Method and device to measure the mechanical input point impedance of organic structures

Abstract

Measuring the mechanical input point impedance of organic structures using inertia shakers such as the electromagnetic transducers used for bone conduction excitation comprises using a pair of acceler

Description

Background of the Invention The present invention relates to the mechanical input point impedance measurement in the field of organic structural vibration excitation as in case of bone conduction stimulation. It describes both a new method and a new device to determine the mechanical input point impedance and further the imprinted force and respective power. Bone conduction devices are used for audiometric diagnostics and hearing rehabilitation in case of a sound conduction hearing impairment since they transmit the signal to the cochlea via structural vibrations, bypassing the outer and middle ear. Moreover, headsets where the sound signal is presented via structure borne transmission are available for leisure and business activities in the commercial sector. A mechanical input point impedance measurement provides information about the resistance of the excited structure to its excitation, and therefore contains information about the coupling of the bone conduction device and the respective frequency dependent transmission of vibrations to the excited structure. For the hearing-impaired patient, it is a necessity that the corresponding frequency dependent hearing loss is compensated for. It is therefore important that dips in the transmitted frequency spectrum caused by the coupling of the exciter, as well as its internal resonances and output power are detected and as well compensated to achieve a balanced hearing rehabilitation. Furthermore, a change in the values of the impedance over time can provide information about a pathological alteration. An example for this can be the change of the skin stiffness in case of the use of a too high contact force during transcutaneous (skin-driven) bone conduction excitation. Especially in case of infants and patients who have difficulty in sensing or communicating their perceptions and discomfort to the caregiver it is important to collect and interpret parameters that can provide such information. State of the Art Previous measurements of the mechanical input point impedance in the field of bone conduction excitation have so far only been possible by introducing an impedance head between the vibration stimulator and the head [1, 2, 3, 4, 5, 6, 7, 8). In most cases, the impedance heads such as Briiel &Kjaer Type 8000 for skin-driven (further referred to as transcutaneous) and Bruel &Kjaer Type 8001 for percutaneous excitation are applied for this purpose [9], The operation principle of these devices can be found in

[10] , For transcutaneous excitation, the mechanical input point impedance is determined by the skin and in case of percutaneous stimulation by the skull bone. Based on early impedance measurements, special calibration units for bone conduction transducers, so called "artificial mastoid" were developed. These are for transcutaneous excitation the Bruel &Kjaer Type 4930 [11, 7] and for percutaneous excitation the Interacoustics Skull Simulator SKS10 [12, 7] as well as the Audiscan Verifit Skull Simulator [13, 7], 27 11 24 The main problem arising by introducing an impedance head between the excitation source and the head for in vivo measurements is the sensitive and instable setup, as well as the internal mass below the force gauge of an impedance head in relation to the mass of the excited organic structure. In case of transcutaneous bone conduction excitation, it is important to keep in mind that the mass below the force gauge of the impedance head must be lower than the mass of the excited skin. Otherwise, this leads to a falsified impedance curve since the measured mass line corresponds to the integrated mass of the impedance head and not to the excited structure. It was found that the load of a bone conductor has a slight impact on its frequency-dependent output performance and accordingly the mechanical impedance in case of transcutaneous [8] as well as percutaneous excitation [7], The rigid skull shows a 10-30 dB higher mechanical impedance than the skin [3] and can be simplified assumed to be a pure mass while the transducer in this case can be seen as a constant force source [7]. In contrast, the skin shows a more complex composition and correspondingly mechanical behaviour. This is also reflected in the comparisons of in vivo impedance measurements with those of an artificial mastoid. While in the case of percutaneous bone conduction stimulation there is a high compliance between the mechanical input point impedance of the subjects with the artificial mastoid [7], in the case of transcutaneous excitation there is a significant deviation in this comparison [8], This shows that the use of an artificial mastoid as load for measuring the mechanical output behaviour of bone conduction transducers for transcutaneous excitation has an insufficient precision and an alternative must be provided [8]. Some approaches in this field can be found, such as in case of glasses with integrated bone conduction headphones which use additional sensors to measure the vibration resulting from the force excitation [14, 15, 16, 17], Furthermore, in the past already a device for measuring the frequency characteristics of a transducer in vivo was invented which is coupled between the bone conductor and the excited structure. It consists of a spring mounted mass, whose defined weight as well as its impedance must be much higher than the output impedance of the transducer to measure via a power transducer the output power of the exciter

