loudspeaker

Abstract

To provide a speed detection device and method for a loudspeaker driver motion feedback.SOLUTION: A loudspeaker driver includes a voice coil 114 and a detection winding 118 coaxially wound around a voice coil former 110, and is driven so as to reciprocate along a reciprocating axis 112 by the application of an electrical signal to the voice coil, and has a first magnetic field arranged to couple with the voice coil and a second magnetic field arranged to couple with the detection winding, and detects a voltage induced in the second detection winding during reciprocating motion in the axial direction in the second magnetic field. The second magnetic field has an orientation periodicity that is circumferential with respect to the reciprocating axis, and the orientation periodicity extends over a portion of the length of the voice coil former along the reciprocating axis.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to the field of loudspeakers, and in particular to detecting the instantaneous velocity of a reciprocating voice coil and voice coil former for driving an acoustic diaphragm from which acoustic waves are radiated. The present invention relates to a method for designing a loudspeaker, a loudspeaker and a voice coil former thereof. [Background Art]

[0002] There are many types of conventional acoustic loudspeakers for converting electrical audio signals into corresponding sound.

[0003] A loudspeaker generally comprises one or more drivers, a housing, and electrical connections, and in many cases also includes circuitry such as a crossover network. The driver to which the present invention relates is a voice coil, that is, a coil of conductive wire helically wound around a rigid, generally cylindrical former. These drivers are located within a magnetic field, and when an electrical audio signal passes through the voice coil, they reciprocate to drive an acoustic diaphragm to radiate sound waves. Such arrangements have been used for over a century and are the subject of, for example, US1707570.

[0004] Ideally, for a loudspeaker, the motion of the voice coil is linearly related to the electrical signal applied to the driver terminals of the loudspeaker, so that when a signal constituting one or more sine waves is applied to the loudspeaker, the resulting motion of the voice coil consists only of the same set of sine waves. However, this is not the case in practical devices, and the transfer function has significant non-linearities. As a result, when one or more sine waves are applied to the driver terminals of a loudspeaker, the resulting voice coil motion also includes harmonics at multiples of the applied sine waves and sum and difference frequencies. This characteristic is undesirable for high-quality audio reproduction.

[0005] Nonlinear characteristics result from the modulation of the transfer function parameters during the operation of a loudspeaker, particularly the modulation as a function of voice coil current, voice coil temperature, and voice coil and diaphragm displacement. The main mechanisms that cause nonlinearity are typically as follows:

[0006] The suspension and ambient stiffness vary as a function of the voice coil position, the motor system strength (BL) varies as a function of the voice coil position, the resistance of the voice coil wire varies as a function of the voice coil temperature, and the change in voice coil inductance as a function of the voice coil position as a function of the state of the motor system's magnetic circuit, which itself is a function of the current and past voice coil position and voice coil current.

[0007] One approach employed in loudspeaker design is to minimize these mechanisms, but this approach typically comes with significant additional cost and complexity. Another approach to reducing the nonlinearity of loudspeaker drivers is to detect the motion of the voice coil and use this detected signal in a negative feedback loop (using appropriate amplifier and control electronics). This second approach, commonly known as "motion feedback," is well-known but not commonly used in commercially available loudspeakers, mainly due to the complexity of the setup and the cost and performance of available motion sensors. US3941932 is an example of motion feedback that uses a piezoelectric accelerometer placed under the driver dust cap to detect the acceleration of the voice coil. Another well-known example of motion feedback, as shown in Figure 1, involves placing an additional sensing winding, wound coaxially with the voice coil, in the magnet motor gap.

[0008] In a conventional velocity-sensing motion feedback configuration schematically shown in Figure 1 as a cross-sectional side view, the loudspeaker has a ferrite ring magnet 2, a steel yoke 4, and a steel front plate 6, which together form a magnetic gap 8, within which a voice coil former 10 reciprocates along an axis 12 to drive a diaphragm (not shown). The voice coil 14 is wound around the voice coil former 10 in a conventional manner and extends along the axis 12 for a distance sufficient to accommodate the reciprocating motion of the voice coil former 10, and the voice coil former is driven by energizing the voice coil 14 with an electrical signal.

[0009] The voltage applied to the voice coil 14 generates a magnetic field that interacts with the magnetic field in the magnetic gap 8, driving the voice coil former 10 to move along the axis 12 according to the well-known principle of electromagnetic induction. The fine wire detection coil 16 is wound coaxially with the voice coil 14.

