GB2153074A

GB2153074A - Multisensor

Info

Publication number: GB2153074A
Application number: GB08428962A
Authority: GB
Inventors: Robert E Stewart
Original assignee: Litton Systems Inc
Current assignee: Northrop Grumman Guidance and Electronics Co Inc
Priority date: 1984-01-23
Filing date: 1984-11-16
Publication date: 1985-08-14
Also published as: IT8547574A0; GB8428962D0; SE8500267L; CA1217648A; SE457569B; SE8500267D0; JPS60162917A; NO844439L; DE3500043A1; FR2558588A1; IT8547574A1; IT1182134B; GB2153074B; DE3500043C2

Abstract

A multisensor of the vibrating- mass type comprises accelerometers (10, 12) mounted in parallel beam suspensions with their input axes (22, 24) substantially colinear. Forces are applied to the accelerometers through piezoelectric elements mounted to the suspensions to induce vibration so that coriolis rate information is included in the output of the system. Output circuitry discriminates between rate and linear acceleration information so that enhanced sensitivity to each parameter is obtained without confusion between coriolis and linear acceleration forces even when the frequency of linear acceleration is close to the modulation vibration frequency. Rate of rotation is sensed about axis (82). <IMAGE>

Description

SPECIFICATION Multisensor The present invention relates to inertial guidance instrumentation and, more particularly, pertains to multisensors suitable for measuring both the linear acceleration and rate of rotation of a moving body.

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 (3 x v; where: A = coriolis acceleration; Ç = angular rate of the rotating coordinate system (second body) to be measured; and v = velocity component perpendicu lar to the axis of rotation.

The foregoing expresses the basic pinciple on which all vibratory gyros as well as spinning wheel gyros are based; namely, a coriolis acceleration force is experienced when a moving mass has a velocity component perpendicular to the axis of rotation of an associated rotating coordinate system. In application, this principle allows the sensing of angular rate with an oscillating pendulum as 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 inertial sensors (all referred to by trademark name): "Gyrotron" of the Sperry Gyroscope Corporation (1940); "A5 Gyro" of Royal Aircraft Establishment; "Vibrating String Gyro" of North American Rockwell Corporation (Autonetics Division, Anaheim, California ); "Viro" of the General Electric Corporation and "Sonic Sell Gyro" of General Motors Corporation (Delco Division). All of 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 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 the orthogonal sensitive axes, additionally complicating such arrangements.

A potential problem inherent in any multisensor comprising one or more vibrated sensors of the inertial mass type results from the fact that acceleration information along the input axis of the sensor(s) is included in the sensor's output. While, for many applications and environments, the frequency of acceleration is predictable and lies outside the bandwidth of concern, confusion will arise when the frequency of linear acceleration along the input axis is close to the frequency of vibration of the sensor.

According to the present invention, there is provided a multisensor comprising, in combination: a) means responsive to acceleration along a first axis; b) means responsive to acceleration along a second axis; c) means for mounting each of said responsive means so that said first axis is substantially colinear with said second axis; and d) means for vibrating said responsive means out of phase with one another along parallel axes, each of said axes being orthogonal to said first and second axes.

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 top view of a single axis multisensor with certain portions removed for purposes of clarity; Figure 2 is in part, a cross section of the single axis multisensor taken along section line 2-2 of Figure 1 and additionally including certain components omitted from Figure 1; Figure 3 is an enlarged partial cross section taken along the line 3-3 of Figure 2 for the purposes of illustrating the manner by which vibration of right and left accelerometers is effected; and Figure 4 is a functional block diagram of rate and acceleration extraction circuitry for use with the multisensor.

Turning now to the drawings, Figure 1 presents a top view of a multisensor in accordance with one embodiment of the present invention. A number of features of the embodiment have been removed from Figure 1 to enhance the clarity of illustration. Such features will be shown and pointed out in subsequent views and accompanying discussion.

The multisensor illustrated includes a right accelerometer 10 and a left accelerometer 1 2 arranged within a case 14, the top of which is removed in Figure 1 to allow visual access. At least one accelerometer may be of a type in which an inertial mass therein is arranged to react to, and thereby provide an indication of, acceleration forces along a predetermined direction. It might further be of the constrained mass type. Alternatively accelerometers of the open loop type or a combination of open and closed loop type sensors may be employed in the multisensor. Additionally, the present invention may be practiced by means of accelerometers including elements whose optical properties are altered during acceleration.

