Campbell Scientific CR5000 Measurement and Control Module - Manual (Page 75)

Section 3. CR5000 Measurement Details
3-21
The resolution gets much worse with the shorter intervals used with higher
speed measurements. As an example, assume that engine RPM is being
measured from a signal that outputs 30 pulses per revolution. At 2000 RPM,
the signal has a frequency of 100 Hz (2000 RPMx(1 min/60 s)x30=1000). The
multiplier to convert from frequency to RPM is 2 RPM/Hz (1 RPM/(30
pulses/60s) = 2). At a 1 second scan interval, the resolution is 2 RPM.
However, if the scan interval were 1 ms, the resolution would be 2000 RPM.
At the 1 ms scan, if every thing was perfect, each interval there would be 1
count. However, a slight variation in the frequency might cause 2 counts
within one interval and none in the next, causing the result to vary between 0
and 4000 RPM!
3.8 Self Calibration
The CR5000 performs a self-calibration of the analog voltage measurements
and excitation voltages. The range gains and offsets and the excitation voltage
output will vary with temperature. The self calibration allows the CR5000 to
maintain its specifications over the temperature range.
Rather than make all of the measurements required to calibrate all
range/integration type combinations possible in the CR5000, the calibration
only measures the range/integration type combinations that occur in the
running CR5000 program. The calibration may occur in three different modes.
1. Compile time calibration. This occurs prior to running the program and
calibrates all integration/range combinations needed. For the 250 usc and
zero integrations multiple measurements are made and averaged to come
up with gain values to use in the measurements. Ten measurements are
made on the zero integration ranges and five measurements for the 250
usec integrations. When this calibration is performed the values in the
calibration table are completely replaced (i.e., no filtering is used).
2. System background calibration. This automatically takes place in the
background while the user program is running. Multiple measurements
are not averaged, but a filter is applied to the new gain/offset values
obtained. The filter is used so that the calibration values change slowly.
The filter combines the newly measured value multiplied by 0.1 with the
previous calibration value by 0.9 to arrive at the new calibration value. A
piece of the background calibration is added to each fast scan in the user
program. The background calibration measurements will be repeated
every 4 seconds or the time it takes to complete them, whichever is
longer. If there is not enough time to do the background calibration, the
CR5000 will display: “ Warning when Fast Scan X is running
background calibration will be disabled.” (X is the number of the fast
scan where the first scan entered in the program is 1, the next scan is 2,
etc.)
3. Calibration under program control. When the calibrate instruction is
included in a program, the calibration is identical to the compile time
calibration. The calibration table values are replaced with those
calculated. The fast integrations have averaging as in the compile
calibrate. When a calibrate instruction is found in any scan the