Load Cell software calibration
Alex Karpov
Jun-27th, 2023 7:35

Let;s start with the Raw Value / mV question:

The Raw Value representation mode gives the value directly from the analog to digital converter.

The IO-LC3 actually has a µV/V representation mode (activated the same way as Raw Value), which gives the relative input voltage in microvolts per volt (e.g. 1,000 means 1 mV/V). Note that both µV/V and Raw Value representation modes are meant for checking sensor connections and not for normal operation.

Soft calibration:
The IO-LC1/3 modules support soft calibration via the Advanced\Calibration\Edit Calibration Point command, which matches raw-value with weight. This method must be done carefully, because it is not as accurate as 'Hard Calibration' at the first place.

Please follow these notes for help:

*Used Load Cell has 2 setpoints: At 100kg he has 0,3765mv and at 2.500kg he has 9,4136mv.

  1. A few 'good to know' points:
    A) There is no need to calibrate more than two points.
    B) Either the order or the point-number assignment can be done in any order.
    C) It is advisable to start with a clean calibration (using the Advanced\Calibration\Clear Calibration command).
    C) Changing the default input range, if necessary, must be done before starting the calibration process.
  2. Calibrating the first point
    The loadcell parameters can lead to one calibration-point: The user can obtain one weight-uV/V pair from the loadcell parameters.
  3. Now there are three steps left in order to calibrate the first point.

    Step 1:
    Scale the loadcell data to an ADC raw-value.
    The following two factors will help scaling the uV/V value. Those factors are averages based on measurements which have been done at our lab on some samples and represent the average A/D converter uV/V to raw-value transfer factor (before multiplying by this factors, the user must first scale the calculated mV/V value to uV/V by multiplying by 1,000). There are two such factors: One for the -20mV to +20mV input range and one for the -80mV to +80mV input range.

    For -20mV to +20mV:
    2103 Raw-Value counts per uV/V
    For -80mV to +80mV: 526 Raw-Value counts per uV/V

    Please note
    that A/D converters differ from each other by nature and that is the main reason why the soft calibration is not as accurate as the hard calibration.

    Step 2: Apply the calibration-point to the IO-LC1/3
    Use the Edit Calibration Point FB to apply the weight-raw value pair to the IO-LC1/3.
  4. Calibrating the second point
    The best way to calibrate the second point is by hard-calibration which will also compensate for dead-load offset. This can be done using the Calibration\Calibrate Point FB. To calibrate the point, a dead-load will be a good 'known weight', since it usually referred to as zero weight. The Calibrate Point FB will acquire the dead-load's raw-value and match it with a user-defined weight (usually zero).
    Note that the user can calibrate a second point using the theoretical method described at 2), keeping in mind that:
    A) If Gross weight is of interest, performing a zero acquisition after the calibration (as explained in 4 below) is a must.
    B) This is the less preferred method, for use only when the dead-load cannot be acquired.
  5. Zero acquisition
    It is recommended to apply the Acquire Zero FB after the calibration and from time to time to further compensate for loadcell offsets. The Save Calibration must be used in order to save the new zero.

Example using the Loadcell (S/N 17041001) data:

Full Scale Load: 2500 Kg

Full Scale Output: 2.161 mV/V

Additional information: "The supplier of the loadcell says the following: For load cell s/n 17041001, the zero reading is 0.006 mV/V …" (not necessarily needed, will be calibrated using dead-load or zero acquisition)

1. Determining the input range:

Approximate absolute full-scale voltage (sensor output) = 2.161mV/V x 5V = 10mV

Hence, the default ±20mV range setting fits fine in this case.

2. Calibrating the first point – Scaling the loadcell data to an ADC raw-value

Taking the full-scale output in mV/V (that is 2.161 mV/V in this case), and scaling it to raw value:

First, we will multiply by 1,000 – getting 2,161 µV/V

Now we wil use the factor for ±20mV range (from above), that is 2103 Raw-Value counts per uV/V:

Raw Value = 2,161 µV/V x 2103 Raw-Value counts per uV/V = 4,544,583 counts

Now we shall add the raw value reading for dead load as explained in Step 2 above and continue from there.

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