CN106403794A - Device used for dynamic and static calibration of eddy current sensor - Google Patents

Abstract

A device for dynamic and static calibration of eddy current sensors. The device has a base on which a vibrating table module, a jig for clamping the sensor to be calibrated, a motion execution module of the jig and a grating ruler assembly are arranged; the working table of the vibrating table module is Install the induction plate and the standard acceleration sensor, the induction plate is located between the worktable and the fixture; the motion execution module consists of a positioning drive assembly that drives the fixture to move radially along the induction plate and a calibration drive assembly that drives the fixture to move axially along the induction plate; During static calibration, the vibrating table module is in a non-working state, and the calibration drive assembly makes the fixture do a linear reciprocating motion close to or away from the induction disk, and the grating ruler assembly obtains the actual displacement of the calibrated sensor on each test point; during dynamic calibration, the calibration The drive assembly is in a non-working state, and the vibrating table module makes the induction disc vibrate sinusoidally. The invention has the advantage of wide calibration frequency range.

Description

Device for dynamic and static calibration of eddy current sensor

technical field

The invention relates to a device for dynamic calibration and static calibration of an eddy current sensor.

technical background

The eddy current sensor can measure the relative displacement between the measured metal conductor and the surface of the sensor probe in a non-contact manner, and is a non-contact linearization measurement tool.

Chinese patent No. 200810036312.2 discloses a fully automatic eddy current sensor dynamic and static calibrator, including a static test unit composed of a static calibration specimen plate and a static calibration probe mounting frame, a dynamic calibration probe mounting frame, a dynamic inclined Disk, a dynamic test unit composed of a dynamic swash plate drive motor. The specific structure is: on the linear module driven by the stepping motor, set a movable guide rail extension rack driven by it, and set the static calibration test piece disk and the dynamic calibration probe installation rack on the guide rail extension rack. In addition, A fixed static calibration probe installation frame is arranged on the instrument panel, and a dynamic swash plate and its main drive motor are correspondingly arranged below the probe installation holes of the dynamic calibration probe installation frame. The disadvantages of this fully automatic eddy current sensor dynamic and static calibrator are:

1. Since the dynamic swash plate is driven by a motor, its rotation speed is r/min ( It is generally difficult to reach more than 10000r/min), so the calibration frequency input to the eddy current sensor is only Hz, that is, the dynamic calibration frequency range of the device is relatively narrow.

2. Static calibration and dynamic calibration are performed by different synchronous motors to perform positioning and calibration movements, the overall mechanical structure is complex, and there is no effective integration of repeated functional modules.

Contents of the invention

In order to overcome the shortcoming of using a dynamic swash plate to dynamically calibrate the eddy current sensor and the calibration frequency range is relatively narrow, the present invention provides a dynamic and static eddy current sensor that uses a vibration table module to dynamically calibrate the eddy current sensor and calibrates the eddy current sensor with a wide frequency range. Calibration device.

A device for dynamic and static calibration of eddy current sensors. The device has a base on which a vibrating table module, a jig for clamping the sensor to be calibrated, a motion execution module of the jig and a grating ruler assembly are arranged; the working table of the vibrating table module is Install the induction disc and the standard acceleration sensor, the induction disc is located between the worktable and the fixture; the motion execution module consists of a positioning drive assembly that drives the fixture to move radially along the induction disc and a calibration drive component that drives the fixture to move axially along the induction disc; During static calibration, the vibrating table module is in a non-working state, and the calibration drive assembly makes the fixture do a linear reciprocating motion close to or away from the induction disk, and the grating ruler assembly obtains the actual displacement of the calibrated sensor on each test point; during dynamic calibration, the calibration The drive assembly is in a non-working state, and the vibrating table module makes the induction disc vibrate sinusoidally.

Further, the position adjustment drive assembly includes a first lead screw mechanism and a guide rail-slider combination; the fixture is fixed on the slider of the guide rail-slider combination; the axial direction of the first lead screw mechanism is parallel to the radial direction of the induction disc, and the guide rail and The first lead screw of the first lead screw mechanism is parallel, and the first nut of the first lead screw mechanism is fixed to the slider; one end of the lead screw is provided with a knob. When the knob rotates with the first lead screw, the first nut takes the slider to translate along the axial direction of the first lead screw. Turn the knob to make the first nut bring the fixture to translate along the radial direction of the induction disc, so that the eddy current sensor to be calibrated is out of the sensing area of the induction disc.

