DE2618759C3 - Accelerometer - Google Patents

Description

The invention relates to an accelerometer according to the preamble of the main claim.

An accelerometer according to the preamble of the main claim is already from IBM Technical Disclosure Bulletin, Volume 10, No. 3, August 1967, pages 209, 210, in which the two Oscillating crystals are arranged in a sealed housing and are preloaded by a weight stand. The weight is supported by an element which has points of contact opposite the Has quartz oscillators in the form of point contacts. The quartz crystals are at this accelerometer not directly but in connection with each other via the weight. One such accelerometer However, it is largely temperature-sensitive, as no measures are provided, which compensate for temperature changes.

In DE-AS 18 16 998 an accelerometer is described with a bending beam clamped on one side, in which a temperature compensation of the Viscosity of the damping fluid is reached. A change in the viscosity of the Damping fluid by changing the amount of damping fluid according to the position of a Executed bimetallic strip, the position of which also changes as a result of temperature changes

ίο The non-linearity of the viscosity of the Damping fluid are taken into account in the event of temperature changes.

The invention is based on the object of creating a simply constructed accelerometer, which is largely independent of temperature. This object is achieved according to the invention by the subject matter of the main claim solved. Further developments of the invention emerge from the subclaims.

In the case of the accelerometer, the quartz crystals are directly connected to one another at their free ends tied together. This makes it possible for the accelerometer to measure the acceleration by sensing the Measures the beat frequency of the two quartz oscillators and thus mutually cancel out the measurement errors that by the displacements or deformations of the quartz crystals on their bearing member as a result of Result in temperature changes. The accelerometer therefore has a very good temperature resistance on. As a result of the bellows and the

ίο the arrangement reached support points results in a stable, adaptable structure. The thermal expansion of the damping medium leads as a result of The variable voltage exerted on the quartz crystal leads to the resonance frequency / temperature characteristic

r> at unit voltages is improved if the quartz crystals are subjected to voltages. About that In addition, the support points can also be shifted manually so that when the Resonance frequency depending on the acceleration of the quartz crystal a correction is possible. Finally, the use of a damping medium prevents damage to the oscillating crystals due to mechanical resonance in the accelerometer and it can also do one in a short time measured acceleration are displayed. When acceleration is exerted on the weight, this weight creates a large moment of inertia; the deflection or displacement of the weight that results from the change in the resonance frequency of the quartz oscillators when measuring the Acceleration exploited.

It is essential that there are changes in the resonance frequency due to the displacement of the bearing member as well as changes in the resonance frequency due to the temperature dependence of the quartz oscillators on each other cancel so that the accelerometer is practically insensitive to temperature.

An embodiment of the invention is shown in the drawing and will be described in more detail below

M). It shows

F i g. 1 is a schematic representation which exemplifies the measuring principle on which the invention is based,
F i g. 2 a longitudinal section through an embodiment

h ^r > form of the invention,

F i g. 3 is a schematic representation which shows the arrangement according to FIG. 2 illustrates temperature compensation made,

F i g. 4 is a graph showing the characteristics of the two used quartz oscillators with regard to the frequency change of the resonance frequency as a function of can be seen from the load,

Fig. 5 is a graph showing the frequency change in Dependence on temperature illustrated and

F i g. 6 is a graph showing the mechanical vibration characteristics of a moving system

Fig. 1 illustrates the measuring principle on which the invention is based and shows schematically the arrangement of two quartz oscillators, the reference number 1 being the first plate-shaped quartz oscillator and the reference number 2 the second plate-shaped quartz oscillator, the reference number 3 an electrode which establishes the connection to an electrical circuit arrangement, the reference number 4 denotes a spacer, the reference number 5 denotes a mounting plate and the letter V / a weight.

The first quartz oscillator 1 and the second / th quartz oscillator 2 are arranged opposite one another and are fixedly fastened with one end to the fastening plate 5 and the other end to the spacer 4 and are thus connected to one another, whereby relative to the weight W on the side of the fastening plate 5 a fixed, immovable end and relative to the weight W on the side of the spacer 4 a movable, free end of the arrangement is formed.

