E V A L U A T I O N AND P R O V I S I O N OF R E L I A B I L I T Y

FOR P R O T O N P R E C E S S I O N M A G N E T O M E T E R S

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Opticomechanical, magnetically modulated, proton precession, and other types of magnetometers are used for measuring weak magnetic fields (below 40 A / m ) and, in particular, the terrestrial magnetic field. The particular feature in the application of these magnetometers consists of their separation from their main base and, therefore, reliability becomes one of their most important operational parameters.

A particularly wide use is made of the proton precession magnetometers, which have considerable advantages as compared with other types. For instance, they have virtually no drift or variations in their sensitivity, it is not necessary to provide a definite orientation of their sensing elements with respect to the measured elements, and they are suitable for measuring absolute values of the magnetic field. These advantages of the proton precession magnet- ometers make them suitable for measuring the terrestrial magnetic field from aircraft, i.e., they can be used as aero- magnetometers.

The separation of proton precession magnetometers from their base and their application as aeromagnetometers complicate the conditions of their utilization and, therefore, one of the basic problems in designing such equipment becomes the provision of a definite level for their reliability.

The proton precession magnetometers usually consist of four basic units [1] comprising a receiving device, a unit for shaping and multiplying the frequency signal, an electronic frequency meter, and a recorder.

The receiving device transducer, which is based on the principle of free nuclear precession, transforms the mea- sured magnetic field into a pulse repetition frequency fx which is directly proportional to the measured magnetic field strength. The very weak signal, of a few microvolts, received from the transducer is amplified in the receiving device to a few millivolts, thus making it possible to transmit the signal over a cable to the input of the shaping and muhiplying unit located in the aircraft. After a repeated amplification and muhiplication the signal nf x is fed to the input of an electronic frequency meter, which counts the number of periods in frequency nfx over a fixed time inter- val. The signal is then converted to a decimal code and the recorder presents the measurement results in the form of a dotted graph. All the units with the exception of the receiving device are on board the aircraft. The receiving device is suspended beiow the aircraft on a cable which at the same time serves as a primary communication channel.

An analysis of the operation experience of the proton precession magnetometers shows that various types of failures appear in distinct forms in different units of the aeromagnetometer and affect in different ways its reliability.

Sudden failures can occur in all the aeromagnetometer units. Selection of reliable component elements and their rational loading serve to discount the possibility of sudden failures of the units on board the aircraft. In princi- ple standby replacements are also possible, but owing to the equipment's limitations in weight and volume they are not used.

The reliability of the receiving device is ensured by the measures mentioned above and by providing a stand- by according to the cold replacement method.

It was found by calculations that the probability of a faultless operation (the absence of sudden failures) of a magnetometer type AYaAM-6 without repairs for 10 h of continuous flying amounts at least to P10 = 0,965.

It is known that the reliability of measuring devices depends not only on sudden failures, but also on gradual failures produced by the errors increasing beyond their tolerances. A particular feature of an aeromagnetometer con- sists of the transmission of information over the measuring channel in the form of pulse frequency fx" The computer circuit is arranged in such a manner that it continues to operate reliably when the amplitude of signals obtained from

Translated from Izmeri tel 'naya Tekhnika, No. 4, pp. 36-37, April, 1968. Original article submitted June 15, 1967.

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the receiving device amplifier drops to 50%. As a resultofa tentative computation of the amplifier circuit with re- spect to gradual failures by the method described in [2] it has been found that the amplifier gain can change in 10h of continuous work by not more than 0.5%, and that the margin of the signal amplitude variations amounts to 50%.

The wear of the recorder's mechanical transmission components can lead to gradual failures which may affect its reliability. As a result of computations by the technique described in [4] it was found that the probability of a recorder's faultless operation without gradual failures over 500 h amounts to Ps00 = 0.999, and for a continuous 10-h flight it amounts to Pl0 m 1. Therefore, the gradual failures which may arise in mechanical transmissions can virtu- ally be discounted.

Thus, gradual failures, which arise only in the receiving device and the recorder's mechanical transmission, hardly affect the reliability of the aeromagnetometer and, therefore, they can be neglected.

The practice of utilizing aeromagnetometers has shown that their reliability is affected considerably by self- rectifying failures (malfunctioning and intermittent failures) which arise due to the noise being commensurate with the useful signals.

Owing to the appearance ofself-rec tifying failures, the information about the measured quantity may be distorted or lost in the cable or the receiving device, since the signal obtained from the transducer is very weak and the am- plification possibilities are limited by the size of the receiving device.

A noiseproof transducer has been developed in order to make the receiving device reliable with respect to in- termittent failures.

The principle of producing a noiseproof transducer consists [1] of providing the coil (coils) with such a configu- ration that its stray field with the current flowing through it is adequately small or equal to zero. In this case, ac- cording to the mutuality principle, external fields will not induce an emf in the coil. These conditions are met by a toroidal coil or a double ellipsoid of revolution.

In order to reduce the effect of intermittent failures in the cable it is necessary to use a screened cable and to apply redundant information. Since the measuring process consists of several measurements of interdependent quan- tities, it is possible to select an operating speed of the equipment with a precise reproduction of the measured quan- tity in the presence of n consecutive hitches. For the solution of this problem it is necessary to know the distribution of interferences and to determine the possible number of consecutive hitches. With a knowledge of the statistics of interferences it is possible to estimate tentatively the reliability of a device with respect to intermittent failures [3].

Thus, the problem of obtaining a given reliability for aeromagnetometers is reduced to eliminating sudden and intermittent failures.

The experience of ensuring the reliability of aeromagnetometers can be used for determining and ensuring the reliability of similar equipment.

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L I T E R A T U R E C I T E D

A. Ya. Rotshtein and V. S. Tsirel', Proton Geomagnetometers [in Russian], Gosgeoltekhizdat, Moscow (1963). A. M. Polovko, Foundations of the Theory of Reliability [in Russian], Izd. Nauka, Moscow (1964). K. A. Iydu, Nauchno-tekhnicheskii informatsionnyi byulleten' , LPI ira. Kalinina, No. 12 (1960). M. N. Ryabinov, Geophysical Equipment, No. 32 [in Rnssian], Izd. Nedra, Leningrad (1967).

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