CA1277518C - Capacitive pressure-sensing method and apparatus - Google Patents
Capacitive pressure-sensing method and apparatusInfo
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- CA1277518C CA1277518C CA000538866A CA538866A CA1277518C CA 1277518 C CA1277518 C CA 1277518C CA 000538866 A CA000538866 A CA 000538866A CA 538866 A CA538866 A CA 538866A CA 1277518 C CA1277518 C CA 1277518C
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- pressure
- electrode means
- resilient
- drum
- sensor
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Abstract
ABSTRACT
A novel capacitive pressure-sensitive sensing tech-nique and apparatus wherein an elastomeric conductive electrode carrying a two-dimensional array of projections is pressure-deformed against a fixed coextensive coopera-tive electrode to generate signals, such as tones and sounds in the application to musical instruments, or visual representations, corresponding to the dynamic pres-sures applied over the two-dimensional surface. A novel drum-like and other musical instruments embodying such novel capacitive sensing techniques and the like.
A novel capacitive pressure-sensitive sensing tech-nique and apparatus wherein an elastomeric conductive electrode carrying a two-dimensional array of projections is pressure-deformed against a fixed coextensive coopera-tive electrode to generate signals, such as tones and sounds in the application to musical instruments, or visual representations, corresponding to the dynamic pres-sures applied over the two-dimensional surface. A novel drum-like and other musical instruments embodying such novel capacitive sensing techniques and the like.
Description
77~a CAP~CITI~E P~CSSURE-SE~SI~G ~ETHO~
A~D AP~ARATUS
The presen~ invention relaces tO pressure-sensin~
methods and apparatus, being more specifically concerned with novel t~o-dimensional capacitive sensors and techni-ques particularly, though by no means e~clusively, applic-able to musical and rhvt~mic inst.ume~ts and other devices responsive to touch and variable forces applied over a e~o-dimensional surface.
Novel capacitive pressure sensors having a resilient shaped curved or tapered conductive electrode that is deformed by an instrument key activation or other pressure into engaging variable capacitive cooperation with a fi~ed electrode electrically separated therefrom are dislosed in U.S. Letters Patent ~o. 4,498,365, of applicant Jeff Tripp hereln, and are most useful for operation by a single lim~-ted region of pressure contat. Such sensors provide con-tinuous sensing, as for electronic tone generation in an instrument, and even enable further pressure variations after the key activation or other pressure coneact, as for such purposes as enabling a second note generation or pitch !7;~
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775~8 or tone variation, in the illustration of usage in an instrument ~eyboard. Clearly o~her applicaeions requiring similar response are also useful.
There are occasions, ho~e~7er, where it is desired to enable pressure to be applied over a two-dimen-sional surface and wieh sensiti~ity to variations in attack or impact and/or response to particular patterns or dynamic shape variations of the pressure over èhe two-dimensional surface. As an illustration, a drum membrane to be activated b~ the impact of a drum stic~, or the sweeping of a drum brush, andlor the sweeping of fingers or the hand with various dynamic pressure patterns and variations over the membrane, require such two-dimensional independent fine-point or region pressure sensing and transducing into electrical signals for the purpose of generating sounds that characterize such pressures and pressure patterns. Similarly, as another illustration, configurations may be designed with multiple electrodes cooperating with a common elastomeric electrode, later described, for the reproducing of vigual paeterns, as for measuring hand, finger or foot prints and variations in movement thereof, again operating with two-dimensional continuous, dynamic sensing.
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1;~'77S1~3 For use in taceile sensors to develop sensory feedback, co~plianc conduceive elastomer pads have been developed with an arrav of tactiles which are voltage e~cited and opera~ bv resistanc2 changes in res~onse to pressure and are scanned on a ro~-column sequence to pro-vide a multi-bit digital signal output for such purposes.
(See, for example, Barry Nright Corporaeion 1984 bulletin ~Sensorflex/Aste~", p. 17, 18). Such'sensors, while two-dimensional, have problems in stability of conductivity over time, require comple~ electronics, and have practical llmits on the size or area that can be monitored in view of the pad resistance involved.
An ob~ect of the present inventlon, accordingly, is to provide a new and improved method of and apparatus for providing such two-dimensional pressure-sensing responses for such applications and others requiring simi-lar responses.
A further ob~ect is to provide novel musical and rhythmic instruments of great flexibility, including ~ -drum-like instruments, resulting from the use of the novel pressure sensing of the ~nvention.
