Abstract:
The invention relates to measuring devices to be used in the measuring of angular velocity and, more precisely, to vibrating micromechanical sensors of angular velocity. In a sensor of angular velocity according to the invention, a mass is supported to the frame of the sensor component by means of an asymmetrical spring structure ( 1 ), ( 2 ), ( 3 ), ( 4 ), ( 22 ), ( 24 ) in such a way, that the coupling from one mode of motion to another, conveyed by the spring ( 1 ), ( 2 ), ( 3 ), ( 4 ), ( 22 ), ( 24 ), cancels or alleviates the coupling caused by the non-ideality due to the skewness in the springs or in their support. The structure of the sensor of angular velocity according to the invention enables reliable measuring with good performance, particularly in small vibrating micromechanical solutions for sensors of angular velocity.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation application of U.S. patent application Ser. No. 12/285,406, filed on Oct. 3, 2008. The disclosure of the prior application(s) is hereby incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to measuring devices to be used for measuring angular velocity and, more precisely, to vibrating micromechanical sensors of angular velocity. The object of the invention is to provide an improved sensor structure enabling reliable measuring with good performance, particularly in small size vibrating micromechanical solutions for sensors of angular velocity. 
     BACKGROUND OF THE INVENTION 
     In measuring angular velocity, the principle of the method of measuring based on a vibrating sensor of angular velocity has proved to be simple and reliable. In a vibrating sensor of angular velocity, a certain known primary motion is induced and maintained in the sensor. The desired motion to be measured by means of the sensor is then detected as a deviation of the primary motion. 
     An external angular velocity in a direction perpendicular to the resonators&#39; direction of motion acting on the sensor induces a Coriolis force in the seismic mass in a direction perpendicular to its direction of motion. The Coriolis force, proportional to the angular velocity, is detected, for example capacitively, in the vibration of the mass. 
     One of the most significant problems in micromechanical vibrating sensors of angular velocity is the so called quadrature signal, which is caused by poor dimensional precision in the structures. In resonators manufactured using the means of micromechanics, there may be found tolerance errors in the perpendicularity of the directions of motion, which in the detection of the sensor of angular velocity cause a signal, called the quadrature signal, of a magnitude, at worst, hundreds of times larger than the angular velocity signal corresponding to the maximum value of the output scale. 
     The angular velocity signal to be measured, being proportional to the speed of the mass, is luckily phase-shifted by 90 degrees in relation to the quadrature signal, whereby the quadrature signal disappears in an ideal demodulation. However, being significantly larger than the signal to be measured, it restricts the dynamics of the signal. Another big disadvantage of the quadrature signal is, that it, if left uncompensated for, significantly degrades the stability of the zero point of the sensor, due to phase shifts in the electronic signals as, for example, the temperature changes. 
     In the sensor, the quadrature signal can be compensated for by using electric forces. One of the known techniques is i.a. feed-forward compensation, in which a force modulated by the detected primary motion is fed back into the detecting resonator at a phase opposite to the quadrature signal. Alternative ways of electrical compensation include, for example, straightening of the direction of motion by a static electric force or by a force generated by a static entity modulated by the motion, which force compensates for the quadrature signal caused by a residual of the spring force. 
     Compensation by means of electric forces constitutes a challenge to the sensor&#39;s electronics. What is required is either accurate phase control or, possibly, large voltages and separate structures within the sensor. 
     Thus, the object of the invention is to provide a structure of a vibrating sensor of angular velocity, in which the compensation for the quadrature signal is implemented directly by mechanical design, without electric forces. 
     Referring to prior art, the Finnish patent publication FI-116543B1 describes a sensor of angular velocity according to prior art, where the seismic masses are connected to support areas by springs and/or stiff auxiliary structures, which give the masses a degree of freedom in relation to an axis of rotation perpendicular to the plane of the disk they are forming, and to at least one axis of rotation extending in the direction of the plane of the disk. 
