# Magmeter Maintenance, Feed-Forward Variations; Robots

## What Maintenance Is Required for a Magmeter? And What's the Difference Between "Positional Feed-Forward" and "Incremental Feed-Forward" in PID Control?

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Dick Furness
rydalhall46@aol.com

A: "Empty pipe detection" is provided in most magnetic flowmeters with cut-off to force the flow reading to zero at no flow to eliminate false counts in the totalizer. In some cases they serve to prevent the coil from overheating.

Magnetic flowmeters need calibration, but using smart meter verification, it is possible means to determine if calibration is really required or if it can wait a few months.

Jonas Berge
jberge@singnet.com.sg

Q: Please can you explain the difference between "positional feed-forward" and "incremental feed-forward" in PID control? What are the pros and cons of these feed-forward algorithms. I understand that positional feed- forward is adding predefined value to the PID output. In what situations can we use with lead lag or without lead lag? What is the "incremental feed forward."

A: Incremental feed-forward is essentially an impulse function that is applied to the manipulated variable only on a change in the load variable--it has no steady-state component. To minimize integrated error following a load change, the feed-forward signal must be applied in the steady state, that is, positionally. It can either be additive with a fixed gain, as typical when controlling liquid level, or multiplicative, where the gain of the multiplier is set by the output of the feedback controller, used when controlling temperature and composition.

Avoid incremental algorithms at all costs—they float and will cause all kinds of trouble and confusion.

Greg Shinskey
shinskey@metrocast.net

A: Google tells me from the list of hits that "positional feed-forward" is associated with robots and "incremental feed-forward" is associated with neural networks.

Where they are explained there is a forest of complex math that I am unable to penetrate. Never heard of them in process control. You should ask your vendor.

Bill Hawkins
bill@iaxs.net

Q: I see that in the proposed index of the fifth edition of your handbook, you plan to have a chapter on robots. Is it because that technology utilizes the principles that were developed by the process control industry? In other words, did they learn from us in the area of modeling?

Harry Crowney
HCrowney@aol.com

A: The short answer is yes; the long answer would take volumes. What I plan to cover in the fifth edition of Volume 1 is only the measurement aspects, not the control algorithms or the manipulated variables. They will come in the second and third volumes. The sensors of this industry are interesting because they are drastically less expensive, lighter and smaller than in the traditional industries, while depending totally on wireless technology. You can see an example below: www.ted.com/talks/a_robot_that_flies_like_a_bird.html.

Béla Lipták
liptakbela@aol.com

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