Abstract:
In a displacement detection device, an acceleration sensor generates at least a first acceleration signal relating to an axis of detection, and a displacement detection circuit is connected to the acceleration sensor has a comparator stage for comparing the acceleration signal with a programmable acceleration threshold and generates a displacement-detection signal. A high-pass filter is arranged between the acceleration sensor and the comparator stage so as to reduce a DC component of the acceleration signal. The cut-off frequency of the high-pass filter is modified according to the type of displacements that are to be detected.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a displacement detection device for a portable apparatus and, more particularly, to a device that simplifies the user interface of the portable apparatus. 
   2. Description of the Related Art 
   As is known, in the last few years, with the increasing use of portable electronic devices (such as laptops, Personal Data Assistants (PDAs), digital audio players, cellphones, digital video cameras, and the like), the need has arisen to simplify the use of such devices and, in particular, to simplify the user interface. In this regard, of particular interest is the possibility of activating given functions or programs of the portable devices with a simple movement imparted thereon by the user. Consider, for example, navigation within a list of options or an address book of a cellphone by simply tilting it or moving it in the direction in which is it is desired to scroll the list or the address book, or again to the possibility of adapting the display of an image on a PDA by simply changing the physical orientation, such as the inclination thereof. 
   In some portable devices it has been proposed to integrate a displacement detection device, which enables activation of functions or programs upon detection of a movement imparted by the user. 
   Displacement detection devices of a known type generally include an accelerometer of a linear type, which can be manufactured using the semiconductor technology (so-called MEMS, or micro-electro-mechanical systems). The linear accelerometer is used for measuring the accelerations acting on the portable apparatus so as to determine, with an appropriate conditioning electronics, the direction in which the portable apparatus has been displaced. 
   As is known, and shown in  FIG. 1 , a linear accelerometer  1  of a MEMS type includes a sensing element  2 , which detects acceleration and generates an electrical signal correlated to the detected acceleration, and a conditioning electronics for conditioning the electrical signal, which generates an output signal. V out  and typically includes a charge integrator  3  and a gain and noise-cancellation stage  4 , which uses, for example, the correlated-double-sampling (CDS) technique in order to eliminate the noise. 
   As shown in detail in  FIG. 2 , the sensing element  2  includes a stator, of which only first and second fixed electrodes  5   a ,  5   b  are shown, and a rotor, made up of a mobile mass  6  and mobile electrodes  7  fixed to the mobile mass  6 . Each mobile electrode  7  is arranged between a respective first fixed electrode  5   a  and a respective second fixed electrode  5   b . The mobile mass  6  is suspended via springs  10  to anchorage elements  11 . 
   The mobile mass  6  can move along an axis  13  that constitutes the preferential axis of detection of the linear accelerometer  1 . 
   As shown in  FIG. 3 , the sensing element  2  can be schematically represented as a first capacitor C 1  and a second capacitor C 2  arranged in series, the capacitances of which are variable as a function of the distance between the mobile electrodes  7  and the fixed electrodes  5   a ,  5   b , and hence as a function of the displacement of the rotor with respect to the stator. In particular, the first capacitor C 1  is formed by the set of the first fixed electrodes  5   a  and by the set of the mobile electrodes  7 , whilst the second capacitor C 2  is formed by the set of the second fixed electrodes  5   b  and by the set of the mobile electrodes  7 . 
   When the linear accelerometer  1  is subjected to an acceleration along the axis  13 , the mobile mass  6  moves along said axis, and it consequently produces a capacitive unbalancing between the first capacitor C 1  and the second capacitor C 2 . This capacitive unbalancing is detected by the conditioning electronics, which then supplies at an output the signal V out . 
   In particular, the displacement of the mobile mass  6  arises even in the presence of a static acceleration (for example, the acceleration of gravity), generating a corresponding capacitive unbalancing, which is detected by the conditioning electronics. Accordingly, even in a rest condition, a non-zero acceleration is detected, having a value equal to that of the component of the acceleration of gravity along the axis  13 . 
