Delay between angle and entered analog on a 6221

Hello

my test setup consists of a (8184 current run LabVIEW RT) embedded PXI controller which uses a PXI-6221 of data acquisition. One of the outputs analog of the 6221 creates a demand for speed that is sent to a servo controller. the servo drives then the test configuration. Between the servo and the remaining test configuration, a quadrature encoder is located. Both the 6221 meters are used to measure the position of the servo (using the trains of pulses A and B) as well as the speed of the servo (frequency of A pulse train). In addition to these counter inputs, some analog inputs of the 6221 serve as well to recored the configuration of test signals.

In this configuration, one of the analog signal has a frequency of n periods per revolution of servo (determined by the mechanical design of the test facility). But when I leave the servo turn to for example 100 rpm, then decelerate from status quo with a constant deceleration and then draw the analog signal against the measured angle, I see clearly that as long as the speed is telling, I n periods per turn (or a passage from my analog AC to zero each 1/2n towers); However, as soon as the slowdown begins, the plot gets stretched along the axis of the angle (so the plot tells me there are less than n analog periods per turn, which is impossible because it would require the mechanical destruction of the facility).
However, I can calculate a position based on speed information signal, as I got to the second counter (by integration of the speed). When I do this for the above velocity profile and draw the analog signal against the calculated angle, I see exactly n times / revolution, no matter how fast the installation program is rotating (just the expected behavior).

So, apparently, the speed measured is "in phase" with analog signals, while the measured angle has a "phase error. Draw the angles measured and calculated against the time tells me the same thing: the measured angle is always late (compared to the calculated angle). The period is not constant throughout the measurement; I've seen values between 30 and 170 ms within a single record. Due to this change of variable, inserting a delay for all channels, but the angle is not a great idea.

Unfortunately, calculation of the position of the speed signal is no option for me, because the direction of rotation is changed during measurement; because speed information I have based simply on a frequency of pulse train, it contains no information management, therefore a calculated position would be unaware of the changes of direction.

Does anyone have an idea whence thios delay and what I can do to fix this problem?
Try a position different methods of decoding or exchanging the meter channels has not made a difference.

Thank you!

Hey Kevin,

attached you will find the last block diagram (no new translation this time, since no changes have been made that require a), which includes your last suggestions (explicitly start the task of AI, remove the excess constraints) with a flat sequence structure that applies all the preparations of task to finish before the start of the first task (I know I could have achieved the same effect of intelligent routing of the signal error, but I think the code is) better readable this way).

Without OPI, LabVIEW tends to start to have, ao and tasks of the angle of the very different moments, which (for some reason that I don't understand - we have a sample clock based calendar here, and the sample clock is run, well after the three mentioned tasks...) tends to cause delays in the order of several 10-100 ms between tasks. With the structure, this problem is eliminated.

The Sub - VI just above DAQmx writing contains the "conduct" code that I used in the last screen shot to "serialize" the requested speeds - in this way, the table that contains the requirements of speed does not need to be rebuilt.

The code that creates a weather channel frequency implicitly timed measure left the VI shown here in a second VI that is running on the host computer, because everything that involves a calculation and does not require any intzeraction with the DAQ hardware is better placed there (the 850 MHz on the PXI controller celeton is slower than 2 definitiely something host GHz Dual-Core system).

The behavior you mentioned for the task "not started" HERE is what wrote using LabVIEW. I thought the block of relaxation that I had at the beginning would treat the start of the task - at least, the code like this figure repeatedly in the examples that come with LabVIEW.

I don't really understand what has caused the problems I've had - it seems that it was the combination of the measure quickly loop iteration with the start of task based on trigger HERE, but I do not understand what are the mechanisms in the background caused the problem (too forced loop was not the cause; the same block diagram works well with an additional loop timer).

If the solution was composed of the following steps:

-remove the old code calculating speed, replace it with a better code out of the main loop. This allows to

-reduce considerably the frequency of the main loop.

-Pull the DAQmx writing out of the loop, and

s ' ensure that all tasks are started explicitly just before the main loop starts to run.

-Possibly remove excess constraints of the main loop.

Tags: NI Hardware

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