Calibration Theory and Practice
instrument systems is necessary to make
displayed information correct and useful. Instrument systems use
measure primary environmental factors (boatspeed, wind, heading,
and others), possibly combine them, and display the results. Stand-alone
systems do not combine inputs, while integrated systems
Most instrument systems provide some means of calibration; either
through hardware (screw-turns) or software commands, or both. Some
calibrations tend to be static; once set, they pretty much stay where
they are, while other calibrations always seem to need changing.
These days, ALL integrated instrument systems (the ones that can output
true wind and current) are computerized. They pretty much follow the
block diagram shown above, although they might not offer all the
options shown. A key characteristic of a properly designed integrated
system is that all inputs and outputs are available in one place for
all functional channels. For instance, heel has an effect on boatspeed
and true wind. If heel were not available to the appropriate
algorithms, this could not be done.
sensor inputs to
make their readings accurate. They adjust for things like boundary
layer (paddlewheels), upwash (masthead units), and installation
variables such as compass deviation or sensor misalignment. Almost all
instrument systems, including stand-alone types, offer some kind of
that process the inputs to produce outputs. Some outputs (the
"primaries") are simply a repeat of the sensor reading, although the
digital domain allows better fidelity, modeling and filtering. Other
outputs are not directly measurable. For example, true wind angle and
speed are a combination of apparent wind and boatspeed, and wind
direction is in turn a combination of true wind angle and heading.
Instrument algorithms tend to be invariable because they are based on
mathematical models, but some systems allow limited control.
the way the algorithms work. For example, the system might allow
switching from paddle to SOG to replace boatspeed, or certain
calculations might be disabled to comply with racing rules. Because
these features are limited to high end systems, they tend to be
controlled by computer commands.
scale or warp
correct for unmeasured or unknown effects (such as wind shear and
gradient), or when input calibrations do not completely correct the
flexible way to
specify calibrations or adjustments which depend on other measured
parameters, e.g. leeway depending on keel extension or trimtab angle.
But since they are
difficult to create and maintain, they should only be used when needed.
One of the problems with "table type" calibration is that you may be
able to determine the value for a few cells, but never get to test all
of them, leaving you with the problem of a 'bumpy' table. Everything
may be fine until you happen to get into an unusual condition, and your
instruments suddenly go haywire. On the other hand, if your boat is
high performance (e.g. big roach, canting keel, forward rudder), lookup
tables are sometimes the only way to go.
Instrument Calibration on the
The Ockam Unisyn™ and Tryad™ systems have many
calibration, options and adjustments providing for all levels of need.
Most Ockam interfaces have Input
screws, used for basic instrument calibration. Many deride them as
old-fashioned and low-tech. Yes, and they’re inconvenient
use them because our low-tech clients can relate to them.
robust, reliable and don’t freak out like RAM-based cals can.
are a solution to the inconvenience factor, though we recommend turning
the screws once the input cals have been figured out.
System Manual section 3 covers these calibrations.
Every hardware cal screw has an equivalent
software command. For
example, sending the command K1=1.06
sets CAL Boatspeed Master to 1.06 regardless of the hardware screw
setting. Sending the command K1=D
returns the calibration to the hardware value.
These commands can be entered by means of the Eye PDA application,
displays, OckamSoft 4,
or even a terminal emulator. They greatly speed the calibration
process, but the settings are vulnerable to memory loss or processor
replacement. Once the calibration settings have been established, it is
recommended that the settings be transferred to the hardware screws.
Software cals are described in the Ockam System
Manual section 4.
AutoCal is a PC application which automates the output of Software Cal
commands based on user-specified independent variable values. In the
table above, CAL Upwash (CALUW) is modified depending on the current
value of true wind speed and angle. In addition to Input Cals,
AutoCal can adjust true wind to correct out
that last bit of wiggle in wind direction and true wind speed.
If you need this additional capability, download
the AutoCal applet
includes a sample spreadsheet and more documentation.
in the T1
AutoCal table is
complete and debugged, you can move the function into the T1, thereby
unloading your PC and the comm channel for other work.
includes instructions for preparing AutoCal.dat for the T1.
you’re racing, it’s not the right time to be
twirling cal or
trying to modify a spreadsheet. In this situation, we recommend using
the ‘Kwik Cal™’. With this function you
can quickly adjust true wind
angle for a perfect wind direction.
Details on how this works are covered in the
Tryad T1 document.
|Sensor correction tables
revision 20.04 (2/1/08), the T1 can now correct for individual sensor
non-linearities by means of sensor correction tables. For more detail,
correction on the T1 (PDF)
DeWiggler is a process for calibrating your instrument system. It comes
in two flavors:
Recommended calibration changes include
comes aboard with a calibration expert. Your instrument system is
adjusted on the spot, and when he leaves, Pro leaves with him. Ockam
charges the expert for the use of Pro.
is a free program you or your dealer install. After logging some
sailing, you email the log files to Ockam for analysis. Calibration
recommendations come back by return email and get entered into your
instruments. Ockam charges you or your dealer for the analysis.
- Boatspeed calibration.
- Compass lubber offset.
- A compass deviation table suitable for
use with the T1 (see above).
- Windspeed calibration.
- Upwash and slope calibrations.
DeWiggler presentation (PDF)