Abstract:
A calculating device of the stiffness measurement device pressurizes an object to be measured with a predetermined pressure and the stiffness of the object to be measured in the squeal frequency band is calculated based on an inclination of a stress-displacement performance curve immediately after a start of depressurization after the pressurization. According to this device, there is no need to oscillate the object with a high frequency band and there is no need to enhance the stiffness of the housing of the measurement device, which leads to a downsizing of the device. Further, there is no need for measuring of acceleration speed of the object to be measured and accelerator can be eliminated to reduce the cost of the stiffness measurement device.

Description:
TECHNICAL FIELD   
       [0001]    This invention relates to a method and a device for measuring the stiffness of an object to be measured and more particularly to a method and a device for measuring the stiffness (equivalent value) of a friction pad material in a squeal frequency band for a brake disc device. 
       BACKGROUND ART   
       [0002]    A disc brake device adapted to a vehicle brake device sometimes generates a so-called squeal noise when the brake is applied. This squeal noise is considered to be generated due to a coupled oscillation of the disc plate and the friction pad material and a study has been made focusing on the frictional contact portion between the disc plate and the friction pad material, between which the force transmitting is made (See non-patent literature 1). At the early stage of the study, a model analysis of a contact point where vibration occurs with the same phase and the same amplitude in the friction contact portion was conducted. According to the analysis, a qualitative tendency of the squeal could be represented, but a restoration moment about the contact point could not be taken into consideration and as a result, the qualitative tendency of the squeal did not agree with the quantitative tendency. Accordingly, in order to solve this problem, it is necessary to conduct the model analysis of the contact surface by measuring the stiffness of the dynamic stiffness of the contact surface. 
         [0003]    In the non-patent literature 1, as a measurement device for measuring the dynamic stiffness of the contact surface, a device equipped with an oscillation device for oscillating the friction pad material under a pressurized state, a load measurement device which measures the load applied to the friction pad material and an acceleration measurement device which measures the acceleration of the friction pad material generated by the oscillation. This dynamic stiffness measurement device first oscillates the friction pad material with a random wave of squeal oscillation frequency band (1 kHz to 5 kHz) and measures the acceleration speed of the friction pad material generated by the oscillation. Thus, the transfer function (Accelerance) from the force to the acceleration is obtained and the resulted value is converted into the dynamic stiffness. Further, a device similar to the device above is disclosed in the patent literature 1, although the device disclosed therein relates to a measurement device for measuring the spring constant. 
       Citation List   
     Patent Literature   
       [0004]    Patent Literature 1: JP4059836B2 
       Non-Patent Literature   
       [0005]    Non-patent literature 1: “Influence of Dynamic Stiffness in a Contact Region on Disk Brake Squeal”, No. 614 of CD-ROM Thesis Paper, Dynamics and Design Conference 2004 Vol. No. 04-5, The Japan Society of Mechanical Engineers. 
       SUMMARY OF INVENTION   
     Technical Problem(s)   
       [0006]    According to the dynamic stiffness measurement device of the above mentioned literatures, it is necessary to first obtain a transfer function (Accelerance) from the force to the acceleration and then to convert the obtained result into the stiffness. Accordingly, this device has a tendency of becoming complicated. Further, according to this device, it is necessary to oscillate the friction pad material with a relatively high frequency and accordingly, it is necessary to enhance the stiffness of housing of the dynamic stiffness measurement device. This may lead to an increase of size of the device. Further, it is necessary to measure the acceleration speed of the friction pad material and an acceleration measurement device has to be equipped, which leads to an increase of cost in manufacturing the dynamic stiffness measurement device. 
         [0007]    Accordingly, this invention was made in consideration with the above-mentioned situation and the objective of the invention is to provide a stiffness measurement method which can easily measure the stiffness (equivalent value) of an object to be measured in the squeal frequency band and to provide a stiffness measurement device which is small in size and low in cost. 
       Solution to Problem(s)  
       [0008]    The method for measuring a stiffness of an object to be measured in a squeal frequency band according to the invention associated with a first aspect includes a supporting step for supporting the object to be measured, a pressurizing step for pressurizing the object to be measured which has been supported, a gradual depressurizing step for gradually depressurizing the object to be measured after a pressure applied thereto by the pressurizing step has reached to a predetermined value, a measuring step for measuring the pressure applied to and the pressure depressurized from the object to be measured and a displacement of the object to be measured when the object to be measured is pressurized and depressurized at the steps of the pressurizing step and the gradual depressurizing step and a calculating step for calculating the stiffness of the object to be measured in the squeal frequency band based on an inclination of a stress-displacement performance curve immediately after a start of the depressurizing step for gradually depressurizing the pressure applied to the object to be measured. 
