Pressing sensor and electronic device for detecting the end of a press without error

A pressing sensor is provided that includes a first sensor element that outputs a voltage corresponding to an amount of displacement in shape of a receiving unit that receives a press operation; a press operation determining unit that determines whether the receiving unit is in a state of receiving a press operation; and a setting unit that, after elapse of a first prescribed time from a point of time at which the press operation determining unit determines that the receiving unit has received a press operation, sets an absolute value of a threshold that is smaller than an absolute value of the threshold before elapse of the first prescribed time, the absolute value of the threshold representing a difference from a ground voltage value for determining whether the receiving unit has ended reception of the press operation.

TECHNICAL FIELD

The present invention relates to a pressing sensor having a piezoelectric element and to an electronic device having the pressing sensor.

BACKGROUND

Patent Document 1 (identified below) discloses a displacement sensor including an elastic material and a piezoelectric element of a flat film attached to a first main surface of the elastic material. Moreover, the piezoelectric element has electrodes formed respectively on both main surfaces of a piezoelectric sheet. According to the displacement sensor described in Patent Document 1, the piezoelectric sheet is displaced in shape by displacement in shape of the elastic material to cause a piezoelectric effect, by which an output voltage corresponding to the amount of displacement is output from the electrodes formed on both surfaces of the piezoelectric sheet.

FIGS. 11(A) to 11(C)are diagrams for explaining fluctuations of a voltage that is generated when a conventional displacement sensor is subjected to a long press operation. InFIGS. 11(A) to 11(C), sections denoted as S represent times during which the displacement sensor detects a press. According to the displacement sensor described in Patent Document 1, for example, when a user presses the displacement sensor, it generates a positive voltage, and when the positive voltage exceeds a given positive threshold (Vth1), the sensor determines that the press has been started, as indicated inFIG. 11(A). When the user removes the user's hand from the displacement sensor, the sensor then generates a negative voltage, and when the negative voltage drops below a given negative threshold (Vth2), the sensor determines that the press has ended. In the present description, negative and positive polarities will be explained on the assumption that a reference voltage is 0 V. In the present description, a case where the reference voltage is defined as a ground voltage will be described.

A case where the displacement sensor described in Patent Document 1 is pressed for a long time is considered. In this case, at the start of the press operation to the displacement sensor, a positive voltage is generated and then a negative voltage is also generated as a result of a restitutive action of the elastic material. For this reason, as shown inFIG. 11(B), if the threshold (Vth2) is set as a threshold small in magnitude on the negative side for determining the end of the press, the negative voltage generated as a result of the restitutive action may drop below the threshold (Vth2), which leads to an error in determining the end of the press. In contrast, as shown inFIG. 11(C), if the threshold (Vth2) is set as a threshold large in magnitude on the negative side for determining the end of the press, because fluctuations of output voltages become smaller when the user's hand is removed slowly, the negative voltage resulting from removal of the hand may fail to drop below the threshold (Vth2), in which case the end of the press cannot be detected.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of an embodiment of the present invention to provide a pressing sensor that, even when the pressing sensor (e.g., a displacement sensor) is subjected to a long press, can detect the end of the long press without an erroneous operation.

According to an exemplary embodiment, a pressing sensor is provided that includes a first sensor element, a press operation determining unit, and a setting unit. The first sensor element is configured to output a voltage corresponding to an amount of displacement in shape of a receiving unit that receives a press operation. The press operation determining unit is configured to determine whether the receiving unit is in a state of receiving a press operation, based on a threshold. After the elapse of a first prescribed time from a point of time at which the press operation determining unit determines that the receiving unit has received a press operation, the setting unit is configured to set an absolute value of a threshold that is smaller than an absolute value of the threshold before the elapse of the first prescribed time, with the absolute value of the threshold representing a difference from a ground voltage value for determining whether the receiving unit has ended reception of the press operation.

