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Design Considerations in the Presence of Magnetic Fields

Greater processing power and the demand for operator safety has driven a dramatic increase in unmanned vehicle use.

Unmanned vehicle use has increased dramatically over the last decade driven by greater processing power and the demand for operator safety.

Unmanned vehicles generally require some form of navigation system that provides a heading reference. Traditional navigation heading solutions included magnetic compasses with mechanical gyroscopes, Global Positioning Systems (GPS) and inertial navigation systems. With the advent of Micro Electro-Mechanical Systems (MEMS) sensor technology, the digital compass emerged as a leader in price-performance, offering a number of advantages to the designer as a method of providing and maintaining accurate heading. The benefits of a digital compass to a designer are many:

Low cost

  • Depending upon the application and heading accuracy required, digital compasses start as low as a few hundred dollars

Small form factor and low mass

  • Today’s digital compasses can be well under two square inches, and weigh as little as 25 grams.

Low power requirement

  • Depending on the application and heading accuracy requirements, digital compasses can operate at less than 50 mw of power

Highly accurate

  • Today’s digital compasses can provide heading accuracy in a static application environment after calibration to as low as 0.3 degrees RMS error

No reliance on GPS to maintain heading

  • A digital compass relies on magnetometers to passively detect the earth’s magnetic field to derive heading and does not rely on the presence of an active

GPS signal

  • Adaptable calibration for a variety of applications and environments

  • The digital compass is very versatile in that it can provide accurate heading in a large number of applications and environments.

Today’s digital compasses provide a practical solution to the challenging needs of many unmanned applications. Digital compasses utilize magnetometers to measure the earth’s magnetic field to provide heading reference relative to magnetic north. As such, their accuracy is subject to degradation in the presence of magnetic interference. Therefore, magnetic materials should be kept away from a digital compass for best performance. In practice this may not always be possible, particularly if the unmanned vehicle passes through stray magnetic fields as part of its course of navigation.

Unmanned Vehicle Digital Compass
Design Considerations

Digital compasses should be mounted away from strong magnetic fields or highly magnetic material. This includes close proximity to batteries, electric motors and electric currents. Batteries typically contain a magnetic signature and electric motors generate magnetic fields that will affect compass performance. It is recommended that these items be placed as far away as practical from the digital compass.

Digital compasses should be calibrated in-house by the supplier to remove any magnetic anomalies associated with the compass itself and to ensure the product’s accuracy. Typically, the magnetometers are calibrated at the factory in a Dycome (see Figure 1) and the gyroscopes are calibrated via a Rate Table (see Figure 2).

 Figure1
Figure 1: The Dycome
Figure2
Figure 2: The Rate Table

The computer-controlled precision factory calibration corrects:

  • Sensor Offset and Sensitivity

  • Sensor Linearity

  • Axial Misalignment in 3-Dimensions

The digital compass should also be calibrated in its environment of end use in the unmanned vehicle itself. Optimally, digital compasses should come equipped with in- field calibration functions to help compensate for nearby magnetic material.

Digital compasses should compensate for both soft and hard iron interference. Hard iron anomalies are generated by ferrous (magnetized) materials and soft iron anomalies are produced by materials that while not magnetized, could still affect the magnetometer’s ability to obtain an accurate heading reference by measurement of the earth’s magnetic fields.

Magnetic Interference Compensation
in the Field

Correcting the impact of magnetic field distortions on heading in unmanned vehicles in service is more challenging. The nature and strength of the magnetic anomalies that may be experienced as an unmanned vehicle passes in close proximity to magnetic material is unpredictable. These magnetic effects may momentarily degrade heading accuracy. The degree of degradation will vary depending on the distance to and quantity of the ferrous material:

  • Amount and shape of the magnetic material

  • Location and orientation relative to compass

  • The degree of magnetization (which can change over time)

  • Uniformity of the magnetization across the material

Digital compass designs must compensate for these magnetic disturbances. This is accomplished by in-field calibration and the utilization of gyroscopes to augment the traditional digital compass design.

In-field Calibration

In-field calibration algorithms enable unmanned vehicles to maintain heading accuracy. When the compass is mounted in the user’s application, any magnetic material (screws, brackets, components, etc) will affect the compass accuracy if not compensated. In- field calibration is required to remove many of these interferences. The digital compass should provide calibration capability in full 3-dimensions.

Ideally, digital compasses should constantly monitor the magnetic field conditions and automatically calibrate for hard and soft magnetic distortions as the product is used. Since the magnetic environment in which the compass is used is typically unpredictable in many situations, it can make continuous calibrations unreliable. It is therefore more desirable to have the end-user control the calibration process to insure that the environment has minimal impact to the calibration process. During a typical in-field calibration, the end-user will orient their product, which contains a digital compass, in different positions and then command the compass to sample the magnetic field conditions at each orientation. Up to twelve magnetic measurements are taken to allow the compass to learn the distortions within the product and compensate for them. The in-field calibration algorithm will adjust the magnetic offsets and scaling so that all of the magnetic measurements are spherical under all rotations. These offsets and scaling parameters are saved and used in normal operation. Any significant modifications to the vehicle after the compass has been calibrated like installation of new hardware, batteries changed, etc., may require the compass to be recalibrated.

Gyroscope Digital Compass Enhancement

The magnetometers utilized in the digital compass design are prone to error when exposed to magnetic interference. Forms of magnetic interference include stray magnetic fields in the path of the unmanned vehicle or from internal transient magnetic effects from power cables or electric motors under varying operating conditions.

Errors from the presence of these stray magnetic fields experienced during the operation of unmanned vehicles can be improved by adding gyroscopes to the digital compass design. Typical gyroscopes used in this application are MEMS devices. Tri- axial angular rate sensors measure rotation rates around three orthogonal axes (X, Y, and Z). These gyroscopes give more information about how the compass is moving so it can properly compensate and provide a more stable heading, pitch, and roll, in the presence of spurious magnetic interference. A smart sensor fusion algorithm is typically used to combine the magnetometer, accelerometer, and gyro measurements into an accurate orientation output. Disturbances in the magnetometer measurements are not typically reflected in the accelerometer and gyro measurements. The sensor fusion algorithm looks for these differences and weights the sensors accordingly to ensure that the compass, and vehicle, stays on course.

Accelerometers used as tilt sensors are included in digital compass designs to compensate for the relative orientation of the magnetometers that measure the earth’s magnetic fields to provide heading. The accelerometers are affected by acceleration due to motion, especially linear motion. An added benefit of the gyroscopes is that since they measure angular rates in X, Y, and Z, they can also be used to overcome the disturbances of motion in the accelerometers. Gyroscopes only provide a relative measurement in that if the compass is not rotating, the gyroscopes outputs are zero. Therefore, the compass relies more heavily on the gyroscopes and magnetometers during periods of motion and the accelerometers and magnetometers during periods of rest. Similar to magnetic disturbances, disturbances in linear acceleration are not typically reflected in the magnetometer and gyro measurements. The sensor fusion algorithm can therefore detect these disturbances and adapt to them to keep the compass orientation output from being affected.

In summary, operating environments can adversely affect magnetic compasses. Time- varying magnetic fields may degrade compass performance. Integration of a digital compass requires a degree of application specific engineering to ensure the best possible accuracy is gained from the digital compass.

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