Further, the calibration drive assembly includes a second lead screw mechanism, a heavy-duty guide rail and a stepper motor; the stepper motor is fixed to the second lead screw of the second lead screw mechanism through a coupling, and the slider of the heavy-duty guide rail is connected to the second nut Fixed; the positioning drive assembly is fixed on the slider of the heavy-duty guide rail through the connecting block. The stepper motor moves along the axial direction of the induction disk with the adjustment drive assembly and the fixture through the second nut, and makes the calibrated sensor move back and forth according to the test requirements during static calibration.

The grating scale assembly includes the grating reading head fixed on the base and the grating scale fixed with the slider of the heavy-duty guide rail. The grating ruler moves with the slider of the heavy-duty guide rail, and the grating reading head reads the actual displacement of the slider in real time. The slider of the heavy-duty guide rail is fixed to the fixture, so the actual displacement of the slider is the actual displacement of the fixture and the sensor to be calibrated on the fixture.

Further, the vibrating table module includes a cylinder body, a front end cover, a rear end cover, a permanent magnet, a central magnetic pole, moving parts and a limiting part; the front end cover and the rear end cover respectively close the front opening and the rear end opening of the cylinder body, and the permanent magnets respectively It is fixed with the rear end cover and the central magnetic pole; the permanent magnet, the central magnetic pole, the rear end cover and the front end cover are arranged coaxially; a through hole is opened on the front end cover, and the head of the central magnetic pole extends into the through hole, A uniform air-gap magnetic field is formed between them;

The moving parts include the moving coil, the moving coil frame and the working table. The moving coil is wound on the moving coil frame. The limit part makes the moving coil coaxial with the central magnetic pole and is located in the air gap magnetic field; the induction disc is fixed on the exposed part of the working table. Front; the standard acceleration sensor is fixed on the back of the work surface.

The coaxial positioning method of the front end cover and the central magnetic pole is: the front end cover has an annular positioning boss, and the positioning boss is coaxial with the through hole (the positioning boss and the through hole can be precisely positioned when the front end cover is made on the same axis), and the positioning The boss cooperates with the positioning ring (that is, a section of the positioning ring is set on the positioning boss), and the positioning ring is fixed with the front end cover; the central magnetic pole is inserted into the positioning ring and contacts the surface of the positioning ring.

The fixing method of the central magnetic pole and the permanent magnet is as follows: the rear end surface of the central magnetic pole is completely attached to the front end surface of the permanent magnet, an axial through hole is arranged in the center of the permanent magnet, an axial screw hole is arranged in the center of the central magnetic pole, and a central pole passes through The through hole is engaged with the screw hole of the central magnetic pole, and the center of the rear end cover is provided with an axial socket, and the other end of the central pole is inserted into the socket and fixed with the rear end cover.

The fixing method of the central pole and the rear end cover is as follows: the threaded part of the central pole is exposed in the socket of the rear end cover, and the fixing nut is screwed tightly with the threaded part. In order to prevent the central pole from being exposed to the rear end cover, a counterbore is arranged on the rear end cover, and the central pole and the fixing nut are located in the counterbore.

The front section of the central magnetic pole is in clearance fit with the moving coil frame, and the cross-sectional shape of the rear section is congruent with that of the permanent magnet.

The fixing method of the induction plate, the standard acceleration sensor and the worktable is: the induction plate is installed on the front of the worktable by the first double-ended stud, and the standard acceleration sensor is installed on the back of the worktable by the second double-ended stud.

Further, the position-limiting component is mainly composed of a supporting frame and a supporting spring; the supporting frame is fixed to the front end cover and is coaxial with the front end cover, and the supporting spring is a reed whose one end is connected to the supporting frame and the other end is connected to the moving coil frame. The reed is circular. Or the reeds are rectangular, and a plurality of rectangular reeds are evenly distributed along the circumference of the moving coil holder. The reeds are fixed to the supporting frame by pressing plates and bolts. The support spring acts as a supporting and positioning function for the moving coil frame, so that the moving coil frame is kept in a balanced position coaxial with the central magnetic pole.

