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Guest DASH

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Hi Dash,


PID control (sometimes also known as '3 term control') provides improvements over simple proportional control.


In a simple system whereby you want to control, say, water pressure using a variable speed drive you would have a pressure transducer providing feedback, and a 'set point' where the desired pressure is entered / set.


Simple proportional control compares the Set Point to the Feedback and generates an Error wich varies the speed of the pump to suit. This simple configuration suits a lot of applications as is, but has some limitations including never being able to exactly match the Feedback to the Set Point and potentially suffering from instability.


Where the requirement is to more accurately control the pressure under perhaps widely and rapidly changing flow requirements, simple proportional control is not sufficient.


Enter PID control.


PID control provides;


'P' for Proportion (as above)

'I' for integral

'D' for differential


The 'I' term 'integrates the error over time, producing an output to suit, thereby progressively matching the Feedback to the Set Point.


The 'D' term responds to sudden changes in either the Feedback (ie flow requirement changes rapidly) or the Set Point. This differential (or rate of change) funtion provides dynamic response minimising sudden errors caused by 'bump disturbances'.


The above is admittedly an overly simplified description, many sources of info on PID control is available, most fairly complex.


BTW, PID control is often built in to VSDs these days, and many PLC manufactures offer a ready to go routine to avoid you having to develop your own.





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  • 3 years later...

Keep an eye on the web page at http://www.LMPhotonics.com/pid.htm This page is currently under construction, but will cover many aspects of PID control.

If you have suggestions, let me know.

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The easiest example for most people to understand and experience PID control is the Cruise Control in your car (actually, it's just PI control, we take over the Derivative function).


Once you set the cruise control for a particular speed, that became your SETPOINT.


On flat ground, nothing much really happens, but as you go up a hill, the car begins to slow down. That drop in speed is your ERROR, recorded as an increase in load on the engine.


The PI controller responds by increasing the throttle (if you have ever rested your foot on the gas pedal, you can feel it get sucked down). How much throttle is applied is PROPORTIONAL to the increased load on the engine, so on a steeper hill where the engine is loaded down more, the throttle increase is proportionally higher to compensate.


If the error persists for too long, in other words the proportional response is not enough, the throttle is increased even more because the error was INTEGRATED into a time allotment and deemed to be excessive beyond the proportional response. The integral function also serves to release the throttle more slowly so that the car is not jerking and lurching as you drive.


If the PI controller is constantly going up and down too much, we (the driver) may DERIVE that the settings need to be adjusted to smooth out the response, such as by holding the gas pedal to the floor, knowing that there is no point in relaxing yet because we see that the hill is too long.

"He's not dead, he's just pinin' for the fjords!"
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That is a good example.

Another example that I use when I am describing it to people is steering the car. We provide the PID control rather than some electronics, but we compesate for any sudden changes in direction - Derivative control and we keep the vehical centered in the lane, - centre position for the steering wheel - Integral control, how much we turn the wheel - proportional control. If we overcompensate, (Too much D) then we swerve from side to side. - watch a new driver. It takes a bit of practice for hem to get their P, I and D terms tuned, and then it becomes automatic. An experienced driver quickly adjusts to the amount he/she has to turn the wheel (P control) on a new or different vehical.


Most of our activities can be analysed in terms of PID control!


Best regards,

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