ShaunR

Hi Shaun,

I'm sorry, my submission deadline is soon enough, so I've been a bit naive with this expecting everything to fall into place in a short space of time... That's not going happen...

My pump is just on-off, 12v dc. An analogue controlled pump would probably have been a sounder option, at least when working with labview. I realize that now...

I have the PID toolkit, so I'll have a look at the 'PID simulator' and the other examples like you said. I have attached the main program & sub vi's. I've also attached the basic program that I started off with(minus PID & PWM) & a manual PWM program that I was also trying to use... Thanks for all the help and I apologize if you thought that I'm trying to get you to do my project for me!! I'm just hoping to pass it when it comes down to the crunch...

Thanks again,

Rich

Sorry for the delay. Had my own projects that weren't falling and needed kicking into place

No need to apologise. I was just trying to let you know that you need to understand what you are doing

OK. So you have a 12 digitally controlled motor. That's your design decision Nothing wrong with that, after all we need to keep within budget don't we?

I'm assuming you have seen the NI PID Examples and seen that they get a lovely smooth control curve. Right?. Just change the sampling time from 50m to, say 500 m.

Does it still look like a nice well controlled curve? What do you see? Remember we haven't changed any PID parameters, or the load we are controlling. Just how often we sample the PV.

What do you think is happening here?

Hi Shaun,

Thanks a mil for all the help - I'd be lost without it:worshippy:! I've attached my vi with the PID & PWM subvi's added in. I'm sure that it'll be wrong somewhere but I'm going to try and tune the PID controller tomorrow and see can I get some sort of response... When it comes to the PID tuning, I spoke to my project supervisor from the college & he said that I would need to work out my values by the 'plant response' method...?

Leaving Ki & Kd at zero, he said to 1st get; Kc = % change in voltage shift of my output

% change in voltage shift of my input

He said that I then calculate the rise time of the slope Kr and the lag Kl. These values are then used in the ziegler nicolls formula to calculate my P, I & D... It sounds like an awfully complicated way of calculating the gains to me, but then again so does most associated with PID...! The 0-500 values on my front panel is just the range of response I got from my cistern arm(pot) from empty to full... Thanks again,

Richie

Hmmm.

I think maybe I have failed miserably to explain the key concepts.

The previous example was to show how you can add a very simple PWM (there is more than 1 type) to an on/off controller that controls a single digital output Cutting and pasting my example and wiring up controls with the same name won't cut it since you want to use a PID controller and not an on/off controller. I'm not doing your project for you, merely trying to show, with he use of examples, how to go about it. The example demonstrates how you can vary a digital output to react a a certain stimulus and since it was available, I used Neds on/off controller .

You could, for example, replace the PID VI in the NI example "PID simulator" with the vi I supplied, but you would still have to figure out a method of calculating the duty-cycle on each iteration in relation to the input (which you haven't). You could, however, replace it with Neds example (no mods needed) and see what the difference is to the PID one.

I would suggest taking the time to view all the PID examples shipped with the PID toolkit (assuming you have it) and understand the theory.

I would even be tempted to rip out the process simulator into another vi and use it to create the step response graph and log the results. Then do my ZN analysis on it to see if I came close to the ones in the example. Then change the process values and repeat.

From your project supervisors comment, I think he is expecting an analogue output (or a PWM simulation of an analogue voltage which isn't the type of PWM I used). Is the pump analogue controlled? If it is. What is the reason for using a digital output when your USB device has analogue outputs (And there are many tutorials on Youtube of PID with an analogue output. using LabView)

One further point. If your posting VIs that uses sub-vis. Can you put the all vis into a single directory and zip up the entire directory so that all the sub-vis are included. I don't have "On-Off Controller-PWM.vi" or "Simple PID- ViFormyproject.vi". I can guess at what they probably are but its always best to be looking at the same code.

Oppps.

