Strain Gauge module has to high offset error

[Rainer Ehrt]

Hi folks,
after Aaron helped to make my FP- bank working (Thx Aaron), the next
problem is the 2 strain gauge modules (FG-SG-140) having far to high
offset errors.

The specified Offset error for 15...35°C is 7,6µV typ. and 28µV max.

My measured Offset errors for the worst module are:

Ch0:...Ch7: +1365, +1302, +1245, +1138, +1277, +1079, +1041, +845 µV

So none of the channels is inside the specification.

How I tested the modules:
V(exc) = 5V, Half bridge completition on, Range = +/- 3,9 mV/V.
One Sub-D Connector with an 120 MF-Resistor as measuring- DMS,
another Sub-D with an 120 MF-Resistor as compensation- dummy.

This way the module at channel 0 measured 1,243 mV/V.
Measuring againe with the resistors exchanged to each
other: 1,486 mV/V
So U(Offset) = (1,243-1,486) / 2 = 1,365 mV for U(exc) = 5V

This is allmost a third of the choosen input range.
Am I right?

Self control:
I have two FP-SG-140 modules and a connector for each DMS and each dummy
input. So I could test each channel separately. To inshure cabeling
between the connectors and the FP-SG-140 are OK, I also changed the
modules to the socket of the other.

Any ideas?

[Rainer Ehrt]

Heres' the same again with the corrected calculation...

Rainer Ehrt wrote:

> Hi folks,
> after Aaron helped to make my FP- bank working (Thx Aaron), the next
> problem is the 2 strain gauge modules (FG-SG-140) having far to high
> offset errors.
>
> The specified Offset error for 15...35°C is 7,6µV typ. and 28µV max.
>
> My measured Offset errors for the worst module are:
>
> Ch0:...Ch7: +1365, +1302, +1245, +1138, +1277, +1079, +1041, +845 µV
>
> So none of the channels is inside the specification.
>
> How I tested the modules:
> V(exc) = 5V, Half bridge completition on, Range = +/- 3,9 mV/V.
> One Sub-D Connector with an 120 MF-Resistor as measuring- DMS,
> another Sub-D with an 120 MF-Resistor as compensation- dummy.
>
> This way the module at channel 0 mea
sured 1,243 mV/V.
> Measuring again with the resistors exchanged to each other: -1,486 mV/V
> So U(Offset) = (1,243-(-1,486)) / 2 = 1,365 mV for U(exc) = 5V
>
> This is allmost a third of the choosen input range.
> Am I right?
>
> Self control:
> I have two FP-SG-140 modules and a connector for each DMS and each dummy
> input. So I could test each channel separately. To inshure cabeling
> between the connectors and the FP-SG-140 are OK, I also changed the
> modules to the socket of the other.
>
> Any ideas?

[pravin b]

Rainer,
Did you missed adding the corrected calculation?

[Rainer Ehrt]

Hi Pravin,
no, the correct calculation is there, but I did hide it (accidantly)
inside the second posting, which shows as "reply", with preceeding ">".
Sorry for confusing.

Original lines:
This way the module at channel 0 measured 1,243 mV/V.
Measuring againe with the resistors exchanged to each other: 1,486 mV/V
So U(Offset) = (1,243-1,486) / 2 = 1,365 mV for U(exc) = 5V

Corrected lines:
> This way the module at channel 0 measured 1,243 mV/V.
> Measuring again with the resistors exchanged to each other: -1,486
mV/V
> So U(Offset) = (1,243-(-1,486)) / 2 = 1,365 mV for U(exc) = 5V

Rainer
###########

Pravin B wrote:

> Rainer,
> Did you missed adding the corrected calculation?

[Aaron_G]

Rainer,

There is a basic problem in how you are testing the module. When you turn the bridge completion on, and swap the resistors postion, you are effectively measuring the relative offset of the two resistors with respect to each other, you are not measuring the offset error of the FP-SG-140 itself.

To measure the offset error of the FP-SG-140, leave bridge completion off, take two resistors that are of equal resistance (+/- whatever percent), wire the first resistor from Vexc to Vin+, wire the second resistor from Vin- to Com, and wire a small jumper between Vin+ and Vin-. This will give you the offset for the range/excitation configuration that you are in.

Regards,
Aaron