Data aquisition help needed

[Powakid42]

Hi, i am a Mechanical Engineering student from Portugal, my project this semestre envolves a strain gauge (wheatstone bridge) , a variable resistance, and a pressure gauge.

I have a NI435-x data aquisition board, and a tbx-68-t terminal block, i dont know much about electronics, so i need to know where do i conect the wires of the strain gauge (i have to wires to power it, i will conect a power unit) and i have the measuring wires, another two, i want to know where i conect these to in the terminal block.

Another question is the pressure gauge, it comes in intensity (amperes) the sensor makes variatons in amperes, i want to turn it in volts variations so i can conect it to the terminal.. how do i do it?

The variable resistance makes variations in ohms, i want to know to i do i turn it in volts.

Thank you for you help

[caz]

Hi to a fellow ME student,

I'm not familiar with your board and terminal block, but I imagine you can find a wiring diagram for the board and the block.  You're connector block will have 2 pins for each analog input channel, a +AI0 and -AI0 for channel 0 if the board does differential voltage measurements.  You would connect a measuring lead from the bridge to one of these pins and the other lead to the other pin.  Which lead you put on the + will determine the sign (positive or negative) voltage that you read.  You will want to keep track of this to know if the strain is compressive or in tension.

It sounds like you have a bridge with a variable resistance to balance the bridge, ie. make the voltage reading zero.  I'm guessing the measuring wires are connected on one end to a point between the 2 resistors in each leg.  These wires already measure voltage.  Changing the resistance of the variable resistor will change the voltage difference between the 2 measuring leads.

For the pressure transducer, if it outputs a current proportional to the pressure, you will have to convert this to a voltage to measure.  This can be done easilty with a resistor.  If you know the maximum output current, you can choose the correct resistor so as not to make a voltage higher than your daq board can digitize.  For instance, if the max current is 1 mA, and the board can digitize up to 10 V, then V=IR would say that you need a 10 kOhm resistor.  Connect the resistor to the 2 leads coming from the transducer and measure the voltage difference on either side of the resistor.  To get accurate pressure readings, you will need to know the resistance really well.  A little hint, a wheatstone bridge is really good at measuring an unknown resistance so maybe you could use your bridge once its working to get an accurate reading of the resistance.

The voltage measurement of the pressure transducer will be connected to the terminal block the same way that you do it for the wheatsone bridge measurement.  If your board supports multiple channels then you will be able to measure strain and pressure simultaneously.

When you have more labView questions please post again and note that there are separate threads for LabVIEW questions and DAQ questions.

cheers,

Drew