Other and further ob~ects will be e~plained hereinafter and will be more particularly delineated in the appended cla~ms.
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In summary, however, from one of its aspects, the invention embraces a capacitive pressure~sensitive sensor having, in combination, a first electrode comprising a thin resilient conductive plastic sheet having a plurality of closely spaced resilient conductive projections protruding from one surface of the sheet and with adjacent regions pressure-deformable by application of pressure thereat from the opposite surface of the sheet, and a second electrode facing and coextensive with the projections and separated from the same by a thin dielectric layer therebetween. Preferred and best mode embodiments and components, including drum instruments and the like, are hereinafter described in detail.
In its method aspect the invention relates to a method of capacitive pressure-sensing, that comprises, dynamically deforming adjacent regions of a conductive resilient plastic two-dimensional array of closely spaced projections in a predetermined direction and in a contour of pressure corresponding to a predetermined pressure pattern extending over one or more regions of the array, with each projection deformed by the pressure thereabove, limiting the contoured deforming of the projections at a fixed~position coe~tensive cooperative capacitive electrode surface separated from the array by a thin dielectric medium, and sensing the dynamic capacitive variations effected by the projections under contoured pressure to generate electrical signals corresponding thereto.
The invention will now be described with reference to the accompanying drawings, Fig. 1 of which is a transverse MLS/bp 4 .
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~775~8 section of a preferred two dimensional capacitive pressure sensor useful for the practice of the invention;
Figs. 2A-2C are experimentally derived variations obtained by drumstick impacting of the sensor of Fig. 1, and Figs. 2D and 2E are outputs for surface pressure-pattern applications thereto;
Figs. 3A-3D are enlarged transverse sectional views of projection configurations useful as electrodes of - 4a -MLS/bp - ~, ' 1'~77~
the sensor of Fig. l;
Fig. 4A is an isomecric view, partially cut awav, of a multi-section drum using effectively a plura-lir,~ of sensor pads or sensing zones for selective and relativelv independent effects;
Fig. 4B is si~ilar to Fig. 4A but employs a single elastomeric pad sensor electrode;
Fig. 4C is a si~ilar view of a bottom section of the drum useful with both of the embodiments of Figs. 4A
and 4B; and Fig. S is a circuit diagram of a preferred sig-nal-processing apparatus for responding to the capacitive variations of the sensors of the instrument of Figs. 4A
and 4B to produce signals that may, for e~ample, be used to control sound generators to generate desired tones and sounds.
Referri~g to Fig. 1, the pressure sen50r, in preferred form, comprises a thln plastic conductive rubber or other resilient elastomeric pad electrode 1, preferably provided with a protective cover layer C, as of Mylar or the like as later more fully discussed, and having a planar surface from one side of which (shown as the bottom surface) curved or otherwise variable thickness or tapered proiections 1' of the same conductive resilient material `' ''' ' '' ' ~ ' ' ~
protrude in a two-dimensional closely spaced preferably uniform array e~tending in close capacitive relationship wieh a coe~tensive two-dimensional conductor elec~rode sur~ace 3, separa~ed from t~e projections 1' bv a thin dielectric layer 2, preferably somewhat resilien~ly defor-mable, also. The electrode surface 3 is shown fixedly disposed on a hard immovable board B, so that pressing of the electrode 1 into mechanical force contact with the lmmovable electrode 3 develops the desired capacitance changes to be measured, and with the electrode 3 limiting the downward depression of the upper elastomeric pad elec-trode. In Figs. 3A-3D, various curved or tapered shapes for the projections 1' are illustrated as substantially hemispherical, as truncated hemispheres with conical or tapered tips, a double conical tip, and a cone with a somewhat rounded tip, respectively.
It has been discovered that when the opposite (upper) surface of the electrode 1 is deformed, as by the finger F in Fig. 1, the curved or tapered projections 1' under the pattern of the finger tip will correspondingly be depressed and de~ormed, compressing their tapered thickness to produce greater capacitive effects, with sub-stantially individual independent projection-deforming selectivity, to simulate the finger contour and the .