In general, the electrical resonance frequency changes (frequency of the natural natural oscillation, jo which is to be referred to below as the electrical resonance frequency) of an oscillating crystal, the so-called thickness oscillations, i.e. oscillations in the direction of its thickness, executes proportional to the Stresses caused by a weight (or r> the amount of deformation due to the stresses of the quartz crystal), regardless of the type of voltage generated by a load, what can be expressed by the following equations:

I /
F.

./ '(ACiA' 2Cij \ dFj

2 /

cn

(D

(2)

4i

In equations (1) and (2), F denotes the resonance frequency, Af the amount of change in the resonance frequency, F the tensile or compressive stress resulting from the force exerted on the quartz crystal , Cij the elastic constant of the quartz crystal, t the thickness of the Quartz crystal and ρ is the density of the quartz crystal.

The deflection of the free end of two oppositely arranged plates due to a load or load W can be determined with the help of the equation for the curve of a bending beam clamped on one side: t> o

Eh \ (

(3)

In equation (3) ν denotes the deflection of the tip of the bending beam, W the load / the length of the long side (horizontal direction) of the quartz crystal, b the length of the short side (non-horizontal direction) of the quartz crystal, Ah the distance between two quartz crystals, t the thickness of the quartz crystal and £ the modulus of elasticity.

It is now known that the voltage F is proportional to the amount of deflection, so that using equations (1) and (3) it follows that the deflection of the tip of a quartz crystal is proportional to both changes in load W and the resonance frequency of the quartz crystal.

F i g. FIG. 4 is a graph showing the measurement results for the two in FIG. 1 shown quartz crystals are applied. In the diagram, (a) denotes the characteristic curve of the first quartz crystal and (b) denotes the characteristic curve of the second quartz crystal. Load H ^

The characteristics (a) and (b) run in opposite directions, but it can be seen that the absolute values of the frequency change ratio Af / f are the same. This is based on the fact that a tensile force acts on the first quartz crystal and a compressive force acts on the second quartz crystal, and the absolute amounts of the tensile force and the compressive force are the same in a range in which the tip displacement of the quartz crystals is not large

In the following, with reference to FIG. 2, fig. 3, fig. 5 and FIG. 6 an embodiment of the Quartz crystal accelerometer according to the invention using the principles described above

In this embodiment, a plate-shaped quartz oscillator (hereinafter also referred to as a quartz oscillator plate called) is used as a quartz resonator or quartz oscillator

In Fig.2 is in the form of a central longitudinal section the quartz crystal accelerometer illustrates, where the reference numeral 1 denotes the first crystal crystal plate, the reference number 2 the second quartz oscillator plate, the reference number 3 an electrode for connecting a electrical circuit arrangement, the reference number 4 a connecting part, the reference number 5 a base block, the Reference number 6 a pin-like element, reference number 7 two bearing journals, reference number 8 a bellows, the Reference number 9 a weight, reference number 10 a displaceable component, reference number U a housing, the reference number 12 an adjustment hole, the reference number 13 a locking screw for the adjustment hole, the reference number 14 a filling hole for a damping medium, the reference number 15 a screw plug for the filling hole, the reference number 16 a filling hole for inert gas, the reference number 17 holes for fastening screws, reference number 18 a first cavity formed by the housing 11, the bellows 8, the weight 9, etc., and the reference numeral 19 denotes a second cavity within the bellows 8.

The fixed ends of the first quartz oscillator plate 1 and the second quartz oscillator plate 2, which with their largest surfaces facing each other are inserted or inserted into the base block 5, while the free ends of the quartz oscillator plates are firmly held together by the connecting part 4.

