Calibration of ShearTrac II sensors

Basic Principles

    In order to calibrate the transducers used in the ShearTrac II system, you will need an independent means for accurately determining the actual condition the transducer is experiencing.  For calibration of all the sensors that are part of your system, you will use the software that you received such as DS, Shear, DSS, Shear.DSS etc... Start your software and load your calibration template that was supplied to you. After that Save As the template file with a new name specifying the new date of calibration. 

    During the calibration process for which the Calibrate X window is used (X stands for Load, Displacement, Cell Pressure or Sample Pressure), you will need to enter (in engineering units) the actual condition being sensed by the transducer and then click the Read button to record (in counts) the transducer output.  After you have done this for a range of values, clicking the Calibrate button.  The program will calculate a calibration factor and offset value (the sensor output at zero condition).  Click the Save button to save the calibration information.  You can then view the results of the calibration on the Calibration Summary window opened by clicking Summary in the Calibrate menu.

    The Calibration Summary window displays the calibration factors and offsets that the program uses to convert the transducer readings from counts to engineering units.  The software control software performs the conversion by first subtracting the offset (in counts) from the transducer reading (in counts) and then multiplying the result by the calibration factor as shown in the equation,

where:

R_E is the sensor reading in engineering units,

F is the calibration factor,

R_C is the sensor reading in counts and

O_C is the offset value in counts.

The calibration factor will depend on the unit system you are using.  The factor will be automatically adjusted if a new system of units is selected.  The current units are shown on the System Monitor window (opened from the View menu).

*CAUTION: When you have finished the calibration of the system sensors, it is important to open the Calibration Summary window and click the Download button to transfer the calibration parameters to the embedded controller of the ShearTrac II unit.  (Alternatively, you can transfer the calibration information using the ShearTrac II keypad after the calibration has been performed.  See your ShearTrac-II hardware manual for how to do this.)  Failure to do so will result in the incorrect factor being used if ShearTrac II are operated manually, and in incorrect values being displayed on the LCD screens when System is chosen from the Monitor menu.

 *Note: You can manually change values in the Calibrate X windows and on the Calibration Summary window.  However, this should be done only if you are certain a value is in error and you know what the correct value should be.

*WARNING: Use great care in altering the calibration factors and offsets for the force transducer.  Incorrect values can cause the system to apply the wrong forces and equipment damage is possible.

Vertical and Horizontal Force Transducers 

*Note: In this section, X is used to stand for either Vertical or Horizontal.

    The force transducers (load cells) are initially calibrated prior to shipment. Re-calibration should be done every year. To perform the calibration, you will need an independent way of measuring force that is applied to the load cell.  We recommended that you use a certified calibration grade load cell.

    The calibration can be performed in any system of units. If the units are changed later, the calibration will automatically be converted to the new units. For example, if you have a calibration grade load cell that reads in pounds, it is most convenient to choose English in the Units window (accessed from the Options menu) before performing the calibration. Then, if your test is to be run in force units of Newtons, you need to change the units to metric before running the test.

The description that follows is for a calibration in Newtons.  Modify these instructions as required for a different force unit.

    To calibrate the vertical load cell, raise the loading mechanism enough so that you can place your calibration grade load cell between the bottom of the water bath box and the vertical load cell.  To calibrate the horizontal load cell, move the water bath box horizontally enough so that you can place your calibration grade load cell between the right end of the water bath box and the horizontal load cell.  Use the appropriate keypad to position the loading mechanism or water bath box. 

    The calibration is performed using both the X Load Control window and the Calibrate X Load window. Bring down the Control menu and select X Load to open the X Load Control window.  Position this window at a convenient location on your screen.  Bring down the Calibrate menu and select X Load to open the Calibrate X Load window. Figure 1 shows the Vertical Load Control and Calibrate Vertical Load windows. 

