WO2010066502A1 - Measuring bridge, measurement unit and rotating mirror - Google Patents

Abstract

The invention relates to a measuring bridge (1), which comprises second impedance-variable elements (R1b..R4b) in addition to the first four impedance variable elements (R1a..R4a) in a bridge arrangement. The first elements (R1a..R4a) are arranged in a measurement region (A) that is prepared for an effect of a first physical magnitude and a change to the said impedances based on said effect. The second elements (R1b..R4b), in contrast, are arranged outside of said measurement region (A). The invention furthermore relates to a measurement unit (3) that comprises a measuring bridge (1) according to the invention and an electronic circuit (2) connected thereto. The invention finally relates to a rotating mirror having a measuring bridge (1) or a measurement unit (3) by means of which a rotation angle of the mirror can be measured.

Description

 description

title

Measuring bridge, measuring unit and rotatably mounted mirror

The invention relates to a measuring bridge comprising four first impedance-changeable elements in bridge arrangement, which are arranged in a measuring range which is prepared for an action of a first physical quantity and a change of said impedances due thereto. Furthermore, the invention relates to a measuring unit in which an electronic circuit is electrically connected to a measuring bridge. Finally, the invention relates to a rotatably mounted mirror

State of the art

Measuring bridges are a well-known element for detecting a wide variety of physical quantities. They are composed of four elements with variable impedance, which are connected together in a closed ring or a square, and have a voltage source or a power source in one diagonal and a voltmeter in the other diagonal. Measuring bridges have the advantage that they are invariable against the influence of disturbances which act equally on all four bridge elements. Unfortunately, this does not apply to disturbances that do not affect all bridge elements equally. In the following, the practical effect of such an inhomogeneous disturbance will be explained by means of an exemplary arrangement. This arrangement is, of course, only one of many possible arrangements in which the described problem occurs in a modified form, and therefore must not be construed as representing the problem only with this one arrangement.

For micromechanical sensors, an electrical resistance (piezoresistor) reacting to a mechanical stress is frequently used as a sensitive element for measuring mechanical stresses. The piezoresistive effect is based on the change of a specific resistance of a piezoelectric material (piezoelectric crystal) by pressure or tension. To measure small changes in resistance are such elements usually connected in a Wheatstone bridge. In micromechanical pressure sensors, for example, four piezoresistors are mounted on a pressure-sensitive deformable membrane. However, piezo-resistors for mechanical stress also react to temperature and light, canceling out the effects of temperature and light, which affect all resistances of an ideal Wheatstone bridge. In practice, this is not the case, since the effect is not equal to all resistances, such as at oblique incidence of light. The result is a falsification of the actual measured variable. For example, in the case of a pressure sensor, the incidence of light by a neon tube can cause a falsification of the sensor output signal of several percent. The photosensitivity of the sensor elements can be reduced by a suitable structure, for example by a closed light-tight housing. Often this is not possible, or not all disturbing influences can be completely avoided by shielding.

The photosensitivity presents a particular problem in particular when using the piezoelectric elements as position detectors for micro-optomechanical components (MOEMS), for example micromirrors. Such mirrors are used, for example, for deflecting a light beam, in particular a laser light beam. As a practical application, so-called "laser scanners" are mentioned here, which are used on the one hand for imaging and on the other hand for measurement purposes (for example, for creating a terrain profile) A mirror which is usually gimbaled allows line-by-line building or line-by-line scanning The deflection of the mirror is often done with a so-called "Galvanometerantrieb".

For this application, there are very high demands on the accuracy of the position detection (detection of the deflection angle of the mirror axes of rotation, approximately at the ends of the torsion springs, on which the mirror is rotatably mounted, or via which the rotatable mounting is accomplished). Due to their high accuracy, piezoresistors are preferred for this task. As transducer elements they are in this application very close to the optical beam path of the laser beam and thus are also subject to its influence, for example due to unwanted reflections and stray light. Outside light from the environment (daylight or room lighting) has a similar effect, but due to the high intensity of laser light affects the disturbing influence of the same particularly detrimental to the measurement accuracy of the bridge out. The proximity to the optical beam path also makes it very difficult to impossible to avoid the visual impact by constructive measures in the housing.