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--~775~3 , various forces e~erted by the various partions of the fin-ger tip on the individual projections immediately there-under. ~ith suitable electronics connected at the out~ut te~minals ~ and 5 o~ t~is var-'abLe capacitor l-1'-7-3, as later described, the application and movement of the fin-ger tip will generate capacitive variations that are readily processed into signals that may control the gener-ation of audio tones or sounds, with audible effects pro-portional to the pressure and corresponding to the attac~
or impact of the finger tip and to surface area dynamic pressure pattern of the moving finger tip on the e1ectrode pad surface 1. With the drum cover layer or head C placed over the silicone rubber or other elastomeric pad elec-trode 1, protection against abrasion or soiling of the pad and the static attraction of dist is provided. Addition-ally, the layer C serves as an electrical insulaeor and isolator to prevent body capacitance from infl~encing the system and to prevent introduction of noise. The layer further acts as a "spreader" cover, useful where there may be high local forces (such as the tip of a drumstick) both to limit the compression sec of the pad and mechanically to ampliry the signal by spreading the impact over a lar-ger area of the capacitor.
Referring to the embodiment of Fig. 4A, impact-ing the dru~t head membrane or cover layer C with a dru~
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~775~8 stick, or wire brush, and/or sweeping the stick, brush, fingers, or hands over the membrane, have been found t~us to generate individual capacitive variations over the t-~o-dimensional surface thac c~n be signal-processed in~o sound patterns corresponding to and in substantially pro-portional response to the pressure patterns applied, and preferably in the continuous pressure-sensing manner of the single sensor units described in said patent. Various signal threshoLds for degrees of depression can be est-ablished as described in said patent, and in connection ~ith Fig. 5, for particular tone or sound effects, includ-ing second striking effects during depression and tone variation effects.
Figs~ 2A-2C show expesimentally obtained visuai representations of output signals generat~d by the capaci-tive changes with this two-dimensional shaped resilient capacitive electrode configuration for light, medium and hard drum stick impacts or strikes of the membrane, dis-played on a print-out connected to the electrode, the sig-nal generation being later described in connection with Fig. 5~ The electrode 1-1' was of silicone carbon-loaded elastomeric plastic sheeting, about a tenth of an inch thic~ and of about 60 Durometer, carrying a two-dimen-' ' : ' ~775~3 sional ar~ay of closely spaced shaped projeceions (1~pro~ections per squar~ inch) pro~ruding about 0.06 inch from a web 1 of abo~le 0.035 inch thickness. The other elecrrode 3 was of 1 mll aluminum foil with tne dielect c f ra ~/~ ~7~ ~ ~ ,~ f ~
layer 2 of '~apcon" (DuPon~polyimide plastic), also abou~
1 mil thick.
The surEace pressure pattern effect is shown ~n Figs. 2D and 2E, the former showing the sensing surface output (arbitrary units) in response to area over which the force is applied, and the latter illustrating the out-put as a function of force applied to the sensing sectors.
Returning to the drum-like application of Fig.
4A, an edge clamp 9 may hold the assembly together and with a dress plate 7 ~Figs. 4A and 4C), which may incor-porate a ground plane. If desired, the electronics for the signal processing may be mounted in the underside of the base board B at B', Fig. 4C, as later described.
Separate sectors or regions oE the drum head C
may be provided as ae 6', 6", eec., Fig. 4A, for different and independent effects at such regions or sectors, and with a for~ed metallic "spider" separator 10 between regions. The spider separator is bonded to the drum head cover C with an adhesive layer 8 to provide a structure thae prevents cross-talk between regions.
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1'c:77~8 The basic system configura~ion, ~hen, is a P.C. board, (1) for e~ample (screened on a polyester film c ,na r~
as of ~ylar,~) which contains the sensing hoteom elec-trode(s) 3, means to connec~ t~e drive signal to the elas-tomer upper electrode(s) 1-1', and means to connect ~o the main electronics; (2) a sheet of dielectric 2 which may or may not be adhesive-bonded to or screened onto the P.C.
board; ~3) the upper electrode~s) l of te~tured conductive elastomer as described above; (4) a top cover or drum head C; (5) electronics which provides drive signal to the elastomer electrode(s) l, supplies the inverse of the drive signal to the other side of the sensing capacitor, monitors for changes in capacitance of the sensor area, and converts such changes to useable electronic signals.
There may be multiple electrodes 3 beneath a single elas-tomeric sheet electrode 1-1', Fig. 4B, to produce a number of independent zones, as well, as later more ~ully e~plained.
Total vertical deflection in the system as cur-rently configured is appro~imately 1/16". The force re~uired to deflect an area ls at least roughly propor-tianaL to the signal produced, and it "gives bac~' force in a manner that makes it an effective pressure sensor.