Figure 1. Vertical Load Control and Calibrate Vertical Load Windows

    On the X Load Control window, enter a value (in counts) that corresponds to a relatively small load.  This will typically be around 32,000 to 33,000 counts.  The right text field will display a value based on the conversion factor currently in the program.  Click the Go button. A positioning will occur until the reading from the system load cell matches (as per previous calibration) the force value you have entered.  When the positioning stops, read the force produced on your calibrated load cell and enter it in the Dial Reading column of the Calibrate X Load window.  Click the large Read button on the Calibrate X Load window.  A value will appear in the Sensor Reading column.  Repeat this procedure for a series of load values that covers the range of the system load cell.

    When you have collected the data needed for the calibration, click the Calibrate button on the Calibrate X Load window. The factor (calibration factor or slope, in engineering units per count) and offset (in cnt) of a best-fit line will be displayed in the right-hand text fields of the Calibrate X Load window together with the standard error and correlation coefficient. If the correlation coefficient is less than 0.9999, you have gotten a bad reading and need to redo the calibration. We recommend that you view the calibration data and the best-fit line in graphical form. To do this, click the Graph button. Check that the data closely fit the line. A poor fit or a bad data point indicates that the calibration should be repeated.

    If you have a good calibration, write down the calibration factor and offset (but do not save them yet). Open the Calibration Summary window (by clicking Summary on the Calibrate menu) and compare the factor and offset to those shown.  (Note that the Calibration Summary window opens with Vertical Load as the default sensor in the top text field.  Be sure to use the pull-down button at the right of the Channel text field and choose Horizontal Load when calibrating the Horizontal Load transducer.)  If they are more than 5% different, you have an error somewhere, or the equipment has changed significantly since the last calibration. (This comparison, of course, assumes that you are performing a routine calibration on a load cell that has been calibrated previously.)

    To save the calibration factors so that they will be displayed on the Calibration Summary window, click the Save button on the Calibrate X Load window.

*Note: If you close the Calibrate X Load window (by clicking the Close button or the X in the upper right corner of the window), the calibration results will not be saved, but neither will they be removed from the widow.  If you do not save the results before you close the window, you can later reopen the window and save the results.  Saving the calibration results will replace the values shown on the Calibration Summary window. 

    After saving the calibration parameters, check to be sure the Factor and Offset values on the Calibration Summary window (see Figure 2) are the same as those on the Calibrate X Load window.

Figure 2. Calibration Summary Window

    Be sure to click the Apply button on the Calibration Summary window shown in Figure A 2.  This will both apply the calibration results and save the calibration data (including standard error and calibration graph results).  Clicking the Download button will download the calibration results to the controller in the ShearTrac-II unit.

*Note: If you close the Calibrate X Load window before clicking Apply and Download in the Calibration Summary window, the calibration factors will not be lost, but the calibration data (including standard error, calibration graphical results, etc.) will be lost.

Vertical and Horizontal Displacement Transducers

*Note: In this section, X is used to stand for either Vertical or Horizontal. The images are from Vertical displacement. 

    The displacement transducers (LVDTs) are initially calibrated prior to shipment.  Recalibration should be done every year. To perform the calibration, you will need a set of precision gauge blocks. The calibration can be performed in any system of units. If the units are changed later, the calibration will automatically be converted to the new units. For example, if you only have gauge blocks that are in inches, you will need to choose inches in the Units window (accessed from the Options menu) before performing the calibration. Then, if your test is to be run in length units of millimeter, you will need to change the units before running the test. 

    The description which follows is for a calibration performed using gauge blocks specified in inches and a displacement transducer with a 1.5 in (38 mm) stroke. Modify these instructions as required for other transducers and/or other gauge blocks.

    The calibration is performed using both the System Monitor window and the Calibrate X Displacement window. Pull down the View menu and click the System option.  This will open the System Monitor window. Pull down the Calibrate menu and click the X Displacement option. This will open the Calibrate X Displacement window.  The System Monitor and Calibrate Vertical Displacement windows are shown in Figure 3.