In addition to light, the inhomogeneously acting interfering field can also be due to temperature or even external mechanical stress, which arises, for example, when gluing or soldering the sensor. Of course, the mentioned interference fields and their causes are only a limited excerpt of the conceivable interference fields and their causes, which due to the abundance at this point can not be listed in their entirety.

Against this background, there is a need for further improved accuracy and resistance.

Disclosure of the invention

The invention provides a measuring bridge according to claim 1, a measuring unit according to claim 8 and a mirror according to claim 10.

Accordingly, a measuring bridge of the aforementioned type is provided, in which four further, second impedance-variable elements are arranged outside the measuring range and are electrically connected to the bridge arrangement.

Accordingly, there is further provided a measuring unit of the initially mentioned type, which is prepared to subtract the signal of a bridge formed by the second elements from the bridge formed by the first elements.

Finally, accordingly, a mirror of the aforementioned type is provided, which comprises at least one inventive measuring bridge or an inventive measuring unit, which is designed to measure the introduced into a bearing of the mirror torque and / or a size dependent thereon.

By providing second elements outside the actual measurement range for the first physical quantity, the influence of the second physical quantity can be taken into account separately. It is assumed that the first and second physical quantities have different effects inside and outside the measuring range. If possible, constructively provide that the first physical quantity in - A -

Essentially only within the measuring range. Likewise, provision must be made for the second physical variable to act sufficiently on the second elements outside the measuring range. In this case, the first physical variable represents the actual size to be measured, while the second variable represents a disturbance variable. The influence of the second physical variable, that is to say the disturbing influence, on the first and second elements can be influenced by the differentiated effect of the first and second physical quantities Measurement of the first physical quantity, ie the actual measured variable, are taken into account.

The measuring unit according to the invention, which comprises an electronic circuit connected thereto in addition to the measuring bridge, is advantageously used in an embodiment of the measuring bridge in which two signals are available, a first, which is essentially the first and second physical variable, ie measurement and disturbance variable in the measuring range, includes and a second, which essentially only the second physical variable, so the disturbance outside the measuring range, includes, yet the invention is advantageously applicable here. In this variant of the invention, the second is subtracted from the first measurement signal with the aid of the electronic circuit, so that essentially the measured quantity remains, which can be provided directly to downstream processing units.

The invention is particularly suitable for use in a rotatable

Mirror for the aforementioned laser scanner, since here a disturbance variable with considerable intensity, namely the laser light, acts on the measuring elements and the measurement of the actual measured variable, namely the rotation angle of the mirror, difficult.

At this point it is additionally noted that, of course, a change of the real part in the case of a non-existent imaginary part is considered as a change of the impedance. The invention therefore relates in particular to resistance measuring bridges.

Advantageous embodiments and modifications of the invention will become apparent from the dependent claims and from the description in conjunction with the figures of the drawing.

It is advantageous if two second elements each having two first elements are connected in parallel and the voltage or current supply of the bridge arrangements is arranged at the node of said parallel branches, wherein the first and second elements have a same-direction impedance change gradient with respect to an action second, from the first different physical size inside and outside the measuring range. In this variant of the invention, in principle, two measuring bridges are arranged in parallel, whereby the bridge composed of the first elements of the first and the second physical variable and the bridge formed of the second elements is influenced only by the second physical quantity. Inhomogeneities of the second physical quantity, for example due to a temperature (scalar) field, act essentially equally on the first and second bridges, which is why the interference of the second physical quantity can be taken into account comparatively easily, for example by subtracting the measurement result of the second bridge of FIG that of the first bridge. Of course, the mentioned principle can be extended arbitrarily. For example, a third bridge may be connected in parallel to account for a different second physical (disturbance) magnitude, if not already possible with the second bridge.

It is furthermore advantageous if in each case a second element is connected in series with a first element in each case and the first elements have an impedance change gradient opposite to the second elements with respect to an influence of a second, different physical quantity within and outside the measuring range. This is another elegant way to eliminate or at least partially compensate for the influence of the second disturbing physical quantity. Here, each first element, which is influenced by the first and second physical quantities, is in each case connected in series with a second element, which is influenced only by the second physical quantity. By providing opposing impedance change gradients, the disturbing influence of the second physical quantity is reduced and ideally even canceled out. Inhomogeneities of the second physical quantity, for example a temperature (scalar) field, act essentially equally on the respectively associated first and second elements and therefore have little or no influence on the measurement of the first physical quantity.