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1~77~8 The system as described can be modified mechanically and electronically to transduce a wider range oE forces and to have a deeper ac~uation distance for applications for which that would be useful, if desired.
The planar nature of the system means that ehe smaller the ratio of activation area to total area of the sensing zone, the smaller the activation signal relative to the "base'-, or resting capacitance of the zone. Since large zones are employed, this base capacitance is lar~e.
~urther, once the rubber projections 1' are fully depressed, no signal increase results from additional force or pressure Because of the limited vertical travel, high-velocity small-area stri~es "top out' quick-ly. The use of the semi-rlgid Mylar cover C for mechani-cal amplification brings additional area of the capacitor into play for both light and heavy strikes of small-area imple-~ents, producing a broader range of differentiable "attacks". The ratio of the area of neighboring capacitor brought into play versus activating implement area is reduced as the activating implement area gets larger, un-til an implement as large as the zone shows no amplifica-t~on effect. In other words, the use of mechanical ampli-fication allows for compressing a broader range of force-- - . - - .
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7~8 -L~-area products (pressure or impac~) into the narro~er range of effective transduc~ion of the sensor/electronics com-binaeion.
Ie is r~or~h noting tha~ the "~eb' of the conduc-tive rubber electrode 1-1' plays a s~milar, although noe ideneical, role, with web thickness adjustable to tailor the system for a specific application.
This construction does reduce the degree of independence of local areas of the surface; but it is this which enables the obtaining of comparable signals from a high velocity small area strike (drumstick) and a large area low velocity strike (a finger). The semi-rigid cover of head layer C perormq a further dynamic unction in the drum. The harder it is hit, the more instantaneously rigid it appears, and the broader the area of the capaci-tor which is affected (again, mechanical amplification).
The cover can vary from none~istent to thin and elastome-ric (protective only), to thin and semi-rigid (thin Mylar), to thicker semi-rigid, to rigid.
The last would be used to make the system area-insensieive for high range low-profile applications such as weighing devices or discrete impact sensorg, or in com-~ blnation ~ith tnothet sengor ln a s~acX to d~rive area-:
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~'~77518 sensitive informa~ion from the top sensor and simuleaneous area-insensicive information from the bottom, or in a stack of many sensors for precise force measuremen~ over longe dis~ances.
The multi-zone or sec~or electronic dru~ instru-ment application of the invention, in its preferred prac-tical configurations, Figs. 4A and 4B, embodies five inde-pendent strike zones 6', 6", 6"-, etc. on its top surface, and five CV (analog Control ~oltage) outputs. It is powered by a 12 volt battery or other d.c. power supply and mounts on a standard tom post via a clamp on the bottom, as later explained. The system is responsive to both steady and i~pulsive forces and with response speed in the tens of microseconds range and a frequency response well into the kilohertz range.
Output is an analog voltage which tracks the changes in capacitance due to striking or pressing the pad; these being scaled to dri~e most existing CV elec-tronic drum '-brains". As before stated, a preferred elec-tronlc circuit for operating with the sensors of Fig. 1, 4A and 4B is shown in Fig. 5, using a bottom section, Fig. 4C, com~on to both of the embodiments of Figs. 4A and 4B. In a practical apparatus, the body of the de~ice is, : :
, ~77~8 for e~ample, a l"-~hic~ par~icle board disc B which has a cavity B' roueed in the bac~ for the electronics. On this is placed a printed circui~ sheet 11, as of a die-cu~
s~ee~ of Mylar, on which is screened a conductive pattern to provide the five bottom elec~rode surfaces 3 for the five zones 6', 6", 6"', etc. The drive signal is connec-ted to the eiastomeric electrodes 1-1', with conductors 4 and 5 for connection of these areas to the electronics E.
The traces travel along a membrane "rail" 11', which wraps around the body to the electronics cavity B'. Over the electrode areas 3 is subsequently screened a urethane-based material which serves as the dielectric layer Z.
This layer is also preferably screened on the tail to pro-vide insulation. Upon the printed circuit sheet are placed five die-cut pieces of the elasto~eric electrode 1-1' and the spider separator 10. The spider separator i5 fastened through the printed circuit sheet into the body with several fasteners F, such as screws. This simul-taneously positions the electrodes 1-1' in position, and electrically connects the pattern 4 to the five electrodes ~ubsequently to provide the drive signal.
