about PID design

P

Thread Starter

Peter Wong

I am a college student and i would like to ask you about pid (proportional integral derivative)controller. i had been assigned a project title related to PID controller. my project is to design a hardware and software (c programming) PID controller to control the fuel rate in boiler.
i am going to use temperature as plant. when the temperature is exceed setpoint then feedback to pid controller and thus cut off voltage supply to electrical valve(on off valve). when the temperature is lower then turn on the valve.
but, i am wondering which type of pid controller i should use since i can afford for the lowest price pid controller. and which brand are famous in Asia area? siemens? farnell? please recommend. thanks.
besides, the design of software of PID controller also make me worry since i am poor in programming C. and how can we program into pid controller? using parallel port?
i would to as also how to do the simulation using matlab? what should i concern more based on my project? and is it any method to know the result or the improvement through PID controller? what i meant is how to analyze the process. do i need to know any specific value of the valve, current value, temperaure measurement before plot graph?
has anyone seen the project before in any website? please be kind to tell me. thanks in advance.

sincerely,
peter wong
 
F

Frank Rytkonen

Peter:
You're missing the whole point of PID control! You say the following:

> i am going to use temperature as plant. when the temperature is exceed setpoint then feedback to pid controller and thus cut off voltage supply to electrical valve(on off valve). when the temperature is lower then turn on the valve.

The scheme you describe is not PID control, it is on-off control. Anyone can set a temperature switch to operate an on-off solenoid valve.
PID control would monitor the analog value of temperature, compare it to a setpoint temperature to determine an error, then process the error using PID algorithms to determine the value of an analog signal to be sent to the final element controller - in your case an electrical valve (but not an on-off valve) that would move to adjust fuel flow to the proper value to maintain burner temperature.
You need a better understanding of PID control. There are some good tutorials at www.controleng.com. under the TUTORIALS menu option on the left side of the page. Look there to get the basics, then start to worry about the details such as hardware, software, etc.
Best of luck!
 
B
You should use a cascade loop with a fair amount of safety overrides as shown in Figure 8.5i on page 1046 of the second (process control) volume of my Instrument Engineers' Handbook, which I'm sure your library has.
Best regards, Bela Liptak
 
D
Dear peter:

The key to decide the PID design is to understand what system you prefer to control.
You had to understand the dynamic equation of the system. The PID is only a compensate mechanism to help the system achieve its control target. It is
like path control, set point control, and so on.
I thought the fuel rate in bolier is nonlinear therefore you had to decide to transfer the overall control system to 1st order or 2nd order linear equation based on your set point.
During the transferfunction you might need to find the matrix.
If you did not understand the overall control system transferfunction therefore it is easy to fail even you tune the PID parameter.
I thought your problem is not how to design PID by software.
The formal way to reduce try and error effort is to understand your mechanism first. Maybe you need a thermal engineer's assistance to
help you calculate the fuel dynamic and steady state equation. The c programming is just to program the overall control system not only for PID loop.
Matlab is a tool for students to learn the simulation quickly. There are many functions ready for you. You could transfer the function into C code.

The PID controller in Asia Omron is the cheapest brand.
As I know the fuel rate in Boiler seems to use the package control not only by PID and the control variable is not only limited in temp.

You could visit the link http://www.fansi.net/Discussion/Tech2.asp?
ItemNo=230&Class1=Facility-FMCS I replied how to tune PID parameter.

Regards,

Daniel Lee 11/9/2000

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J

Johan Bengtsson

Hmm, what are you about to do? pick one:

- make your own PID controller using some electronic components like OP-amps, resistors and capacitors (this takes some electronic design but no programming)

- make your own PID controller using some programmable chip like a PIC or something and write the program for that one (this takes some electronic design, less than in the previous example, and some programming (C or assembler))

- buy a standard PID controller
(this takes no electronic design or programming at all)

- use a computer (normal PC) and write the PID as a program in that one (may take some electronic design depending on how you plan to do the I/O and some programming)

- use an existing program in a PC and only tune it
(may take some electronic design, but probably not and no programming)

- use a PLC and implement a PID controller in that one (takes no electronic desing and no programming in C, but some in the language used by the PLC)

- anything other


After that you do of course have to tune it, but that is a later question.

Are you planning to control the valve with an analog or a digital signal?


