LabVIEW PID Code explained?

Hi NI Community,
I'm working on understanding the code behind the LabVIEW PID. First, let me say that YES I understand the theory of a classical PID control and understand how Kp, Ki, and Kd should affect my system. I'm interested in some of the underlying code behind the PID (DBL).vi in LabVIEW.
Specifically, integrator error in this VI is defined as
IntError = Error + LastError *(Kp/Ki)*0.5*(1/60)*dt + LastIntError
I'm simpliflying some of the logic but that's the meat of it. What are the 0.5 and 1/60 attempting to do in this equation? The dt is in units of seconds. Also, shouldn't this IntError be multiplied by Ki (e.g. Igain) at somepoint as well to follow the model of a Classical PID control?
These links were a great starting point but I'm trying to connect the dots between this theory and the actual code running in LabVIEW:
http://zone.ni.com/devzone/cda/tut/p/id/6440
http://zone.ni.com/devzone/cda/tut/p/id/3782#toc1
Thanks,
Craig

Hello Craig,
The PID toolkit is based on the "academic" version of PID where it uses proportional gain affecting integral time and derivative time. Those time are in minutes instead of seconds, that is the reason you have 1/60 on the equation.
Also, during the process of discretization of the integrator (making the continuous integrator discrete), you have basically 3 ways to discretize it: forward, backward and trapezoidal (also known as bilinear or tustin). The best method is the trapezoidal, which is based on getting the following formula: (see page 2-2 on the PID manual - http://www.ni.com/pdf/manuals/372192d.pdf)
IntegralError (i) ={0.5* [error (i) - error (i-1)]} + IntegralError (i-1)
Then, by changing the time to minutes and making the integral gain = proportinal gain / integral time, then you would get to our current implementation.
Also, notice that there are in general 3 methods of representing PID algorithms, which we call: academic, parallel and series. See http://digital.ni.com/public.nsf/allkb/6E76EC47B831FAED86256E4E007347DC
http://zone.ni.com/reference/en-XX/help/371894D-01/lvctrldsgn/cd_pid_mdl_vi/
Please let me know if you have any more questions.
Barp - Control and Simulation Group - LabVIEW R&D - National Instruments

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Engine Dynamometer Brake Control, DAQ & LabVIEW PID

