Airmar NMEA 2000® Depth, Датчики - Manual (Page 26)

DST200 User Manual revision 1.000
Proprietary and Confidential
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This PGN does not respond to ISO Requests (PGN 59904).
The Speed Calibration PGN is intended to correct first-order nonlinearities in the paddlewheel speed sensor for a given installation. Speed Calibration is a piecewise linear transfer
function that maps the raw pulse frequency coming from the paddlewheel sensor (measured in pulses per second, or Hertz) to an output speed curve (measured in meters per
second). Each data point in the piecewise linear function is the endpoint of a line segment. The input, or domain of the function, is the raw paddewheel rotational speed in Hertz,
and the output, or range of the function, is the calibrated speed in meters per second.
The data points in this PGN must be ordered with input frequencies monotonically increasing.
To change the Speed Calibration curve, use the Command Group Function PGN 126208. The following example will serve to illustrate the process.
The pulse rate (in Hertz per knot) of a given paddlewheel speed sensor can be plotted against the actual vessel speed in knots. Figure 1, below, shows an example of a simple
4-point calibration curve for a typical transom-mount paddlewheel sensor.
Figure 1: Pulse Rate versus Speed
The table on the left (below) corresponds to the above graph. This relationship can also be understood as Pulse Frequency (in Hertz) versus actual vessel speed (in meters per
second). The table on the right (below) shows the same data recast to these units.
To determine the pulse frequency produced by the paddlewheel sensor at different speeds, use the transmitted proprietary PGN 65409, along with a separate speed measuring
device (such as a GPS). Collect Pulse Frequency versus Speed data for various speeds to generate a table like the one on the right, above.
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Speed (knots) Pulse Rate (Hz/kt)
3.75 3.25
10 7.5
15 6.5
30 6.5
Frequency (Hz) Speed (m/s)
12.2 1.93
75.0 5.14
97.5 7.72
195.0 15.43