GB2146775A - Accelerometer system - Google Patents

Abstract

An accelerometer system suitable for measuring low- frequency accelerations in inertial systems is also able to measure high-frequency vibrational accelerations which give rise to e.g. sculling effects in inertial platform arrangements employing three such accelerometer systems. The system comprises a low-frequency inertial quality accelerometer 11 and a high-frequency, e.g. piezo-electric, accelerometer 15, the outputs being filtered by low- and high-pass filters (20, 22) respectively and additively combined (24), the level of combined output signal being maintained over the band of frequencies of interest. The filters may be arranged to give a flat response at a cross-over frequency chosen to be removed from the higher and lower frequencies mainly desired to be measured. Three such systems may be combined with their sensitive axes orthogonally arranged. <IMAGE>

Description

SPECIFICATION Accelerometer system This invention relates to accelerometer systems, particularly of the type employed in inertial platforms.

Inertial platform arrangements are well known employing a plurality of gyroscopes and accelerometers arranged to monitor linear and rotational motion of the platform. The accelerometers employed to sense motion of the platform are required to measure accurately zero acceleration, unidirectional acceleration and low-frequency vibrational acceleration of the platform consistent with motion of a vehicle carrying it. Such accelerometers will hereinafter be referred to as 'inertial quality' or 'low-frequency' accelerometers having a typical operating bandwidth from zero to several hundred Hertz.

Within an inertial platform construction there are several sources of error due both to imperfections of components and due to external forces, particularly what are referred to in this specification as high-frequency vibrations, that is, vibrations at frequencies above the normal operating range of the inertial quality accelerometers.

Such sources of error and the inducing motions are discussed in the text books, for instance by N. Fernandez and G.R. Macomber in 'Inertial Guidance Engineering' published by Prentice-Hall Inc., Englewood Cliffs, N.J., U.S.A. One effect, sculling motion, is of particular interest in that high-frequency iinear and rotational vibrations about two orthogonal axes result in a non vibrational displacement component about a third orthogonal axis, such component being within the operating bandwidth of the low-frequency inertial quality accelerometer.

With the commonly employed gimbalmounted gyro systems vibrational errors tend to be small and give rise to effects which are readily eliminated. However in the more recently deveioped mechanically dithered laser gyro systems the levels of such vibrational motions are higher and unavoidable. This is particularly true of strapdown systems in which the platform and its components are rigidly fixed with respect to a carrying vehicle.

To mitigate the effects of such high-frequency vibrations on the low-frequency accelerometers it becomes necessary to measure those high-frequency vibrational accelerations along the three orthogonal axes of the inertial quality accelerometer and derive therefrom correction to be applied to the low-frequency accelerations measured.

It is apparent that this may be achieved in a number of ways. Firstly, each low-frequency inertial quality accelerometer may be constructed with an operating bandwidth extending to higher frequencies in excess of one kiloHertz, but such a device is complex and essentially expensive to produce whilst compromising on accuracy and/or reliability at the extremes of operating range which are by definition of most interest.

Secondly, in addition to the inertial quality accelerometers employed at low-frequencies, high-frequency accelerometers, that is, with an operating range of tens of Hertz to several kiloHertz, ones may be employed solely to detect vibrational movements within the system. Signals produced by the high-frequency accelerometers are processed by application to suitable algorithms to provide corrections to the signals produced by the low-frequency accelerometers.

The use of separate high- and low-frequency accelerometers does lead to other potential sources of error. The magnitudes of signals produced by all six accelerometers vary from device to device and the correction algorithm has to be tailored as a function of the accelerometers' characteristics. If any of the accelerometers are changed or their characteristics vary then scale factor errors are introduced requiring the whole system to be re-calibrated.

It is an object of the present invention to provide a single-axis accelerometer system suitable for use in an inertial platform operable over a wider frequency range of vibrational accelerations than known inertial quality accelerometers and which mitigates the disadvantages of the above outlined arrangement.

It is also an object of the present invention to provide an inertial platform arrangement including a triaxial combination of such singleaxis accelerometer systems.

According to a first aspect of the present invention a single-axis accelerometer system comprises a low-frequency accelerometer (as herein defined) responsive to acceleration forces acting thereon in a predetermined sensitive direction, a high-frequency accelerometer (as herein defined), fixed in relation to the low-frequency accelerometer and responsive to high-frequency vibrational acceleration forces acting thereon in said predetermined sensitive direction and signal processing means operable to receive acceleration related signals from both the high-frequency and lowfrequency accelerometers and including filter means operable to inhibit signals from the high-frequency accelerometer occuring in a frequency band at which the low-frequency accelerometer operates and to inhibit signals from the low-frequency accelerometer occuring in a frequency band at which the highfrequency accelerometer operates and signal combining means operable to combine the filtered signals of the two accelerometers to provide a single substantially uniform signal in both the low-frequency inertial band and highfrequency vibration band of interest.

