Sensor module and method for correcting sense output signal therefrom

A sense signal outputted from a sensor element and a reference voltage having a constant voltage level are selectively inputted to an amplifier, and amplified signals thereof are sequentially outputted as A/D-converted data by an A/D converter. An average of a predetermined number of A/D-converted data corresponding to the reference voltage is calculated, and a correction value is obtained by subtracting the average from one of the A/D-converted data corresponding to the reference voltage. Corrected data is obtained by subtracting the correction value from each A/D-converted data corresponding to the sense signal outputted from the sensor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a sensor module including a sensor element, an amplifier and an analog to digital (A/D) converter.

2. Description of the Related Art

Various sensors including an acceleration sensor, an angular velocity sensor, etc., a differential amplifier for amplifying sense signals outputted from those sensors, and an A/D converter for A/D-converting the amplified sense signals are packaged into one sensor module, which is well known to those skilled in the art. This sensor module can be readily incorporated into another device, and contribute to reducing the number of components of the device and miniaturizing the device.

As a technique for increasing precision of an output signal in an apparatus using a sensor, in Japanese Patent Laid-open Publication No. H9-43264 is disclosed an acceleration detection apparatus which has an acceleration sensor for obtaining an output signal corresponding to an acceleration in a traveling direction of an automobile. This acceleration detection apparatus comprises means for retaining, as a correction signal, the output signal from the acceleration sensor when the number of engine rotations of the automobile is constant, namely, the acceleration in the traveling direction of the automobile is 0, and thereafter removing the correction signal from the output signal from the acceleration sensor. The correction signal includes unnecessary components not related to the acceleration in the traveling direction of the automobile, such as a drift component occurring in the output signal from the acceleration sensor due to a temperature variation or other environmental variations, or a gravitational acceleration component applied to the acceleration sensor when the automobile travels on an uphill road. These unnecessary components are removed by the removing means. Therefore, this acceleration detection apparatus can reduce an error which will occur due to such unnecessary components.

SUMMARY OF THE INVENTION

A sensor module always suffers what is called a fluctuation in a sense output signal resulting from the fact that an output signal from a component other than a sensor, namely, an operational amplifier or A/D converter varies due to a certain factor. That is, a fluctuation component is ceaselessly introduced into a sense output signal finally outputted from the sensor module, thereby making it difficult to obtain the sense output signal with high precision.

An object of the present invention is to provide a sensor module including various sensors, a differential amplifier, an A/D converter, etc., which is capable of removing a fluctuation component resulting from an output variation in a signal processor provided downstream of the sensors, such as the differential amplifier or A/D converter, and obtaining a higher precision of sense output signal, and a method for correcting the sense output signal from the sensor module.

In accordance with an aspect of the present invention, the above and other objects can be accomplished by the provision of a sensor module comprising a sensor element for generating a sense signal corresponding to a sensed amount, an amplifier for amplifying the sense signal and outputting the amplified signal, and an analog to digital (A/D) converter for sequentially A/D-converting the amplified signal with a predetermined timing to obtain A/D-converted data, and sequentially outputting the obtained A/D-converted data, wherein the sensor module further comprises: reference voltage generation portion for generating a reference voltage having a constant voltage level; an input signal selection portion for selectively supplying either one of the sense signal and reference voltage to the amplifier; an averaging portion for calculating an average of a predetermined number of A/D-converted data corresponding to the reference voltage; a correction value generation portion for subtracting the average from one of the A/D-converted data corresponding to the reference voltage and outputting a result of the subtraction as a correction value; and a correction portion for subtracting the correction value from each A/D-converted data corresponding to the sense signal to obtain corrected data, and outputting the obtained corrected data as a sense output signal.

