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This guide has be done in order to assist the user to calibrate Hall Effect / LVDT transducers.

Introduction

The GDS Calibrator has been designed to give simple, accurate linear calibrations.
The unit is based around a high accuracy micrometer head and a linear guide.
The Calibrator has a maximum travel of 50mm with a resolution of 5um.

Mounting the hall effect transducer

Before inserting a transducer ensure the micrometer is set somewhere in the middle of its travel range. This allows for easy adjustment later on in the calibration process.

The Calibrator has purpose-designed attachments for mounting GDS Hall Effect transducers. Each Transducer (chip) should be calibrated against its corresponding magnet arm, with which it will be used in practice.

The Hall Effect transducer chip is mounted in the same style of holder as for placement on a triaxial sample.

Fit the magnet arm into the groove in the holding platen and tighten the two bolts holding the arm in place. (3)

The arm is then adjusted into position over the Hall Effect chip. The position of the arm can be altered vertically and horizontally. Using the horizontal adjustment position the magnet near the center of the Hall Effect chip. Using the vertical adjustment adjust so that the magnet block is parallel with the Hall Effect chip. (4)

Plug in the Hall Effect chip to the data acquisition device and adjust the lateral position of the chip to give a reading close to the zero gauss position of the transducer.


Gently tighten all of the retaining bolts.

Note this position then, using the micrometer, run the carriage throughout the quoted linear range of the transducer. Check that the magnet remains in contact with the chip and parallel to it throughout the range.

Mounting the LVDT transducer

The GDS displacement transducer calibrator is a high-quality versatile displacement transducer calibrator. A high accuracy micrometer head is used to set a series of reference displacements while the transducer is held in position.


Depending on the model chosen the micrometer may have a range of 50mm with a resolution of 5μm and accuracy of ±5μm or a range of 25mm with resolution 1μm and accuracy of ±3μm.


Typically the 50mm travel micrometer will be used with external potentiometer transducers, while the 25mm version is more suitable for use with LVDTs.

Performing a calibration

For AdvDCS V2 devices, please refer to the next section.

GDS Lab includes a wizard for entering tabulated calibration data and performing point-by-point calibrations where a single sensitivity value is not available or where there the relationship between raw output and engineering units is not linear. 

To access the wizard, click on the transducer you want to calibrate in the Object Display, as shown in the image on the right.

Under the Calibration tab, click on Calibrate Transducer

If you wish to store the old calibration details you must first create a new transducer.  The wizard will overwrite the calibration information for the currently selected transducer. Follow the steps shown below until you reach the calibration wizard main window.

The screenshot shows the wizard’s main window. 

For tabulated calibration data you input the ‘Applied Value’ and ‘Read Value’, clicking ‘Accept’ after each pair is entered.  To perform a point-by-point calibration you enter the applied value manually, then having selected ‘Autoread’ click accept when you have applied the force/displacement etc.

The graph and error values will update with each new data point.  When you have entered sufficient data points click next to continue. 

You can then choose whether to use a single best fit line for the calibration or linear interpolation between each data point (Look-up Table).  The wizard will remind you of the filename used and what the equivalent sensitivity value is for the entered information.

Finally, the new calibration details will be set and you are returned to the calibration tab.

Calibration for AdvDCS v2 (Calamari) devices

To change, update or create a new calibration for a transducer, open the transducer window by clicking on the transducer name in the Object/Hardware Display window.

In the image, for example, we open the Load Cell transducer window.

Select the Calibration tab. The transducer window shows the calibration applied to the selected transducer.

The figure on the right shows, among other parameters, the Sensitivity and Offset parameters, which are associated with a Linear calibration. Other calibration types can be used, for example:

  • Table-based calibration

  • Polynomial calibration

  • Calculated calibration (not modifiable by the user)

  • CalibrationNone, which provides the raw reading of the transducer without any modification.

If you need to switch the current calibration for a different one already existing, create a new calibration from scratch, or modify or update the current calibration, press Change/Modify Calibration.

If the button is greyed out and cannot be pressed, this means that the calibration has been locked by GDS. In this case, contact support@gdsinstruments.com and ask for that parameter to be unlocked.

Select calibration file

After pressing the Change/Modify Calibration button, a new window opens with a list of calibrations stored in the device.

This option is also the one that needs to be selected if you wish to create a new calibration from scratch without modifying existing ones, for example when adding a brand new transducer to the system.

