# Patent

## Title
Method for the mircomechanical measurement of acceleration and a micromechanical acceleration sensor

## Abstract

The invention relates to measurement devices used in the measurement of acceleration, and more specifically, to micromechanical acceleration sensors. The invention seeks to offer an improved method for the measurement of acceleration directed to three or two dimensions using a micromechanical acceleration sensor as well as an improved micromechanical acceleration sensor. Using this invention, the functional reliability of a sensor can be monitored in constant use, and it is suitable for use particularly in small-sized micromechanical acceleration sensor solutions measuring in relation to several axes.

## Background
FIELD OF THE INVENTION
The invention relates to measurement devices used in the measurement of acceleration, and more specifically, to micromechanical acceleration sensors. The invention seeks to offer an improved method for the micromechanical measurement of acceleration as well as an micromechanical acceleration sensor, which is suitable for use in small-sized micromechanical acceleration sensor solutions, and which is particularly suited for use in small-sized micromechanical acceleration sensor solutions measuring in relation to several axes.
BACKGROUND OF THE INVENTION
Measurement based on a micromechanical acceleration sensor has proven to be in principle a simple and reliable manner of measuring acceleration. In the case of a micromechanical acceleration sensor, measurement is based, for example, in the capacitance measurement on a change of a gap between two surfaces of a electrode pair of a sensor caused by acceleration. The capacitance between the surfaces, i.e. the storage capacity of the electrical charge, depends on the surface area of the surfaces and the distance between the surfaces. A measurement based on a micromechanical acceleration sensor such as a capacitance measurement can be used even with particularly small acceleration measurement areas.
Small-sized micromechanical acceleration sensors are often used in particularly critical application sites, such as, for example, in ABS and ESP systems (ABS, Antilocking Brake System; ESP, Electronic Stability Program) used in the automobile industry. For this reason, it is vitally important to assure the proper function of the capacitative acceleration sensors at start-up as well as in constant use. In demanding application sites there is a desire to know all deviations exceeding the device specifications in the functionality of the acceleration sensor after factory calibration. These types of deviations can be, among others, offset deviations, sensitivity deviations or physical damages.
Currently are known some micromechanical acceleration sensor solutions according to known art measuring in relation to several axes. For example, Finnish patent application publications FI 20030206 and FI 20030207 describe capacitative acceleration sensor solutions according to known art measuring in relation to several axes.
In acceleration sensor solutions according to known art, a disadvantage has been the observation of the functional reliability of the sensor at start-up as well as in constant use. In acceleration sensor solutions according to known art, a known self-testing arrangement can be used, in which the acceleration sensor is subjected to high voltage. The high voltage causes an electrostatic force between the measurement electrode and the mass of the sensor, resulting in bending of the spring and shifting of the mass, which changes the capacitance of the sensor, which can be measured using an ASIC (ASIC, Application Specific Integrated Circuit).
Using the self-testing arrangement described above offset deviations or sensitivity deviations due to time or temperature cannot be measured. Additionally, measurement of acceleration must be interrupted during self-testing.
In micromechanical acceleration sensor solutions according to known art and their self-testing methods, a disadvantage has also been the implementation of high voltage parts in the circuit structures used, which creates many disadvantages in the design and realization of circuit technology related to the acceleration sensor.
In demanding application sites of micromechanical acceleration sensors there is a clearly growing need for micromechanical acceleration sensors of greater functional reliability than earlier solutions, which are suitable for use in reliable measurement of acceleration particularly in small-sized micromechanical acceleration sensor solutions measuring in relation to several axes.
SUMMARY OF THE INVENTION
The object of the invention is an improved method for implementing continuous self-testing of a micromechanical acceleration sensor as well as an improved micromechanical acceleration sensor. Using this invention, the functional reliability of a micromechanical acceleration sensor can be monitored in constant use, and it is suitable for use particularly in small-sized micromechanical acceleration sensor solutions measuring in relation to several axes.
According to the first feature of the invention, a method is provided for the measurement of acceleration using the values of acceleration on the sensor measurement area, directed at the sensor, to the direction of three dimensions using a micromechanical acceleration sensor, which acceleration sensor comprises at least four units of sensor measurement devices measuring in different measurement directions, of which four vectors describing the measurement directions of the sensor measurement devices no three vectors belong to the same level such that in the method the continuous self-testing of the acceleration sensor during start-up and/or operation is implemented such that

    
    
        to the accelerations {right arrow over (a)}1, {right arrow over (a)}2, {right arrow over (a)}3, . . . {right arrow over (a)}n obtained as measurement results of the sensor measurement devices are given the multipliers k1, k2, k3, . . . kn such that, when the sensor is functioning perfectly, the value of the expression k1{right arrow over (a)}1+k2{right arrow over (a)}2+k3{right arrow over (a)}3+ . . . +kn{right arrow over (a)}n calculates to {right arrow over (0)},
        the accelerations {right arrow over (a)}1, {right arrow over (a)}2, {right arrow over (a)}3, . . . {right arrow over (a)}n an given by the sensor measurement devices are measured at given intervals,
        the value of the expression k1{right arrow over (a)}1+k2{right arrow over (a)}2+k3{right arrow over (a)}3+ . . . +kn{right arrow over (a)}n is calculated,
        the value of said expression is compared to a pre-defined threshold value, and
        if the value of said expression deviates from the value {right arrow over (0)} more than said pre-defined threshold value, an error message is given.
    
    


Preferably, said four measurement directions of the sensor measurement devices described by said four vectors are selected so that the earth gravitational force has an essential component affecting in each of the said four measurement directions.
Preferably, the measuring directions of the sensor measurement devices measuring in different directions are selected symmetrically such that the accelerations obtained as measurement results of the sensor measuring devices measuring in four different directions can be presented as equations:

{right arrow over (a)}1=c·{right arrow over (a)}x+c·{right arrow over (a)}z 

{right arrow over (a)}2=c·{right arrow over (a)}y+c·{right arrow over (a)}z 

{right arrow over (a)}3=−c·{right arrow over (a)}x+c·{right arrow over (a)}z 

{right arrow over (a)}4=−c·{right arrow over (a)}y+c·{right arrow over (a)}z,

and that to the accelerations {right arrow over (a)}1, {right arrow over (a)}2, {right arrow over (a)}3 and {right arrow over (a)}4 obtained as measurement results of the sensor measuring devices are given the multipliers k1, k2, k3 and k4 such that k1=k3=1 and k2=k4=−1.

Preferably, the calculation of said vector expressions is implemented using scalar values. Preferably, an error message is given when m number of deviations of the value of said expression from the value {right arrow over (0)} more than said pre-defined threshold value have occurred.
According to the second feature of the invention, a method is provided for the measurement of acceleration using the values of acceleration on the sensor measurement area, directed at the sensor, to the direction of three dimensions using a micromechanical acceleration sensor, which acceleration sensor comprises at least four units of sensor measurement devices measuring in different measurement directions, of which four vectors describing the measurement directions of the sensor measurement devices no three vectors belong to the same level such that in the method the continuous self-testing of the acceleration sensor during start-up and/or operation is implemented such that

    
    
        to the corresponding scalar values a1, a2, a3, . . . an of the accelerations {right arrow over (a)}1, {right arrow over (a)}2, {right arrow over (a)}3, . . . {right arrow over (a)}n obtained as measurement results of the sensor measurement devices are given the multipliers k1, k2, k3, . . . kn such that, when the sensor is functioning perfectly, the value of the expression k1a1+k2a2+k3a3+ . . . +knan calculates to 0,
        the scalar values a1, a2, a3, . . . an of the accelerations {right arrow over (a)}1, {right arrow over (a)}2, {right arrow over (a)}3, . . . {right arrow over (a)}n given by the sensor measurement devices are measured at given intervals,
        the value of the expression k1a1+k2a2+k3a3+ . . . +knan is calculated,
        the value of said expression is compared to a pre-defined threshold value, and
        if the value of said expression deviates from the value 0 more than said pre-defined threshold value, an error message is given.
    
    


Preferably, said four measurement directions of the sensor measurement devices described by said four vectors are selected so that the earth gravitational force has an essential component affecting in each of the said four measurement directions.
Preferably, an error message is given when m number of deviations of the value of said expression from the value 0 more than said pre-defined threshold value have occurred.
According to the third feature of the invention, a method is provided for the measurement of acceleration using the values of acceleration on the sensor measurement area, directed at the sensor, to the direction of three dimensions using a micromechanical acceleration sensor, which acceleration sensor comprises at least four units of sensor measurement devices measuring in different measurement directions, of which four vectors describing the measurement directions of the sensor measurement devices no three vectors belong to the same level such that in the method the continuous self-testing of the acceleration sensor during start-up and/or operation is implemented in the linear or linearized measurement such that

    
    
        to the signal levels s1, s2, s3, . . . sn corresponding to the scalar values a1, a2, a3, . . . an of acceleration obtained as measurement results of the sensor measurement devices are given the multipliers k1, k2, k3, . . . kn such that, when the sensor is functioning perfectly, the value of the summation expression
    
    






  
    
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calculates to 0,

    
    
        the signal levels s1, s2, s3, . . . sn given by the sensor measurement devices are measured at given intervals,
        the value of the summation expression
    
    






  
    
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is calculated,

    
    
        the value of said summation expression is compared to a pre-defined threshold value, and
        if the values of said summation expressions deviate from the value 0 more than said pre-defined threshold value, an error message is given.
    