/Johan Bengtsson

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> ---------- Forwarded message ----------
> From: Frank Rytkonen <[email protected]>
>
> Peter:
> You're missing the whole point of PID control! You say the following:
>
> > i am going to use temperature as plant. when the
> temperature is exceed setpoint then feedback to pid controller and
> thus cut off voltage supply to electrical valve (on off valve). when
> the temperature is lower then turn on the valve. <
>
> The scheme you describe is not PID control, it is on-off control. Anyone
> can set a temperature switch to operate an on-off solenoid valve.

(and it sounds like my furnace rheostat at home - hardly something to be reinvented)

> PID control would monitor the analog value of temperature, compare it to a
> setpoint temperature to determine an error, then process the error using
> PID algorithms to determine the value of an analog signal to be sent to
> the final element controller - in your case an electrical valve (but not an
> on-off valve) that would move to adjust fuel flow to the proper value to
> maintain burner temperature.

He *could* use PID control to drive an on-off valve, if it was done in a PWM style. That is, the analog output would be converted into a percentage time on, time off duty cycle for the on-off valve.

Most of us could relate to this type of control when watching (and listening to) a
microwave oven when running at less than 100%. You can hear the cook-rest cycles.

If you can be comfortable with (sometimes) having the valve off when below the set point, and valve on when above, (normal behavior of PID and/or PWM), the system should work fine.

But select a PWM cycle rate appropriate to the controlled device. I'd need sub-millisecond
PWM if I was sending an audio signal to a speaker. But I'd probably choose a cycle in the
minutes range if I were driving my furnace. Also watch time-off cycles so that you don't turn
the device back on too soon (like the compressor in a refrigerator or air conditioner). There may
also be a minimum on-time before there is actually any real work done by the device (it may
have a "cycling up" delay by its nature).

HTH

Rufus
 
P
To all all helpful friends,

Thanks for your effort to help me. I really appreciate all of you. Your opinions and suggestions gave me some understanding on my project, indeed. Thanks.

What i would like to mention is my main task is focus on control the fuel rate into a boiler according to any plant using PID controller. Since i thought the temperature is the easiest, so it is chosen. althought I will built the hardware but I am not attempted to built the all real boiler system. What Mr.[Harrold, Dave (Cahners - CHI)] mentioned really scare me. ("You are going to find boiler control is not as simple as just controlling the fuel." )

The fuel that that I am going to use is fluid such as oil ( do u think can work?) since using gas is quite danger.

I had asked some manufacturers about PID controller. I think I am going to use omron's PID controller. Most probably is CK5. Second choice is using PIC (my project requirement include c programming) and the the last choice is using PLC or PC. But i don't know how PIC and PC can perform PID function. Do u mind to tell me? The first choice will be PIC if OMRON's PID controller is not involved programming.

"PID control would monitor the analog value of temperature, compare it to a setpoint temperature to determine an error, then process the error using PID algorithms to determine the value of an analog signal to be sent to the final element controller - in your case an electrical valve (but not an on-off valve) that would move to adjust fuel flow to the proper value to maintain burner temperature."

---I think this is what i need to do actually. :)
By the way the electrical valve is mean analog ac voltage supply valve?

"Are you planning to control the valve with an analog or a digital signal? "--Johan Bengtsson
---what is difference? Is the PID controller supply ac output? I though it supplied only digital output like PC. ;)

"The fuel rate in bolier is nonlinear therefore you had to decide to transfer the overall control system to 1st order or 2nd order linear equation based on your set point. "--Daniel Lee
--- I really don't know how to transfer my system to mentioned equation. and it is impossible for me to find any thermal engineer to assist me. :(

I hope you all will help me to clear my doubt. Thanks in advanced.

sincerely,
peter wong
 
R
Not quite right specially in Temp controllers you will find rather two point actuators (heating element) or three point heat/off/cool. They
are certainly controlled by PID algorithm using 2 or 3 point step algorithm. The PID calcultes the on/off percentage of your heating and or cooling element.

Cheers Rolf
RG Electrical Engineering & Consulting
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Peter,

The major problem with your project is the goal. In a boiler you don't control burner temperature. You want to vary a control valve which delivers gas or coal to the burner. Oil provides the flame that burns the fuel, but is a digital on/off control. Also the boiler temp is controlled by throttle pressure or turbine speed demand, this complicates the goal of your project, I would suggest using a more simple example, such as a steam flow control valve. This uses a simple PV, to make the project more interesting, try compensating the flow rate with pressure and temperature inputs.
 
hi mr.

I am university student. and I am looking for an PID controller design for position control

also I need to know the circuit implementation and how to make the circuit and what chips do I need to use?
Please help me if you can
and reply to let me know
Maher
 
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