SA Instrumentation and Control received some interesting feedback from a reader last month, which we have decided to publish:
“Thank you for Michael Brown’s control loop series. I find his writing both entertaining and enlightening and have to admit these are my favourite articles in SAI&C. I have been working with positioning systems for more than 30 years and my experiences in the industry have much in common with Michael’s. Particularly as most of the theories are applicable to motion systems, and, like Michael, I encounter similar problems when it comes to a proper understanding of them by engineers in industry.
PID filtering is the standard control method. With a suitable feedback transducer, usually an encoder mounted directly on the drive motor shaft, there is no dead time and tight control is achieved.
Some of the major pitfalls I come across include:
* Bad matching of the load and rotor inertias – practical values lie in the range of 1:1 to 10:1. Less than 1:1 means the motor is oversized and above 10:1 can result in slow response or instability.
* Lost motion (also known as hysterisis or backlash) between the motor and the feedback transducer is an automatic cause of instability.
* Likewise, lost motion in the coupling of the motor to the load gives rise to stability problems as the load inertia causes a bump every time direction is changed or speed is altered.
Because of the rapid dynamics of motion systems, theoretical tuning methods are easily demonstrated and work well; Bode plots are particularly helpful. As a result of this, correctly sized and installed systems should be easy to tune.
Unfortunately the quantities involved are not intuitive and normally require proper calculation. This gives rise to heated arguments with the unenlightened:
* “Inertia is a mythical quantity. I can push the load with one finger.”
* “Any system with a feedback transducer can be controlled.”
* “Why is it unstable? All that is needed is to switch off the motor at the correct point.”
I could go on, but I will not.
Best Regards,
Glyn Craig, Techlyn”
Andrew [Ashton] is going to investigate the possibility of Glyn Craig writing a series of articles that will alternate on a monthly basis with Mike Brown’s Control Loop.
Thank you to Mike for making us aware of this possibility and opening the door for discussion.
This issue
Schneider has added Ethernet connectivity to its range of TeSys T intelligent motor automation products. In this month’s cover story we examine the product’s underlying technologies and the new benefits associated with the addition of the Ethernet capability.
We have interesting submissions from experts at both National Instruments and Emerson. National Instruments has sent us a very good overview of Green Engineering and the way its LabVIEW software is being used, together with PACs, as an example of how Green Engineering has improved old processes and technologies at Nucor Steel. Emerson, in the meanwhile, continues to expand on the benefits of wireless networks and cites some real world success stories.
Our feature on industrial networking has attracted its usual volume of attention with suppliers like Beckhoff, Phoenix Contact and Siemens, along with many others, keen to showcase the latest product offerings.
And then, something I always look forward to at the beginning of the year: Jim Pinto’s prognostications on the state of the market and the future of technology. In his column this month he punts his top-5 technology trends for 2009 in the context of the current global market contraction.
I hope you enjoy the reading.
Steven Meyer, Editor
SA Instrumentation & Control
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