    


Preferably, said four measurement directions of the sensor measurement devices described by said four vectors are selected so that the earth gravitational force has an essential component affecting in each of the said four measurement directions.
Preferably, an error message is given when m number of deviations of the value of said expression from the value 0 more than said pre-defined threshold value have occurred.
According to the forth feature of the invention, a method is provided for the measurement of acceleration using the values of acceleration on the sensor measurement area, directed at the sensor, to the direction of two dimensions using a micromechanical acceleration sensor, which acceleration sensor comprises at least three units of sensor measurement devices measuring in different measurement directions, of which three vectors describing the measurement directions of the sensor measurement devices no two vectors are in the same direction such that in the method the continuous self-testing of the acceleration sensor during start-up and/or operation is implemented such that

    
    
        to the accelerations {right arrow over (a)}1, {right arrow over (a)}2, . . . {right arrow over (a)}n obtained as measurement results of the sensor measurement devices are given the multipliers k1, k2, . . . kn such that, when the sensor is functioning perfectly, the value of the expression k1{right arrow over (a)}1+k2{right arrow over (a)}2+ . . . +kn{right arrow over (a)}n calculates to 0,
        the accelerations {right arrow over (a)}1, {right arrow over (a)}2, . . . {right arrow over (a)}n given by the sensor measurement devices are measured at given intervals,
        the value of the expression k1{right arrow over (a)}1+k2{right arrow over (a)}2+ . . . +kn{right arrow over (a)}n is calculated,
        the value of said expression is compared to a pre-defined threshold value, and
        if the value of said expression deviates from the value {right arrow over (0)} more than said pre-defined threshold value, an error message is given.
    
    


Preferably, said three measurement directions of the sensor measurement devices described by said three vectors are selected so that the earth gravitational force has an essential component affecting in each of the said three measurement directions.
Preferably, the measuring directions of the sensor measurement devices measuring in different directions are selected symmetrically such that the accelerations obtained as measurement results of the sensor measuring devices measuring in four different directions can be presented as equations:

{right arrow over (a)}1=d·{right arrow over (a)}x 

{right arrow over (a)}2=d·{right arrow over (a)}y 

{right arrow over (a)}3=−d·{right arrow over (a)}x 

{right arrow over (a)}4=−d·{right arrow over (a)}y,

and that to the accelerations {right arrow over (a)}1, {right arrow over (a)}2, {right arrow over (a)}3 and {right arrow over (a)}4 obtained as measurement results of the sensor measurement devices are given the multipliers k1, k2, k3 and k4 such that k1=k2=k3=k4=1.

Preferably, the calculation of said vector expressions is implemented using scalar values. Preferably, an error message is given when m number of deviations of the value of said expression from the value {right arrow over (0)} more than said pre-defined threshold value have occurred.
According to the fifth feature of the invention, a method is provided for the measurement of acceleration using the values of acceleration on the sensor measurement area, directed at the sensor, to the direction of two dimensions using a micromechanical acceleration sensor, which acceleration sensor comprises at least three units of sensor measurement devices measuring in different measurement directions, of which three vectors describing the measurement directions of the sensor measurement devices no two vectors are in the same direction such that in the method the continuous self-testing of the acceleration sensor during start-up and/or operation is implemented such that

    
    
        to the scalar values a1, a2, . . . an of the accelerations {right arrow over (a)}1, {right arrow over (a)}2, . . . {right arrow over (a)}n obtained as measurement results of the sensor measurement devices are given the multipliers k1, k2, . . . kn such that, when the sensor is functioning perfectly, the value of the expression k1a1+k2a2+ . . . +knan calculates to 0,
        the scalar values a1, a2, . . . an of the accelerations {right arrow over (a)}1, {right arrow over (a)}2, . . . {right arrow over (a)}n given by the sensor measurement devices are measured at given intervals,
        the value of the expression k1a1+k2a2+ . . . +knan is calculated,
        the value of said expression is compared to a pre-defined threshold value, and
        if the value of said expression deviates from the value 0 more than said predetermined threshold value, an error message is given.
    
    


Preferably, said three measurement directions of the sensor measurement devices described by said three vectors are selected so that the earth gravitational force has an essential component affecting in each of the said three measurement directions.
Preferably, an error message is given when m number of deviations of the value of said expression from the value 0 more than said pre-defined threshold value have occurred.
According to the sixth feature of the invention, a method is provided for the measurement of acceleration using the values of acceleration on the sensor measurement area, directed at the sensor, to the direction of two dimensions using a micromechanical acceleration sensor, which acceleration sensor comprises at least three units of sensor measurement devices measuring in different measurement directions, of which three vectors describing the measurement directions of the sensor measurement devices no two vectors are in the same direction such that in the method the continuous self-testing of the acceleration sensor during start-up and/or operation is implemented in the linear or linearized measurement such that

    
    
        to the signal levels s1, s2, . . . sn corresponding to the scalar values a1, a2, . . . an of acceleration obtained as measurement results of the sensor measurement devices are given the multipliers k1, k2, . . . kn such that, when the sensor is functioning perfectly, the value of the summation expression
    
    






  
    
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calculates to 0,

    
    
        the signal levels s1, s2, . . . sn given by the sensor measuring devices are measured at given intervals,
        the value of the summation expression
    
    






  
    
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is calculated,

    
    
        the value of said summation expression is compared to a pre-defined threshold value, and
        if the values of said summation equations deviate from the value 0 more than said pre-defined threshold value, an error message is given.
    
    


Preferably, said three measurement directions of the sensor measurement devices described by said three vectors are selected so that the earth gravitational force has an essential component affecting in each of the said three measurement directions.
Preferably, an error message is given when m number of deviations of the values of said summation expressions from the value 0 more than said pre-defined threshold value have occurred.
According to the seventh feature of the invention, a micromechanical acceleration sensor is provided for the measurement of acceleration using the values of acceleration on the sensor measurement area, directed at the sensor, to the direction of three dimensions, which acceleration sensor comprises at least four units of sensor measurement devices measuring in different measurement directions, of which four vectors describing the measurement directions of the sensor measurement devices no three vectors belong to the same level such that the acceleration sensor comprises means for implementing continuous self-testing during start-up and/or operation, these means comprising a sensor measurement block, a sensor self-testing block, a sensor comparison block and an alarm block such that

    
    
        the sensor measurement block comprises means for measuring accelerations directed to several different directions at given intervals using the sensor measurement devices, and for giving the values of the accelerations {right arrow over (a)}1, {right arrow over (a)}2, {right arrow over (a)}3, . . . {right arrow over (a)}n obtained as measurement results to the sensor self-testing block,
        the sensor self-testing block comprises means for giving the multipliers k1, k2, k3, . . . kn to the accelerations {right arrow over (a)}1, {right arrow over (a)}2, {right arrow over (a)}3, . . . {right arrow over (a)}n obtained as measurement results of the sensor measurement devices such that, when the sensor is functioning perfectly, the value of the expression k1{right arrow over (a)}1+k2{right arrow over (a)}2+k3{right arrow over (a)}3+ . . . +kn{right arrow over (a)}n calculates to 0,
        sensor self-testing block further comprises means for calculating the value of the expression k1{right arrow over (a)}1+k2{right arrow over (a)}2+k3{right arrow over (a)}3+ . . . +kn{right arrow over (a)}n to the accelerations a1, a2, a3, . . . {right arrow over (a)}n obtained as measurement results of the sensor measurement devices and measured at given intervals, and means for giving the value of said expression to the sensor comparison block,
        the sensor comparison block comprises means for comparing the value of said expression k1{right arrow over (a)}1+k2{right arrow over (a)}2+k3{right arrow over (a)}3+ . . . +kn{right arrow over (a)}n to a given pre-defined threshold value, and
        the sensor comparison block further comprises means for giving an error message to the alarm block, if the value of said expression deviates from the value {right arrow over (0)} more than said pre-defined threshold value.
    
    


Preferably, said four measurement directions of the sensor measurement devices described by said four vectors are selected so that the earth gravitational force has an essential component affecting in each of the said four measurement directions.
Preferably, the measurement directions of the sensor measurement devices measuring in different measurement directions are chosen symmetrically such that the accelerations obtained as measurement results of the sensor measurement devices measuring in four different directions can be presented as equations:

{right arrow over (a)}1=c·{right arrow over (a)}x+c·{right arrow over (a)}z 

{right arrow over (a)}2=c·{right arrow over (a)}y+c·{right arrow over (a)}z 

{right arrow over (a)}3=−c·{right arrow over (a)}x+c·{right arrow over (a)}z 

{right arrow over (a)}4=−c·{right arrow over (a)}yc·{right arrow over (a)}z,

and that to the accelerations {right arrow over (a)}1, {right arrow over (a)}2, {right arrow over (a)}3 and {right arrow over (a)}4 obtained as measurement results of the sensor measurement devices are given the multipliers k1, k2, k3 and k4 such that k1=k3=1 and k2=k4=−1.

