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參數(shù)資料
| 型號(hào)： | HMC1055 |
| 廠商： | Electronic Theatre Controls, Inc. |
| 英文描述： | 3-AXIS COMPASS SENSOR SET |
| 中文描述： | 3軸指南針傳感器系列 |
| 文件頁(yè)數(shù)： | 11/12頁(yè) |
| 文件大小： | 406K |
| 代理商： | HMC1055 |
HMC1055
Advance Information
Heading Computation
SENSOR PRODUCTS
Once the magnetic sensor axis outputs are gathered and the calibration corrections subtracted, the next step toward
heading computation is to gather the pitch and roll (tilt) data from the MEMSIC MXS3334UL accelerometer outputs.
The MXS3334UL in perfectly horizontal (zero tilt) condition produces a 100Hz, 50 percent duty cycle Pulse Width
Modulated (PWM) digital waveform from its Doutx and Douty pins corresponding to the X and Y sensitive axis. These
output pins will change their duty cycle from 30% to 70% when tilted fully in each axis (±1g). The scaling of the PWM
outputs is strictly gravitational, so that a 45 degree tilt results in 707 milli-g’s or a slew of ±14.1% from the 50% center
point duty cycle.
With the MXS3334UL’s positive X-axis direction oriented towards the front of the user’s platform, a pitch downward
will result in a reduced PWM duty cycle, with a pitch upward increasing in duty cycle. Likewise, the Y-axis arrow is 90
degrees counter-clockwise which results in a roll left corresponding to a decreasing duty cycle, and roll right to an
increasing duty cycle.
Measuring the pitch and roll data for a microcontroller is reasonably simple in that the Doutx and Douty logic signals
can be sent to microcontroller digital input pins for duty cycle measurement. At firmware development or factory
calibration, the total microcontroller clock cycles between Doutx or Douty rising edges should be accrued using an
interrupt or watchdog timer feature to scale the 100Hz (10 millisecond) edges. Then measuring the Doutx and Douty
falling edges from the rising edge (duty cycle computation) should be a process of clock cycle counting. For example,
a 1MHz clocked microcontroller should count about 10,000 cycles per rising edge, and 5,000 cycle counts from rising
to falling edge would represent a 50% duty cycle or zero degree pitch or roll.
Once the duty cycle is measured for each axis output and mathematically converted to a gravitational value, these
values can be compared to a memory mapped table, if the user desires the true pitch and roll angles. For example, if
the pitch and roll data is to be known in one degree increments, a 91-point map can be created to match up
gravitational values (sign independent) with corresponding degree indications. Because tilt-compensated compassing
requires sine and cosine of the pitch and roll angles, the gravitational data is already formatted between zero and one
and does not require further memory maps of trigonometric functions. The gravity angles for pitch and roll already fit
the sine of the angles, and the cosines are just one minus the sine values (cosine = 1 – sine).
The equations:
X’ = X * cos(
I
) + Y * sin(
T
) * sin(
I
) – Z * cos(
T
) * sin(
I
)
Y’ = Y * cos(
T
) + Z * sin(
T
)
Create tilt compensated X and Y magnetic vectors (X’, Y’) from the raw X, Y, and Y magnetic sensor inputs plus the
pitch (
I
) and roll (
T
) angles. Once X’ and Y’ are computed, the compass heading can be computed by equation:
Azimuth (Heading) = arctan (Y’ / X’)
To perform the arc-tangent trigonometric function, a memory map needs to be implemented. Thankfully the pattern
repeats in each 90° quadrant, so with a one-degree compass resolution requirement, 90 mapped quotients of the arc-
tangent function can be used. If 0.1° resolution is needed then 900 locations are needed and only 180 locations with
0.5° resolution. Also, special case quotient detections are needed for the zero and inifinity situations at 0°, 90, 180°,
and 270° prior to the quotient computation.
After the heading is computed, two heading correction factors may be added to handle declination angle and platform
angle error. Declination angle is the difference between the magnetic north pole and the geometric north pole, and
varies depending on the latitude and longitude (global location) of the user compass platform. If you have access to
Global Positioning Satellite (GPS) information resulting in a latitude and longitude computation, then the declination
angle can be computed or memory mapped for heading correction. Platform angle error may occur if the sensors are
not aligned perfectly with the mechanical characteristics of the user platform. These angular errors can be inserted in
firmware development and or in factory calibration.
Solid State Electronics Center
www.magneticsensors.com
(800) 323-8295
Page 11
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