Strain Gage and Load Cell devices, and bridge completion resistors,
often have an internal fixed offset error which is
calibrated out using a technique called "Balancing".
This involves applying 0.0 force, measuring the
voltage from the bridge, and then subtracting this voltage from future measurements.
This one-point zero calibration can be done
on all strain gages and load cells with one command,
or can be done on each individually, as described below.
One can also do two-point calibration where one calibrates both at 0 and at
a high calibration point (e.g. 80Kg with 0 to 100Kg load cell),
as described here.
Balance All Gages at One Time
To balance (i.e. one-point zero calibration) ALL strain gage and load cell sensors at one time:
Make sure you are using instruNet software version ≥ v3.7 (file "iNet32.dll").
To check your version, select ABOUT in the instruNet World HELP menu; and to update for
free, click here.
Set up your sensors and save your settings in a .prf file via the SAVE button in the NETWORK page.
Apply 0 force to ALL strain gage and load cell sensors.
Select "Balance Gages" under the "Hardware" menu. This causes
instruNet to measure the voltage from strain gage and load cell
sensors, store it in the CONSTANTS settings VINIT fields, and then subtract this voltage
from future measurements such that 0 is recorded when 0 force is applied.
One can also set up the Network page Calibrate button to balance gages, as described
here.
Try several different stimuli
and see if they agree with the measured values. If they don't, click
here.
Save your calibration settings in a .prf file by pressing the SAVE
button in the NETWORK page (e.g. in file "balanced force.prf").
When doing measurements in the future, use this file.
If you want to calibrate a second (high) point,
click here.
Balance Each Gage Separately
To balance (i.e. one-point zero calibration) each strain gage and load cell sensor SEPARATELY:
Make sure you are using instruNet software version ≥ v3.7 (file "iNet32.dll").
To check your version, select ABOUT in the instruNet World HELP menu; and to update for
free, click here.
Set up your sensors and save your settings in a .prf file
via the SAVE button in the NETWORK page (e.g. with file name "force.prf").
Find your channel in the NETWORK page, press it's
Quick Setup
button, apply 0 force to your sensor, and then press the ZERO BALANCE button.
Try several different stimuli and see if they agree with the measured values. If they don't, click
here.
Save your new zero calibration settings by pressing the SAVE
button in the NETWORK page (e.g. in file "balanced force.prf").
When doing measurements in the future, use this file.
If you want to calibrate a second (high) point, continue with the below instructions.
Two-Point Calibration of Strain Gage & Load Cell
Two-point Strain Gage & Load Cell calibration involves establishing a zero point
via the Balance command as
described above. And then establishing a second point by applying a large stimuli and
measuring the response in the Quick Setup dialog box.
Perform one-point zero Balancing as described
above.
Find your channel in the NETWORK page and press it's
Quick Setup button.
Apply a large known stimulus to the sensor and
set the High Calibrate parameter to this value.
For example, if you apply 80.00Kg to a 0 to 100Kg load cell and are displaying Kg units, then set
High Calibrate to 80.00.
Press the HIGH CALIBRATE button to measure and lock-in the voltage associated with this value.
Try several different stimuli
and see if they agree with the measured values. If they don't, click
here.
Press the SAVE button in the NETWORK page to save your new Calibration values in a .prf settings file.
One can test a load cell or strain gage by applying a known force (e.g. 0) and see if instruNet
measures it properly. If it does not, then consider:
Make sure you are using instruNet software version ≥ v3.7 (file "iNet32.dll").
To check your version, select ABOUT in the instruNet World HELP menu; and to update for
free, click here.
If you have a fixed offset error and have not yet balanced (i.e. zero calibrated),
then do so now as described above.
Apply the known force which is not measuring correctly, click on the channel in the
Network page, and then press the SENSOR REPORT button.
At the bottom of the list you will see printed either "RANGE OK" or "RANGE ERROR". RANGE ERROR indicates the system is trying to measure a voltage that exceeds
the electrical measurement range (e.g. trying to read 52mV on ±40mV measurement range). This could be caused by a wiring error, bad sensor, or large offset from non-similar bridge resistors. To fix a too-large-offset problem,
increase the measurement min/max parameters in the Hardware settings area to force a larger voltage measurement range (e.g. move from ±40mV to ±80mV).
If your sensor does not measure 0 after you select Balance Gages with 0 force applied, look for RANGE ERROR after pressing the SENSOR REPORT button (while applying 0 force).
One can check wiring by measuring voltages with a handheld volt meter. One should see
a small voltage (< ±80mV) between the instruNet Vin- and Vin+ screw terminals, and see half the
excitation voltage (within ±100mV) between instruNet GND and Vin+. For example, one should measure 1.55V to 1.75V between GND and Vin+ with 3.3V excitation.
One can also set up the instruNet channel to measure volts (Voltage sensor type)
on the ±5V measurement range to see if
instruNet is measuring the same voltage as the handheld meter.
The instruNet Wiring field would be set to "Vin+ - Vin-"
to measure the voltage across the bridge (should be < ±80mV) and would be
set to "Vin+ - GND" to measure the voltage at one of the
bridge side nodes (should be ~Vexc/2).
Apply 0 force to a load cell or strain gage sensor,
record on paper the instruNet reading (e.g. uStrain, Kg),
select "Recalibrate instruNet" under the instruNet World "Hardware" menu,
and note how the measured value changes.
The change is due to instruNet recalibrating its internal electronics, which
typically adds several microvolts of additional accuracy due to the correcting of internal thermal drifts.
If your sensor originally balanced correctly yet you later see a small error with 0 force applied (and the
above "Recalibrate instruNet" does not fix it), then you probably are seeing thermal drift (i.e. change due to temperature)
from the sensor device itself or from the bridge completion resistors.
Sensors
heat up after applying excitation voltage (e.g. at power on); therefore, it is sometimes helpful to wait (e.g. 1 hour) for this heating to
settle before balancing (zero calibration), and before checking your work (at a later date) with 0 force.
Sensor heating, and its associated thermal drift, is worse with
larger excitation voltages; therefore, lower
excitation voltage is often preferred over larger
(e.g. 3.3V is often better than 10V).
If you want detailed information on how your sensor has been set up, then
press the Sensor Report button in the lower right corner of the Channel Setup dialog.
If working with i100 hardware (not i4xx/i60x), add 0.1uF capacitors to reduce the effects of RFI.
Click on your channel in the Network page and select Mapping in the
Settings menu of the Channel Options dialog. Does your mapping look correct?
The displayed value is in External units and is being mapped from
Internal units,
as described here.
For more hints on sensor troubleshooting, click
here.
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