[0010] As the voice coil former 10 reciprocates within the magnetic gap 8 with the detection coil 16, the magnetic field within the gap 8 induces a voltage in the detection coil 16, which can be measured and used to determine the instantaneous velocity of the voice coil former 10.

[0011] The detection winding moves with the voice coil, and theoretically, when the detection winding passes through the magnetic field in the magnet motor gap, the induced motion EMF, ε, is as follows: motion This generates an output voltage proportional to the movement speed of the voice coil.

[0012]

number

[0013] Here, (BL)sc is the sensitivity coefficient (which is the average magnetic flux density (B) across the detection voice coil winding multiplied by the length (L) of the speed detection winding),

number

[0014] The motion feedback approach using secondary windings has several advantages over the approach using accelerometers, as follows:

[0015] The detected velocity is almost accurately represented as axial voice coil motion up to very high frequencies and does not include parasitic self-resonance or resonance from the sensor mounting base, which are often problematic with other sensors such as accelerometers.

[0016] The speed detection winding has a low impedance output and does not require proximal electrical amplification in the driver's moving parts.

[0017] As a result, half of the speed sensing winding moves backward and the other half moves forward, so locking motion of the movable driver component (i.e., motion not parallel to the reciprocating axis) tends not to be detected (this is advantageous because small amounts of locking are common in many drivers and have little impact on performance).

[0018] Since the speed detection winding does not carry a large current, extremely thin wires can be used, which means they occupy almost no space in the moving assembly and add only a small amount of mass.

[0019] The conventional speed detection winding approach shown in Figure 1 has two major problems that significantly impair performance and limit its use. First, for the detection voltage Vs to be linearly related to the voice coil speed, the sensitivity coefficient (BL)sc in Equation 1.1 must be constant. Figure 1 is drawn with an overhanging speed detection winding configuration such that the length A of the magnet motor gap is shorter than the length D of the speed detection winding. This means that the driver's range of motion (the movement of the driver away from the "stationary" position, as shown in Figure 1) is less than the distance C, but the average magnetic flux density received by the detection winding is approximately constant. If the range of motion is greater than the distance C, the average magnetic flux density changes (because only a portion of the winding is in the motor gap), and the sensitivity of the speed detection winding decreases dramatically. To address this cause of nonlinearity, the winding height of the speed sensing winding could be increased to exceed the winding height of the voice coil; however, this is not useful because it would require increasing the length of the former, increasing the weight of the driver and the size of the loudspeaker, and increasing the clearance between the former and the steel yoke to avoid collisions during operation.

[0020] Secondly, an additional mutual EMF exists at the terminals of the speed detection winding due to transformer-like coupling with the current flowing through the voice coil. A more precise description of the speed detection winding voltage is as follows:

[0021]

number

[0022] Here, M is the mutual inductance between the voice winding and the velocity sensing winding, and i is the current flowing through the voice coil. This mutual inductance effect is well known and contaminates the detected signal in typical drivers to such an extent that the effective feedback bandwidth is significantly limited. FIG. 2 shows a simulation of the velocity sensing winding voltage and the constituent EMF of a typical low-inductance loudspeaker having a velocity sensing winding wound directly over the voice coil, clearly showing how mutual inductance dominates the detected voltage at high frequencies. A large dip in the detected signal exists at the frequency where the motion EMF and the mutual EMF have the same value.

[0023] The value of mutual inductance depends on the self-inductance of each coil and the magnetic coupling between the two coils, as expressed in the following formula.

[0024] [Numerical formula]

[0025] Here, L e is the self-inductance of the voice coil, and L esc is the self-inductance of the velocity sensing winding, and k is the coupling coefficient having a value between 0 and 1 representing the proportion of magnetic flux coupled from one coil to the other. The mutual EMF is proportional to the square of the number of turns of the voice and velocity sensing windings, and BL and (BL)sc are proportional to the number of turns. This means that the mutual EMF is particularly high in drivers with a large number of turns in the voice coil. For example, FIG. 3 (showing the measured velocity sensing winding voltage and the actual velocity (measured by laser) of a loudspeaker with high inductance and strong coupling between the voice coil and the velocity sensing winding) shows that in some cases the velocity signal is completely overwhelmed by the mutual EMF, resulting in poor sound quality, and this arrangement cannot be used for velocity sensing in the audible frequency band. A velocity sensing motion feedback approach that is suitable for all loudspeakers including high inductance voice coils and addresses or improves on the problems of conventional systems is needed. SUMMARY OF THE INVENTION

[0026] The present invention is based on the realization that by providing a primarily radial magnetic field to energize and move a voice coil in the axial direction, and by providing an auxiliary magnetic field along at least a portion of the voice coil axis that has a higher-order circumferential periodicity than the primarily radial magnetic field, it is possible to provide a speed detection device that is relatively small, lightweight, easy to manufacture, and not as troubled as conventional speed detection motion feedback devices.

