DE8801694U1 - Device for measuring acceleration - Google Patents

Description

DIPL-INQ. PETER OlfT^PATfNTANWALT o-raso Leonberg Representative at the European Paper Association, H &jfeo4ifl ^atefy Afcrnfey Tiroler Straße 1S

2119/ot/mü 29.01.1968

Harr Edmund Zottnik, Rotenacker 11. 7145 Markgröningen Mr. K.-F. KolleV/ Jakobstraße 18/ 7016 Gerllnqen

Device for acceleration measurement State of the art

The invention is based on a device for measuring acceleration according to the preamble of claim 1.

In a known acceleration sensor of this type (DE-OS 33 06 813), which is particularly suitable for use in a short-term accident data recorder, at least one bending beam is provided which is fastened to a clamping body on one side. The other end of the bending beam forms a capacitance sensor with a stationary counter plate. This capacitor, which consists of a plate (as a seismic mass) which moves due to inertial forces when accelerations are applied, and a fixed plate, is part of an oscillating circuit which changes its frequency upwards or downwards in a way that can be evaluated depending on the positive or negative acceleration applied.

Other well-known acceleration sensors are so-called shock or vibration meters, ballistic sensors or miniature sensors; a quartz acceleration sensor designed as a so-called "low impedance" contains a

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Crystal on which a pressure corresponding to the acceleration can be exerted, resulting in a direct voltage change in a supply voltage applied to the quartz, or without a supply voltage a charge shift in the quartz (piezoelectric effect; piezoresistive effect) ·

It is also known to use rotor systems that work with the so-called Wiegand effect for acceleration encoders, particularly under harsh operating conditions, whereby Wiegand wires embedded in the rotor are scanned contactlessly by a sensor head. This also makes it possible to record accelerations.

In principle, accelerations can only be measured indirectly through their effects on a seismic mass, for example, whereby the force exerted by a known mass subjected to acceleration on a solid base can be measured either directly (piezo element) or indirectly (spring travel). The problem with acceleration sensors used in motor vehicles, such as accident data recorders, but also in safety equipment in the motor vehicle sector is the fact that they have to meet very high requirements while maintaining the relatively high accuracy required, but on the other hand they have to be particularly cost-effective for extensive use, have to have calibration accuracy over a certain period of time, for example at least two years, have to have a bus-capable output signal and, as mentioned, have to meet the vehicle application specifications while being highly sensitive and accurate.

The invention is therefore based on the object of specifying a method and a device for acceleration measurement, which ensures that a high level of functional reliability is combined with high measurement accuracy at a low cost and that problem-free evaluation

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tion of the won« . Signals can be made. Advantages of the invention

The invention solves this problem with the characterizing features of claim 1 or subclaim 4 and has the decisive advantage that the novel concept results in surprising cost-effectiveness combined with particularly high precision and a wide range of possible applications.

In the preferred embodiment, an acceleration sensor according to the invention requires only a guide tube, three magnets, a sensor that responds to changes in magnetic field lines, usually a field plate and a housing that accommodates these parts. Practical tests and calculations have shown that, depending on the dimensions and design of the magnets, acceleration values of up to 30 g and far beyond can be reliably recorded and discriminated against without any problems. Signs of aging and a correspondingly strong temperature change

&Idigr; turgang-related problems practically do not occur; Likewise, lateral acceleration influences, friction^-

# influences, breakaway torque/starting conditions extremely low

&iacgr; and remain negligible. As far as temperature problems occur,

occur, the temperature gradient is absolutely reversible and

is reproducible and taken into account in the characteristic curve field

■ can.

The preferred embodiment does not use springs, which may be subject to aging or corrosion, and uses air damping, which is always available, where necessary.

Another advantage is that the magnets used for storage, drive and provision of the measurement signal build up a "spring force" which increases with increasing deflection

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increases progressively, i.e. shows a behavior that allows the use of such an acceleration sensor in large measuring ranges. The limit of the measurable acceleration is determined by the mass of one of the magnets acting as a piston and by the available magnetic force. By choosing suitable geometric dimensions and materials, the area of application can be set and specified over a wide range.

