usb 6002 using the 8 analog ports

[fablion]

Hi,

I am quite new user of DAQExpress and Ni USB 6002.

I managed to read data from 4 pressure transducers at the same time. The + port of each port is connected to the + port of the transducer and the - port to the - port of the transducer. A resistor of 470 ohms connects + and - port of the NI USB 6002. Each transducer is powered by its own 24 V power supply.

Is it possible to increase the number of pressure transducer connected to the NI to 7 or 8 equipments? (for instance plugging together all the - ports of the transducer to 1 port of the NI usb 6002, and connect 7 + ports of transducers to the 7 other ports of the NI USB.

Sorry if this question seems to be simple or stupid.

Regards

Fabien

[Tom_Powers]

You have to set up the acquisition as referenced single ended mode rather than differential mode.  If you use express VIs to create the task (yuck) then it will come up that way when you select more than 4 channels. If you right click on it and select create DAQmx Code it will create code so that you can figure out what they did.

Alternately you can create the

find the menu items for

DAQmx Create Task

then wire the task into a shift register in a loop the size of the number of channels and put

DAQmx Create Channel

with AI Voltage selected with the channel names and physical channels incremented by and RSE selected as the input terminal configuration selected.  Then put in a vi.

DAQmx Timing

Then put in

DAQmx Read selecting Analog 1D Wfm Nchan Nsamp

to get the data.

Wiring wise with differential you put the plus minus terminals are connect to:

AI 0 and AI 4

AI 1 and AI 5

AI 2 and AI 6

AI 3 and AI 7

For single ended you use AI 0 through AI 7 for the positive inputs and connect the negative wires, which it is assuming are all ground connections , to one of the 5 AI GND connections.   You will have to double up on some of the wires.

[fablion]

Hi @Tom_Powers,

Thanks for your quick answer.

The pressure transducers convert pressure in current, not in voltage. As your answer and the manual pages (https://zone.ni.com/reference/en-XX/help/370466AH-01/measfunds/refsingleended/) deals with voltage, I wonder if this is appropriate to my purpose.

Furthermore, the minus port of my transducer is connected to the minus port of the power supply (see link below), not the ground port. SO I don't think it is the same.

https://fr.rs-online.com/web/p/alimentations-de-laboratoire/1757368

Best regards, and happy wishes!

Fabien

[Tom_Powers]

First on the concept that the output is a current.  Let's assume that it is a 4 to 20 mA output, which is common.  If so you use a 250 Ohm load resistor and and it will convert the 20 mA to 5V and the 4 mA to 1V.  You can adjust the resistor up to maybe 500 Ohms and get 10V to 2V.  If you go this high you have to check the compliance of the output of your 4 - 20 mA loop, ,where compliance is defined as the maximum output voltage.  Put a resistor across each input.  Measure the value of each resistor with an Ohm meter if you want to calibrate things properly.

On the ground.  You should be tying the black terminal of that power supply to the green terminal in order to ground it.  Otherwise you are using a floating power supply.  While there may be a reason to use a floating power supply the normal configuration is to tie either the black terminal (for a positive supply) or the red terminal (for a negative supply) should be tied to the green terminal.  If you hook it up this way then you have a ground referenced signal.

In theory if you do not connect the black terminal to the green terminal then you can connect the "return" signal of your output to the GND terminal of the USB-6002 effectively referencing your signals to the ground of the USB-6002.   My approach would be to connect the black terminal to the green terminal on the power supply.

[fablion]

Thanks for your answer, even a 1st of January ! 🙂

To be sure to well understand (sorry, my english is not perfect and my knowlegde in electricity was in schools 🙂 😞

first loop: 

• + port of the power supply --> port + of the pressure transducer
• Port - of the pressure transducer --> "port +" of a 470 ohm resistor (we generally use this resistor in differential mode)
• "Port -" of the resistor --> Port AI0 of the NI USB 6002
• Common AIGND of the NI USB 6002 -->  Port - of the power supply
• Port - of the power supply --> port GND of the power supply

Second loop:

• + port of the power supply --> port + of the pressure transducer
• Port - of the pressure transducer --> "port +" of a 470 ohm resistor (we generally use this resistor in differential mode)
• "Port -" of the resistor --> Port AI1 of the NI USB 6002
• Common AIGND of the NI USB 6002 -->  Port - of the power supply
• Port - of the power supply --> port GND of the power supply

and so on.