Preferably, the calculation of said vector expressions is implemented using scalar values. Preferably, an error message is given when m number of deviations of the value of said expression from the value {right arrow over (0)} more than said pre-defined threshold value have occurred.
Preferably, the sensor comparison block is connected to the sensor self-testing block. Further preferably, the sensor measurement block is connected to the sensor self-testing block. Further preferably, the sensor alarm block is connected to the sensor comparison block.
According to the eighth feature of the invention, a micromechanical acceleration sensor is provided for the measurement of acceleration using the values of acceleration on the sensor measurement area, directed at the sensor, to the direction of three dimensions, which acceleration sensor comprises at least four units of sensor measurement devices measuring in different measurement directions, of which four vectors describing the measurement directions of the sensor measurement devices no three vectors belong to the same level such that the acceleration sensor comprises means for implementing continuous self-testing during start-up and/or operation, these means comprising a sensor measurement block, a sensor self-testing block, a sensor comparison block and an alarm block such that

    
    
        the sensor measurement block comprises means for measuring accelerations directed to several different directions at given intervals using the sensor measurement devices, and for giving scalar values a1, a2, a3, . . . an of the accelerations {right arrow over (a)}1, {right arrow over (a)}2, {right arrow over (a)}3, . . . {right arrow over (a)}n obtained as measurement results to the sensor self-testing block,
        the sensor self-testing block comprises means for giving the multipliers k1, k2, k3, . . . kn to the scalar values a1, a2, a3, . . . an of the accelerations {right arrow over (a)}1, {right arrow over (a)}2, {right arrow over (a)}3, . . . {right arrow over (a)}n obtained as measurement results of the sensor measurement devices such that, when the sensor is functioning perfectly, the value of the expression k1a1+k2a2+k3a3+ . . . +knan calculates to 0,
        the sensor self-testing block further comprises means for calculating the value of the expression k1a1+k2a2+k3a3+ . . . +knan to the accelerations {right arrow over (a)}1, {right arrow over (a)}2, {right arrow over (a)}3, . . . {right arrow over (a)}n obtained as measurement results of the sensor measurement devices and measured at given intervals, and means for giving the value of said expression to the sensor comparison block,
        the sensor comparison block comprises means for comparing the value of said expression k1a1+k2a2+k3a3+ . . . +knan to a given pre-defined threshold value, and
        the sensor comparison block further comprises means for giving an error message to the alarm block, if the value of said expression deviates from the value 0 more than said pre-defined threshold value.
    
    


Preferably, said four measurement directions of the sensor measurement devices described by said four vectors are selected so that the earth gravitational force has an essential component affecting in each of the said four measurement directions.
Preferably, an error message is given when m number of deviations of the value of said expression from the value 0 more than said pre-defined threshold value have occurred.
Preferably, the sensor comparison block is connected to the sensor self-testing block. Further preferably, the sensor measurement block is connected to the sensor self-testing block. Further preferably, the sensor alarm block is connected to the sensor comparison block.
According to the ninth feature of the invention, a micromechanical acceleration sensor is provided for the measurement of acceleration using the values of acceleration on the sensor measurement area, directed at the sensor, to the direction of three dimensions, which acceleration sensor comprises at least four units of sensor measurement devices measuring in different measurement directions, of which four vectors describing the measurement directions of the sensor measurement devices no three vectors belong to the same level such that the acceleration sensor comprises means for implementing continuous self-testing during start-up and/or operation, these means comprising a sensor measurement block, a sensor self-testing block, a sensor comparison block and an alarm block such that

    
    
        the sensor measurement block comprises means for measuring accelerations directed to several different directions at given intervals using the sensor measurement devices, and for giving the signal levels s1, s2, s3, . . . sn corresponding to the scalar values a1, a2, a3, . . . an of acceleration obtained as measurement results to the sensor self-testing block,
        the sensor self-testing block comprises means for giving the multipliers k1, k2, k3, . . . kn to the signal levels s1, s2, . . . s3, . . . sn corresponding to the scalar values a1, a2, a3, . . . an of acceleration obtained as measurement results of the sensor measurement devices such that, when the sensor is functioning perfectly, the value of the summation expression
    
    






  
    
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calculates to 0,

    
    
        the sensor self-testing block further comprises means for calculating the value of the summation expression
    
    






  
    
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to the signal levels s1, s2, s3, . . . sn corresponding to the scalar values a1, a2, a3, . . . an of acceleration obtained as measurement results of the sensor measurement devices and measured at given intervals, and means for giving the value of said expression to the sensor comparison block,

    
    
        the sensor comparison block comprises means for comparing the value of said summation expression
    
    






  
    
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to a given pre-defined threshold value, and

    
    
        the sensor comparison block further comprises means for giving an error message to the alarm block, if the value of said expression deviates from the value 0 more than said pre-defined threshold value.
    
    


Preferably, said four measurement directions of the sensor measurement devices described by said four vectors are selected so that the earth gravitational force has an essential component affecting in each of the said four measurement directions.
Preferably, an error message is given when m number of deviations of the value of said expression from the value 0 more than said pre-defined threshold value have occurred.
Preferably, the sensor comparison block is connected to the sensor self-testing block. Further preferably, the sensor measurement block is connected to the sensor self-testing block. Further preferably, the sensor alarm block is connected to the sensor comparison block.
According to the tenth feature of the invention, a micromechanical acceleration sensor is provided for the measurement of acceleration using the values of acceleration on the sensor measurement area, directed at the sensor, to the direction of two dimensions, which acceleration sensor comprises at least three units of sensor measurement devices measuring in different measurement directions, of which three vectors describing the measurement directions of the sensor measurement devices no two vectors are in the same direction such that the acceleration sensor comprises means for implementing continuous self-testing during start-up and/or operation, these means comprising a sensor measurement block, a sensor self-testing block, a sensor comparison block and an alarm block such that

    
    
        the sensor measurement block comprises means for measuring accelerations directed to several different directions at given intervals using the sensor measurement devices, and for giving the values of the accelerations {right arrow over (a)}1, {right arrow over (a)}2, . . . {right arrow over (a)}n obtained as measurement results to the sensor self-testing block,
        the sensor self-testing block comprises means for giving the multipliers k1, k2, . . . . kn to the accelerations {right arrow over (a)}1, {right arrow over (a)}2, . . . {right arrow over (a)}n obtained as measurement results of the sensor measurement devices such that, when the sensor is functioning perfectly, the value of the expression k1{right arrow over (a)}1+k2{right arrow over (a)}2+ . . . +kn{right arrow over (a)}n calculates to 0,
        the sensor self-testing block further comprises means for calculating the value of the expression k1{right arrow over (a)}1+k2{right arrow over (a)}2+ . . . +kn{right arrow over (a)}n to the accelerations a1, a2, . . . an, obtained as measurement results of the sensor measurement devices and measured at given intervals, and means for giving the value of said expression to the sensor comparison block,
        the sensor comparison block comprises means for comparing the value of said expression k1{right arrow over (a)}1+k2{right arrow over (a)}2+ . . . +kn{right arrow over (a)}n to a given pre-defined threshold value, and
        the sensor comparison block further comprises means for giving an error message to the alarm block, if the value of said expression deviates from the value {right arrow over (0)} more than said pre-defined threshold value.
    
    


Preferably, said three measurement directions of the sensor measurement devices described by said three vectors are selected so that the earth gravitational force has an essential component affecting in each of the said three measurement directions.
Preferably, the measurement directions of the sensor measurement devices measuring in different measurement directions are chosen symmetrically such that the accelerations obtained as measurement results of the sensor measurement devices measuring in four different directions can be presented as equations:

{right arrow over (a)}1=d·{right arrow over (a)}x 

{right arrow over (a)}2=d·{right arrow over (a)}y 

{right arrow over (a)}3=−d·{right arrow over (a)}x 

{right arrow over (a)}4=−d·{right arrow over (a)}y,

and that to the accelerations {right arrow over (a)}1, {right arrow over (a)}2, {right arrow over (a)}3 and {right arrow over (a)}4 obtained as measurement results of the sensor measuring devices are given the multipliers k1, k2, k3 and k4 such that k1=k2=k3=k4=1.

Preferably, the calculation of said vector expressions is implemented using scalar values. Preferably, an error message is given when m number of deviations of the value of said expression from the value {right arrow over (0)} more than said pre-defined threshold value have occurred.
Preferably, the sensor comparison block is connected to the sensor self-testing block. Further preferably, the sensor measurement block is connected to the sensor self-testing block. Further preferably, the sensor alarm block is connected to the sensor comparison block.
According to the eleventh feature of the invention, a micromechanical acceleration sensor is provided for the measurement of acceleration using the values of acceleration on the sensor measurement area, directed at the sensor, to the direction of two dimensions, which acceleration sensor comprises at least three units of sensor measurement devices measuring in different measurement directions, of which three vectors describing the measurement directions of the sensor measurement devices no two vectors are in the same direction such that the acceleration sensor comprises means for implementing continuous self-testing during start-up and/or operation, these means comprising a sensor measurement block, a sensor self-testing block, a sensor comparison block and an alarm block such that

    
    
        the sensor measurement block comprises means for measuring accelerations directed to several different directions at given intervals using the sensor measurement devices, and for giving the scalar values a1, a2, . . . an of the accelerations {right arrow over (a)}1, {right arrow over (a)}2, . . . {right arrow over (a)}n obtained as measurement results to the sensor self-testing block,
        the sensor self-testing block comprises means for giving the multipliers k1, k2, . . . kn to the scalar values a1, a2, . . . an of the accelerations {right arrow over (a)}1, {right arrow over (a)}2, . . . {right arrow over (a)}n obtained as measurement results of the sensor measurement devices such that, when the sensor is functioning perfectly, the value of the expression k1a1+k2a2+ . . . +knan calculates to 0,
        the sensor self-testing block further comprises means for calculating the value of the expression k1a1+k2a2+ . . . +knan to the accelerations {right arrow over (a)}1, {right arrow over (a)}2, . . . {right arrow over (a)}n obtained as measurement results of the sensor measurement devices and measured at given intervals, and means for giving the value of said expression to the sensor comparison block,
        the sensor comparison block comprises means for comparing the value of said expression k1a1+k2a2+ . . . +knan to a given pre-defined threshold value, and
        the sensor comparison block further comprises means for giving an error message to the alarm block, if the value of said expression deviates from the value 0 more than said pre-defined threshold value.
    