The accelerometers are situated within a cavity 1 6 formed interior to the case 14. Each accelerometer is fixed to a three part bracket, the right accelerometer being fixed to the bracket that includes a finger 1 8 and beam 1 9 and the left accelerometer 1 2 being fixed to the bracket that includes a finger 20 and beam 21. Each combined bracket-and-accelerometer assembly is sandwiched between a pair of spaced-apart flexible side beams that include piezoelectric elements bonded thereto for effecting predetermined vibratory sensor motion. The side beams are not shown in Figure 1 to permit a clear view of each accelerometer and bracket assembly.

The accelerometers 10 and 1 2 are arranged within the cavity 1 6 in such a way that, at rest, their input axes, 22 and 24 respectively, are substantially colinear. This may be seen more clearly in Figure 2, a cross sectional view generally taken along line 2-2 of Figure 1 and including some elements removed from the prior figure for purposes of clarity. In Figure 2 one can view the right and left parallel beam suspensions comprising spaced apart side beams in pairs, 25, 26, and 27, 28, respectively that sandwich the right and left accelerometer-and-bracket assemblies. (It is to be noted that the right bracket assembly is completed by means of a lower finger 30 and the left bracket is completed by a lower finger 32).

The masses of the right and left assemblages of accelerometer, bracket and sidebeam pairs are substantially the same to minimize loading at the case mountings 34, 36, 38 and 40. Such pairing of masses tends, to a first order, to compensate linear (pure translational) vibration forces. Identical holes 42 and 44 are provided in the beams 19 and 21, the hole 42 essentially serving only to equalize mass while the hole 44 accommodates a magnet 46 that corresponds to a magnet 48 fixed to the right accelerometer 10.

Each of the magnets 46 and 48 interacts with a case-fixed pair of coils that, taken together, act as the multisensor velocity pickoff. In the instance of the magnet 48, its vibration with the right accelerometer 10 induces current in velocity pickoff coils 50 and 52 that are secured to a case-fixed bracket 54. (The bracket 54 additionally provides the location of a right acceleration restoring amplifier 56.) The vibration of the left accelerometer 1 2 and magnet 46 induces current in velocity pickoff coils 58 and 60 that are associated with the bracket 62. The left acceleration restoring amplifier 64 is secured to the case-fixed bracket 62.

Figure 3 is an enlarged partial cross sectional view taken along the line 3-3 of Figure 1 for the purpose of illustrating the means employed to effect vibration of the accelerometers 10 and 1 2. As can be seen in this view, the right accelerometer 10 is maintained in secure relation between side beams 25 and 26 of the right parallel bar suspension by means of the spaced apart fingers 1 8 and 30 of the retaining bracket. The sidebeams 25 and 26 extend the length of the cavity 1 6 and are fixed at their ends to the opposed beam support flexures 66 and 68. The side beams are each of generally W-shaped cross section with outwardly-facing reinforced portions integral with thin, web-like members.

Piezoelectric elements 70, 72, 74, 76 are bonded to the web-like portions of the side beams by adhesive means such as epoxy or the like. Metallized contacts as shown are plated to the piezoelectric elements in pairs.

As is well known, such piezoelectric material is subject to predictabie and reproducible deformation in response to positive and negative electrical potentials. For example, by application of negative and positive electrical potentials to properly polarised elements in accordance with the combination indicated in Figure 3, net forces will be applied to the side beams tending to force each upwards at its midpoint. Conversely, by the reversal of signs of the indicated potentials, the combinations of sidewall, bracket and accelerometer will be forced downwards. Thus, by appropriate sequencing of polarities of the elecrical signals applied to the sidewalls, the accelerometer 10 (and the accelerometer 12) can be caused to vibrate up and down at a preselected frequency and amplitude.

The vibrations of the accelerometers 10 and 1 2 are induced, with a 180 degree phase difference, to occur along the parallel axes 78 and 80 shown in Figure 2. As a result of the above-described coriolis acceleration forces that are induced in a vibrating system, the vibrations of the accelerometers 10 and 12 along the indicated axes will induce measurable acceleration forces proportional to the rate of rotation of the multisensor in the direction of the input axis of each accelerometer.Thus, the outputs of the accelerometers 10 and 1 2 will contain a measure of the rate of rotation of the system about axis 82, shown in Figure Figure 4 is a schematic diagram of electrical circuitry for determining both linear acceleration along the input axes of the accelerometers 10 and 1 2 and rotation about axis 82 with great accuracy by utilizing the output generated by a multisensor in accordance with the preceding discussion. By processing the signals as shown, one attains accuracy of measurement that would otherwise be jeopardized in a coriolis type multisensor when accelerations along the accelerometer input axis are experienced at frequencies approximating the modulation frequency of vibration of the accelerometer.

