excitation voltage for load cell

[sfjd]

Hi,
I was trying do a custome volatage task for my HBM load cell (Type: C6A : capacity: 100 tonne). Is it necessary that I calibrate the instrument usiing the calibration wizard provided under the calibration tab inside MAX? My lab technical person told me that when I give the excitation voltage correctly and given my custom scale, the SCXi 1520 and 1314 (on a 1052 chasis and 6251 DAQ)should indicate the correct load. But this is not happening to my case. I have a few questions:

1. In the HBM data sheet for C6 force transducer, I searched for excitation volatage, it gives a range (say 0.5 V to 10 V). What wall I enter for the loadcell. I tried with 1 V , 2 V, ... 10 V. Each time when I click the calibrate wizard, I get different different values for known load values. But all the time the indicated load reading by NI is far from the actual value. I have applied the loads through an UTM and read the values using an HBM indicator. To check the HB indicator I also connected anothe indicator of different sort. Both showed the same reading. But when I connect it to the NI, it sensed different values: I will write the steps I followed to measure load value:

1. Create a custom voltage task in MAX.
2. Change Max: 100000  kg(f) (100 tonne capacity load cell: 2mV/V output: excitation 0.5 V to 10 V); Min to 0 kg(f)
3. Full bridge; Excitation: 1 V( I tried with 2, 3,4,5,6,7,8,9,10 but each time with different result).
4. Custom scale: Map range
    prescaled: Maximum: 2 mV/V multiplied by 1 V excitation = 2m
                      Minimum: 0
    Mapped value : Maximum = 100k  ; Minimum: 0
    Units: volts to kg(f)
5. Sampling mode: continuous sampling rate 1000; sampling frequency: 1000
5. Click Calibration under the third tab (after settings, device,)
6. I kept two load cells of 100 tonne capacity each of the same make one above the another between the cross heads of the UTM. One load cell was connected to an HBM load indicator and the other to the SCXI 1520 and 1314 terminal block channel 0.
7. No load applied; In the calibration table of DAQ task in MAX, the sensor showed zero.
    5 tonne applied:   sensor indicated 3.5 tonne in the calibration table (different 1.5 tonne)
    10 tonne applied: sensor indicated 8.2 tonne. (difference 1.8 tonne)
    15 tonne applied: sensor indicated ; sensor indicated 15.2 tonne
    20 tonne: indicated 22 tonne:
    25 tonne applied : indicated 27 tonne:

8. The above values showed dffrerent result when I changed the excitation voltage to 2 V and redo the calibration. I have done this upto 10 V excitation, each time it gives some absurd result.

I am totally fed up with these measurement system. Did I make any mistakes in the above process? I don't know. It would be of great help if somebody could help.

Regards,
Rajeevan
  

[MickeyD]

Hi Rajeevan,

The strain gauge will return different voltages depending on the excitation you put in to it, so that is not unexpected.  The problem though is that you are not seeing the values you expect for any given excitation.  With your other indicators was there any excitation being passed to the strain gauge and if so what voltage was that. 

Can you just choose one of the excitation voltages then take the readings and input the calibration values.  Then use the task you have created and just measure other values this should give you precise and stable readings.  Using the calibration tab you might see the differences you are describing, but by calibrating them you should not see them in the final data.  The key to getting your application working is to find out what excitation voltage the sensors you feel are giving you correct readings are using, then we can set up your SCXI system to do the same.

Have a great day,

Michael D
Applications Engineering
National Instruments

[sfjd]

Dear Michael,

I also used the recomended value of excitation (5 V) for the load cell (HBM, Type C6A, capacity 1 MN). Then also it showed varying result. During calibration using UTM, if the NI instument showed 96 tonnes for an applied load of 90 tonnes, though we make a calibration, how reliable is the measured data? I mean, a difference of 6 tonnes is too much.

Other indicator is not connected the present load cell connected to the NI. How I measure load is given below:

1. Place one load cell above the other(same make and capacity).
2, Top load cell connected to NI
3. Bottom load cell connected to HBM indicator.
4. Start applying load ove the load cells using UTM.

I don't think any excitation is passed from the HBM indicator to the NI.

I also have trivial question. My load cell is 100 tonnes capacity. I am going to measure only in the range zero to 100 tonnes.

Do I need to give the min under settings as -100 tonnes and max to +100 tonnes. Or can I give min as zero and max as +100?

Regards,
Rajeevan

[Ed W]

Hi Rajeevan,

There are several other tests that you can do for these types of measurements. 

1. Measure the excitation lines with a multimeter or another external device to verify that the proper excitation levels are output.  If they are not correct, there may be a hardware problem with your device.
2. Once those excitation levels have been verified, connected them to your load cell, and measure the load cell output with a multimeter.  You should not see any discrepancy between the load cell output level on the multimeter to that of the NI device.

Please post back with the results of these tests, and we can continue troubleshooting from there.

Ed W.

Applications Engineer

National Instruments

[MSS]

This is very common when you use HBM indicator as a reference. If you would have calibrated and stored the linearized values earlier against the testing instrument to be calibrated.

when you change the excitation voltage the feedback voltage also will change so the indicator will indicate different values for different excitation . If you want to use different excitation you have to calibrate it for all that excitation voltage that is going to be used.

Could you post the values in mv  for atleast five different loads and for two trials for particular excitation voltage.