Just noticed a feature (bug..it is the wee hours is my excuse) The over set-point check is inverted (sorry about that).

This is what it should be

Thanks everybody for all the help and advise:rolleyes:! I'm going to focus on trying to make my process work with one of two programs;

1 - My initial program with the on-off controller added in(cheers anon),

2 - An edited version of the FPGA PID PWM program(thanks for pointing that out Shaun).

I've run into a bit of a snag with wiring my DAQ assistant Ai/p and Do/p into the FPGA program... My analog input subbed in nicely into the PID loop, but in the PWM generation loop there are 2 terminals to wire into my digital output(direction bit & PWM output), where as I only have one digital output to generate on my DAQ assistant. Also, when I did try to wire up the DAQ assistant to one of the terminals it gives the error;

'2 terminals connected of different types. Type of source is boolean(T or F). Type of sink is 1-D array of boolean(T or F)'.

I understad that one is a scaler type and one is array type, but even by clearing the error I can't be sure that I'd wire it up correctly... Am I on the right track? I've attached the FPGA program. Thanks again guys,

Richie (Labview & PID rookie)!!

Unfortunately. You have the software, but not the hardware. You need FPGA hardware to run the FPGA stuff on. I only mentioned it to illustrate that NI also have a digital solution.

But all is not lost.

If you remember back to my example, I said we just need a way to open the valve for a period the PID algo tells us. This seems to be the bit you are having problems with.

For the digital output. You already Have an on/off controller example supplied by Ned. But what we really want is a PWM On/Off Controller then it would give us the opportunity to vary the on/off outside the dead-band. PWM always sounds complicated. It really isn't and to demonstrate I've modified Neds example.

The bottom half of the loop is Neds on/off controller. I've just modified it a bit so it isn't reliant on the previous iteration of the loop. It's functionally identical.

The top half is a timed gate which allows the signal from the on/off controller through for a specified period of time. So now we have an on off controller that we can vary the pulse width in the time domain. And that's PWM. Generally we wouldn't have it all in the same VI and the PWM would run asynchronously to the controller. But for our purposes its actually a bonus.

If we set the duty cycle to 100% (note I've used duty cycle and made it a percentage) then it will behave as Neds On/Off controller) then we have our on/off controller back again.

If only we could figure out a way to turn the error between the setpoint and the process value into a percentage......

It's unfortunate your hardware doesn't have a configurable counter/timer output otherwise you could have used that.

Thanks.Do you have a sample to send me?

There are several examples shipped with labview . Use the NI example finder and you will find them under:

>Building User Interfaces

> Listboxes and Tables

Shaun,

The fellow I work with is taking a controls class from the same prof I did 20 years ago (so I know he is worth listening to). He was reported to have made a provocative statement to the effect:

People are always trying to come up with some special algorithm for control, but after screwing around, and around, and around, they always find out the PID would do it better.

Who? The Prof? Or your colleague?

(my words again)

He believe meant that to cover even the asymmetrical case (that should be defined in more detail probably).

What I meant was that there are certain processes that on/off controllers simply cannot control. They become unstable and oscillate.

As an example of an asymmetric system (just to clarify after all.....aren't they all?). I once had an environmental chamber that was convection heated but nitrogen cooled. So. To ramp up (say) 5 °C would take 15 mins at 100% output, but with a 2 second injection of liquid nitrogen the temp would drop by 5°C in about 10 seconds. Especially since some dork had designed the thing with the temperature probe near the injector.

I suspect he is very good at ferreting out the P, the I , and the D. But that is a weighty statement. I will probably keep it in mind and not give up too soon.

Mike

What you have to bear in mind is that PID isn't 1 algorithm. Its 3 algorithms cascaded. And you can choose which benefits from the 3 you want to include. You could (for example) use only the P and I terms or the P and D or just the P. Its the flexibility that makes it attractive since there is no "one size fits all" solution for every system.

If maths is your forte then I would suggest reading P, PI, PID Control.

