Pid Implementation

[lchamarthi 2]

Hello, I am new to the PID & I would like to take suggestion in the implementation My temperature range: 0----250 So Set Value will be in this range given from HMI. Module resolution is from 0----0FA0 ( which is equivalent to 0----10v) Generally I planned to implement like this without using PID instruction If Suppose SV=250 PV=100 difference=SV-PV=+150 This Maximum difference will be 250 for 250 Centigrade-------0FA0 (10V output) So 150.. -------= (150 *10)/250 =6V (interms of volts) Gas valve will open proportionately 0--10v. Can I use SCL instruction for this manipulation? The above calucation will be going on as long as difference carries positive sign. If it is Negative sign, immediately we will stop/close (0 Volts output) the Gas valve & will continue till reaches Zero. It is the general idea but i don't know what kind of problems i will face with this? Also I would like to know what are the advantages of PID over above? I really looking for the earliest possible reply Thanks in Advance Lakshmi Edited 16 Mar 2006 by lchamarthi

[edpuente 0]

HI: If you are controlling temperature, why don´t you use a temp controller card, it easier. Anyway there is the PID instruction. In the programming manual you can find what parameters to use.... What PLC are you using? Regards Edgar

[lchamarthi 2]

Hello edpuente, Thank you for the suggestion, We don't need that many (4 or 2 loop) inputs.We have only one temperature input that to accurcy in controlling is not crucial. (which is not precised). Moreover seperate TCXXX card will cost more. I would like to know comments on my approach without using PID instruction. Even I couldn't able to understand the PID instruction what they are saying. Could anybody give me hand? Thanks in advance Lakshmi

[Sleepy Wombat 27]

One- You could look at sliding mode control...... Two - If you don;t care about accuracy then simple on off control would probably suffice (be careful of the thermal properties of the substance you are heating so you do not over shoot to much) On off control in its simpliest form would turn the digital output on until setpoint is reach then wait for the temp to drop back down say a couple of degrees then turn output on again. The difference is called the Hysteris..

[lchamarthi 2]

Hi Sleepy Wombat, Thanks for clarifying the difference between On-Off control & continious control application. I have the sufficient Hot airflow (we generate) through a valve/damper onto a material. Obviously Temp depends on the amount of air that we allowed from the valve/damper. There is no subtance for heat production. Just my job is to control only the percentage opening of Valve depends on the error. This valve is Analog output. I hope u understood the application. Is my approach without PID works? If PID is required, could anybody help me in understanding the PID Parameters that I need to take care. waiting for suggestion Regards Lakshmi

[crb8501 0]

Lakshmi, ON/OFF control will work and will be similar to the temperature control in a home. The downside of ON/OFF control is that the valve will always be "hunting" or, if you prefer, oscillating around the setpoint. If this is acceptable and the valve maintenance issue (from moving often) is not a problem, then I think your plan will work. You have described (in your original post) what is called a proportional only control (P controller or ON/OFF control ). If you choose to use the PID algorithm and the temperature response is fairly fast, then a PI controller (set derivative = 0) will work well. Does the setpoint ever change or will it be the same always ? regards, randy

[lchamarthi 2]

Really I welcome all of your valuable suggestions. So now I convinced to implement PID for my application. But confusing how to get the Constants P =?, I =? & D= ? values to get good response. Is it Trial & Error method? Could anybody help me? Lakshmi Edited 20 Mar 2006 by lchamarthi

[PdL 71]

Provide the PID instruction with a setpoint value and a process value and set it in auto tune mode. It will control the valve until the temp overshoots (positive and negative) get smaller and smaller. Once finsihed, you can store the parameters and perhaps adjust them a bit yourself, but you have a good start indication of the values.

[lchamarthi 2]

Thanks PdL, Auto Tuning function very useful.I refered Instruction manual & I could able to find one Auto tune Instruction PIDAT(191) for CJ1M-CPU13. Here in this, C, C+1,.C+2...C+8 are PID parameters. The settings of these parameters i really confusing how to program for autotune function. Please send me sample PID code if anybody have?/Help me understanding PID parameters settings for Auto tune functions. Lakshmi

[PdL 71]

From the top of my head, put #8000 ( = bit 15 > AT command ) on parameter word C+9 (command word). Activate PIDAT, when finished with autotuning C+9 bit 15 is reset to 0. Now C+1 to C+3 hold the tuned P, I, and D constants. Don't have any program examples at hand here... Hope this helps!

[lchamarthi 2]

PdL, Thanks for the quick response,Information provided & Now I can go ahead based on your inputs. Lakshmi

[PdL 71]

Here's some additional reading on PID basics.