GB2096326A - Force-balance pendulum accelerometers - Google Patents

Abstract

A force-balance, pendulum accelerometer includes a pendulum (5) in the form of a disc having a pivot axis which intersects the disc, a mounting which may comprise a pair of wires (2a) which suspend the disc at points on its periphery, wherein the centre of gravity of the disc is spaced from the pivot axis, and at least two force-balance, electrostatic plates (3b, 4b) spaced from the surface of the disc for applying an electrostatic balance force to the disc. <IMAGE>

Description

SPECIFICATION Improvements in or relating to accelerometers The present invention relates to accelerometers and in particular relates to force-balance, pendulum accelerometers used, for example, in inertial guidance systems for the aerospace applications.

A known force-balance accelerometer includes a small pendulous mass supported on a pair of flexural pivots. Force balancing is effected by using two permanent magnets operating in a push-pull manner. An accelerometer of this type is described on page 11 of Von Nostrand's Scientific Encyclopedia (5th Edn). A moving coil and permanent magnet arrangement may be used to effect force balancing.

As an alternative to using magnetic force balancing it has been proposed that an electrostatic force balancing arrangement may be used in a suspended pendulum accelerometer.

However, it has hitherto been found difficult to generate sufficiently large electrostatic forces to force balance the pendulum of a practical accelerometer.

It has further been proposed to immerse an electrostatically balanced accelerometer with a liquid such as silicone oil to increase relative permittivity. However a number of disadvantages are associated with oil filled accelerometers. One disadvantage is that silicone oil, for example, has a density which changes by 0.1 % C and its relative permittivity falls with increasing temperature. A further disadvantage is that it is necessary to employ some form of expansion bellows with an oil filled accelerometer.

The present invention provides an electrostatically force balanced accelerometer which has reduced drive voltage requirements.

According to the present invention a forcebalancing, pendulum accelerometer includes a pendulum comprising a thin sheet having a pivot axis which intersects the sheet, wherein the centre of gravity of the sheet is spaced from the pivot axis, and at least two force-balance electrostatic plates closely spaced from the surface of the sheet for applying an electrostatic balance force to the sheet.

The pendulum may be a thin flat disc and may have a relatively massive rim portion remote from the pivot axis. The pivot axis may be diametral and the disc may be suspended or hinged, may have an aperture, preferably adjacent to the rim thereof to provide an off-diameter centre of gravity.

The pendulum may be suspended by a torsion wire or wires or may be suspended by a pair of wires attached to the sides of the pendulum and extending in a plane generally normal to the pendulum when in the null position. Preferably, however, the pendulum is hinged about the pivot axis.

The force balance electrostatic plates may be pulse proportionally driven by a servo having feedback input from electrostatic position sensor plates.

Embodiments of the invention will now be described by way of example only with reference to the drawings of which: Figure 1 is a schematic side view of an accelerometer in accordance with the invention.

Figure 2 is a plan view of a pendulum used in the accelerometer shown in Figure 1.

Figure 3 is a front view of the accelerometer shown in Figure 1.

Figure 4 is a plan view of an alternative form of pendulum.

Figure 5 shows an idealised voltage waveform of a pulse produced by a digital control system of the accelerometer.

Figure 6 is a graph showing errors due to nonlinearity characteristics of the pulse of Figure 5.

Figure 7 which is schematic shows a pendulum position sensing circuit of the accelerometer of Figure 1.

Figures 8 and 9 which are also schematic show alternative forms of pendulum position sensing circuits.

Figure 10 which is schematic-shows a servo circuit forming part of the accelerometer of Figure 1.

Figure 11 is a plan view of a further form of pendulum embodying hinged mounting.

Figure 12 is a sectional view of X-X of the pendulum of Figure 11.

Figure 13 is a sectional view of Y-Y of part of the pendulum of Figure 11, drawn to a larger scale.

Figure 1 shows an accelerometer in accordance with the invention, which includes a cylindrical, plastics body 1 which houses a disc shaped pendulum 5, a pair of force balance electrostatic plates 3a, 3b, and a pair of electrostatic position sensor plates 4a, 4b. The accelerometer further includes a servo unit 12, the circuit of which is shown in Figure 9, which has electrical outputs connected to the force balance plates 3a, 3b, and has inputs (not shown) from each of the position sensor plates 4a, 4b.

The pendulum 5, which is shown in detail in Figure 2, is suspended by a pair of fine, taut wires 2a, 2b which pass through apertures 11 a and 1 b in the pendulum (see Figure 2). Figure 3 shows the disposition of the wires 2a, 2b which are mounted at their ends on the inner wall of the body 1.

Figure 2 shows the pendulum 5 which is composed of aluminium and is generally disc shaped, having a diameter of 22 mm and a mass of 0.5 gm. The pendulum has an arcuate portion 8 which provides out-of-balance mass so that the centre of gravity of the pendulum is spaced from its suspension axis, s-s, between suspension points 1 a, 11 b. The pendulum was formed from a flat sided metal disc by etching quadrant areas 6 to half thickness to leave a raised circumferential portion 7 and radial webs 9 and 10 to provide mechanical stiffening.

Figure 4 shows an alternative pendulum configuration wherein the required spacing of centre of gravity from suspension axis s-s, is obtained by using an apertured disc 20 in which an elongate aperture 24 is provided. The pendulum has a raised circumferential portion 21 and radial webs 22 having generally the same configuration as in the pendulum of Figure 3.

A study of analogue proportional control for electrostatic force balance of pendulum showed that small angular displacements of the pendulum lead to excessively high force unbalance values.

The servo of Figure 1 and Figure 9 employs a pulse proportional system by applying a fixed voltage for a time period which is force related.