Hi all, I am in the middle of a project to design, build and test a controller for an eddy current engine dynamometer. I have an idea of how the inputs, outputs and overall process go, but am not sure how to best implement the necessary features in LabVIEW. I have access to a NI USB-6211 DAQ, and a PC with LabVIEW 8.6, DAQmx drivers and the PID toolkit installed.
On the electrical and mechanical side, an SCR firing board takes a 0-5VDC analog control signal to vary the amount of current passed through 380V three-phase electrical lines hooked into a large field coil. Varying the input 0-5VDC signal results in a directly related variation of input current into the field coil, which in turn affects the strength of the magnetic field generated by the coil. A large ferromagnetic rotor spins concentrically within the coil. If the magnetic field increases, eddy currents are induced in the rotor causing it to slow down and heat up, and vice versa. The engine-under-test is bolted to the large rotor and is put under load by the effects of the induced magnetic field and eddy currents.
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Power sweep tests are used by all engine and chassis dynamometers to gather an evenly distributed collection of data across the engine's usable RPM range. The idea is that the engine should be forced to accelerate its RPM at the same rate from just off-idle to rev limit. Bolted to a dyno and given its druthers, most engines will accelerate more slowly off-idle and more quickly in their upper RPM power bands. Load must be controlled so that the engine can spin as freely as possible down low in the RPM range, and be forced to maintain constant acceleration as it tries to pull away in the upper RPM range. Human, manual control of rotary knobs can provide a respectable effort in this situation, but the problem becomes very apparent when comparing back-to-back, "identical" tests on the same engine, with the same operator. Repeatability of torque and power measurement tests is very important to understanding how distinct changes to the engines mechanical and fluid systems affect its torque output, along with other symptoms.
I hope the background is helpful.
There are RPM and Torque inputs into LabVIEW for the engine under test. In the design stage, I figured I would be able to implement a PID controller in LabVIEW to vary the SCR firing board's 0-5VDC control signal. The PID loop would control the 0-5VDC signal so as to allow the RPM of the engine-under-test to accelerate as closely as possible to an operator-chosen rate. The USB-6211 DAQ has two analog outputs, one of which can be used for the 0-5VDC control signal. The DAQ also has two digital counter circuits. One of them is used for counting and determining the continually changing frequency of a TTL pulse train driven by engine-under-test RPM. Lastly, one of eight analog inputs is used to measure a 0-5VDC analog input signal from a strain gage signal conditioner indirectly measuring engine-under-test torque output.
I worked with LabVIEW as a student in school, but never attempted to design VI's from scratch until now. I spent the last week or so practicing with the DAQmx Assistant and later the broken-out DAQmx LabVIEW code for bench-testing the counter and analog inputs for RPM and Torque. I plan to begin experimenting with PID controls this week, but I have stumbled into a few trouble spots with the DAQ input code already.
As of right now, it seems that the PID control loop will only use RPM data, not engine torque data. I would like to make sure that the sampling settings being used provide just the right amount of coverage, not using more DAQ or PC resources than necessary. I figure this will assure the sampling process and controller will run as close to real-time as possible without relatively large-scale changes to the system. Due to mechanical limitations of the dynamometer, the engines under test will never exceed 3600 RPM. A variable reluctance sensor is positioned closely to a 60-toothed trigger wheel bolted to the dyno's rotating assembly. The VR waveform is passed through a LM1815 based VR signal conditioning circuit to produce a TTL pulse train. This digital signal is then piped into of the counter inputs on the USB-6211 DAQ.
(3600 Revolutions per Minute * 60 Teeth per Revolution) / 60 Seconds per Minute = 3600 Teeth per Second (Hz)
The maximum frequency of the RPM signal will be 3600Hz. I started to try different DAQmx Timing settings. It seems the three main options are Timing Source, Timing Rate and Number of Samples. I have the book "LabVIEW for Everyone," and read Chapter 11 on LabVIEW with DAQmx, but I had trouble figuring out exactly how these three fields affect a Counter input, as compared to the examples in the book covering analog inputs. If it's not too much trouble, could anyone shed any light on this?
In case it's interesting, here are some pictures of the engine dynamometer and some of the older data equipment that is being replaced.
Engine Dyno Pictures
Thank you for any help!
Rob

CoastalMaineBird wrote:
As it happens, I am involved with a large project that controls engines, as well. I have two control loops, one controls dyno drive based on speed, and the other controls throttle based on torque. Usually only one is in PID mode at a time, the other is open loop. I have to run the engine thru a prescribed speed-torque cycle and measure how much exhaust pollution occurs.
We do "Torque Maps" which are sweeps similar to what you describe. You put the throttle 100% ON, and ramp up the dyno speed, all the while capturing data. You get a map of how much torque you can obtain at any given speed.
I do it on a PXI box to avoid timing issues. When I last tried to use the host computer to do this, Windows was just not able to devote the time to getting the data right. The PXI box guarantees that I can control the loop at 100 Hz (I have tested it up to 1000 Hz).
Anyway, to your specific question:
I started to try different DAQmx Timing settings. It seems the three main options are Timing Source, Timing Rate and Number of Samples.
The counters are general-purpose, so they can be configured in many different ways.
The TIMING SOURCE is where the basic timebase comes from. If you're measuring a high-frequency signal, you want a higher-frequency timing source (so you have resolution to spare). If you're measuring a low-frequency signal, you want a low-freq timebase (so you don't run out of counter bits). If your max input is 3600 Hz (277 uSec), then a 1 MHz timebase (1 uSec) will give you one part in 277 resolution WORST CASE. A 10-MHz timebase will be 10 times as good.
I don't know where you are getting the TIMING RATE and # SAMPLES inputs, can you explain?
That's a very interesting project you are working on, certainly a lot more involved and impressive. I think I saw a few screenshots of the LabVIEW interface on your website and wow!...that is really nice. The emissions analysis equipment can get very pricey, but I can't think of a more timely test to be directly involved in at this point in time.
I briefly researched standalone LabVIEW system options and real-time capabilities. I am hoping that the barebones nature and low operation count of the VI will allow a Windows host PC to be sufficient for controlling the process. This is largely an economic motivation to keep the overall project cost down. If it turns out that PXI hardware is necessary, it will also be possible to look down that path at that point in time.
When I first looked at LabVIEW DAQmx reference materials, I suspectec I would need to go beyond the DAQ Assistant and break down DAQmx tasks into their lower level start, read, write, stop and clear components. After reading more and working with additional LabVIEW PID examples, it looks like I may be able to stick with DAQ Assistant sub-VI's. I was confused at the time by the different DAQmx timing components and chose to begin with a question on the counter input. Today, a DAQ Assistant sub-VI seems to be handling the counter input well enough.
I most likely need to focus on PID parameter tuning at this time. If it turns out that the timing configuration of the counter RPM input is preventing the system from working as required to perform a power sweep, or follow a Torque Map like you mentioned, then I will need to revisit it. For now I'm just going to be happy that its portion of the system seems to be working.
Message Edited by el bob on 03-24-2009 01:45 PM