Figure 3. System Monitor and Calibrate Vertical Displacement Windows

    On the System Monitor window, you will see count readings for displacement in the left text boxes of the rows labeled Vertical and Horizontal.  Using the appropriate LCD screen and keypad on the ShearTrac-II unit, make the appropriate position change so that the displayed count value in the appropriate text box is about 45,000 counts.

    Gently push the rod into the body of the LVDT, and place the thinnest gauge block (0.0625 in for the example being given) between the rod and the underside of the platen. Release the rod so that it holds the gauge block in place. (The shaft will now be the thickness of the gauge block farther into the body of the LVDT than when you started.) Note that the Displacement count reading (on the System Monitor window) has decreased. Be sure the unit for the column headed dial reading is the same as the thickness unit of the gage block you are using.  Click the top entry field of the Dial Reading column and type the thickness value of the gauge block as a negative number. (The count values will decrease as you add thicker and thicker gage blocks.  The gage block values must be entered as negative umbers so the slope of the calibration line, which is the calibration factor, will be positive.)  Click the large Read button. A count value will appear in the Sensor Reading column.

    Remove the gauge block and replace it with the next thicker block (0.1 in for the example being given). Move the cursor down one row in the Dial Reading column, and type the thickness value (as a negative number) of the new gauge block. Click the large Read button. Repeat this process sequentially inserting thicker and thicker gauge blocks until you have covered the range of the LVDT. You may need to stack two or more gauge blocks together to get small increment increases, but it is best to use a single block as often as possible.

    When you have finished, click the Calibrate button on the Calibrate Displacement window. The factor (calibration factor or slope, in units chosen per count) and the offset (in cnt) of a best-fit line will be displayed in the right-hand text fields of the Calibrate Displacement window together with the standard error and correlation coefficient. If the correlation coefficient is less than 0.9999, you have gotten a bad reading and need to redo the calibration. We recommend that you view the calibration data and the best-fit line in graphical form. To do this, click the Graph button. Check that the data points lay close to the best-fit the line. A poor fit or a bad data point indicates that the calibration should be repeated.

    If you have a good calibration, write down the calibration factor and offset (but do not save them yet). Open the Calibration Summary window (by clicking Summary on the Calibrate menu), and compare the factor and offset to those shown. (Note that the Calibration Summary window opens with Vertical Load as the default sensor in the top text field.  Be sure to use the pull-down button at the right of the Channel text field and choose X Displacement when calibrating the X displacement transducer.) If they are more than 5% different, you have an error somewhere, or the equipment has changed significantly since the last calibration. (This comparison, of course, assumes that you are performing a routine calibration on a displacement transducer that has been calibrated previously.)

    To save the calibration factors so that they will be displayed on the Calibration Summary window, click the Save button on the Calibrate Displacement window.

*Note: If you close the Calibrate X Displacement window (by clicking the Close button or the X in the upper right corner of the window), the calibration results will not be saved, but neither will they be removed from the widow.  If you do not save the results before you close the window, you can later reopen the window and save the results.  Saving the calibration results will replace the values shown on the Calibration Summary window. 

    After saving the calibration parameters, check to be sure the Factor and Offset values on the Calibration Summary window (see Figure 4) are the same as those on the Calibrate Displacement window.

Figure 4. Calibration Summary Window for Vertical Displacement

    Be sure to click the Apply button on the Calibration Summary window shown in Figure A 4.  This will both apply the calibration results and save the calibration data (including standard error and calibration graph results).  Clicking the Download button will download the calibration results to the controller in the ShearTrac-II unit.

*Note: If you close the Calibrate X Displacement window before clicking Apply and Download on the Calibration Summary window, the calibration factors will not be lost, but the calibration data (including standard error, calibration graphical results, etc.) will be lost.

Should you have any issues with the above steps or further questions, please open a ticket by clicking "New Support Ticket". 

GeoComp Support Team