GB2276241A

GB2276241A - Rate of rotation sensor

Info

Publication number: GB2276241A
Application number: GB9410061A
Authority: GB
Inventors: Erich Zabler; Jiri Marek
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1990-10-13
Filing date: 1994-05-17
Publication date: 1994-09-21
Anticipated expiration: 2011-09-19
Also published as: GB2276241A8; GB9410061D0; GB2276241B

Abstract

The sensor comprises an element of layered construction which has a vibrator 30 formed as one layer and located on a lamellar base 10 formed as another layer. Means are provided for exciting the vibrator in a first direction of vibration 1 that is oriented parallel to the main surfaces of the base. The vibrator can be deflected, by Coriolis force, in a direction 2 perpendicularly to the main surfaces of the base. Means are also provided for capacitively or piezoresistively measuring the deflections of the vibrator in the direction perpendicular to the main surfaces of the base. <IMAGE>

Description

2276241 Rate-of- otation Sensor The present invention relates to a

rate-ofrotation sensor.

To measure the speed of rotation of a vehicle essentially around the vertical axis, for example, in order to regulate the vehicle movement dynamics or even for navigational purposes, it is already known to measure small rates of rotation in the region of several degrees per second with sensors in which a tuning-fork structure which is oriented parallel to the axis of rotation is excited to perform vibrations in a plane perpendicular to the axis of rotation. During a rotation about the axis of rotation, the Coriolis force acts on the vibrating tuning-fork prongs perpendicularly to the axis of rotation and perpendicularly to the direction of excitation, i.e. to the deflection of the prongs in the absence of a rotational movement. The rate of rotation can be,measured and evaluated by means of the deflection, due to the Coriolis force, of the prongs perpendicular to the direction of excitation.

Non-prepublished German patent application No. 40,224,953 describes various developments of a rate-ofrotation sensor having a sensor element which is formed by structuring from a single-crystal silicone wafer and has at least one vibrator, preferably a pair of vibrators which are connected to a fixed frame by one or more links. The vibrators are capable of vibratin4 in two mutually perpendicular directions. Various possibilities for exciting the vibrator in a first direction of vibration which is in the plane of the wafer are described, such as, for example, electromagnetic excitation, thermomechanical excitation and various possibilities for electrostatic excitation.

2 This rate-of-rotation sensor is furthermore equipped with means for measuring deflections of the vibrators in the second direction of vibration.

A paper entitled "Laterally Driven Polysilicon Resonant Microstructures" by William C. Tang, Tu-Cuong H. Nguen and Roger T. Howe in Sensors and Actuators, 20 (1989) 25-32 describes various polysilicon structures, deposited on bases, which are capable of vibrating and methods of producing them.

According to the present invention, there is provided a rate-of-rotation sensor comprising a sensor element of layered construction, one layer of which is formed by at least one vibrator, and another layer of which is formed by a lamellar base, means for exciting the vibrator in a first direction of vibration which is oriented parallel to a main surface of the base; at least one structural element, which is capable of vibration and which forms the at least one vibrator, being applied to a main surface of the base, the structural element being deflectable perpendicularly to the main surface of the base, and means for capacitively or piezoresistively measuring the deflections of the structural element perpendicular to the main surface of the base.

A sensor in accordance with the invention has the advantage that the vibrator can be constructed as a link with a high height-to-width ratio depending on the thickness of the base, so that a large deflection Can take place in the first direction of vibration and transverse deflections, which give rise to interfering signals, are at the same time substantially avoided. This special construction of the vibrator advantageously makes possible a rigid, precise and interference-proof guidance in the first direction of vibration. At the 3 same time, the structural element, which can be deflected perpendicularly to the first direction of vibration and which is applied to the vibrator and serves as an acceleration sensor for the Coriolis acceleration perpendicular to the first direction of vibration, is so constructed that it has a high sensitivity. It is particularly advantageous to connect the acceleration-sensitive structural element to the vibrator by means of links which are aligned parallel to the first direction of vibration. As a result of this, transverse deflections of the structural element due to the movement of the vibrator can substantially be avoided. The measuring effect of a sensor constructed according to the invention can be reinforced positively by exciting the vibrator to perform high-frequency vibrations and by as large a deflection as possible of the structural element perpendicular to the first direction of vibration of the vibrator.

