Synchronise cDAQ with USB 6009

[samfau2]

Hello,

I appreciate this question has probably come up before but I couldn't find any definitive advice.

I have a cDAQ chasis with a 9237 bridge module for pressure measurements via transducers. I also have a valve with a limit switch to indicate fully open and fully closed. The switches will be connected to a USB 6009 to measure the closing time of the valve. One switch breaks when it starts closing and one switch makes when it finishes closing, both circuits will be connected to analog inputs on the 6009.

Ideally, I would like to be able to synchronise the pressure measurements relative to the valve closure. I.e. mark the point at which the valve started and finished closing on the plot of pressure measurements over time. The valve closure will be rapid (60ms) and I will be sampling pressure at 50KS/s.

How easy is synchronise the two DAQ devices to allow this to be performed accurately? If this is possible, could you point me in the direction of advice/guidance to achieve it?

Alternatively... As I am only using 2 channels of the 9237, is it possible to use the other two channels for the valve limit switches. I am unsure how this works due to the fact the NI 9237 is a bridge based module (i.e. it has AI0+ and AI0- as opposed to just AI0 on the 6009).

 Hope this makes sense. Thanks.

[Tom-S]

Hi,

To synchronise two DAQ devices the must be able to share a clock. Unfortunatley between your two devices true hardware sychronisation is not possible, as they have no way of sharing a clock. You may be able to use software synchronisation however by using software timing within your code to save the time when the valve closes and then plot this time against the pressure plot. 

It may be better to use your alternative solution as you will then be able to do true hardware sychronisation as both elements can use the cDAQ onboard clock. You can find information on how to sychronise to DAQmx channels on the following white paper.

http://www.ni.com/white-paper/5471/en/

I hope this helps,

TomS

Applications Engineering

National Instruments UK

[samfau2]

Thank you Tom that's very helpful. I will have a look at that white paper.

Sam

[samfau2]

Hello again,

With regards to your previous post.. It would now be beneficial to use up all 4 channels of the NI9237 with transducers. Therefore leaving no room for the

switch inputs.

If I were to use a the 6009, to what degree of syncronisation could be achieved and how easy would this be to perform? I only require the valve open/close signal to be accurate to around 1ms.

I have had the switch circuit working perfectly on the USB-6009 but when using the 9237 I get a lot of signal noise/interferance when the switch is open and it erractically jumps between the top and bottom of the input range. This is a separate issue but just wondering if you had any ideas what might be causing it? Is it likely that it's because the 9237 is designed for measuring bridge based signals and not straight voltage signals?

Thanks

Sam