900240 Rev. B
3/01
page 5
Figure 2. RS485 to Computer Connection
The HMR3000 Compass module’s RS-485 interface is half duplex, i.e. transmit and receive circuits share the same physical
pair of wires. The HMR3000 must disable its transmitter to allow characters to be received from a host system. If the unit is
operating in
Run
mode, i.e. generating repetitive output, then the
Run / Stop
pin (pin # 6) should be forced low before the host
attempts to transmit a command.
See details in Description of hardware interrupt section.
2.11
Installation
To get optimum performance when installing HMR3000, follow the guidelines listed below for your vehicle or platform.
Location—
Install the HMR3000 as far as possible from any source generating a magnetic field and far from ferrous
metal objects. Honeywell magnetic sensors used in HMR3000 have a large field range of
±
1G (
±
100
µ
T), compared to
0.65 G (65
µ
T); the maximum of earth’s total magnetic field, and therefore would not saturate in most platforms.
Calibration and compensation routines in the compass can effectively compensate for static magnetic fields
superimposed on the earth’s field components, which are used for heading calculations. However, compasses can not
compensate for the effects of varying fields produced by dc and ac currents.
Level—
Since the HMR3000 is electronically gimbaled it is not necessary to mount the compass leveled. However, to get
the maximum possible tilt range, the compass should be mounted level when the vehicle or platform is in normal
operation.
Orientation—
Forward direction of the compass can be oriented at any angle from that of the platform. Use Deviation
parameter to convert the compasses magnetic heading to true or magnetic heading of the vehicle/platform
2.12
Calibration
All magnetic compasses have to be calibrated in order to compensate for magnetic fields other than the earth’s field
components to get accurate heading. These additional magnetic fields are generated by the host and therefore depend on
the compass mounting location. By performing a simple procedure, the HMR3000 can compensate for steady, static
magnetic fields known as hard iron fields. Field components found after a calibration are only valid for the particular
orientation and location of the compass. A re-calibration is necessary after a relocation of the compass or if the platform
has changed its magnetic character.
Compass calibration is performed by following a calibration procedure specified by the manufacturer. During this
procedure the compass collects data required for the compensation algorithms. The goal of the calibration procedure is to
sample the magnetic field components for many possible orientations of the host system. Rotating the host system
through 360 degrees or driving in a circle (in the case of a vehicle) will enable the compass to sample its magnetic
environment. The HMR3000 can be calibrated by either using the built in calibration method or by using the PC Demo
Interface software program. The calibration procedure for both these methods is the same.
Built-in Calibration Method—
This method uses an iterative procedure to calculate the hard iron offsets. In most
situations 275 iterations would produce good results. The calibration procedure has to continue until this iteration count
is reached.
To put the HMR3000 into calibration mode (issue #F33.4=0*51<CR><lf>):
Slowly rotate the host system through a full circle in a gentle motion while changing roll and pitch as much as the
host will allow. Generally this procedure will take over two minutes.
Check the iteration count by issuing #I26C?*31<CR><lf>. HMR3000 will reply with a #
nnnn
*hh<CR><lf>
message, where
nnnn
is the value of the iteration count.
If this value is less than 275
continue with the calibration procedure until that number reaches 275.
At the end of this procedure issue a command to save the results in the EEPROM (#F2FE.2=1*67<CR><lf>)
Put the unit back into Operate mode (issue #F33.4=1*50<CR><lf>)