   Since in general it is desired to determine the displacements of the portable apparatus along three axes (corresponding to the three axes x, y and z of a set of Cartesian axes fixed with respect to the portable apparatus), the displacement detection device generally has three uniaxial linear accelerometers, each of which detects the component of the acceleration acting on the portable apparatus along one direction of detection. Alternatively, in an equivalent way, a single accelerometer provided with three axes of detection may be envisaged. In any case, three acceleration signals A x , A y  and A z  are generated, which represent the component of the acceleration along the x, y, and z axis, respectively. 
   The displacements of the portable apparatus are then determined by an appropriate processing circuit, which processes the acceleration signals A x , A y  and A z , and in particular compares them with respective fixed acceleration thresholds. Exceeding of one of the acceleration thresholds indicates that the portable apparatus has undergone a displacement in the corresponding direction. 
   As previously described, due to the acceleration of gravity, the accelerometers have a non-zero output even in the absence of any acceleration imparted by the user on the portable apparatus. In particular, the acceleration signals A x , A y  and A z  due to the sole acceleration of gravity have different values according to the inclination of the portable apparatus, in so far as the components of the acceleration of gravity along the axes x, y and z, respectively, differ each time. Consequently, exceeding of the acceleration thresholds by the acceleration signals A x , A y  and A z , in the presence of an acceleration imparted on the portable apparatus, depends upon the initial position (and in particular upon the orientation) of the portable apparatus. 
   This phenomenon entails disparity of operation according to the initial resting position of the portable apparatus, as determined by the user. Furthermore, this problem is aggravated by the fact that each user has a personal way of holding a portable apparatus, and the differences increase, for example, between right-handed users and left-handed users. 
   To solve the aforesaid problem, it has been proposed to set, in an initial step prior to displacement measurements, a reference position, corresponding to the resting position of the portable apparatus, and then to refer the detected displacements to said reference position, in a differential manner. A circuit diagram of a processing circuit implementing this solution is shown in  FIG. 4 . 
   In detail, the processing circuit, designated by  20 , comprises three registers  14 - 16 , three adders  17 - 19 , and three threshold comparators  21 - 23 . The registers  14 - 16  receive at input a respective acceleration signal A x , A y  and A z  from an accelerometer  25 , and store an initial value thereof. The adders  17 - 19  each receive at input a respective acceleration signal A x , A y  and A z  and the output of a respective register  14 - 16 . The three threshold comparators  21 - 23  receive at input the signal outputted by a respective adder  17 - 19 , compare it with an acceleration threshold A th , and supply at output a respective logic signal that is the result of the comparison. This logic signal represents detection of a significant acceleration along the corresponding axis. The acceleration threshold A th  is stored in a dedicated threshold register  25 . 
   Following an appropriate external initialization command (designated by INIT in  FIG. 4 ), the initial values of the acceleration signals A x , A y  and A z  in the resting position of the portable apparatus are stored in the registers  14 ,  15  and  16  (these initial values will be used as reference values). Then, the reference values stored in the respective register are subtracted from the acceleration signals A x , A y  and A z  each time detected by the accelerometer  25 . In this way, the variation of the respective acceleration signal A x , A y  and A z  with respect to the resting position is provided at input to the threshold comparators  21 ,  22 ,  23 , and hence the displacement detection does not (in theory) depend upon the initial position (i.e., upon the acceleration of gravity), but only upon the displacements imparted on the portable apparatus. 
   A drawback of the described processing circuit  20  is linked to the fact that it is necessary to automatically determine when new reference values are to be set in the registers  14 ,  15  and  16 . Furthermore, once the reference position has been established, the user must maintain the portable apparatus in said position to ensure correct operation of the displacement detection device. In fact, the described circuit is not able to adapt automatically to the change of posture of the user, including for example the inevitable (in so far as involuntary) movements of the wrist that cause a variation in the portable apparatus orientation. Furthermore, in the worst case (i.e., when said involuntary movements are greater than the acceleration threshold), the involuntary movements could trigger an undesirable activation event. It is, consequently, evident that a solution of this type actually makes unnatural the use of functions activated by movement, which instead should simplify the man-machine interface. 
   Another proposed solution envisages setting the acceleration threshold each time the portable apparatus is displaced. This solution is, however, far from practicable, unless high computing power is available. 