     Further, referring to prior art, the Finnish patent publication FI-116544B1 describes a sensor of angular velocity according to prior art, where at least one pair of electrodes is formed in association with the edge of the seismic mass, which pair of electrodes forms two capacitances with the surface of the mass, so that, as a function of the angle of rotation of the mass&#39;s primary motion, one capacitance of the pair of electrodes increases and the other capacitance of the pair of electrodes decreases. 
     SUMMARY OF THE INVENTION 
     The objective of the invention is to provide such an improved vibrating sensor of angular velocity, which enables reliable measuring with a good performance, particularly in solutions with a small vibrating sensor of angular velocity, and in which the compensation for the quadrature signal is implemented by mechanical design without electric compensation, or, alternatively, in combination with the electric compensation methods mentioned above. 
     According to a first aspect of the invention, a vibrating micromechanical sensor of angular velocity is provided, which comprises at least one seismic mass and, associated with the mass, a moving electrode, which mass possesses a primary motion, which is to be activated, and, in addition to the primary motion, at least one degree of freedom in relation to a detection axis, or detection axes, essentially perpendicular to the primary motion, and which mass, or which masses, is/are supported to the frame of the sensor component by means of a spring structure such, that the spring structure is asymmetric such, that the coupling, conveyed by the spring, from one mode of motion to another cancels or alleviates the coupling caused by non-ideality due to skewness of the springs or their support. 
     Preferably, one corner of the spring structure is etched off. Alternatively, one or more compensation groove is etched into the spring structure. Further, alternatively, one or more compensation cavity is etched into the spring structure. Further, alternatively, one or more compensation groove or compensation cavity is etched into at least one attachment spot for the spring structure. Further, preferably, the compensation grooves or the compensation cavities are suitably dimensioned such, that they effectively straighten the end portion of a skewed spring. 
     Alternatively, one edge of the spring structure is serrated. Alternatively, both edges of the spring structure are serrated. Further, preferably, the serration is suitably dimensioned to be one-sided or asymmetric such, that the serration twists the bending axis of the spring. 
     Preferably, the spring structure is designed to be asymmetric such, that the coupling from one mode of motion to another, conveyed by the spring, cancels or alleviates the coupling caused by non-ideality due to an inclination relative to the perpendicular to the disk of the groove of the DRIE etching process. 
     According to a second aspect of the invention, a method is provided for manufacturing, by means of micromechanical disk structures, a vibrating micromechanical sensor of angular velocity comprising at least one seismic mass and, in association with the mass, a moving electrode, which mass possesses a primary motion to be activated and, in addition to the primary motion, at least one degree of freedom in relation to a detection axis, or detection axes, essentially perpendicular to the primary motion, and which mass, or which masses, is/are supported to the frame of the sensor component by means of a spring structure such, that the spring structure of the sensor of angular velocity is made asymmetric by etching. 
