External trigger with SignalExpress

[DropTower]

Hi, I have a question regarding using an external trigger in SignalExpress

I want to do some basic impact testing using a small drop tower and I need to be able to use an optical sensor as a external trigger to initiate recording data from my force sensor. Included is a rough illustration of the setup. There is crosshead in a guided free fall and a force sensor resting on a platform below. As the crosshead falls, it passes by an optical sensor. In short, I want to start recording data just before impact (ie. as the crosshead passes the optical sensor) and continue recording for a duration of 5s.

I have a NI cDAQ 9178 and the NI 9205 that I am using. I'm fairly new to SignalExpress but I have spent hours reading the manual, looking at examples, searching the web and fiddling with SignalExpress trying to figure out how to use an external trigger. So far, I have found a number of ways to use internal triggering, but nothing on external triggering. Is it even possible with SignalExpress? And if so, what do I need to do?

Thanks

[DFGray]

External triggering is highly dependent upon your actual hardware.  In your case, with a 9205 in a 9178, your only external triggering options are analog.  However, this should work for you, provided your required sampling rate is not above 125kSamples/sec (which is half the 9205's sample rate).  Try the following:

1. Wire your optical sensor output to channel 0 of the 9205.
2. Wire your force sensor output to channel 1 of the 9205.
3. In SignalExpress, drop a DAQmx Acquire step and take two voltage measurements - channels 0,1 on the 9205
4. Set Acquisition Mode to N Samples, Sample to Read so you get 5s of data, and Rate (Hz) to your needed sample rate.
5. On the Triggering tab, select Trigger Type = Analog Edge and Trigger Source = channel 0 (will be something like cDAQ1Mod1_ai0).
6. Set the Slope and Level to appropriate values based on your optical sensor.
7. Run...

You will get two sets of data - one for the optical sensor and one for the force sensor.  From your description, you can toss the optical sensor data and only use the force sensor data from channel 1.

Note that only channel 0 of your 9205 module can be used for triggering, so you must wire your optical sensor to it.

Good luck.  Let us know if you have further questions.

[DropTower]

Thank you, this information was most helpful!

I followed your instructions and it works well. The triggering problem is now solved. However, I noticed something odd with the data. I have attached two screen shots to help explain what I'm seeing. For both screen shots the hardware set up was the same and both the optical sensor and force sensor were in a RSE connection configuration. I also tried NRSE and differential, but got the same results.

First I set up SignalExpress to measure only the signal from the force sensor and recorded a typical impact. Everything looks good - initially the signal is constant at zero, it spikes with the impact and goes back down to zero.This is shown in the first screen shot.

Next, I set up a new project in SignalExpress to measure both signals (one from the optical sensor and one from the force sensor) and set up the trigger according to your instructions. I let it run without applying an impact to the force sensor (ie. dropped the crosshead, let it fall past the optical sensor but stopped its fall before it hit the force sensor) and displayed both sets of data. From this I would expect a pulse for the optical sensor signal and a constant signal near zero for the force sensor. The force sensor signal itself should be independent from and unaffected by the optical sensor pulse. The second screen shot shows what I get instead. The pulse from the optical sensor looks good, the triggering is correct, but the signal from the force sensor is not constant and looks more like the charge-discharge cycle of a capacitor. I don't understand what is happening here.

I have tried changing a few different things that I thought might be causing the problem, but I have not found anything so far that makes a difference. Maybe the sensors aren't connected properly, I am not sure. Do you have any suggestions?

I will mention one more thing in case it is contributing to the problem. So far I have not actually been using an optical sensor, but instead I am using a 2.6V voltage supply with an on/off switch. I just toggle the voltage supply on, then off, to mimic a pulse from an optical sensor. Eventually, when things are working better I will replace it with the actual optical sensor.

[DFGray]

Given the evidence, it appears your two channels are capacitively coupled.  As you said, the second channel is charging and discharging based on the results of the first.  This could be due to a couple of reasons.

1. Your setup has capacitive coupling between the input wires.  Without actually seeing your setup, this will be hard to diagnose.  If this is the issue, consult your nearest electrical engineer for help.
2. You may be seeing expected crosstalk between the two channels of the 9205.  The 9205 only has one analog to digital converter, so multiplexes between them.  Given the large differences in input signals between the two channels, you could be seeing the results of not having enough settling time between taking measurements.  You can look at the specs on your 9205 and compare to your acquisition rates to see if this is the issue.  If this is the problem, simply reduce your acquisition rate. It appears, from your data, that you can do this.

Good Luck!