GB2151022A - Two axis multisensor - Google Patents

Abstract

A two axis rate and linear acceleration detecting multisensor is formed by mounting a pair of accelerometers of the constrained mass type within a case. The accelerometers are mounted orthogonal to each other in vibratory units responsive to an electromagnet therebetween. Rate is determined from the coriolis acceleration force experienced by the accelerometers which vibrate 180 degrees out of phase to minimize signal distortions resulting from transference of vibrational energy to the case and mountings. <IMAGE>

Description

SPECIFICATION Two axis multisensor The present invention relates generally to the field of inertial guidance instrumentation and, particularly, to multisensors for use in measuring both the linear acceleration and rate of rotation of a moving body with respect to two axes.

A number of attempts have been made to utilize an inertial mass to detect the rate of rotation of a body. Generally, such attempts have been based upon the coriolis acceleration experienced by a vibrating or rotating body fixed to a second body whose rotation is to be sensed. Coriolis acceleration is described by the following equation: A = 2 x v; where A = coriolis acceleration; w = angular rate of the rotating coordinate system (second body) to be measured; and v = velocity component perpendicular to the axis of rotation.

The foregoing expresses the basic principle on which all vibratory gyros as well as spinning wheel gyros are based, namely, upon the acceleration experienced by a mass having a component of velocity perpendicular to the axis of rotation of the rotating coordinate system to which it is attached. The sensing of angular rate with an oscillating pendulum was first demonstrated by Leon Foucault in the early 1 850's. Since then a number of attempts have been made to apply coriolis acceleration principles to the design of rate and rate integrating gyros.

Prominent among the attempts to develop a rate sensing gyro in accordance with the foregoing principles have been the following (all referred to by trademark name): "Gyrotron" of the Sperry Gyroscope Corporation (1940); the "A5 Gyro" of Royal Aircraft Establishment, the "Vibrating String Gyro" of North American Rockwell Corporation (Autonetics Division, Anaheim, California); "Viro" of the General Electric Corporation and the "Sonic Bell Gyro" of General Motors Corporation (Delco Division). All the above-mentioned, with the exception of Gyrotron, began development in the early 1 960's.

In general, the above-named systems rely upon either a rotating body or an unconstrained vibrating body to supply the velocity component v perpendicular to the axis of rotation of the second body. The acceleration force experienced by such a rotating or vibrating body is then measured in some manner to provide the coriolis acceleration A. Knowing the coriolis acceleration and the velocity of a force-sensing element, one can then simply determine the rate of rotation of the body.

Vibrating bodies offer obvious advantages over rotating assemblages in terms of mechanical simplicity. In order to arrange a rotatable inertial instrument, such as an accelerometer, having sensitivity to coriolis acceleration, ball bearings, slip rings, spin motors and the like must be provided. Further, a rotational arrangement must be referenced in phase with the case in which it is mounted to resolve the input angular rate into two orthogonal sensitive axes.

Present day attempts to measure rotation via the use of a vibrating inertial sensor have been implemented by means of open loop vibrating mechanical systems in which the displacement of an unconstrained vibrating inertial mass upon experiencing coriolis acceleration generates an electrical signal proportional to the coriolis force. Such systems operate as tuning forks wherein the tines vibrate at frequency v and are deflected in a perpendicular plane by an amount proportional to A.

Such systems, while less complex mechanically than rotating systems, have proven to be subject to inaccuracies resulting from the orthogonal movements required of the vibrating open loop force detecting mechanisms of the "Vibrating string" variety.

According to one aspect of the present invention, there is provided a multisensor, responsive to the linear acceleration and rate of rotation of a body, comprising a constrained mass sensor responsive to linear acceleration along a first preselected axis, a constrained mass sensor responsive to linear acceleration along a second preselected axis, means for arranging said constrained mass sensors so that said first preselected axis is orthogonal to said second preselected axis, means for vibrating said sensors, and means responsive to the coriolis acceleration forces exerted upon said sensors.

According to a further aspect, the present invention provides a method for sensing the rate of rotation and acceleration of a body comprising the steps of: a) providing first and second constrained mass inertial sensors responsive to linear ac celeration forces; and b) arranging said first and second sensors with respect to said body so that said sensors are responsive to linear acceleration forces experienced by said body along respective, orthogonal, axes; then c) vibrating said first and second sensors at a preselected frequency; and d) measuring the linear and coriolis acceleration forces exerted upon each of said constrained mass inertial sensors.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 is a partial view, in exploded perspective, illustrating the relative arrangement of accelerometers in accordance with one embodiment of the invention; and Figure 2 is a side sectional view of the multisensor of Fig. 1.