    


Preferably, said three measurement directions of the sensor measurement devices described by said three vectors are selected so that the earth gravitational force has an essential component affecting in each of the said three measurement directions.
Preferably, an error message is given when m number of deviations of the value of said expression from the value 0 more than said pre-defined threshold value have occurred.
Preferably, the sensor comparison block is connected to the sensor self-testing block. Further preferably, the sensor measurement block is connected to the sensor self-testing block. Further preferably, the sensor alarm block is connected to the sensor comparison block.
According to the twelfth feature of the invention, a micromechanical acceleration sensor is provided for the measurement of acceleration using the values of acceleration on the sensor measurement area, directed at the sensor, to the direction of two dimensions, which acceleration sensor comprises at least three units of sensor measurement devices measuring in different measurement directions, of which three vectors describing the measurement directions of the sensor measurement devices no two vectors are in the same direction such that the acceleration sensor comprises means for implementing continuous self-testing during start-up and/or operation, these means comprising a sensor measurement block, a sensor self-testing block, a sensor comparison block and an alarm block such that

    
    
        the sensor measurement block comprises means for measuring accelerations directed to several different directions at given intervals using the sensor measurement devices, and for giving the signal levels s1, s2, . . . sn corresponding to the scalar values a1, a2, . . . an of acceleration obtained as measurement results to the sensor self-testing block, the sensor self-testing block comprises means for giving the multipliers k1, k2, . . . kn to the signal levels s1, s2, . . . sn corresponding to the scalar values a1, a2, . . . an of acceleration obtained as measurement results of the sensor measurement devices such that, when the sensor is functioning perfectly, the value of the summation expression
    
    






  
    
      ∑
      
        i
        =
        1
      
      n
    
    ⁢
    
      
    
    ⁢
    
      
        k
        i
      
      ⁢
      
        s
        i
      
    
  
  -
  
    k
    0
  




calculates to 0,

    
    
        the sensor self-testing block further comprises means for calculating the value of the summation expression
    
    






  
    
      ∑
      
        i
        =
        1
      
      n
    
    ⁢
    
      
    
    ⁢
    
      
        k
        i
      
      ⁢
      
        s
        i
      
    
  
  -
  
    k
    0
  




to the signal levels s1, s2, . . . sn corresponding to the scalar values a1, a2, . . . an of acceleration obtained as measurement results of the sensor measurement devices and measured at given intervals, and means for giving the value of said expression to the sensor comparison block,

    
    
        the sensor comparison block comprises means for comparing the value of said summation expression
    
    






  
    
      ∑
      
        i
        =
        1
      
      n
    
    ⁢
    
      
    
    ⁢
    
      
        k
        i
      
      ⁢
      
        s
        i
      
    
  
  -
  
    k
    0
  




to a given pre-defined threshold value, and

    
    
        the sensor comparison block further comprises means for giving an error message to the alarm block, if the value of said expression deviates from the value 0 more than said pre-defined threshold value.
    
    


Preferably, said three measurement directions of the sensor measurement devices described by said three vectors are selected so that the earth-gravitational force has an essential component affecting in each of the said three measurement directions.
Preferably, an error message is given when m number of deviations of the value of said expression from the value 0 more than said pre-defined threshold value have occurred.
Preferably, the sensor comparison block is connected to the sensor self-testing block. Further preferably, the sensor measurement block is connected to the sensor self-testing block. Further preferably, the sensor alarm block is connected to the sensor comparison block.



BRIEF DESCRIPTION OF THE DRAWINGS
In the following the invention and its preferred embodiments are described in detail with reference, by way of an example, to the accompanying FIG. 1, which shows a micromechanical acceleration sensor solution according to the invention.



DETAILED DESCRIPTION OF THE INVENTION
A micromechanical acceleration sensor solution according to the invention is implemented using sensor measurement devices measuring in several different directions. For measurement of acceleration directed to three dimensions there must be at least four units of these sensor measurement devices measuring in several different directions. Additionally, in this case, the measurement directions of the sensor measurement devices must be selected such that of vectors describing the measurement directions of the sensor measurement devices measuring in several different directions four vectors can be selected such that of these four vectors no three vectors belong to the same level.
One can select said four measurement directions of the sensor measurement devices described by said four vectors so that the earth gravitational force has an essential component affecting in each of the said four measurement directions. This means that none of the selected four measurement directions are perpendicular to or almost perpendicular to the earth gravitational force. As the earth gravitational force affects in each of the said four measurement directions, this increases the micromechanical acceleration sensor's capability to detect errors.
In conventional Cartesian rectangular space coordinates the unit vectors in x-, y- and z-directions are {right arrow over (i)}, {right arrow over (j)} and {right arrow over (k)}. The accelerations {right arrow over (a)}i obtained in the selected directions of the sensor measurement devices can be divided into components in x-, y- and z-directions {right arrow over (a)}i={right arrow over (a)}xi+{right arrow over (a)}yi+{right arrow over (a)}zi or alternatively using unit vectors in x-, y- and z-directions {right arrow over (i)}, {right arrow over (j)} and {right arrow over (k)} described as {right arrow over (a)}i=axi{right arrow over (i)}+ayi{right arrow over (j)}+azi{right arrow over (k)}. As the angles between the selected directions of measurement and x-, y- and z-directions can be selected αi, βi and χi, wherein αi describes the angle between {right arrow over (a)}i and {right arrow over (a)}xi, and correspondingly, βi describes the angle between {right arrow over (a)}i and {right arrow over (a)}yi, and further, χi describes the angle between {right arrow over (a)}i and {right arrow over (a)}zi. Correspondingly, ai describes the scalar reading of acceleration of the acceleration vector {right arrow over (a)}i in the direction of measurement.
In the method according to the invention for the measurement of acceleration directed to three dimensions using a micromechanical acceleration sensor, the accelerations obtained as measurement results of the sensor measurement devices measuring in several different directions can, when divided into components in x-, y- and z-directions, be described as equations:

{right arrow over (a)}1=a1 cos α1·{right arrow over (i)}+a1 cos β1{right arrow over (j)}+a1 cos χ1·{right arrow over (k)}

{right arrow over (a)}2=a2 cos α2·{right arrow over (i)}+a2 cos β2{right arrow over (j)}+a2 cos χ2·{right arrow over (k)}

{right arrow over (a)}3=a3 cos α3·{right arrow over (i)}+a3 cos β3{right arrow over (j)}+a3 cos χ3·{right arrow over (k)}

. . .

{right arrow over (a)}n=an cos αn·{right arrow over (i)}+an cos βn{right arrow over (j)}+an cos χn·{right arrow over (k)}

Because the measurement directions of the sensor measurement devices are selected in advance, the values of the constants of the equation set cos α1, cos α2, cos α3, . . . cos αn, cos β1, cos β2, cos β3, . . . cos βn and cos χi, cos χ2, cos χ3, . . . cos χn are also known already prior to measurement. Additionally, for the directional cosines applies: (cos αi)2+(cos βi)2+(cos χi)2=1.
The vector {right arrow over (a)}i can also be expressed in the form {right arrow over (a)}i=ai·{right arrow over (u)}i, where ai is the scalar value of the vector {right arrow over (a)}i, and correspondingly, {right arrow over (u)}i is the unit vector in the direction of measurement, that is stated using the directional cosines as {right arrow over (u)}i=cos αi·{right arrow over (i)}+cos βi·{right arrow over (j)}+cos χi·{right arrow over (k)}.
When acceleration {right arrow over (a)}=ax{right arrow over (i)}+ay{right arrow over (j)}+az{right arrow over (k)} affects the sensor, the scalar value ai of the measurement result vector {right arrow over (a)}i can be calculated as a dot product of the acceleration vector a affecting the sensor and the unit vector {right arrow over (u)}i in the direction of measurement:

ai={right arrow over (a)}·{right arrow over (u)}i=ax cos αi+ay cos βi+az cos χi.

Correspondingly, the measurement result vector {right arrow over (a)}i can be calculated as a projection of the acceleration vector a affecting the sensor in the direction of measurement:

{right arrow over (a)}i=({right arrow over (a)}·{right arrow over (u)}i)·{right arrow over (u)}i=(ax cos αi+ay cos βi+az cos χi)·(cos αi·{right arrow over (i)}+cos βi·{right arrow over (j)}+cos χi·{right arrow over (k)}).

