it with other instruments which are already automatic, like variometers and proton magnetometers, it will be possible to install absolute magnetic observatories ...
DEVELOPMENT OF AN AUTOMATIC DECLINATIONINCLINATION MAGNETOMETER SEBASTIEN A. VAN LOO13 JEAN L. RASSON Institut Royal Météorologique de Belgique
Abstract. The first results in the design of an automatic DIM are presented. This instrument should be completely operational in 3 years. By associating it with other instruments which are already automatic, like variometers and proton magnetometers, it will be possible to install absolute magnetic observatories, all around the Earth, even in inaccessible places like on islands and on the ocean floor, since there is no need for an operator or maintenance. Automation is difficult because several key components generate considerable magnetic disturbances. Solutions to carry out the operations of rotation of the sensor, precision reading of the angles, and the pointing of an azimuth reference without disturbing the magnetic field, are proposed.
Keywords: DIM, absolute magnetic observation, declination, inclination, theodolite, fluxgate, piezoelectric motor, electronic angular encoder, automation
1. Introduction Many automatic instruments are able to provide recordings of the value of the total geomagnetic field as well as its variations. But the declination and the inclination still must be measured manually by an observer, using a DIM (declination-inclination magnetometer). If this instrument could be automated, it would become possible to establish completely autonomous magnetic observatories, working without need of an operator or maintenance (Rasson 1996). The Earth could then be totally and uniformly covered with magnetic observatories, by adding new stations to the current ______ 13
To whom correspondence should be addressed at: Institut Royal Météorologique de Belgique, Centre de Physique du Globe, B-5670 Dourbes, Belgium. Email: sebvl@oma be 177 J.L. Rasson and T. Delipetrov (eds.), Geomagnetics for Aeronautical Safety, 177–186. © 2006 Springer. Printed in the Netherlands.
S.A. VAN LOO AND J.L. RASSON
178
network, with installations far from base observatories, at inaccessible places like the ocean floor (70% of the Earth’s total surface), high altitudes, deserted areas, etc. Since January 2004, we have worked on the development of an absolute, automatic instrument for measurement of the direction of the geomagnetic field. At the beginning of 2006, the first phase of the project will be completed. The objective of this first phase is to obtain a completely functional and automatic instrument, with a total error on the measured angles D and I smaller than 6 arc-seconds, and using a far target as azimuth reference.
Figure 94. At left, a prototype of the theodolite; At right, a plan of the final theodolite.
The second phase, which ends in January 2008, will be devoted to the development of an automatic gyroscopic North-seeker, which will be used as azimuth reference for the instrument (Chave 1995). The errors on the measured angles will then be kept smaller than 6 arc-seconds for I, and smaller than 20 arc-seconds for D (Table 22). Table 22. Specifications of the automatic declination-inclination magnetometer. Time
Error on D
Error on I
Azimuth reference
Jan 2006 Jan 2008
< 6 arc-seconds < 20 arc seconds
< 6 arc-seconds < 6 arc-seconds
automatic pointing of a far target automatic gyroscopic North-seeker
The instrument will be similar to a robotized DIM system. The fundamental principles leading to the automation of the measurement are first presented. Then technological solutions to minimize error are proposed so that the instrument will meet the high precision and magnetic cleanliness
DEVELOPMENT OF AN AUTOMATIC DIM
179
constraints. Last, the electronic system for reading the angles, the use of non-magnetic piezoelectric motors, and the automatic pointing of the target are covered in depth. 2. Automation of the measurement An automatic measurement must have the same metrological qualities as a manual measurement. Thus, the same operations should be reproduced (see Table 23). In accordance with the traditional method of measurement with a DIM, the declination and the inclination are measured in 4 different positions (Rasson 2005). The instrumental errors should be equivalent to those of a traditional theodolite. The target is also measured in two positions. The execution of this protocol for each measurement ensures the absolute character of the result. Table 23. Operations to carry out in order to make an absolute measurement of the direction of the geomagnetic field. 1.
Synchronization with universal time.
2.
Leveling of the instrument.
3.
Pointing an azimuth reference (2 positions).
4.
Measurement of the declination (4 positions).
5.
Measurement of the inclination (4 positions).
6.
Pooling the results with those of scalar magnetometer, and variometer.
