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Tweaking AutoCal

Did you ever notice how some boats seem to have strange instrument readings only in certain conditions? Like having boatspeed be off by a half knot only when going between 4 and 6 knots, or the wind readings being off only at certain apparent wind angles? A lot of times, the instruments are actually working correctly, but the peculiarities of the boat’s shape moving through a fluid (either water or air) affect the motion of that fluid, and produce unavoidable local transitory effects on the sensors. These local effects then give rise to the anomalous instrument readings.

So how do you compensate for these local transitory effects? Short of redesigning the boat or moving the sensors, you can use AutoCal tables. AutoCal tables produce correction factors for an instrument reading in certain ranges. For instance, the 0.5 knot error between 4 and 6 knots could be removed by using an AutoCal table.

An important thing to note is that AutoCals should be used as an adjunct to good calibration, not a replacement! The base calibrations should be refined to the point where it is nearly impossible to remove additional errors before resorting to AutoCal tables. AutoCals were developed in response to the need by high-end racing yachts to remove that last bit of error from the calibrations to produce the best instrument readings possible. Keep in mind that many of these programs have one or more people devoted entirely to keeping the electronics in superb operating conditions, so they often reach the limits of the base calibrations.

In days of yore, the AutoCals were done externally to the system processor by an attached PC. The PC would run a piece of software (like OckamSoft) that looked at the instrument readings, and then send a correction factor when warranted. This of course introduces another potential point of failure in the instrument system, which tends to make it a bit more delicate – not something that’s particularly desirable on a boat. With the introduction of the T1 Tryad processor, it became possible to run AutoCal tables internal to the processor and eliminate the need for the external PC sending correction factors. The AutoCal tables are stored on the internal CompactFlash memory card, along with other system information. The only problem with this is that it is difficult to change AutoCals on the fly, as the system needs to be shut off to remove the CompactFlash for file transfer. Another thing to note is that the external PC is still required. It’s best to test and qualify the AutoCal tables using the external PC before moving the AutoCal tables to the T1’s internal CompactFlash memory card.

However, many of the high-end programs demand quicker and easier changes to the AutoCal tables, so we’ve developed a quicker method. This new method allows you to create an AutoCal table and download it directly to the internal memory on the T1, without the need to turn off the system and remove the ComapctFlash! For the end user, this new “live edit” method uses an Excel spreadsheet with a line graph to visually build an AutoCal table. There’s also an API for use by other navigational programs to incorporate this flexibility into their own structure – there will probably be at least one program incorporating this feature in the near future. You can download the end user utility here. Note that this live edit capability requires an update to the T1 firmware to operate correctly.

We also have a “Robocal” program that’s been sitting quietly waiting for a nibble. It helps you determine the correct base calibration for your instrument system using a PC connected to the Ockam data stream. If anyone is interested, write me at ——- and I’ll send you a copy to try.

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