An Improved Version of the Fluxgate Compass Module

Acta Polytechnica Vol. 47 No. 4–5/2007 An Improved Version of the Fluxgate Compass Module V. Petrucha Satellite based navigation systems (GPS) are widely used for ground, air and marine navigation. In the case of a malfunction or satellite signal inaccessibility, some back-up navigation system is needed. An electronic compass can provide this function. The compass module described in this paper is designed for precise navigation purposes. The compass module is equipped with electronic tilt error compensation, and includes everything in one package - electronics with digital output, sensors. A typical application of this compass is in underground drilling. A critical parameter in this application is heading accuracy. A reading error of 1 degree can cause a displacement of 1.8 metres in the target area (length of tunnel 100m). This is not acceptable in an urban conglomeration, and therefore a more accurate heading sensing device must be used. An improved version of this electronic compass is being finished. Keywords: navigation, azimuth, electronic compass, fluxgate, magnetometer. 1 Navigation 2 Magnetometer Satellite navigation systems can determine an absolute position on the Earth’s surface. An electronic compass must be a part of an inertial navigation system to be able to do the same. The output value from an electronic compass is the azimuth. The azimuth can be calculated using equation (1), where HEY and HEX are the horizontal parts of the magnetic vector and D is the declination in the measurement location (see Fig. 1). æH ö (1) Y = arctan çç EY ÷÷ - D. è H EX ø The azimuth calculated using equation (1) is correct only when the magnetic sensors are in the horizontal plane (pitch = 0, roll = 0). This cannot be easily mechanically assured in underground drilling applications. Therefore tilt sensors must be introduced into the system. The data from the magnetometers is then mathematically compensated for the actual measured pitch and roll. Three MEMS accelerometers are used as tilt sensors. The main part of the compass is a sensor of the Earth’s magnetic field. Three types of sensors are typically used for geomagnetic field sensing. Hall-effect magnetometers are used in applications where the cost of the sensor, its dimensions and power consumption are critical, e.g. watches and mobile phones. An AMR sensor (Honeywell HMC1001) offers higher accuracy but it is still very difficult to achieve the desired error limits (<±0.5 degree). A fluxgate sensor is the best choice for applications where accuracy is the most critical parameter. The miniature PCB fluxgate sensors used in this compass have smaller dimensions (34×16×1.2mm), lower power consumption (important for a battery operated device) and lower price (in case of mass production). Three types of PCB fluxgates sensors are shown in Fig. 2 (type A on the left with an excitation coil around the whole core, type B on the right with an excitation coil only on the two sides, type C where the excitation coil is equally distributed around the whole core with a higher count of compensation coil turns). This difference in sensor excitation coil distribution has a considerable impact on the sensor properties. A type A sensor with the excitation coil equally distributed has lower non-linearity error. However, a higher compensation current is needed because of the lower count of sensing-compensation coil turns. A type B sensor is used in the compass module, as the lower compensation current (for the Earth’s magnetic field it is 28 mA) means easier design of the compensation loop. A common operational amplifier can supply such a current. Fig. 1: Azimuth, heading components Fig. 2: Three types of PCB fluxgate sensors developed at CTU. Type A on the left, type B on the right, and C at the bottom 18 direction and magnetic vector © Czech Technical University Publishing House http://ctn.cvut.cz/ap/ Acta Polytechnica Vol. 47 No. 4–5/2007 A type C sensor has been developed, which combines the best from type A and type B. This sensor, with low non-linearity error and small compensation current, is used in the new version of the electronic compass. 2.1 Magnetometer electronics Fluxgate sensors are usually excited using a sine wave signal. In order to suppress the power consumption, pulse excitation is used. Typical signal evaluation electronics for sine wave excitation is the second harmonic detector. When operating with pulse excitation, some other type of evaluation has to be used. A differential switched integrator seems to be a good choice [1]. Various excitation signal patterns were tested (Fig. 3). The excitation frequency is constant (10 kHz), while the pulse width (e.g. 12 %) and the phase between excitation and evaluation sync. signals are changed. Another problem is the temperature bias stability of the PCB fluxgate sensors used here. The compass consumes less than two watts of electric energy, but this still causes considerable selfheating. By using resistors with a very small temperature coefficient (±3 ppm) as a current to voltage converter (magnetometers operate in a closed loop) and with a stable voltage reference (±3 ppm), the temperature stability is mainly influenced by the sensors themselves. Fig. 4 shows the temperature stability of the magnetometers. Simple measuring equipment was used to test PCB fluxgate magnetometer linearity. The test field (±55000 nT) was generated with a Helmholtz coil driven by the power supply with an IEEE488 interface. The magnetometer output voltage digitized by the ADC of the compass was sent to the computer and processed with MS Excel. The measured non-linearity depends on the actual environmental conditions (presence of magnetic disturbance). A typical non-linearity value was ±0.05 % of full scale (see Fig. 5). 40 20 e [nT] 0 -60000 -40000 -20000 0 20000 40000 60000 -20 -40 -60 B [nT] Fig. 3: From top to bottom: excitation current (600 mAp-p), synchronization signals for evaluation electronics, sensor output response (B ~ 40 mT) A three-channel PCB fluxgate magnetometer is used in this compass. The construction of such a device is a demanding challenge. At least a four-layer PCB must be used for the three-channel magnetometer evaluation electronics. Because of the unavailability of SMD components (resistors with low temperature coeficient, high quality capacitors), mixed components were used. The second version is constructed entirely with SMT. Only three connectors are “through hole” types. Fig. 5: PCB fluxgate magnetometer linearity error (channel X, ±0.036 % F.S.) 2.2 Compass electronics The whole system consists of three magnetometers, three accelerometers, six delta-sigma ADCs (ADS1210) and two microcontrollers (ATMEL ATTiny2313) – the first one is used for data acquisition and communicates through a serial line with the master system (e.g. a PC), and the second is used in an excitation unit. An improved version contains ATMEGA16 in the ADC module (t (...truncated)