Since in the end position, i.e. at maximum acceleration, there is a large-area contact between the middle piston magnet and one of the end magnets, the inventive concept also enables extremely high overloads of the acceleration sensor without causing irreversible damage or even interference.

The measures listed in the subclaims enable advantageous developments and improvements to the invention. The conversion of the resistance changes in the field plate resulting from a change in acceleration into a bus-compatible output signal is particularly advantageous when the field plate resistance is installed as a frequency-determining element in an oscillating circuit of sufficient quality. If, in such an embodiment, the periods of the respective measuring frequency are summed up in an integration interval determined by the system, then a counter reading is obtained in each interval which already represents the current measured value.

The measured value (zero value) obtained in the resting state when the acceleration is zero is stored by the system as the current calibration value and used as the basis for the subsequent signal value generation. The calibration value therefore also contains f o r e s t i o n o r e s t i o n o f aging, temperature

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drift etc. accumulated errors, so that the measurement can be carried out at any time practically error-free.

All measured values are related to the frequency of the zero value by simple subtraction, so that the number of digits is simultaneously reduced and the respective sign of the acceleration acting is obtained.

The difference formation also advantageously ensures that the signal obtained is reduced to the actual signal size, which can then be used for further processing.

drawing

An embodiment of the invention is shown in the drawing and is explained in more detail in the following description. They show:

Fig. 1 is a section along the line I-I of the preferred embodiment of an acceleration measuring device according to the invention shown in Fig. 2;

Fig. 2 shows the cross section along the line H-II of Fig. 1; in

Fig. 3 is a schematic representation of the resulting change in the magnetic lines at two different positions of the movable piston magnet, including their effects on the transducer element (field plate) and

Fig. 4 shows a possible embodiment of an oscillating circuit by which the changing resistance of the sensor in Fig. 3 can be converted into a correspondingly changing measuring frequency.

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Description of the embodiments

The basic idea of the present invention is to use a piston magnet movably mounted within a guide tube as the seismic mass that is subjected to the acceleration effect and to assign to the system a sensor that reacts sensitively to a variation in magnetic field lines.

A particularly advantageous embodiment of this basic idea then consists in magnetically pre-tensioning the piston magnet in its central position by means of magnetic force suspension within two additional stationary magnets arranged on either side of it, i.e. by the effect of the field lines emanating from all three magnets, and at the same time thereby realizing "magnetic spring elements" which in turn fulfill two tasks simultaneously, namely on the one hand the "magnetic spring travel formation" and on the other hand, through their field line courses changing as a result of a movement of the piston magnet, simultaneously influencing the sensor analogously to the path covered by the piston magnet (deflection) and thus to the acceleration acting.

In Fig. 1, the acceleration sensor is designated 10; it comprises an inner guide tube 11, for example made of brass or another suitable non-magnetic material. In the advantageous embodiment of the present invention, the guide tube 11 has a circular cross-section and a generally cylindrical shape and serves as a bearing sleeve for a piston magnet 12 that is displaceably mounted in both directions according to arrows A and B inside the guide tube. In order to ensure easy movement of the piston magnet 12 without major frictional influences, but

01/29/1988 - To ensure that the diameter dimensions (outer diameter of the piston magnet 12 in relation to the guide tube inner diameter) are not too large, the guide tube (or piston) can be coated with a suitable, particularly slippery material such as polytetrafluoroethylene (PTFE) or made entirely of a suitably slippery material with a low coefficient of friction. The guide tube can be made of brass, for example, but can also be made of non-magnetic, and in some cases even magnetically conductive, material. In this case too, no adhesion forces occur between the guide tube and the piston magnet due to the magnetic circuit. A preferred non-magnetic material is high-alloy (austenitic) V2A steel, which is easy to polish and provides good protection against external magnetic influences.

In any case, the piston magnet ^1.2 is freely movable in both directions inside the guide tube 11, whereby it is supported "in a resilient manner" by the effect of the magnetic field lines emanating from it and from two stationary magnets arranged at the ends of the guide tube. The two outer end magnets closing the guide tube are designated 13a and 13b; the polarity of the magnets is, of course, selected such that on the mutually facing surfaces of each stationary band magnet 13a, 13b and the piston magnet 12 there are poles of the same name, i.e. which repel each other; the polarization can therefore be designed as shown in Fig. 1 (N - north pole; S - south pole).

