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Active Transducer Protection Part 1: Mechanical Overload

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Active Transducer Protection Part 1: Mechanical Overload Active Transducer Protection Part 1: Mechanical Overload Wolfgang Klippel, KLIPPEL GmbH, Dresden, Germany The generation of sufficient acoustical output by smaller audio systems requires maximum exploitation of the usable working range. Digital preprocessing of audio input signals can be used to prevent a mechanical or thermal overload generating excessive distortion and eventually damaging the transducer. The first part of two related papers focuses on the mechanical protection defining useful technical terms and the theoretical framework to compare existing algorithms and to develop meaningful specifications required for the adjustment of the protection system to the particular transducer. The new concept is illustrated with a micro-speaker and the data exchange and communication between transducer manufacturer, software provider and system integrator are discussed. technical paper, which addresses the specification of transducer characteristics to simplify the 1. INTRODUCTION communication between transducer manufacturer, In many audio applications, the required acoustical software provider and system integrator. Test output has to be reproduced at sufficient quality by procedures and rating methods have been developed in using smaller, lighter and efficient transducers. The this paper to specify the maximal voice coil excursion maximum voltage provided by available power Xmax required for tuning the protection systems in smart amplifiers is usually large enough to move the voice amplifiers. Two related papers presented here follow coil to the maximum mechanical boundaries generating this approach and investigate this interesting topic in a mechanical overload and impulsive distortion, which greater detail to improve the definition of the technical have a high impact on the perceived sound quality. The terms and to develop a common framework for heating of the coil is a second mechanism, which may evaluating different protection schemes. damage the transducer. Micro-speakers as used in personal audio devices are a critical example because Attenuation element Transducer audio amplifier the suspension system provides no natural protection of signal Variable the coil as found in normal woofers with a progressive w(t) High-pass spider. Amplifiers used in mobile phones can provide v(t) significantly more power than the micro-speaker can handle safely. This discrepancy causes the need for Mechanical Tv,max Thermal Displacement xrel active protection, which is the most important feature of Attenuation Attenuation smart amplifiers. Control Control The basic requirements and most important goals of X protection systems are: p± voice coil temperature Tv(t) Protection Transducer Limits States • Reliable protection of the transducer against mechanical and thermal overload for any audio input. • Maximum of acoustical output • Minimum of additional signal distortion and other Figure 1 Input information required by an active artifacts degrading sound quality protection system • Minimum delay increasing the latency in the audio path The first paper presented here addresses the mechanical • Cost-efficient implementation protection while the following paper is dedicated to the thermal protection. The mechanical overload can be A variety of ideas and solutions for this important and prevented by activating a high-pass filter as depicted in challenging issue has been developed so far and is Figure 1 and attenuating only low frequencies which accessible as patent literature [1-16]. The paper contribute to the voice coil displacement xrel(t). The presented by Vignon and Scarlett [17] is the first thermal protection requires the attenuation of low and high frequency components which increase the voice L xabs (t) L (2) coil temperature Tv. Mechanical and thermal control systems activating the corresponding attenuation The boundaries L+ and L- depend on the particular elements require the absolute voice coil position xabs(t) or relative voice coil displacement x (t) and design, but are considered as time invariant and rel independent of the particular device under test (DUT). temperature T (t), which are the critical state variables v The positive and negative clearances indicating the two overload conditions. The information can be provided by direct measurement of the X clear (t) L X 0 (t) transducer state or by transducer modeling. (3) X clear (t) L X 0 (t) The attenuation control also shown in Figure 1 represent the distance between the instantaneous rest compares the measured or modeled state variables with position X0(t) and the upper and lower boundaries L±, protection limits, which describe the limits of the and give the theoretical limits of the voice coil permissible working range. displacement xrel(t). However, the attenuation control system uses the maximum positive and negative 2. DEFINITIONS protection limits X p (t) xrel (t) X p (t) The instantaneous position of the voice coil (4) X clear (t) M xrel (t) X clear (t) M x (t) X (t, DUT) x (t) (1) abs 0 rel which are the clearance values reduced by safety margins M+ and M- to cope with inaccurate state depends on the voice coil rest position X0(t,DUT) and information and the reaction time of the attenuation the voice coil displacement xrel(t) generated by the audio control system. signal w(t). The voice coil rest position X0(t,DUT) is a (slowly) varying transducer parameter depending on the particular device under test (DUT) due to production xabs spread. It also changes over time t due to aging, viscoelasticity, gravity, load and climate. L+ M+ xabs Xclear+(t) upper boundary Xp+(t) xAC (t) L+ xrel (t) safety 2XAC M + margin xDC (t) upper upper X 0 (t) Xclear+(t) X (t) clearance protection limit 0 xrel (t) X 0 Xp+(t) Xp-(t) X 0 (t) rest position Xclear-(t) Xp-(t) M- lower X (t) clearance clear- L lower - protection limit safety M - margin L- lower boundary Figure 3 DC-component xDC(t) and AC-component xAC(t) of the voice coil displacement and the influence of an offset ΔX0(t) in the voice coil rest position Figure 2 Limits and other characteristics describing the permissible displacement of the voice coil. The mechanical protection shall ensure that the While reliable protection under all circumstances is the instantaneous voice coil position stays in the primary goal, the protection system shall reduce the permissible working range: input signal only to prevent an overload situation. An overreaction of the protection system would reduce acoustical output, which is related to the AC close to the geometrical center of the boundaries L+ and displacement : L- generating symmetrical clearances: xAC (t) xrel (t) xDC (t) (5) X clear X clear L L / 2 (11) The DC component xDC(t) shifts the voice coil away Furthermore, the transducer manufacturer spends from the instantaneous rest position X0(t) but generates significant effort to reduce the asymmetries in the no acoustical output. The DC component xDC(t) which nonlinear stiffness Kms(x) and force factor Bl(x) and to may be in the same dimension as the AC components keep the generated DC displacement xDC(t) small. xrel(t) is generated by transducer nonlinearities Unfortunately, production spread and the influence of rectifying the audio signal [21]. Contrary to the voice acoustical load in the audio system, aging of the coil rest position X0(t), the DC component xDC(t) is a suspension, gravity and climate may generate an offset fast varying signal component depending on the in the rest position: instantaneous properties of the audio signal. This is a critical problem in transducers with a soft suspension, X o (t) X 0 (t) X 0 (12) especially in micro-speakers, as investigated by Vignon This offset reduces the clearance on one side of the coil et. al. [17]. and generates asymmetries in the nonlinearities which A useful criterion for assessing the performance of the may additionally increase the DC component xDC(t). protection system is the nominal maximum amplitude XAC(t) which limits the desired AC-signal 3. OVERVIEW ON PROTECTION SCHEMES X AC xAC (t) X AC (6) 3.1. Attenuation Control while assuming that the audio signal w(t) has a symmetrical probability density function pdf(w(t)). The attenuation control element in Figure 1 activates the The end-of-line test completing the manufacturing high-pass filter which attenuates the low frequency process can provide valuable statistical information components in the audio signal w(t) to keep the which describe the initial properties (t=0) of devices following peak value of the displacement signal within under tests (DUTs) such as the mean initial voice coil the protection limits Xp+ and Xp+. The mechanical rest position attenuation control element receives the instantaneous displacement signal xrel(t) depending on the attenuated 1 ND audio signal v(t). This feedback structure can cope with X X (t 0,n ) (7) 0 N 0 DUT the time-variant and nonlinear transfer responses of the D nDUT attenuator and transducer and the properties of the and the mean initial positive and negative clearances instantaneous audio signal w(t). N D x 1 rel X (t ) X X (t 0,n ) (8) peak 0 clear N clear DUT D nDUT Xe(t) Envelope averaged over a representative number ND of DUTs. Protection The Golden Reference DUT selected in end-of-line limit testing has properties close to those initial mean values. Xp+(t) The initial value of the maximum positive and negative protection limits of the displacement t X p (t 0) X clear M x (t) 0 t (9) rel X p (t 0) X clear M Xp-(t) are the initial clearances reduced by the safety margins Protection M+ and M-. limit dx ( t ) The transducer manufacturer is interested in maximizing rel dt Envelope the nominal maximum amplitude XAC by placing the voice coil at the optimum rest position X L L / 2 (10) 0 Figure 4 Anticipation of the peak displacement at an overshoot case The attenuation control system has to cope with the xrel(t) with the protection limits Xp± and attenuates the restoring force and inertia of the mass-spring system input signal w(t) and the delayed input signal w(t-τ) generating a low-pass characteristic between terminal simultaneously.
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