According to a second aspect of the present invention an inertial platform arrangement includes three single-axis accelerometer systems as defined in the preceding paragraph arranged with their sensitive axes mutually orthogonal.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1(a) is a schematic block diagram of a single-axis accelerometer according to the present invention, Figure 1(b) is a schematic block diagram of the measuring and signal processing element including the accelerometer of Figure 1 (a) associated with an inertial platform, and Figure 2 is a schematic perspective view of a strapdown inertial platform including a laser gyro arrangement and three accelerometer systems according to the present invention showing the disposition of the individual accelerometers.

Referring to Figure 1 (a) the single-axis accelerometer system 10 of the present invention comprises a conventional low-frequency inertial quality accelerometer 11, for example of the pendulous type designated FA2 and manufactured by the applicant company, having an operating bandwidth extending from zero (or D.C.) up to several hundred Hertz vibration. The accelerometer 11 includes a capture circuit 1 2 arranged to give a differential voltage output at terminals 13, 14 proportional to an acceleration force detected as acting along a sensitive axis of the accelerometer. The terminal 1 3 may be considered the positive terminal with respect to the terminal 14, that is, defined by the polarity of output voltage for acceleration in a predetermined direction along the sensitive axis.

The system also includes a high-frequency vibration measuring accelerometer 15, such as a miniature piezo electric type 2250A manufactured by Endevco Corp. San Juan Capistrano, California, U.S.A., operable to measure vibrational accelerations in the ranges of tens of Hertz to thousands of Hertz.

The high-frequency accelerometer produces an alternating output voltage of amplitude proportional to the vibrational acceleration amplitude by way of an output amplifier 1 6 having differential output terminals 1 7 and 1 8. The output terminal 1 7 provides a signal voltage positive with respect to the terminal 1 8 for acceleration along a sensitive axis in the same predetermined direction as mentioned above for the low-frequency accelerometer and the high-frequency accelerometer 1 5 is rigidly fixed with respect to the body of low-frequency accelerometer 11 so that their sensitive axis extend in the same direction.

The differential outputs of the accelerometers are applied to signal processing means indicated generally at 1 9 and comprising filter means 20 and signal combining means 21.

The filter means 20 comprises a low pass filter network 22 consisting of series resistor R1 connected to output 1 3 followed by a shunt capacitor C1 connected to the output 1 4 of the low-frequency accelerometer and a high-pass filter network 23 comprising a series capacitor C2 connected to output 1 8 followed by a shunt resistor R2 connected to the output 1 7. The outputs 14 and 1 7 of the accelerometers are connected to each other and the components R1 and C2 to opposite polarity inputs of a differential input amplifier 24 which comprises the signal combining means 21 and which provides an accelerometer system output signal at 25.

The output signal levels of the accelerometers are adjusted during manufacture so that the system output signal is of substantially constant amplitude for accelerations both at the low-frequency inertial, range and the high-frequency vibration range. Although not essential the filter components may be chosen and matched with respect to each other such that the filters have a common cut-off frequency removed from the aforementioned higher and lower-frequency bands at which the power spectrum is greatest. Ideally the filter network components are chosen such that the combined, system output, signal remains of substantially constant amplitude in the vicinity of the cross-over point.

The precise form of the accelerometer system, that is, choice of high- and low-frequency accelerometers, construction of filter means and signal combining means is open to variation, but such a system provides an effective single-axis accelerometer having an operating bandwidth in excess of known inertial quality accelerometers and constructed as a single unit in which scale factor considerations, associated with changes in either one of the component accelerometers, is mitigated by changing the complete calibrated accelerometer system.

The accelerometer system described may be provided with self-checking means (not shown) where there is overlap in the operating bands of the high and low-frequency accelerometers. Band pass filter means may be provided to isolate the signals from each accelerometer in the common band and comparison means compare them to indicate a departure from conformity of response to the two accelerometers.

The physical construction of the accelerometer system is such that the high-frequency accelerometer with the low-frequency accelerometer and may be readily mounted on or in the body thereof with little increase in the original volume occupied. Furthermore, provided the sensitive axes are suitably aligned and both accelerometers are subject to the same acceleration then there is considerable freedom in mounting the two accelerometers in the system so that when a plurality are employed, for example in a three axis combination, a more efficient arrangement may be obtained.

