ADCS-601™ OPERATION AND MAINTENANCE MANUAL
© 2005 DH Instruments, Inc. Page 142
See Section 7.2.5.2 for the recommended procedure to verify the FPG8601 load
cell linearity.
7.2.5.2 LINEARITY VERIFICATION PROCEDURE
To verify the FPG8601 load cell linearity proceed as follows:
Remove the FPG8601 piston-cylinder following the procedure described in
Section 6.3.2.
Attach the Calibration and Linearization Bracket (CLB) to the FPG8601
gimbal assmbly using the two piston retaining nuts Verify that the FPG8601
is level.
Go to the ADCS Tools FPG Run Screen by pressing the [FPG8601] button
on the Main Toolbar.
Zero the FPG8601 using the [Zero] button the FPG Run Screen.
Record the value of <N> after zeroing.
Place a second 500 g mass on top of the first 500 g mass. Wait for
stabilization and record the value of <N>. Note: it is normal for the value of
<N> not to correspond exactly to the mass loaded.
Repeat mass placement and recording <N> until all the masses of the 2 kg
mass set have been loaded.
Repeat process changing order of placement of masses to include different
points in the span as desired (for example, after zeroing, load the 1 x 100 g
and 2 x 200 g, rather than the 500 g to check more points in the low end of
the span.
Evaluate the linearity verification data using the information in Section 7.2.5.3.
Remove the masses and CLB from the FPG8601.
Reinstall the piston-cylinder and lower mounting post (see Section 6.3.6).
7.2.5.3 EVALUATING LINEARITY VERIFICATION DATA
The objective of the linearity evaluation using the CLB and FPG8601 mass set is
not to verify the absolute measurement uncertainty of the load cell, only its
linearity. Due to relative adjustments to the load cell output to take into
consideration local gravity, air density and other factors, it is normal for one
measurement count of the FPG8601 load cell not to correspond to a whole
number mass value and for the output in counts when a mass is loaded not to
agree in the absolute sense with the true value of the mass.
The linearity evaluation can be considered a purely proportional excersize. With
a perfectly linear device, if an input of 1 results in an output of x, an input of 2 will
result in an output of exactly 2x, etc.
The linearity tolerance of the load cell is ± (2 ppm + 2 counts).
There are several ways to use the data taken in Section
7.2.5.2 to evaluate the
linearity of the load cell. One
is to perform a best fit, linear regression of the true
mass load and load cell output data and compare the residuals to the linearity
tolerance. A simplified method follows:
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