From here, you have the following options:

  1. Select a different calibration without any modification
    In this case, select the desired calibration and press OK. This option is usually chosen when switching between transducers on a particular input channel. For example, a system may come with 2 different load cells with different full-scale values and this option allows to select the appropriate calibration for the load cell being used at the moment.

  2. Create a new calibration
    By pressing Add New Item, a new empty calibration is generated. Press OK to go back to the transducer window and manually enter the calibration parameters. If you prefer to perform a point-to-point calibration, click onView/Modify Selected Calibration to open the calibration wizard.

  3. View or edit an existing calibration
    Click on an existing calibration and then press on View/Modify Calibration. This will open the calibration wizard. The wizard will populate the data with the last user-performed calibration data. Factory calibrations do not provide any calibration data points.

The Calibration Wizard

The Calibration Wizard can be used to populate and inspect the calibration data points. Linear and polynomial regressions are generated automatically by the wizard.

This section will detail the steps to follow to generate a new point-by-point calibration using the calibration wizard.

Factory calibrations do not provide calibration data points

Info panel

In this panel, you can set a custom name for the calibration and an optional description. The former could be used to identify the type of transducer this calibration points to, whilst the latter could be used to store the serial number of the device and any additional information. The Type combo-box shows the type of calibration (e.g. linear, polynomial, etc.).

Add or modify calibration data points

Data table

Below the info panel, on the left, sits the calibration data point table. It contains data points collected during the calibration procedure and it allows the manual editing of the calibration points.

The field Applied refers to the calibrated units the transducer is currently subjected to, e.g. 1 kN or 50 kPa. The field Read is the raw reading of the transducer, which most often is in units of mV.

The figure on the right shows an example with 3 data points. The Applied, Read and Use? fields can be modified.

Data insertion panel

A new data row can be added by manually inserting data in the blank row at the bottom of the table or, preferably, by using the Applied Value and Read Value input box shown in the figure on the right.

Press Insert to add the pair of applied and read value to the data table.
The Autoread button, if enabled, automatically feeds the Read Value box with data collected automatically by the device. This allows for a fast point-by-point calibration procedure.

The Full Scale Value field is used to set the full scale value (in mV) of the transducer which will be used by the system to select the optimal input stage configuration for maximum accuracy. The Effective Full-Scale field shows the current full scale of the input.

Make sure that the full scale value matches the transducer full scale value as stated in the calibration certificate. If you are unsure on the exact full scale value, calculate it by dividing the maximum rated value of the transducer by the sensitivity. For example, for a 5kN load cell with a sensitivity value of 0.2597 kN/mV, the full scale value will be equal to:

FSV = 5 / 0.2597 = 19.253 mV ≃ 20mV

If the transducer being calibrated already has a valid calibration, the effective full scale value will revert back to the full scale value of the original calibration. In order to continue with the new full scale value, save the calibration and re-open the calibration wizard.

To delete a calibration data row, select it and then press Delete Row. Alternatively, you can un-set the checkbox under the Use? column to preserve the calibration row; in this case, the data row will be ignored when generating the final calibration.

Graph and regression results

At the bottom of the calibration wizard, you will find the graph and, where applicable, the regression results from the current calibration points. For a linear calibration, the results indicate the slope (which will determine the sensitivity of the final calibration), the intercept (which will determine the offset), the maximum error from linearity and its associated data point, and the R2 coefficient which determines the goodness of the linear fit (the closer to 1, the better).

For polynomial calibrations, this panel also shows the polynomial order chosen for positive and negative raw values.

Import/Export calibration

Calibration can be imported from file by clicking on Load from File. Supported formats are .ccal and .xlsx (both generated by a previous calibration on AdvDCS devices or provided by GDS staff).

To export the calibration in Excel xlsx format, click on Export to File.

Save calibration

Once finished with the calibration process, click on Save to save and apply the calibration to the transducer. The transducer window will show the new calibration parameters, which can be manually edited if needed. Calibration data points will be automatically saved to disk under C:\GDSLab\Calibration Files\ with the following file name format:

[TransducerName].ccal

Select the new or updated calibration

After closing the Calibration Wizard, select the new or updated calibration and click Ok.

The new or updated calibration should show in the transducer window. You can edit any calibration parameter without affecting the saved calibration file.

Always double-check that the calibration parameters (e.g. sensitivity, offset, limits) are within the expected range for the transducer.

 

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