In this configuration, the threshold at a point of time at which the press operation determining unit determines that the receiving unit has ended reception of the press operation is set to be smaller in its difference from the ground voltage than the threshold at the point of time at which the press operation determining unit determines that the receiving unit has received the press operation. Even when the user presses the receiving unit for a long time and then removes the user's finger slowly from the receiving unit, because the absolute value of the threshold is set as a small value, the press operation determining unit is configured to determine that the receiving unit has ended reception of the press operation. Thus, even when the pressing sensor (e.g., a displacement sensor) is subjected to a long press, the end of the long press can be detected without an erroneous operation.

Moreover, an electronic device includes the above pressing sensor according to an exemplary embodiment.

In this configuration, by disposing the pressing sensor on an operating unit or the like of the electronic device, the end of a long press can be detected without an erroneous operation when the operating unit is subjected to the long press.

According to the exemplary embodiment of the present invention, even when the displacement sensor is subjected to a long press, the end of the long press can be detected without an erroneous operation.

DETAILED DESCRIPTION

FIG. 1(A)is a perspective view of an electronic device according to a first embodiment.FIG. 1(B)is a sectional view of the electronic device ofFIG. 1(A)that is taken along I-I line.FIG. 2(A)is a diagram for explaining an arrangement of a piezoelectric element according to the first embodiment.FIG. 2(B)is a block diagram of a pressing sensor. InFIG. 2(A), to depict the interior of the electronic device, its top face and part of its internal structure are omitted. The electronic device shown inFIG. 1(A)is an example of the present invention. However, it is noted that the electronic device depicted inFIG. 1(A)can be modified properly in shape or the like according to specifications.

As shown inFIGS. 1(A), 1(B), and2(A), an electronic device1includes a housing2of a substantially parallelepiped shape. The housing2has a side face3, a top face4, a bottom face7, and an opening8. The opening8is formed on a part of the top face4. The electronic device1includes a tabular display5placed in the opening8of the housing2. In one aspect, the electronic device1can be configured to have no top face4. For example, the electronic device1may be configured such that the top of the housing2is entirely formed as the opening8and that the display5is disposed in such a way as to close the opening8.

The display5is configured to function as an operation surface on which a user makes a touch operation using a finger, pen, or the like. In the following description, the width direction (i.e., a lateral direction) of the housing2is defined as an X direction, the length direction (i.e., vertical direction) of the same as a Y direction, and the thickness direction (i.e., height or vertical direction) of the same as a Z direction.

The housing2has a receiving unit9disposed on a part of the exterior of the side face3. The electronic device1includes a piezoelectric element20disposed on the interior of the side face3that corresponds to the receiving unit9. The piezoelectric element20is pasted or otherwise secured to the interior of the side face3, using, for example, an adhesive tape, bond, or the like. The piezoelectric element20is referred to as a first sensor element according to the present disclosure.

The electronic device1includes a circuit6disposed inside the housing2. As shown inFIG. 2(B), the circuit6has a CPU21, a timer22, a RAM23, and a ROM24. The CPU21has a press operation determining unit25and a setting unit26(also referred to as a threshold setting unit).

According to the exemplary aspect, the CPU21is configured to read programs (e.g., software code) out of the ROM24, which is a memory medium to the RAM23, to carry out and execute various operations described below.

The circuit6is electrically connected to the piezoelectric element20and is configured to detect a voltage generated by the piezoelectric element20. The circuit6, together with the piezoelectric element20, makes up a pressing sensor100according to the exemplary embodiment.

When the receiving unit9receives a press operation, the receiving unit9is displaced in shape due to the force of the pressing operation. As a result of displacement in shape of the receiving unit9and of the housing2, the piezoelectric element20is likewise displaced in shape. The piezoelectric element20outputs a voltage corresponding to an amount of displacement in shape of the receiving unit9. The pressing sensor100causes the circuit6to detect the voltage that the piezoelectric element20generates as a result of its displacement in shape, thereby detects the press.