[0027] Accordingly, the present invention provides a method for measuring the instantaneous velocity of a loudspeaker driver reciprocating in a magnetic field, wherein the driver comprises a voice coil wound coaxially on a former and a detection winding wound coaxially on the former, the driver is driven to reciprocate along a reciprocating axis by the application of an electrical signal to the voice coil, the method comprising providing a first magnetic field arranged and adapted to be coupled primarily to the voice coil, providing a second magnetic field arranged and adapted primarily to be coupled primarily to the detection winding, and detecting a voltage induced in the second detection winding as it reciprocates axially in the second magnetic field, the second magnetic field having an orientation periodicity that is circumferential with respect to the reciprocating axis, and the orientation periodicity extends along the reciprocating axis over at least a portion of the length of the former.

[0028] In this way, the magnetic circuit provides two magnetic flux distributions, one of which interacts almost independently with the voice coil and the sense winding, respectively. The first magnetic flux distribution corresponds to the magnetic gap of a typical motor system, optimized to maximize coupling with the voice coil, and has a substantially radial magnetic field. The second magnetic flux distribution is optimized to maximize coupling with the sense winding. The sense winding is designed to minimize coupling to the first region of concentrated flux, and the second magnetic gap is designed and positioned relative to the voice coil so that the voice coil minimizes coupling to the second region of concentrated flux. The two magnetic fields can be embodied in several different ways (some of which will be described later), allowing for the existence of an overall magnetic field that varies in a plane perpendicular to the loudspeaker axis at one or more positions along the loudspeaker axis, which in turn allows for the use of different sense winding arrangements to detect the instantaneous velocity of the voice coil former when the loudspeaker is in use, which is an improvement over conventional velocity-sensing motion feedback loudspeakers. A loudspeaker based on the principles of the present invention exhibits reduced nonlinearity in the detection voltage / voice coil velocity relationship and extremely low mutual inductance.

[0029] The first and second magnetic fields may be positioned at different locations along the axis. Additionally or alternatively, the first and second magnetic fields may be superimposed. Offsetting the two magnetic fields with respect to the reciprocating axis helps to increase the linearity range of the sensing winding. Superimposing the magnetic fields allows for a more compact design.

[0030] Such an arrangement allows the sensing winding to be configured such that it does not interact with the magnetic driving field generated by the voice coil when an electrical signal is applied to drive the voice coil (for example, by ensuring that the sensing winding is perpendicular to the reciprocating axis at virtually all points). As a result, the coupling coefficient between the voice coil and the speed sensing winding approaches zero, and the mutual inductance also approaches zero. Consequently, the mutual emf also approaches zero, and the speed sensing winding voltage is dominated by the motion emf signal, even in drivers with voice coils having very high inductance. The sensing winding is preferably configured to have a circumferential periodicity that matches the second magnetic field.

[0031] The first magnetic field may be primarily radial with respect to the reciprocating axis, and the second magnetic field may have a small circumferential fluctuation component, so that when the two magnetic fields are superimposed, there is a reference radial flux, and when moving circumferentially, there is a region of radial flux slightly higher than the reference radial flux and a region of radial flux slightly lower than the reference radial flux. When the two magnetic fields are not superimposed but instead separated axially, the first flux is substantially constant around the circumference of the magnetic gap over a first axial distance, and the second magnetic field has a region of slightly positive radial flux and a region of slightly negative radial flux over the circumferential and second axial distances.

[0032] In another embodiment, the present invention provides a loudspeaker in which a voice coil is wound coaxially on a former, and together they are adapted to reciprocate along the reciprocating axis within a gap in a magnet arrangement to radiate acoustic energy, such that when an electrical signal is applied to the voice coil, an acoustic diaphragm connected to the former reciprocates along the reciprocating axis, and the voice coil comprises a voice coil extending a first distance along the axis and the former, and a detection winding arranged coaxially on the former and extending a second distance along the reciprocating axis. The magnet arrangement is adapted and configured to generate two magnetic fields, a first magnetic field primarily driving the voice coil to reciprocate and positioned adjacent to it in the axial direction at a first distance, and a second magnetic field positioned adjacent to it in the axial direction at a second distance and adapted primarily to couple with the sensing winding, the sensing winding being arranged around the former in the form of an even number of loops, the even number of loops being arranged separately around the former but electrically connected to form a single winding, and each loop extending around a loop axis substantially perpendicular to the reciprocating axis.

