GB2222684A

GB2222684A - Acceleration sensor

Info

Publication number: GB2222684A
Application number: GB8918889A
Authority: GB
Inventors: Herman Winner
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1988-08-20
Filing date: 1989-08-18
Publication date: 1990-03-14
Anticipated expiration: 2009-08-18
Also published as: JPH02112766A; FR2635588A1; GB8918889D0; DE3828307A1; GB2222684B

Description

- 1 Acceleration sensor 2222684

Prior art

The invention is based on an acceleration sensor of the generic type of the main claim. In the case of a known acceleration sensor, on a leaf spring there is a seismic mass, which is deflected to a greater or lesser extent depending on the magnitude and direction of the acceleration occurring. With the aid of a Hall element, fastened to the seismic mass, a measuring signal proportional to the acceleration is generated. The movement of the seismic mass is damped by an eddy-current brake arranged at the free end of the bending spring. The acceleration sensor has, however, the disadvantage that only a limited damping is possible. The damping cannot be adapted to the individual conditions, in particular in the case of relatively high resonant frequencies.

Advantages of the invention The acceleration sensor according to the invention with the characterizing features of Claim 1 has, in comparison, the advantage that very large and very different damping values can be realized. The acceleration sensor can be adapted easily to the individual conditions, so that it always measures in the relatively optimum resonant frequency range. In this case, an increase in the bandwidth of the sensor can be tealized without loss of sensitivity for the lower frequencies. By using a current source as driver of the plunger coil, a compensation of the change in resistance of the plunger coil, and consequently a relatively good temperature stability of the damping, is possible. This temperature stability of the damping can also be increased with the aid of temperature-dependent resistors. The frequency response and the phase response can be smoothed in a is simple way. The space requirement of the acceleration sensor itself is low.

Advantageous further developments of the acceleration sensor specified in Claim 1 are possible by the measures listed in the subclaims.

Drawing Exemplary embodiments of the invention are represented in the drawing and described in more detail in the following description. Figures 1 to ' 3 show longitudinal sections of the mechanical part of acceleration sensors and Figure 4 shows a circuit arrangement for the active damping of acceleration sensors.

Description of the exemplary embodiments

In Figure 1, 10 denotes an acceleration sensor, the bending spring 11 of which is fastened at one end to bar 12. At the free end lla of the bending spring 11, metal block is arranged as seismic mass 13. Furthermore, on the bending spring 11 there are, in its bending region, four strain-sensitive sensor resistors 14 connected in a Wheatstone circuit, as represented in more detail in Figure 4. In this case, two sensor resistors are arranged parallel and two sensor resistors are arranged perpendicular to the axis of the bending spring 11. On the underside of the bending spring 11, there is fastened in the region of the seismic mass 13 an annular, crosssectionally U-shaped holder 15, around the legs 16 of which a coil 17 is wound. The holder 15 is designed in such a way that its legs 16 engage between the legs of a cross -sectionally W-shaped cup magnet 18 comprising a permanent magnet. The two outer legs 19 of the cup magnet 18 have the south pole, while the middle leg 20 represents the north pole. Of course, a correspondingly reversed poling, north pole on the outside, south pole on the inside is also possible. Furthermore, the cup magnet may also consist of soft iron with permanent magnets.

In Figure 4, a closed electric circuit is represented, into which the sensor resistors 14 and the 1 1 w - 3 plunger coil 17 are connected. The centre taps 22, 23 of the Wheatstone bridge circuit of the resistors 14 are connected to a bridge amplifier 24, from which the measuring signal is fed to an active filter 25 with highpass characteristic. Between the bridge amplifier 24 and the high- pass filter 25, the measuring signal is fed via an electric parallel branch 26 to an adder 27. The highpass filter 25 is connected via an electric line 28 to a voltage-controlled current source 29 of the plunger coil 17. Furthermore, from a junction 30 after the high-pass filter 25, an electric line 31 leads to the adder 27. In the adder 27, a simple, weighted addition of the signal of the bridge amplifier 24 and of the signal of the highpass filter 25 is performed, so that, up to the mechanical resonant frequency f. of the bending spring/mass system, a smooth frequency response ( 1 dB) with little phase rotation (< 20 degrees) is possible. From the adder 27, the measuring signal is fed to a triggering unit (not shown), which controls the passenger protection devices of the motor vehicle, such as for example belt tensioner, air bag, rollover bar, hazard warning system, central locking. The anti-lock system for the brakes and the chassis may also be controlled.

