Abstract:
A sensor for detecting a rotation rate of an object, including: a sensor element, which is designed to vibrate at an angle to the rotation axis of the rotation rate to be detected at an excitation frequency on a resetting element fastened in a spatially fixed manner to the object, such that the sensor element is deflected at a reaction frequency in a reaction direction at an angle to the rotation axis and at an angle to the vibration direction owing to the Coriolis force; and a measuring transducer, which is designed to detect the vibration in the reaction direction wherein the vibratable sensor element is formed in such a manner that a comparison of a temperature-dependent displacement of a frequency spacing between the excitation frequency and the reaction frequency and a temperature-dependent position of the sensor element on the object satisfies a predefined condition.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. National Phase Application of PCT/EP2013/076498, filed Dec. 13, 2013, which claims priority to German Patent Application No. 10 2012 224 081.0, filed Dec. 20, 2012, the contents of such applications being incorporated by reference herein. 
     FIELD OF THE INVENTION 
     The invention relates to a sensor for detecting a rotation rate of an object, a method for detecting a rotation rate of an object, a control device for carrying out the method, and a vehicle. 
     BACKGROUND OF THE INVENTION 
     A rotation rate sensor for detecting a yaw rate and a pitch rate of a vehicle is known from DE 10 2010 002 796 A1, which is incorporated by reference. The rotation rate sensor comprises an encoder element, which is configured to oscillate at an angle to the axis of rotation of the rotation rate to be detected at an excitation frequency on a restoring element, which is fastened in a stationary manner on the object, so that the encoder element is deflected, as a result of the Coriolis force, at an angle to the axis of rotation and at an angle to the oscillation direction in a reaction direction at a reaction frequency. In this case, a measurement pickup is configured to detect the oscillation in a reaction direction, so that the rotation rate can be detected based on the detected oscillation, because it is dependent on the detected oscillation. 
     SUMMARY OF THE INVENTION 
     An aspect of the present invention improves this circuit of the current sensors in series between power source and consumer. 
     According to one aspect of the invention, a sensor for detecting a rotation rate of an object comprises an encoder element, which is configured to oscillate at an angle to the axis of rotation of the rotation rate to be detected at an excitation frequency on a restoring element, which is fastened in a stationary manner on the object, so that the encoder element is deflected, as a result of the Coriolis force, at an angle to the axis of rotation and at an angle to the oscillation direction in a reaction direction at a reaction frequency, and a measurement pickup, which is configured to detect the oscillation in a reaction direction. In this case, the oscillatory encoder element is designed such that a comparison of a temperature-dependent shift of a frequency interval between the excitation frequency and the reaction frequency and a temperature-dependent location of the encoder element on the object meets a predetermined condition. 
     The measurement pickup can directly detect the oscillation of the encoder element in the reaction direction. Alternatively, the encoder element could also be coupled to a further mass, however, which is excited to oscillate in the reaction direction by the encoder element. The measurement pickup could then detect the oscillation of this second mass. As long as no excitation takes place in the reaction direction, this second mass could then optionally be stationary. 
     The specified sensor is based on the consideration that the oscillation of the encoder element caused by the Coriolis force at an angle to the excited oscillation at the excitation frequency is subject to temperature effects, which results in an incorrect detection of the rotation rate of the object originating from the oscillation caused by the Coriolis force. 
     However, it is recognized in the scope of the specified sensor that the temperature effects are essentially induced by two different mechanical effects. On the one hand, temperature-related heat movements in the object result in mechanical tensions, which have an influence on the location of the encoder element in relation to the object, which finally results in a different oscillation and therefore corruption of the rotation rate to be detected. On the other hand, the temperature also has an influence on the frequency of the oscillation of the encoder element caused by the Coriolis force, which also results in a different oscillation and therefore corruption of the rotation rate to be detected. 
