Signal acquisition

[zaharoff]

Hi,

I'm using a NI PCI 6601 board card with a SCB-68 box for a velocity measurement. In fact, I have 2 Photo-Electrics sensors, and I would like to measure the time that something takes to get from on to the other. I have made all the connexion to the card, but I can't get any signal in Labview. I've tried to use the DAQ Assistant, but it don't get any signal. I think I will have to programm something for the time measurement between the 2 signals, but the first step would be to get these signals.

Thanks.

Benjamin.

[PNHorn]

Hi Zaharoff,

I need to know a couple of bits of information before we can move on with this. First of all, what signals are the Photo-Electric Sensors giving out when the object moves past it and makes it give out a pulse. I would assume it is giving out a 0-5v TTL signal, which would be ideal. If this is not the case then please let me know and we may have to look into some other areas to get the correct signal levels.

Next, what do you have it connected to on the SCB-68. For period measurements you have two options depending on how accurate you and the signal and how much programming you want to do. The two options both use the counter pins on your 6601 card.

Before we move on, please check that you have the correct pin assignments for the pins. If we assume you are using CTR 0 for the start measurement function and CTR 1 for the stop measurement function then the following pin assignments are what you need.

CTR 0: source pin - Pin 2

CTR 0: gate pin - Pin 3

CTR 1: source pin - Pin 7

CTR 1: gate pin - Pin 8

These pin numbers are a straight through connection to the pins on your terminal block so use the same screw terminal numbers as you have pin numbers on the DAQ card.

Now, the two options you have are:

1: Permanently enable the gate pin on both counters (this is actually the default state so just don't wire anything to the gate terminal on each counter and it should work fine), then take the signal from the source pin on each counter and use software timing to time the gap between the two pulses. This would be a very simple method of doing the timing but as it is done in software you are liable to get slightly poor readings as the operating system could decide to go off and randomly check the CD drive or look for virus's.

2: Take a known frequency into the source pin and then create a system where you enable the gate pin on the first pulse, and the disable the gate pin on the second pulse - all on the same counter. As you know the frequency and you can count how many pulses went through while the gate was open you can calculate the time taken for the object to pass between the two Photo-Electric Sensors, you simply multiply the period of the pulse by the number of pulses.

Hope this helps you get sorted, if you have further questions then please feel free to get back to me.

Best regards,

Peter H.

Applications Engineer

NIUK.

[zaharoff]

Hi,

Yes indeed, the signal out of the sensor is a 0-0.433V signal. But I don't know where to plug the cable out of the sensor : gate pin or source pin ?

Then I don't know which mode chose in labview while setting the DAQ. I think this is in the Counter Input menu, but then, I can't chose between all the choices : edge counting, frequency, period, ....I've tried all of them, but no signal in the test window.

Do I have to connect something to the DAQ in the labview block diagramm window to get the time between the 2 sensors ? Do I have to set 2 DAQs ?

Thanks.

Benjamin.

[AnalogKid2DigitalMan]

A signal with the range 0-0.433V is too small to trigger the counter. It accepts TTL logic levels (0-5V). You will have to modify the photodiode circuit or add amplification/buffering in order to have the signal compatible with the 6601.

[zaharoff]

Well, thanks for your help. I'll try to find this to get a 0-0.5V signal.

Benjamin.

[AnalogKid2DigitalMan]

Ben:

You keep getting the values wrong- TTL is near zero (0) volts for a logic low. Near FIVE (5) volts for a logic high.

Actual defined ranges are 0 to 0.8 volts for logic low and 2 to 5 volts for logic high. Suggest pulling up the output of the photodiode to 5V, if possible.

Good luck and happing timing!

[zaharoff]

Yes, I've tested the sensors output, and it is about 11.95 V. But I had to insert this signal in a circuit with resistors and zener diodes, that's why at the end I have a very low signal. I may found some help in my labo, to get something to amplify the signal.

Anyway, tanks a lot for your help again.

ben.