The press operation determining unit25is configured to determine whether the receiving unit9is in a state of receiving a press operation, based on a threshold. This will be described in detail later on. After the elapse of a first prescribed time (t1) from a point of time at which the press operation determining unit25determines that the receiving unit9has received a press operation, the setting unit26sets an absolute value of the threshold smaller than an absolute value of the threshold before the elapse of the first prescribed time (t1). It is noted that the piezoelectric element20and the circuit6do not always need to be placed in the housing2and can be placed outside the housing2in an alternative aspect. The piezoelectric element20will hereinafter be described.

FIG. 3(A)is an exploded perspective view of the piezoelectric element according to the first embodiment, andFIG. 3(B)is a sectional view of the piezoelectric element.FIG. 4is an explanatory diagram of the piezoelectric film according to the first embodiment. As shown inFIGS. 3(A) and 3(B), the piezoelectric element20has a piezoelectric film10of a flat film, and a first electrode11and a second electrode12of flat films. It is noted that inFIGS. 3(A) and 3(B), constituent elements other than the piezoelectric film10, the first electrode11, and the second electrode12are not depicted for clarity purposes.

The piezoelectric film10has a first main surface14and a second main surface15. Similar to the piezoelectric film10, the first electrode11and the second electrode12are each formed into a rectangular shape in a plan view. The first electrode11is disposed on the first main surface14of the piezoelectric film10. The second electrode12is disposed on the second main surface15of the piezoelectric film10.

It is preferable that when the piezoelectric element20is seen from its top, at least either the first electrode11or the second electrode12be superposed completely on the piezoelectric film10or located inside a plane parallel with the piezoelectric film10in a plan view. This arrangement inhibits short circuit at the ends of the first electrode11and the second electrode12. Either the first electrode11or the second electrode12is a ground electrode according to the exemplary embodiment. The piezoelectric element20, therefore, outputs a voltage based on a reference voltage at the ground electrode. In the present description, for convenience, the voltage at the ground electrode is assumed to be 0 V.

FIG. 4is the explanatory diagram of the piezoelectric film according to the first embodiment, showing a plan view of the piezoelectric film. As shown inFIG. 4, the piezoelectric film10may be a film made of chiral polymers. According to the first embodiment, the chiral polymers are polylactic acid (PLA) polymers, especially, are poly-L-lactic acid (PLLA) polymers. A PLLA polymer as a chiral polymer has a principle chain of a helical structure. Uniaxially drawing PLLA to orient its molecules gives PLLA piezoelectricity. When the tabular surface of the piezoelectric film10is pressed, uniaxially drawn PLLA, i.e., the piezoelectric film10generates charges. Charges generated by the piezoelectric film10are converted into a voltage through the circuit6. At this time, the amount of the voltage detected at the circuit6depends on the amount of displacement of the tabular surface in a direction perpendicular to the tabular surface, the displacement being caused by the press.

According to the first embodiment, the direction of uniaxial drawing of the piezoelectric film10(PLLA) makes an angle of 45 degrees against the Y direction and the Z direction, as indicated by an arrow901inFIG. 4. This angle of 45 degrees includes, for example, angles ranging from 45 degrees plus 10 degrees to 45 degrees minus 10 degrees. In this manner, a press to the piezoelectric film10causes it to generate a voltage.

Because PLLA becomes piezoelectric when subjected to a molecule orientation process, such as drawing, it does not need to be subjected to a polling process, whereas other polymers or piezoelectric ceramics, such as polyvinylidene difluoride (PVDF), must be subjected to the polling process to make them piezoelectric. The piezoelectricity of PLLA, which does not belong to a ferromagnetic material group, is not developed by ion polarization, which gives such ferromagnetic materials as PVDF and lead zirconate titanate (PZT) piezoelectricity, but originates from the helical structure characteristic of molecules making up PLLA. For this reason, PLLA does not have pyroelectricity that other ferromagnetic piezoelectric materials have. Havin no pyroelectricity, PLLA is not affected by the temperature of the user's finger or frictional heat. This allows the piezoelectric element20to be formed as a thin film. While PVDF shows time-dependent fluctuations of its piezoelectric constant, which may drop significantly in some cases, the piezoelectric constant of PLLA is extremely stable, not fluctuating depending on time passage. Thus, without being affected by the surrounding environment, displacement in shape caused by a press can be detected with high sensitivity.