     Based on this finding, the specified sensor is based on the idea of reconciling the two above-mentioned temperature-related effects with one another, so that in total they still result in a change of the oscillation of the encoder element and therefore corruption of the rotation rate, but the reconciliation is to be carried out such that the oscillation of the encoder element is temperature-independent as a whole. The corruption could therefore be determined beforehand and taken into consideration during the analysis of the oscillation and therefore during the determination of the rotation rate. 
     In the scope of this idea, various determining factors are available in the specified sensor, to set the predetermined condition. These determining factors can comprise, for example, the mass of the encoder element, the spring stiffness of the restoring element, the relative location of the encoder element, for example, in relation to the axis of rotation, or arbitrary other adjustable system parameters in the specified sensor. The fulfillment of the condition can be verified, for example, by suitable dimensioning beforehand, by simulations beforehand, or by test experiments using prototypes. 
     In a refinement of the specified sensor, the predetermined condition comprises an observation of the comparison in an approximation. The approximation can comprise, for example, a linearization, a series expansion, or another arbitrary simplification of the comparison of the temperature-dependent shift of the frequency interval between the excitation frequency and the reaction frequency and the temperature-dependent location of the encoder element on the object. In this manner, the above-mentioned reconciliation of the spring constant to the remaining components of the specified sensor can be substantially simplified. In this case, the simplification can be based on the analytical expressions known to a person skilled in the art for the temperature-dependent shift of the frequency interval between the excitation frequency and the reaction frequency and for the temperature-dependent location of the encoder element. 
     In a special refinement of the specified sensor, the predetermined condition comprises an error interval, in which the comparison, in particular the approximated comparison, must fall within a predetermined temperature range. That is to say, in the scope of the specified sensor, complete temperature independence is not achieved, but the temperature dependence of the rotation rate determinable using the specified sensor can be specified with a defined tolerance range, whereby the specified sensor can be selected specifically for an application, because the tolerance range is known. 
     In another refinement of the specified sensor, the encoder element is a capacitance electrode, which oscillates in the reaction direction in relation to a further capacitance electrode, wherein the measurement pickup is configured to detect the oscillation in the reaction direction based on a capacitance between the two capacitance electrodes. In this manner, the oscillation can be electrically detected via the capacitance in a simple manner. 
     In a special refinement of the specified sensor, the restoring element has a spring constant, which is dependent on an electrical voltage between the two capacitance electrodes. The voltage can soften the spring in a manner known per se in this case, which is known to a person skilled in the art under the term “spring softening” effect. 
     In a preferred refinement of the specified sensor, the voltage is set such that the comparison meets the predetermined condition. That is to say, the voltage between the capacitance electrodes represents a further of the determining factors listed above as examples, to meet the condition mentioned at the outset. 
     In a particularly preferred refinement of the specified sensor, the reaction oscillation induces a change ΔĈ of the capacitance between the two capacitance electrodes with Δf as the frequency interval, Ω as the rotation rate, x 0  as the amplitude of the oscillation at the excitation frequency, d as the spacing of the capacitance electrodes, and C s (0) as the open-circuit capacitance between the two capacitance electrodes, which meets the following condition in a first approximation 
                 Δ   ⁢           ⁢     C   ^       ≈         Ω   ⁢           ⁢     x   0         2   ⁢   π   ⁢           ⁢   Δ   ⁢           ⁢   fd       ⁢       C   s     ⁡     (   0   )           ,         
wherein, as a predetermined condition, the term
 