The further construction is completed by cap-shaped covers 14a, 14b on both sides, which hold the end magnets 13a, 13b flush so that they sit in them, for example glued in. The whole then forms a closed whole with a central elongated housing part 15, for example square in cross-section, whereby the guide tube 11 sits in a hole 1Sa in the central housing part 15 and the covers 14a, 14b on both sides, which can also be rectangular or square in cross-section, also engage on both sides by means of offset inner ring collars 16a, 16b in the hole 1Sa of the middle housing part 15 and can thus also secure the guide tube 11 in its position, for example.

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The covers 14a, 14b on both sides and the middle housing part 13 together form the housing of the acceleration sensor and at the same time both the magnetically closed circuit (back connection for the components arranged inside the housing) and an effective shield against external magnetic influences to which an electrical sensor element responds. This sensor element is designed in Fig. 1 as a field plate 17 placed on the edge of one of the end magnets 13b and, for example, glued or otherwise attached. Under certain circumstances, the guide tube can also be omitted entirely and its function is taken over by the housing, namely the middle housing part 15.

The basic function of the present invention is as follows. If the acceleration sensor shown in Fig. 1 is subjected to an acceleration from the direction of the arrow C or C, i.e. axially, whereby the structure of the acceleration sensor means that it only reacts and can react to accelerations or acceleration components running in its axial direction, then the piston magnet as a measuring piston is deflected against the "spring force" of the mutually repelling magnetic fields on both sides, i.e. it is displaced in the plane of the drawing in Fig. 1 either to the left or to the right depending on the acceleration acting, which simultaneously results in a deformation of the magnetic field line course, which also takes place in a special way on the surface of the stationary end magnets 13a, 13b opposite the piston magnet.

The two magnetic fields under consideration here repel each other, so when the piston magnet approaches one of the stationary end magnets 13a, 13b, the curvature of the field lines increases, particularly on the surface of the respective end magnet, and thus also the field density in the measuring plane selected here, which corresponds to the surface of the end magnet.

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magnet and on which the sensor is located in its design as a field plate. These changes can be recorded and evaluated as resistance changes with the field plate 17.

In this context, the following should be noted: in the preferred embodiment, the invention uses only one field plate 17 at the preferred location in the illustration of the figure, i.e. at the edge of one of the end magnets, since this is where a particularly strong change in the field lines occurs, which will be discussed further below using the illustration of the figure. However, it is understood and within the scope of the invention to also use several sensors, for example in a uniform arrangement distributed peripherally around the edge of one of the end plates, or to provide sensors on the other end plate in the same number and distribution as on the one and then to evaluate the change in resistance resulting from the change in the field lines in any connection. The change in resistance naturally also occurs where the two magnets (end magnet and piston magnet) move away from each other - in this case the density of the field lines is lower. It is therefore also possible to connect the field plates against each other in their resistance configuration or to design them in the form of bridge circuits.

However, empirical studies have shown that it is sufficient to use a field plate sensor in the arrangement shown in Fig. 1 and Fig. 2, whereby an acceleration change of ± 10 m/s ² results in a resistance change of 5 % or 3 % depending on the direction.

These changes are completely sufficient, with the stronger change occurring when the piston magnet approaches the

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the band magnet provided with the field plate, since the compression of the magnetic field lines results in a greater change in density than the "thinning" of the field lines when the respective magnets are relatively far away from each other.

This Uariat-inn &Lgr;&agr;*· PeIrIl &iacgr;&eegr;&iacgr; en heH a Pnfli rinnnänrleriinn

or deflection of the piston magnet can be imagined as shown in Fig. 3 using two discrete positions of the piston magnet 12, once in position 12A (solid line) and once in position 12B (dashed line).

If the two magnets are further apart, the repelling field lines run approximately as shown at 18; if the piston magnet approaches the opposite end magnet with the field plate 17 up to position 12B, the field lines, which are constant in number, press together much more strongly and therefore penetrate the field plate 17 in a greater concentration and with a greater curvature, i.e. over a longer path, as indicated by the dashed line at 18'. This results in the resistance changes, which then have to be converted into an evaluable output signal.