[18] . This device was additionally extended by an accelerometer mounted to the seismic mass of the transducer to be subsequently able to calculate the respective output force of the transducer

[18] , However, the velocity caused by the imprinted force to the structure is not captured [7]. With the mentioned state of the art inventions, it was so far not possible to measure the mechanical input point impedance without additional use of an impedance head. The present invention comprises besides the determination of the mechanical input point impedance of the organic structure the advantage of the measurement of the velocity (which is the integrated acceleration signal) for the force excitation. The clear novelty and technical advantage of the present invention is: • the built-in impedance measurement and further determination of imprinted force and respective power based on the usage of electromagnetic bone conduction transducer where an accelerometer at is attached to the seismic mass m1 and a second accelerometer a2 which is attached to the actuator mass m2 of the electromagnetic bone conduction transducer, 27 11 24 • the fixation of actuator mass m2 supported by a small plate 6 to ensure good fixation and balanced contact pressure onto the surface of the organic structure by means of an elastic or steel band 7, • the usage of a data interface 16 to record and process acceleration data from aA and a2 for further processing in a computing unit 17 • A reduction of the total weight of the actuator mass m2 accelerometer a2 resting on the skin as exemplarily shown using the "Balanced Electromagnetic Separation Transducer" in Figure lb, which is achieved using a lightweight aluminum indenter, interconnecting the different components of m2 and with a recess to embed accelerometer a2. Since no additional impedance head is required, the setup can be designed highly integrated and small. Due to its lightweight and small design, the device can be mounted at any position using an elastic- or steel-headband. Since a correlation between the velocity and the respective sound perception is assumed [8], the device can accordingly further be used for bone conduction hearing aid fitting of infants and other patients with difficulties to communicate their perceptions based on objective measurements. Subsequently, it is possible to determine the imprinted force and power from the recorded acceleration spectra and derived impedance. Future bone-conduction devices may incorporate the presented construction and method outlined for mechanical input point impedance measurement, to subsequently allow for balancing the vibration transmission to the head and detection of changes in each the transducer, the coupling and the imprinted organic structure. Description of the Invention The functioning of standardized bone conductors is based on the principle of inertial vibration. A seismic mass is connected to an actuator mass m2 by one or more spring elements. The actuator mass m2 is placed on the structure to be excited. By the electromagnetic interaction between an integrated permanent magnet and one or two voice coils, forces Ft and F7 are generated, acting upon mA and m2. In dependence on the polarization of the current flowing through the coil results an attraction or repulsion of the two masses mA and m2 to each other, whereby both forces F_A and F7 are acting with the same amplitude in opposite directions. This allows for the calculation of the mechanical input point impedance by measuring the respective velocities of each the seismic mass m1 and the actuator mass m2. This can be achieved by attaching an accelerometer to each of the masses. Figure 1 shows such a setup using a simplified structural drawing of two standardized inertial shakers used for bone conduction excitation. In Figure la a state-of-the-art electromagnetic transducer

[19] , and in Figure lb the "Balanced Electromagnetic Separation Transducer" [20, 21, 22] are displayed, both equipped with the corresponding accelerometers. In case of the first example in Figure la, the accelerometers are attached to each of the masses. In the second example in Figure lb, a lightweight accelerometer as the PCB piezotronics Model 352A73 is embedded into an indenter which interconnects the components of m2. As shown in Fig. lb, the indenter 13 comprises a recess to embed the accelerometer a? with the mass m2. The accelerometer is attached to mj. 27 11 24 The seismic mass m1 based on the state-of-the-art electromagnetic bone conduction transducer example in Figure la