                 C   S     ⁡     (   0   )         Δ   ⁢           ⁢   fd           
is less in a predetermined temperature range than a predetermined error threshold. In this manner, it can be detected by computation beforehand whether the above-mentioned predetermined condition is fulfilled or not.
 
     According to a further aspect of the invention, a method for detecting a rotation rate of an object using an encoder element, which is configured to oscillate at an angle to the axis of rotation of the rotation rate to be detected at an excitation frequency on a restoring element, which is fastened in a stationary manner on the object, so that the encoder element is deflected, as a result of the Coriolis force, at an angle to the axis of rotation and at an angle to the oscillation direction in a reaction direction at a reaction frequency, comprises the steps of detecting the oscillation in the reaction direction, determining the rotation rate based on the detected oscillation, and setting a spring constant of the restoring element such that a comparison of a temperature-dependent shift of a frequency interval between the excitation frequency and the reaction frequency and a temperature-dependent location of the encoder element on the object meets a predetermined condition. 
     The specified method can be restricted by steps which correspond in meaning to the features of the dependent claims of the specified sensor. 
     According to a further aspect of the invention, a control device is configured to carry out a method as claimed in any one of the preceding claims. 
     In a refinement of the specified control device, the specified device has a memory and a processor. In this case, the specified method is stored in the form of a computer program in the memory and the processor is provided for executing the method when the computer program is loaded from the memory into the processor. 
     According to a further aspect of the invention, a computer program comprises program code means to carry out all steps of one of the specified methods when the computer program is executed on a computer or one of the specified devices. 
     According to a further aspect of the invention, a computer program product contains a program code, which is stored on a computer-readable data carrier and which, when it is executed on a data processing unit, carries out one of the specified methods. 
     According to a further aspect of the invention, a rotation rate sensor comprises a specified control device. 
     According to a further aspect of the invention, a vehicle comprises a specified control device and/or a specified rotation rate sensor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above-described properties, features, and advantages of this invention and the manner in which they are achieved will become more clear and comprehensible in conjunction with the following description of the exemplary embodiments, which are explained in greater detail in conjunction with the drawings, wherein: 
         FIG. 1  shows a diagram to illustrate the Coriolis effect on an oscillating mass, 
         FIG. 2  shows a schematic circuit diagram of a vehicle having a rotation rate sensor, and 
         FIG. 3  shows a schematic circuit diagram of the rotation rate sensor from  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the figures, identical technical elements are provided with identical reference signs and are only described once. 
     Reference is made to  FIG. 1 , which shows a diagram to illustrate the Coriolis effect on an oscillating mass  2 . 
     The oscillating mass  2  will be used at a later point as an encoder element  2  for a rotation rate sensor  4 , which will be described in greater detail in the scope of  FIGS. 2 and 3 . 
     As is known per se, the Coriolis effect acts in rotating systems, such as the disk  6  shown in  FIG. 1 , which is to rotate at an angular velocity  8 . The encoder element  2  is fastened between an axis of rotation  10  and an external circumference  12  of the disk  6  via two springs  14  as restoring elements  14 . Therefore, the encoder element  2  also rotates on the rotating disk  6 . 
     The Coriolis effect states in a manner known per se that when a linear movement  16  of the encoder element  2  is observed perpendicularly to the axis of rotation  10  from a system at rest in relation to the rotating disk  6 , the encoder element must apply a force perpendicularly to the movement  16  and perpendicularly to the axis of rotation  10  to maintain this linear movement  16 . If this force is not applied, the encoder element  2  is then moved perpendicularly to the movement  16  and perpendicularly to the axis of rotation  10  with a Coriolis force  18 . 