It is advantageous if the ratio of the impedance change gradient the

Inverse of the ratio of the influence of the second physical quantity inside and outside the measuring range corresponds. The second physical quantity may not be the same within the measurement range as outside the measurement range. Since the measuring range is also provided for the action of the first physical variable, the second physical variable will often have a stronger effect there as well. The following is assumed because of the ease of representation of this condition, although the resulting teaching is applicable to other circumstances. The first element is thus more strongly influenced by the second physical quantity than the second element. However, if the impedance change gradient of the second element, that is to say its sensitivity, is increased in comparison with the second physical variable, this unequal influence can be compensated for. This can be done by suitable choice of material or suitable dimensioning. For example, a larger sensor is in principle more sensitive to acting influences, which is why the desired sensitivity can also be achieved with a less sensitive material.

It is favorable if the first physical size is one of the group: light intensity,

Temperature, electric field, magnetic field, activity, mechanical stress or pressure. These are sizes that are very often measured with measuring bridges. Nevertheless, the listed sizes represent only a limited portion of the conceivable possibilities, which may not be construed as if the invention is limited to this section only.

It is furthermore advantageous if the second physical variable is or are one or more of the group: light intensity, temperature, electric field, magnetic field, activity, mechanical stress or pressure. These are quantities that often occur as disturbances in measuring bridges. With regard to the exemplary character of the enumeration, what has already been said above applies.

In an advantageous embodiment of the invention, the first and second elements are piezoelectric sensors and the first physical variable is a mechanical stress and the second physical variable is a light intensity and / or temperature. This is one of many conceivable combinations of elements and physical quantities, the suitability of which stands out in particular with regard to the mentioned mirrors for the laser scanners. It is determined by a mechanical stress, namely in the storage of the rotatable mirror whose angle of rotation. The piezoelectric sensors used are, however, at the same time sensitive to the acting laser light per se or the resulting increase in temperature and temperature distribution.

In an advantageous embodiment of the measuring unit according to the invention, the electronic circuit is further prepared to weight the bridge signals before the subtraction inversely proportional to the ratio of the influence of the second physical quantity inside and outside the measuring range. As already mentioned, the second physical size within the measuring range may not be the same as outside the measuring range. It has been stated above that for this purpose the impedance change gradients of the elements, ie their sensitivity, can be adapted accordingly. Additionally or alternatively, however, it is also possible to weight the bridge signals of two bridges by means of an electronic circuit before they are subtracted. This may be easier to accomplish than adjusting the elements themselves. By suitable design, an adjustable weighting can be provided here, which facilitates the initial calibration, or even a calibration due to sensor drift.

The above embodiments and developments of the invention can be combined in any manner.

The present invention will be explained in more detail with reference to the exemplary embodiments indicated in the schematic figures of the drawing. It shows:

1 shows a first variant of a measuring bridge according to the invention or a first measuring unit according to the invention;

FIG. 2 shows a second variant of a measuring bridge according to the invention or a second measuring unit according to the invention;

FIG. 3 shows a third variant of a measuring bridge according to the invention; Figure 4. an inventive, rotatably mounted mirror.

In the figures of the drawing are identical and functionally identical elements and features - unless otherwise stated - provided with the same reference numerals.

FIG. 1 shows a first measuring bridge according to the invention, in which two second elements R1 b, R3b / R2b, R4b, each with two first elements R1a, R3a / R2a, R4a, are connected in parallel in a bridge arrangement. The voltage supply U is arranged at the node of said parallel branches, which is why there are essentially two parallel Wheatstone bridges. A Wheatstone bridge is located within the measuring range A (shown in dashed lines) within which a first physical quantity can act, the second Wheatstone bridge is outside this range A and is substantially invariant to detuning due to the first physical quantity. Finally, a first and second differential amplifier or

Instrumentation amplifier V1 and V2 shown, which with their positive input with each an output of the Wheatstone bridge formed by the first elements R1 a..R4a and with their negative input each having an output of the Wheatstone bridge formed by the second elements R1 b..R4b are connected.

