Trigger based on Position when using analog feedback

Question

I am developing an application that uses two hydraulic cylinders to move a model in a pitching and plunging motion.
My hardware consists of a P4 computer running windows XP SP2. The motion controller is a PCI-7350, 2 axis board.
The two hydraulic cylinders are equipped with analog position sensors which provide the primary feedback signal for the motion control.
The drive signal from the motion controller is used to open or close a hydraulic valve, closing the loop. The motion portion works great.
I am using buffered contouring to generate a precise motion on the two axis using a vector space. The update rate is 10mSec and the motion appears to be correct. Is the contour update rate a precise 10mSec if the PID is running at 62.5 uSec?
My problem is I need to generate a trigger when the controller is on specific positions of the contour. I can't use breakpoints as I am using analog feedback instead of an encoder. I wrote an onboard program that monitors the contour buffer and toggles the breakpoint line when the controller is on or beyond the specific point. It runs ok and is close but, I am getting an occasional timing discrepency of roughly 16 mSecs. ie the trigger will occasionally be  late by more than 1 contour period.
I tried increasing the size of the buffer to hold all the points as I thought the delays might be occuring when the buffer was being updated. No change in performance. I tried assigning more time to the trigger program as it is the only on board program, but again it seemed to make no difference. I reduced host communications to a minimum, no change.
I have attached my entire application and a test sample waveform  in a zip file. You can run the application without any hydraulic cylinders if you set max following error to 32768. You should have a trigger pulse on the breakpoint line every 500 mSec.
Any pointers would be greatly appreciated.
Attachments:
M22PitchPlunge.zip ‏524 KB

Answer 1

Brian,
Yes, you can reach exactly 10 mS update interval with a PID rate
of 62,5 µSec. You can calculate the actual interval with the forumla
provided here.
Of course the accuracy is limited by the accuracy of the onboard
oscillators of the board but this shouldn't be really relevant.
Generating triggers based on 2D position data is not a trivial
task. I have tried to examine your program but it's a bit hard to read.
I had a look at your onboard program. I don't understand exactly how
you determine if the axes have reached a trigger position or not. It
looks like you are only reading values from buffers. For an exact match
with the physical positions you should read the real position values
from the axes and compare them with your trigger position buffer. In
your comparison algorithm you will need to add some tolerance to allow
some deviation from the ideal trigger position (see image).
There are several possible solutions for your application. You could
either try to implement the more advanced comparison in your onboard
program but I'm not sure if this will result in the performance you
need. The 7350 provides only a limited amount of CPU power for onboard
programs. At last it all comes down to the question how good your
deterministic real-time behavior needs to be. If some ms delay and
jitter are ok for you then you probably could stick with this approach.
If you need a better real-time behavior you should consider using
either an additional real-time board like the PCI-7041/6040E that could run the 2D trigger algorithm or a PCI-7830R board that could run the algorithm on an FPGA.
There are even more options including the NI SoftMotion Development Module to design your own motion controller using LabVIEW RT but again, this depends on your application needs.
Best regards,
Jochen Klier
National Instruments Germany
Message Edited by Jochen on 09-09-2005 10:47 AM
Attachments:
Image1.jpg ‏19 KB