An advantageous refinement of the invention is to use a single-crystal silicon wafer having (110)- or (100)-crystal orientation as the base since these can be structured easily with methods known from micromechanics by dry- or wet-chemical etching. Links having walls constructed perpendicularly to the wafer surface are particularly advantageously suitable as vibrators. They can easily be formed by structuring from (110)-oriented silicon wafers by anisotropic electrochemical etching by means of KOH. Designs independent of the crystal orientation can also be achieved by dry-chemical etching (trenching). Particularly advantageous as structural elements capable of vibration are polysilicon structures or structures of single-crystal silicon, as they can be produced in thin-film technology by known methods in a manner such that they have a high sensitivity. The 4 production of the rate-of-rotation sensors in silicon according to the invention is, in addition, particularly advantageous since it permits an integration of the associated evaluation circuit on the sensor element.

In order that the invention may be well understood there will now be described some embodiments thereof, given by way of example, reference being made to the accompanying drawings, in which:

Figure 1 shows the perspective representation of a portion of a sensor element; Figures 2a to d show various designs of a structural element; and Figure 3 shows the perspective representation of a further sensor element.

Referring first to Figure 1, there is shown a portion of a sensor element which has been produced in the form of a double tuning fork by structuring from a lamellar base 10. one side of the tuning-fork structure consists of a vibrator 13 which is connected to the base 10 on two sides by means of four links 14. Both the vibrator 13 and the suspension links 14 are constructed with the full thickness of the base 10. It is also possible to connect the vibrator 13 to the base by means of links 14 on one side only so that an open tuningfork structure is produced.

The vibrator can be excited to perform vibrations in a first direction of vibration, as indicated by the arrow 1, which is in the plane of the base with the aid of means not shown in greater detail in the drawing. This can be done, for example, electrostatically, electrodynamically or even thermoelectrically, as described in the aforesaid nonprepublished German patent application No. 40,224,953.

Applied to the vibrator 13 above a link 23 is a structural element 21 constructed as a tongue. The tongue 21 is oriented parallel to the base surface and can be deflected perpendicularly to the base surface, so that accelerations perpendicular to the base surface can be measured with the tongue 21. If the vibrator 13 vibrates in the first direction of vibration 1 and if the sensor element is subjected to a rotational movement around an axis of rotation 3 which is oriented perpendicularly to the first direction of vibration 1 and perpendicularly to the direction of deflection 2 of the tongue 21, a Coriolis acceleration acts on the tongue 21 perpendicularly to the base surface in the direction 2. This Coriolis acceleration, which results in a deflection of the tongue 21, can either be measured piezoresistively by piezoresistances arranged on the tongue 21, or, as shown in Figure 1, capacitively. For this purpose, a region of the surface, facing the tongue 21, of the vibrator 13 is prepared as one electrode 17 of a plate capacitor which is connected to a terminal 191 by means of a lead 181. The other electrode side of the plate capacitor is formed by the tongue 21 itself, which is connected to a terminal 192 by means of a lead 182. A deflection of the tongue 21 in the direction 2 results in a change in capacitance of this plate capacitor and can be evaluation by means of an evaluation circuit not shown here.

The structure shown in Figure 1 of the sensor element can be produced particularly advantageously in single-crystal silicon bases 10 and polysilicon structures or single-crystal silicon structures deposited thereon. The vibrator 13 and the links 14 can easily be produced by structuring from the silicon base by dry- or wet-chemical etching, which can be done, for example, by electrochemically etching a membrane and 6 then structuring the membrane. (110)-oriented silicon wafers are particularly suitable for producing structures having side walls perpendicular to the base surface since the structures can then be produced by wet-chemical etching by means of KOH. If dry-chemical etching (trenching) is used, such links can also be produced independently of the crystal orientation. The structural elements capable of vibrating and arranged on the vibrators and also their connecting links to the base substrate can advantageously be produced in polysilicon or single-crystal silicon since then structural elements which contribute to a high sensitivity of the rate-of-rotation sensor can be produced by depositing silicon on an auxiliary layer, for example an oxide layer, which serves as so-called "sacrificial layer" and then removed again by underetching the silicon structure. In addition to tongues oriented next to and parallel to the base surface, bridge-like plates arranged on the vibrator and attached at two sides are also suitable as structural elements.