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-77~8 A spacer ring 12 $s placed around the periphery of the assembly~ with a die-cuC adhesive film 8 placed over the spider separaeor, and the head C is placed onto the assembly, follo~ed by the dress ring 9, which is not qet swaged over on the bottom. The assembly is iltverted, the dress plate 7 is installed, and the dress ring is swaged to its final configuration. On an access plate 13 are installed the five output iacks and the power jack J
and the two potentiometers P, for all of which, terminals are later identified in the circuit of Fig. 5. These are connected to the electronics E mounted on the bottom B' of the access plate. The membrane tail is then connected to the electronics and the access plate is fastened to the body with the tom cla~tp 14 fastened in pOsitiolt to com-plete the assembly.
If the single elastomeric pad version of Fig. 4B
is used, the die-cut elastomeric electrodes, the splder, and the adhesive film disappear and are replaced by a sin-gle molded pad on which are defined five zones of elec-trode 1-l' separated by segments of solid conductive rubber 1~. Pasteners are driven through these solid sec-tions, through the printed circuit sheet, and into the body simultaneously to lock the assembly in posicion and , .
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-: . - ' - :-, ~ z77~8 connect the conductor 4 to ~he electrodes 1-1'.
In the application of the invention to single zone sensors, the invention provides considerable novelty in t~at it can (1~ produce similar signals from similar inputs at different points on the surrace, (2) simulta-neously transduce the resultant of area and pressure at all points on the surface, and (3) provide continuous output proportional to either static or dynamic pressure patterns on its surface. Nhat it cannot distinguish is (1) the location on its surface of a pressure input, (2) the force being applied at any specified point on its surface, or (3) whether the area-pressure pattern is a large area/low force or a s~all area/large force. In order to develop this in-formation, it is necessary to use multiple second elec-trodes, as later described.
Output is an analog voltage which tracks the changes in capacitance due to scriking or pressing ehe pad;
these being scaled to drive most e~isting CV electronic drum "brains." As before stated, a preferred electronic circuit for operating with the sensors of Figs. 1, 4A and 4B i9 shown ln Fig. 5, using a source of high frequency AC
~oltage and ~easuring the degree of AC current flow. The degree of flow is given by the equation: I-2 EFC, ~ .
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1'~775~8 whera I is the current flow in amperes, E is the applied AC (assumed sine ~ave) voltage, F is its frequenc~J, and C
is ehe capacitance of ehe sensor 1-1'-2-3 in Farads.
Typical values or these variables in the drum applicat~on of the invention are as follows:
E=8 volts F~lOOKHz C=300pF
I-l.OmA
Thus the magnitude of current flow represents the instan-taneous amount of capacitance which, in turn~ reflects the instantaneous product of force and area applied to the sensor. There are several methods for "subtracting out"
the "base capacitance" that exists when no force is applied. The preferred method is to app'y an equal but 180 degrees out-of-phase voltage through a fixed capacitor equal to the base capacitance and connec~ the combination to the YenYor output. At rest, the two capacitive currents cancel giving zero net current. When pressure increases the current through the sensor, the net current increa5es away ~rom ~ero, giving a usable output.
At the top of Fig. 5 a push-pull sine wave power oscillator is shown consisting of two transistors Tl, T2, ' ' - , , :' , : .: ' .
:, : ' . -- , ~'~775~8 ne~ork resistors Rl-~s, a center-tapped choke coil CT and a parallel capacitor C'. The co~bina~ion of the coil CL
inductance (250 microhenrv) and the capacitance C' (.01 micro~arad) produces a resonan~ tank circuit with a reso-nant frequency of approximately lOOKHz. The base-to-col-lector resistors R3 and Rs (22K ohm) provide feedback necessary to start and sustain oscillation, while the base-to-e~itter resistors R2 and R4 (4.7 ~ohm) li~it over-drive on the transistor bases. The series resistor Rl (470 ohms) simulates a current source which improves the oscillator's nearly perfect (approximately 1% dlstortion) sine wave. Since the center tap of the coil CT is grounded, the ends of the coil provide precisely out-of-phase sine waves of equal amplitude to the remaining cir-; cuitry. The oscillator output, labelled "Drive Out" goes to the common plate of the sensors (the conductive rubber pad 1-1' of Fig. 4B~ for example) while the opposite oscillator output goes to the signal processing circuitry now to be explained.
The remaining circuitry consists of five similar circuits for the ive sensor pads or sensor sectors, the clrcuit for sensor (pad) #l (say sector 6', for example,) being illustratively desctibed. The pressure sensor is .