   BRIEF SUMMARY OF THE INVENTION 
   The disclosed embodiments of the present invention provide a displacement detection device that will enable the drawbacks described above to be overcome and, in particular, that will enable detection operations to be made independent of the initial position assumed by the portable apparatus incorporating the detection device. 
   In accordance with one embodiment of the invention, a displacement detection device is provided that includes an acceleration sensor generating a first acceleration signal relating to a first axis of detection; and a displacement detection circuit connected to the acceleration sensor and including a first comparator stage for comparing the first acceleration signal with a first acceleration threshold and generating a first displacement detection signal, the displacement detection circuit further including a first filtering circuit arranged between the acceleration sensor and the first comparator stage and configured to reduce a DC component of the first acceleration signal. 
   In accordance with another aspect of the foregoing embodiment, the first filtering circuit is formed of either a high-pass filter or a proportional-integral controller or a band-pass filter. 
   In accordance with another aspect of the foregoing embodiment, the displacement detection circuit includes a second comparator stage for comparing the first acceleration signal with a second acceleration threshold lower than the first acceleration threshold. 
   In accordance with yet a further aspect of the foregoing embodiment, the first and second acceleration thresholds are settable, and the displacement detection circuit includes a first memory element storing the first acceleration threshold and a second memory element storing the second acceleration threshold. 
   In accordance with another aspect of the foregoing embodiment, the displacement detection circuit includes an enabling stage connected to the first comparator stage and receiving an enabling signal, the enabling stage configured to supply at an output the first displacement-detection signal when an enabling signal satisfies a given condition. 
   In accordance with yet another aspect of the foregoing embodiment, the acceleration sensor includes a linear accelerometer with three axes of detection further generating a second acceleration signal and a third acceleration signal, the first, second, and third acceleration signals relating to a component of acceleration along a respective axis of detection; and wherein the displacement detection circuit further includes a second comparator stage and a third comparator stage for comparing, respectively, the second and third acceleration signals with the first acceleration threshold and generating a second displacement-detection signal and a third displacement-detection signal; and a second and third filtering circuit arranged, respectively, between the acceleration sensor and the second and third comparator stages and configured to reduce a DC component, respectively, of the second acceleration signal and of the third acceleration signal. 
   In accordance with yet another aspect of the foregoing embodiment, a microprocessor is provided that is connected to the displacement detection circuit, and the acceleration sensor, the displacement detection circuit, and the microprocessor are integrated in a single chip. 
   In accordance with another embodiment of the invention, a method for detection of the displacement of a portable apparatus is provided. The method includes detecting at least a first acceleration signal correlated with the acceleration of the portable apparatus along a first axis of detection; comparing the first acceleration signal with a first acceleration threshold; generating a first displacement-detection signal based on the comparison between the first acceleration signal and the first acceleration threshold; and attenuating by filtering a DC component of the first acceleration signal before comparing the first acceleration signal with the first acceleration threshold. 
   In accordance with another aspect of the foregoing embodiment, the method further includes modifying a cut-off frequency associated with the attenuating by filtering, the modifying the cut-off frequency carried out before the attenuating by filtering. 
   In accordance with another aspect of the foregoing embodiment, the modifying includes selecting a mode of operation chosen between at least a first mode associated with fast movements of the portable apparatus, and a second mode associated with slow movements of the portable apparatus, the modifying step further including increasing the cut-off frequency in the first mode and decreasing the cut-off frequency in the second mode. 
   In accordance with another aspect of the foregoing embodiment, the method further includes detecting a second acceleration signal and a third acceleration signal correlated with the acceleration of the portable apparatus along a second axis of detection and a third axis of detection, respectively; attenuating by filtering a DC component of the second acceleration signal and of the third acceleration signal; and after attenuating by filtering, comparing the second acceleration signal and the third acceleration signal with the first acceleration threshold for generating, respectively, a second displacement-detection signal and a third displacement-detection signal. 
   In accordance with another embodiment of the invention, a circuit is provided that includes an input circuit receiving an external acceleration signal and generating therefrom a filtered acceleration signal having a reduced DC component; a detection circuit receiving the filtered acceleration signal and further receiving a threshold signal and generating in response thereto a detection signal; and a microprocessor receiving the detection signal and generating in response thereto an actuation signal. 
