The Question:
Must We Abandon Magnetic Encoders?Weve used 12-bit magnetic, absolute rotary encoders with pretty good success. We like their durability. We have the opportunity to sell our equipment to customers in more difficult operating environments, and where there is a lot more electrical noise than weve been used to. Were told we are asking for trouble by expecting the magnetic encoders to perform dependably and need to switch to optical encoders and fiberoptics. Wed like to hear pros and cons from experienced users.
---From February 2005 Control Design
The Answers:
Theyre Tough EnoughMagnetic encoders can handle environments that will fail opticals in short order (fluid immersion, dust etc.) Magnetics are immune to those external effects, and are quite robust. New technologies today have brought about the integrated Hall Effect IC that has 1,024 counts/rev. in absolute or incremental modes.
Paul Moffatt, Quincy Compressors, http://www.quincycompressors.com/
Stay Attracted to Magnetics
You are worried about proper functioning of the chip under electrically noisy environment. Are you more worried about EMC noise or about magnetic fields generated by currents in the vicinity of the chip?
It would be interesting to know which chip you were using that is causing the trouble. Our magnetic rotary encoder is, due to the technology used, pretty insensitive to external magnetic fields. It has all sensors on-chip, and no external components except buffer caps are required, which is beneficial to EMC noise as well.
Josef Janisch, product manager, sensors and automation, Austriamicrosystems, www.austriamicrosystems.com
Get Application-Specific
In general, magnetic encoders are better for harsh environments, with higher shock and vibration ratings, higher temperature and humidity ratings, etc. Things that break (shock) and blind (humidity) optical encoders, have little effect on magnetic ones. However, magnetic encoders are susceptible to strong external magnetic fields. The magnetic fields found on motors and brakes, however, are not usually strong enough to affect magnetic encoders, as the magnets typically have cans over them internally to protect them. However, optical encoders generally provide higher accuracy and repeatability, and definitely have the potential for higher resolution. The relative cost will depend on whether one is talking about single-turn or multi-turn encoders. So, the lowest-cost design that will meet the performance requirements of the application is the best design. Sometimes that will be optical encoders, and sometimes that will be magnetic. Keep in mind that many multi-turn encoders are optical on the single-turn stage, but use magnets on the multi-turn stages.
When it comes to magnetic vs. optical encoder reliability, there are no absolutes (pun intended). The nature of the application will determine what aspect of the encoder is most likely to be subject to failure, and therefore what aspect of the encoder design is most important. If its shaft load, then the bearings need to be considered, and optical vs. magnetic is generally not even an issue. If its temperature, then one might also need to consider the code disk material--both a glass optical encoder or magnetic encoder might work fine, but plastic disk optical encoder wouldnt be suitable. If its humidity and possible condensation (e.g., on the code disk or magnets), magnets may have an edge over optics (Think of wearing glasses and walking into a warm building after being outside in the cold for a long time--glasses fog up; the same thing can happen to an optical encoder). If the application is subject to shock and vibration, then magnetic encoders are usually better than optical disks that may break.
Fiberoptic-based encoders are relatively new and still somewhat expensive. They take the electronics out of the encoder and move them to the control side, where environmental conditions are more benign. So for applications that are harmful to electronics, they may offer some advantages. However, as with anything else, there will be application for which they are preferred, and others for which they are not.
Scott Hewitt, manager, Sick Stegmann Inc., www.stegmann.com
Augusts Problem:
What Are the Limits to Embedded XP?
For some time now, we have used a VxWorks real-time kernel with Windows NT to control five axes of motion. The specs tell us we can use XP Embedded without a third-party kernel, and, as a nice bonus, significantly reduce the memory-gluttony of NT. What does real-life performance experience tell us about whether this is a good idea?
Send us your comments, suggestions, or solutions for these problems. Well present them in the August 2005 issue and on ControlDesign.com. Send visuals, tooa sketch is fine. E-mail us at [email protected] or mail to The Answer to Your Problems, CONTROL DESIGN, 555 W. Pierce Rd., Suite 301, Itasca, IL 60143. Please include your company, location, and title in the response.
Have a problem youd like to pose to the readers? Send it along, too.
