Drop shock measurement system and acceleration sensor element used in the same

The drop impact measuring system has i) a plurality of bimorph-type acceleration sensor containing a plurality of free vibrating sections each of which has individual draw-out electrodes; ii) a switch section for selecting output from the bimorph-type acceleration sensors; iii) an amplifying circuit for amplifying a signal applied via the switch section from the acceleration sensors; and iv) a logic circuit for logically evaluating the output from the amplifying circuit and controlling the switch section according to the result acquired from the logical evaluation.

TECHNICAL FIELD

The present invention relates to a drop impact measuring system used for a drop impact test on mobile electronic equipment including a mobile phone, and also relates to an acceleration sensor element employed for the drop impact measuring system.

BACKGROUND ART

For mobile electronic equipment including a mobile phone, a notebook size computer, a mobile cassette player, compact disc (CD) player, and mini disc (MD) player, dropping of the equipment is an ever-present danger when considering its intended use. Accordingly, protecting the equipment from impact caused by dropping the equipment has now been a growing need. A typical failure comes from distortion of a motherboard mounted on the equipment due to drop impact, by which some on-board components have shorts in the wiring, or come off the board. Therefore, to protect the equipment from such accidents, the following steps should be taken: i) selecting a material and a structure of electronic equipment to be tested; ii) determining a drop height and direction; iii) simulating drop impact acceleration applied to each section of the equipment; and then iv) getting feedback from the result and improving the design of the inner structure, for example, the position and method of installing a circuit board. However, the drop impact acceleration during falling greatly varies between different falling objects, and the acceleration applied to an object has significant consequence to the impact force. Furthermore, mechanical vibration and its corresponding frequency caused by the drop impact greatly depends on the structure of an object. The reasons above have been obstacles to detecting drop impact acceleration applied to a falling object.

DISCLOSURE OF THE INVENTION

The drop impact measuring system has i) a plurality of bimorph-type acceleration sensors containing a plurality of free vibrating sections each of which has individual draw-out electrodes; ii) a switch section for selecting output of the bimorph acceleration sensors obtained through the draw-out electrodes; iii) an amplifying circuit for amplifying at least one of voltage and current applied via the switch section from the bimorph acceleration sensors; and iv) a logic circuit for logically evaluating the output from the amplifying circuit and controlling the switch section according to the result acquired from the logical evaluation.

A bimorph acceleration sensor element is formed of the free vibrating sections having cantilever beams. Signals generated at the free vibrating section are fed through the draw-out electrodes to the switch section.

The bimorph acceleration sensor element is also formed of the free vibrating section having both-ends-supported beams. Signals generated at the free vibrating section are fed through the draw-out electrodes to the switch section.

BEST MODE FOR CARRYING OUT OF THE INVENTION

First Exemplary Embodiment

The present invention relates to method and apparatus which is used in the development of electronic equipment. It is known that electronic equipment (e.g. consumer electronics) is prone to damage when exposed to physical shock. Such physical shock may occur, for example, when the electronic equipment is accidentally dropped on a hard surface. Thus, it is desirable to design and build electronic equipment in a manner so that it can receive as much physical shock as possible and still continue to operate. As discussed in the “Background of the Invention,” exposing electronic equipment to physical shock can cause the equipment to malfunction by distorting the equipment's motherboard or causing shorts in the wiring.

Thus, during the design phase of electronic equipment, it is desirable to place some sort of sensor and then to drop the equipment (with the sensor in place). The sensor can provide an indication of the amount of physical shock sustained by the equipment as a result of, for example, being dropped. Based on the information provided by the sensor, the physical design of the equipment can then be modified in order to decrease the effects of physical shock.

One problem with this approach is determining what type of sensor to place in the equipment for this type of testing. For this purpose, an acceleration sensor element may be used. It is desirable, however, to select an acceleration sensor which, when dropped with the electronic equipment being tested, does not have its vibration frequency bandwidth threshold exceeded. In accordance with the present invention, a method and apparatus are provided to test electronic equipment with various free vibrating sections until a free vibrating section is identified which does not exceed its vibration frequency bandwidth when exposed to physical shock. Having selected that free vibrating section, that free vibrating section in an acceleration sensor as development of the electronic equipment continues.