[0033] The position of the loop around the former provides the detection winding with circumferential periodicity of sensitivity. This sensitivity periodicity preferably coincides with the periodicity of the second magnetic field.

[0034] The first and second magnetic fields and the first and second distances do not necessarily have to overlap in the direction of the reciprocating axis (instead they are offset along the reciprocating axis), or the first and second magnetic fields and the first and second distances may overlap in the direction of the reciprocating axis. As described above, there are advantages to offsetting or superimposing the magnetic fields.

[0035] The second detection winding may be formed on the outer surface of the former in one or more layers, each including a plurality of separate coils arranged circumferentially around the former, with each coil including a plurality of adjacent turns extending around a loop axis. The outer surface of the former may include the outermost surface and / or the innermost surface in the radial direction of the former. The circumferentially adjacent coils may be turned in alternating directions around their respective axes to match the change in the magnitude of the radial magnetic flux, which can be achieved by alternating the radial magnetic polarity in the second magnetic field. The circumferentially invariant primary and voice coil magnetic fields may produce induced voltages within the individual loops, but no net EMF is generated from the secondary winding formed by the loops due to the alternating polarity of the loops.

[0036] Two or more coils may be located on superimposed adjacent layers, or they may be superimposed such that the majority of turns are located on a single layer, with small portions of each turn providing an intersecting path in a second adjacent layer where the turns of one coil intersect with the turns of another coil.

[0037] The detection winding may comprise two or more printed layers, where printed coils in one layer are circumferentially aligned with printed coils in adjacent layers around the reciprocating axis. Preferably, a portion of each coil is aligned perpendicular to the reciprocating axis, and the turns in the portion of each coil are spaced further apart from the reciprocating axis by a greater distance than the turns forming the rest of the coil. This allows the sensitivity coefficient (BL)sc to be kept substantially constant over a very wide axial range of the detection winding location. The first distance is preferably smaller than the second distance, which helps to keep the sensitivity of the detection winding constant along the reciprocating axis.

[0038] The magnet arrangement may include separate first and second magnets for generating first and second magnetic fields, or it may be a single integrated magnet adapted to generate a magnetic field equivalent to the combined first and second magnetic fields.

[0039] In a further embodiment, the present invention also provides a former on which a loudspeaker voice coil is wound coaxially, wherein the former and the voice coil are adapted to reciprocate along the reciprocating axis within a gap in a magnet arrangement such that an acoustic diaphragm connected to the former reciprocates along the reciprocating axis to radiate acoustic energy, the former includes a detection winding formed on the outer and / or inner surface of the former in one or more printed circuit layers including a plurality of separate detection coils arranged circumferentially around the former, each detection coil including a plurality of adjacent turns, the detection coils are arranged separately around the former but are electrically connected to form a single winding, and each detection coil extends around a loop axis substantially perpendicular to the reciprocating axis.

[0040] Such an arrangement is ideal for operation as a velocity sensing winding in a magnetic gap formed by a magnet arrangement adapted and configured to generate two magnetic fields: a first magnetic field primarily for driving the voice coil to reciprocate, and a second magnetic field positioned axially adjacent to the first magnetic field and adapted primarily for coupling with the sensing winding. The former may comprise two or more printed layers, where printed coils in one layer are circumferentially aligned with printed coils in adjacent layers around an axis. Parts of each coil may be aligned perpendicular to the reciprocating axis, and the turns of the part of each coil are spaced further apart from each other with respect to the reciprocating axis than the turns forming the rest of the coil.

