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HMR3000 User's Guide
3.4 HMR3000 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 rates. The fluidic ilt 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 offsets for the three magnetic measurements; and the gain and
offset compensation for tilt measurements. Tilt measurements are linearized to account for the non-linear characteristics
of the inclinometer function. All five measurements, TiltX, TiltY, MagX, MagY, and MagZ, are low-pass filtered, using an
IIR filter (Infinite Impulse Response), depending on the setting for the TC1. This filter may be disabled by setting the time
constant to zero.
Magnetic sensor operation includes a Set/Reset pulse routine to achieve high sensitivity magnetic measurements. The
active area of the MR element is Set and Reset periodically by 3 amp current pulses through the magnetic sensor
set/reset straps. 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.
3.5 HMR3000 Interface Pin Descriptions
The table below shows pin assignments for the 9-pin D-shell connector. Different pins are used to supply either regulated
5V
or unregulated 6V to 15V power. Only one of the two power pins (9 or 8) should be connected in a given installation.
dc
Name
TxD / A
RxD / B
GND
6-15V
5V
Oper / Calib *
Run / Stop *
Ready / Sleep *
Cont / Reset *
*A dash following a signal name is used to denote that the signal is asserted active low.
In this case, when pin 1 is low the Calibrate function is selected.
3.6 HMR3000 Serial Data Communication
The HMR3000 serial communications are governed by a simple, asynchronous, ASCII protocol modeled after the NMEA
0183 standard. Either an RS-232 or an RS-485 electrical interface can be used. ASCII characters are transmitted and
received using 1 start bit, 8 data bits (LSB first), no parity (MSB always 0), and 1 stop bit; 10 bits total per character. Baud
rate can be any one of 1200, 2400, 4800, 9600, 19200, 38400.
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In/Out
Pin
Description
Out
2
RS-232 transmit out / RS-485 transmit-receive signal
In
3
RS-232 receive in / RS-485 transmit-receive return
In
5
Power and signal common
In
9
Unregulated power input
In
8
Regulated power input
In
1
Operate / Calibrate-* input (open = Operate)
In
6
Run / Stop- input (open = Run)
In
4
Ready / Sleep- input (open = Ready)
In
7
Continue / Reset- input (open = Continue)
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