FIG. 1shows a circuit diagram of a drop impact measuring system in accordance with a first embodiment of the present invention. Acceleration sensors101,102, and103are connected to switches106,107, and108, respectively. Sensors101through103are also connected to ground109. By closing one of switches106through108, an output signal from one of acceleration sensors101through103is fed into amplifying circuit104to be amplified. Amplifying circuit104outputs a signal, which corresponds to acceleration, via terminal110. Logic circuit105evaluates whether or not the signal from amplifying circuit104stays within a predetermined frequency threshold and whether resonance occurs or not, and then accordingly outputs logically evaluated result. Logic circuit105determines which one of switches106through108should be closed, according to the logical evaluation, thereby controlling switches106through108.

FIGS. 5A through 5Dshow the structure of an acceleration sensor element, which is the major component of acceleration sensors101through103.FIGS. 5A through 5Dshows an acceleration sensor element having a both-ends-supported beam structure.FIG. 5Ais a top view;FIG. 5Bis a bottom view;FIG. 5Cis a side view; andFIG. 5Dis a perspective view. Each of free vibrating sections545through547shown inFIGS. 5A through 5Dhas a bimorph structure in which distortion and mechanical vibration occurs when an impact force is applied. Free vibrating sections545,546, and547have main electrodes501,502, and503on each upper surface thereof, and have main electrodes520,521, and522on each lower surface thereof, respectively. Draw-out electrodes504and505are connected with main electrode501; draw-out electrodes506and507are connected with main electrode502; and draw-out electrodes508and509are connected with main electrode503to establish electrical connections, respectively. Free vibrating sections545through547generate electric charges according to distortion in shape due to their bimorph structures. The electric charge generated on each upper surface of free vibrating sections545,546, and547is carried to main electrodes501,502, and503, respectively, and further carried to draw-out electrodes504and505;506and507;508and509, respectively. On the other hand, the electric charge generated on each lower surface of free vibrating sections545,546, and547is carried to main electrodes520,521, and522, respectively, and further carried to draw-out electrodes523and524;525and526;527and528, respectively. The electric charges carried to main electrodes520through522are opposite in polarity to those carried to main electrodes501through503. Draw-out electrodes523through528are extended to the bottom of supporters541through544, via the side surfaces of the supporters, and are exposed at the bottom of the each supporter.

The deformation of free vibrating sections545through547creates oppositely polarized electric charges: one is drawn out by draw-out electrodes504through509disposed on the upper surface of free vibrating sections545through547; the other is drawn out by draw-out electrodes523through528exposed at the bottom of supporters541through544.

The reason why draw-out electrodes504through509and523through528are formed as a part of free vibrating sections545through547is as follows: such a structure can avoid cancellation of the drawn out electric charge due to stress distribution in free vibrating sections545through547. in the structure above, for example, each width of draw-out electrodes504and505is determined less than one-fifth of that of main electrode501.

Free vibrating sections545,546, and547have lengths of L3, L2, and L1, respectively, and in which L3is the longest, and L1is the shortest. Free vibrating sections545through547are electrically separated each other.

FIG. 2Ashows the circuit diagram of acceleration sensors101through103shown inFIG. 1. Acceleration sensor element201is the one the same as shown inFIGS. 5A through 5D. One of draw-out electrodes523and524shown inFIGS. 5A through 5Dis connected to ground205ofFIG. 2A. On the other hand, one of draw-out electrodes504and505shown inFIGS. 5A through 5Dis connected to resistor202ofFIG. 2A. The electric charge obtained from draw-out electrode504or505is converted through resistor202into current. Besides, feeding the current through resistor202allows resistor202to generate voltage. In the case that resistor202is required to have large resistance value more than 1 MΩ, employing field-effect transistor (FET)203allows the circuit to have lower impedance. The source of FET203is connected, through terminal206, to the positive side of a power-supplying unit; the gate is connected to the acceleration sensor element201and resistor202; and the drain is connected to resistor204. The other terminal of resistor204is connected to ground205. The potential of the drain is taken out through terminal207. Forming the circuit like this allows terminal207to have lower output impedance.

The acceleration sensor element shown inFIGS. 5A through 5Dalso has free vibrating sections546and547. Each signal obtained from the two sections is similarly processed, as shown inFIG. 2A.