The period is proportional to, and gives a measure of, the sensed acceleration. The shape of a typical pulse waveform is shown in Figure 5. Since the rise and fall slopes of an actual pulse P, differ from an idealised pulse V as is shown in Figures 5 and 6 a non-linearity introduced into the force balancing of the pendulum. A further non-linearity arises from asymmetry due to differences between gap distances of positive and negative plates 3a, 3b. Adjustment of pick-off null is provided to minimise the latter non-linearity.For a current drive of 20,us, and a plate capacitance of 1 5 pf the rise time t is given by cV 15x10-'2x400 t== =0.3 y sec 20x10-3 Assuming equal rise and fall times and a 10 y sec pulse the nett velocity loss (from the ideal acceleration time integral) is 1% since the force is proportional to the square of the linearly ramped voltage. This is not a serious error provided that the vibration levels do not make frequent excursions into '10 g plus' regions causing rectification effects which manifest themselves as bias.

Figure 7 shows a position sensing circuit which senses the angular displacement of the pendulum and has an output which is fed to the servo 12.

The pendulum 5 is earthed via its suspension wires 2a, 2b. The circuit includes earthed pick-off and drive guard plates 30, a differential transformer 31 operating at 100 KHz having its output connected to drive plates 32, 33, which correspond to the plates 4a, 4b, in Figure 1. A pair of live screen plates 34, 35 are closely spaced from plates 32, 33, respectively.

An alternative circuit is shown in Figure 8, and includes earthed frame plates 40 and a common mode voltage transformer.

A further alternative circuit is shown in Figure 9 is arranged so that there are active voltages on both pendulum and frame, and employs an earthed transformer.

Figure 10 shows the circuit of the servo 12 of Figure 1.

An out-of-balance current sensor coil 60 feeds a preamplifier having an a.c. output which is phase sensitively detected in a demodulator 61.

The output from demodulator is fed into a clocked switch unit 64 via comparators 62 and 63 to provide pulsed output to the force plates 3a, 3b.

Figure 11 shows a further form of pendulum 70 which is hinged mounted. The pendulum is formed from aluminium alloy sheet which is selectively etched to provide a sheet having stiffener ribs 72, 73, 74, 75 which extend outwardly from a central rib 88 which extends along a hinged axis P, and transverse rib 76 which extends in a direction normal to the axis P, and having an outer rib 71 which is generally parallel to, and spaced from, the central rib 88. The ribs 71 to 76 and 88 are 0.5 mm thick and the areas of the sheet which extend between the sheets are 0.12 mm thick. Figure 12 shows the pendulum 70 in cross section and shows the extent of the transverse rib 76 and the outer rib 71.

The pendulum 70 is mounted by means of their hinges 82, 83 which extend from fixed mounting plates 80 and 81 respectively. The hinge 82 is shown in cross section and to a larger scale in Figure 13. The hinge comprises a mounting plate 80 which has a bore 84 which receives a clamping bolt (not shown) for mounting the plate 80 on a fixed platform (not shown). The pendulum 70 is connected to the plate by means of a thin hinge element 86 composed of nickel sulphamate and having a fold film 87 on its upper surface. The hinge is formed by forming the plate 80 integrally with the pendulum 70 so that the plate 80 and the central rib 88 of the pendulum form and continuous member of aluminium alloy having a thickness of 0.5 mm. The layer 87 of gold is formed on the underside of the pendulum at the hinge region as shown in Figure 13 and the layer of nickel sulphamate hinge element 86 formed on the gold layer 87. A channel 89 is formed by etching away aluminium alloy to leave the pendulum 70 supported by the hinge element 86 which extends from the plate 80 and has a thickness of 50 ym. The hinge 83 is formed in exactly the same manner as that for the hinge 82.

The mass of the pendulum, hinges, and mounting plates was 0.5 gms.

Modifications or variations of the embodiments described above will be apparent to persons skilled in the art to which the invention relates. For example, the position of the pendulum 70 could be detected by optical means.

The sheet which forms the pendulous mass may be composed of materials other than aluminium alloy, for example silicon.

Claims (11)

Claims

1. A force-balance, pendulum accelerometer including a pendulum which comprises a thin, generally flat sheet having a pivot axis which intersects the sheet, wherein the centre of gravity of the sheet is spaced from the pivot axis, and at least two force-balance electrostatic plates closely spaced from the surface of the sheet for applying an electrostatic balance force to the sheet.

2. An accelerometer as claimed in claim 1 wherein the sheet includes stiffening ribs which extend from the pivot axis.

3. An accelerometer as claimed in claim 1 or claim 2 wherein the sheet includes a rib extending generally parallel to and spaced from the pivot axis to provide an off-axis centre of gravity.

4. An accelerometer as claimed in any of the previous claims wherein the sheet is a disc and wherein the disc is hinged or suspended about the pivot axis at points at or near the disc periphery.

5. An accelerometer as claimed in any previous claim wherein the sheet is suspended by a pair of wires attached to the sides of the sheet and extending in a plane generally normal to the plane of the sheet when in a null position.

6. An accelerometer as claimed in any previous claim wherein the sheet is mounted on a pair of low stiffness hinges on the pivot axis.

7. An accelerometer as claimed in claim 6 wherein the hinges are composed of nickel sulphamate.

8. An accelerometer as claimed in any previous claim wherein the force-balance electrostatic plates are driven by a servo having feedback input from a sheet position measurement circuit.

9. An accelerometer as claimed in claim 8 wherein the servo provides a pulse-proportional drive to the electrostatic plates.

10. An accelerometer as claimed in claim 8 wherein the sheet position measurement circuit has an output which varies with the value of the capacitance between the sheet and a sensor plate spaced from the sheet.

11. An accelerometer substantially as claimed herein with reference to the drawings.