   In accordance with a further embodiment of the invention, a circuit for processing an acceleration signal is provided, the circuit including an input circuit configured to receive an external acceleration signal and to generate a filtered acceleration signal; a first threshold signal circuit configured to store and output a first threshold signal; a comparator circuit configured to receive the filtered acceleration signal and the first threshold signal and to output a detection signal; and an enabling circuit configured to receive the detection signal and an external enabling signal and to generate a logic signal in response thereto. 
   In accordance with another aspect of the foregoing embodiment, the circuit further includes a plurality of input circuits, each configured to receive an external acceleration signal corresponding to a different direction of acceleration, and a plurality of comparator circuits each receiving as input the first threshold signal and a respective one of the plurality of acceleration signals; a plurality of enable circuits each configured to receive a detection signal from a respective comparator circuit and to further receive a respective external enabling signal and to output a logic signal corresponding to the respective acceleration signal. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     For a better understanding of the present invention, a preferred embodiment thereof is now described, purely by way of non-limiting example and with reference to the attached drawings, wherein: 
       FIGS. 1-3  are schematic representations of a linear accelerometer of a known type; 
       FIG. 4  shows a circuit diagram of a displacement detection circuit of a known type; 
       FIG. 5  shows a block diagram of a portable apparatus incorporating a displacement detection device; 
       FIG. 6  shows a first embodiment of a processing circuit of the displacement detection device of  FIG. 5 ; 
       FIG. 7  shows a second embodiment of the processing circuit of  FIG. 6 ; 
       FIG. 8  shows a possible variation of the processing circuit of  FIG. 6 ; and 
       FIG. 9  shows a variation of the block diagram of  FIG. 5 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 5  is a schematic illustration of a portable apparatus  28  provided with a displacement detection device  30  and with a microprocessor  31  in accordance with one embodiment of the invention. The displacement detection device  30  includes an accelerometer  32  of a linear type, and a processing circuit  34 . In particular, the accelerometer  32  and the processing circuit  34  are integrated in a single chip. 
   The accelerometer  32  is of a known type. In one embodiment, it is a MEMS device made with the semiconductor technology and has three axes of detection x, y and z so as to generate three acceleration signals A x , A y  and A z . Each acceleration signal is correlated to the acceleration detected along a respective axis of detection. For example, the accelerometer  32  can be realized as described in “A low-g 3 axis digital output accelerometer for emerging automotive applications”, B. Vigna, et al., AMAA 2004. 
   The processing circuit  34  receives at input the acceleration signals A x , A y  and A z , performs appropriate processing operations, and supplies at an output the logic signals corresponding to the displacements of the portable apparatus from the reference position. 
   The microprocessor  31  receives the logic signals and determines the direction in which the portable apparatus  28  has moved so as to activate corresponding functions or programs. 
   For convenience of illustration, the processing circuit  34  will now be described with reference to a single axis of detection (such as the axis x, not shown) along which the displacements of the portable apparatus  28  are detected. The processing circuits corresponding to axes y and z have in any case the same structure. 
   The processing circuit  34  (see  FIG. 6 ) comprises a threshold comparator  36 , a first register  37  storing an acceleration threshold A th , a high-pass filter  38 , receiving at input the acceleration signal A x  from the accelerometer  32  (not shown in  FIG. 6 ), and an enabling stage  39 . The high-pass filter  38  can, for example, be realized via a PI (proportional-integral) controller. 
   In detail, the threshold comparator  36  has a first input connected to the first register  37 , a second input connected to the output of the high-pass filter  38 , and an output connected to the enabling stage  39 . The enabling stage  39 , preferably comprising logic gates, has a first input connected to the output of the threshold comparator  36 , a second input receiving an enabling signal EN x  of a logic type, and an output connected to the microprocessor  31  ( FIG. 5 ). 
   The processing circuit  34  further comprises a modification stage  40 , which is connected to the high-pass filter  38  and has the purpose of modifying the cut-off frequency f t  of the high-pass filter  38 , on the basis of a selection by a user. 
   Operation of the processing circuit  34  is described below. 
