U3-LV pressure and displacement sensors setup

6 posts / 0 new

Thu, 10/22/2015 - 09:32

kreecki

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Dear LJ Support,

I have U3 LV and I would like to measure and record hydraulic pressure and linear displacement.

My hardware would include:

1. DC power supply of 10 V as per below link:

1.1. FAR-1-220 http://www.omega.co.uk/temperature/pdf/PSR-24S.pdf

2. Pressure gauge as per below link:

2.1. PX312-3KGV http://www.omega.co.uk/pressure/pdf/PX302.pdf

3. Linear displacement potentiometer:

3.1. LP803-03 http://www.omega.com/pressure/pdf/LP803.pdf

My questions are:

1. Can I connect all the sensors as per diagram on below picture?

1.1. Do I need any other type of resistors / bridges / amplifiers that I have to connect to LJ in order to measure the signal from the above sensors assuming they are powered by 10 V DC source as per (1.1)?

1.2. When connecting the pressure sensors should I just connect the power supply (1.1) to Excite +/- on pressure sensor and output +/- to LJ AIN1/GND?

1.3. When connecting (3.1) displacement potentiometer should I connect 10 V DC supply to GRD / V-in and V-out to LJ FIO 1 and LJ GND to sensor Ground?

I really appreciate any help on this.

Tom

File Attachment:

LJ connections.PNG

Thu, 10/22/2015 - 16:27

LabJack Support

Problems immediately jump out

Problems immediately jump out with both sensors.

1. Pressure: This is a raw bridge type sensor. Lots of info in the following old forum topic, but you will need an LJTick-InAmp to handle this with the U3:

http://forums.labjack.com/index.php?showtopic=4376

https://labjack.com/support/datasheets/accessories/ljtick-inamp

2. Potentiometer: This is connected properly, but the problem is that it will give you a 0-10V signal if you are varying the pot across its entire range. The low voltage analog inputs on the U3-LV are limited to 0-2.4 volts normally, or you can set them to the 0-3.6 special range. If you can keep your pot moving in the area where the signal is 0-3.6 you should be good, but if you need to move the pot across its entire range you need to change something. Either reduce the excitation voltage to the pot, or attenuate the output signal with something like the LJTick-Divider.

Fri, 10/23/2015 - 04:46

kreecki

Thank you for the advice.I

Thank you for the advice.

I have modified my schematic drawing.

Can you please have a look to see if this will work?

How do you calculate required gain of the voltage signal that needs to be set on the LJTick-InAmp?
My pressure sensor output signal will be in range 0-100mV at 10 V DC supply therefore if i selected 11 gain will it be 0-1.1 V?

Tom

File Attachment:

LJ connections_Rev1.PNG

Fri, 10/23/2015 - 08:19

LabJack Support

Use the signal range tables

Use the signal range tables for the LJTIA:

https://labjack.com/support/datasheets/accessories/ljtick-inamp/appendix-a

In your case you run into a problem right away. Your Vcm is 5 volts, and if you use the calculator linked on that page I think you will find that common mode voltage does not work at all with the LJTIA.

So, remove your power supply connections from Vexc+ and Vexc-, and instead connect LJTIA-VREF to Vexc+ and LJTIA-GND to Vexc-. This will excite your sensor with 2.5 volts, which means the Vcm of your bridge is 1.25 volts. I see that the PX312 mentions an excitation voltage of 5 to 15 volts. The max spec of 15 volts is likely important, but typically a bridge circuit does not have a minimum voltage, so I suspect that this will work fine with 2.5 volts.

So with Vexc=2.5V, the output of your pressure sensor at rated pressure will be 25 mV. Looking at the LJTIA table with Voffset=0.4V and Vcm=1.25V, if you selected a gain of x201 then an input signal of 0.0104V would result in an output signal of 2.5V. The best option is this case is a gain of x51, where an input signal of 0.0412V will result in an output signal of 2.5V. Keep in mind the normal input limit of the U3-LV analog input is about 2.4 volts, so this gain of x51 should work best with your signal which goes up to 0.025V.

Fri, 10/23/2015 - 09:22

kreecki

Thanks for corrections.All

Thanks for corrections.

All this is new for me, hence my trivial questions.
Revised schematic attached. Pls have a look.

I basicly thought if the manufacturer specifies 5-15 V than that needs to be maintained.....

What with the accuracy of measurements of the sensor like this when half of the recomended minimum voltage is provided?

Once i will have all hardware organized i will be back with more questions.

File Attachment:

LJ connections_Rev2.PNG

Mon, 10/26/2015 - 15:19

LabJack Support

Revised schematic looks good.

If you ask the manufacturer, they will likely just say something like "we only specify accuracy if Vexc is 5 to 15 volts".

I my experience, it is quite rare for a bridge circuit to specify a minimum voltage. They usually are simple resistive circuits, and simply specify a max voltage. With a simple resistive bridge, the accuracy should not be affected by how low Vexc is, but perhaps there is something special about this bridge?

If you want to use 10 volt excitation, you could use the EI-1040 amp with the U3, or use a U6/T7 instead which has a built-in amp that will be fine with Vcm=5V.