     Preferably, the etching mask is designed such, that it compensates for non-idealities occurring over the surface of the disk, caused by the manufacturing process. Preferably the DRIE etching technique (DRIE, Deep Reactive Ion Etching) is used in the manufacturing. Preferably, in the manufacturing, non-idealities of the DRIE etching process are utilized, such as the ARDE effect (ARDE, Aspect Ratio Dependent Etch rate). Preferably, a two-stage DRIE etching process is utilized in the manufacturing, by means of which the depth of the etched groove or cavity can be suitably dimensioned. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Below, the invention and its preferable embodiments are described in detail with exemplifying reference to the attached drawings, of which: 
         FIG. 1  shows a section through a spring structure used in supporting the seismic mass of a vibrating sensor of angular velocity according to the invention, 
         FIG. 2  shows a perspective view of the spring structure used in supporting the seismic mass of a vibrating sensor of angular velocity according to the invention, 
         FIG. 3  shows a section through an alternative spring structure used in supporting the seismic mass of a vibrating sensor of angular velocity according to the invention, 
         FIG. 4  shows a perspective view of the alternative spring structure used in supporting the seismic mass of a vibrating sensor of angular velocity according to the invention, 
         FIG. 5  shows a perspective view of a second alternative spring structure used in supporting the seismic mass of a vibrating sensor of angular velocity according to the invention, 
         FIG. 6  shows a perspective view of a third alternative spring structure used in supporting the seismic mass of a vibrating sensor of angular velocity according to the invention, 
         FIG. 7  shows a perspective view of a fourth alternative spring structure used in supporting the seismic mass of a vibrating sensor of angular velocity according to the invention, 
         FIG. 8  shows a perspective view of the structure of a vibrating sensor of angular velocity according to the invention, 
         FIG. 9  shows a perspective view of the structure of a vibrating sensor of angular velocity with two axes according to the invention, 
         FIG. 10  shows a perspective view of a fifth alternative spring structure used in supporting the seismic mass of a vibrating sensor of angular velocity according to the invention, 
         FIG. 11  shows a perspective view of a sixth alternative spring structure used in supporting the seismic mass of a vibrating sensor of angular velocity according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In a vibrating sensor of angular velocity according to the invention, the primary motion to be activated is the vibration of at least one seismic mass and an associated moving electrode. In addition to the primary motion, the seismic mass possesses another degree of freedom in relation to a detection axis essentially perpendicular to the primary motion. 
     Further, the sensor of angular velocity according to the invention comprises a seismic mass and an associated moving electrode, which mass is supported to the frame of the sensor component by means of a spring structure. 
     The moving electrode in the primary motion mode of the vibrating sensor of angular velocity according to the invention is activated into vibration. Thus, the coupling caused by the Coriolis force activates the detection motion mode. The motion axes of the primary motion mode and the detection motion mode, or the detection motion modes, are essentially perpendicular to each other. Due to the known non-ideality, there will be a coupling between the modes in the absence of angular velocity activation. 
     In one structure of a vibrating sensor of angular velocity according to the invention, the primary motion to be activated is the vibration of at least one seismic mass and an associated moving electrode. Correspondingly, the motion of the detection motion mode can then occur, for example, essentially in the plane of the disk. Alternatively, the motion of the detection motion mode can occur essentially perpendicularly to the plane of the disk. 
     The vibrating sensors of angular velocity according to the invention are typically manufactured by means of disk structures. Typically a multitude of structures of sensors of angular velocity are manufactured onto a central disk by means of, for instance, etching techniques, which central disk then, for example, is sealed by disks on top and underneath. The sensors of angular velocity are diced out of the finished disk package structure. 
     In sensors having the structure of vibrating sensors of angular velocity, manufactured by means of the DRIE etching technique (DRIE, Deep Reactive Ion Etching), in which sensors one mode of motion is in the plane of the disk and the other is perpendicular to the plane of the disk, the quadrature signal is caused by an inclination of the DRIE groove in relation to the normal to the disk. The phenomenon is well repeatable and its distribution over the disk is known. 
     Since the distribution over the disk is known, compensation, according to the invention, of the structures of the vibrating sensors of angular velocity can be implemented by means of the manufacturing mask, whereby there will be no need for individual mechanical tuning of the structures of the sensors of angular velocity. 
     The phenomenon causing the quadrature signal in the structure of the vibrating sensor of angular velocity according to the invention is a known one, such as, for example, the quadrature signal caused by the inclination of the DRIE etching process groove in relation to the normal to the disk, and thus the quadrature signal distribution over the disk is also known and repeatable. 
     In the solution according to the invention, the quadrature signal is compensated for by designing the springs to be asymmetrical such, that the coupling, conveyed by the springs, from one mode of motion to another cancels or significantly alleviates the coupling caused by the non-ideality due to the quadrature signal. 