Is it necessary to connect all Power Supply GND port together?

Thanks a lot for your quick help.

[Tom_Powers]

First I do not know what kind of pressure transducer you are using. 

I ASSUMED, and that assumption may be wrong, that it was the type which you apply DC power supply to, it has some conditioning electronics and outputs a 4-20 mA signal on two other wires.  Such a transducer generally 4 terminal, those being input voltage, input voltage return, output current and output current return. 

It is possible that you have a different type of sensor in which case I would need to understand what it is when it comes to how to make the measurement.  Can you provide a link to a data sheet?

Oh and when describing circuits in words it is best to do so in terms of Nets where a net is where multiple wires are connected together.  In the 4 wire sensor case the Nets are

Vsource -> Power supply red, sensor 0 to 7 input voltage

Ground -> Power supply black, Power supply green, Sensor 0 through 7 input return

I_out0 -> Sensor output +, R0-1, AI-0

I_ret0 -> Sensor output -, R0-2, AI_Gnd (one of 5)

I_out1 -> Sensor output +, R1-1, AI-1

I_ret1 -> Sensor output -, R1-2, AI_Gnd (one of 5)

. . .

I_out7 -> Sensor output +, R7-1, AI-7

I_ret7 -> Sensor output -, R7-2, AI_Gnd (one of 5)

One can debate the merits of connecting all of the I_ret# nets to the Ground net.

[fablion]

The pressure transducer used:
https://www.emerson.com/fr-fr/catalog/automation-solutions-fr-fr/rosemount-3051-coplanar-pressure-tr...

The power supply is this on (24V DC):

https://fr.rs-online.com/web/p/alimentations-de-laboratoire/1757368

[fablion]

[Tom_Powers · Accepted solution]

It is odd to me that it only has two terminals and is a 4-20 mA output, but I can deal with that.  I can not find a matching manual in English that has that figure as figure 4-1.  The one that I found (attached) shows something that is of interest in figure 5-2 which may mean that you can put the sampling resistor where the Ammeter and connect it to the USB-6002.  I suspect that the question becomes "In the unit is the extra terminal labeled "TEST" or "Vout Low Power".

The way that I interpret figure 4.1 is that the resistor B is the 250 Ohm resistor that I suggested. 

The earth connection is the green terminal on the power supply. 

I can do away with the ammeter and volt meter if you would like or leave them in when you build up the first channel just to check the circuit then remove them after you check that things work.

Without the ammeter and voltmeter you would have the nets.

V24 -> PS-Red, Trans-Pwr_Plus

Gnd-> PS-black, PS-Green, Rb-2, USB-6002-AI gnd,

Signal- Trans-Pwr_Neg, Rb-1, USB-6002-AI 0

If you put the ammeter and voltmeter in you still want to connect AI 0 to Rb-1.

You can use any resistor value between 250 and 500 Ohms you just have  to scale your current.  If you use a 250 Ohm resistor use a 1/2W resistor just to improve reliability.  You will have to scale your voltage read-back by the resistance so you should measure it or use 1% devices.   The scaling is

I = V / R

Do not be surprised if the signal is noisy.  In such cases I would take 100 points at 1 kS/s and use the average value.

[fablion]

After reading again your previous (not the last one) answer and looking after what my colleagues are doing (the resistor is connected directly on the NI USB in the + and - ports), I realize that the resistor have to be connected in parallel and not in serie (it is probably not well described. In fact, it is in serie and this is the NI which is in parallel on the resistor, right?). So if I well understand everything properly, I should connect everything like this:

Thanks for your confirmation, your quick and very appreciated answers and your patience!

Regards

Fabien