Further, a protective cover is provided outside the limiting part, and the protective cover includes a cover body and a rubber film, and the cover body and the front end cover are fixed and coaxial; the rubber film is in the form of a ring, and the head of the moving coil holder is exposed to the rubber film, and the rubber film It is fixed with the cover body by ring pressure plate and screws. The dust cover prevents dust and other sundries from falling into the movement gap, prolonging the service life of the vibration table module.

When the present invention is in use, the standard sensor is first installed on the back of the worktable, and then the worktable is installed on the moving coil frame, and then the induction plate is installed on the front of the worktable with double-ended studs, and then the waiting The calibration sensor is mounted and fixed on the sensor fixture. The sensor is positioned and adjusted through the motion execution module: first, turn the knob to rotate the first lead screw, and drive the sensor fixture to move through the first nut, and then drive the sensor to move to a position aligned with the center of the induction plate to ensure that the sensor is in the center of the induction plate. within the sensing area. Then control the stepper motor to drive the second lead screw to rotate, and drive the sensor fixture and sensor on the heavy-duty guide rail to move back and forth through the second nut, and read the DC voltage output by the eddy current sensor in real time until the voltage value is in the sensor. Within the linear working voltage range of the eddy current sensor, the positioning adjustment of the eddy current sensor is completed.

When performing dynamic calibration of the eddy current sensor, the sinusoidal current signal into the moving coil, the effective length of the moving coil conductor and the induction strength of the magnetic field in which the conductor is located Under certain circumstances, the electromagnetic force on the moving coil It changes in a standard sinusoidal law, so the moving coil will drive the fixedly connected moving coil frame, worktable and induction plate under the electromagnetic force. Under the action of the same frequency sinusoidal vibration along the axis direction. According to the output voltage of the standard acceleration sensor installed on the worktable, the actual vibration level of the vibration table is calculated, and by gradually increasing/decreasing the amplitude of the input current signal, the vibration level of the vibration table gradually reaches the target vibration level. Then by changing the frequency of the input current signal , the frequency response characteristics of the sensor to be calibrated can be calibrated, because the input signal frequency Changes can be made so that wider frequency band calibrations can be done for eddy current sensors.

When performing static calibration of the eddy current sensor, according to the parameter requirements of the static calibration, such as stroke points, step length, and motion direction, similar to the sensor positioning process, the motion execution module is used to drive the ball screw to rotate by controlling the stepping motor, and The sensor fixture and sensor on the heavy-duty guide rail are driven by the screw nut to move forward and backward. In addition, the linear grating scale is fixed on the side of the heavy-duty guide rail, and moves forward and backward together with the sensor to be calibrated, and cooperates with the reading head installed and fixed on the base. Get the actual displacement of the sensor to be calibrated at each test point . According to the actual output voltage of the sensor and the actual displacement D of the linear grating ruler, the reference sensitivity S of the sensor can be obtained through calculation. The motion execution module can be used as the sensor positioning execution part of dynamic and static calibration, and can also be used as the calibration motion execution part of static calibration.

The beneficial effects of the present invention are:

1. The vibration table is used to dynamically calibrate the sensor, compared with the swash plate dynamic calibration method

By changing the frequency of the input current signal, the sensor to be calibrated can be calibrated at different frequencies, making it suitable for wider frequency band calibration.

2. The motion execution module can be used as the sensor positioning execution part of dynamic and static calibration,

It can also be used as the calibration execution part of the static calibration, which can conveniently and quickly realize the dynamic and static calibration of the sensor respectively, and improve the calibration efficiency of the sensor.

Description of drawings

Figure 1 The overall schematic diagram of the calibration device.

Figure 2 The front view of the structure of the motion execution module.

Figure 3. Top view of the structure of the motion execution module.

Fig. 4 Schematic diagram of the structure of the vibration table module.

detailed description

As shown in Figure 1, the dynamic and static automatic calibration device of the eddy current sensor includes a vibration table module 1, an induction disc 2 connected to the work surface of the vibration table, a standard acceleration sensor 5 connected to the work surface, a sensor fixture 3, a motion execution module 4 and Fix the base 6 of the vibration table and the motion execution module.

The motion execution module is composed of a positioning drive assembly that drives the clamp to move radially along the induction disk 2 and a calibration drive assembly that drives the clamp to move axially along the induction disk 2 .