     As a special case of the solution, a asymmetrical spring in the structure, according to the invention, of the vibrating sensor of angular velocity can be produced by using other non-idealities of the DRIE etching process. As examples of such non-idealities of the DRIE etching process, the ARDE effect (ARDE, Aspect Ratio Dependent Etch rate), and the wedge-like character of the profile, can be mentioned. Thus, compensation according to the invention is accomplished without any additional process steps. 
     The  FIGS. 1 through 11  show examples of spring structures of the structure of the vibrating sensor of angular velocity according to the invention, by means of which spring structures the quadrature signal can be compensated for. 
       FIG. 1  shows a section through a spring structure used in supporting the seismic mass of a vibrating sensor of angular velocity according to the invention. The spring structure of the sensor of angular velocity according to the invention is depicted with the numeral  1 . A corner of the spring structure  1  according to the invention is etched off. The spring structure according to the invention is asymmetrical such, that the coupling, conveyed by the spring, from one mode of motion to another cancels or significantly alleviates the coupling caused by non-ideality due to the quadrature signal. 
       FIG. 2  shows a perspective view of the spring structure used in supporting the seismic mass of a vibrating sensor of angular velocity according to the invention. The spring structure of the sensor of angular velocity according to the invention is depicted with by the numeral  1 . A corner of the spring structure  1  according to the invention is etched off. 
     In manufacturing the spring structures according to the invention shown in  FIGS. 1-2 , the etching mask can be designed such that it compensates for non-idealities occurring over the disk caused by the manufacturing process. One of these non-idealities is, for example, the non-ideality caused by the inclination of the DRIE etching process groove in relation to the normal to the disk. Thus, the size of the cut-off corner of the spring structure  1  according to the invention varies over the disk. 
       FIG. 3  shows a section through an alternative spring structure used in supporting the seismic mass of a vibrating sensor of angular velocity according to the invention. The alternative spring structure of the sensor of angular velocity according to the invention is depicted with the numeral  2 . One or more compensation grooves have been etched into the alternative spring structure  2  according to the invention. The alternative spring structure  2  according to the invention is asymmetrical such that the coupling, conveyed by the spring, from one mode of motion to another cancels or significantly alleviates the coupling caused by non-ideality due to the quadrature signal. 
       FIG. 4  shows a perspective view of the alternative spring structure used in supporting the seismic mass of a vibrating sensor of angular velocity according to the invention. The alternative spring structure of the sensor of angular velocity according to the invention is depicted with the numeral  2 . One or more compensation grooves have been etched into the alternative spring structure  2  according to the invention. 
     In manufacturing the alternative spring structures according to the invention, shown in  FIGS. 3-4 , the etching mask can be designed such that it compensates for non-idealities occurring over the disk caused by the manufacturing process. One of these non-idealities is, for example, the non-ideality caused by the inclination of the DRIE etching process groove in relation to the normal to the disk. Thus, the dimensioning of the compensation groove of the alternative spring structure  2  according to the invention varies over the disk. 
     The compensation groove of the alternative spring structure  2  according to the invention can be etched in the same DRIE etching as the other structures. In manufacturing the alternative spring structures  2  according to the invention, due to the ARDE effect, the compensation groove would not be etched all the way through the disk, but rather, the depth of the groove can be suitably dimensioned. Alternatively, a groove of a suitable depth can, for example, be etched by means of a two-stage etching process. 
       FIG. 5  shows a perspective view of a second alternative spring structure used in supporting the seismic mass of a vibrating sensor of angular velocity according to the invention. The second alternative spring structure of the sensor of angular velocity according to the invention is depicted with the numeral  3 . One or more compensation cavities have been etched into the second alternative spring structure  3  according to the invention. The second alternative spring structure  3  according to the invention is asymmetrical such that the coupling, conveyed by the spring, from one mode of motion to another cancels or significantly alleviates the coupling caused by non-ideality due to the quadrature signal. 
