HMR3400 Datasheet, PDF (4/9 Pages) Honeywell Accelerometers – Digital Compass Solution

HMR3400
longer valid. Should the part be accidentally bent, return for recalibration is possible or align the part vertical to recapture
most of the accuracy. Correct flat orientation of the compass modules is with the pins pointing downward.
CIRCUIT DESCRIPTION
The HMR3400 Digital Compass Solution includes all the basic sensors and electronics to provide a digital indication of
heading. The HMR3400 uses three magnetic sensors and includes an accelerometer to provide tilt (pitch and roll) sensing
relative to the board’s horizontal (flat) position. HMR3400 is derived from the HMR3300 product baseline, and is a shrink
in terms of size and flexibility of power supplied.
The HMR3400 circuit starts with Honeywell HMC1021Z and HMC1022 single and two-axis magnetic sensors providing X,
Y, and Z axis magnetic sensing of the earth’s field. The HMC1022 provides the horizontal components (X and Y), and
HMC1021Z provides the vertical (Z) axis component of the incident magnetic field into cartesian magnitudes at orthognal
angles. These sensors are supplied power by a constant current source to maintain best accuracy over temperature. The
sensor output voltages and constant current sensor supply voltage are provided to a multiplexed 16-bit Analog to Digital
Converter (ADC) integrated circuit. A microcontroller integrated circuit periodically queries the multiplexed ADC and
performs the offset corrections and computes the heading. This microcontroller also performs the external serial data
interface and other housekeeping functions such as the calibration routine. An onboard EEPROM integrated circuit is
employed to retain necessary data variables for best performance.
The HMR3400 contains an additional pair of data inputs from the ±2g accelerometer (Analog Devices ADXL213) is
received by the microcontroller. These tilt inputs (pitch and roll) are added to sensor data inputs to form a complete data
set for a three dimensional computation of heading. If the board is held horizontal, the pitch and roll angles are zero and
the X and Y sensor inputs dominate the heading equation. When tilted, the Z magnetic sensor plus the accelerometer’s
pitch and roll angles enter into heading computation.
The power supply for the HMR3400 circuit is designed for regulated +5 volt provi sion from a regulated supply source. The
power supply is a dual ground (analog and digital) system to control internal noise and maximize measurment accuracy.
APPLICATION NOTES
When To Calibrate
The HMR3400 comes with an optional user hard-iron calibration routine to null modest intensity hard-iron distortion. For
many users in cleaner magnetic environments, the factory calibration will be better and yield more accurate readings than
after a user calibration.
The calibration routine is not cure-all for nasty magnetic environments. If a needle compass is thrown off from true
readings, then it is very likely the HMR3400 will have poor accuracy too. Most compass error sources come from ferrous
metals (steel, iron, nickel, cobalt, etc.) located too close to the compass location and are known as soft -irons creating soft -
iron distortion. Soft-iron distortion will change the intensity and direction of the magnetic fields on any nearby compass,
and the calibration routine can not remove these flux concentration and bending errors. A good rule of thumb is to keep
soft-irons at least two largest dimensions away from the compass. For example, a half-inch stainless steel panhead bolt
should be at least an inch away from the HMC1021Z and HMC1022 sensor locations.
Other nasty magnetic environments are man-made AC and DC magnetic fields created from nearby motors and high
current conductors. These fields should also require compass or source relocation when possible. In some cases, ferrous
metal shielding may help if the shield material is thin and far enough away from the compass.
Hard-iron distortion can be calibrated out, and is composed of soft -irons that are also magnetized and create remnant
(stray) magnetic flux. Classic hard-iron distortion typically comes from large vehicle chassis components and engine
blocks that have up to ±2 gauss on the parts. Locating the compass away from hard and soft-irons is the first line of
defense to preserve accuracy, and the calibration routine will null out the remaining hard-iron influences.
Calibration Procedure
For the HMR3400, one complete turn in a level plane is the starting point to expose the XY sensors to all headings to
compute the calibration offsets. Since the compass collects data at a 8 samples per second rate, a sample per degree of
rotation or slower is a good guideline. If slow turns are not possible, multiple faster turns are a good substitute. The
goodness of the calibration or the amount of hard-iron present is found by checking the Xof, Yof, and Zof values after the
calibration routine is complete. In known clean magnetic environments, the horizontal values (XY = level, YZ = upright)
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