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Working with PID Control (S71214) [Gradient]

Created by: Paras at: 12/13/2014 10:42 AM (7 Replies)

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12/13/2014 10:42 AM	Rate (0)
Paras Posts: 21 Rating: (0)	Hello ! I'm using S71200 CPU 1214 and using PID Control on inlet of a tank through a proportional (4-20 mA) pneumatic valve. I've to control the flow of liquid into the tank with respect to time i.e. e.g. 15 Litre liquid in 3 minutes. for this i've to open and close the valve according to the time provided i.e. the provided liquid must finish in the provided time. and i have to work on gradient [linear graph between quantity(litres) and time] (Please see attachment). My quries are : 1. Should i use PID , PI or PD ??? 2. Should i use PID 3_Step or PID Compact ??? 3. How do i work on this gradient ??? Attachment Gradient.jpg (313 Downloads)
Paras Posts: 21 Rating: (0)
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12/13/2014 11:26 AM	Rate (1)
William_B Posts: 5225 Rating: (1195)	Paras Hello ! I'm using S71200 CPU 1214 and using PID Control on inlet of a tank through a proportional (4-20 mA) pneumatic valve. I've to control the flow of liquid into the tank with respect to time i.e. e.g. 15 Litre liquid in 3 minutes. for this i've to open and close the valve according to the time provided i.e. the provided liquid must finish in the provided time. and i have to work on gradient [linear graph between quantity(litres) and time] (Please see attachment). My quries are : 1. Should i use PID , PI or PD ??? 2. Should i use PID 3_Step or PID Compact ??? 3. How do i work on this gradient ??? Hi Paras, Why ask your question in "Meet & Talk"? It is better suited in Step7 (TIA). How much do you know about PID / PI loops in general? I will give you a hint to question 3. Whichever control loop you use (PID / PI are not the only options), you can dynamically change the setpoint value. So steadily over time you increase your request for the volume. Try this [LINK]. Also try "ramp Step7" as search term in this forum. Greetings. w
William_B Posts: 5225 Rating: (1195)
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This contribution was helpful to 1 thankful Users Paras

12/13/2014 1:34 PM	Rate (1)
hdhosseini Diamond Expert Joined: 1/28/2009 Last visit: 10/26/2024 Posts: 6852 Rating: (1365)	Paras Hello ! I'm using S71200 CPU 1214 and using PID Control on inlet of a tank through a proportional (4-20 mA) pneumatic valve. I've to control the flow of liquid into the tank with respect to time i.e. e.g. 15 Litre liquid in 3 minutes. for this i've to open and close the valve according to the time provided i.e. the provided liquid must finish in the provided time. and i have to work on gradient [linear graph between quantity(litres) and time] (Please see attachment). My quries are : 1. Should i use PID , PI or PD ??? 2. Should i use PID 3_Step or PID Compact ??? 3. How do i work on this gradient ??? New points regarding your question: You can try PI or PID to see which suits your application but my initial preference for test would be PI controller for a ramp response. If both inputs and outputs are continuous, you can use PID Compact. I do agree to use the same function proposed by "William B." for setpoint while I think same scenario should be considered for process value.Before the real test, check in the simulation if the gradient of process input will help a better regulation or itself without further processing.In each cycle ((New_value- old value)/Time_slice) will be the slope of tangent line to the process curve. And new point, if the PID follows the set value within the time span (after a short transient response), what should happen after the 15 minute passes?In this case, I suppose the volume transfered and the control valve can be closed .We need more clarifications for better suggestions. I hope this helps, Hamid Hosseini
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This contribution was helpful to 2 thankful Users IBN-Service Paras