In accordance with another aspect of the present invention, there is provided a method for correcting a sense output signal from a sensor module, the sensor module including a sensor element for generating a sense signal corresponding to a sensed amount, an amplifier for amplifying the sense signal and outputting the amplified signal, and an A/D converter for sequentially A/D-converting the amplified signal with a predetermined timing to obtain A/D-converted data, and sequentially outputting the obtained A/D-converted data, the method comprising: inputting a reference voltage having a constant voltage level to the amplifier and obtaining an average of a predetermined number of A/D-converted data corresponding to the reference voltage; subtracting the average from one of the A/D-converted data corresponding to the reference voltage to obtain a correction value; and inputting the sense signal to the amplifier, subtracting the correction value from each A/D-converted data corresponding to the sense signal to obtain corrected data, and outputting the obtained corrected data as the sense output signal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, preferred embodiments of the invention will be described in conjunction with the annexed drawings.FIG. 1is a block diagram showing the configuration of a sensor module according to an exemplary embodiment of the present invention. A sensor10may be, for example, a piezoresistor-type three-axis acceleration sensor. The piezoresistor-type acceleration sensor has a structure consisting of a frame, mass and beam which are formed from silicon as a base material using a micro electro-mechanical system (MEMS) technique, in which piezoresistors are disposed on the beam. A piezoresistor is an element whose resistance varies due to a variation in the number of carriers or mobility in a semiconductor crystal lattice when an external mechanical force is applied to the crystal lattice so as to deform it. When an acceleration is applied to a sensor chip, the mass moves and deformation occurs on the beam. Stress occurs in the piezoresistors on the beam due to the deformation, resulting in a variation in resistance. In the piezoresistor-type acceleration sensor, a bridge circuit is formed with the piezoresistors to output the variation in the resistance of the piezoresistors as an electrical signal. That is, a sense voltage Vs having a voltage level corresponding to the acceleration applied to the sensor element is generated between output terminals of the sensor10.

A reference voltage generator11is a direct current (DC) voltage generation circuit that generates a reference voltage Vref having a constant voltage level between output terminals thereof. The reference voltage generator11may be implemented with, for example, a bandgap circuit, etc. such that the reference voltage Vref can always be maintained at a stable DC voltage level, not dependent on an ambient temperature or supply voltage.

An input switching circuit12is a switching circuit that switches an input voltage to a differential amplifier13. To this end, the input switching circuit12includes a first switch SW1afor switching the input/cutoff of the sense voltage Vs from the sensor10to the differential amplifier13, and a second switch SW2afor switching the input/cutoff of the reference voltage Vref from the reference voltage generator11to the differential amplifier13. The switching of each switch is carried out based on a control signal supplied from a control circuit15.

The differential amplifier13has two input terminals, and amplifies and outputs a difference between voltages applied to the two input terminals. The sense voltage Vs supplied from the sensor10or the reference voltage Vref supplied from the reference voltage generator11is outputted as a signal amplified by a predetermined amplification factor by the differential amplifier13. The reason why the signal amplification is performed using the differential amplifier13in this manner is that the sense voltage Vs from the sensor10is weak and thus needs to be raised to a voltage level required for A/D conversion thereof. Also, because the differential amplifier13amplifies a difference between signals at the input terminals thereof, even though noise is introduced into the signals, it hardly matters in that it is difficult to appear as an electrical signal difference.

An A/D converter14samples the amplified signal of the sense voltage Vs or reference voltage Vref supplied from the differential amplifier13at a predetermined period to convert the amplified signal into a digital amount corresponding to the magnitude thereof, and sequentially outputs the converted digital amount as A/D-converted data. The A/D converter14may be, for example, a known consecutive comparison type A/D converter, and samples and holds the amplified signal of the sense voltage Vs or reference voltage Vref supplied from the differential amplifier13, converts the sampled and held signal into a digital amount while performing comparison beginning with a most significant bit, and sequentially outputs the converted digital amount.

An output switching circuit16includes a first switch SW1band a second switch SW2b, each of which is turned on/off based on a control signal supplied from the control circuit15. With this configuration, A/D-converted data A(x) corresponding to the sense voltage Vs from the sensor10, sequentially outputted from the A/D converter14, is outputted from an output terminal Q1through the first switch SW1b, and A/D-converted data D(x) corresponding to the reference voltage Vref from the reference voltage generator11, sequentially outputted from the A/D converter14, is outputted from an output terminal Q2through the second switch SW2b.

The control circuit15supplies control signals to the input switching circuit12and the output switching circuit16to control ON/OFF of each switch of the input switching circuit12so as to alternately supply the sense voltage Vs and the reference voltage Vref to the differential amplifier13with a predetermined timing, and to synchronize an ON/OFF timing of the first and second switches of the output switching circuit16with an ON/OFF timing of the first and second switches of the input switching circuit12so as to bisect output sources of an A/D-converted output of the sense voltage Vs and an A/D-converted output of the reference voltage Vref, as stated above. That is, the control circuit15operatively associates the ON/OFF timing of the output switching circuit16with the ON/OFF timing of the input switching circuit12such that an A/D-converted output corresponding to the sense voltage Vs from the sensor10is outputted from the output terminal Q1through the first switch SW1band an A/D-converted output corresponding to the reference voltage Vref from the reference voltage generator11is outputted from the output terminal Q2through the second switch SW2b.