The signals that create the vibrations of the accelerometers are provided along conductors 88 and 90 by a driver circuit 86. Currents induced in the right and left pickoff coil pairs actuate the driver 86 in a self-resonant circuit arrangement. For example, the sensed vibration of the left accelerometer 12, converted into a corresponding sinusoidal current proportional to velocity through interaction of the magnet 46 with left pickoff coils 58, 60 is shown in the Figure to be applied as an input to the driver circuit 86.In addition, the signal induced in the pickoff coils serves as a demo adulation reference signal by application to a demodulator 92. (As should be apparent, the coriolis acceleration signal, a cross product, is oscillatory with frequency equal to that of the frequency of vibration of the sensing accelerometer and amplitude proportional to the input angular rate. Thus, the extraction of angular rate or velocity information requires demodulation of a sinusoidal signal).

The outputs of the right and left accelerometers are fed, in parallel, to both a differential amplifier 94 and a summing amplifier 96.

As the accelerometers are vibrated 1 80 degrees out-of-phase, the component portions of their signal outputs that relate to the measurement of coriolis acceleration are of opposite sign while the portions relating to linear acceleration are not so effected and are of like sign. Thus the output of the differential amplifier 94, a measure of the difference between the accelerometer outputs, is solely a measure of coriolis acceleration and, hence, rotation, since the portions of the outputs responsive to linear acceleration are cancelled regardless of the relationship between the frequencies of these two individual component portions of accelerometer output.As a further consequence of the equal and opposite senses of the coriolis or rate components of the sensor outputs, the output of the differential amplifier 94 provides twice as sensitive a measure of rotation as the output of a single component accelerometer of the multisensor.

The rate output is then applied to the demodulator 92 which, as discussed above, utilizes the induced sinusoidal current of the velocity pickoff coils as its demodulations reference. The demodulated rate output is then applied to a filter 98 for final extraction of the rate signal.

As a further consequence of the opposite senses of the coriolis components of the outputs of the right and left accelerometers, the output of the summing amplifier 96, to which the accelerometer outputs are applied, contains no rate information and is twice as sensitive a measure of linear acceleration along the coincident accelerometer input axes as is the output of a single one of the accelerometers 10 and 1 2. This output signal is not demodulated (unlike the rate signal) since it is a direct measure of acceleration, whether or not such acceleration is vibratory in nature. This signal is then applied to filter 100 for extraction of acceleration information therefrom.

Thus it is seen that there has been provided an improved multisensor of the vibratory type that achieves enhanced sensitivity to both acceleration and rotation and is not susceptible to errors that might otherwise be induced when the frequency of acceleration coincides with or is very close to the modulated frequency of the vibrated sensor. While this invention has been described with respect to its presently preferred embodiment, its scope is by no means thereby so limited. Rather, this invention is intended to embody all variations falling with the language of the set of claims that follows and their equivalents.

Claims

1. A multisensor comprising, in combination: a) means responsive to acceleration along a first axis; b) means responsive to acceleration along a second axis; c) means for mounting each of said responsive means so that said first axis is substantially colinear with said second axis; and d) means for vibrating said responsive means out of phase with one another along parallel axes, each of said axes being orthogonal to said first and second axes.

2. A multisensor as defined in Claim 1, wherein at least one of said means responsive to acceleration is an accelerometer.

3. A multisensor as defined in Claim 1 or 2, wherein at least one of said means responsive to acceleration is inertial mass.

4. A multisensor as defined in Claim 1, 2 or 3, wherein said means for mounting comprises a parallel beam suspension.

5. A multisensor as defined in any one of the preceding claims, wherein said means for vibrating includes a plurality of piezoelectric elements.

6. A multisensor substantially as hereinbefore described with reference to Figures 1 to 3 of the accompanying drawings.

7. A multisensor as defined in any one of the preceding claims, in combination with circuitry for determining, from signals from the means responsive to accelerator, linear acceleration along said colinear first and second axes and rotation about an axis at right angles to the first and second axes.

8. A multisensor as defined in Claim 7 and comprising a demodulator and means for forming, and supplying to the demodulator, the difference between the outputs of the means responsive to acceleration, such that the demodulator is operable to form a signal representing coriolis acceleration, and hence rotation.

9. A multisensor as defined in Claim 8, wherein there are pickoff means for producing for the demodulator a demodulation reference signal having a frequency determined by the frequency of vibration of the means responsive to acceleration.

10. A multisensor as defined in Claim 9, wherein the pickoff means is coupled to the means for vibrating to provide a self-resonant vibration system for the means responsive to acceleration.

11. A multisensor as defined in Claim 7, 8, 9 or 10, and comprising means for forming a signal representing linear acceleration from an additive combination of the outputs of the means responsive to acceleration.

1 2. A multisensor as defined in Claim 7, wherein said circuitry is substantially as hereinbefore described with reference to Figure 4 of the accompanying drawings.