This is most readily appreciated by example in which the accelerometer system of Figure 1 is employed in an inertial platform arrangement. Such arrangements are well known and the principal components are illustrated schematically in Figure 1 (b) which together with Figure 1 (a) shows three single-axis accelerometer systems 10, 10' and 10" carried by and having sensitive axes aligned in the orthogonal X, Y and Z axes of the inertial platform.

The platform also carries three single-axis gyros 30, 30' and 30" each also aligned to measure rotational forces with respect to the orthogonal X, Y and Z axes. The outputs of accelerometers and gyros are fed to a computer shown generally at 31 which uses standard techniques to compute the spatial position of the platform and does not require further description. The computer is also organised to take into account the receipt of the accelerometer signals in the higher frequency vibration band and from them to use algorithms based on standard error correction equations, to compute and apply low-frequency corrections to the low-frequency accelerometer signals of other axes.

Referring now to Figure 2, an inertial platform is indicated schematically comprising a cluster 40 formed by the three accelerometer systems 10, 10' and 10" carried on the inner walls of a cube structure and the three gyro packages 30, 30' and 30" attached to the exterior walls of the structure. The gyros are compact laser gyros and mechanically dithered to effect operation. They are also operated in the strapdown mode, that is, the platform is not suspended to maintain orientation in space by movement with respect to a vehicle carrying the platform, but is constrained to move with the vehicle being susceptible both to vibrations transmitted from the vehicle and those generated by operation of the gyros.

The accelerometer systems 10, 10' and 10" are constructed such that the high-frequency accelerometer 1 5 is displaced from the main bulk of the low-freguency accelerometer 11 enabling all three accelerometer systems to be located so that the low-frequency accelerometers are mounted with their centres of percussion close to the centre of gravity of the platform and the high-frequency accelerometers are substantially coexistant in space.

The use of a single-axis accelerometer system in accordance with the first aspect of the present invention thus enables construction of an inertial platform of improved performance.

Clearly other forms of platform may employ such single-axis accelerometer system which systems may also be employed in any arrangement in which a wider operating bandwidth is required and potential scale factor errors through the use of more than one accelerometer are to be avoided.

Claims (10)

1. A single-axis accelerometer system comprising a low-frequency accelerometer (as herein defined) responsive to acceleration forces acting thereon in a predetermined sensitive direction, a high-frequency accelerometer (as herein defined), fixed in relation to the low-frequency accelerometer and responsive to high-frequency vibrational acceleration forces acting thereon in said predetermined sensitive direction and signal processing means operable to receive acceleration related signals from both the high-frequency and lowfrequency accelerometers and including filter means operable to inhibit signals from the high-frequency accelerometer occuring in a frequency band at which the low-frequency accelerometer operates and to inhibit signals from the low-frequency accelerometer occuring in a frequency band at which the highfrequency accelerometer operates and signal combining means operable to combine the filtered signals of the two accelerometers to provide a single substantially uniform signal in both the low-frequency inertial band and highfrequency vibration band of interest.

2. A single-axis accelerometer system as claimed in claim 1 in which the filter means comprises a high pass filter network coupled to the output of the high-frequency accelerometer and a low pass filter network coupled to output of the low-frequency accelerometer, the cut off points of the filters being at a frequency above normally encountered inertial vibrations and below normally encountered system vibrations.

3. A single-axis accelerometer system as claimed in claim 2 in which the filter networks are chosen such that the combined signal is of substantially constant amplitude at the crossover point.

4. A single-axis accelerometer system as claimed in any one of claims 1 to 3 in which the combining means is a summing amplifier.

5. A single-axis accelerometer system as claimed in any one of the preceding claims in which the high-frequency accelerometer is a miniature piezo-electric accelerometer.

6. A single axis accelerometer as claimed in any one of the preceding claims including self checking means comprising filter means operable to pass signals from the low- and highfrequency accelerometers in a band common to both accelerometers and comparison means operable to compare the signals received from such accelerometer and indicate any difference in their responses.

7. A single-axis accelerometer system substantially as herein described with reference to and as shown by Figure 1 of the accompanying drawings.

8. An inertial platform arrangement including three single-axis accelerometer systems as claimed in any one of the preceding claims arranged with their sensitive axes mutually orthogonal.

9. An inertial platform arrangement as claimed in claim 8 in which the single-axis accelerometer systems are disposed with the high-frequency accelerometers substantially co-existant.

10. An inertial platform arrangement substantially as herein described with reference to and as shown in Fig, 1 (b) and Fig. 2 of the accompanying drawings.