The position adjustment drive assembly includes the combination of the first lead screw mechanism and the guide rail 416-slider 417; the fixture is fixed on the slide block 417 of the guide rail 416-slider 417 combination; To be parallel, the guide rail 416 is parallel to the first lead screw 418 of the first lead screw mechanism, and the first nut 411 of the first lead screw mechanism is fixed to the slider 417; one end of the first lead screw 418 is provided with a knob 49. When the knob 49 rotates with the first lead screw 418 , the first nut 411 with the slider 417 translates along the axial direction of the first lead screw 418 . Turn the knob 49 to make the first nut 411 translate along the radial direction of the induction disk 2 with the clamp 3 , so that the eddy current sensor to be calibrated is out of the sensing area of the induction disk 2 .

The calibration drive assembly includes a second lead screw mechanism, a heavy-duty guide rail 410 and a stepper motor 41; the stepper motor 41 is fixed with the second lead screw 45 of the second lead screw mechanism through a coupling, and the slide block of the heavy-duty guide rail 410 is connected to the second lead screw 45 of the second lead screw mechanism. The second nut 47 is fixed through the connecting block 44; the position adjustment drive assembly is fixed on the slider of the heavy-duty guide rail 410 through the connecting block. The stepper motor 41 moves along the axial direction of the induction disk 2 with the adjustment drive assembly and the fixture 3 through the second nut 47, and makes the sensor to be calibrated move back and forth according to the test requirements during static calibration.

The base 6 is a box, and the calibration drive assembly is located inside the box of the base 6 . There is an opening on the top of the box so that the calibration drive assembly can be connected and moved with the adjustment drive assembly.

The grating ruler assembly includes a grating reading head 415 fixed on the base, a grating mounting plate 414 fixedly connected to the heavy-duty guide rail 410 , and a grating ruler 413 fixed to the grating mounting plate 414 . The grating ruler 413 moves with the grating mounting plate 414 of the heavy-duty guide rail 410, and the grating reading head 415 reads the grating mounting plate 414 in real time, that is, the actual displacement of the heavy-duty guide rail 410. The heavy-duty guide rail 410 is fixed with the fixture, so the grating mounting plate 414 The actual displacement is the actual displacement of the fixture and the sensor to be calibrated clamped on the fixture.

The vibrating table module 1 includes a permanent magnet 11 , a central magnetic pole 13 , a cylinder 12 , a front end cover 15 , a rear end cover 116 , moving parts, limiting parts and the like. The central magnetic pole 13 , the cylinder body 12 and the front and rear end covers 15 and 116 are all made of magnetically permeable materials, and a uniform air gap magnetic field is generated between the central magnetic pole 13 and the front end cover 15 . The front end cover 15 and the rear end cover 116 are installed and fixed on the base 6 by bolts, the moving parts are installed in the gap between the central magnetic pole 13 and the front end cover 15, the moving parts can move along the axial direction in the gap, and the limiting parts are installed by bolts On the front cover 15.

The limiting component includes a support frame 17 and a support spring 112 . The support frame 17 is installed and fixed on the front end cover 15 by bolts, and the support springs 112 evenly distributed around are connected to the moving parts and the support frame 17 through the pressure plate 113 and bolts, so as to adjust the tightness of the support spring 112, thereby limiting the position and maintaining balance role of location. There is a protective cover 16 outside the support frame 17, and the protective cover 16 is installed on the front end cover 15 by bolts. On the end face of the protective cover 16, the rubber film 114 is placed under the annular pressure plate 115 and fixed with screws to prevent dust, etc. The effect of debris falling into the movement gap.

The moving parts include a moving coil 18, a moving coil frame 19, a working surface 110 and an induction plate 2, the moving coil 18 is fixedly connected with the moving coil frame 19, the working surface 110 is connected with the moving coil frame 19 by bolts, and the induction plate 2 is connected by a double The head stud 111 is connected and fixed with the work surface 110 . The moving coil frame 19 is cylindrical and has a housing cavity inside. The standard acceleration sensor 5 is installed on the end surface of the worktable 110 through a stud 111 and is located in the housing cavity of the moving coil frame 19 . The center of the permanent magnet 11 has a through hole, the center of the center pole 13 has a threaded hole, and the two ends of the center pole 118 have threads, and the center pole 118 passes through the through hole of the permanent magnet 11, and then is screwed on the threaded hole of the center pole 13 , and finally the center pole 118 is screwed and fixed on the rear end cover 116 through the round nut 117, which mainly plays a role in positioning the permanent magnet 11 and the center pole 13. The positioning ring 14 is made of non-magnetic material, and one end is matched with the central magnetic pole 13, and the other end is fixed on the front end cover 15 by bolts, so as to position the central magnetic pole 13.