It was necessary to design and use a non-magnetic theodolite. Instead of a telescope, the theodolite is equipped with a directional magnetic sensor (fluxgate), and with a laser to point at the target. To make 4 positions of measurement for declination and inclination, the sensor must be able to make a complete rotation around the horizontal and vertical axes. Finally, the angular position of the sensor must be measured very precisely. 3. Technological solutions The two principal problems to overcome are avoiding magnetic parts or parts which cause a magnetic disturbance, and designing a precision device (from the mechanical and electronic points of view). Ferromagnetic materials cannot be used in construction, nor can electric lines conveying detectable DC current. Electronic circuits must be kept far away from the magnetic sensor. Figure 94 shows the present status of the theodolite. Its final version has not yet been realized. A device for controlling and correcting the leveling is also under development.
180
S.A. VAN LOO AND J.L. RASSON
The signals produced by the electronic acquisition system (readings of the angle, fluxgate, level, and pointing at the target) are collected by a microcontroller which uses analog to digital converters. Signals are then processed, and instructions are sent to the motor drivers in order to carry out the desired operation (Figure 95). The data storage, time control, and user interface are ensured by a computer, connected to the microcontroller via a USB bus.
Figure 95. Interactions between the different subsystems.
3.1. THE ANGULAR ENCODERS
In order to electronically evaluate angles, optical angular encoders are used. One system is used for each of the two orthogonal axes of the theodolite.
Figure 96. General diagram of an optical encoder.
A graduated disc, fixed on one axis of the theodolite, rotates between a light source and a detection system (Figure 96). Gratings, with the same period as the graduated disc, are placed behind the light source in order to amplify the signal by the optical moiré effect. There are four gratings and one photodiode for each graduated disc. The gratings are shifted by a quarter of a period (Figure 97). By subtracting the light signals c from a,
DEVELOPMENT OF AN AUTOMATIC DIM
181
and d from b, we obtain two sinusoidal signals free from the common mode. The disc is also equipped with a third track which produces only one reference pulse per rotation.
Figure 97. Graduated disc and gratings.
Since discs with 2500 graduations are used, a resolution of 0.144° is obtained (simply by counting the graduations). Then, because the two sinusoidal signals are in phase quadrature (Figure 98), calculating the arctangents of the signals sine/cosine leads to an analog signal having a linear dependence on the angle. Depending upon the quality of the encoders, the electronic disturbances, and the mechanical alignment of the system, a precision of up to 1 arcsecond can be achieved. The reference pulse is used to make this incremental encoder absolute. Figure 98. Electric signals allowing (a) the period count and (b) the continuous evaluation of the angle by interpolation between the graduation period increments.
Good signals lead to good precision. So the errors related to encoder and electronics quality, like amplitude modulation and undesired offset, are corrected in real-time by a digital processing algorithm (Figure 99). Errors, related to mechanical misalignment of the encoder compared to the rotation axis, are corrected by placing two encoders around the same disc 180° apart (Figure 100). Taking the average of the two measured angles provides a result free from eccentricity errors.
182
S.A. VAN LOO AND J.L. RASSON
Figure 99. Signals before (a) and after shaping (b).
Figure 100. Two encoders placed around the horizontal axis.
DEVELOPMENT OF AN AUTOMATIC DIM
183
Available encoders are generally not magnetically clean, and cannot be placed symmetrically in pairs on the same disc. Some parts (the detector board and others) have to be replaced by specially designed circuits (Figure 101).
Figure 101. Example of a detector board for angular encoders where a ready-made IC is used as a detector, and linear amplifiers are included on the board.
Preliminary tests show that the error can easily be made lower than 3.6 arc-seconds. More rigorous tests are presently under development. 3.2. THE PIEZOELECTRIC MOTORS
The movement around the axis of the theodolite is driven by piezoelectric motors, which can be bought in totally non-magnetic versions. The rotational movement of the shaft is obtained by pressing its base against an annular piezoelectric crystal, on the surface of which a revolving traveling wave is maintained (Figure 102). This traveling wave is obtained by stimulating the crystal with two high voltage signals (300Vpp), one cosine and one sine, at a frequency of about 40 kHz. In this way, power is produced as a small, non-disturbing AC current. Sometimes, a slow speed is necessary, primarily because of the computing and reaction times of the electronic circuits (for example when the angle has to be calculated precisely, or when a position has to be reached very finely). Other times, in order to save time, large displacements can be carried out at high speed. Smooth accelerations and decelerations are
184
S.A. VAN LOO AND J.L. RASSON
also necessary to avoid vibrations at start and stop. For these reasons it is very important to have total control of the rotation speed. The motor shafts can be used directly as axes for the theodolite, with no need for a transmission or reduction system. Three parameters of the motor drive sine waveforms can be varied to control the motor rotation speed: amplitude, phase, and frequency. Changes in amplitude led to a loss of torque at slow speed. Tests varying the phase demonstrated that speed variation was strongly non linear, and repeatability was too low for effective control. Adjusting the frequency of the excitation signals allowed us to obtain satisfactory motor speed control with adequate torque, linearity, and repeatability.