Turning now to the drawings, Fig. 1 is an exploded perspective view of basic parts of an embodiment of the invention, pertaining to the preferred relative orientations of the inertial force sensing means that comprise the heart of the multisensor. The force sensing means comprises an orthogonal arrangement of upper and lower accelerometers 10 and 12 respectively. Each accelerometer is preferably of the force balance type in which a mass, such as a pendulous mass, is oriented to react to an acceleration force acting along a predetermined axis, known as its input axis.Unlike an open loop type of force detection mechanism, such mass is constrained by the action of reactive "forces" so that, rather than effecting a measurable displacement, the force acting on the mass is a measurable function of the energy required to enable the forcers to maintain the null position of the mass as it experiences acceleration forces. The pickoff sensors, which may be any of a number of conventional electro-mechanical transducers, produce resultant electrical signals proportional to the force (acceleration) sensed by the reactive inertial mass within the accelerometer.

While a wide range of inertial accelerationsensing instruments may be accomodated and function within the scope of the invention, the apparatus as illustrated in Fig. 1 utilizes two A4 MOD IV accelerometers of the pendulous, force-balance type. This accelerometer is in production and presently available from Litton Systems, Inc. of Beverly Hills, California. Each of the upper and lower accelerometers 10 and 1 2 is shown to be fixed to a corresponding upper or lower bracket 14, 16 comprising (in the instance of the illustrated lower bracket 16) a central backing member 1 8 sandwiched between two transversely-oriented flanges 20 and 22. The height of each overall bracket structure exceeds that the accelerometer fixed to it and each is mounted so that it extends both below and above such accelerometer.As will be seen, such arrangement allows the accelerometers to be mounted within the case of a multisensor in such a way that a suspension is effected, minimizing the possibility of deleterious mechanical feedback between accelerometer and case. Holes 24, 26, 28, 30, 32 and 34 are provided within the elements of the brackets for bolts that secure the brackets to the accelerometers and to an armature/diaphragm, disclosed in Fig. 2.

While the conventional inner workings of the accelerometers 10 and 12 are not shown, input axes 36 and 38 define the orientations of sensitivity to acceleration forces. Double headed arrows 40 and 42 indicate the colinear directions of vibration of the accelerometers while rotation of the body to which the multisensor case is fixed is measured about the indicated orthogonal rotation-sensitive axes 44 and 46. Thus, referring back to the equation for coriolis acceleration, the system shown in Fig. 1 is seen to impose a predetermined vibratory velocity v upon force-detecting accelerometers 10 and 1 2 along collinear axes 40 and 42, sense orthogonal rotations co about accelerometer axes 44 and 46 and experience coriolis acceleration forces A along input axes 36 and 38.Additionally, the multisensor system will, of course, detect noncoriolis induced linear acceleration forces along the input axes 36 and 38. Such accelerations can be distinguished from the rate measuring coriolis forces by appropriate selection of the frequency of vibration of the accelerometers coupled with conventional demodulation techniques, discussed below.

The multisensor system as illustrated and discussed above is shown in more detail in Fig. 2, giving a cross-section of a cylindrical case 48 of a multisensor including an assemblage as shown in Fig. 1. The instrumentation within the cylindrical case 48 is substantially orthogo-symmetrical about a horizontal axis 50; that is, corresponding elements of the instrumentation above the axis 50 are rotated by ninety degrees from those below the axis.

This is shown, of course, in Fig. 1.

Covers 52 and 54 seal the multisensor case. As is seen in Fig. 2, the bracket 14 securing the upper accelerometer 10 includes a central bracking member 56 joined to transversely-oriented flanges 58 and 60.

Each accelerometer-and-bracket assembly is bolted at top and bottom to a substantially disc-shaped diaphragm/armature having reinforced center and edge portions separated by a relatively thin annular diaphragm formed therewith to form independent double diaphragm suspensions both above and below the horizontal axis 50. Armature/diaphragms 62 and 64 are bolted to, and supply the sole support of, the upper bracket-and-accelerometer assembly while armature/diaphragms 66 and 68 provide the sole support for the lower bracket-and-accelerometer assembly.

Cylindrical spacers 70 and 72 separate the edges of the armature/diaphragms, completing a pair of independent vibratory units within the case 48, the upper vibratory unit comprising upper accelerometer 10-andbracket assembly sandwiched between armature/diaphragms 62 and 64 and surrounded by the cylindrical spacer 70 and the lower vibratory unit comprising lower accelerometer 12-and-bracket assembly sandwiched between armature/diaphragms 66 and 68 and surrounded by the cylindrical spacer 72.