In the method according to the invention, the continuous self-testing of the acceleration sensor during start-up and/or operation is implemented such that to the accelerations {right arrow over (a)}1, {right arrow over (a)}2, {right arrow over (a)}3, . . . {right arrow over (a)}n obtained as measurement results of the sensor measurement devices are given the multipliers k1, k2, k3, . . . kn such that, when the sensor is functioning perfectly, the value of the expression k1{right arrow over (a)}1+k2{right arrow over (a)}2+k3{right arrow over (a)}3+ . . . +kn{right arrow over (a)}n calculates to 0.
In the method according to the invention, the accelerations {right arrow over (a)}1, {right arrow over (a)}2, {right arrow over (a)}3, . . . {right arrow over (a)}n given by the sensor measurement devices are measured at given intervals, after which the value of the expression k1{right arrow over (a)}1+k2{right arrow over (a)}2+k3{right arrow over (a)}3+ . . . +kn{right arrow over (a)}n is calculated. The value of the expression is compared to said given pre-defined threshold value, and if the value of said expression deviates from the value {right arrow over (0)} more than said pre-defined threshold value, an error message is given. Typically, an error message is given when m number of deviations of the value of said expression from the value {right arrow over (0)} more than said pre-defined threshold value have occurred.
In the method according to the invention, the calculation of the vector equations and vector expressions of acceleration can in practise be implemented using scalar values. In place of the equation k1{right arrow over (a)}1+k2{right arrow over (a)}2+k3{right arrow over (a)}3+ . . . +kn{right arrow over (a)}n=0 can be written the equation k1a1+k2a2+k3a3+ . . . +knan=0. If into this is inserted projections of the dominant acceleration vector {right arrow over (a)}=ax{right arrow over (i)}+ay{right arrow over (j)}+az{right arrow over (k)} in each measurement direction, the equation obtained is:




  
    
      ∑
      
        i
        =
        1
      
      n
    
    ⁢
    
      
        k
        i
      
      ⁢
      
        a
        i
      
    
  
  =
  
    
      
        ∑
        
          i
          =
          1
        
        n
      
      ⁢
      
        
          k
          i
        
        ⁡
        
          (
          
            
              
                a
                x
              
              ⁢
              cos
              ⁢
              
                
              
              ⁢
              
                α
                i
              
            
            +
            
              
                a
                y
              
              ⁢
              cos
              ⁢
              
                
              
              ⁢
              
                β
                i
              
            
            +
            
              
                a
                z
              
              ⁢
              cos
              ⁢
              
                
              
              ⁢
              
                χ
                i
              
            
          
          )
        
      
    
    =
    0
  




This has to be implemented using the value of any dominant acceleration {right arrow over (a)}, from which follows an equation set for solving the multipliers ki



  
    
      
        ∑
        
          i
          =
          1
        
        n
      
      ⁢
      
        
          k
          i
        
        ⁢
        cos
        ⁢
        
          
        
        ⁢
        
          α
          i
        
      
    
    =
    0
  




  
    
      
        ∑
        
          i
          =
          1
        
        n
      
      ⁢
      
        
          k
          i
        
        ⁢
        cos
        ⁢
        
          
        
        ⁢
        
          β
          i
        
      
    
    =
    0
  




  
    
      
        ∑
        
          i
          =
          1
        
        n
      
      ⁢
      
        
          k
          i
        
        ⁢
        cos
        ⁢
        
          
        
        ⁢
        
          χ
          i
        
      
    
    =
    0.
  



Multipliers ki can be found, if the directions of measurement are selected to be adequately different in direction. Using multipliers found in this way, in the method according to the invention, the value of the expression k1a1+k2a2+k3a3+ . . . +knan can be calculated and compared to a pre-set threshold value.
Typically, in the linear or linearized measurement of acceleration, acceleration is measured as a linear signal value, wherein the scalar value ai of each acceleration can be calculated from the signal level in the following manner:

ai=bi·(si−s0i).

In the method according to the invention, in place of the equation based on acceleration results k1a1+k2a2+k3a3+ . . . +knan=0 an equation based on signal values can be used:

k1s1+k2s2+k3s3+ . . . +knsn−k0=0.

If into this is inserted the signal values




  
    s
    i
  
  =
  
    
      
        a
        i
      
      
        b
        i
      
    
    +
    
      s
      
        0
        ⁢
        i
      
    
  




corresponding to the acceleration measurement results from the previously solved equation as well as the scalar values ai of the measurement result vector {right arrow over (a)}i of the dominant acceleration vector {right arrow over (a)}=ax{right arrow over (i)}+ay{right arrow over (j)}+az{right arrow over (k)} in each measurement direction, the equation obtained is:





  
    
      
        ∑
        
          i
          =
          1
        
        n
      
      ⁢
      
        
          k
          i
        
        ⁢
        
          s
          i
        
      
    
    -
    
      k
      0
    
  
  =
  
    
      
        
          ∑
          
            i
            =
            1
          
          n
        
        ⁢
        
          
            
              k
              i
            
            
              b
              i
            
          
          ⁢
          
            (
            
              
                
                  a
                  x
                
                ⁢
                cos
                ⁢
                
                  
                
                ⁢
                
                  α
                  i
                
              
              +
              
                
                  a
                  y
                
                ⁢
                cos
                ⁢
                
                  
                
                ⁢
                
                  β
                  i
                
              
              +
              
                
                  a
                  z
                
                ⁢
                cos
                ⁢
                
                  
                
                ⁢
                
                  χ
                  i
                
              
            
            )
          
        
      
      +
      
        
          ∑
          
            i
            =
            1
          
          n
        
        ⁢
        
          
            k
            i
          
          ⁢
          
            s
            
              0
              ⁢
              i
            
          
        
      
      -
      
        k
        0
      
    
    =
    0.
  




This has to be implemented using the value of any dominant acceleration {right arrow over (a)}, from which follows an equation set for solving the multipliers ki



  
    
      
        ∑
        
          i
          =
          1
        
        n
      
      ⁢
      
        
          
            k
            i
          
          
            b
            i
          
        
        ⁢
        cos
        ⁢
        
          
        
        ⁢
        
          α
          i
        
      
    
    =
    
      
        
          ∑
          
            i
            =
            1
          
          n
        
        ⁢
        
          
            k
            i
          
          ⁢
          
            h
            ix
          
        
      
      =
      0
    
  




  
    
      
        ∑
        
          i
          =
          1
        
        n
      
      ⁢
      
        
          
            k
            i
          
          
            b
            i
          
        
        ⁢
        cos
        ⁢
        
          
        
        ⁢
        
          β
          i
        
      
    
    =
    
      
        
          ∑
          
            i
            =
            1
          
          n
        
        ⁢
        
          
            k
            i
          
          ⁢
          
            h
            iy
          
        
      
      =
      0
    
  




  
    
      
        ∑
        
          i
          =
          1
        
        n
      
      ⁢
      
        
          
            k
            i
          
          
            b
            i
          
        
        ⁢
        cos
        ⁢
        
          
        
        ⁢
        
          χ
          i
        
      
    
    =
    
      
        
          ∑
          
            i
            =
            1
          
          n
        
        ⁢
        
          
            k
            i
          
          ⁢
          
            h
            iz
          
        
      
      =
      0
    
  




  
    
      
        ∑
        
          i
          =
          1
        
        n
      
      ⁢
      
        
          k
          i
        
        ⁢
        
          s
          
            0
            ⁢
            i
          
        
      
    
    =
    
      
        k
        0
      
      .
    
  



The constant k0 describes the offset constant of the entire sensor arrangement, which can be used to allow consideration, for example, for a calibration error of the zero point of the sensor as well as influences on the measuring event caused by variations in humidity or temperature. Constants hix, hiyand hiz describe the sensitivities of the measurement signal si of the sensor in x-, y- and z-directions.
In an alternate method according to the invention, the continuous self-testing of the acceleration sensor during start-up and/or operation is implemented in the linear or linearized measurement such that when the x-, y-, and z-directional signal measurement sensitivities hix, hiy and hiz of the sensor measurement devices and the offset constant k0 are known in advance, to the signal levels s1, s2, s3, . . . sn corresponding to the scalar values a1, a2, a3, . . . an obtained as measurement results of the sensor measurement devices are given the multipliers k1, k2, k3, . . . kn such that, when the sensor is functioning perfectly, the value of the summation equation




  
    
      ∑
      
        i
        =
        1
      
      n
    
    ⁢
    
      
        k
        i
      
      ⁢
      
        s
        i
      
    
  
  -
  
    k
    0
  




calculates to 0.

In the method according to the invention, the signal levels s1, s2, s3, . . . sn given by the sensor measurement devices are measured at given intervals, after which the value of the summation equation




  
    
      ∑
      
        i
        =
        1
      
      n
    
    ⁢
    
      
        k
        i
      
      ⁢
      
        s
        i
      
    
  
  -
  
    k
    0
  




is calculated. The value of the summation equation is compared to said given pre-defined threshold value, and if the value of said equation deviates from the value 0 more than the pre-defined threshold value, an error message is given. Typically, an error message is given when m number of deviations of the value of said summation expression from the value 0 more than said pre-defined threshold value have occurred.

