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METSYSTEM

Cascading PID

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Good day, I have been scratching my head for a while now and hope that someone can help me with a control function. The Process Variable is a single Analogue input 4-20mA (Dissolved Oxygen) The Control Variable is 3x Analogue outputs 4-20mA (Air blower speed) The setpoint is a fixed integer from an HMI. I have been told that cascading PID loops is a good way of doing this due to the huge change in the process varable throughout the day. Is this a good method, and if so are then any usefull tips for it with regards to SLC500 logic. Many thanks in advance Alex

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Some reading material for you. http://www.controlguru.com/2007/070607.html http://www.google.com/#hl=en&sclient=psy-ab&q=control+guru+cascade&oq=control+guru&gs_l=hp.1.2.0l2j0i30l2.2417.7198.1.9870.5.4.1.0.0.0.656.1797.3-2j1j1.4.0...0.0...1c.M2uEXNfJwQE&pbx=1&bav=on.2,or.r_gc.r_pw.r_qf.&fp=b752d12a099bb300&biw=1270&bih=687

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I don't see enough IO for a cascade loop. I typically try to avoid using them when I can but in the right application they work well. With the right IO your application may be a good fit. For cascade control you need a Primary Loop and a Secondary Loop. In your case the PV for the Primary is the Dissolved Oxygen value. The output from the primary loop is the setpoint for the secondary loop. If I am understanding the system correctly the secondary loop would need to be a flow control loop whose output controls the speed of the blower. The missing variable is the flow value for the secondary loop PV. The terms Primary / Secondary are often interchanged with Outer / Inner. Switch out of cascade and tune the Secondary / Inner / Flow loop so that setpoint changes are smooth with little to no oscillation. Once that's done you can tune the outer loop.

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Guys thanks for your help, It would appear I was barking up the wrong tree with cascading loops. What I should do is use one PID controller. Start the first blower, if that blower cant maintain then drop it down to the low speed setpoint with the PID in manual, then bring the second blower in with the first and put th PID back to auto so they ramp together. Apply the same with the third blower as well. I will see what I can do Cheers

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