The following table shows a "spring force curve" measured on an example, i.e. the allocation of the distance between the middle, movable piston magnet 12 and one of the end magnets, the forces in grams acting to create this distance and the acceleration calculated from the respective force.

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Distance (mm) Force (g) 30 0 20 4 12 10 10 14 8th 20 6 30 5 40 4 60 1 180

Acceleration r. (m/s*)

6.6

Θ 12 10 16,6

f> 10 14 23,3

33.3 49.9 66.5 99.8 299.3

I Since a practical example shows the following

ti numerical values regarding dimensions or weight

I, which are only given here for the sake of completeness and which do not restrict the invention to this, the application range can also be roughly outlined on the basis of |i these values.

f The piston magnet has a diameter of 10 mm and a length of 15 mm and weighs 5.9 g;

?; if you compare these 5.9 g with the 180 g in the table, then you get an application range of about 30 times the acceleration of gravity, which corresponds to an acceleration of about 300 m/s ²

_; corresponds.

£ Different dimensions will, of course, produce different values.

In order to convert the resistance changes occurring in the field plate during acceleration into a bus-compatible output signal, the field plate resistance is best installed as a frequency-determining element in an oscillating circuit of sufficient quality.

It is proposed as shown in Fig.

a · &igr; «j · > • «ti

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a feedback oscillating circuit 20, consisting of an inverter 21 with hysteresis, the field plate 17' fed back from the output of the inverter 21 to its input and a capacitor 18 connected against Hasse. Such an oscillating circuit with a Schmitt trigger characteristic and with a downstream second buffer element 22 for decoupling is preferably constructed as a temperature-compensated and therefore very stable CHOS module and generates a high frequency square wave output voltage, the basic function being easy to understand that the capacitor 18 is charged via the field plate 17* until the response threshold of the "Schmitt trigger" is reached; then the switchover takes place, the capacitor is discharged until the lower trigger threshold is reached again.

&Lgr;.?. output side puff build» tein 22 then results in a

Zero or center frequency (at zero displacement of the piston,

magnets) of, for example, 50 MHz, i.e. very high frequency, j

with the excellent integration capabilities;

Such a circuit also offers a particularly high resolution \

can be realized if one considers the extreme values of the influencing j

acceleration with frequency changes of about 3 % · on one side and 5 % on the other.

The characteristic is non-linear, the resulting "spring force" acts progressively up to the stop, so that even maximum deflections can be perfectly recorded and converted into a corresponding measuring signal. There are also hardly any errors in the area of the oscillating circuit and its control, since the effect of aging and temperature changes on the field plate is negligible, the same applies to the magnetic field forces and friction. The only factor that plays a role in the temperature resistance of the magnets used is the very high Curie temperature, which is therefore never attainable under normal circumstances;

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The field plate can be used at least between - 30° to about + 100°.

The further processing of the high-frequency output signal of the oscillating circuit according to Fig. 4 can then be carried out in such a way that the frequency of the oscillating circuit is counted, whereby the pulse changes occurring within a counting interval are added, so that a clear signal integration results; the counter readings are recorded every 100 ms, for example, and processed by forming the difference, whereby a current calibration value stored in the system is used as the zero value.

This zero value is obtained in the resting state when acceleration b = 0 is applied and is used as the basis for all subsequent value calculations. The calibration value also contains all errors that have accumulated during the operating time due to aging etc. or compensates for them, so that from this point of view, too, a very high level of accuracy of the measured values determined is ensured.

All measured values are related to the zero frequency of the calibration value by simple subtraction, which drastically reduces the number of digits of the measured values that are used; this reduction gives the actual signal size. At the same time, the valid sign of the acceleration is obtained by subtraction and, by constantly re-adjusting the current calibration value of the zero value measurement (which is carried out repeatedly during operation), a calibration in the sense of automatic calibration is achieved. The field plate resistance as well as the coercive field strength of the magnets are temperature-dependent, so that it can be useful to record the characteristic fields of the sensors. The current temperature of the systems is recorded with the

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records or evaluates when the measured value is generated. To do this, an identical oscillating circuit, in which a PTC or NTC as a frequency-determining element supplies a signal that the UDS (accident data recorder) stores (after each recording). This means that the element temperature response at the time of recording is known and the position of the characteristic curve in the characteristic field is given. Knowledge of the system temperature is also desirable because of the other components of the UDS.