When the present invention is in use, first the standard sensor 5 is installed on the inner side of the work surface 110, then the work surface 110 is installed on the moving coil frame 19, and then the induction disc 2 is installed on the work surface 110 through the stud 111, Then the sensor to be calibrated 7 is installed and fixed on the sensor fixture 3, and the sensor to be calibrated 7 is positioned and adjusted through the motion execution module. First, the knob 49 is turned to make the ball screw 418 rotate, and the sensor fixture 3 is driven by the screw nut 412 to move, and then Drive the sensor 7 to be calibrated installed on the fixture 3 to move it to a position aligned with the center of the induction disc 2, ensure that the sensor 7 to be calibrated is in the sensing area of the induction disc 2, and then control the stepping motor 41 to drive the ball screw 45 rotation, and drive the sensor fixture 3 on the heavy-duty guide rail 410 and the sensor 7 to be calibrated to move back and forth through the lead screw nut 47 and the connecting block 44, and read the DC voltage output by the sensor 7 to be calibrated in real time until the output voltage value is at The linear operating voltage range of the sensor 7 to be calibrated indicates that the positioning and adjustment of the sensor 7 to be calibrated is completed at this time, and the next dynamic and static calibration can be performed.

When performing dynamic calibration of the eddy current sensor, the sinusoidal current signal Into the moving coil 18, the effective length of the conductor of the moving coil 18 and the induction strength of the magnetic field in which the conductor is located Under certain circumstances, the electromagnetic force received by the moving coil 18 It changes in a standard sinusoidal law, so the moving coil 18 will drive the moving coil frame 19, the work table 110 and the induction plate 2 which are fixedly connected to each other under the electromagnetic force. The standard sinusoidal vibration of the same frequency is made along the axial direction under the action of the force. And through the output voltage of the standard acceleration sensor 5 installed on the worktable 110, the actual vibration level of the vibration table is calculated, and the amplitude of the input current signal is gradually increased/decreased, so that the vibration level of the vibration table gradually reaches the target vibration level. Then by changing the frequency of the input current signal , then the frequency response characteristics of the sensor can be calibrated, because the input signal frequency Changes can be made so that wider frequency band calibrations can be done for eddy current sensors.

When carrying out the static calibration of the eddy current sensor, according to the parameter requirements such as the number of travel points, step size, and motion direction of the static calibration, similar to the sensor positioning process, the motion execution module is used to drive the ball screw 45 to rotate by controlling the stepping motor 41 , and drive the heavy-duty guide rail 410 through the lead screw nut 47 and the connecting block 43, so that the sensor fixture 3 on the heavy-duty guide rail 410 and the sensor 7 to be calibrated move forward and backward according to the requirements, and the linear grating ruler 413 is fixed on the side of the heavy-duty guide rail 410, Move back and forth together with the sensor 7 to be calibrated, and cooperate with the grating reading head 415 installed and fixed on the base 6 to obtain the actual displacement of the sensor 7 to be calibrated at each test point . According to the actual output voltage of sensor 7 to be calibrated and the actual output displacement D of the linear grating scale 413, the reference sensitivity S of the sensor 7 to be calibrated can be obtained through calculation. The motion execution module can be used not only as the sensor positioning execution part of dynamic and static calibration, but also as the calibration movement execution part of static calibration, which can conveniently and quickly realize the dynamic and static calibration of the sensor respectively, and improve the calibration efficiency of the sensor.

The content described in the embodiments of this specification is only an enumeration of the implementation forms of the present invention, and the protection scope of the present invention should not be regarded as limited to the specific forms stated in the embodiments. Equivalent technical means that a person can think of based on the concept of the present invention.