A memory17is a storage medium for storing some of the A/D-converted data D(x) corresponding to the reference voltage Vref, sequentially supplied through the second switch SW2bof the output switching circuit16. In an initialization operation upon power-on to be described later, the memory17stores, for example, 100 sampled data, among the A/D-converted data corresponding to the reference voltage Vref, sequentially supplied.

An averaging circuit18calculates an average Dave of the A/D-converted data of the reference voltage Vref stored in the memory17and retains the calculated average.

A hold circuit19holds one A/D-converted data D included in the A/D-converted data D(x) corresponding to the reference voltage Vref, sequentially supplied through the second switch SW2bof the output switching circuit16, based on a control signal and outputs the held data.

A first subtracter20subtracts the average Dave retained by the averaging circuit18from the A/D-converted data D of the reference voltage Vref held by the hold circuit19and outputs a result of the subtraction as a correction value E (=D−Dave).

A second subtracter21subtracts the subtraction result of the first subtracter20, or the correction value E, from the A/D-converted data A(x) corresponding to the sense voltage Vs from the sensor10, sequentially supplied through the first switch SW1bof the output switching circuit16, and outputs a result of the subtraction as corrected data M(x) (=A(x)−E). This corrected data M(x) is a final sense output signal from the sensor module of the present invention.

Next, the operation of the sensor module with the above-stated configuration according to the present embodiment will be described with reference toFIGS. 2 and 3.FIGS. 2 and 3are timing charts illustrating the operations of the respective functional blocks constituting the sensor module.FIG. 2illustrates an initialization operation of the sensor module when the sensor module is powered on, andFIG. 3illustrates an acceleration sensing operation of the sensor module.

First, the initialization operation of the sensor module will be described with reference toFIG. 2. Upon application of power to the sensor module, the initialization operation of the sensor module is started. In this initialization operation, the averaging circuit18calculates an average of the reference voltage Vref outputted from the reference voltage generator11within a certain period and retains the calculated average. That is, upon application of power to the sensor module, the control circuit15supplies control signals to the input switching circuit12and the output switching circuit16to turn off the first switch SW1aof the input switching circuit12and turn on the second switch SW2athereof and turn off the first switch SW1bof the output switching circuit16and turn on the second switch SW2bthereof (S1). As a result, the reference voltage Vref generated between the output terminals of the reference voltage generator11is applied to the differential amplifier13. Then, the differential amplifier13generates an amplified signal by amplifying the reference voltage Vref by a predetermined amplification factor, and supplies the generated amplified signal to the A/D converter14.

The A/D converter14samples the amplified signal of the reference voltage Vref supplied thereto at a predetermined sampling period and sequentially outputs the sampled signal as A/D-converted data D(x) (S2). Also, the A/D-converted data D(x) is superimposed by a fluctuation component as the input voltage is passed through the differential amplifier13and the A/D converter14.

The A/D-converted data D(x) corresponding to the reference voltage Vref, sequentially outputted from the A/D converter14, is supplied to the memory17through the second switch SW2bof the output switching circuit16. The memory17stores for example, 100 data, among the A/D-converted data of the reference voltage Vref periodically supplied correspondingly to the sampling period of the A/D converter14(S3). Also, the data stored in the memory17is retained therein until the sensor module is powered off.

The averaging circuit18extracts the 100 data stored in the memory17, calculates an average Dave of the extracted data and retains the calculated average (S4). A fluctuation component can be offset by averaging a plurality of A/D-converted data superimposed by the fluctuation component by means of the averaging circuit18. That is, the average Dave calculated by the averaging circuit18is defined as a true value of the reference voltage Vref after amplification from which the fluctuation component was removed, and is used to extract the fluctuation component in the sensing operation of the sensor module of the present invention to be described below. When the calculation of the average Dave is completed, the initialization operation is ended.

Also, the average Daveis calculated when a normal calibration temperature of the sensor is measured (when a sensitivity offset value of the sensor is acquired), and then stored in a nonvolatile memory or the like. Therefore, it is also possible to correct a variation in the reference voltage Vref resulting from a temperature variation without carrying out the measurement again upon application of power.