If (like me) you only want the bullet points and prefer practical examples then try PID Tutorial is

Yes, I'd love to see that idea implemented. Usually though, I take the situation as a hint I ought to put the loops in their own VIs. Not always convenient, but helps a lot for when you need to do stepping or execution highlighting.

It also gives you the option of running them in different priorities and execution systems.

We should have a jokes/humour section

Right, but if they're hooked up to a PID loop, they're using a PWM driver to convert the PID's continuous signal (duty cycle) into the device's discrete output (on or off). PID is meaningless in a digital system; if you can't change the magnitude of your output based on the magnitude of the error,

Ahhh. But you can.

Consider the lilly a tank that requires 10 litres to fill completely and all we have is a valve that only has 2 positions (open and closed i.e. digital control). Our hypothetical tank is (say) being drained at a rate of 1 litre per minute however, our valve, when fully opened, is capable of supplying 2 litres per minute. If we open that valve for 10 minutes then we will fill the tank to the brim and it all gets rather messy, what with the overspill and everything. However, if we open the valve for 30 seconds, we will supply the tank at the same rate as it is being emptied (on a minute by minute basis).

Now. the problem is, in all probability there will be times when there is nothing in the tank (due to timing differences, magic and the law of jam-butty) . So its desirable to open the valve for (say) 40 seconds so there is always more than is being consumed. Ultimately the tank will fill so as we approach the a pre-determined limit we want to switch back to 30 second "pulses".

We haven't changed the magnitude of the output (the valve can only be on or off). But what we have done is opened it for more or less time. We have "pulsed" the valve and by changing (or "modulating") how long we keep in open for ( "width" of the pulse), controlled the system and we can all go home .

For the above, PID control is simply a calculation (derived from the current level in the tank) that continuously calculates how long we leave the valve open for to achieve and maintain a desired steady state (a level in the tank).. What it gives us is a continuously varying duty PWM signal (which is varying at the sampling rate) , where the period is 1 minute and has a steady state at 50% duty cycle (30 seconds).

It doesn't really matter what interface we use to control the valve itself (analogue, digital, a hammer or telekinesis). Just as long as we can open and close it for an amount of time the PID algorithm tells us to.

Perhaps a better phrasing would have been "PID is designed to drive a continuous signal, not a discrete one."

Nope. Since you are still advocating that the type control interface is implicitly linked to the process.

Ovens and kilns, for example, use contractors, SSRs and normal relays, all of which are digital.

NI have an example of PWM PID control for their FPGA boards.PID Control Reference Example for LabVIEW FPGA

They also have a note on their PID autotune function from the PiD control toolkit which uses a relay PID Autotuning VI and Limitations (it uses Ziegler-Nichols apparently )

But I suppose I'd better answer the OPs original question

Choosing PID parameters is a bit of a black art. Manufacturers of PID controllers all have their own algorithms for determining the P, I and D terms (usually called "Autotune") and generally they will be biased to their particular industry and products.. One thing they all have in common though, is that they will analyse one or more step functions as you have seen with the Ziegler-Nichols. But it is worth bearing in mind that this "Autotune" is not a magic bullet. It is a "first best guess" and time will still need to be taken to optimise for the particular process.

So. If the manufacturers can't get it spot on for every process. Then I don't think we can expect to.

You've read up on Z-N (if its good enough for NI then its good enough to start with eh? ). and its probably time to bite the bullet and see what happens. Use your calculated values and log the tanks (level?) process variable. Start off by using a low setpoint so you don't overfill and inspect the log record to see how much overshoot, ripple and offset there was (you never know, it might be acceptable as is!). You've been wise enough to install a manual valve so if things go really askew you can just shut it off. If you didn't have any problems, Increase the P a large amount and start running for that valve:lol:

PID is designed to drive an analog signal, not a digital one.