[> as as the inertial mass of the accelerometer acting weight 9 has a screwing device and is with the displaceable element 10 by means of a Screwed connection, the weight 9

is designed such that it is rotated by means of a screwdriver in the direction of the horizontal of the two quartz oscillator plates 1 and 2 moved by means of the displaceable element 10, the out of the Weight 9 and the two quartz crystal plates 1 and 2 existing arrangement sliding between the on the Weight 9 provided support pin 7 and the cylindrical pin 6, which is fixed to the connecting part 4 is attached, is held.

The bellows 8 is firmly connected to the base block 5 and the sliding element 10, and the second Cavity 19 is made up of the base block 5, the bellows 8, the displaceable element 10 and the weight 9 educated.

The case 1! is provided with an insert hole 12, via which the weight 9 is slid in the direction of the two quartz crystal plates 1 and 2, to adjust the position of the weight 9, and also screw holes 17 are in the housing 11 provided to attach the accelerometer to an object to be measured while im Base block 5 also has a filling opening 16 for filling inert gas, such as. B. dry nitrogen, is provided in the second cavity within the bellows 8, so that the housing 11 and the Base block S surround two oscillating quartz plates 1 and 2, which are enclosed by the bellows 8 and thus the interior of the arrangement is hermetically sealed.

After the position of the weight 9 is set via the adjustment hole 12, this hole is by means of a screw plug 13 closed. A filling hole 14 is provided on this closure 13, through the e! - damping medium in the form of a viscous substance, 'ie ζ. B. Siiikonöl, in the first space 18 ... '! · Can be, whereupon the Filling hole 14 is closed by the screw plug 15.

The filling hole 16 provided in the base block 5 is z. B. closed by synthetic resin after inert gas has been filled into the second cavity 19 within the bellows 8.

The reason that the weight 9 is movable in the horizontal direction of the quartz crystal plates or is arranged displaceably, consists in the fact that this results in deviations with regard to the force-frequency dependency and mechanical resonance frequency by changing the position of the bearing journals can be compensated or corrected when forces occur, with these deviations or Error due to a certain spread or division of the two at the same acceleration Quartz crystal plates 1 and 2 acting forces can arise and then to differences with regard to the resonance frequency lead. In particular, the displacement of the weight 9 due to a force in a bend of the two quartz crystal plates 1 and 2 implemented, the scope or amount of the Deflection can be changed by moving the positions of the bearing pin 7 and thus the Tensile force and the compressive force acting on the first quartz oscillating plate 1 and the second quartz oscillating plate 2 can act, set or be regulated.

The damping medium filled into the first cavity 18 restricts the mechanical vibration of the Weight 9, causing damage to the oscillating quartz plates due to mechanical resonance as well as with the usual display of a measured value is prevented.

If the mass corresponding to the weight is denoted by m, the spring constant or elastic constant of the ^r ) oscillating quartz plate with k and the viscosity resistance due to the damping medium with R , then the following applies to the equation of motion of the oscillating quartz plate in the case of a bending beam clamped on one side:

dv ² R dv A * A

. , + - ■■ - ·. - + χ ~ cos «* /, (4)
d / ~ »π df m m

where ω is the angular frequency of the mechanical oscillation of the moving system including the quartz plate and A is a constant.
Solving equation (4) gives:

, ".Ν = i / e '" cos (l "" ² , - K ² t + P)

m 1 4 K ² , .r + (, ..?, -m ² ) ²
"'where:

κ - ^R ■ ^{k 2}

2 m in

J "I 2 fc-2

ωό and ü) i are the angular frequencies of the mechanical Vibration of the moving system including

j-, borrowed the quartz plates for R = £ 0 and R = O, P is the initial speed

In equation (5), the expression in question with regard to the damping medium is the second Term, so that if ß denotes the amplitude and

4Ii also applies:

= U ■ r =

m ni _n = -

A [Ar ¹ L * + (1 -if) ²

results.
, o The relationship between Ω and

/ Jl Ι'ΐΓι

is in Fig. 6 shown graphically.