         [0009]    The invention associated with a second aspect pertains to a stiffness measurement device which measures the stiffness of an object to be measured in a squeal frequency band. The stiffness measurement device includes a support device which supports the object to be measured, a pressurizing device which can pressurize and/or depressurize the object to be measured which has been supported by the support device, a pressure measurement device which measures a pressure to be applied to or depressurized from the object to be measured, a displacement measurement device which measures a displacement of the object to be measured when the object to be measured is pressurized or depressurized and a calculating device which calculates the stiffness of the object to be measured in the squeal frequency band based on an inclination immediately after a start of a depressurization in a stress-displacement performance diagram by obtaining the stress-displacement diagram upon the depressurization after the pressurization of the object to be measured to a predetermined pressure level. 
         [0010]    The invention associated with a third aspect is characterized in that stiffness measurement device as set forth in the second aspect includes the pressurizing device which gradually depressurizes the pressure of the object to be measured and the displacement measurement device which measures the displacement of the object to be measured at the time of a gradual depressurizing. 
         [0011]    The invention associated with a fourth aspect is characterized in that the object to be measured as set forth in the second aspect or the third aspect includes a friction pad material of a disc brake and the calculating device which calculates the stiffness of the object to be measured in the squeal frequency band based on the inclination that the displacement of the object to be measured has reached to an amplitude of oscillation of squeal of the friction pad material. 
         [0012]    According to the invention associated with the first aspect, the stiffness of the object to be measured in the squeal frequency band is calculated based on an inclination of a stress-displacement performance curve (diagram) immediately after a start of depressurization in the step for gradually depressurizing a pressure applied to the object to be measured, after a pressurization of the object to be measured to a predetermined pressure level. Accordingly, the stiffness of the object to be measured in the squeal frequency band can be easily measured without obtaining a transfer function (Accelerance) transferring from a force to acceleration and converting the obtained result of the transfer function into the dynamic stiffness, which conventionally was necessary. 
         [0013]    According to the invention associated with the second aspect, the calculating device calculates the stiffness of the object to be measured in the squeal frequency band based on the inclination immediately after the start of depressurization in a stress-displacement diagram upon a gradual depressurization after pressurization of the object to be measured to a predetermined pressure level. Accordingly, since there is no need to oscillate the object to be measured with a high frequency and further no need to enhance stiffness of the housing as required in the conventional device, the device as a whole can be down-sized. Further, according to the invention, there is no need to measure the acceleration of the object to be measured as was necessary for the conventional device, the acceleration measurement device can be omitted which leads to a cost reduction. 
         [0014]    According to the invention associated with the third aspect, the displacement of the object to be measured when the object is gradually depressurized. Accordingly, a displacement measurement device which can measure a displacement of an object relatively in a low frequency band can be used. 
         [0015]    According to the invention associated with the fourth aspect, the stiffness of a contact surface between the disc plate and the friction pad material in a squeal frequency band can be obtained. Accordingly, by analyzing the contact surface model, the squeal generated by coupled oscillation between the disc plate and the friction pad material can be quantitatively represented. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS   
         [0016]      FIG. 1  is a general view of a stiffness measurement device according to an embodiment of the invention; 
           [0017]      FIG. 2  is a flowchart explaining an operation of the stiffness measurement device according to the embodiment. 
           [0018]      FIG. 3  is a cross sectional view of the stiffness measurement device according to the embodiment taken along the line A-A in  FIG. 1 , showing a state that the measurement preparation has been completed; 
           [0019]      FIG. 4  is a stress-displacement diagram measured by the stiffness measurement device according to the embodiment; 
           [0020]      FIG. 5  is a graph showing a relationship between the stiffness in the squeal frequency band measured by using a sine wave by the pressurizing device of the stiffness measurement device according to the embodiment of the invention and the dynamic stiffness measured by a conventional dynamic stiffness measurement device. 
           [0021]      FIG. 6  is a graph showing a relationship between the stiffness in the squeal frequency band measured by using a triangle wave by the pressurizing device of the stiffness measurement device according to the embodiment of the invention and the dynamic stiffness measured by a conventional dynamic stiffness measurement. 
           [0022]      FIG. 7  is a stress-displacement diagram showing the relationship between the stress and the displacement measured by changing the frequency by the pressurizing device of the stiffness measurement device according to the embodiment. 
           [0023]      FIG. 8  is a graph showing ratio of the stiffness in the squeal frequency band measured by changing the frequency by the pressurizing device of the stiffness measurement device according to the embodiment relative to the dynamic stiffness measured by the conventional measurement device. 