   The acceleration signal A x  generated by the accelerometer  32  is filtered in the high-pass filter  38  and supplied to the threshold comparator  36 , which first calculates its absolute value and then compares it with the acceleration threshold A th , generating at output a logic signal (designated by X′). The logic signal X′ has, for example, a high value, in the case where the absolute value of the acceleration signal A x  is higher than the acceleration threshold A th  (a situation corresponding to the detection of an acceleration greater than the threshold set along the axis x). The acceleration threshold A th  is settable by the user, by writing in the first register  37 . 
   When the enabling signal EN x  assumes a pre-set logic value, for example high, the enabling stage  39  supplies at output a displacement-detection signal X corresponding to the logic signal X′ received by the threshold comparator  36 . Otherwise, the enabling stage  39  supplies at output a pre-set logic signal, for example low, actually disabling detection of displacements along the axis x. This function is useful for disabling one or more of the axes of detection and hence simplifying the processing by the microprocessor  31 , in the case where the direction of displacement of the portable apparatus  28  is known with certainty. In any case, the displacement-detection signal X is sent to the microprocessor  31 , which processes it (possibly together with the displacement-detection signals Y and Z corresponding to the other axes of detection y and z) to determine the direction of displacement of the portable apparatus  28 . 
   In particular, the high-pass filter  38  eliminates (or in any case reduces) the DC component (i.e., at zero-frequency) of the acceleration signal A x  received at input due to the acceleration of gravity, so that detection of displacement is actually independent of the initial position of the portable apparatus  28 . Furthermore, the modification stage  40  enables modification of the cut-off frequency f t  of the high-pass filter  38  so as to adapt operation of the processing circuit  34  to the type of application/function that it is desired to provide. For example, if it is desired to detect slow movements made by the user, it will be necessary to select a sufficiently low cut-off frequency f t  in such a way that the low-frequency components of the corresponding acceleration signals will have an amplitude sufficient to be detected. A higher cut-off frequency f t  will, instead, be selected in the case where the movements of the user are faster. 
   In the described processing circuit  34 , an acceleration signal A x  close to the acceleration threshold A th  causes continual detections of exceeding of the threshold and multiple switching of the displacement-detection signal X. 
   In order to prevent the above situation, according to a further aspect of the present invention, it is proposed to use two different acceleration thresholds, an external acceleration threshold A th     —     e  and an internal acceleration threshold A th      —     i , which is lower than the external acceleration threshold A th     —     e , so as to perform a displacement detection with hysteresis. 
   The processing circuit  50  shown in  FIG. 7  implements this functionality. 
   In detail, the processing circuit  50  includes a high-pass filter  51 , a first threshold comparator  52  and a second threshold comparator  53 , a set/reset register  54 , a modification stage  57  connected to the high-pass filter  51 , and an enabling stage  55 . The high-pass filter  51 , each of the threshold comparators  52 ,  53 , the modification stage  57 , and the enabling stage  55  are equivalent to the corresponding elements described in connection with  FIG. 6 . 
   In the specific case, the first threshold comparator  52 , in addition to being connected to the output of the high-pass filter  51 , is connected to a first register  56 , which stores the external acceleration threshold A th     —     e . The second threshold comparator  53 , in addition to being connected to the output of the high-pass filter  51 , is connected to a second register  58 , which stores the internal acceleration threshold A th     —     i . 
   The result of the comparisons executed by the first and second threshold comparators  52 ,  53  is supplied to the set input and reset input, respectively, of the set/reset register  54  which, in a per se known manner, also receives a clock signal CK. The output of the set/reset register  54  is connected to the enabling stage  55 . 
   The processing circuit  50  further includes a displacement register  60 , which is connected to the output of the enabling stage  55 , and a signaling logic  62 , which is connected to the displacement register  60 . 
   Operation of the processing circuit  50  is described hereinafter. 
   When the absolute value of the acceleration signal A x  is greater than the external acceleration threshold A th     —     e , the output of the first threshold comparator  52  assumes a high logic value, and sets in a set condition the set/reset register  54 , the output of which consequently also assumes the high logic value. Then, only when the acceleration signal A x  falls below the internal acceleration threshold A th     —     i , the output of the second threshold comparator  53  assumes a high logic value and sets in a reset condition the set/reset register  54 , the output of which switches to the low logic value. Accordingly, the oscillations of the displacement-detection signal X are eliminated. Through the enabling stage  55  and the enabling signal EN x  it is again possible to disable detection of displacements along the axis x. 