     This Coriolis force  18  is used in the rotation rate sensor  4  to detect the angular velocity  8  of, for example, the rotating disk  6 , since the Coriolis force  18  is dependent in a manner known per se on the movement  16  and the angular velocity  8 . To measure the angular velocity  8 , the encoder element  2 , which is held via the restoring elements  14  on the rotating disk  6 , is excited in a defined manner into an oscillating movement  16 . Due to the oscillating, periodically reversing movement  16 , the Coriolis force  18  also periodically reverses, and causes the encoder element  18  to oscillate perpendicularly to the movement  16  and perpendicularly to the axis of rotation  10 . The oscillation of the encoder element  2  in the direction of the Coriolis force  18 , which is therefore dependent on the angular velocity  8 , is detected in the rotation rate sensor  4  and used to determine the angular velocity  8  in a manner known per se, as is explained, for example, in document DE 10 2010 002 796 A1. 
     Before the rotation rate sensor  4  will be described in greater detail, an exemplary embodiment for the rotation rate sensor  4  is to be shown. 
     Reference is made to  FIG. 2 , which shows a schematic view of a vehicle  20  having a vehicle dynamics controller known per se. Details of this vehicle dynamics controller can be inferred, for example, from DE 10 2011 080 789 A1, which is incorporated by reference. 
     The vehicle  20  comprises a chassis  22  and four wheels  24 . Each wheel  24  can be decelerated in relation to the chassis  22  via a brake  26 , which is fastened in a stationary manner on the chassis  22 , to decelerate a movement of the vehicle  20  on a road (not shown in greater detail). 
     In this case, it can occur in a manner known to a person skilled in the art that the wheels  24  of the vehicle  20  lose their ground adhesion and the vehicle  20  even moves away from a trajectory, which is predefined, for example, via a steering wheel (not shown in greater detail), due to understeer or oversteer. This is prevented by control circuits known per se such as ABS (antilock braking systems) and ESP (electronic stability programs). 
     In the present embodiment, the vehicle  20  has speed sensors  28  on the wheels  24  for this purpose, which detect a speed  30  of the wheels  24 . Furthermore, the vehicle  20  has the rotation rate sensor  4 , which, for example, can output the yaw rate of the vehicle  20  as an angular velocity  8 , for example. As explained in DE 10 2010 002 796 A1, which is incorporated by reference, the rotation rate sensor can detect up to two rotation rates of the vehicle  20  and thus, for example, also output the pitch rate or the roll rate of the vehicle, depending on the direction in which the oscillating movement  16 , which is shown in  FIG. 1 , of the encoder element  2  is excited. 
     Based on the detected speeds  30  and the angular velocity  8 , which is formed as the yaw rate, a controller  32  can determine, in a manner known to those skilled in the art, whether the vehicle  20  is slipping on the roadway or even deviating from the above-mentioned predefined trajectory and can react accordingly thereto using a controller output signal  34  known per se. The controller output signal  34  can then be used by a positioning unit  36  to activate actuators, such as the brakes  26 , by means of positioning signals  38 , these actuators reacting to the slipping and the deviation from the predefined trajectory in a manner known per se. 
     The controller  32  can be integrated, for example, in an engine controller, which is known per se, of the vehicle  20 . The controller  32  and the positioning unit  36  can also be designed as a shared control unit and can optionally be integrated in the above-mentioned engine controller. 
     Reference is made to  FIG. 3 , which shows a schematic view of the rotation rate sensor  4 , which is designed as a micromechanical sensor. 
     The rotation rate sensor  4  comprises a housing  40 , which encloses a cavity  42 . In the present embodiment, the housing  40  comprises a carrier substrate  44  and a cover  46 . The carrier substrate  44  can be formed from silicon, while in contrast the cover  46  can be formed from a heterogeneous glass-silicon composite. In this manner, the cavity  42  can be closed using a wafer bonding method known per se under vacuum, for example, by glass frit soldering or anodic bonding. 
     Two encoder elements  2 , which are fixed on the housing  40  via the restoring elements  14 , are located inside the cavity  42 . 
     In such micromechanical sensors, it is typical to excite the encoder elements  2  to oscillate in the direction of the movement  16  to be excited via capacitive comb drives known per se or via piezoelectric structures. 
     In the present example, the encoder element  2  is a capacitor plate, which forms a capacitance  49  in each case with a capacitor plate  48  formed as a readout element  48 . For the sake of comprehensibility, the encoder elements  2  are referred to hereafter as encoder capacitor plates  2  and the readout elements  48  are referred to as readout capacitor plates  48 . The readout capacitor plates  48  are fastened in parallel to the encoder capacitor plates  2  on the cover  46  in this case. A vertical distance  50  between encoder capacitor plate  2  and readout capacitor plate  48  corresponds to a capacitor plate spacing  50 , of which only one is indicated in  FIG. 3  for the sake of comprehensibility. The capacitor plate surface  52  of the encoder capacitor plates  2  and readout capacitor plates  48  define the capacitances  49  in this case together with the capacitor plate spacing  50 . 