A signal from terminal207is fed to the corresponding one of switches106through108.

In the embodiment, ID numbers1through3are assigned to the acceleration sensors having free vibrating sections545through547, respectively, and a letter “S” indicates the ID number: S=1 in acceleration sensor101containing free vibrating section545; S=2 in acceleration sensor102containing free vibrating section546; and S=3 in acceleration sensor103containing free vibrating section547. Similarly, a letter “N” represents the total number of acceleration sensors: N=3 in the acceleration sensors shown inFIGS. 5A through 5D.

Although the explanation above introduces the structure in which i) generating current by moving the electric charge obtained from one of draw-out electrodes504and505; ii) converting the current into voltage by feeding through resistor202; and then iii) amplifying the voltage, it is not limited thereto: the structure in which current is directly amplified is also acceptable.

Acceleration sensor element201generally bears capacitance—working the capacitance with resistor202inevitably forms a filter that cuts off low band frequencies. Therefore, the signals generated in acceleration sensor element201have decreased low band frequencies. In this case, the low-band cut-off frequency, “Fc” is given by the expression below.
Fc=1/(2π×R×Cs)  (Expression. 1),
where, Cs represents capacitance of acceleration sensor element201; R represents resistance of resistor202.

FIG. 3is a flow chart illustrating the routine of measuring drop impact in the circuit shown inFIG. 1. The purpose of this routine is to identify the free vibrating section most suitable for measuring drop impact for electronic equipment having a drop impact detection device. In the first step300, an acceleration sensor including the free vibrating section that measures the lowest mechanical vibration frequency bandwidth is selected from free vibrating sections545through547of different lengths as shown inFIGS. 5A through 5D; acceleration sensor101including free vibrating section545measures the lowest mechanical vibration frequency bandwidth, that is, “S” takes on 1.

In step301, among switches106through108, only one switch associated with the acceleration sensor selected in step300is closed, whereby sensor switching is performed. At the moment, since “S” retains the value of 1, only switch106ofFIG. 1is closed.

In step302, the drop down test begins.

In step303, drop impact is measured. Amplifying circuit104amplifies signals from acceleration sensor101generated by the drop impact and then outputs the measured result (i.e., impact acceleration) through terminal110shown inFIG. 1.

In step304, logic circuit105evaluates whether the output signal from amplifying circuit104stays within a vibration frequency bandwidth threshold or not, and whether resonance occurs or not.

In step306, if the impact acceleration stays within the vibration frequency bandwidth threshold, the acceleration is displayed in step312. A display for showing the impact acceleration is not shown inFIG. 1.

On the other hand, if the impact acceleration exceeds the vibration frequency bandwidth threshold, the procedure goes to step307. In step307, the value currently stored in S is compared with the value stored in N.

If the comparison finds that the value of S equals to that of N—which means that each of the output signals generated from free vibrating sections545through547, shown inFIGS. 5A through 5D, via amplifying circuit104exceeds the vibration frequency bandwidth threshold in each of the free vibrating sections, the procedure goes to step311, where an error indication is shown on the display.

On the other hand, if S does not reach N, the procedure goes to step310where the value of S (that indicates a sensor number) is updated. That is, S is incremented by 1 in step310, and then the procedure goes back to step301. According to the updated S, next acceleration sensor is selected in step301, and the drop impact measurement is resumed from step302.

FIG. 4is another flow chart illustrating the routine of measuring drop impact in the circuit shown inFIG. 1. According to the flow chart shown inFIG. 3, if the result of the drop impact measurement obtained at step303exceeds the vibration frequency bandwidth threshold of S representing a sensor number, the value of S is incremented by 1 in step310, and then the drop-impact measurement is resumed at step302. On the other hand, in the flow chart ofFIG. 4, even if the first result of the measurement exceeds the vibration frequency bandwidth threshold of S, the logic circuit evaluates output signals to find a proper sensor number, and then the drop impact measurement is resumed with the acceleration sensor corresponding to the sensor number.

In step400, S initially takes on 1, and the drop impact test begins at step401. Next, in step402, amplifying circuit104amplifies the output signal of acceleration sensor101that has been entered through switch106.

