I am running into problems using DAQmx on a PXI-1031 system.
Specifically, I am unable to read data collected by a DAQmx task into a
timed loop. In my setup I have 8 analog input channels that are read
and stored using two separate loops:
1. Store ch0-ch7 data at 2kHz
2. Read ch6 data in a 16ms timed loop
I am able to implement the storage fine, by initializing an 8
channel 2kHz continuous-sample DAQ task (outside any loops), then
reading 200 samples/channel using DAQmx Read (Analog 2D DBL NChan
NSamp) in a while loop (this gives a 100ms period for the 'storage'
loop).
The problem arises when I try to read ch6 data into the 16ms
deterministic loop. The ch6 data represents control feedback for an
actuator running at 16ms, therefore it only needs to read the most
recent data value (i.e. it does not need to store any extra data
between 16ms periods). It would seem easiest if there were a way to
simply query the current data coming from the 2kHz DAQ task, but I have
tried this with another DAQmx Read block off the existing 2kHz task,
with no success. Moreover, my understanding is that even if it could
read from the same task, it would remove data from the buffer -
stealing this data away from the storage loop.
The only way I have been able to successfully implement this:
- increase the speed of the storage loop to <16ms period (by reducing the number of samples to read per iteration)
- write one ch6 data element (DBL) to a local variable
- read this local variable in the 16ms control loop
This
seems like a very ad hoc solution, with the drawback that it forces the
storage loop to work much harder than necessary. I am certain there is
a better way than this, though I cannot find any information in the
help files. Can anyone suggest a better implementation for reading
analog data into a timed loop?