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1'~775~8 connected externally between the terminals labelled "Drive Outpu~" and 'Pad l In". Capacieive current propor~iona~
to the sensor's capacitance thus flo~s into the "Pad 1 In"
terminal. A~ t~e same time, capac'tive cur;en~ o~ oppo-site phase from the opposite side of the oscillator flows into "Pad 1 In' through a series resistance capacitance networ~ combination in which the resistor value is fixed and the capacitor (C") value can be varied over a limited range. In practice, the capacitor is ad~usted so that its value e~uals the sensar's base capacitance, as before explained. The resistor effects more complete cancella-tion of the two currents by accounting for the finite resistance of the conductive rubber pad 1-1'. Perfect balance is achieved only when both C" and the resistance are matched. In practice, the resistance is only a s~all portion of the total impetance, so exact resistance match is not overly important t20% resistance mismatch has little effect).
As pressure is applied to the sensor, the net current into the "Pad 1 In" ter~inal increases away from zero. This current flo~ develops a small AC voltage across the resistor ~" (4.7g). The AC voltage is recti-fied by a diode D (lN270 ger3anium) and the resul~ing DC
voltage is held an a .OlOF capacitor, sa labelled. The .
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~ ~7~8 ~o--germanium diode 3 is used eo avoid the threshold efrect of silicon diodes due to their relativel~ high (0.6 Volts) forward voltage drop. During times of greater pressure, the positive DC voltage de~Jeloped ac oss the .OluF ca?aci-tor is higher. During times OI lesser pressure, the charge of the capacitor leaks away slowly through the diode D over a period of several milliseconds. In this manner, the .OlOF capacitor tends to hold the value of pressure peaks momentarily. The relatively small capaci-tor voltage (generally under a volt) is increased six-fold by an operational amplifier A (L~ 358) and ~eedback net-work Rf and Rf' (lOOK and 22X ohms, respectively, for example). The ampliSier output voltage is finally applied to the "Pad l Out' terminal through a lR ohm protective resistor Ro~ This voltage (and those of the other 4 channels) is then routed to a syntheslzer which responds in a desirable manner to changes in the voltage leveL, as is well known.
In actual use, it is desirable to be able to ad-~ust the circuit sensitivity and response to pressure.
Overall sensitivity o the sensors 15 altered by changing the output voltage of the oscillator Tl-T2, which is accomplished by changing the oscillator's power supply voltage. This is shown accompLished by externally -' - . ' . ' ' . , , .
775~8 cannecting a potentiometer Pl (lKohm) to the 'Sens.Xi", Sens.Wipe", and 'Sens.Low' terminals. The 470-ohm resis-tor connected to '-Sens.Low'- li~its the adjustment to a 3 eo-l range. A threshold e~fect can also be had bv var,v-ing the DC volta~e a~ the "Thresh.l7ipe" terminal. ~hen this voltage is zero, the final output voltage is a faith-ful six-times copy of the rectified AC voltage appearing across the .OlO~F filtering capacitor. As it is made posi-tive, the output voltage (which cannot be negative) will not increase from zero until the recitified voltage increases past a threshold related to the voltage at the "Thresh.Wipe" terminal (bottom left of Fig. 5). This is also accomplished e~ternally by connecting a lR ohm poten-tiometer P2 to the three "Thresh." terminals. The 15K
resistor connected to "Thresh.~i" limits the threshold ad-justment to a useful range.
Summarizing the operation of Fig. 5, therefore, the oqcillator signal (lOOKHz) i9 connecced to all ehe conductive rubber electrodes through "Drive Out.'- The amplitude of that '-drive'- signal iq controlled by poten-tiometer Pl connected to the ehree terminals '-Sens. Hi, 'Low, and Wipe(r)". The second electrode(s) 3 for each of the five sensing zones is connected to one of five ,, "' ' ' ' ~ , :
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- ' : ' ' ' ' ': '' ,' ' ' ~ ' ~775~8 duplicate circuics ehrough the inpucs labelled Pad 1"
through 'Pad 5.' These circuits measure the AC capaci-tive current across each sensor by converting it to an AC
voltage ac-oss the 4.7~ resistor. This AC voltage is con-verced to a DC voltage by the diode D, then is amplified and sent to the output jacks through ehe '-~ad Out' termi-nals. Each of these circuits receives the inverse drive signal; each variable capacitor is adjusted until the two drive signals cancel and the capacitive current (and thus the voltage output of each resting system) is as close to zero as possible. "The smallest signal which will produce a response may be controlled by ad~usting the 'Threshhold' potentiometer."