In step403, logic circuit105evaluates whether the output signal from amplifying circuit104stays within a vibration frequency bandwidth threshold or not, and whether resonance occurs or not.

In step404, if the logic circuit evaluates that the impact acceleration stays within a vibration frequency bandwidth threshold, the acceleration is displayed in step408. A display for showing the impact acceleration is not shown inFIG. 1.

On the other hand, if the impact acceleration exceeds the vibration frequency bandwidth threshold of the current sensor, logic circuit105selects a proper sensor number according to the output signal of amplifying circuit104in step406.

In step407, a switch corresponding to the sensor number selected above is closed to resume the drop impact test.

In step408, the drop impact is measured and displayed.

An acceleration sensor for detecting drop impact is required to have differently ranged vibration frequency bandwidths, that is, desirably to have various frequencies, not only one.

In the acceleration sensor element shown inFIGS. 5A through 5D, suppose that “fr” represents the resonance frequency; “L” represents the length of the free vibrating section; “t” represents the thickness; and “α” represents a constant, “fr” is given by the expression below:
Fr∝α×T/L2(Expression 2).

That is, the resonance frequency “fr” varies inversely with the square of the length of the free vibrating section. The acceleration sensor element shown inFIGS. 5A through 5Dhas free vibrating sections545through547of different lengths, thereby offering various vibration frequency bandwidths. The acceleration sensor element shown inFIGS. 5A through 5Dis thus ready to detect various resonance frequencies within each of the free vibrating section bandwidths.

Employing such structured acceleration sensor element for a drop impact measuring system can easily detect impact acceleration, even in the case that a mechanical vibration frequency at the drop down of an object is unpredictable in advance. Accordingly, it will contribute to easy structural design of the housing.

Each ofFIGS. 7A through 7C, andFIGS. 8A through 8Dshows an acceleration sensor element having a cantilever beam structure.

FIG. 7Ais a top view,FIG. 7Bis a side view, andFIG. 7Cis a perspective view. The structure in which supporter744holds each one end of free vibrating sections741through743forms into a cantilever beam. Free vibrating sections741,742, and743have main electrodes701,702, and703on the upper surfaces thereof, and main electrodes721,722, and723on the lower surfaces thereof, respectively. Draw-out electrodes724,725, and726, which are formed on the bottom of supporter744, are electrically connected with main electrodes721,722, and723, respectively.

Each ofFIGS. 8A through 8Dshows another acceleration sensor element having the cantilever beam structure.FIG. 8Ais a top view,FIG. 8Bis a bottom view, andFIG. 8Cis a perspective view. The structure in which supporter843holds each one end of free vibrating sections841and842forms into a cantilever beam. Free vibrating section841has main electrode801on the upper surface thereof, and main electrode821on the lower surface thereof. Similarly, free vibrating section842has main electrode802on the upper surface thereof, and main electrode822on the lower surface thereof. Draw-out electrodes803and804are electrically connected with main electrodes801and802, respectively. Draw-out electrode823, which is formed on the bottom of supporter843, runs across side844of the supporter and reaches main electrode821to have electric connections. Similarly, draw-out electrode824, which is also formed on the bottom of supporter843, runs across the side opposite to side844of the supporter and reaches main electrode822to have electric connections.

When comparisons are made between the acceleration sensor element formed into a cantilever beam structure shown inFIGS. 7A through 7CandFIGS. 8A through 8C; and the acceleration sensor element formed into a both ends-supported beam structure shown inFIGS. 5A through 5D, the cantilever beam structure generates four to five times more amount of output electrical charge than the both ends-supported beam structure does, provided they have same dimensions. Therefore, amplifying circuit104ofFIG. 1, which is necessary for the acceleration sensor element having the both ends-supported beam shown inFIGS. 5A through 5D, can be eliminated by using the cantilever beam structure. Eliminating the amplifying circuit can provide a low-cost drop impact measuring system. Besides, the parallel arrangement of free vibrating sections741through743allows the whole structure of the acceleration sensor element to be compact, accordingly providing a downsized drop impact measuring system.

Employing the structure explained in the first embodiment can provide the drop impact measuring system that easily measures an impact acceleration of even more than 1000G having great variations in resonance frequency. At the same time, the structure of the embodiment also provides an acceleration sensor element capable of detecting impact acceleration of various resonance frequencies.

