Real-time PID control and continuous buffered acquisition. Does it work together?
I use the "real-time PID control.vi" that acquires single AI point and generate single AO point in every "while loop" iteration to control a piezo actuator motion. Simultaneously I need to do (in another loop) a continuous buffered acquisition of another two AI channels (in the same rate as first AI channel).
It does not work together.
Does anybody know how to fix it?
Platform: LabVIEW 6.1
Board: PCI-6024E
Hi,
I would try to avoid the idea of using a separate loop since the PID
loop is a continuous acquisition and will require un-interrupted access
to the Analog DAQ system of the board. The last thing it needs is for
you to attempt to start other AI functions at the same time.
Configure your AI in the PID loop to read in multiple channels (Just add
some more channels to the Input Channels Control. Separate the data as
it comes out of the AI single Scan function using Index Array or other
Array functions into that needed for the PID and that needed for your
other data (the example is already doing this to make sure it only gets
1 channel of data into the PID). If you don't need to sample all your
channels of data all the time you can throw some data away as
required.
Obviously your other channels are now arriving one sample at a time so
if you might need to build an array, insert into an array or use Auto
Index on the loop if you are data logging.
Regards,
Dave R.
varmih wrote:
> I use the "real-time PID control.vi" that acquires single AI point and
> generate single AO point in every "while loop" iteration to control a
> piezo actuator motion. Simultaneously I need to do (in another loop) a
> continuous buffered acquisition of another two AI channels (in the
> same rate as first AI channel).
> It does not work together.
> Does anybody know how to fix it?
>
> Platform: LabVIEW 6.1
> Board: PCI-6024E
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Hi,
I found a real-time PID control in LabView examples, for E Series boards. I used one of this boards a time ago and used this control. It works very well.
Now, I have a M series board (PCI-6221, LabView 7.0) and I would like to use a similar program. I tried to use the DAQmx assistant and look at the examples, but I did not find anything similar.
Could you please take a look in this example (attached) and help me in configuring the PCI-6221 to work in similar way?
I really apreciate your support.
Thanks,
Jeferson
Attachments:
Real-Time PID Control2.vi 131 KB
simple PID.vi 53 KBJeferson,
I no longer have LabVIEW 7.0 on my machine, but I know that in LabVIEW 7.1, DAQmx installs an example called "PID Control-Single Channel.vi." This should be exactly what you are looking for. It can be found in the LabVIEW Example Finder (Help->Find Examples) by searching for "PID."
Basically, this VI uses hardware timing (the rate will depend on your computer specifications) to acquire a single point, calculate the PID output, then output that value on the next sample clock. Please let us know if you have any additional questions!
Hope this helps,
Ryan Verret
Product Marketing Engineer
Signal Generators
National Instruments -
I have a big delay in my real time pid control
hi ,
since i don't have a digital input module i tried to use the serial port RS232 on my Crio to read the data from an absolute encoder in order to perform PID control for a dc motor, but the problem is that the data took too match time to be read; there is a big delay between the first read position and the next one,and when i change the slide controller the indicator took time to change, although it should be instantinouse, here is
a snapshot of my vi.
please i need help.Hi Moho,
Looking at your code I see a couple of things that might be causing the issue. How long is the delay that you are experiencing? You are trying to transfer 2000 bytes every loop which will cause some delay. You also have a lot of mathematical operations that can take time between loop iterations. Also what is the VI that you have manipulating your waveform? Have you looked into the PID VIs?
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What is the difference between buffered acquisition and non buffered acquisition?
What is the difference between buffered acquisition and non buffered acquisition? How will I decide whether I want buffered acquisition or non buffered acquisition?
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Shared Variables for Real-TIme Robot Control
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DavidHi David,
I'm curious why you decided to build a CIN instead of developing the code in
LabVIEW. Is there some functionality that that LabVIEW couldn't
provide? Can you provide some more information about the LabVIEW
Real-Time target you're using? What type of IO are you using?
It is impossible to get LabVIEW Real-Time performance on a desktop PC running
an OS other than LabVIEW Real-Time. Even running a timed loop in LabVIEW
for Windows won't guarantee a jitter free application. Also, no TCP based
network communication can be deterministic. This means Network Shared
Variables are also not deterministic (they use a TCP for data transport) and I
advise against using them as a means to send time critical control data between
a Windows host and a LabVIEW Real-Time application.
In general, I would architect most LabVIEW-based control applications as
follows:
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LabVIEW Real-Time application would accept set points and/or commands from the
'host' (desktop PC). The Real-Time controller should be capable of
running independently or automatically shutting down safely if communication to
the PC is lost.
- Write a front-end user interface in LabVIEW that runs on the desktop
PC. Use Shared Variables with the RT-FIFO option enabled to send new set
points and/or commands to the LabVIEW Real-Time target.
Shared variable buffering and RT-FIFOs can be a little confusing. Granted
not all control applications are the same, but I generally recommend against
using buffering in control applications and in LabVIEW Real-Time applications
recommend using the RT-FIFO option. Here's why: Imagine you have a
Real-Time application with two timed loops. Time-loop 'A' calculates the
time critical control parameters that get written to hardware output in
timed-loop 'B'. Loop 'A' writes the outputs to a RT-FIFO enabled variable
with a RT-FIFO length of 50. Loop 'B' reads the outputs from the shared
variable, but for some reason, if loop 'B' gets behind then the shared variable
RT-FIFO will now contain several extra elements. Unless loop 'b' runs
extra fast to empty the RT-FIFO, loop 'B' will now start outputting values that
it should have output on previous cycles. The actual desired behavior is
that loop 'B' should output the most recent control settings, which means you
should turn off buffering and set the RT-FIFO length to 1.
There is also a clear distinction between buffering and the RT-FIFO
option. The RT-FIFO option is used to add a non-blocking layer between
network communication and time-critical code in LabVIEW Real-Time
applications. It also provides a safe mechanism to share data between two
loops running in a Real-Time application without introducing unnecessary
jitter. Network buffering is a feature that allows a client to receive
data change updates from the server even if the client is reading the variable
slower than the server is writing to it. In the example I presented above
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entirely within the Real-Time application. However, it would be
appropriate to send control set points from a Windows PC to the Real-Time
application using network published shared variables with the RT-FIFO option
enabled. If it is critical that the Real-Time application executed all
commands in the sequence they were sent then you could enable an appropriate
buffer. If the control application only needs the latest set point
setting from the Windows host then you can safely disable network buffering
(but you should still enable the RT-FIFO option with a length of 1 element.)
Network buffering is especially good if the writer is 'bursty' and the reading
rate is relatively constant. In the robot application I can imagine buffering
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case, you may write the sequence values to the variable very quickly, but the
Real-Time controller would read the set points out as it proceeded through the movements.
The following document presents a good overview of shared variable
options: http://zone.ni.com/devzone/cda/tut/p/id/4679
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LabVIEW R&D
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