           [0024]      FIG. 9  is a graph showing ratio of the static stiffness measured by changing the frequency by the pressurizing device of the stiffness measurement device according to the embodiment relative to the static stiffness measured by the conventional measurement device. 
       
    
    
     DESCRIPTION OF EMBODIMENTS   
       [0025]    The embodiment of the present invention will be explained below with reference to the attached drawings. As shown in  FIG. 1 , the stiffness measurement device  1  according to the embodiment includes a support device  10  which supports an object to be measured P (for example, a test piece cut from a friction pad material of a disc brake device), a pressurizing device  20  which pressurizes the object to be measured P supported by the support device  10  or depressurizes the pressure of the object to be measured P which has been pressurized, a pressure measurement device  30  which measures the pressure to be applied to or depressurized from the object to be measured P, a displacement measurement device  40  which measures a displacement of the object to be measured P when the object to be measured P is pressurized or depressurized and a control device  50  which inputs the measurement data sent from the pressure measurement device  30  and the displacement measurement device  40 . 
         [0026]    The support device  10  includes a housing  11 , a mount  12  for mounting the object to be measured P and a fixing screw  13  which fixes the object to be measured P to the mount  12 . The housing  11  is formed to be a box-shape having upper portion  11   a , a bottom portion  11   b  side portions  11   c  and  11   d  and a back portion  11   e.  The mount  12  is fixed to the bottom portion  11   b  of the housing  11  together with the pressure measurement device  30 , the structure of which will be explained later. The fixing screw  13  includes a handle portion  13   a  and a male screw portion  13   b  provided on the handle portion  13   a  and projecting downwardly therefrom to penetrate through the upper portion  11   a  of the housing  11  located lower side relative to the handle portion  13   a  of the fixing screw  13 . The male screw portion  13   b  is mated with a female screw portion  11   f  which is provided in the upper portion  11   a  and penetrating therethrough. 
         [0027]    The pressurizing device  20  includes four columnar actuators  21 ,  22 ,  25  and  26  and two fixing plates  23  and  24  for fixing the actuators  21 ,  22 ,  25  and  26  by sandwiching by the two fixing plates. The actuators  25  and  26  are located behind the actuators  21  and  22  (See  FIG. 3 ), respectively. These actuators  21 ,  22 ,  25  and  26  are the actuators such as layered piezoelectric actuators which are extendible or compressible by an electric power of sine waves or triangle waves. The four actuators  21 ,  22 ,  25  and  26  are arranged at four corners of the fixing plates  23  and  24  in parallel with each other, respectively. The upper surfaces and the under surfaces of the four actuators  21 ,  22 ,  25  and  26  are fixed to the fixing plates  23  and  24 , respectively. 
         [0028]    The pressurizing device  30  is for example, a crystal piezoelectric type force sensor and is fixed to the upper surface of the bottom portion  11   b  of the housing  11 . The pressurizing device  30  includes a sensor portion (not shown) on an upper portion. The mount  12  is mounted on this sensor portion. 
         [0029]    The displacement measurement device  40  includes a sensor portion  41 , a support arm  42  which supports the sensor portion  41  at one end portion, a support shaft  43  movably supporting the other end portion of the support arm  42  in an up-down direction and a stand  44  from which the support shaft  43  is erected. The sensor portion  41  is an eddy current type displacement sensor which can measure the displacement amount of the object to be measured P which is depressurized or pressurized by the extendible or compressible actuators  21 ,  22 ,  25  and  26  by the electric power of relatively low frequency sine waves or triangle waves. The support arm  42  is slidable along the support shaft  43  and is fixed thereto by a screw (not shown) at any desired height position. 
         [0030]    The control device  50  inputs the measurement data from the pressure measurement device  30  and includes a pressure control device  51  which controls the pressurization and depressurization operation of the pressurizing device  20 , a calculating device  52  which calculates the stiffness of the object to be measured in the squeal frequency band by inputting the measurement data from the pressure measurement device  30  and the displacement measurement device  40  and a memory device  53  which memorizes pressure control program for controlling the above pressure. 