   The displacement register  60  has the purpose of storing the displacement-detection signal X. Preferably, when also displacements along the axes y and z are detected, the displacement register  60  is single, and stores also the displacement-detection signals Y and Z. The microprocessor  31  can then read periodically the contents of the displacement register  60 , for example via a serial interface, and process the displacement-detection signal X (and possibly the displacement-detection signals Y and Z) to determine the direction of displacement of the portable apparatus  28 . 
   Furthermore, the signaling logic  62  has the purpose of controlling the contents of the displacement register  60  and issuing an interrupt signal I for the microprocessor  31  when a variation in the data stored in the displacement register  60  occurs, so as to signal to the microprocessor  31  the need to perform a new reading. Once the microprocessor  31  has performed said reading, the signaling logic  62  disables the interrupt signal I, which remains disabled until a new change in the data stored in the displacement register  60  occurs. 
   The described displacement detection device has the following advantages. 
   First, it makes it possible to disregard the value of the DC component of the acceleration, for example due to the acceleration of gravity, and to assess the amplitude of the sole movements made by the user. In this way, it is in effect a constant resetting of the reference position, starting from which the displacements of the portable apparatus are to be detected. 
   The possibility of modifying the cut-off frequency of the high-pass filter enables adaptation of the displacement detection device to the requirements and to the type of movements of the user, as well as to the type of portable apparatus. 
   Furthermore, the whole displacement detection device can be integrated in a single chip, with obvious advantages in terms of savings in area occupation and of simplification of the management software of the portable apparatus. 
   Finally, it is clear that modifications and variations can be made to what is described and illustrated herein, without thereby departing from the scope of the present invention, as defined in the annexed claims. 
   In particular, as previously described, the displacement detection device can detect displacements along the three axes x, y and z, as shown in  FIG. 8 . 
   Furthermore, the displacement detection device can be readily modified for detecting not only the direction of displacement of the portable apparatus, but also the sense or positive/negative movement of the displacement. In fact, it is sufficient to use two comparators that compare the raw value (instead of the absolute value) of the acceleration signal filtered by the high-pass filter, one of which with a positive acceleration threshold and the other with a negative acceleration threshold. In this way, two output logic signals are obtained, which represent exceeding of the acceleration threshold in a first sense or in a second sense of the displacement. Likewise, in the case where a displacement detection with hysteresis is performed, the processing circuit will comprise four threshold comparators, two of which are receiving positive acceleration thresholds, and the other two of which are receiving negative acceleration thresholds. Alternatively, the sense of the displacement can be recognized by the sign of the digital word at output from the accelerometer, i.e., by the value of the most significant bit. 
   Furthermore, instead of the high-pass filter, a band-pass filter can be provided or any circuit commonly known to persons skilled in the art for suppression (or attenuation) of the DC component of a signal. 
   The described displacement detection device can be used not only for the purposes of simplification of the human interface, but also for reactivating a portable apparatus from the stand-by condition (thus implementing the so-called “wake-up” function). In this case, the detection of an acceleration greater than the pre-set threshold causes reactivation of functions of the portable apparatus previously de-activated to save energy. In particular, if the threshold is selected appropriately, the small involuntary movements performed by the user keep the portable apparatus in an operative condition while it is used. 
   In addition, the displacement detection device can be used also in systems for control and compensation of vibrations, for example, inside electrical household appliances. In this case, in fact, it is not necessary to know the DC acceleration to which the apparatus is subjected, but rather the displacement with respect to a resting position, so as to minimize the vibrations. 
   Finally, in the variation shown in  FIG. 9 , the microprocessor  31  is integrated within the displacement detection device  30 . In this way, within a single chip, all the necessary processing operations are carried out to determine the direction, and in some applications the sense or vector of the displacement of the portable apparatus. 
   All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, are incorporated herein by reference, in their entirety. 
   From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.