recognized standards for motion control are evolving, albeit slowly. The pace of progress is affected by technological complexity and manufacturers' desire to protect their investments.
Frank J. Bartos, executive editor email@example.com
Typical Standards Sources
SERCOS N.A. .
In some cases, downloading of publications is free.