Embed labview generated code with c++ gui application

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Tonph wrote:
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This October, we are holding the LabVIEW Example Code Contest 2010. LabVIEW users can demonstrate the cool things they've been able to accomplish with LabVIEW by submitting new example code. The best example code wins an iPad.
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Data Acquisition
File I/O
Math/Analysis
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Games
Submissions will be accepted from October 1 through 17. Judges will choose finalists for each category, then the NI Community will vote for their favorite example codes. Father of LabVIEW Jeff Kodosky will choose the overall winner.
Read the rules, ask questions, and submit your entries in the LabVIEW Example Code Contest 2010 Group

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I would suggest two possible links on the lower center of this page:
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Ladies and Gentlemen,
I am trying to run a LabVIEW-generated C code {Calling a LabVIEW-built DLL in C to Acquire, Analyze, and Present Data (http://zone.ni.com/devzone/cda/epd/p/id/1515)} in VxWorks 5.5.1. However, I have the following errors in using Tornado 2.2.1 to compile:
vxrm *.o *.rpo ctdt.c symTbl.c vxApp* *.out *.pl
vxrm ..\prjComps.h ..\prjParams.h ..\prjConfig.c ..\linkSyms.c
vxrm ..\libs.nm ..\libs.size
ccsimpc -g -mpentium -ansi -fno-builtin -fno-defer-pop -I. -IC:\WindRiver\Tor2.2-PPC\targe
t\h\ -DCPU=SIMNT -DTOOL_FAMILY=gnu -DTOOL=gnu -Wall -c ..\callLVDLL.c
In file included from ..\extcode.h:14,
                 from ..\LV_AAP.h:1,
                 from ..\callLVDLL.c:23:
..\platdefines.h:308: #error
..\platdefines.h:339: parse error
In file included from ..\extcode.h:15,
                 from ..\LV_AAP.h:1,
                 from ..\callLVDLL.c:23:
..\fundtypes.h:17: parse error
..\fundtypes.h:43: stdint.h: No such file or directory
In file included from ..\fundtypes.h:61,
                 from ..\extcode.h:15,
                 from ..\LV_AAP.h:1,
                 from ..\callLVDLL.c:23:
..\lv_prolog.h:37: parse error
In file included from ..\extcode.h:15,
                 from ..\LV_AAP.h:1,
                 from ..\callLVDLL.c:23:
..\fundtypes.h:157: parse error
In file included from ..\fundtypes.h:252,
                 from ..\extcode.h:15,
                 from ..\LV_AAP.h:1,
                 from ..\callLVDLL.c:23:
..\lv_epilog.h:17: parse error
In file included from ..\extcode.h:15,
                 from ..\LV_AAP.h:1,
                 from ..\callLVDLL.c:23:
..\fundtypes.h:361: parse error
..\fundtypes.h:370: parse error
..\fundtypes.h:386: parse error
In file included from ..\LV_AAP.h:1,
                 from ..\callLVDLL.c:23:
..\extcode.h:40: parse error
..\extcode.h:49: parse error
..\extcode.h:57: parse error
In file included from ..\extcode.h:116,
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                 from ..\callLVDLL.c:23:
..\lv_prolog.h:37: parse error
In file included from ..\LV_AAP.h:1,
                 from ..\callLVDLL.c:23:
..\extcode.h:526: parse error
..\extcode.h:959: parse error
In file included from ..\extcode.h:1080,
                 from ..\LV_AAP.h:1,
                 from ..\callLVDLL.c:23:
..\lv_epilog.h:17: parse error
make: *** [callLVDLL.o] Error 0x1
Done.
I would appreciate your help in this matter. "LV_AAP.h" and "callLVDLL.c" will be furnished upon request OR you can download from
http://zone.ni.com/devzone/cda/epd/p/id/1515
The rest of the header files are in my directory:
C:\Program Files\National Instruments\LabVIEW 2010\cintools
Regards,
Chi