Such an acceleration sensor can be used with particular advantage in the field of motor vehicles due to its resistance to aging and the high accuracy of the values it provides, for example for signal transmission for universal data recorders, tachographs or other recording devices, whereby the acceleration sensor is evaluated in its signal output when existing sensors can no longer be evaluated due to an accident. The acceleration sensor according to the invention can also be used for any safety devices in the field of motor vehicles, for example for determining the use of anti-skid control, for controlling the behavior of anti-lock braking systems (ABS), for controlling belt tensioners, airbags, etc.

Investigations have shown that no noticeable errors occur due to the frictional influences of the piston magnet on the inner guide walls of the guide tube, since the friction as sliding friction is essentially constant and included in the characteristic curve, while the response threshold for the static friction lies in an acceleration range of less than 1 m/s ¹ and is therefore negligible. With such low accelerations, the value provided by, for example, conventional displacement sensors can be used anyway,

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Since there is no slippage here that would require the use of

Acceleration sensors according to the present invention are required.

All features presented in the description, the following claims and the drawing can be essential to the invention both individually and in any combination with one another.

Claims (9)

DIPL-ING. PETER OTTE P/EENTANV!^LT ^: .. ^:; .. ^: D-7250Leonberg Representative at the European Patent Office / European Patent Attorney Tiroler Straße 2119/ot/mü 29.01.1988 Mr. Edmund Zottnik, Rotenacker 11, 7145 Markgröningen Mr. K.-F. Kolley, Jakobstraße 18, 7016 Gerungen S ection claims 1. Device for measuring acceleration, in particular for recording acceleration values, for controlling/regulating motor vehicle functions including accident data recorders, anti-skid control (ASR), control of anti-lock braking systems (ABS), for controlling seat belt sensing devices, airbag functions and the like, with a seismic mass element subject to the effect of acceleration and movably guided in the direction of the acting acceleration, characterized in that on both sides of a slidingly mounted piston magnet as a seismic mass element, end magnets (13a, 13b) are arranged which hold the piston magnet freely movable in the manner of a spring-preloaded state by the repulsive effect of magnetic poles of the same name, and that in the area of magnetic field lines which change in their density when the piston magnet (12) is deflected by an acceleration effect, at least one magnetic field-sensitive sensor is arranged. 2t Device according to claim 1/ characterized in that the sensor is a field plate resistor (17). 3. Device according to claim 1, characterized in /2 Ü 2119/ot/mü 29.01.1988 - 2 - the sensor is a Hall generator. 4. Device according to claim 1, characterized in that the piston magnet (12) is slidably mounted in a non-inductive guide tube (11), that ^one end of the guide tube has end magnets (13a, 13b) with magnetic poles of the same name as the side of the piston magnet (12) facing it and therefore producing a repulsive effect, and that the at least one sensor is arranged on the edge of the surface of one of the end magnets (13a, 13b) facing the piston magnet. 5. Device according to claim 4, characterized in that the end magnets (13a, 13b) are accommodated in cap-shaped housing covers (14a, 14b) which close off on both sides a central housing part (15) which contains the guide tube (11) for the piston magnet (12) in a bore (15a). 6. Device according to claim 4 or 5, characterized in that the cap-shaped covers on both sides (14a, 14b) and the middle housing part (15) consist of a magnetically permeable material and at the same time form a magnetically closed return circuit and a shield against external interference magnetic fields. 7. Device according to claim 5 or 6, characterized in that the inner wall of the guide tube (11) is coated with a sliding layer (PTFE) or consists of such a layer. 8. Device according to claim 1 or 2, characterized in that the at least one field plate resistor ■ »· · 1 r ' ·■ «&ngr; ·· · · ftf' .If* •!«Si ■ > ·· •!•••t&igr;&igr; . &igr; · · 2119/ot/mu 29,01.1988 &igr; i f I t (17) controls an oscillating circuit (20) as a frequency-determining element. 9. Device according to claim 8, characterized in that an inverter/amplifier (21) with hysteresis is provided and the field plate resistor (17) is arranged in its feedback branch, with a capacitor (18) connected to ground and a further amplifier element (22) connected downstream for buffering.