When pressure is applied to a sensor zone, the capacitance is changed, the capacitive current increases, and the DC voltage on the output rises. When the pressure . i5 removed, the output returns to zero. A rapid strike produces a "pulse" with a rapid rise and fall, Figs. 2A-C, and slow pressure simply produces a proportional slow in-crease in the voltage of the ouCput. This type of analog output, called CV in the music industry, as before stated, is connected to a sound generator which accepts the CV in-put, with the level of control of sound depending entirelY
on the capabilities of the sound generator.
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1'~7'7~8 _~3_ ~ he pri~ary target sound generaeors are CV elec-tronic drum 'brains", and these show differene respOnses based on the characteristics of their input circuitry If the inputs to the ''brain" are AC coupled, for instance, then only sharp stri~es twhere the DC output simulates AC) will result in sound generation. If, however, the 'brain"
inputs are DC coupled, any signal which exceeds a particu-lar voltage threshold uill produce a sound. It is on these systems that the drum of the invention produces spe-cial effects, since, unlike conventional piezoelectric controllers, the systems of the invention sustains a vol-tage proportional to pressure. MainCaining pressure on a pad holds the output voltage above the threshold voltage of the "brain", and continuous sound or repetitive trig-gering of sounds may occur. If pitch is modified by the voltage amplitude of the input signal, then fluctuation of the pressure on a pad produces corresponding changes oP
the pitch of the sound.
As before stated, earlier electronic drum con-trollers (drum pads) use piezoelectric crystals as the transducers. While the trangducer of the present inven-tion generates continuous signals relative to an absoluee baseline, the piezo transducers generate transient signals - ' ~ ' .
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~:77~i'18 _~4_ proporeional to rate o~ change. They generaee a voltage when physically discorted, and the more rapidly and drama-tically they are "bent", the higher the voltaae genera-ted. ~owever, as soon as the dis~orsin~ s~ops, even i they are held in a ben~ position, they cease to generate a voltage, and the output drops to zero. It is for this reason that they, unlike the present invention, are un-able to provide continuing control based on pressure following the intitial strike. Further, since they oper-ate on rate of change, slow distortion does not generaee a useable signal. For these reasons, they are especially appropriate as transducers for applications where only a trigger signal is required, and this signal is to be generated by significant impact, but they are not particu-larly appropriate for keyboard-like controllers "when con-tinuing control of sound is desired."
While, therefore, the tevice of ~he invention when struck with a drumstick produces an output wavefor~
resembling that produced by conventional eleceronic drum controllers which use a piezoelectric crystal as a trans-ducer, ùnlike p~ezoelectric systems, the system of the invention continues to produce signals proportional to residual pressure, allowing continued conerol of the sound generating device afeer the initial strike. Purther, it effectively transduces less abrupt dynamic forces which .
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~2775l8 _~5_ ~ould be inadequato to produce a useful signal from a piezoeleceric syste~.
The controller of the invention also works wi~h s~nt~esize-s r~hich produce o~her than rhv~hm sounds and are set up to use CV (Control Voltage) dnputs. With these, the range of potential effects multiplies, since the voltage of the inpue may be programmed to control a variety of musical parameters.
The circuit of Fig. 5 is completely analog. To incorporate digital signal processars, each output, Pither before or after a~plification, is put through an ADC (Ana-log-to-Digital Convereer). A microprocessor (or other well-known digital signal processing circuitry) monitors the resulting digital repre~entations of variations of the pad capacitance over time and constructs corresponding digital control signals according to pre-programmed rules of logic (software).
It is al90 possible to modify the system as des-cribed to output digital conerol signals according to ~IDI
t~usical Instrument Digital Interface) or other communica-tions protocols. This is acco~plished by processing each of the discrete circuit output~ through an ADC to produce digital representations of the variations of sensor capa-citance over ti~e. A microprocessor or other digital - . : . .
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77~i~8 ~? 6-signaL processing circuitry monitors these digital repre-sentaeions and conseructs corresponding di~ital control signals according to pre-programmed rules of logic. Addi-tional control deqices (switc~es, slide potentiometers, displays, etc.) and appropriate hardware and software mav be incorporated to allow users to modify the aforemen-tioned pre-programmed rules of logic. Other protocols are possible for communications with computers and robots.
