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PID control

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Hello, I want to know if there is any good method to do PID control without using loop controller. I want to control temperature and pressure PID control. Because loop controller is expensive, is there any other alternative method? Ting

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A loop controller is expensive compared to what ? What type of loop controllers are you looking out.... What is required to be controlled... Ie you say pressure etc ... Is that with a modulating valve ? On off control, or analogue modulation etc....what are the devices... How much money or budget have you got to spend on the process for the control ?

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I am using Omron's PLC unit, and the LCB loop controller unit's price is at least comparative to the CPU itself. And if a simple pressure control by tuning valve is enough, the controller is not necessary, I think. Because at least a direct P, I, and D calculation can make the PID control possible. Ting

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There is a regular PID instruction, and a PID with autotune (PIDAT), check the manuals, but you still need an analog in/out module. Geir

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I would agree with Geir The PIDAT instruction should be a very good fit for a pressure control application, if the control loop is not exceptionally complicated. The LCU for the CJ is a good fit for more complex process loops such as feed forward, cascade control, etc. PMCR

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Thank you very much. You reassure me that LCU is for some complicated scenario, and a simple PID control loop would be done by PID instruction. My control loop is a single simple loop - tune the valve according to the difference of pressure's setting value and current value. According to your advice, the PID instruction is just for this, is it? Ting

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Yes, the PID instruction is for exactly this. There are 2 different instructions: PID and PIDAT. PID is the older version. PIDAT is the newer version, and does everything that PID does, and adds Auto Tuning (that's the 'AT' part of PIDAT). I would strongly suggest PIDAT. When you go to implement PIDAT, give us some information about your control loop, and we can help you with the handful of parameters needed to set it up. It can be a little confusing at first, but once you have it setup, it works great! The information that we would need is: What is the numeric range of the input data. ie 0 - 2000, 0- 4000, 0 - 12000, etc. This would be the data coming from the input device. What do you want the range of the output to be? 0 - 2000, 0 - 4000, etc. This would be the data going to the output (if it is an analog output). Is this a Forward Loop (meaning that you want the output to go UP as the input goes UP), or Reverse (meaning that you want the output to go DOWN as the input goes UP).

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Hi All; I always found the PID instructions tedious to use so I wrapped it inside a function block. While I was at it I added features like Manual/Auto, Loop Enable, Bumpless transfer. Now its a real loop controller. I think the function block feature is the single best thing to happen in the Omron world in the last 25 years. Jim

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That's great! I agree FBs are very useful. It begs the question: can you share your function block? I have created one that takes the output of PID and time proportions it for a digital output. You can pass the output of the PID function, the output resolution of the PID instruction, and the time base (control period) of the digital output into the FB, and you get a digital signal and a MV for monitoring.

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