JET PROPULSION LABORATORY INTEROFFICE MEMORANDUM
     
TO: Dave Gruel S & I DFM 97-001
FROM: Tim Schofield 25th March 1997
SUBJECT: ASI/MET Accelerometer - Data Conversion Expressions.
REFERENCE: (1). AIP data conversion expressions - Revision B, S & I DFM 96-014, 18th December 1996. From: Tim Schofield To: Distribution


This memo gives calibration expressions for the six ASI/MET science and engineering accelerometers, based on the laboratory calibration work of 2/96 and measurements taken in flight during spacecraft spin down of 12/12/96.

A linear, temperature-dependent conversion from counts to acceleration is used for each accelerometer head and gain setting, so that:

Acceleration in g's = (Counts - A(T)) / B(T)
where: A(T) = A0 + A1T is the accelerometer bias in counts,
B(T) = B0 + B1T is the accelerometer scale factor in counts/g,
T is head temperature in centigrade,
Counts lie in the range -8192 to +8191 (If counts > 8191, counts = counts -214).
1 g = 9.795433 m.s-2,

The table below gives A0, A1, B0, and B1 for each head and gain setting. The temperature sensor channel identifications associated with each head are also given. Temperature sensor conversions are given in reference (1).

Head Sx Sy Sz Ex E+yz E-yz
Accelerometer Channel W-2001 W-2002 W-2003 L-2001 L-2002 L-2003
Temperature Sensor W-4001 W-4002 W-4003 L-4050 L-4051 L-4052
 
40 g A0 Counts 2.75 -0.97 0.83 1.35 -0.02 -0.66
Range A1 Counts/°C -0.023 -0.003 -0.003 -0.005 -0.000 -0.001
 
  B0 Counts/g -184.118 -185.581 -183.928 -182.776 -183.450 -183.924
  B1 Counts/g/°C -0.028 -0.025 -0.030 -0.032 -0.032 -0.029
 
800 mg A0 Counts -4.17 8.40 1.58 12.33 -4.93 -6.67
Range A1 Counts/°C -0.012 -0.011 -0.003 0.024 0.043 0.019
 
  B0 Counts/g -9008.76 -9031.40 -8978.62 -8938.97 -8933.65 -8976.08
  B1 Counts/g/°C -0.805 -0.731 -0.822 -0.798 -0.697 -0.729
 
16 mg A0 Counts -313.76 408.90 51.90 562.73 -336.07 -287.37
Range A1 Counts/°C -0.284 -0.512 0.095 -0.854 1.336 0.401
 
  B0 Counts/g -445743.0 -446319.0 -444297.0 -442228.0 -441730.0 -444094.0
  B1 Counts/g/°C -34.0 -33.0 -37.0 -35.0 -38.0 -26.0

Notes on the Calibration Table.

  1. Only A0 on the 16 mg range has been corrected for offset measurements in space, performed at temperatures in the 11-12°C range. The combined effect of temperature differences and offset changes for the other gain settings was generally one count or less.
  2. A(T) is represented quite poorly by a linear expression for the 16 mg range. Errors can be as high as 2 or 3 counts. Offsets will therefore be most accurate at 11-12°C and will diverge slightly at other temperatures. For the other gain settings, errors are 1 count or less.
  3. So far, 16 mg offsets have remained fairly stable in space. It is likely however that further small adjustments will have to be made.