An electromagnet 74 is positioned in the center of the case 48 by means of an in wardly-extending radial flange 76 and cup or bush 78 formed therewith. A conventional acceleration restoring amplifier 80 mounted on the flange 76 receives pickoff signals generated within the accelerometers and, in response, provides control signals to forcers within the accelerometers that act upon the pendulous mass. The necessary conductors for the aforesaid are not shown in Fig. 2; however, electrical communication is provided exterior to the multisensor by means of upper and lower conductors 82 and 84 which are in electrical communication with the sensing apparatus of the upper and lower accelerometers 10 and 1 2 respectively through soldered contact pads 86 and 88.Each conductor includes six individual conductors; one pair of conductors relates to the excitation of a light emitting diode portion of the pickoff sensor; another pair is associated with the output of a photodiode portion of the pickoff; and the third pair provides current to the accelerometer forcer mechanism.

The elecromagnet 74 drives the upper and lower double-diaphragm vibratory units defined above by activating and deactivating electromagnetic fields which alternately attract and release the diaphgragms 64 and 66. As a consequence of the driving of the diaphragms, the vibratory units, including associated accelerometers, are caused to oscillate in the vertical plane. Further, in accordance with the positioning of the electromagnet 74 between the diaphragms 64 and 66, the two units and associated accelerometers vibate out of phase by 1 80 degrees. By vibrating out of phase, the units, each having identical resonant frequencies, exert equal and opposite vibrtional forces thereby minmizing the vibrational energy coupled to the case 48 to avoid mounting sensitivities.

The ouput of each accelerometer is a signal containing both rate information and linear acceleration (along each accelerometer's input axis) information. The individual demodulation of the two types of information is relatively straightforward as a consequence of the differing frequencies of the rate of rotation and acceleration signals. The output rate information is modulated at the preselected frequency of accelerometer vibration while linear acceleration of interest can be expected to be either constant or to lie within a relatively low and predictable frequency range. The frequency of vibration of the double diaphragm suspensions is chosen to be high relative to system bandwidth requirements to permit the filtering of the modulated rate signal from the accelerometer output.Angular rate information is obtained by capacitively coupling the accelerometer output to a bandpass amplifier centered about the modulation frequency. The output of the bandpass amplifier is applied to the input of a demodulator, the reference signal for the demodulator being chosen to be in phase with the velocity of the vibrating unit. The output of the demodulator is then filtered to provide a d.c. voltage proportional in amplitude to the angular rate applied with a polarity sensitive to the direction of applied angular rate.

Thus it is seen that there has been provided to the inertial instrumentation art an apparatus which is capable of measuring rotation in two orthogonal planes and acceleration in two orthogonal directions. By using such a multisensor, one is able to obtain the advantages of a vibratory apparatus in terms of lesser complexity than that obtainable with a rotary oscillatory arrangement, while avoiding the inherent drawbacks of former vibrating arrangements.

While the invention has been described in its presently preferred embodiment, its scope is not to be so limited. Rather the invention is intended to encompass that defined in the following set of claims.

Claims (10)

1. A multsensor, responsive to the linear acceleration and rate of rotation of a body, comprising a constrained mass sensor responsive to linear acceleration along a first preselected axis, a constrained mass sensor responsive to linear acceleration along a second preselected axis, means for arranging said constrained mass sensors so that said first preselected axis is orthogonal to said second preselected axis, means for vibrating said sensors, and means responsive to the coriolis acceleration forces exerted upon said sensors.

2. A multisensor as defined in Claim 1, wherein said means for vibrating is arranged to vibrate said sensors out-of-phrase with one another.

3. A multisensor as defined in Claim 1 or 2, wherein said sensors responsive to linear acceleration comprise first and second accelerometer and said means for arranging comprises respective double diaphragm suspensions.

4. A multisensor as defined in Claim 3, wherein said means for vibrating includes an electromagnet mounted intermediate said double diaphragm suspensions.

5. A multisensor as defined in Claim 3 or 4, wherein the vibration frequency of said double diaphragm suspensions is high relative to the system bandwidth of the multisensor.

6. A multisensor as defined in any one of Claims 1 to 5, wherein said first and second sensors are pendulous force-balance accelerometers.

7. A multisensor substantially as hereinbefore described with reference to the accompanying drawings.

8. A method for sensing the rate of rotation and acceleration of a body comprising the steps of: a) providing first and second constrained mass inertial sensors responsive to linear acceleration forces; and b) arranging said first and second sensors with respect to said body so that said sensors are responsive to linear acceleration forces experienced by said body along respective, orthogonal, axes; then c) vibrating said first and second sensors at a preselected frequency; and d) measuring the linear and coriolis acceleration forces exerted upon each of said constrained mass inertial sensors.

9. A method as defined in claim 8, wherein the vibrating step further comprises the step of vibrating said first and second sensors out-of-phase.

10. A method for sensing the rate of rotation and acceleration of a body substantially as hereinbefore described with reference to the accompanying drawings.