According to the exemplary embodiment, the first electrode11and the second electrode12formed respectively on both main surfaces of the piezoelectric film10can be provided as metal electrodes made of aluminum, copper, or the like. When these electrodes are needed to be transparent, the first electrode11and the second electrode12may be made of a material with high transparency, such as indium tin oxide (ITO) and poly-3,4-ethylenedioxythiophene (PEDOT). The first electrode11and second electrode12of the above structure can obtain charges generated by the piezoelectric film10, as a voltage and output a press detection signal indicative of a voltage value corresponding to a press quantity.

FIG. 5is a diagram for explaining a relationship between time and a generated voltage in a case where the pressing sensor of the first embodiment is subjected to a long press operation.

When the receiving unit9receives a press operation, the piezoelectric element20outputs a voltage based on the reference voltage (e.g., a ground voltage) at the ground electrode, as indicated inFIG. 5. In the beginning, when displaced in shape by the press operation, the piezoelectric element20generates a positive or negative voltage. In this embodiment, a case where the piezoelectric element20in this condition generates a positive voltage will be described. When the generated voltage exceeds a first threshold (Th1) on the positive side, the press operation determining unit25determines that the receiving unit9is in a state of receiving a press operation. For convenience, an elapsed time at this point of determination is defined as a time (t0). A threshold on the negative side in a steady state is defined as a fourth threshold (Th4).

After elapse of the time (t0), the generated voltage peaks at a certain point of time. The voltage at this point of time is defined as Vmax. Afterward, reacting to the press operation, the piezoelectric element20makes a restitutive action, and thereby generates a negative voltage. It is preferable that at a second prescribed time (t2) after the time (t0) at which it is determined that the receiving unit9has received the press operation, the setting unit26switch the current threshold to set a third threshold (Th3) of which the absolute value representing a difference from the ground voltage value is larger than the absolute value of the fourth threshold (Th4). For example, when the voltage generated by the piezoelectric element20peaks, the setting unit26switches the fourth threshold (Th4) on the negative side to set a new threshold, i.e., the third threshold (Th3) larger than the fourth threshold (Th4) in magnitude on the negative side. This threshold switching prevents the press operation determining unit25from erroneously detecting a negative voltage generated as a result of the restitutive action made in reaction to the press operation. The fourth threshold (Th4) may be set in advance as the threshold equal to the third threshold (Th3).

It is also preferable that the setting unit26set the third threshold (Th3) in accordance with the maximum Vmax of the voltage output from the piezoelectric element20. The magnitude of the restitutive action made in reaction to the press operation is roughly proportional to the magnitude of the press operation. For this reason, by setting the third threshold (Th3) with reference to the maximum Vmax of the voltage generated by the press operation, detection of a negative voltage generated as a result of the restitutive action made in reaction to the press operation can be prevented more accurately.

At a point of time at which the first prescribed time (e.g., t1) has passed from the point of time at which the press operation determining unit25determines that the receiving unit9has received the press operation, that is, a point of time at which a prescribed time has passed from the point of time at which the setting unit26sets the third threshold (Th3), the setting unit26switches the current threshold to set a second threshold (Th2) smaller in magnitude on the negative side than the third threshold (Th3).

It is preferable that a period during which the setting unit26maintains the third threshold (Th3) as the current threshold, that is, a period between the second prescribed time (t2) and the first prescribed time (t1) be set with reference to the maximum Vmax of the voltage generated by the press operation. The length of a period during which the restitutive action made in reaction to the press operation lasts is roughly proportional to the magnitude of the press operation. For this reason, by setting the second prescribed time (t2) and the first prescribed time (t1) with reference to the maximum Vmax of the voltage generated by the press operation, detection of a negative voltage generated as a result of the restitutive action made in reaction to the press operation can be prevented more accurately.