For the following considerations, it is assumed by way of example that the first and second elements R1a..R4a and R1b..R4b are piezoresistive semiconductor elements, the first physical quantity is a mechanical stress and the second physical quantity is a light intensity and / or a temperature is. The light intensity and / or temperature thereby changes in the direction indicated by the arrow T, that is decreases in this direction. In this example, the same direction

Impedanzänderungsgradienten for the first and second elements R1 a..R4a and R1 b..R4b with respect to an action of the second physical quantity, so here light and / or temperature provided. By suitably constructive measures it is provided that the mechanical stress essentially does not act substantially in the measuring range A and outside. On the other hand, the light intensity and / or temperature should, if possible, have the same effect within and outside the measuring range A. This can be achieved, for example, by making the measuring area A significantly thinner than the surrounding area. Therefore, a mechanical stress has a much stronger effect in the measuring range A than it does outside, while the light equally affects the measuring range A and the range outside.

The effect of the two amplifiers V1 and V2 is that the signal of the second bridge, that is to say the interference signal, is subtracted from the signal of the first bridge, so that the output voltage Ua is applied between the outputs of the two amplifiers V1 and V2 acting mechanical stress without disturbing the light and / or temperature represents. The elimination of the influence of light or temperature from the difference signal succeeds the better the closer the first and second resistors R1a..R1d, R2a..R2d are arranged to one another, ie R1a to R1b, R2a to R2b and so on. In this way, inhomogeneities of the light intensity or of the temperature act essentially the same on the first and the second

Measuring bridge. In terms of design, a compromise has to be made between this requirement and the requirement that the first physical quantity acts essentially only within the measuring range A, that is to say on the second elements R1 b... R 4 b.

Of course, the mentioned principle can be extended as desired. For example, a third bridge may be connected in parallel to give a different second physical (disturbance) magnitude take into account, if this further disturbance can not be compensated with the help of the second elements R1 b..R4b.

The measuring unit 3 shown in FIG. 2 has the same measuring bridge 1 as in FIG. 1, but a different electronic circuit 2. In this case, the signal of the bridge formed by the second elements R1 b..R4b is applied to the third amplifier V3 and the Signal of the bridge formed by the first elements R1 a..R4a led to the fourth amplifier V4. Subsequently, the output signals of the two amplifiers V3 and V4 are weighted by means of second potentiometers P1 and P2 and fed to the inputs of a fifth amplifier V5 such that the weighted signal of the second bridge is subtracted from the weighted signal of the first bridge. At the output of the fifth amplifier V5 thus there is an output voltage Ua, which again represents the mechanical stress acting in the measuring range A without disturbing the light and / or the temperature. The two potentiometers P1 and P2 (in principle one of them would also be sufficient for the weighting) can be set so that the proportion of the positive / negative input signal at the fifth amplifier V5, which is determined by the second physical variable, ie by the light and / or the temperature caused is the same, even if the second physical quantity inside and outside the measuring range A acts differently.

It should be noted at this point that opposing impedance change gradients can also be provided for the first and second elements R1a..R4a and R1b..R4b. In this case, the signals of the two bridges would have to be added instead of subtracted.

FIG. 3 shows a further variant of the measuring bridge 1 according to the invention, in which a second element R1 b..R4b is connected in series with a respective first element R1a..R4a, and the first elements R1a..R4a one to the second elements R1 b..R4b have opposite impedance change gradients with respect to the second physical quantity. Here, the influence of the second physical variable, ie the disturbance, is reduced or eliminated by the disturbance increasing / decreasing the voltage across the first elements R1a..R4a and lowering / increasing the voltage across the second elements R1b .. R4b causes, so that the total influence of said voltages is reduced, if not eliminated. Advantageously, this is in principle no further electronic circuit necessary, it is enough a simple

Differential amplifier or instrumentation amplifier for the measurement of the bridge voltage. In addition, the ratio of the impedance change gradients of the first and second elements R1a..R4a and R1b..R4b can be adjusted to the reciprocal of the ratio of the influence of the second physical quantity inside and outside the measurement range A when the second physical quantity is inside and outside the measurement range A acts differently.

Of course, the mentioned principle can be extended arbitrarily. For example, a third resistor may each be connected in series in order to take into account a different second physical (disturbance) quantity, provided that this additional disturbance variable can not already be compensated for with the aid of the second elements R1b..R4b.

In a further preferred variant of the invention, the measures of FIG. 2 and FIG. 3, ie both the weighting of the bridge output signals and the weighting of the impedance change gradients, are provided. An adaptation of the measuring unit 3 to different effects of the second physical quantity inside and outside the measuring range A is particularly easy by the two degrees of freedom.