         [0031]    Next, operation of the measurement by the stiffness measurement device  1  will be explained hereinafter with reference to the flowchart illustrated in  FIG. 2 . First, as a preparation process work, the friction pad material is cut out to make a test piece to be an object to be measured P. The cut test piece is mounted on the mount  12  positioned lower side of the fixing plate  24  of the pressurizing device  20 . Then the handle portion  13   a  of the fixing screw  13  is rotated to have the lower end portion of the male screw portion  13   b  to be brought into contact with the upper surface of the fixing plate  23  so that the object to be measured P (test piece) is fixed between the fixing plate  24  and the mount  12  by sandwiching therebetween. Then the sensor portion  41  of the displacement measurement device  40  is inserted between the actuators  21 ,  22 ,  25  and  26  of the pressurizing device  20 . Then the support arm  42  is slidably moved along the support shaft  43  to fix a position (height position) where the lower end portion of the sensor portion  41  is separated with a predetermined distance (which is a distance enable to sense) from the upper surface of the fixing plate  24  of the pressurizing device  20 . The change of the distance between the sensor portion  41  and the fixing plate  24  is measured as the displacement of the object to be measured P. Thus, as shown in  FIG. 3 , the supporting process for the object to be measured P is completed (step S 1 ). 
         [0032]    The pressure control device  51  of the control device  50  pressurizes the friction pad material (object to be measured P) by controlling the pressurizing device  20  (at the step S 2 ). In more detail, the pressure control device  51  reads out the pressure control program from the memory device  53  and then supplies the actuators  21 ,  22 ,  25  and  26  with the sine wave electric power of a frequency with 0.1 Hz in a direction that the amplitude thereof becomes large so that the actuators are extended to pressurize the object to be measured P by the fixing plate  24 . 
         [0033]    The pressure control device  51  judges whether the pressure received by the object to be measured P reached to a predetermined pressure value or not (at the step S 3 ) and if the pressure is judged to have reached to the predetermined pressure level, the object to be measured P is depressurized by controlling the pressurizing device  20  (at the step s 4 ). In more detail, when the pressure of the object to be measured P reaches to the value of 1 MPa which corresponds to the pressure upon braking operation, the pressure control device  51  supplies the actuators  21 ,  22 ,  25  and  26  with the sine wave electric power 0.1 Hz in a direction where the amplitude of frequency is reduced. Thus, the actuators  21 ,  22 ,  25  and  26  are compressed to gradually depressurize the pressing force to the object to be measured P to finally depressurize the applied pressure thereto on the fixing plate  24 . 
         [0034]    The calculating device  52  obtains a relationship between the stress and displacement of the object to be measured P by inputting the pressure and the displacement of the object to be measured P when pressurized or depressurized from the pressure measurement device  30  and the displacement measurement device  40  (at the step S 5 ). Then the stiffness of the object to be measured P in the squeal frequency band based on the inclination immediately after the start of depressurization in a stress-displacement diagram is obtained and the obtained stiffness is memorized in the memory device  53  (at the step S 6 ). In more detail, the calculating device  52  calculates and obtains the stress “p” and displacement “x” relationship diagram upon pressurizing and depressurizing the object to be measured P as shown in  FIG. 4 . It is noted that by gradually depressurizing, the change of the curve of the stress “p” and displacement “x” relationship diagram upon depressurization becomes gentler than the change upon pressurization of the object to be measured P as shown in  FIG. 4 . First, the inclination Δx/Δp is obtained based on the stress change amount Δp when the displacement change amount Δx of the object to be measured P immediately after the start of depressurization from a predetermined pressure “pe” in the curve of the stress “p” and displacement “x” relationship diagram reached to a predetermined amplitude of squeal oscillation of the friction pad material, for example the amplitudes of 0.1 μm, 1.0 μm and 10 μm. Thus obtained inclination value is inversed to be defined as the stiffness in the squeal frequency band and memorized in the memory device  53 . Thus, the measurement operation of the stiffness measurement device  1  is completed. 
         [0035]    The comparison result of dynamic stiffness of four objects to be measured P are shown in  FIG. 5 . The comparison was made between the measurement of the stiffness measured by the stiffness measurement device according to the embodiment of the invention and the dynamic stiffness measured by a conventional dynamic stiffness measurement device. As clearly shown in  FIG. 5 , the stiffness of the object to be measured P in the squeal frequency band obtained by the stiffness measurement device  1  according to the embodiment and the dynamic stiffness of the object to be measured P obtained by the conventional dynamic stiffness measurement device are mutually related one for one with each other. Therefore, an analysis on a model of contact surface between the disc plate and the friction pad material can be made by using the stiffness of the object to be measured P in the squeal frequency band which is obtained by the stiffness measurement device  1  according to the embodiment of the invention. 