[19] , is composed of the pole shoes 1, permanent magnet 2, coils 3, spacers 4, screws and the rigid part of the yoke 5, whereas the actuator mass m2 is defined by the flexible part of the yoke 5. In case of the "Balanced Electromagnetic Separation Transducer" [20, 21, 22] depicted in Figure lb, m1 is made up of the permanent magnets 2, the housing 10, as well as an additional block 8 for the mounting of accelerometer at. m2 is composed of the coil 3, a ferrite rod 9, spacers 11, centering slides which also act as springs 12, an indenter 13 including a centering sleeve to adjust accelerometer a2 and a screw. The device is connected to the structure to be excited by a steel- or an elastic-band 7, whereby a small surface 6 is inserted for good fixation and balanced contact pressure. It is important to ensure that the total mass of m2 and a2 is lower than the mass of the excited organic structure (e.g., skin), as otherwise this total mass is reflected in the resonance frequency and massline instead that of the actual object of measurement. A corresponding reduction of the mass is shown by the example of the "Balanced Electromagnetic Separation Transducer" in Figure lb, which is achieved using a lightweight aluminum indenter, interconnecting the different components of m2 and with a recess to embed accelerometer a2 resting on the skin. Moreover, the accelerometer a2 must be a compromise between a lightweight construction and sufficient sensitivity. A good solution in this respect is the device PCB piezotronics Model 352A73 with a weight of 0.3g and a sensitivity of 5 ^—7

[23] . Measurement and Data Acquisition: Figure 2 shows the block diagram of the proposed method for the determination of the mechanical input point impedance. In the first step the device is connected to a system for signal generation, recording and analysis (e.g. Simcenter Testlab

[24] ). The signals of the accelerometers are recorded and accordingly defined in the measurement system as input for a specified output which drives the transducer 1. 3. For a correct measurement, the measurement and acquisition parameters must be defined in the measuring system 3. These include the excitation signal, which must be a broadband signal, the definition of the bandwidth and resolution, the number of averages and the excitation mode. Moreover, the recording functions must be selected, which should comprise the respective frequency response function (FRF) including the coherence 3. A calibration measurement of the device in unloaded condition 4 must be performed with the previously defined measuring parameters to obtain the appropriate correction parameters to correct for the mass of m2 (see equation (7) and (8)). This can be done, for example, by suspending the device on elastic bands. The same measuring parameters defined before are then used for the actual measure of the organic structure. Therefore, the device is attached via an elastic- or steel-band with a previously specified contact force 5 (e.g. 5.4 ± 0.5 N as ANSI standard). The consideration of the coherence between the averaged spectra of at and a2 is indispensable to check the correctness of the measurement data. For the postprocessing, the frequency response function of cq to a2 already calculated by the measuring system is extracted for both, the calibration as well as the in vivo measurement 6. These data will then be used to calculate the mechanical input point impedance of the structure as well as the imprinted force and according power using an appropriate post processing program 7 (e.g. MATLAB

[25] ). Parameter Determination: Both inertia shaker constructions depicted in Figure la and lb can thus be simplified to the mechanical structure drawing in Figure 3. The mechanical input point impedance describes the resistance of a structure to its excitation