Correspondingly, in the measurement of acceleration directed to two dimensions there must be at least three units of these sensor measurement devices measuring in several different directions. Additionally, in this case, the measurement directions of the sensor measurement devices must be selected such that of vectors describing the measurement directions of the sensor measurement devices measuring in several different directions three vectors can be selected such that of these three vectors no two vectors are in the same direction.
One can select said three measurement directions of the sensor measurement devices described by said three vectors so that the earth gravitational force has an essential component affecting in each of the said three measurement directions. This means that none of the selected three measurement directions are perpendicular to or almost perpendicular to the earth gravitational force. As the earth gravitational force affects in each of the said three measurement directions, this increases the micromechanical acceleration sensor's capability to detect errors.
In conventional Cartesian rectangular plane coordinates the unit vectors in x- and y-directions are {right arrow over (i)} and {right arrow over (j)}. The accelerations {right arrow over (a)}i obtained in the selected directions of the sensor measurement devices can be divided into components in x- and y-directions {right arrow over (a)}i={right arrow over (a)}xi+{right arrow over (a)}yi or alternatively using unit vectors in x- and y-directions {right arrow over (i)} and {right arrow over (j)} described as {right arrow over (a)}i=axi{right arrow over (i)}+ayi{right arrow over (j)}. As the angles between the selected directions of measurement and the x- and y-directions can be selected αi and βi, wherein αi describes the angle between {right arrow over (a)}i and {right arrow over (a)}xi, and correspondingly, βi describes the angle between {right arrow over (a)}1 and {right arrow over (a)}yi. Correspondingly, {right arrow over (a)}i describes the scalar reading of acceleration of the acceleration vector {right arrow over (a)}i in the direction of measurement.
In the method according to the invention for the measurement of acceleration directed to two dimensions using a micromechanical acceleration sensor, the accelerations obtained as measurement results of the sensor measurement devices measuring in several different directions can, when divided into components in x- and y-directions, be described as equations:

{right arrow over (a)}1=a1 cos α1·{right arrow over (i)}+a1 cos β1·{right arrow over (j)}

{right arrow over (a)}2=a2 cos α2·{right arrow over (i)}+a2 cos β2·{right arrow over (j)}

. . .

{right arrow over (a)}n=an cos αn·{right arrow over (i)}+an cos βn·{right arrow over (j)}. 

Because the measurement directions of the sensor measurement devices are selected in advance, the values of the constants of the equation set cos α1, cos α2, cos α3, . . . cos αn and cos β1, cos β2, cos β3, . . . cos βn are also known already prior to measurement. Additionally, for directional cosines applies: (cos αi)2+(cos βi)2=1.
The vector {right arrow over (a)}i can also be expressed in the form {right arrow over (a)}i=ai·{right arrow over (u)}i, where ai is the scalar value of the vector {right arrow over (a)}1, and correspondingly, u, is the unit vector in the direction of measurement, that is stated using the directional cosines as {right arrow over (u)}i=cos αi·{right arrow over (i)}+cos βi·{right arrow over (j)}.
When acceleration {right arrow over (a)}=ax{right arrow over (i)}+ay{right arrow over (j)} affects the sensor, the scalar value ai of the measurement result vector {right arrow over (a)}i can be calculated as a dot product of the acceleration vector {right arrow over (a)} affecting the sensor and the unit vector {right arrow over (u)}i in the direction of measurement:

ai={right arrow over (a)}·{right arrow over (u)}i=ax cos αi+ay cos βi.

Correspondingly, the measurement result vector {right arrow over (a)}i can be calculated as a projection of the acceleration vector {right arrow over (a)} affecting the sensor in the direction of measurement:

{right arrow over (a)}i=({right arrow over (a)}·{right arrow over (u)})·{right arrow over (u)}i=(ax cos αi+ay cos βi)·(cos αi·{right arrow over (i)}+cos βi·{right arrow over (j)}).

In the method according to the invention, the continuous self-testing of the acceleration sensor during start-up and/or operation is implemented such that to the accelerations {right arrow over (a)}1, {right arrow over (a)}2, . . . . {right arrow over (a)}n obtained as measurement results of the sensor measurement devices are given the multipliers k1, k2, k3, . . . kn such that, when the sensor is functioning perfectly, the value of the expression k1{right arrow over (a)}1+k2{right arrow over (a)}2+ . . . +kn{right arrow over (a)}n calculates to {right arrow over (0)}.
In the method according to the invention, the accelerations {right arrow over (a)}1, {right arrow over (a)}2, . . . {right arrow over (a)}n, given by the sensor measurement devices are measured at given intervals, after which the value of the expression k1{right arrow over (a)}1+k2{right arrow over (a)}2+ . . . +kn{right arrow over (a)}n is calculated. The value of the expression is compared to said given pre-defined threshold value, and if the value of said expression deviates from the value {right arrow over (0)} more than said pre-defined threshold value, an error message is given. Typically, an error message is given when m number of deviations of the value of said expression from the value {right arrow over (0)} more than said pre-defined threshold value have occurred.
In the method according to the invention, the calculation of the vector equations and vector expressions of acceleration can in practise be implemented using scalar values. In place of the equation k1{right arrow over (a)}1+k2{right arrow over (a)}2+ . . . +kn{right arrow over (a)}k={right arrow over (0)} can be written the equation k1a1+k2a2+ . . . +knan=0. If into this is inserted projections of the dominant acceleration vector {right arrow over (a)}=ax{right arrow over (i)}+ay{right arrow over (j)} in each measurement direction, the equation obtained is:




  
    
      ∑
      
        i
        =
        1
      
      n
    
    ⁢
    
      
        k
        i
      
      ⁢
      
        a
        i
      
    
  
  =
  
    
      
        ∑
        
          i
          =
          1
        
        n
      
      ⁢
      
        
          k
          i
        
        ⁡
        
          (
          
            
              
                a
                x
              
              ⁢
              cos
              ⁢
              
                
              
              ⁢
              
                α
                i
              
            
            +
            
              
                a
                y
              
              ⁢
              cos
              ⁢
              
                
              
              ⁢
              
                β
                i
              
            
          
          )
        
      
    
    =
    0.
  




This has to be implemented using the value of any dominant acceleration {right arrow over (a)}, from which follows the equation set for solving the multipliers ki



  
    
      
        ∑
        
          i
          =
          1
        
        n
      
      ⁢
      
        
          k
          i
        
        ⁢
        cos
        ⁢
        
          
        
        ⁢
        
          α
          i
        
      
    
    =
    0
  




  
    
      
        ∑
        
          i
          =
          1
        
        n
      
      ⁢
      
        
          k
          i
        
        ⁢
        cos
        ⁢
        
          
        
        ⁢
        
          β
          i
        
      
    
    =
    0.
  



Multipliers ki can be found, if the directions of measurement are selected to be adequately different in direction. Using multipliers found in this way, in the method according to the invention, the value of the expression k1a1+k2a2+ . . . +knan can be calculated and compared to a pre-set threshold value.
Typically, in the linear or linearized measurement of acceleration, acceleration is measured as a linear signal value, wherein the scalar value ai of each acceleration can be calculated from the signal level in the following manner:

ai=bi·(si−s0i)

In the method according to the invention, in place of the equation based on acceleration results k1a1+k2a2+ . . . +knan=0 an equation based on signal values can be used:

k1s1+k2s2+ . . . +knsn−k0=0.

If into this is inserted the signal values




  
    s
    i
  
  =
  
    
      
        a
        i
      
      
        b
        i
      
    
    +
    
      s
      
        0
        ⁢
        i
      
    
  




corresponding to the acceleration measurement results from the previously solved equation as well as the scalar values ai of the measurement result vector {right arrow over (a)}i of the dominant acceleration vector {right arrow over (a)}=ax{right arrow over (i)}+ay{right arrow over (j)} each measurement direction, the equation obtained is:





  
    
      
        ∑
        
          i
          =
          1
        
        n
      
      ⁢
      
        
          k
          i
        
        ⁢
        
          s
          i
        
      
    
    -
    
      k
      0
    
  
  =
  
    
      
        
          ∑
          
            i
            =
            1
          
          n
        
        ⁢
        
          
            
              k
              i
            
            
              b
              i
            
          
          ⁢
          
            (
            
              
                
                  a
                  x
                
                ⁢
                cos
                ⁢
                
                  
                
                ⁢
                
                  α
                  i
                
              
              +
              
                
                  a
                  y
                
                ⁢
                cos
                ⁢
                
                  
                
                ⁢
                
                  β
                  i
                
              
            
            )
          
        
      
      +
      
        
          ∑
          
            i
            =
            1
          
          n
        
        ⁢
        
          
            k
            i
          
          ⁢
          
            s
            
              0
              ⁢
              i
            
          
        
      
      -
      
        k
        0
      
    
    =
    0.
  




This has to be implemented using the value of any dominant acceleration {right arrow over (a)}, from which follows an equation set for solving the multipliers ki



  
    
      
        ∑
        
          i
          =
          1
        
        n
      
      ⁢
      
        
          
            k
            i
          
          
            b
            i
          
        
        ⁢
        cos
        ⁢
        
          
        
        ⁢
        
          α
          i
        
      
    
    =
    
      
        
          ∑
          
            i
            =
            1
          
          n
        
        ⁢
        
          
            k
            i
          
          ⁢
          
            h
            ix
          
        
      
      =
      0
    
  




  
    
      
        ∑
        
          i
          =
          1
        
        n
      
      ⁢
      
        
          
            k
            i
          
          
            b
            i
          
        
        ⁢
        cos
        ⁢
        
          
        
        ⁢
        
          β
          i
        
      
    
    =
    
      
        
          ∑
          
            i
            =
            1
          
          n
        
        ⁢
        
          
            k
            i
          
          ⁢
          
            h
            iy
          
        
      
      =
      0
    
  




  
    
      
        ∑
        
          i
          =
          1
        
        n
      
      ⁢
      
        
          k
          i
        
        ⁢
        
          s
          
            0
            ⁢
            i
          
        
      
    
    =
    
      
        k
        0
      
      .
    