     In manufacturing the second alternative spring structure  3  according to the invention, shown in  FIG. 5 , the etching mask can be designed such that it compensates for non-idealities occurring over the disk caused by the manufacturing process. One of these non-idealities is, for example, the non-ideality caused by the inclination of the DRIE etching process groove in relation to the normal to the disk. Thus, the dimensioning of the compensation cavities of the second alternative spring structure  3  according to the invention varies over the disk. 
     The compensation cavities of the second alternative spring structure  3  according to the invention can be etched in the same DRIE etching as the other structures. In manufacturing the second alternative spring structures  3  according to the invention, due to the ARDE effect, the compensation cavities would not be etched all the way through the disk, but rather, the depth of the cavities can be suitably dimensioned. Alternatively, a groove of a suitable depth can, for example, be etched by means of a two-stage etching process. 
       FIG. 6  shows a perspective view of a third alternative spring structure used in supporting the seismic mass of a vibrating sensor of angular velocity according to the invention. The third alternative spring structure of the sensor of angular velocity according to the invention is depicted with the numeral  4 . The third alternative spring structure according to the invention comprises attachment spots  5 ,  6 . One or more compensation groove or compensation cavity have been etched into at least one attachment spot  5 ,  6  of the spring structure  4 . The third alternative spring structure  4  according to the invention is asymmetrical such that the coupling, conveyed by the spring, from one mode of motion to another cancels or significantly alleviates the coupling caused by non-ideality due to the quadrature signal. 
       FIG. 7  shows a perspective view of a fourth alternative spring structure used in supporting the seismic mass of a vibrating sensor of angular velocity according to the invention. The fourth alternative spring structure of the sensor of angular velocity according to the invention is depicted with the numeral  7 . The fourth alternative spring structure  7  according to the invention comprises attachment spots  8 ,  9 . One or more compensation groove or compensation cavity have been etched into both attachment spots  8 ,  9  of the spring structure  7 . The fourth alternative spring structure  7  according to the invention is asymmetrical such that the coupling, conveyed by the spring, from one mode of motion to another cancels or significantly alleviates the coupling caused by non-ideality due to the quadrature signal. 
     In manufacturing the presented spring structures  4 ,  7 , shown in  FIGS. 6-7 , the etching mask can be designed such that it compensates for non-idealities occurring over the disk caused by the manufacturing process. One of these non-idealities is, for example, the non-ideality caused by the inclination of the DRIE etching process groove in relation to the normal to the disk. Thus, the dimensioning of the compensation grooves or compensation cavities of the presented spring structures  4 ,  7  varies over the disk. 
     The compensation grooves or compensation cavities of the spring structures  4 ,  7  shown in  FIGS. 6-7  can be etched in the same DRIE etching as the other structures. In manufacturing of the presented spring structures  4 ,  7 , due to the ARDE effect, the compensation grooves or compensation cavities would not be etched all the way through the disk, but rather, the depth of the grooves or cavities can be suitably dimensioned. Alternatively, a groove of suitable depth can, for example, be etched by means of a two-stage etching process. In the presented spring structures  4 ,  7 , suitably dimensioned compensation grooves or compensation cavities effectively straighten the end portion of a skewed spring. 
       FIG. 8  shows a perspective view of the structure of a vibrating sensor of angular velocity according to the invention. The vibrating masses of the sensor of angular velocity according to the invention are depicted with the numerals  10  and  11 . The masses  10 ,  11  of the sensor of angular velocity are supported at their attachment spots  12 ,  13  by means of spring structures. At the opposite end  14  at the mass side of the attachment spot  12  of the spring structure of the sensor of angular velocity or, alternatively, at the end  15  at the attachment spot  13  side of the spring structure, one or more compensation grooves or compensation cavities  14 ,  15  are etched. The spring structure of the invention is asymmetrical such that the coupling, conveyed by the spring, from one mode of motion to another cancels or significantly alleviates the coupling caused by non-ideality due to the quadrature signal. 