After elapse of the first prescribed time (t1), the setting unit26switches the current threshold to set the second threshold (Th2). When the finger is removed slowly from the receiving unit9in a long press condition in which little voltage fluctuations result, peaks develop in a relatively broad range, where the peaks are on the same positive side as the peak developing at detection of the press operation is and on the negative side opposite to the positive side, as shown inFIG. 5. Because the absolute value of the second threshold (Th2) is set as a relatively small value, even if a voltage change caused by removal of the finger from the receiving unit9is small, the resulting voltage peak is well below the second threshold (Th2) (i.e., in the negative side). The press operation determining unit25is thus able to determine that the receiving unit9has ended reception of the press operation when the resulting voltage passes the second threshold (Th2). The pressing sensor100, therefore, can detect a pressing time, which is denoted as S inFIG. 5, in a long press operation. In this case, a point of time at which the press operation has ended is defined as a time (t3).

According to an exemplary aspect, it is preferable that the second threshold (Th2) be of a size that prevents the press operation determining unit25from picking noise created by a shift of the finger or the like during the long press to the receiving unit9. This allows the press operation determining unit25to accurately determine the end of the press operation even if the user's hand is shaky a little.

Moreover, it is preferable that when the press operation determining unit25determines that the press operation has been ended, the setting unit26switch the current threshold from the second threshold (Th2) to the fourth threshold (Th4) for the steady state at a time (t3). This allows smooth detection of a press operation even if the user subsequently carries out a long press operation again.

According to this embodiment, a plurality of the receiving units9may be formed and each of them may be disposed in any given location other than the display5in the housing2.

FIG. 6is a diagram for explaining arrangement of a piezoelectric element according to a second embodiment.FIG. 7is a diagram for explaining a relationship between time and a generated voltage in a case where a pressing sensor of the second embodiment is subjected to a long press operation. The pressing sensor of the second embodiment will hereinafter be described. The same constituent elements as constituent elements included in the pressing sensor100of the first embodiment will be omitted in further description for clarity purposes. InFIG. 6, therefore, out of the constituent elements of the pressing sensor of the second embodiment, only the constituent elements different from constituent elements of the pressing sensor100are depicted. InFIG. 7, a chart on the lower side shows output from a different sensor element.

As shown inFIG. 6, a pressing sensor200of the second embodiment includes a sensor element60, in addition to the piezoelectric element20. The sensor element60is equivalent to a second sensor element according to the present invention. Similar to the piezoelectric element20, the sensor element60is pasted to the interior of the side face3, using, for example, an adhesive tape, bond, or the like. The sensor element60is a sensor element that is configured to detect its displacement in shape or the like. In other words, the sensor element60is an auxiliary sensor element that detects displacement in shape of the piezoelectric element20indirectly when the user makes a press to the receiving unit9. As the sensor element60, for example, a piezoelectric film sensor, a proximity sensor, an illumination sensor, an acceleration sensor, a gyro sensor, a pressure sensor, or the like can be used.

When the receiving unit9of the housing2receives a press operation such that a part having received the press operation is near the piezoelectric element20, the sensor element60, which receives an effect of the press operation, is configured to generate certain signals. For example, when the sensor element60is a pressure sensor, the sensor element60is displaced in shape as a result of displacement in shape of the piezoelectric element20. As shown in a chart on the lower side ofFIG. 7, a signal from the sensor element60is detected at the start and end of a long press.

The press operation determining unit25thus detects not only the output from the piezoelectric element20, but also signals from the sensor element60. This allows the setting unit26to determine a point of time at which the setting unit26switches the threshold based on a signal from the sensor element60. Thus, the setting unit26can set a point of time of switching the threshold more accurately.

If a part other than the receiving unit9receives a press operation on the side face3of the housing2, the sensor element60generates a signal different from a signal that the sensor element60generates when the receiving unit9receives a press operation. The press operation determining unit25thus determines whether a signal generated by the sensor element60is the signal that the sensor element60generates when the receiving unit9receives a press operation. Through this process, only the case of the receiving unit9receiving a press operation can be detected more accurately. When the press operation determining unit25determines that the sensor element60has generated a signal different from the signal that the sensor element60generates when the receiving unit9receives a press operation, the setting unit26is stopped from switching the threshold. This suppresses unnecessary power consumption.

