|
|
 A New Sensor ASIC The first commercially available improvement on fluxgate magnetometer technology in Tim White, Compasses are now being incorporated into a variety of products—zipper pulls, watches, and the digital navigation systems of luxury vehicles. The latter application in particular presents unusual design challenges. Because the compass must operate accurately in the presence of dynamic magnetic fields that change unpredictably, it must be able to detect small changes in an external magnetic field while canceling out any magnetic noise present in the vehicle. Furthermore, the vehicle’s own magnetic noise constantly changes in strength and direction as it comes near outside sources that are also magnetized. Large buildings and overhead power lines can modify a passing vehicle’s magnetic field, as can a drive through the car wash. Solving the noise problem requires a circuit that can constantly null out the spurious magnetic noise and at the same time accurately resolve the signal. Design Improvements on the Magnetometer Magnetometers, in addition to detecting the presence of a magnetic field, can in certain applications determine the strength and direction of that field. This capability makes magnetometers well suited to the digital compass design described here.
A magnetic sensor ASIC (see Photo 1) developed by Precision Navigation Inc. (Santa Rosa, California) and American Microsystems Inc. consists of two nonlinear coils that require balanced drivers, slew rate controlled changes in current, and a method of measuring sensor outputs. A single sensor circuit is used to drive and monitor both sensors, which also helps reduce production costs. A digital state machine tracks which of the two sensors is being accessed at any particular time, and stores the data in the appropriate register. A test circuit allows an onboard A/D converter to monitor the sensor coil continuity in the vehicle and to report problems through error codes back to a central processor, and ultimately to the compass display. The new ASIC and sensor combination is unusual in that it eliminates a lot of complex analog processing. It does not require the high current pulse often associated with magnetic sensors. In fact, low power requirements allow the ASIC to operate off a button battery. There is also no requirement to process low-amplitude voltages or currents through high-gain amplifiers, integrators, and analog filters. All offsets and noise are corrected digitally through an onchip digital state machine. The sensitivity of the circuit is adjusted with digital registers rather than with analog gain setting capacitors or resistors.
The ASIC interfaces to a special class of nonlinear coils that have the unique property of being able to change inductance with small changes in a magnetic field. They do not require a separate drive and sensing coil; instead, both functions are implemented in one coil. The ASIC is also capable of driving two coils that are oriented perpendicular to each other, allowing it to sense both a sine and cosine wave. To save costs, the same analog circuit is shared by both sensors. Theory of Operation The block diagram in Figure 1 shows the main components of the ASIC circuit. The magnetic oscillator block is an oscillator that changes frequency based on the amount of magnetic flux present in the sensor core. The coil is part of an inductance and resistance (LR) circuit used in an oscillator. As the magnetic flux changes, the inductance of the LR oscillator varies and directly affects the frequency. The circuit then becomes a simple magnetic flux-to-frequency converter. Four oscillators in one, it is reconfigured through control signals from the state machine to drive current first in one polarity, and then reversed for Coil X. Matching the current drive for both source and sink as it comes out of the drive pins was a critical requirement during development of the circuit. With a drive current of ~5 mA, the two resistors on either side of the coil had to be fairly well matched at ±1% to eliminate problems with cancellation of random magnetic noise. The state machine then disables Coil X and repeats for Coil Y. The drive outputs are balanced drivers. The oscillator is allowed to free-run, and the output cycles are monitored with an up-counter. The current is then reversed and output cycles are monitored with the counter switched so as to count down. The final count is an indication of the magnetic field direction. The number of counts relative to the 4 MHz state machine clock is an indication of field strength. Measuring cycles for the same period of time in both polarities of the coil has the effect of making a fully differential measurement. This cancels the effect of magnetic noise. Since magnetic noise tends to be random and the Earth’s magnetic field is constant, the noise tends to be averaged out in a process similar to that of averaging out noise in a dual-slope A/D converter, but this circuit is based on frequency rather than voltage. Any random field interference near the sensors is eliminated. Steady-state interference is adjusted out of the system by two registers within the state machine that determine how long the oscillator is sampled by the state machine. Digital adjustment of sensor sensitivity can also be accomplished with the registers. The sensitivity registers allow the system to be adjusted for measurements in different ranges. This flexibility allows the same sensor ASIC to be used in a number of applications. When an application calls for a magnetic sensor with reduced sensitivity, the same sensor ASIC can be used by digitally adjusting the resolution of the device. Applications A primary global supplier of compass circuits for rear view mirrors and headliner displays is currently using the ASIC, as have all three major U.S. and most of the major Japanese automakers since the 1998 model year. As an ASIC, it is customized for each application but the technology is commercially available. The same circuit design is currently being evaluated for use in detecting the magnetic signature of physical objects, such as paper currency and buried land mines. Its effectiveness is also being examined for home security applications, and as an alternative to Hall effect sensors, which are limited by low resolution and limited range, and are less sensitive to the Earth’s magnetic field by orders of magnitude. The new ASIC provides 16 bits of heading information for each coil. As a result, compasses that use the new magnetic sensor ASIC are accurate to within ±2°. New Developments The original circuit was designed for 5 V operation. A newer version has already been implemented to work down to 2.5 V and a current of only 1 mA. The newer version also allows for a third, Z, Coil, which permits a full 3D measurement. This allows the circuit to work in portable devices and allows them to be held in any position. The design is being implemented in a wristwatch and other battery-operated applications. The new low-power circuit still requires only one magnetic oscillator. The newer 2.5 V part has been implemented in a 0.8 micron CMOS mixed-signal process. Summary Most applications of the fluxgate sensor have been limited to low-volume, high-accuracy applications. At the opposite end, Hall effect sensors have dominated the high-volume, low-cost area. The sensor and ASIC combination presented in this article provides the performance of a fluxgate sensor without the need for complex analog drive and sensing circuitry. The current, voltage, and cost are much less than those options with fluxgate implementations.
Tim White is System Architect, American Microsystems Inc., 2300 Buckskin Rd., Pocatello, ID 83201; 208-233-4690, fax 208-234-6626, white@poci.amis.com |
|
|
