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Control Loops

11/29/2011 12:40 PM

friends why we r using cascade control and how to choose between cascade and and simple conrol loop

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#1

Re: Control Loops

11/29/2011 12:48 PM

Given that control is about getting what is wanted from what is available, how about a description of both of these for the forum to work on?

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#2

Re: Control Loops

11/29/2011 4:05 PM

Process control is a complex area. Generally there is a controller and something that is controlled (ie. a flow analyser and a pump, where the pump is controlled to the flow measured by the flow analyser). This will typically be controlled by either Proportional, Integral or derivative control loop (PID) or a combination of the three.

In my experiance, I 've seen many proportional, prop. & int., and full PID control loops. Most often a PI control loop is able to control things well enough for my needs.

But when a PID loop becomes too difficult to tune, I will program in a cascade loop. Often this is because there is an excessively large lag, and / or some other complex process between the analyser and the device.

Some examples of where I've used this ...

Process water treatment plant consisting of ... Feed Pump --> Feed Tank --> MicroFiltration --> Buffer Tank --> Reverse Osmosis Plant --> Product Water Tank. The client wanted the level in the Product Water Tank to control the speed of the Feed Water pump. Note: the client is always right & also an idiot. Because of the complexity of the microfiltration plant (start/stop/backwash operation) & the constant speed operation of the RO, we could only get this to work by using a cascade control.

Another example is dosing chlorine into water entering a very large storage tank (several days residence time). There were chlorine analysers on the inlet to the tank, and outlet of the tank. The chlorine dosing was controlled under PID control on the inlet to the tank to a setpoint that was adjusted by cascade loop to the chlorine level in outlet of the tank. Outcome was very good long term control.

Hope this helps.

Anthony

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#3
In reply to #2

Re: Control Loops

12/05/2011 3:15 PM

A simple form of cascade control would be an oil fired furnace where the temperature controller sets the setpoint of the oil feed controller be it a VSD pump or control valve etc.

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