Claims (10)

1. be used for eddy current sensor dynamic static calibration device it is characterised in that：This device has pedestal, and setting on pedestal is shaken Dynamic platform module, the fixture of clamping sensor to be calibrated, the Motor execution module of fixture and grating scale assembly；The work of vibration table module Make to install inductive disks and standard acceler on table top, inductive disks are located between work top and fixture；Motor execution mould Block by drive fixture along inductive disks radial motion positioning drive component and drive fixture along inductive disks axially-movable calibration drive Dynamic assembly；When doing static calibration, for off working state, calibrate drive component makes fixture do closer or far from sense to vibration table module The straight reciprocating motion that should coil, grating scale assembly obtains the actual displacement being calibrated sensor in each test point；Do dynamic school On time, for off working state, vibration table module makes inductive disks do sinusoidal vibration to calibration drive component.

2. be used for as claimed in claim 1 the device of eddy current sensor dynamic static calibration it is characterised in that：Positioning driving group Part includes the first screw mechanism and guide rail-slide block combination；Fixture is fixed on the slide block of guide rail-slide block combination；First leading screw machine The axial direction of structure is radial parallel with inductive disks, and guide rail is parallel with the first leading screw of the first screw mechanism, and the of the first screw mechanism One nut and slide block are fixed；Leading screw one end arranges knob.

3. be used for as claimed in claim 2 the device of eddy current sensor dynamic static calibration it is characterised in that：Calibration driving group Part includes the second screw mechanism, heavily loaded guide rail and motor；Motor passes through the second of shaft coupling and the second screw mechanism Leading screw is fixed, and the slide block of heavily loaded guide rail is fixed with the second nut；Positioning drive component is fixed on heavily loaded guide rail by contiguous block On slide block.

4. be used for as claimed in claim 3 the device of eddy current sensor dynamic static calibration it is characterised in that：Grating scale assembly Including the grating reading head being fixed on pedestal and the grating scale fixing with the slide block of heavily loaded guide rail.

5. the device for eddy current sensor dynamic static calibration as described in one of claim 1-4 it is characterised in that：Vibration Platform module includes cylinder, drive end bearing bracket, rear end cap, permanent magnet, central magnetic pole, moving component and limiting component；Drive end bearing bracket and rear end Lid closes off front opening and the open rearward end of cylinder, and permanent magnet is fixed with rear end cap and central magnetic pole respectively；Permanent magnet, in Heart magnetic pole, rear end cap and drive end bearing bracket are coaxially disposed；Through hole is opened up on drive end bearing bracket, the head of central magnetic pole stretches in this through hole, in Form uniform air-gap field between heart magnetic pole and through hole；

Moving component includes moving-coil, moving-coil frame and work top, and moving-coil is wound on moving-coil frame, and limiting component makes moving-coil and center Magnetic pole is coaxial and is located in air-gap field；Inductive disks are fixed on the front exposed of work top；Standard acceler is solid The back side due to work top.

6. be used for as claimed in claim 5 the device of eddy current sensor dynamic static calibration it is characterised in that：Drive end bearing bracket has The positioning boss of annular, positioning boss is coaxial with through hole, and positioning boss is coordinated with locating ring, and locating ring is fixed with drive end bearing bracket；In Heart magnetic pole be inserted in locating ring and with positioning annular face contacts.

7. be used for as claimed in claim 6 the device of eddy current sensor dynamic static calibration it is characterised in that：Central magnetic pole Rear end face is fitted completely with the front end face of permanent magnet, the centrally disposed axial through hole of permanent magnet, the centrally disposed axle of central magnetic pole To screw, a central strut engaged through through hole with the screw of central magnetic pole, the centrally disposed axial jack of rear end cap, The other end of this central strut is inserted this jack and is fixed with rear end cap.

8. be used for as claimed in claim 7 the device of eddy current sensor dynamic static calibration it is characterised in that：Central strut Threaded portion exposes to the jack of rear end cap, and fixing nut is screwed with threaded portion.

9. be used for as claimed in claim 5 the device of eddy current sensor dynamic static calibration it is characterised in that：Limiting component master To be made up of support frame and support spring；Support frame and drive end bearing bracket are fixed and coaxial with drive end bearing bracket, and support spring is one End connects support frame, the other end connects the reed of moving-coil frame.

10. be used for as claimed in claim 9 the device of eddy current sensor dynamic static calibration it is characterised in that：Limiting component Outside protective cover is set, protective cover includes cover body and rubber film, and cover body and drive end bearing bracket are fixed and coaxially；Rubber film is in annulus Shape, the head of moving-coil frame exposes to rubber film, and rubber film is fixed with cover body by annular pressing plate and screw.