PID is designed to control a process to alleviate overshoot, ripple and offsets. It has little to do with the interface.

A simple approach is an On-Off controller with deadband. An example of such a controller is attached.

Indeed. First choice is always an on/off controller (they're cheap and easy). But the choice on this type of control depends on whether the process can tolerate overshoot and ripple rather than digital or analogue. Some processes that have significant lag or asymmetric properties cannot be controlled at all this way.

You could also try opening the registry key in read-only mode - A lot of HKLM is read-only to regular users (without UAC elevation) and if your just reading a path this shouldnt be an issue.

One of the inputs to the open registry key gives you an enum with things like read-key, write-key, etc - choose the one that looks the most like "read only" (I dont have a windows + LV machine around right now)

KEY_READ

Hello,

I would like to install and run on a Windows 7 PC an application built on my developpement PC (LabVIEW 8.5.1 under XP).

I had some problems with the front panel fonts solved as described on another post by adding on the .ini file:

appFont="Tahoma" 13dialogFont="Tahoma" 13systemFont="Tahoma" 13

The data acquisition works normally.

The program should open some acquired data directly on OpenOffice scalc. To do that it looks on the registry the Path of scalc. There seems to be some problems accessing the registry: The vi "Open Registry Key.vi" working normally on XP returns a -604 error on Windows 7.

Any hints?

Thank you.

Error -604 is "Access Denied". Its probably the UAC (User Account Control) which was first introduced in Vista and consequently first disabled in Vista .

Try disabling UAC and see if that helps.

Disabling UAC

The LAVA CR Web front-end (what we already have now) accepts Zip files or Packages. This will not change.

We don't have a VIPM compatible LAVA CR repository yet and I don't know if we will ever have one. But I would love to have one and if the LAVA community yells loud enough (hint hint) maybe we will. Who knows

But in this hypothetical future, if we did have a VIPM compatible LAVA CR repository, then only packaged submissions would go into that repository since that is the only format compatible with the VIPM repository. However we would continue accepting Zip files and packages into the LAVA CR Web front-end.

Sweet. Thanks for clarifying.

Currently LAVA CR accepts submissions as Zip files or Packages. Only packages would be submitted to the LAVA (VIPM compatible) Repository.

Hmmm.

Does that mean you would have 2 repostories one for packages and one for zip files? Or do you mean that you will not accept a submission UNLESS it is packaged for VIPM?

*exactly*! And then we could talk about how VIPM users can either automatically or manually connect to the repository.

How would you handle the submissions to the LAVA repository that aren't packaged in a VIPM distribution?

It doesn't fit on my screen.

Better put in a capital req for that 42" LCD TV monitor.

I still don't know exactly what a "neat curve" means.

At the risk of tryin' to walk with both feet in mouth. Maybe he means something like this?

Sales and Marketing:

Engineering:

Anyone out there have a better way of separating the noise from the spikes?

6 Sigma analysis.Anything greater than +- 3 standard deviations from the mean (or in this case +3 STD).

Works well for these waveforms.

No, the WM_Seticon returns a handle with the old icon.

You can drop this VI:

To set or Reset the icon of a front panel vi!

Here's some little demo code:

That sets and resets the icon of a FP window.

Note that if you don't reset the icon it will be in memory until the FP window is closed.

Can anybody tell me how the code runs on:

LabVIEW 32 on Windows64-bit

LabVIEW 64 on Windows64-bit

Ton

Works fine on both x64 an x32 using Win7 and Vista ( once I'd replaced all the ogk stuff )

Yes that definitly works!

Thanks for correcting me.

Now comes the bonus question:

Mac

Linux

Ton

Buy me a MAC and LabView for MAC and Linux and I'll tell you

BTW. No problems changing the icon back again?

Maybe named after the top level .lvlib would be better?

How about "Eric"? That's a good solid name.

Change the "uType " to "1" (icon) instead of "0" (bitmap) and load an ".ico" file.