Figure 102. General diagram of a rotary piezoelectric motor.
3.3. THE AZIMUTH REFERENCE
In order to reference the horizontal angle measurements to True North the theodolite must acquire a known azimuth reference. This process is traditionally performed by an observer who points the telescope at a far target. To automate the process, the following method is presented. A laser diode module is installed in place of the telescope. It points toward a corner cube reflector which is centered at the point whose azimuth is precisely known (actually the visual target). According to the properties of the corner cube reflector, an incident light ray is reflected along the incoming beam, but offset by a distance, e, (Figure 103) depending on the angle, D between the incident ray and the line which connects the center of the corner cube to the vertical axis of the theodolite. Two solar cells are positioned around the laser in order to evaluate the offset of the reflected
DEVELOPMENT OF AN AUTOMATIC DIM
185
ray. The difference of light touching the two solar cells is directly related to the pointing of the center of this electronic target: when the reflected ray returns precisely in the center, the laser exactly points to the center of the target.
Figure 103. The corner cube reflector.
The goal of the second phase of the project is to replace this azimuth reference system with an automatic gyroscope.. This would allow the instrumentation to work in a closed system with no need to connect to external references.
References Chave, A.D., Green, A.W., Evans, R.L., Filloux, J.H., Law, L.K., Petitt, R.A., Rasson J.L., Schultz, A., Spiess, F.N., Tarits, P., Tivey, N. and Webb, S.P. (1995). Report of a Workshop on Technical Appoaches to Construction of a Seafloor Geomagnetic Observatory, Technical Report WHOI-95-12, Woods Hole Oceanographic Institution, Woods Hole, USA. Rasson J.L. (1996). Progress in the design of an automatic DIflux, in Proceedings of the VIth Workshop on Geomagnetic Observatory Instruments, Data Acquisition and Processing (JL Rasson Ed.), Publ. Sci. et Techn. No 003, Institut Royal Meteorologique de Belgique, Brussels p190-194. Rasson J.L., (2005). About Absolute Geomagnetic Measurements in the Observatory and in the Field, Publication Scientifique et Technique No 040, Institut Royal Meteorologique de Belgique, Brussels, 43 p
DISCUSSION
Question (Jordan Zivanovic): Is the microcontroller with 8 gates or more? Answer (Sebastien van Loo): I currently use a microcontroller with a 16 bit digital port, having 8 analog inputs (ADCs), and a USB interface Question (Jürgen Matzka): How to find the zero-position of the fluxgate sensor (slow movement or stepwise moving)?
186
S.A. VAN LOO AND J.L. RASSON
Answer (Sebastien van Loo): Piezoelectric motors offer the possibility to rotate so slowly that the zero-position of the sensor can be found by moving continuously. Question (Spomenko J. Mihajlovic): What about magnetic influence of electronic parts. Can you use photo-resistors? Answer (Sebastien van Loo): The majority of the electronic systems are kept far away from the sensor. For the circuits which must be closer, like the angular encoders, I take many precautions to minimize the disturbances, like avoiding current loops, and choosing SMD-packaged parts. Actually, I use photodiodes rather that photo-resistors (angular encoders, target pointing). But if the use of photo-resistors appeared essential later, I think that it would be possible to find some models which are magnetically clean enough. Question (Valery Korepanov): How do you find true azimuth in small closed volume? Answer (Sebastien van Loo): Initially, the azimuth reference will be obtained, by the automatic pointing of a far target. The second phase of the project is devoted to the replacement of this system by an automatic gyroscope. It would then be possible to obtain true azimuth in a small volume. Question (Angelo de Santis): In your automatic system have you considered the possibility to make an absolute measurement of D and I practically simultaneously by placing the fluxgate element at a given nonzero inclination with respect to horizontal plane and rotating it at the usual four positions of zero-current findings? Answer (Sebastien van Loo): The measurement algorithm that I chose consists in measuring the declination while the fluxgate is placed horizontally and the inclination while the fluxgate is in the magnetic meridian. But the instrument can be programmed to execute any other algorithm, without need of hardware adaptations.