[0041] The present invention will be described below with reference to the attached figures. [Brief explanation of the drawing]

[0042] [Figure 1(a)] This is a schematic diagram of a conventional loudspeaker with a speed detection winding. [Figure 2] Figure 1 is a graph showing the simulation of the speed winding voltage and configuration EMF frequency response of the loudspeaker. [Figure 3] This graph shows the measured speed detection winding voltage and actual speed of a conventional loudspeaker with high inductance and strong coupling between the voice coil and the speed detection winding. [Figure 4] This is a schematic cross-sectional view of one embodiment of the speed-detecting loudspeaker according to the present invention. [Figure 5] Figure 4 is a schematic plan view of the loudspeaker. [Figure 6a-b] Figure 4 is a schematic diagram of the magnetic field of the loudspeaker, and Figures 6a and 6b show the direction of the magnetic field at different points along the axis of the loudspeaker. [Figure 6c-e] Figure 4 is a schematic diagram of the magnetic field of the loudspeaker, and Figures 6c and 6d show two magnetic fields that combine at one or more points along the axis of the loudspeaker to generate the magnetic field shown in Figure 6e. [Figure 7] This is a schematic diagram of a detection winding arrangement with two layers. [Figure 8] Figure 7 shows one layer of the detection winding arrangement. [Figure 9] Figure 7 shows the other layers of the detection winding. [Figure 10] This graph shows the detected winding voltage in a conventional loudspeaker and the detected winding voltage and actual speed of the loudspeaker according to the present invention. [Figure 11] This graph shows the basic SPL (sound pressure level) and THD (total harmonic distortion) SPL of a loudspeaker with a conventional voltage amplifier, compared to a current amplifier with negative feedback from a speed sensor. [Figure 12] This is a schematic diagram of another example of a detection winding arrangement. [Figure 13] Figures 13a and 13b are schematic diagrams of possible loudspeaker drive magnet configurations for providing the magnetic field shown in Figure 6e, with and without a voice coil former, respectively. [Figure 14]Figures 14a and 14b are schematic diagrams of other possible loudspeaker drive magnet configurations for providing the magnetic field shown in Figure 6e, with and without a voice coil former, respectively. [Modes for carrying out the invention]

[0043] Figures 1 to 3 relate to prior art and are described in the introduction above.

[0044] Figures 4 and 5 show one embodiment of the speed-detecting loudspeaker according to the present invention.

[0045] This loudspeaker has a ferrite ring magnet 102, a steel yoke 104, and a steel front plate 106, which are coupled to form a magnetic gap 108, within which a voice coil former 110 reciprocates along an axis 112 to drive a diaphragm (not shown). The voice coil 114 is wound around the voice coil former 110 in a conventional manner and extends along the axis 112 for a distance sufficient to accommodate the reciprocating motion of the voice coil former 110, and the voice coil former is driven by energizing the voice coil 114 with an electrical signal (this electrical signal is usually generated by an electrical and / or electronic circuit outside the loudspeaker housing (however, a crossover circuit etc. may be present inside the housing)).

[0046] Those skilled in the art will understand the principles and apparatus involved in the generation and transmission of these signals to loudspeakers, and since these are not directly related to the present invention, they will not be described further herein.

[0047] The voltage applied to the voice coil 114 generates a magnetic field that interacts with the magnetic field in the magnetic gap 108, driving the voice coil former 110 to move along the axis 112. The detection winding 118 extends beneath the voice coil 114 around the voice coil former 110 (either the inner or outer surface of the voice coil former 110 or both) and is formed as a printed circuit board (PCB) that extends axially beyond the voice coil 114 (the laminated sandwich structure of conductive and insulating layers forming the PCB itself may constitute the former). At the front end of the magnetic gap 108, above the T-yoke poles, there are an even number (four are shown) of neodymium (NdFeB) magnets 120 arranged as shown, with alternating magnetic polarity. Similar to conventional speed detection designs, as the voice coil former 110 reciprocates within the magnetic gap 108 with the detection winding 118, a voltage is induced in the detection winding 118 by the magnetic field generated by the NdFeB magnet 120, and this voltage can be measured and used to determine the instantaneous speed of the voice coil former 10. In the illustrated embodiment, four cylindrical neodymium magnets 120 are added in a magnetic quadrupole orientation to form a secondary magnetic field that generates a motion EMF within the speed detection winding proportional to the speed of the voice coil former 110. The detection winding is printed on the voice coil former 110 using flexible PCB technology. This approach is lightweight and allows for complex winding patterns (detailed below). The axial spread of the voice coil and speed detection track is indicated by dimensions D1 and D2, the secondary magnetic gap is indicated by A2, and the overhang of the detection winding is indicated by C2. Since the detection magnetic field is located above the primary magnetic field, and the magnetic field height A2 can be lower, the overhang C2 can be substantially higher than in conventional configurations. This allows (BL)sc to be linear over a much wider range of voice coil displacements than in conventional loudspeakers.