The optimum value of the viscosity resistance R is determined according to the characteristic curve B in FIG. 6 received, so that for

at the accelerometer

R = 14.2 χ 10- ² ~ 14.7 χ 10- ² (dyn · sec / cm)
results

The viscosity resistance R is also determined by the shape and dimensions of the weight, since the following applies:

R = Ct.

where C is a constant determined by the shape and dimensions of the weight and e is the coefficient of dynamic viscosity. For the proposed embodiment of the invention, it was found that a value of ε = 120 · 10 ^-3 Pa · s for forming the value of R is suitable.

Since the coefficient of dynamic viscosity for water is "1", the coefficient to use is the dynamic viscosity is 120 times the coefficient of water.

Such a value of the coefficient of dynamic viscosity of ε = 120 · 10 ³ Pa · s can easily be achieved by using commercially available silicone oil.

Except for liquids such as B. silicone oil, powder material can also be used, as in generally any material can be used that has a dampening effect.

The inert gas is filled into the second cavity within the bellows 8 from one usual reason. By arranging quartz oscillators in an atmosphere of inert gas intends to slow down the aging and to prevent oxidation of the electrode 3 and thus the To increase the operational reliability of the arrangement.

The reason for using the bellows 8 that allows the weight 9 to move in the horizontal direction of the two swinging quartz plates 1 and 2 move, consists in particular in the absorption of the increase in volume due to thermal Expansion of the damping medium.

If an object on which measurements are to be made moves with a certain acceleration in the direction of arrow A, the accelerometer also moves with the same acceleration in the direction of arrow A. At this point in time, such a force is applied to weight 9 exerted that it remains in its original position due to its inertia or its inertia, and this force acts in the same way as the load exerted on the two quartz crystal plates 1 and 2 in the opposite direction to the arrow A. The first and second quartz oscillators 1 and 2 are bent because the free ends attached to one another move in a direction opposite to the direction of arrow A. Therefore, a tensile force acts on the first quartz oscillator plate 1 and a compressive force acts on the second quartz oscillator plate 2, and the corresponding resonance frequency changes in proportion to the amount of acceleration, such as the characteristics a and b in FIG. 4 can be found

As in the present embodiment of the invention, the acceleration due to the beat frequency is determined between the first quartz oscillating plate 1 and the second quartz oscillating plate 2, the acceleration is achieved by roughly twice the variation of the resonance frequency of a quartz oscillating plate recorded

The acceleration and the force acting on the quartz crystal plate due to the acceleration are proportional to each other, and the force acting on the quartz crystal plate and the resonance frequency the quartz crystal plates are also in proportion to each other so that it is easy to get out of the Change the resonance frequency to derive the amount of acceleration.

^r i In the following, the change in the characteristic values of the oscillating quartz plates due to temperature changes will be explained.

Due to the changes in the ambient temperature, the base block 5 and the connecting part 4 exercise

ι D forces from the tensions in the first oscillating quartz plate 1 and the second quartz crystal plate 2, so that even if the temperature characteristics the quartz crystal plates themselves are good, their resonance frequency changes. However, since the changes

; -; the resonance frequency of the first quartz crystal plate and the second quartz crystal plate are substantially the same (the changes in the resonance frequency of the both quartz crystal plates 1 and 2 are ideally exactly the same, the arrangement consisting of the

»O mounting plate 5, the connecting element 4 and the two quartz crystal plates 1 and 2, exactly matched), and the change of the Beat frequency is recorded after mixing, changes in the resonance frequency cancel each other out

2 ^r > are mutually exclusive due to changes in temperature, so that a temperature-resistant accelerometer is obtained.

In addition, the quartz crystal plates can, for. B. so-called AT cuts, with a simple

«Ι rotated AT cut (rxl) + 35 ° 15 ' itself has a good resonance frequency / temperature characteristic, but the voltage changes as a function of the resonance frequency, ie the force sensitivity, has a temperature dependency. The curve d in

j-> F i g. 5 shows such an example in which the change in the resonance frequency as a function of temperature changes runs as a straight line with regard to temperature changes.
To avoid measurement errors due to the temperature-dependent

4 (To avoid this sensitivity to force, the thermal expansion of the damping medium exploited.