  



The constant k0 describes the offset constant of the entire sensor arrangement, which can be used to allow consideration, for example, for a calibration error of the zero point of the sensor as well as influences on the measuring event caused by variations in humidity or temperature. Constants hix and hiy describe the sensitivities of the measurement signal si of the sensor in x- and y-directions.
In an alternate method according to the invention, the continuous self-testing of the acceleration sensor during start-up and/or operation is implemented in the linear or linearized measurement such that when the x- and y-directional signal measurement sensitivities hix and hiy of the sensor measurement devices and the offset constant k0 are known in advance, to the signal levels s1, s2, . . . sn corresponding to the scalar values a1, a2, . . . an obtained as measurement results of the sensor measurement devices are given the multipliers k1, k2, . . . kn such that, when the sensor is functioning perfectly, the value of the summation equation




  
    
      ∑
      
        i
        =
        1
      
      n
    
    ⁢
    
      
        k
        i
      
      ⁢
      
        s
        i
      
    
  
  -
  
    k
    0
  




calculates to 0.

In the method according to the invention, the signal levels s1, s2, . . . sn given by the sensor measurement devices are measured at given intervals, after which the value of the summation equation




  
    
      ∑
      
        i
        =
        1
      
      n
    
    ⁢
    
      
        k
        i
      
      ⁢
      
        s
        i
      
    
  
  -
  
    k
    0
  




is calculated. The value of the summation equation is compared to said given pre-defined threshold value, and if the value of said equation deviates from the value 0 more than the pre-defined threshold value, an error message is given. Typically, an error message is given when m number of deviations of the value of said summation expression from the value 0 more than said pre-defined threshold value have occurred.

FIG. 1 on shows a micromechanical acceleration sensor solution according to the invention. A micromechanical acceleration sensor solution according to the invention comprises a sensor measurement block 1, a sensor self-testing block 2, a sensor comparison block 3 and an alarm block 4.
A micromechanical acceleration sensor solution according to the invention comprises means 1-4 for implementing continuous self-testing during start-up and/or operation such that first, in the sensor measurement block 1, the accelerations {right arrow over (a)}1, {right arrow over (a)}2, {right arrow over (a)}3, . . . an are obtained as measurement results of the sensor measurement devices measuring in three directions. Next, in the sensor self-testing block 2, the multipliers k1, k2, k3, . . . kn are given to the accelerations {right arrow over (a)}1, {right arrow over (a)}2, {right arrow over (a)}3, . . . {right arrow over (a)}n obtained as measurement results of the sensor measurement devices such that, when the sensor is functioning perfectly, the value of the expression k1{right arrow over (a)}1+k2{right arrow over (a)}2+k3{right arrow over (a)}3+ . . . +kn{right arrow over (a)}n calculates to 0.
In a micromechanical acceleration sensor solution according to the invention, the accelerations {right arrow over (a)}1, {right arrow over (a)}2, {right arrow over (a)}3, . . . {right arrow over (a)}n given by the sensor measurement devices are measured in the sensor measurement block 1 at given intervals, after which the value of the expression k1{right arrow over (a)}1+k2{right arrow over (a)}2+k3{right arrow over (a)}3+ . . . +kn{right arrow over (a)}n is calculated in the sensor self-testing block 2. Further, in the sensor comparison block 3, the value of the expression is compared to said given pre-defined threshold value, and if the value of said expression deviates from the value {right arrow over (0)} more than said pre-defined threshold value, an error message is given to the sensor alarm block 4.
In solution according to the invention, the sensor comparison block 3 can be connected to the sensor self-testing block 2. Further, the sensor measurement block 1 can be connected to the sensor self-testing block 2. Further, the sensor alarm block 4 can be connected to the sensor comparison block 3.
In a comparable manner, an alternative solution according to the invention comprises means 1-4 for implementing continuous self-testing during start-up and/or operation such that first, in the sensor measurement block 1, the accelerations {right arrow over (a)}1, {right arrow over (a)}2, . . . {right arrow over (a)}n are obtained as measurement results of the sensor measurement devices measuring in different measurement directions. Next, in the sensor self-testing block 2, the multipliers k1, k2, . . . kn are given to the accelerations {right arrow over (a)}1, {right arrow over (a)}2, . . . {right arrow over (a)}n obtained as measurement results of the sensor measurement devices such that, when the sensor is functioning perfectly, the value of the expression k1{right arrow over (a)}1+k2{right arrow over (a)}2+ . . . +kn{right arrow over (a)}n calculates to {right arrow over (0)}.
In an alternative micromechanical acceleration sensor solution according to the invention, the accelerations {right arrow over (a)}1, {right arrow over (a)}2, . . . {right arrow over (a)}n given by the sensor measurement devices are measured in the sensor measurement block 1 at given intervals, after which the value of the expression k1{right arrow over (a)}1+k2{right arrow over (a)}2+ . . . +kn{right arrow over (a)}n is calculated in the sensor self-testing block 2. Further, in the sensor comparison block 3, the value of the expression is compared to said given pre-defined threshold value, and if the value of said expression deviates from the value {right arrow over (0)} more than said pre-defined threshold value, an error message is given to the alarm block 4.
Also in an alternative solution according to the invention, the sensor comparison block 3 can be connected to the sensor self-testing block 2. Further, the sensor measurement block 1 can be connected to the sensor self-testing block 2. Further, the sensor alarm block 4 can be connected to the sensor comparison block 3.
In the method according to the invention for the measurement of acceleration directed to three dimensions using a micromechanical acceleration sensor, the measurement directions of the sensor measurement devices measuring in different measurement directions can, for example, be selected symmetrically such that the accelerations obtained as measurement results of sensor measurement devices measuring in four different directions can be presented as equations:

{right arrow over (a)}1=c·{right arrow over (a)}x+c·{right arrow over (a)}z 

{right arrow over (a)}2=c·{right arrow over (a)}y+c·{right arrow over (a)}z 

{right arrow over (a)}3=−c·{right arrow over (a)}x+c·{right arrow over (a)}z.

{right arrow over (a)}4=−c·{right arrow over (a)}y+c·{right arrow over (a)}z.

In this case, the continuous self-testing of the acceleration sensor during start-up and/or operation can be implemented such that to the accelerations {right arrow over (a)}1, {right arrow over (a)}2, {right arrow over (a)}3 and {right arrow over (a)}4 obtained as measurement results of the sensor measurement devices are given the multipliers k1, k2, k3 and k4 such that k1=−k3=k3=−k4. According to the fundamental idea of the invention, in this way, when the sensor is functioning perfectly, the equation k1{right arrow over (a)}1+k2{right arrow over (a)}2+k3{right arrow over (a)}3+k4{right arrow over (a)}4={right arrow over (0)} is implemented. The equation can also be written in a more clear form k1({right arrow over (a)}1+{right arrow over (a)}3−({right arrow over (a)}2+{right arrow over (a)}4))={right arrow over (0)}.
Further, the continuous self-testing of the acceleration sensor during start-up and/or operation can be implemented such that to the accelerations {right arrow over (a)}1, {right arrow over (a)}2, {right arrow over (a)}3 and {right arrow over (a)}4 obtained as measurement results of the sensor measurement devices are given the multipliers k1, k2, k3 and k4 such that k1=k3=1 and k2=k4=−1. According to the fundamental idea of the invention, in this way, when the sensor is functioning perfectly, the equation k1{right arrow over (a)}1+k2{right arrow over (a)}2+k3{right arrow over (a)}3+k4{right arrow over (a)}4={right arrow over (0)} is implemented. The equation can also be written in a more clear form {right arrow over (a)}1+{right arrow over (a)}3−({right arrow over (a)}2+{right arrow over (a)}4)={right arrow over (0)}.
In the method according to the invention, the calculation of the vector equations and vector expressions of acceleration is in practise often implemented using scalar values.
In an alternative method according to the invention for the measurement of acceleration directed to two dimensions using a micromechanical acceleration sensor, the measurement directions of the sensor measurement devices measuring in several different directions can, for example, be selected symmetrically such that the accelerations obtained as measurement results of sensor measurement devices measuring in four different directions can be presented as equations:

{right arrow over (a)}1=d·{right arrow over (a)}x 

{right arrow over (a)}2=d·{right arrow over (a)}y 

{right arrow over (a)}3=−d·{right arrow over (a)}x 

{right arrow over (a)}4=−d·{right arrow over (a)}y.