[0048] Figures 6a and 6b show the magnetic field orientations generated in the regions of the ring magnet 102 and the front plate 106, and the region of the neodymium magnet 120, respectively. In this particular embodiment, the two regions are separated axially, resulting in little interaction between the two magnetic fields, and the first magnetic field of the primary gap 108 (Figure 6a) is almost entirely radial, and therefore operates in the same manner as a conventional motor system. The secondary magnetic field has circumferential periodicity.

[0049] In this embodiment, the magnetic field generated by the secondary magnetic circuit has a substantially quadrupole orientation in which the radial magnetic field polarity changes twice around the circumference of the magnetic gap. It should be noted that many other secondary magnetic field shapes are possible, such as substantially dipole, substantially octupole, etc., any shape in which the radial magnetic field polarity changes an even number of times in essence. The secondary magnetic field must strictly match the winding arrangement of the detection winding so that (BL)sc is high enough to provide sufficient velocity sensitivity. The quadrupole secondary magnetic field is a preferred embodiment because it is the lowest-order secondary magnetic field that eliminates the oscillation (of the voice coil former when it is reciprocating) in the detected signal, but other embodiments are also possible if they are arranged geometrically or circumferentially to prevent oscillation.

[0050] It should be noted that the first and second magnetic fields do not need to be axially separated, and in other embodiments, there may be interaction between the two magnetic fields, and the two regions may overlap. This does not adversely affect the performance of the sensing winding, provided that the winding loop is positioned so as not to couple with the magnetic field generated when current flows through the voice coil. An example of this is shown in Figures 6c to 6e, where Figure 6c shows the radial flux of the first magnetic field, Figure 6d shows the radial flux of the second magnetic field, and Figure 6e shows the radial flux when the first and second magnetic fields are superimposed in the same axial position. In this quadrupole example, two regions of slightly higher radial flux and two regions of slightly lower radial flux are located around the circumference of the magnetic gap. The sensing winding is configured to have a circumferential periodicity that matches the second magnetic field.

[0051] Figure 7 shows the PCB Gerber file for a quadrupole detection winding consisting of two PCB layers (the figure is a 2D representation of the winding formed circumferentially around the voice coil former). Figures 8 and 9 show the track arrangement on the individual layers more clearly. The arrangement here involves a detection winding with eight series-connected helices formed as four loops 126 on each of the two PCB layers. The four loops of the layers in Figures 8 and 9 are superimposed axially and circumferentially such that adjacent loops 126 within the layer alternate in the direction they turn, and pairs of superimposed loops turn in the same direction. This winding arrangement is optimized in two aspects. Firstly, the lower part of the winding is located in the secondary magnetic field (marked D2 in Figure 7).

[0052] The individual tracks of the sensing winding in this region are positioned further apart than the rest of the tracks and are intended to be oriented substantially perpendicular or perpendicular to the reciprocating axis when in use, matching the quadrupole magnetic field and designed to provide a nearly constant (BL)sc over a very wide range of coil positions. Secondly, this winding arrangement is optimized to minimize coupling with the magnetic field generated when current flows through the voice coil. In this case, the motor system is substantially axially symmetric, and the magnetic field from the voice coil is also substantially axially symmetric. The sensing winding arrangement has an equal number of aligned / superimposed pairs of clockwise and counterclockwise tracks (around the axis of the voice coil loop), and therefore there is zero coupling with the magnetic field from the voice coil. There are many variations of the same helical arrangement with minor modifications such as the order and direction of the helix, and it will be understood that the same approach can be used to develop sensing windings with any even order.

[0053] Figure 10 shows the improved detection winding performance using the present invention compared to a conventional detection winding wound directly on the voice coil and using the same magnetic gap. The frequency of the notch where motion EMF and mutual EMF are equal increased by 1.5 octaves.

[0054] Figure 11 shows a comparison of the output and THD of the prototype loudspeaker using the speed sensor described above, first driven by a conventional low-impedance (voltage output) amplifier, and then driven by a high-impedance (current output) amplifier with negative feedback from the speed sensor signal. In this case, the amount of negative feedback was adjusted to approximately match the response of the low-impedance amplifier. The linear outputs of both systems were similar, but the system with negative feedback from the speed sensor showed substantially reduced distortion.

