9211 question; and also how do I create a simple PI control?

[oilyfingers]

Hi

Firstly, with the 9211 4 channel thermocouple input module, I presume it's okay to connect different types of thermocouple to different channels, at the same time? (eg channel 1 - T type, channel 2 - E type etc). Also why is the sampling rate for this '14 Sample/sec' - seems low - but I can apparently run it at much higher rates and labview doesn't complain? 

Secondly, I want to implement a simple 14 channel PI temperature controller. I've had a look at some of the examples but 'PID Control-Multi Channel.vi' (for example) seems only to be a proportional controller, no integral term. Perhaps I should just write my own? Can anyone point me in the right direction?

Dave

[oilyfingers]

I've managed to solve the second question (by using the PID toolkit) but can anyone answer the first question? Can I use different thermocouple types at the same time (on different channels) and what is the best way to set this up - to use a virtual task?

[Mark M · Accepted solution]

Hi Dave,

Hope you are well today!

Yes, you can use different types of thermocouples connected to several channels on the same 9211 in one task. You can simply create a virtual task for a group of channels in MAX to realise this. 9211 takes measurement from channel one after the other by scanning through them (time division multiplexing), rather than taking all measurements simultaneously. This is due to 9211 has only one ADC, cold-junction compensation circuit and auto-zero circuit.

Now, depending on whether you intend to use CJC and/or auto-zero feature or not, the maximum sampling rate varies slightly. A comparison table can be seen on page 20 in the datasheet of 9211.

I suggest the reason why 9211 does not support higher sampling rates was due its designed applications. 9211 was designed for general temperature measurements from the ground, and there is no point to sample temperature of an point say 20,000 times every second, because in many scenarios the temperature does not change that quickly. Also, by lowing the sampling rate, the noise immunity performance can be increased on the ADC.

Say if I do a temperature monitoring application, I would normally refresh my temperature reading every second, or every half of a second and that should provide an accurate indication to my environment. However, If you are looking to do measurements in some extreme environments, alternative DAQ devices/sensor should be considered.

Hope this helps,

Message Edited by Mark M on 11-27-2008 03:04 PM

[oilyfingers]

Thanks - presumably if I do sample at a faster rate eg 140Hz, the 9211 will simply return the same value 10 samples in a row ( in this case, since 140Hz / 14Hz = 10)?

Is there any detriment to sampling faster than the maximum rate for the hardware?

Dave

[Mark M]

Hi Dave,

I presume you have not bought and used a 9211 yet? In reality if you set a sampling rate that is out of its specifications, the driver (DAQmx) will report a fault and will not continue the task.

For example, if you set it to sample at 140 Hz for 140 samples, the DAQmx will return with the following errors:

Error -200077 occurred at Test Panel

Possible Reason(s):

Measurements: Requested value is not a supported value for this property.

Property: SampClk.Rate
You Have Requested:  140.000000
Valid Values Begin with:  2.793968e-3
Valid Values End with:  12.000000

As you can see, in the error report, it also indicates the maximum and minimum values you can specify for this parameter. In this case, the min rate is 2.79e-3 samples/s and the max rate is 12 samples/s.

Hope this helps,

Message Edited by Mark M on 11-28-2008 04:43 PM

[oilyfingers]

actually, I am using a 9211 right now, and also a 9205 (analog input module) in a compact DAQ frame. The 9205 can sample at much higher rates and I have one ADC loop running a a few hundred Hz, sampling both the 9211 channels and the 9205 channels at the same time. And it works fine! This is why I asked the question....