The high-pass filter 25 has a predetermined cutoff frequency f.. In the range of low frequencies of the excitation of the bending spring 11, i.e. f < f., the high-pass filter 25 acts as differentiator of the signal of the bridge amplifier 24, proportional to the deflection of the seismic mass 13. As a result, due to the negative feedback via the voltagecontrolled current source 29 and the plunger coil 17, a force proportional to the velocity of the seismic mass 13 is brought about, which corresponds to a viscous damping.

Above the high-pass cutoff frequency f., i.e. in the region of the frequency f > f., on the other hand, a force proportional to the deflection of the seismic mass 13 is exerted by the plunger coil 17 on the seismic mass 13. As a result, the effective rigidity of the bending spring 11 is indirectly increased, which leads to a higher resonant frequency of the mass 13/spring 11 system. For an optimal configuration of the active damping of the acceleration sensor 10, the cutoff frequency fjj of the active high-pass filter 25 and the feedback factor, which is brought about by the properties and the quality of the electric lines, of the current source 29 and of the plunger coil 17, must be matched.to each other.

In Figure 2, an acceleration sensor 10a is represented which has, unlike in Figure 1,, a double bending spring 35 formed in the shape of an S. This bending spring 35 consists of two parallel-arranged bending springs 35a and 35b, which are fastened with the aid of the block 36 and between which the seismic mass 13 is arranged at the free end. The sensor resistors 14 are fitted on one of the two bending springs 35a, 35b. Due to the S-shaped design of the bending spring 35, the seismic mass 13 executes an essentially perpendicular movement during the deflection. As a result, the plunger coil is moved virtually perpendicularly to the magnet f ield generated by the cup magnet 18, as a result of which a greater and more accurate signal is possible. Unlike this, the seismic mass 13 in Figure 1 is rotated slightly, which means that the mass 13 is moved on a path running around the suspension point of the bending spring 11 as centre point.

In Figure 3, an alternative design of the acceleration sensor 10 according to Figure 1 is represented. The seismic mass consists of a permanent magnet 38, which is arranged on the underside of the bending spring 11. The plunger coil 17 is fastened on a stationary holder 15a. It is, furthermore, also possible to use the S-shaped double bending spring from Figure 2 in the case of the design according to Figure 3.

The measuring signal fed to the high-pass filter can also be picked off capacitively, no bridge amplifier being necessary.

:1 -

Claims

1. Acceleration sensor (10) with a spring (11)/mass (13) system for a vehicle, in particular motor vehicle, for the triggering of safety equipment or for control, with a damping device for the spring/mass system, characterized in that the damping device is an active damping, which consists of an active electronic circuit, having a high-pass filter (25) acting as differentiator, and of an electrodynamic drive (17, 18).

2. Sensor according to Claim 1, characterized in that the measuring signal of the spring (11)/mass (13) system and the outward signal of the high-pass filter (25) is fed to a passive or active network (27) for addition, and the signal produced is evaluated in an evaluation circuit.

3. Sensor according to Claim 1 and/or 2, characterized in that a voltagecontrolled current source (29) is connected in the circuit between the high-pass filter (25) and the drive (17, 18).

Sensor according to one of Claims 1 to 3, characterized in that the measuring signal is picked off with the aid of strain-sensitive resistors (14) arranged on the bending spring (11) and is fed to a bridge amplifier (24).

5. Sensor according to one of Claims 1 to 4, characterized in that the drive is a cup magnet (19) with a plunger coil (17).

6. Sensor according to one of Claims I to 4, characterized in that the drive consists of a permanent magnet (38), which at least partially represents the seismic mass, and a stationary coil (17).

7. Sensor according to one of Claims 1 to 6, characterized in that the spring is an S-shaped designed bending spring (35) with two parallel arranged spring - 5 bra.nc.-les (35a, 35b).

8. Any of the acceleration sensors substantially as herein described with reference to the accompanying dtawings.

Pub1tahed 1990 at The Patent OMCO.StLte HOUMA,15;7 t HEIgh Holburn. London WC1R 4TP. PUrthercoples may be obtainedtromThe PatentOMes. Sales Branch. St Mary Cray. Orpington. Ksn,. 13RS 3RD. Printed by MWtiplex techniques Itd. St Mary Cray. Kent. Con. 187