[0055] Figure 12 shows another detection winding arrangement having four coils formed using two PCB track layers to allow the windings within each loop 128 to intersect. Compared to the arrangements in Figures 7 through 9, this arrangement allows for half the number of turns for a given track spacing, which has the disadvantage of reducing speed sensitivity by half. However, the advantage is that as the voice coil former moves, a more equal length of the detection winding is immersed in the secondary magnetic field. Also, this winding arrangement allows the upper part of the coil (the portion positioned away from the two magnetic gaps) to have approximately the same number of turns clockwise and counterclockwise around the axis of the voice coil, which helps minimize electromagnetic coupling between the detection and the voice coil.

[0056] As is evident, there are many possible configurations for the secondary magnetic field and gap. In the example in Figure 4, a completely separate set of neodymium magnets is used to generate the secondary magnetic field, but it is also possible to generate magnetic fields for both the primary and secondary gaps using a single magnetic circuit. Figures 13 and 14 show two possible alternative configurations (for clarity, Figures 13a and 14a show configurations where a voice coil is present and no former is present, while Figures 13b and 14b show configurations where a former is present and the sensing winding is visible). In the example in Figure 13, a series of alternating pairs of opposing notches 122 and ridges 122 are formed on the upper plate 106' of the magnet configuration to generate the secondary magnetic field and energize the sensing winding 118 (in this case, the configurations of the motor and coil must be carefully designed to minimize mutual inductance). Figure 14 shows an alternative configuration to Figure 4, in which a pair of neodymium magnets 120 generates a magnetic field for the secondary gap. To increase the magnetic flux applied to the detection winding and reduce the stray magnetic field, an upper plate 106 with notches and ridges may be provided as shown in Figure 13, or steel strips may be added. If there is a large interaction between the primary and secondary gaps and the magnetic circuit, the coil shape needs to be optimized to minimize the mutual inductance.

[0057] The detection windings schematicly shown in Figures 7 to 9 and 12 are not planar. The portion of the detection winding that is activated by moving within the second magnetic field (D2 in Figure 7) is axial and circumferential, and is "wound" around the voice coil former (which is cylindrical in Figures 13b and 14b) such that it is substantially perpendicular to the reciprocating axis at all circumferential positions. On the other hand, as can be seen from Figures 13b and 14b, the loops of each detection winding are not planar but curved in one dimension.

[0058] Of course, it will be understood that many modifications are possible to the embodiments described above without departing from the scope of the present invention. For example, the present invention has been described mainly with reference to cylindrical voice coils and formers.

[0059] However, the present invention also applies to non-circular arrangements such as oval, elliptical, or racetrack-shaped (figure-eight, or rounded triangle / square / polygon), planar, or hexagonal voice coils and formers, or any shape that is symmetrical in one or two orthogonal directions and located in a plane substantially perpendicular to the voice coil axis, with a central hole. An array of magnets may be used to energize the voice coil gap, and the present invention can be applied to other types of motors or actuators incorporating suitable drive coils, including drivers, voice coil actuators, and dual or multiple voice coil drivers having multiple coils and / or multiple gaps. The second magnetic field may be offset from the first magnetic field, and it is possible to make the overhang of the sensing winding perpendicular to the axis (D2 in Figure 7) higher than the voice coil. As described, both magnetic fields can be generated using a single magnetic circuit, each magnetic field can be generated using separate magnetic circuits, or a combination of multiple magnetic circuits can be used to generate the first and second magnetic fields in combination. Although only embodiments having one second magnetic field are described, there may be one or more second magnetic fields spaced apart along the axis. This invention primarily describes, in this specification, the most common former and voice coil arrangements in which the voice coil is wound around the outside of the voice coil former.

[0060] However, the principle of the present invention can be similarly applied to other former and voice coil arrangements, such as when the voice coil former is located around the outside of the voice coil, when there are two voice coils, one outside and one inside, or when there are two voice coil formers, one outside and one inside, of the voice coil. The term “around” as used above and in the claims should be interpreted as encompassing all of these alternative arrangements and does not mean that the elements described as being around other elements are only located around the outside, but also include arrangements where the elements are around the inside.

[0061] If different modifications or alternative arrangements are described above, it should be understood that embodiments of the present invention may incorporate such modifications and / or alternatives in any suitable combination.