FIG. 8is a diagram for explaining arrangement of a piezoelectric element according to a third embodiment.FIG. 9is a diagram for explaining a relationship between time and a generated voltage in a case where a pressing sensor of the third embodiment is subjected to a long press operation. The pressing sensor of the third embodiment will hereinafter be described. The same constituent elements as constituent elements included in the pressing sensor200of the second embodiment will be omitted in further description for clarity purposes. InFIG. 8, therefore, out of the constituent elements of the pressing sensor of the third embodiment, only the constituent elements different from constituent elements of the pressing sensor200are depicted. InFIG. 9, a chart on the lower side shows output from a different sensor element.

As shown inFIG. 8, a pressing sensor300of the third embodiment includes a sensor element80in place of the sensor element60. The sensor element80is equivalent to a third sensor element according to the present invention. The sensor element80is a sensor element that detects minute tremors of a living body (so-called living body's tremors) to obtain tremor information.

When the user makes a press to the receiving unit9, the sensor element80detects living body's tremors that are transmitted through the housing2to the sensor element80. For example, as shown inFIG. 9, in a period in which the user's finger is in contact with the housing2, the sensor element80keeps outputting a signal. Based on detected living body's tremors, the press operation determining unit25determines whether the signal output from the sensor element80is a signal that the sensor element80generates when the receiving unit9receives a press operation. Through this process, only the case of the receiving unit9receiving a press operation can be detected more accurately.

The size of the signal changes depending on the distance of a part having received the press operation to the sensor element80on the side face3of the housing2. For this reason, whether the receiving unit9has been subjected to the press operation can be determined based on a positional relationship between the sensor element80and the piezoelectric element20. Based on the magnitude of detected living body's tremors, therefore, the receiving unit9having been subjected to the press operation can be detected more clearly.

FIG. 10(A)is an explanatory diagram of a first modification,FIG. 10(B)is an explanatory diagram of a second modification, andFIG. 10(C)is an explanatory diagram of a third modification of the exemplary embodiments. The first to third modifications are different from the first embodiment only in that their second thresholds (Th2) on the negative side are each different from the second threshold (Th2) of the pressing sensor100of the first embodiment. These second thresholds (Th2), therefore, will be described and other constituent elements will not be described for clarity purposes.

In the pressing sensor100of the first embodiment, the second threshold (Th2) remains constant for a given period. In contrast, the second threshold (Th2) of the first modification increases in proportional to a time elapsed during a given period (t1 to t3), as shown inFIG. 10(A). The second threshold (Th2) of the second modification increases step by step during the given period (t1 to t3), as shown inFIG. 10(B). The second threshold (Th2) of the third modification increases in a curved pattern during the given period (t1 to t3), as shown inFIG. 10(C). In this manner, the second threshold (Th2) can be changed into various patterns of threshold with respect to a time elapsed. The second threshold (Th2), therefore, can be set properly in accordance with the shape, hardness, or the like of the housing2. By increasing the second threshold (Th2) in proportional to a time elapsed in such a manner as depicted inFIGS. 10(A) to 10(C), erroneous detection caused by the effects of unnecessary noise can be further reduced.

It is noted that the structures according to the above exemplary embodiments can be applied not only to a smart phone, for example, but also to various electronic devices. According to the above embodiments, providing the housing with an opening, which is necessary in the case of a mechanical switch, is unnecessary. The electronic device of the above embodiments is thus used preferably as a device required to be waterproofing, such as a switch set in a washing machine or a bathroom.

Moreover, it is noted that the above exemplary embodiments have been described exemplarily in their all aspects and that the embodiments do not pose any limitation. The scope of the present invention will be clarified not by the above embodiment but by claims. The scope of the present invention includes what is claimed by claims.

DESCRIPTION OF REFERENCE SYMBOLS