Next, the acceleration sensing operation of the sensor module after completion of the initialization operation will be described with reference toFIG. 3. The control circuit15supplies control signals to the input switching circuit12and the output switching circuit16to turn off the first switch SW1aof the input switching circuit12and turn on the second switch SW2athereof and turn off the first switch SW1bof the output switching circuit16and turn on the second switch SW2bthereof (S11). As a result, the reference voltage Vref generated between the output terminals of the reference voltage generator11is applied to the differential amplifier13. Then, the differential amplifier13generates an amplified signal by amplifying the reference voltage Vref by a predetermined amplification factor, and supplies the generated amplified signal to the A/D converter14. The A/D converter14samples the amplified signal of the reference voltage Vref supplied thereto at a predetermined sampling period and sequentially outputs the sampled signal as A/D-converted data D(x) (S12). The A/D-converted data D(x) is superimposed by a fluctuation component as the input voltage is passed through the differential amplifier13and the A/D converter14. The A/D-converted data D(x) corresponding to the reference voltage Vref, outputted from the A/D converter14, is supplied to the hold circuit19through the second switch SW2bof the output switching circuit16. The hold circuit19holds one data D1included in the A/D-converted data D(x) sequentially supplied thereto with a timing based on a control signal and outputs the held data (S13). The A/D-converted data D1held by the hold circuit19is supplied to the first subtracter20. The first subtracter20subtracts the average Dave acquired in the initialization operation from the A/D-converted data D1and outputs a result of the subtraction as a correction value E1(=D1−Dave) (S14). That is, because the average Dave is a true value of the reference voltage Vref from which the fluctuation component was removed, as stated previously, and D1is A/D-converted data of the reference voltage Vref including the fluctuation component, it is possible to extract only the fluctuation component by subtracting Davefrom D1. Namely, the correction value E1outputted from the first subtracter10represents the magnitude of the fluctuation component at a data D1acquisition time. Also, because the data stored in the memory17and the average Dave of the averaging circuit18are those set and retained in the initialization step, they are not changed in the acceleration sensing operation.

Then, the control circuit15supplies control signals to the input switching circuit12and the output switching circuit16to turn on the first switch SW1aof the input switching circuit12and turn off the second switch SW2athereof and turn on the first switch SW1bof the output switching circuit16and turn off the second switch SW2bthereof (S15). As a result, the sense voltage Vs generated between the output terminals of the sensor10is applied to the differential amplifier13. Then, the differential amplifier13generates an amplified signal by amplifying the sense voltage Vs by a predetermined amplification factor, and supplies the generated amplified signal to the A/D converter14. The A/D converter14samples the amplified signal of the sense voltage Vs supplied thereto at a predetermined sampling period and sequentially outputs the sampled signal as A/D-converted data A(x) (S16). Also, the A/D-converted data A(x) is superimposed by a fluctuation component as the input voltage is passed through the differential amplifier13and the A/D converter14. The A/D-converted data A(x) corresponding to the sense voltage Vs from the sensor10, outputted from the A/D converter14, is sequentially supplied to the second subtracter21through the first switch SW1bof the output switching circuit16. The second subtracter21subtracts the correction value El (=D1−Dave) outputted from the first subtracter20from each of the A/D-converted data of the sense voltage Vs sequentially supplied thereto and outputs a result of the subtraction as corrected data M(x) (=A(x)−E1) (S17). As stated above, the correction value E1supplied from the first subtracter10represents the magnitude of the fluctuation component at the data D1acquisition time. The subtracter21removes the fluctuation component by subtracting the correction value E1from each of the A/D-converted data of the sense voltage Vs superimposed by the fluctuation component. In this manner, the fluctuation correction is carried out with respect to the sense output signal.

