Dear all,

I'm currently in the process of connecting a Solartron DC50 LVDT to a dataTaker DT80G for some linear displacement measurements.

Here's what I've found out so far, do correct me if I'm wrong about any of these information:

1. DT80G will only tolerate LVDT outputs of +/- 2.5 VDC?

2. The output of the excitation terminals on DT80G are +/- 4.5V?

3. When connecting LVDT to one of the analog channels on DT80G, the excitation leads on that particular channel (* and #) charge the LVDT. Therefore the output voltage from LVDT is fed into the + and - channels on DT80G?

4. I'm currently using DeLogger 4 for feeding programming instructions into DT80G.

   Do I set the Signal type to Bridge (1+V(BR,2,V) 1BGV(4W)) in DeLogger for taking readings from LVDT? Or do I set the signal type to Voltage?

   What about the excitation voltage? Is excitation voltage supplied in Signal type:Voltage settings? Or is it only done when using Signal type:Bridge?

Based on the information above, I've been doing some testing with LVDT both on DT80G and a multimeter and I'm getting inconsistent results. Here are the details:

1. LVDT connected to DT80G (Excitation * and #, LVDT output + and -) -----> Signal type:Bridge ------> fluctuating output both in ppm readings and mV readings (p/s: are the mV readings read from DT80G display the voltage output from LVDT?)

2. I connected LVDT excitation leads to a 4.7V DC power supply to obtain LVDT outputs of +/- 1.6V (measured with multimeter) with range of +/-62mm. Based on the excitation voltage of DT80G of 4.5V, a similar output from LVDT would also be read by DT80G if LVDT were to be excited by DT80G and read of its output by DT80G, weren't it?

The specifications of the LVDT are given below:
Brand: Solartron DC50
Type: DC Captive Armature
Sensitivity: 60 mV/V/mm at 10V DC (typical)
Energising current at 10V: 40mA
Input voltage range: 9-24 V (quite higher than what I obtained readings from).

I hope I haven't given too much information as to confuse you guys, but I really need some help in getting this LVDT connected to DT80G, if it were physically possible.

Thanks guys!

Good afternoon ca2n,

More information the better:

1. Input Range
   The Low Voltage range is +/- 3 VDC and the High Voltage range is +/- 30 VDC

2. Yes an unregulated 4.5 VDC

3. If programmed to do so.

4. No.
   A Linearly Variable Differential Transformer (LVDT) is not a strain bridge and so your configuration will not work.

I would recommend you connect an appropriate 9 to 24 VDC power supply as recommended by LVDT specification and read as a Voltage input.

Cheers,
Roger

Thanks for the feedback, Roger. I greatly appreciate it.

I went on and used a separate power supply to excite LVDT. Output from LVDT is now measured by DT80 with success.

My next issue is concerning a load cell that I'm exciting with an external power supply, the output of which I'm feeding into DT80 as Voltage measurements.

Would it be technically correct in taking the Bridge output from the load cell to be the ratio between the output voltage from the load cell (taken by DT80) and the input voltage supplied from the power supply (in ppm units)?

Hence, in so doing, would the interpretation of the bridge output into engineering terms still be the same as in the following:

Given:

Load cell sensitivity: 3mv/V @ 100N
Minimum output of load cell = 0.0 ppm @ 0N
Maximum output of load cell = (3.000 * 10^6)/1000 = 3000 ppm @ 100N full scale
Therefore, 1 ppm = 100N/3000ppm = 0.0333 N

Example of data:

Supply (excitation) voltage to the load cell = 12.11 V (from the power supply)
Output voltage from the load cell = 7 mV (taken by DT80)
Load cell output = (7/12110) * 10^6 = 578.03 ppm
Therefore, the load in Newtons = 578.03 * 0.0333 = 19.2 N

Thanks!

Good morning ca2n,

Good to see the LVDT is working. I hope you are using a very high quality instrumentation power supply for powering the load cell. It would be easier use the BGI channel type and power the load cell directly from the dataTaker.

Cheers,
Roger

Thanks, Roger.

Yes, I do acknowledge the advantages of using current excitation for the load cell. However, from a few test runs that I have conducted, I have seen that using BGI channel from the dataTaker will lead to ppm readings multiplied by a factor of 10 over the "traditional" computed (V-out/V-exc)*10^6 calculations. For example, at similar test conditions:
Using BGI:

ppm = 5700
Load cell sensitivity = 3.0 mV/V @ 100 N ------> 1 ppm = 0.03333 N
------> 5700 ppm = 5700 * 0.03333 = 190 N

Using the voltage channel and supplying the load cell from an external power supply (as from my previous post):

ppm = 570
Load cell sensitivity = 3.0 mV/V @ 100 N ------> 570 ppm = 570 * 0.03333 = 19.0 N

As you can see, in using BGI channel, the calculated maximum load cell has been exceeded (i.e. 190 N). I'll have to admit that there may be something wrong in the external power supply that I'm using. But, how is it that the output using BGI exceeds of that specified by the load cell itself?

Thanks!

Good afternoon ca2n,

3mV/V is 3000 PPM so unless you have over ranged your load cell the channel configuration is probably wrong.

1. BGI configurations
   Please check you have BGI(4W) selected as your channel type. The default settings for BGI are for 1/2 bridge configuration so you have to specify the 4 wire option and specify the correct gauge resistance (default is 350 Ohms). For details of the wiring to use please refer to configuration B4 on page 298 of DT80 manual (UM-0085-B7)

   To check the gauge resistance please measure the resistance between either the Sig +/- or the Power +/-.

2. BGV configuration.
   Please make sure you are reading the excitation Voltage on the low voltage range. If you are using configuration B1 on page 296 of DT80 manual (UM-0085-B7) and the excitation at DT80 does not exceed 3 VDC.

Cheers,
Roger

I have everything set up and everything is running smoothly. Thanks for all your help, Roger. To sum up, the following are my connection details:

LVDT:

Type: Solartron DC50
Voltage input: 12.11 V DC via external power supply
Voltage output: Read by DT80G (HV
````)

Load Cell:

Type: Interface SM-100N
Output: 3.0 mV/V @ 100 N
Input: DT80G (BGI, 4W)
Output: DT80G (BGI, 4W)
````

p/s: The discrepancies between BGI and voltage readings was no discrepancy at all. It was wrong maths. Silly me.