[26] : Z(&>) = =— — (1) — v(<k>) LmJ where the velocity v is obtained by integrating the measured acceleration signal. The impedance matrix for the mechanical equivalent circuit of the inertial shaker exciting the head as illustrated in Figure 3 can be deduced to: 27 11 24 i * to * m, H-- i*a) k i*a) k i*a) t * to * (m2 + tnstructure) + ~ From the internal force equilibrium based on the mechanism of inertia shakers where and F? act with the same amount in opposite directions results: £2 = (3) Solving and F? from equation (2) under the condition (3) it is obtained that: = i * a) * m1 * + * (v^ — V2) (4) and F7 = (Structure + i * to * m2) * V? * fy-, £2) (5) Now the driving point impedance of the structure can be calculated by: Structure = ~* m1 * I * 0) - m2 * I * 0) (6) While mt can be estimated from a weigh of the respective components and slight deviations have only a minor impact, the precise determination of the mass m2 is extremely important for a correct calculation of the mechanical input point impedance. Accordingly, a calibration measure of the transducer in unloaded condition must be performed, meaning that the two masses m1and m2 can oscillate freely, the ratio of the respective acceleration signals are recorded over the desired frequency range. This can be achieved by suspending the device via an elastic-band from the ceiling, which is attached to the housing of the transducer, so no load arises to m2. Since for the impedance of the transducer in unloaded condition holds that: I p, \ ^unloaded = *i*a)-m2*i*a) = 0 ''^'unloaded (7) m2 can correspondingly be calculated by: m2 = —m1 * ( —I (8) unloaded Through the calibration measurement it is therefore possible to determine the corrected mechanical input point impedance of the measured organic structure: ^Structure * — * J&J Vo / I . . —L / unloaded-* (9) 27 11 24 Furthermore, knowing the mechanical input point impedance and the velocity caused by the imprinted force to the structure this force (10) as well as the according power (11) can be determined: F. ^Structure * ^Structure * ¥2 (10) (11) For better understanding, figures are provided: Figure la shows a state-of-the-art electromagnetic transducer as described in reference

[19] Figure lb shows a state-of-the-art "Balanced Electromagnetic Separation Transducer" as described in reference [20, 21, 22] Figure 2 shows the flowchart for the measurement and data acquisition Figure 3 shows a simplified mechanical structure drawing of both transducers shown in Figure la and lb Figure 4 shows the mechanical input point impedance measurement setup with the new device Reference Numbers: 1. Pole shoes 2. Permanent magnet 3. Coil 4. Spacer which allows the ends of the yoke to deflect 5. Yoke 6. Surface (e.g. washer) for the fixation of the device by reference 7 7. Elastic- or steel-band for attachment 8. Additional block to mount accelerometer a2 9. Ferrite rod 10. Housing 11. Spacer with conductive properties 12. Centering slides acting as springs 13. Additional indenter which interconnects the components of moving mass m2 and embeds the accelerometer a2 14. Electromagnetic bone conduction transducer 15. Wires 16. Measuring system with signal generation, recording and analysis function 17. Computing unit 27 11 24 References [1] Flottorp, Gordon, and Sigurd Solberg. "Mechanical impedance of human headbones (forehead and mastoid portion of the temporal bone) measured under ISO / IEC conditions." The Journal of the Acoustical Society of America 59.4 (1976): 899-906. [2] Hussein, Hany Mohamed Gamal-Eldin. "Measurement of the mechanical impedance of human soft tissue in vivo." Louisiana State University and Agricultural &Mechanical College, 1979. [3] Hakansson, Bo, Peder Carlsson, and Anders Tjellstrbm. "The mechanical point impedance of the human head, with and without skin penetration." The Journal of the Acoustical Society of America 80.4 (1986): 1065-1075. [4] Cortes, Diana. "Bone conduction transducer: Output force dependency on load condition." Chalmers University of Technology, 2002. [5] Stenfelt, Stefan, and Richard L. Goode. "Transmission properties of bone conducted sound: measurements in cadaver heads." The Journal of the Acoustical Society of America 118.4 (2005): 2373-2391. [6] Taschke, Henning. „Mechanismen der Knochenschallleitung." Winter Industries; 1., Edition (2006), ISBN-10 : 3866240864 [7] Hakansson, Bo, et al. "The mechanical impedance of the human skull via direct bone conduction implants." Medical Devices (Auckland, NZ) 13 (2020): 293. [8] Nie, Yafei, et al. "Measurement and modeling of the mechanical impedance of human mastoid and condyle." The Journal of the Acoustical Society of America 151.3 (2022): 1434-1448. [9] Bruel &Kjaer Type 8000&8001: https: / / www.bksv.com / - / media / literature / Product-Data / bp0244.ashx