In this case, the continuous self-testing of the acceleration sensor during start-up and/or operation can be implemented such that to the accelerations {right arrow over (a)}1, {right arrow over (a)}2, {right arrow over (a)}3 and {right arrow over (a)}4 obtained as measurement results of the sensor measurement devices are given the multipliers k1, k2, k3 and k4 such that k1=k3 and k2=k4. According to the fundamental idea of the invention, in this way, when the sensor is functioning perfectly, the equation k1{right arrow over (a)}1+k2{right arrow over (a)}2+k3{right arrow over (a)}3+k4{right arrow over (a)}4={right arrow over (0)} is implemented. The equation can also be written in a more clear form k1({right arrow over (a)}1+{right arrow over (a)}2)+k2({right arrow over (a)}3+{right arrow over (a)}4)={right arrow over (0)}.
Further, the continuous self-testing of the acceleration sensor during start-up and/or operation can be implemented such that to the accelerations a1, a2, a3 and a4 obtained as measurement results of the sensor measurement devices are given the multipliers k1, k2, k3 and k4 such that k1=k2=k3=k4. According to the fundamental idea of the invention, in this way, when the sensor is functioning perfectly, the equation k1{right arrow over (a)}1+k2{right arrow over (a)}2+k3{right arrow over (a)}3+k4{right arrow over (a)}4={right arrow over (0)} is implemented. The equation can also be written in a more clear form k1({right arrow over (a)}1+{right arrow over (a)}2+{right arrow over (a)}3+{right arrow over (a)}4)={right arrow over (0)}.
Still further, continuous self-testing of the acceleration sensor during start-up and/or operation can be implemented such that to the accelerations {right arrow over (a)}1, {right arrow over (a)}2, {right arrow over (a)}3, and {right arrow over (a)}4 obtained as measurement results of the sensor measurement devices are given the multipliers k1, k2, k3 and k4 such that k, =k2=k3=k4=1. According to the fundamental idea of the invention, in this way, when the sensor is functioning perfectly, the equation k1{right arrow over (a)}1+k2{right arrow over (a)}2+k3{right arrow over (a)}3+k4{right arrow over (a)}4={right arrow over (o)} is implemented. The equation can also be written in a more clear form {right arrow over (a)}1+{right arrow over (a)}2+{right arrow over (a)}3+{right arrow over (a)}4={right arrow over (0)}.
Using this invention, an improved method is provided for the measurement of acceleration directed to three or two dimensions using a micromechanical acceleration sensor as well as an improved micromechanical acceleration sensor. Using this invention, the functional reliability of a sensor can be monitored in constant use, and it is suitable for use particularly in small-sized micromechanical acceleration sensor solutions measuring in relation to several axes.

## Claims

1. A method for the measurement of acceleration, the values of acceleration applied to the sensor being within the sensor measurement range, in the direction of three dimensions using a micromechanical acceleration sensor, which acceleration sensor comprises at least four units of sensor measurement devices measuring in different measurement directions, of which four vectors describing the measurement directions of the sensor measurement devices no three vectors belong to the same plane, wherein in the method, the continuous self-testing of the acceleration sensor during start-up and/or operation is implemented such that
to the accelerations {right arrow over (a)}1,{right arrow over (a)}2,{right arrow over (a)}3 and {right arrow over (a)}4 obtained as measurement results of the sensor measurement devices are given the multipliers k1, k2, k3 and k4 such that, when the sensor is functioning perfectly, the value of the expression k1{right arrow over (a)}1,+k2{right arrow over (a)}2+k3{right arrow over (a)}3+ . . . +kn{right arrow over (a)}n calculates to {right arrow over (0)},
the accelerations {right arrow over (a)}1,{right arrow over (a)}2,{right arrow over (a)}3,. . . {right arrow over (a)}n given by the sensor measurement devices are measured at given intervals,
the value of the expression k1{right arrow over (a)}1+k2{right arrow over (a)}2+k3{right arrow over (a)}3+ . . . +kn{right arrow over (a)}n is calculated,
the value of said expression is compared to a pre-defined threshold value, and
if the value of said expression deviates from the value {right arrow over (0)} more than said pre-efined threshold value, an error message is given.

2. A method according to claim 1, wherein the measurement directions of the sensor measurement devices measuring in different directions are selected symmetrically such that the accelerations obtained as measurement results of the sensor measurement devices measuring in four different directions can be presented as equations:


{right arrow over (a)}1=c·{right arrow over (a)}x+c·{right arrow over (a)}z 



{right arrow over (a)}2=c·{right arrow over (a)}y+c·{right arrow over (a)}z 



{right arrow over (a)}3=−c·{right arrow over (a)}x+c·{right arrow over (a)}z 



{right arrow over (a)}4=−c·{right arrow over (a)}y+c·{right arrow over (a)}z,


and that to the accelerations {right arrow over (a)}1,{right arrow over (a)}2,{right arrow over (a)}3 and {right arrow over (a)}4 obtained as measurement results of the sensor measurement devices are given the multipliers k1, k2, k3 and k4 such that k1=k3=1 and k2=k4=−1.

3. A method according to claim 1, wherein the calculation of said vector expressions is implemented using scalar values.

4. A method according to claim 1, wherein an error message is given when m number of deviations of the value of said expression from the value {right arrow over (0)} more than said pre-defined threshold value have occurred.

5. A method according to claim 1, characterized in that said four measurement directions of the sensor measurement device described by said four vectors are selected so that the earth gravitational force has an essential component affecting in each of the said four measurement directions.

6. A micromechanical acceleration sensor for the measurement of acceleration, the values of acceleration applied to the sensor being within the sensor measurement range, in the direction of three dimensions, which acceleration sensor comprises at least four units of sensor measurement devices measuring in different measurement directions, of which four vectors describing the measurement directions of the sensor measurement devices no three vectors belong to the same plane, wherein the acceleration sensor comprises means for implementing continuous self-testing during start-up and/or operation, these means comprising a sensor measurement block, a sensor self-testing block, a sensor comparison block and an alarm block such that
the sensor measurement block comprises means for measuring accelerations directed to several different directions at given intervals using the sensor measurement devices, and for giving the values of the accelerations {right arrow over (a)}1, {right arrow over (a)}2, {right arrow over (a)}3, . . . {right arrow over (a)}n obtained as measurement results to the sensor self-testing block,
the sensor self-testing block comprises means for giving the multipliers k1, k2, k3, . . . kn to the accelerations {right arrow over (a)}1, {right arrow over (a)}2, {right arrow over (a)}3, . . . {right arrow over (a)}n obtained as measurement results of the sensor measurement devices such that, when the sensor is functioning perfectly, the value of the expression k1{right arrow over (a)}1+k2{right arrow over (a)}2+k3{right arrow over (a)}3+ . . . +kn{right arrow over (a)}n calculates to {right arrow over (0)},
sensor self-testing block further comprises means for calculating the value of the expression k1{right arrow over (a)}1+k2{right arrow over (a)}2+k3{right arrow over (a)}3+ . . . +kn{right arrow over (a)}n to the accelerations {right arrow over (a)}1, {right arrow over (a)}2, {right arrow over (a)}3, . . . {right arrow over (a)}n obtained as measurement results of the sensor measurement devices and measured at given intervals, and means for giving the value of said expression to the sensor comparison block,
the sensor comparison block comprises means for comparing the value of said expression k1{right arrow over (a)}1+k2{right arrow over (a)}2+k3{right arrow over (a)}3+ . . . +kn{right arrow over (a)}n to a given pre-defined threshold value, and
the sensor comparison block further comprises means for giving an error message to the alarm block, if the value of said expression deviates from the value {right arrow over (0)} more than said pre-defined threshold value.

7. A micromechanical acceleration sensor according to claim 6, wherein the measurement directions of the sensor measurement devices measuring in different directions are selected symmetrically such that the accelerations obtained as measurement results of sensor measuring devices measuring in four different directions can be presented as equations:


{right arrow over (a)}1=c·{right arrow over (a)}x+c·{right arrow over (a)}z 



{right arrow over (a)}2=c·{right arrow over (a)}y+c·{right arrow over (a)}z 



{right arrow over (a)}3=−c·{right arrow over (a)}x+c·{right arrow over (a)}z 



{right arrow over (a)}4=−c·{right arrow over (a)}y+c·{right arrow over (a)}z, 


and that to the accelerations {right arrow over (a)}1, {right arrow over (a)}2, {right arrow over (a)}3 and {right arrow over (a)}4 obtained as measurement results of the sensor measurement devices are given the multipliers k1, k2, k3 and k4 such that k1=k3=1 and k2=k4=−1.

8. A micromechanical acceleration sensor according to claim 6, wherein the calculation of said vector expressions is implemented using scalar values.

9. A micromechanical acceleration sensor according to claim 6, wherein an error message is given when m number of deviations of the value of said expression from the value {right arrow over (0)} more than said pre-defined threshold value have occurred.

10. A micromechanical acceleration sensor according to claim 6, wherein the sensor comparison block is connected to the sensor self-testing block.

11. A micromechanical acceleration sensor according to claim 6, wherein the sensor measurement block is connected to the sensor self-testing block.

12. A micromechanical acceleration sensor according to claim 6, wherein the sensor alarm block is connected to the sensor comparison block.

13. A micromechanical acceleration sensor according to claim 6, wherein the sensor measurement block is connected to the sensor self-testing block.

14. A micromechanical acceleration sensor according to claim 6, wherein the sensor alarm block is connected to the sensor comparison block.

15. A micromechanical acceleration sensor according to claim 6, characterized in that said four measurement directions of the sensor measurement devices described by said four vectors are selected so that the earth gravitational force has an essential component affecting in each of the said four measurement directions.