     To measure the capacitances  49 , a readout voltage  54  is applied in each case between an encoder capacitor plate  2  and a readout capacitor plate  48 . 
     For example, if the cover  46  bulges toward the rotation rate sensor  4  as a result of intrinsic or external tensions, the capacitor plate spacing  50  thus changes. In the event of rising temperatures, this capacitor plate spacing  50  could be enlarged, for example, which can be induced, for example, by the glass component in the cover  46 , which has a greater coefficient of thermal expansion than the carrier substrate  44  made of silicon. By using polysilicon and/or other typical materials in micromechanics and/or by way of influences from assembly and packaging technology on the housing  40 , however, the capacitor plate spacing  50  can also become less in the event of rising temperatures. The sign of the relationship between capacitor plate spacing  50  and temperature can also be dependent on the design of the rotation rate sensor  4  and the housing  40 , for example, on the structure fixations and the design of the restoring elements  14 . 
     The temperature dependence on the measurement result of the rotation rate sensor  4  can be substantially masked by means of the following considerations. 
     To be able to measure the capacitance  49  between the encoder capacitor plates  2  and the readout capacitor plates  48 , as already explained, a readout voltage  54  must be applied to the capacitor plates  2 ,  48 . This readout voltage  54  additionally exerts an electrostatic force along the direction of the Coriolis force  18  on the encoder capacitor plate  2 , which reduces the spring constant of the overall oscillatory system. 
     A resonant frequency of the encoder capacitor plates  2  in the direction of the direction of the Coriolis force  18 , referred to as the readout direction hereafter, is thus determined, on the one hand, by the mechanical natural resonance in the readout direction and, on the other hand, is reduced by the applied readout voltage  54 . The readout direction is also to be provided hereafter with the reference sign  18  of the Coriolis force for better comprehensibility, since both point in the same direction. 
     If the capacitor plate spacing  50  now increases because of the thermal deformation, the capacitance  49  becomes less and therefore the electrostatic force also becomes less. This in turn causes an increase of the overall spring constant of the oscillatory system, which causes an increase of the resonant frequency of the encoder capacitor plates  2  in the readout direction  18 . The resonant frequency in the direction of the excited movement  16 , referred to as the excitation direction hereafter, remains unchanged, since the electrostatic force action acts along the readout direction  18  and therefore perpendicularly to the excitation direction. The excitation direction is also to be provided hereafter with the reference sign  16  of the excited movement  16  for better comprehensibility, since both point in the same direction. 
     However, due to the change of the resonant frequency in the readout direction  18 , a differential frequency between the resonant frequency in the readout direction  18  and the resonant frequency in the movement direction  18  is also changed. As is known, for example, from DE 10 2010 002 796 A1, however, this differential frequency has a direct influence on the sensitivity of the rotation rate sensor  4 . Therefore, if the capacitor plate spacing  50  increases as a result of tensions, the sensitivity of the rotation rate sensor  4  does sink in inverse proportion to the capacitor plate spacing  50 , but it increases in the opposite direction due to a shrinking differential frequency, however, because the electrostatic force decreases and therefore the resonant frequency increases in the readout direction  18 . The behavior is also observable in the case of decreasing capacitor plate spacing  50 , since the sensitivity of the rotation rate sensor  4  increases due to the lower capacitor plate spacing  50 , but it is simultaneously reduced again, since the differential frequency becomes greater. 
     The temperature-related errors and the temperature-related effects resulting therefrom on the sensitivity of the rotation rate sensor  4  thus mutually compensate for one another. 