12/13/2014 2:28 PM	Rate (1)
IBN-Service Posts: 8946 Rating: (999)	Paras Hello ! ...15 Litre liquid in 3 minutes. ... for this i've to open and close the valve according to the time provided .... [/Quote] Is your valve just on/off or do you have a proportional valve...? For me, it only makes sense with a proportional valve. So you must consider that you do not control the amount or thetime but you must control the FLOW of liquid! P, PI, PID is possible. For first setup, use P(I), you may add D - structure later to make the flow regulation more "smoothly".
IBN-Service Posts: 8946 Rating: (999)
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This contribution was helpful to 2 thankful Users hdhosseini Paras

12/14/2014 5:48 AM	Rate (1)
William_B Posts: 5225 Rating: (1195)	Paras, What you are asking (fill tank within time) is strange. You are sure that the level at time is what is required? Two gentlemen (not me) that are authorities on the subject have put down their thoughts for you. Please take their words seriously. Please allow me to put down some alternate thoughts.. but ignore it until you see that your control needs more. IBN-Service is hinting on the selection of the actuators (components that are controlled). The selection of the sensors and actuators as well as the properties of the liquid and properties of the system, plays a very big part in the control - just as much as the control philosophy you are using. Talking about control.. This system could be broken down into two parts: flow control and level control. From empty to almost full the system can handle flow control with corrections made by looking at the level. When the level is almost at target, the level control can take over to finalise the level. The above introduces a hybrid control between rule based and process controls. Complexities are introduced that is better avoided. Sometimes you do not have a choice. Filling a container with liquid can make it difficult to measure the level. The sensor element may give a variation in measurement that creates problems. But as the container gets more full, the less the measurement is disturbed. If you have a mathematical model of valve opening percentage to liquid flow, this might help in the first stages of filling. Be careful as in the dynamic of the system the valve opening to flow may not be linear, the upstream pressure might change and position of the valve in the tank may have an influence. The best would be to have a type of mathematical model as feedforward to the control loop that handles the flow control. A flow measurement can be used as feedback.. avoiding the level feedback in the first stage of the control. Thereafter the level control loop can take over. Everything that has been said by any of us is used at your own risk. None of us takes any responsibility for the correct interpretation and implementation of what was suggested. Please make sure you do your research and test your coding. Make (face-to-face) an experienced field technician or engineer part of your thought process. Good luck. w
William_B Posts: 5225 Rating: (1195)
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This contribution was helpful to 1 thankful Users Paras

12/14/2014 9:59 AM	Rate (1)
IBN-Service Posts: 8946 Rating: (999)	William B. Paras, What you are asking (fill tank within time) is strange. You are sure that the level at time is what is required? Strange? I don't think ist strange... For me, it sounds like a typical dosing - application. Mostly done with linear pumps / syringebut can be done as well with PID controlled pump.
IBN-Service Posts: 8946 Rating: (999)
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12/14/2014 10:29 AM	Rate (0)
William_B Posts: 5225 Rating: (1195)	IBN-Service William B. Paras, What you are asking (fill tank within time) is strange. You are sure that the level at time is what is required? Strange? I don't think ist strange... For me, it sounds like a typical dosing - application. Mostly done with linear pumps / syringebut can be done as well with PID controlled pump. Thank you IBN-Service for clearing that up for me. If you could be so kind to help with another question: Is a reactant always added in a linear way (volume increasing linearly)?
William_B Posts: 5225 Rating: (1195)
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12/14/2014 10:37 AM	Rate (0)
Paras Posts: 21 Rating: (0)	Thanks all for all your help ! I've attached a pic made by me of the problem i've asked for. As you can see the mixing/measuring tank is filled with a quantity of water [Provided by operator (HMI)] which is controlled by a level sensor and when its done, the operator mixes some chemicals or colour may be and starts the dosing process after providing the Time of dosing from HMI. So that all the amount of mixture is dosed evenly into the Main tank (Linear graph as shown in my first post). I need to work on PWM on this valve because this valve is not proportional but a complete shut off valve. Kindly help me out as i've provided complete details. Thanks in advance. Attachment plant design.jpg (260 Downloads)
Paras Posts: 21 Rating: (0)
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