         [0036]    Further, instead of using frequency of 0.1 Hz sine wave electric power, a frequency of 0.1 Hz triangle wave electric power is supplied to the actuators  21 ,  22 ,  25  and  26  to obtain the stiffness of four objects to be measured P in the squeal frequency band and the dynamic stiffness of four objects to be measured P obtained by the conventional dynamic stiffness measurement device. The comparison result of the stiffness obtained by the embodiment of the invention and the dynamic stiffness obtained by the conventional dynamic stiffness measurement device is shown in  FIG. 6 . As clear from  FIG. 6 , the stiffness of the object to be measured P in the squeal frequency band obtained by using the frequency of 0.1 Hz triangle wave electric power and the dynamic stiffness of the object to be measured obtained by the conventional dynamic stiffness measurement device are mutually related one for one and accordingly, an analysis on a model of contact surface between the disc plate and the friction pad material can be made by using the stiffness of the object to be measured P in the squeal frequency band which is obtained by using frequency of 0.1 Hz triangle wave electric power. 
         [0037]    Still further, other than the frequency of 0.1 Hz sine waves, frequencies of 0.002 Hz and 10 Hz sine waves electric power supplied to the actuators  21 ,  22 ,  25  and  26  are shown in  FIG. 7  as the stress “p” and the displacement “x” relationship diagram. The result of the comparison between the stiffness of the object to be measured P in the squeal frequency band and the dynamic stiffness of the object to be measured P is shown in  FIG. 8 . It is noted that the stiffness in  FIG. 8  was obtained based on the inclination in the stress “p” and displacement “x” relationship diagram when the displacement change amount Δx of the object to be measured P immediately after the start of depressurization from the predetermined pressure level reached to the value of 0.2 μm. As seen from the graph in  FIG. 7 , tendencies of the stress “p” and displacement “x” relationship diagrams in each case where the 0.002 Hz, 0.1 Hz and 10 Hz frequency sine waves electric powers are supplied to the actuators  21 ,  22 ,  25  and  26  are approximately the same and as apparent from  FIG. 8 , the stiffness of the object to be measured in the squeal frequency band is approximately the same with the dynamic stiffness of the object to be measured P obtained by the conventional dynamic stiffness measurement device and accordingly, an analysis on a model of contact surface between the disc plate and the friction pad material can be made by using the stiffness of the object to be measured P in the squeal frequency band which is obtained by using frequencies of 0.002 Hz and 10 Hz frequency waves electric power. 
         [0038]      FIG. 9  shows the result of comparison between the static stiffness p/x obtained from the stress p and the displacement x when the frequency of 0.002 Hz, 0.1 Hz and 10 Hz sine waves electric power in a direction where the frequency oscillation becomes large, is applied to the actuators  21 ,  22 ,  25  and  26  (upon pressurization operation) and the static stiffness of the object to be measured P obtained by a conventional static stiffness measurement device (for example, an Instron make 5582 Type material testing machine). As apparent from  FIG. 9 , each static stiffness of the objects to be measured P is approximately the same with the static stiffness of the object to be measured P measured by the conventional static stiffness measurement device. Thus, the stiffness and the static stiffness of the object to be measured P in the squeal frequency band can be obtained simultaneously according to the stiffness measurement device  1  of the embodiment of the invention. 
         [0039]    As explained in the embodiment of the invention, the pressurizing device  20  is operated to pressurize/depressurize the object to be measured P by using sine wave and triangle wave electric powers to be supplied to the actuators  21 ,  22 ,  25  and  26  to make simple in structure. If as long as a gradual depressurizing can be available, the pressurizing operation need not be carried out with the same way as the depressurizing operation. For example, the pressurization may be carried out very quickly, instead. According to this embodiment, layered piezoelectric actuators are used for the actuators  21 ,  22 ,  25  and  26 . However, a hydraulic type actuator can be used, instead. In addition, according to the embodiment, an eddy current type displacement sensor is used for the displacement measurement device  40 , however, laser type or electrostatic type displacement actuator may be used, instead. 
         [0040]    According to the stiffness measurement device  1  of the embodiment of the invention, the stiffness of the object to be measured P in the squeal frequency band can be measured without any particular exclusive use of dynamic stiffness measurement device, merely using an existing static compression testing machine. The stiffness of the object to be measured P in the squeal frequency band can be easily and simply measured. There is no need to oscillate the object to be measured with a high frequency as was necessary in a conventional method and accordingly, there is no need to enhance the stiffness of the housing of the device, which may lead to the downsizing of the device. Further, according to the conventional device, if the object to be measured P has to be oscillated with a high frequency, the output of the oscillator has to be enhanced to cope with a large object to be measured and accordingly, the size of the object has to be limited to an extent. According to the stiffness measurement device  1  of the embodiment of the invention, since it is not necessary to oscillate with a high frequency, the size of the object to be measured can be extended such as for example, to a full-size friction pad material.