[10] Patent US3222919A:" Mechanical impedance measuring system", ENDEVCO CORPORATION (1965)

[11] Bruel &Kjaer Type 4930: https: / / www.bksv.eom / - / media / literature / Product-Data / bp0266.ashx

[12] Interacoustics Skull Simulator SKS10 https: / / www.interacoustics.com / hearing-aid-fitting / affinity / support / introduction-to-the-skslO-skull-simulator 27 11 24

[13] Audiscan Verifit Skull Simulator https: / / www.audioscan.com / en / skull-simulator /

[14] Patent US10070224B1: "Crosstalk cancellation for bone conduction transducers", Oculus VR, LLC(2018)

[15] Patent US10419843B1: "Bone conduction transducer array for providing audio", Facebook Technologies, LLC (2019)

[16] Patent US10133358B1: "Fitting detection for a bone conduction transducer (BCT) using an inertial measurement unit (IMU) sensor", GOOGLE LLC (2018)

[17] Patent WO2022147905A1: "METHOD FOR OPTIMIZING WORK STATE OF BONE CONDUCTION", SHENZHEN SHOKZ CO., LTD. (2022)

[18] Patent SE452238B: "Frequency characteristics measurement appts, for hearing aid", HAKANSSON BO; CARLSSON PEDER (1987)

[19] Patent WO0180598A1: "BONE-CONDUCTION TRANSDUCER AND BONE-CONDUCTION SPEAKER HEADSET THEREWITH", DOWUMITEC CORPORATION; LEE, SANG, CHUL; KOO, BON, YOUN (2001)

[20] Patent EP1266540B9: "ELECTROMAGNETIC VIBRATOR", OSSEOFON AB; HAAKANSSON, BO (2001)

[21] Patent US2012083860A1: "BONE CONDUCTION TRANSDUCER WITH IMPROVED HIGH FREQUENCY RESPONSE", HAKANSSON BO; OSSEOFON AB (2012)

[22] Hakansson, Bo EV. "The balanced electromagnetic separation transducer: A new bone conduction transducer." The Journal of the Acoustical Society of America 113.2 (2003): 818-825.

[23] https: / / www.pcb.com / contentstore / docs / PCB_Corporate / Vibration / Products / Manuals / 352A7 3_070A71.pdf

[24] https: / / www.plm.automation.siemens.com / global / en / products / simcenter / testlab.html

[25] https: / / www.mathworks.com / ?s_tid=gn_logo

[26] Moser, M. (Ed.). (2009). Messtechnik derAkustik. Springer-Verlag. P: 430

Claims

01 10 241. A method to determine the mechanical point impedance of organic structures using an electromagnetic bone conduction transducer, characterized byin a first step an accelerometer at is attached to a seismic mass mr and a second accelerometer a2 to an actuator mass m2 of an electromagnetic bone conduction transducer,in a second step, a mechanical input point impedance measurement device is connected to a measuring system with signal generator, recording and analysis function,in a third step, measurement- and acquisition parameters comprising the definition of the bandwidth and resolution, the number of averages, the excitation mode, the recording functions, which should comprise the frequency response function (FRF) including the coherence are entered by a user or imported from a parameter storage,in a fourth step, correction parameter for the mass of m2 is determined by means of a calibration measurement of the device in unloaded condition with previously defined measuring parameters,in a fifth step, the here presented device to measure the mechanical input point impedance is attached to the organic structure and measurements with the measurement- and acquisition parameters defined in the third step are carried out,in a sixth step, the values for impedance, force, and power are determined using equ. (9), (10), (11) and graphically displayed.

2. A device to determine the mechanical point impedance of organic structures by means of an electromagnetic bone conduction transducer as used for bone conduction excitation, characterized byan accelerometer at which is attached to a seismic mass m1 and a second accelerometer a2 which is attached to an actuator mass m2 of the electromagnetic bone conduction transducer,the fixation of the actuator mass m2 by means of an elastic- or steel-band 7 supported by a small plate 6

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