       FIG. 9  shows a perspective view of the structure of a vibrating sensor of angular velocity with two axes according to the invention. The vibrating mass of the sensor of angular velocity with two axes according to the invention is depicted with the numeral  16 . The mass  16  of the sensor of angular velocity with two axes is supported at its attachment spot  17  by means of spring structures. At the ends  18 ,  20  of the spring structure of the sensor of angular velocity with two axes, opposite to the attachment spot  17 , or, alternatively, at the ends  19 ,  21  at the attachment spot  17  side of the spring structure, one or two compensation grooves or compensation cavities  18 - 21  are etched. The spring structure of the invention is asymmetrical such that the coupling, conveyed by the spring, from one mode of motion to another cancels or significantly alleviates the coupling caused by non-ideality due to the quadrature signal. 
       FIG. 10  shows a perspective view of a fifth alternative spring structure used in supporting the seismic mass of a vibrating sensor of angular velocity according to the invention. The fifth alternative spring structure of the sensor of angular velocity according to the invention is depicted with the numeral  22 . One edge of the fifth alternative spring structure  22  according to the invention is serrated  23 . The fifth alternative spring structure  22  according to the invention is asymmetrical such that the coupling, conveyed by the spring, from one mode of motion to another cancels or significantly alleviates the coupling caused by non-ideality due to the quadrature signal. 
     In manufacturing the fifth alternative spring structure  22  according to the invention, shown in  FIG. 10 , the etching mask can be designed such that it compensates for non-idealities occurring over the disk, caused by the manufacturing process. One of these non-idealities is, for example, the non-ideality caused by the inclination, in relation to the normal to the disk, of the DRIE etching process groove. Thus, the dimensioning of the serration  23  of the fifth alternative spring structure  22  according to the invention varies over the disk. The serration  23  of the fifth alternative spring structure  22  according to the invention can be etched in the same DRIE etching as the other structures. 
       FIG. 11  shows a perspective view of a sixth alternative spring structure used in supporting the seismic mass of a vibrating sensor of angular velocity according to the invention. The sixth alternative spring structure of the sensor of angular velocity according to the invention is depicted with the numeral  24 . Both edges of the sixth alternative spring structure  24  according to the invention are serrated  25 ,  26 . The sixth alternative spring structure  24  according to the invention is asymmetrical such that the coupling, conveyed by the spring, from one mode of motion to another cancels or significantly alleviates the coupling caused by non-ideality due to the quadrature signal. 
     In manufacturing the sixth alternative spring structure  24  according to the invention, shown in  FIG. 11 , the etching mask can be designed such that it compensates for non-idealities occurring over the disk caused by the manufacturing process. One of these non-idealities is, for example, the non-ideality caused by the inclination of the DRIE etching process groove in relation to the normal to the disk. Thus, the dimensioning of the serrations  25 ,  26  of the sixth alternative spring structure  24  according to the invention varies over the disk. The serrations  25 ,  26  of the sixth alternative spring structure  24  according to the invention can be etched in the same DRIE etching as the other structures. 
     The DRIE etching profile in the spring structures  22 ,  24  shown in  FIGS. 10-11  is, in practice, in addition to the inclination, slightly wedge-shaped, i.e. the grooves widen in the depth direction, whereby the serration patterns differ between the upper and lower surfaces of the spring. In the solution according to the invention, the serration  23 ,  25 ,  26  can be suitably dimensioned to be one-sided or asymmetrical, whereby the serration  23 ,  25 ,  26  twists the bending axis of the spring  22 ,  24 . 
     The solution according to the invention can be used for compensating for the quadrature signal of all such sensors of angular velocity, in which the primary motion is a vibration of at least one seismic mass and an associated moving electrode, and in which the mass, in addition to the primary motion, possesses a second degree of freedom in relation to a detection axis, or detection axes, essentially perpendicular to the primary motion.