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Calibration
Overview
Instrument
Calibration Theory and Practice
Calibration |
of
instrument systems is necessary to make
displayed information correct and useful. Instrument systems use
sensors to
measure primary environmental factors (boatspeed, wind, heading,
position, heel
and others), possibly combine them, and display the results. Stand-alone
systems do not combine inputs, while integrated systems
do.
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
adjustments or
options shown. A key characteristic of a properly designed integrated
system is that all inputs and outputs are available in one place for
calibration by
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. |
Input calibrations |
correct
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
input calibration. |
Algorithms |
are the
computer codes
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. |
Calculation options |
(when
available) change
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. |
Output adjustments |
scale or warp
outputs to
correct for unmeasured or unknown effects (such as wind shear and
gradient), or when input calibrations do not completely correct the
sensor inputs. |
Look-up tables |
are a
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.
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Instrument Calibration on the
Ockam System
The Ockam Unisyn™ and Tryad™ systems have many
types of
calibration, options and adjustments providing for all levels of need.
Hardware Cal |
Most Ockam interfaces have Input
Cal
screws, used for basic instrument calibration. Many deride them as
old-fashioned and low-tech. Yes, and they’re inconvenient
too. We
use them because our low-tech clients can relate to them.
They’re
robust, reliable and don’t freak out like RAM-based cals can.
Software
Cals
are a solution to the inconvenience factor, though we recommend turning
the screws once the input cals have been figured out.
The Ockam
System Manual section 3 covers these calibrations.
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Software Cal |
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,
Matryx
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.
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AutoCal |
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
which
includes a sample spreadsheet and more documentation.
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AutoCal
in the T1 |
When your
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.
The AutoCal
applet
includes instructions for preparing AutoCal.dat for the T1.
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Kwik Cal™
(T1 only)
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When
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.
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Sensor correction tables
(T1 only) |
Beginning
with software
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,
see
Sensor
correction on the T1 (PDF) |
DeWiggler™ |
DeWiggler is a process for calibrating your instrument system. It comes
in two flavors:
- DeWigglerPro™
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.
- DeWigglerAnalyst™
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.
Recommended calibration changes include
- 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)
Web
page |
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