Because the fluctuation component always varies in magnitude, the correction value acquisition and correction process is carried out at intervals of a predetermined period. That is, after acquiring a predetermined number of A/D-converted data of the sense voltage Vs from the sensor10, the control circuit15again supplies control signals to the input switching circuit12and the output switching circuit16to turn off the first switch SW1aof the input switching circuit12and turn on the second switch SW2athereof and turn off the first switch SW1bof the output switching circuit16and turn on the second switch SW2bthereof (S18). As a result, the reference voltage Vref is again applied to the differential amplifier13. The A/D converter14converts an amplified signal of the reference voltage Vref supplied thereto into A/D-converted data D(x) (S19) and supplies the A/D-converted data to the hold circuit19through the output switching circuit16. The hold circuit19holds one data D2included in the new A/D-converted data D(x) sequentially supplied thereto with a timing based on a control signal and outputs the held data (S20). The first subtracter20subtracts the average Davefrom the A/D-converted data D2held by the hold circuit19and outputs a result of the subtraction as a new correction value E2(=D2−Dave) (S21). The new correction value E2represents the magnitude of the fluctuation component at a data D2acquisition time. The correction value may be updated one after another.

Then, the control circuit15controls the input switching circuit12and the output switching circuit16to operate in synchronism. Specifically, the control circuit15supplies control signals to the input switching circuit12and the output switching circuit16to turn on the first switch SW1aof the input switching circuit12and turn off the second switch SW2athereof and turn on the first switch SW1bof the output switching circuit16and turn off the second switch SW2bthereof (S22). As a result, new A/D-converted data A(x) corresponding to the sense voltage Vs from the sensor10is obtained (S23). The second subtracter21subtracts the new correction value E2from each of the new A/D-converted data A(x) sequentially supplied thereto, so as to output, as the sense output signal, corrected data M(x) from which the fluctuation component occurring in the new period was removed (S24).

As described above, in the initialization operation, the sensor module of the present invention passes a reference voltage Vref having a constant voltage level through the differential amplifier13and the A/D converter14to acquire A/D-converted data superimposed by a fluctuation component, and averages the acquired A/D-converted data to obtain an average Davecorresponding to a true value of the reference voltage Vref from which the fluctuation component was offset. In the sensing operation, the sensor module of the present invention specifies the fluctuation component by subtracting the average Davefrom A/D-converted data D corresponding to the reference voltage Vref at a certain time. Then, the sensor module corrects the output of the sensor by subtracting the fluctuation component from each A/D-converted data corresponding to the sensor output, and outputs the correction result as a final sense output signal. Therefore, it is possible to eliminate the effect of the fluctuation component and obtain a high precision of sense output signal. Also, because the extraction of the fluctuation component, namely, the acquisition of the correction value is periodically performed as stated above, it is possible to properly perform the correction with respect to the fluctuation component always varying. Further, it is preferable that a period in which the reference voltage generator11is connected to the differential amplifier13for acquisition of the correction value E is as short as possible.

FIG. 4Ais a graph illustrating a transition of a sense output signal from the sensor module of the present invention under the condition that an acceleration is 0, which plots a moving average of 100 data measured for 22 hours.FIG. 4Bis a graph illustrating, as a comparative example, a transition of a sense output signal from a conventional sensor module with no fluctuation correction function under the same condition. Also, a broken line shown in each graph represents an ideal value of the sense output signal. As apparent from comparison between the two graphs, it can be seen that an output variation from the ideal value is significantly reduced by performing fluctuation correction with respect to the sense output signal.FIG. 4Cillustrates standard deviations of the two sense output signals. It can be understood fromFIG. 4Cthat the deviation of the sense output signal, namely, the fluctuation component superimposing the sense output signal is reduced by almost half owing to the effect of the fluctuation correction.

Also, although the sensor10has been described for illustrative purposes in the above embodiment to be an acceleration sensor, it is applicable to all sensors including an angular velocity sensor, temperature sensor, magnetometric sensor, pressure sensor, etc. Also, the A/D converter14is not limited to a consecutive comparison type A/D converter, but may be an A/D converter of any other type such as a charge equilibration type or dual integral type. In addition, although the sensor module has been described in the above embodiment to acquire A/D-converted data of a reference voltage Vref, specify a fluctuation component based on the acquired A/D-converted data, and then receive the output of the sensor and subtract the fluctuation component from the received sensor output, it may receive and retain the sensor output in advance, and then specify the fluctuation component and subtract the fluctuation component from the retained sensor output.

The invention has been described with reference to the preferred embodiments thereof. It should be understood by those skilled in the art that a variety of alterations and modifications may be made from the embodiments described above. It is therefore contemplated that the appended claims encompass all such alterations and modifications.

This application is based on Japanese Patent Application No. 2008-065446 which is hereby incorporated by reference.