Second Exemplary Embodiment

FIG. 1also shows a circuit diagram of the drop impact measuring system of the second embodiment.FIG. 2Bshows a circuit diagram of the acceleration sensor of the second embodiment.FIGS. 3 and 4are flow charts illustrating the routine of detecting drop impact acceleration in the drop impact measuring system of the second embodiment. Each ofFIGS. 6A through 6Cshows an acceleration sensor element having a structure of cantilever beam of the second embodiment.

FIG. 6Ais a top view,FIG. 6Bis a bottom view, andFIG. 6Cis a perspective view. Each one end of free vibrating sections641through644are supportably fixed by supporter645. Free vibrating sections641,642,643, and644have main electrodes601,602,603, and604on the upper surfaces thereof, and have main electrodes620,621,622, and623on the lower surfaces thereof, respectively. Main electrodes601,602,603, and604are electrically connected with draw-out electrodes605,606,607, and608, respectively. Similarly, main electrodes620,621,622, and623are electrically connected with draw-out electrodes624,625,626, and627, respectively. Draw-out electrodes624through627are exposed from the bottom of supporter645. Free vibrating sections641,642,643, and644are of the same length, which are represented by L4, L5, L6, and L7, respectively.

Using the acceleration sensor element shown inFIGS. 6A through 6C, the acceleration sensor is formed, as shown inFIG. 2B. That is, acceleration sensor element220is formed of free vibrating section641; main electrodes601and620; and draw-out electrodes605and624. Of four acceleration sensor elements shown inFIGS. 6A through 6C, one is used for sensor element220; any one of the rest three is to be employed for sensor element221. Draw-out electrodes624through627are connected to ground225. On the other hand, draw-out electrodes605through608, which are electrically connected in parallel, and then connected to resistor222. Resistors222and224, FET223, terminals226and227shown inFIG. 2Bact the same as resistors202and204, FET203, terminals206and207shown inFIG. 2A, respectively, therefore the explanation of the components above will be omitted. Such structured acceleration sensor serves as acceleration sensor101shown inFIG. 1.

Although acceleration sensors102and103are formed in the same manner with sensor101, each acceleration sensor element used for these three sensors has a different frequency bandwidth range for mechanical vibration. That is, the acceleration sensor can measure acceleration values in wide-ranged frequency bandwidth.

To measure greater drop impact acceleration, miniaturizing the acceleration sensor element and increasing mechanical strength seems to be an effective way. According to a prior-art acceleration sensor element, however, miniaturization of a sensor element has lowered electric capacity and thereby degraded durability of noise characteristics of the sensor element itself Whereas the sensor element introduced in the second embodiment, by virtue of the structure in which the free vibrating sections of same resonance frequencies are electrically connected in parallel, has no decrease in electric capacity if the sensor element itself is miniaturized. Therefore, the structure described in the second embodiment can provide a high-noise-durability acceleration sensor element and a drop impact measuring system using the sensor element.

A prior-art acceleration sensor element, as described above, lowers electric capacity as it is miniaturized. As a result, the “Fc” given by the Expression 1, which represents low-band cut-off frequency, takes on a large value. Whereas the acceleration sensor element of the second embodiment has no decrease in electric capacity, thereby keeping the low-band cut-off frequency “Fc” from getting high.

Although the acceleration sensor of the second embodiment is formed of the acceleration sensor element having the shape illustrated inFIGS. 6A through 6C, it is not limited thereto: the similar effect can be obtained by the structure having free vibrating sections741,742, and743(FIGS. 7A through 7C) of same lengths, i.e., L10=L9=L8; the structure having free vibrating sections841and842(FIGS. 8A through 8C) of same lengths, i.e., L11=L12; or the structure having free vibrating sections545,546, and547(FIGS. 5A through 5D) of same lengths, i.e., L3=L2=L1.

INDUSTRIAL APPLICABILITY

The drop impact measuring system and the acceleration sensor element employed for the system of the present invention can cope well with a wide range of mechanical vibration frequencies and large impact accelerations. Besides, the acceleration sensor element can be miniaturized and improved in durability of noise characteristics.