The production of the sensor element in silicon makes possible the integration of parts of the evaluation circuit on the sensor element. It is particularly advantageous in this connection that, for example, a rigid electrode side of the plate capacitor, which is provided on the surface of the vibrator 13, can be produced by simple diffusion into the base substrate. The same applies to the leads 181, 182.

Figures 2a to d show various designs of the structural element 21 with suspension links 23. Depending on the application, a suspension on one side (see Figure 2b), a suspension on two sides (see Figure 2a and d), or even a suspension on four sides (see Figure 2c) can be chosen. If the structural element 21 7 serves only as acceleration detector for the Coriolis acceleration, that is to say it is applied to a vibrator 13 produced by structuring from a base 10, it is particularly advantageous to align the suspension links 23 parallel to the first direction of vibration 1, as indicated by the arrow 1. As a result of this, interfering transverse deflections of the structural element 21 in the first direction of excitation 1 can be substantially avoided. The suspension on four sides, shown in Figure 2c, of the structural element 21 by four radially arranged suspension links 23 is advantageous since all transverse deflections are uniformly prevented as a result. The acceleration sensitivity perpendicular to the surface of the vibrator 13 can also be advantageously increased by a design of the structural element 21 in accordance with Figure 2d. Here the structural element 21 is connected on two sides via links 23 to the vibrator 13. The connection of the acceleration-sensitive plate 21 to the links 23 is, however, not direct, but is formed by two thin bendable bars 231.

Figure 3 shows a portion of a further sensor element having a cut base 10. In this exemplary embodiment, the base 10 is not structured. Here it may also be a single-crystal silicon wafer or another substrate to which structural elements and signal measurement means can be applied in a suitable manner. Applied to the base 10 is a bridge-like structure xhich is formed essentially by a plate-type seismic mass 30 connected on two sides to the base substrate by means of four links 31. This structure can be excited in a first direction of vibration. as indicated by the arrow 1, which can be done electrostatically. for example. The excitation means are not shown here in greater detail.

8 During a rotation movement of the sensor element around an axis of rotation 3 which is situated in the plane of the base and perpendicularly to the first direction of vibration, a Coriolis acceleration acts on the vibrating seismic mass 30 perpendicularly to the base surface. The deflection, resulting therefrom, of the seismic mass 30 perpendicular to the base surface can be measured piezoresistively or capacitively and is a measure of the angular velocity of the rotation.

The sensor element shown in Figure 3 is produced in silicon. 11 denotes a charge carrier diffusion into the base 10 which serves to insulate a part of the surface, situated beneath the plate-type seismic mass 30, of the base electrically from the plate-type seismic mass 30 so that this part of the surface, together with the seismic mass 30, forms a capacitor by means of whose change in capacitance the deflection of the seismic mass 30 in a second direction of vibration perpendicular to the base surface can be measured.

Attention is drawn to our copending patent application No. 9120061.8 (Serial No. 2249174) from which the present application has been divided.

0 9

Claims

A rate-of-rotation sensor comprising a sensor element of layered construction, one layer of which is formed by at least one vibrator, and another layer of which is formed by a lamellar base, means for exciting the vibrator in a first direction of vibration which is oriented parallel to a main surface of the base; at least one structural element, which is capable of vibration and which forms the at least one vibrator, being applied to a main surface of the base, the structural element being deflectable perpendicularly to the main surface of the base, and means for capacitively or piezoresistively measuring the deflections of the structural element perpendicular to the main surface of the base.
2. A rate-of-rotation sensor as claimed in claim 1, wherein the structural element is constructed as a plate-type seismic mass which is connected to the base on one side or two sides in a bridge-like fashion by means of links and which is oriented parallel to the main surface of the base.
3. A rate-of-rotation sensor as claimed in claim 1 or claim 2 wherein the base is a single-crystal silicon base having (110)- or (100)-crystal orientation.
4. A rate-of-rotation sensor as claimed in any of the preceding claims, wherein the structural element is constructed as a polysilicon structure or as a singlecrystal silicon structure.
5. A rate-of-rotation sensor as claimed in claim - 10 3 wherein the structural element can be produced by depositing a polysilicon layer or a single-crystal silicon layer on auxiliary layer bases applied to subregions of the base surface and by subsequently removing the auxiliary layer base by underetching the applied silicon layer.
6. A rate-ofrotation sensor substantially as herein described with reference to the accompanying draVings.