Finally, FIG. 4 shows a mirror 4, a first frame 5 and a second frame 6. The mirror 4 is connected via torsion springs 7 to the frame 5, which in turn is connected to the frame 6 via torsion springs 8. The axes of

Torsion springs 7 and 8 are e.g. arranged at right angles to each other, so that the mirror 4 can be pivoted in any direction. The entire assembly can be made in one piece, so that an introduced into the torsion springs 7 and 8 torque, which is caused by rotation of the mirror 4, also in the connected to the torsion springs 7 and 8 frames 5 and 6 is initiated. To the

Connection points of the torsion springs 7 and 8 with the frame 5 and 6 is ever a measuring bridge 1a..1 d (alternatively one measuring unit), which measures the deformation at each connection point and a signal to a higher-level processing unit (not shown) passes, which can calculate the angle of rotation of the mirror 4 in both axes from the measured values determined. In this variant, the twist angle is determined with a torque-dependent variable, namely on the deformation caused thereby in frame 5 or 6. It is also conceivable, however, to directly measure the torque, for example by mounting the measuring bridges 1 a..1 d directly to the torsion springs 7 and 8. It is also conceivable to use only one measuring bridge per axis, which minimizes the computational effort, the achievable Accuracy but also lowers. Finally, it should be noted that the invention relates to a variety of applications, even if the invention has been described only with reference to a specific application. However, it is easy for the skilled person and it is therefore within the scope of his ability to adapt the invention to his needs.

Claims

Claims: A measuring bridge (1) comprising four first impedance-variable elements (R1a..R4a) in bridge arrangement, which are arranged in a measuring range (A), which is responsible for an action of a first physical quantity and a change of the said Impedances is prepared, wherein four further, with respect to their impedance variable elements (R1 b..R4b) outside this measuring range (A) are arranged and electrically connected to the bridge assembly. 2. measuring bridge (1) according to claim 1, characterized in that each two second elements (R1 b, R3b / R2b, R4b) with two first elements (R1 a, R3a / R2a, R4a) are connected in parallel and the voltage or the power supply of the bridge arrangements is arranged at the nodes of said parallel branches, wherein the first and second elements have a same-direction impedance change gradient with respect to a second, from the first different physical size inside and outside the measuring range (A). 3. measuring bridge (1) according to claim 1, characterized in that in each case a second element (R1 b..R4b), each with a first element (R1 a..R4a) is connected in series and the first elements (R1 a .. R4a) have an impedance change gradient opposite to the second elements (R1b..R4b) with regard to an action of a second, different from the first physical variable inside and outside the measuring range (A). 4. measuring bridge (1) according to claim 3, characterized in that the ratio of the impedance change gradient corresponds to the inverse of the ratio of the influence of the second physical quantity inside and outside the measuring range (A). 5. measuring bridge (1) according to one of the preceding claims, characterized in that the first physical quantity is one of the group: light intensity, temperature, electric field, magnetic field, activity, mechanical stress or pressure. 6. measuring bridge (1) according to one of claims 2 to 5, characterized in that the second physical quantity is or are one or more of the group: light intensity, temperature, electric field, magnetic field, activity, mechanical stress or pressure. 7. measuring bridge (1) according to one of the preceding claims, characterized in that the first and second elements piezoelectric sensors (R1 a..R4a, R1 b..R4b) and the first physical quantity is a mechanical stress and the second physical quantity Light intensity and / or temperature is. 8. measuring unit (3), comprising a measuring bridge (1) according to claim 2 and an associated electronic circuit (2) which is prepared, the signal of the bridge formed by the second elements (R1 b..R4b) of the Subtract the bridge formed by the first elements (R1 a..R4a). 9. measuring unit (3) according to claim 8, characterized in that the electronic circuit (2) is prepared to weight the bridge signals before subtraction inversely proportional to the ratio of the influence of the second physical quantity inside and outside the measuring range (A). 10. mirror, which is rotatably mounted at least about an axis, with at least one measuring bridge according to one of claims 1 to 7 or a measuring unit according to one of claims 8 to 9, which is adapted to the introduced into a bearing of the mirror torque and / or measure a size dependent thereon.