Claims

1. A method for measuring the instantaneous velocity of a loudspeaker driver reciprocating in a magnetic field, wherein the driver has a voice coil helically wound coaxially with a former and a detection winding arranged coaxially with the former, the driver is driven to reciprocate along a reciprocating axis by the application of an electrical signal to the voice coil, and the method is: To provide a first magnetic field that is primarily arranged and adapted to be coupled with the voice coil, To provide a second magnetic field that is primarily arranged and adapted to be coupled with the detection winding, and To detect the voltage induced in the second detection winding when it reciprocates axially in the second magnetic field. Includes, A method wherein the second magnetic field has an orientation periodicity that is circumferential with respect to the reciprocating axis, and the orientation periodicity extends along the reciprocating axis over at least a portion of the length of the former.

2. The method according to claim 1, comprising arranging the first and second magnetic fields at different positions along the axis.

3. The method according to claim 1, comprising superimposing the first and second magnetic fields.

4. The method according to claim 1, comprising applying the second detection winding to the former in such a pattern that the magnetic drive field generated in the voice coil when the electrical signal is applied to drive the voice coil does not couple with the second detection winding.

5. The method according to claim 1, wherein the first magnetic field is mainly radial with respect to the reciprocating axis.

6. A voice coil wound helically coaxially around a former, which together are adapted to reciprocate along the reciprocating axis within a gap in the magnet arrangement to radiate acoustic energy, such that when an electrical signal is applied to the voice coil, an acoustic diaphragm connected to the former reciprocates along the reciprocating axis, and the voice coil extends a first distance along the axis and the former, A detection winding is coaxially arranged with the former and extends a second distance along the reciprocating axis. Equipped with, The magnet arrangement is adapted and configured to generate two magnetic fields, a first magnetic field primarily driving the voice coil to reciprocate and positioned adjacent to it in the axial direction at a first distance, and a second magnetic field positioned adjacent to it in the axial direction at a second distance and adapted primarily to couple with the sensing winding, the sensing winding being arranged around the former in the form of an even number of loops, the even number of loops being arranged separately around the former but electrically connected to form a single winding, each loop extending around a loop axis substantially perpendicular to the reciprocating axis, in a loudspeaker.

7. The loudspeaker according to claim 6, wherein the first and second magnetic fields and the first and second distances overlap in the direction of the reciprocating axis.

8. The loudspeaker according to claim 6, wherein the first and second magnetic fields and the first and second distances do not overlap in the direction of the reciprocating axis.

9. The loudspeaker according to claim 6, wherein a second detection winding is formed on the outer surface of the former in one or more layers comprising a plurality of separate coils arranged circumferentially around the former, each coil comprising a plurality of adjacent turns extending around a loop axis.

10. The loudspeaker according to claim 9, wherein adjacent coils in the circumferential direction turn in alternating directions.

11. The loudspeaker according to claim 8, comprising two or more printed layers, wherein printed coils in one layer are circumferentially aligned with printed coils in adjacent layers around a reciprocating axis.

12. The loudspeaker according to claim 9, wherein a portion of each coil is aligned perpendicular to the reciprocating axis, and the turns in the portion of each coil are spaced apart from each other with respect to the reciprocating axis by a greater distance than the turns forming the rest of the coil.

13. The loudspeaker according to claim 6, wherein the detection winding is in the form of a printed circuit formed on the inner or outer surface of the former.

14. The loudspeaker according to claim 6, wherein the first distance is smaller than the second distance.

15. The loudspeaker according to claim 6, wherein the magnet arrangement includes separate first and second magnets for generating the first and second magnetic fields.

16. A former for a loudspeaker, the former comprising a voice coil helically wound coaxially with the former, the former and the voice coil being adapted to reciprocate along the reciprocating axis within a gap in a magnet arrangement such that an acoustic diaphragm connected to the former reciprocates along the reciprocating axis to radiate acoustic energy, the former comprising a detection winding formed on the outer and / or inner surface of the former in one or more printed circuit layers comprising a plurality of separate detection coils arranged circumferentially around the former, each coil comprising a plurality of adjacent turns, the detection coils being separately arranged around the former but electrically connected to form a single winding, and each detection coil extending around a loop axis substantially perpendicular to the reciprocating axis.

17. The former according to claim 16, comprising two or more print layers, wherein a printed detection coil in one layer is circumferentially aligned with a printed detection coil in an adjacent layer around the axis.

18. The former according to claim 16, wherein a portion of each detection coil is aligned perpendicular to the reciprocating axis, and the turns of the portion of each detection coil are spaced further apart from each other with respect to the reciprocating axis than the turns forming the rest of the detection coil.

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