This increases with the arrangement according to FIG. 2 damping medium filled into the first cavity 18

4 ^r ) its volume due to a rise in temperature, the weight 9 shifts in the direction of the quartz oscillator plates 1 and 2, which state in FIG. 3 is illustrated

In Fig. 3 illustrates the weight 9 shown in solid lines the state before the displacement, while the weight 9 'shown in dashed lines illustrates the state after the displacement
When the weight 9 shifts, the slide

"; ^r j support journals 7 on the pin-like element 6 and finally assume the position indicated by the reference number T. The distance between the base block 5 and the support journals thus changes with a temperature increase from 11 to IX. For the same, on the

bo bearing journal force exerted (i.e. the force acting on the bearing pin is exerted during the displacement of the weight 9 due to an acceleration) the tensile force or the compressive force, which are exerted on the two quartz crystals 1 and 2, on

b5 reason of a temperature rise small accordingly the change in the resonance frequency (absolute amount) becomes smaller, like the straight line in FIG. 4 to On the other hand, there is the change in

The resonance frequency rises in a straight line due to the temperature dependence of the quartz oscillator plates when the temperature rises, as in the straight line labeled d in FIG. 5, a change in the resonance frequency due to a displacement of the bearing journals and a change in the resonance frequency due to the temperature dependency of the quartz oscillating plate cancel each other out, so that the quartz oscillating accelerometer according to the invention has a very low temperature dependency.

The volume of the first cavity 18 is calculated taking into account the coefficient of thermal expansion of the damping medium used and the resonance frequency / temperature characteristic curve (temperature dependency) the quartz crystal determines. In particular, the volume of the first cavity 18 becomes (Volume of the damping medium) determined so that the change in the resonance frequency due to a displacement of the bearing pin 7 to the weight 9, which is caused by a change in volume of the damping medium due to a change in temperature as well as the change in the resonance frequency of the quartz crystal plates themselves due to a change in temperature annihilate or cancel each other out.

The in Fig.5 c curve designated the temperature dependence illustrates the force sensitivity of the embodiment of the invention the accelerometer, the good characteristics in a wide temperature range (-20 ° C to +60 ⁰ C).

ifizii 3 sheets of drawings

Claims (5)

Patent claims: 1. Accelerometer with two quartz crystals, one with the other at one end moving object connected bearing member are attached, while the other Ends of the quartz oscillators are freely oscillating, with a weight to exert a bias on the free ends of the quartz crystal, a device for the mechanical connection of the free ends of the quartz crystals and with a tightly sealed housing, marked by arch, that the free ends of the quartz crystals (1, 2) are firmly connected to one another are that the surfaces of the quartz oscillators are directly opposite each other and a free gap specify that an expandable in the direction of the quartz crystal bellows (8) between the fixed Ends of the quartz crystal and the weight (9) is provided, which receives the quartz crystal, that the connecting device (4, 6) is arranged between the weight and the quartz oscillators and defines contact points with respect to the quartz oscillators along the direction of expansion of the bellows (8) are displaceable and that in the space (18) between the weight and the bellows on the one hand and the sealed housing (11) on the other hand, a temperature-dependent damping medium is used 2. Accelerometer according to claim 1, characterized in that the quartz crystal (1, 2) surrounding bellows (8) is fluid-tight and defines a cavity (19). 3. Accelerometer according to claim 2, characterized in that the cavity (19) with is filled with an inert gas 4. Accelerometer according to at least one of the preceding claims, characterized in that that the weight (9) on the coupling device (4,6) in the direction of the quartz crystal (1, 2) is adjustable. 5. Accelerometer according to at least one of the preceding claims, characterized in that that the weight (9) via the coupling device (4, 6) at the expandable end of the Bellows (8) is arranged.