     The resonant frequency in the readout direction  18 , as set forth above, is established by the natural resonant frequency of the oscillatory system in the readout direction, which may be reduced by electrostatic forces as a result of the readout voltage, however. To implement the above-mentioned ideas and the compensation of the temperature-related effects in the rotation rate sensor  4 , the readout voltage  50  can therefore be set so that the electrostatic force increases the differential frequency enough that the desired differential frequency results. To set the differential frequency independently of the applied readout voltage  50  between the encoder capacitor plate  2  and the readout capacitor plate  48 , additional capacitors can also be applied, which can also be used, for example, to prevent the mechanical crosstalk between the excitation direction  16  and the readout direction  18 . 
     Due to this in situ compensation of the sensitivity change, it is no longer necessary, for precise measurements of the angular velocity  8 , to implement complex compensation methods in a signal analysis circuit connected to the rotation rate sensor  4  and to reconcile using a large number of temperature support points, for example, via complexly developed characteristic curves. In the ideal case, if the resonant frequency in the excitation direction  16  is precisely adapted to the resonant frequency in the readout direction  18 , a calibration of the sensitivity of the rotation rate sensor  4  via the temperature can be omitted. 
     The above-described in situ compensation of the temperature errors is very robust with variation of the determining factors, such as the resonant frequencies in the excitation direction  16  and the readout direction  18 , the applied readout voltage  54 , etc. A sensitivity drift of at most ±5% can already be achieved, without additional complex compensation mechanisms being necessary in the ASIC, for example, additional controllers, a calibration of the sensitivity drift of the rotation rate sensor  4  via temperature, or a general polynomial reconciliation of the sensitivity drift, wherein the temperature is measured by a temperature sensor. Sensitivity drifts of at most ±3% would also be possible without problems in this case, however. 
     However, the above-mentioned compensation mechanisms can further increase the temperature stability of the rotation rate sensor  4 , so that the rotation rate sensor  4  can also be used for high-precision applications. 
     Furthermore, the option exists of considering additional effects when setting the operating point: on the basis of the change of the elastic coefficients over temperature, the two resonant frequencies in the excitation direction  16  and in the readout direction  18  can vary over temperature. Also, on the basis of different chucking conditions over temperature, the possibly existing pre-tension of the restoring elements  14  can change, which can result in a shift of the resonant frequencies in the excitation direction  16  and in the readout direction  18 . If these relationships are known and are to be approximately described by a function, the pre-tension can thus be set so that the sensitivity drift is also minimal under these variable conditions. 
     To at least test the reconciliation of the temperature-related errors in the resonant frequency in the readout direction  18  and the temperature-related errors of the capacitor plate spacing  50  beforehand by computer, the following formula has proven to be helpful: 
     
       
         
           
             
               
                 Δ 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   C 
                   ^ 
                 
               
               ≈ 
               
                 
                   
                     Ω 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       x 
                       0 
                     
                   
                   
                     2 
                     ⁢ 
                     π 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     Δ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     fd 
                   
                 
                 ⁢ 
                 
                   
                     C 
                     s 
                   
                   ⁡ 
                   
                     ( 
                     0 
                     ) 
                   
                 
               
             
             , 
           
         
       
     
     Therein, in addition to constants which are known per se, ΔĈ corresponds to the change of the capacitance  49  between the encoder capacitor plate  2  and the readout capacitor plate  48  as a result of the movement of the encoder capacitor plate  2  in the readout direction, Δf corresponds to the differential frequency between the resonant frequency in the readout direction  18  and the resonant frequency in the excitation direction  16 , Ω corresponds to the angular velocity  8 , x 0  corresponds to the amplitude of the oscillation of the encoder capacitor plate  2  in the movement direction  16 , d corresponds to the capacitor plate spacing  50 , and C s (0) corresponds to the open-circuit capacitance of the capacitance  49 . 
     In this formula, the term 
                 C   S     ⁡     (   0   )         Δ   ⁢           ⁢   fd           
is to be kept temperature-independent.