To PID or not to PID

proclaimed to the world:


True. Did I infer otherwise? You can destabilize a plant in an open loop system if the final control elements are sticking. The same is true in a closed system too, more so in fact.

An interesting way of looking at it. I would put it thus: A close loop system's focus is on the value of the controlled process directly, while an open loop infers the process. (excuse the excessive use of the word "infer")

I've seen this before and am trying to remember where. Perhaps it was being used to auto syncro generators in a grid. A similar system I built for a looped chain conveyor system where the load was shared between motors positioned throughout the loop. The chain slack was the problem. For some reason I do a lot of correcting designs after they have been installed in plants and they do not work properly. This system had VSDs for each motor with a common speed signal. The original designer had supposed that the motors would run at the same speed, sharing the load. Actually the motor with the heavier load had more slippage and ended up taking all of the load and all the slack ended up in one take up bin. The conveyor still worked but was dragging all of the chain around a turn which wore the guides. If the designers had specified synchronous motors it might have worked. I changed it so that the feedback was motor load, not speed and put sensors in the chain slack bins to trim the loads as necessary to maintain slack in each bin. Each motor load ended up being trimmed to take just the load of moving the chain for the section it handled. Crude, but sometimes crude works the best.
Be well,
HoP
The preceding message represents personal opinions and/or advice that may prove incorrect or harmful. But then maybe not. Feel free to disregard.
------- Words have no Warranty ------ ------- No View without Merit ------
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Tim Wescott wrote:

In materials lab in college, we had a screw press that measured applied force with a platform scale. The motor-driven screw applied strain at a constant rate, and we were supposed to record the force by balancing and reading the scale every 20 seconds. The lab instructor insisted that accurate readings were possible (though he never offered to demonstrate). I rigged a sensor to measure the scale arm's position, a solenoid to hold it centered, and a strip-chart recorder with timing marks to record the current needed to do that. The servo amplifier I built wasn't in the books, but I needed it to deal with the prominent resonances in the mechanical system. We had great lab reports!
There's a name for that design philosophy. "null measurement", I think.
Jerry
--
Engineering is the art of making what you want from things you can get.

  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Francis wrote:

I can see how my remark might seem offensive; it was certainly brusque. I apologize for any offense.
NNExE is "north-northeast by east", a compass direction. My point was that in a well designed system, one measures the result. Assuming, without a confirming measurement, that the command is carried out is at best unwise. The constant-displacement pump is indeed a good source of flow information _provided_ that there is an accurate way to measure the pump's actual speed and be assured that its seals don't leak.
Jerry
--
Engineering is the art of making what you want from things you can get.

  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wot he said
and beware of 'pump slippage'
in the specs there will be an allowable pressure gain for no slip operation
if you exceed this you will need to measure the output to see xxactly what is being out put
otherwise volume and pump speed are reasonably proportional
I would process in the sensor as you suggest and use PD to drive pump motor controller
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.