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3.3
HMR3000 PROCESS CONTROL BLOCK DIAGRAM
User configurable parameters that control the measurement and heading calculation process is denoted at the bottom of
the diagram in italic font.
Sensors
A, B
Tilt
Magne-
tometer
X, Y, Z
Set/Reset Interval
Mag Sample Rate
Figure 4. Process control diagram and user configurable parameters
Measurement Sequence
In normal operation, the microprocessor takes a set of seven measurements, four tilt and three magnetic, that are
combined to produce heading, roll, and pitch data.
A set of magnetic measurements can be taken at 110, 55, 27.5, or 13.75 Hz rate. Tilt sensor is driven with a constant 55
Hz pulse. Raw data are normalized, linearized, and filtered at the 13.75 Hz rate. Normalization includes gain matching,
offset nulling, and hard-iron compensation for the three magnetic measurements; and gain and offset compensation for
tilt. Tilt measurements are linearized to account for the non-linear characteristics of the inclinometer. All five
measurements, TiltX, TiltY, MagX, MagY, and MagZ, are low-pass filtered, using IIR filter, depending on the setting
for the TC1. This filter may be disabled by setting the time constant to zero.
Magnetometer operation includes a Set/Reset pulse to achieve high sensitivity magnetic measurements. The active area
of the MR element is Set or Reset periodically by a 3 amp current pulse through the patented on chip coils. The
periodicity of this Set Reset operation can be changed to achieve high heading accuracy or to conserve power.
Compass heading is calculated 13.75 times per second from the 5 filtered measurements. A form of non-linear
smoothing can be applied to the current heading to produce a smooth heading. The transfer function of the algorithm is a
high degree of smoothing applied for small changes in heading, i.e. noise, while little or no smoothing is applied to
larger, more significant changes by setting the parameters of this non linear filter.
In may not make sense to use the IIR filter and the non-linear smoothing for a given application. If the compass is
mounted on a vessel that cannot change direction quickly, then it is probably better to use the IIR filter and disable
smoothing. On the other hand, smoothing works well for hand-held applications or where noise is a problem.
Microprocessor
IIR Filter
Non Linear
Heading
Calculation
TC1
Filter time constant TC1
Heading filter parameters L and S
Alarm and warning levels
Hard iron coefficients
Strobe mode count
900240 Rev. B
3/01
Output
Filter
RS232
RS485
L and S
Message Type
Message Rate
6 NMEA Messages
ASCII
page 8
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