Hello Che,
Thanks for your response. My real-time target hardware is MVME5100. However, at present I want to see whether a C code {see the attached callLVDLL.c} generated from LabVIEW can be compiled in VxWorks 5.5.1/Tornado 2.2.1. The reason why I am using an old version of VxWorks/Tornado is my company is transitioning to NI PXIe hardwares and LabVIEW software, and phasing out all of VME boards {Power 4e, Power 7e, and MVME5100}.
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Thanks for your help,
Chi
Attachments:
callLVDLL.c ‏3 KB
LV_AAP.h ‏1 KB

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I have a vi code written in labview 7 and I open that in labview 2012, modify the code and save it. But when I open that in labview 7, it doesn't open. Is that any way that I can save the code in 2012 so which can be open in labview 7?

dragondriver wrote:
altenbach wrote:
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(details)
Easier might be to post the VI in the version conversion board.
I don't have labview 8, so there is no way to do so！? ...
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Labview PID loop

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=====================
LabVIEW 2012

Using labview C code

Hello,
I am a bit confused at writing the C code for the CIN. The3 code labview
gives me is:
CIN MgErr CINRun(LStrHandle var1) {
My code that I need to use needs the string input that IU had sent to the
CIN. What is the actual name of the variable that the string is passed as?
Is it LStrHandle, or var1, or the entire LStrHandle var1? Because my code
uses
messagesend(char), where char must be this variable that labview is sending.
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JMH

JMH wrote:
> Hello,
>
> I am a bit confused at writing the C code for the CIN. The3 code labview
> gives me is:
> CIN MgErr CINRun(LStrHandle var1) {
>
> My code that I need to use needs the string input that IU had sent to the
> CIN. What is the actual name of the variable that the string is passed as?
> Is it LStrHandle, or var1, or the entire LStrHandle var1? Because my code
> uses
> messagesend(char), where char must be this variable that labview is sending.
> I would apreciate any help I can get. This is the last issue I have with
> my project before I can start debugging. Thanks
>
> JMH
Let's first back up and make some changes.
1) Decide how many and the types of inputs / outputs the CIN will use.
2) Create the Controls / Indicators / Constants on the Fr
ont Panel / diagram
2A) NAME ALL OF THESE ITEMS
3) Drop the CIN on the diagram start wiring the items to the CIN
[Right click on a terminal of the CIN and select 'Add Parameter' to get more
terminals]
4) Right click on the CIN and select 'Create .c file'
Now the Variables have names you can understand. IF you skip 2A then LabVIEW
will name
the variables as var1, var2, ......varN for the number of variables
(parameters) you add.
So to answer the question var1 in this case is the string coming into the CIN.
The LStrHandle is the datatype (in this case a pointer to a pointer to the
string ) of var1.
I don't want to offend you but I get the impression that you are not
comfortable
with C code and CINs. These are not for the faint of heart. I would strongly
suggest
you look for another solution to the problem before resorting to using CINs.
Of course if I have misinterpreted here you are more than welcome to send me a
lashing.
Good Luck
Kevin Kent
Attachments:
Kevin.Kent.vcf ‏1 KB

LabVIEW PID Code explained?

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