Techniques for doing this are well known to those skilled in this art.
Other iterations are also possible including different outer shapes, different modes of construction, different shapes of strike zones, different numbers of strike zones, versions deviating from strictly flat con-struction, and versions optimized for playing with the hands (e.g.-congas) rather than with stlcks or mallets.
An another e~ample in the musica~ instrument field, the "sandwich" electrode 1-1'-2-3 discussed above may be incorporated into a guitar pickguard with two or three small sensitive zones which may be struck or strummed to generate CV signals for control of drum mach-ines or driving ~IDI converters. The electronics may be placed in a cavity under the pickguard.
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~'~775~3 Another iteration of this product allows the use of one or ~ore 'roving pads 1-1' 2-3 which may be placed on the surface of the guitar in a selected location such as under the right arm or on the plaver's hand or o~her part or his body with an appropriate fastening mechanism and which uses ehe installed electronics to perform a function similar to that of the captive pads in the pick-guard. Electronics may be modified, ~urthermore, to pro-duce either ~IDI signals or otherwise digitally encoded information which may subsequently be used to control ~IDI
music devices, guitar effects, stage appliances, etc.
Differently shaped actuation pads 1-1', differ-ent nu~bers of actuation pads, pad locations on other parts of a guitar, and unctionally similar systems for mounting independently or on other instruments are also clearly useable.
If it is ~lesired to render the system more insensitive so that absoluce pressure or impact is trans-duced, a rigid layer may be applied above the resilient pad electrode 1-1'. The drum-like instrument, moreover, may function as a keyboard with effects such as those described in said paeent--holding the signal by holding the pressure on the head and controlling pitch or tone - : :
.
i,~775~8 variation by wobbling the pressure, etc.
As before stated, the inve~tion ~ay be also used for other purposes than instruments, including providing visual or picture presentations of pressure variations and patterns as on a printout or cathode ray tube; and ic is useful more generally as input sensors for telefactoring, force monitors for purposes such as closing valves and the like, and contact monitors for mobile vehicles, among oeher applicaeions.
Further modifications will also occur to those skilled in this art, and such are considered to fall with-in the spirit ant scope of the invention as defined in the appended claims.
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A~D AP~ARATUS
The presen~ invention relaces tO pressure-sensin~
methods and apparatus, being more specifically concerned with novel t~o-dimensional capacitive sensors and techni-ques particularly, though by no means e~clusively, applic-able to musical and rhvt~mic inst.ume~ts and other devices responsive to touch and variable forces applied over a e~o-dimensional surface.
Novel capacitive pressure sensors having a resilient shaped curved or tapered conductive electrode that is deformed by an instrument key activation or other pressure into engaging variable capacitive cooperation with a fi~ed electrode electrically separated therefrom are dislosed in U.S. Letters Patent ~o. 4,498,365, of applicant Jeff Tripp hereln, and are most useful for operation by a single lim~-ted region of pressure contat. Such sensors provide con-tinuous sensing, as for electronic tone generation in an instrument, and even enable further pressure variations after the key activation or other pressure coneact, as for such purposes as enabling a second note generation or pitch !7;~
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775~8 or tone variation, in the illustration of usage in an instrument ~eyboard. Clearly o~her applicaeions requiring similar response are also useful.
There are occasions, ho~e~7er, where it is desired to enable pressure to be applied over a two-dimen-sional surface and wieh sensiti~ity to variations in attack or impact and/or response to particular patterns or dynamic shape variations of the pressure over èhe two-dimensional surface. As an illustration, a drum membrane to be activated b~ the impact of a drum stic~, or the sweeping of a drum brush, andlor the sweeping of fingers or the hand with various dynamic pressure patterns and variations over the membrane, require such two-dimensional independent fine-point or region pressure sensing and transducing into electrical signals for the purpose of generating sounds that characterize such pressures and pressure patterns. Similarly, as another illustration, configurations may be designed with multiple electrodes cooperating with a common elastomeric electrode, later described, for the reproducing of vigual paeterns, as for measuring hand, finger or foot prints and variations in movement thereof, again operating with two-dimensional continuous, dynamic sensing.
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1;~'77S1~3 For use in taceile sensors to develop sensory feedback, co~plianc conduceive elastomer pads have been developed with an arrav of tactiles which are voltage e~cited and opera~ bv resistanc2 changes in res~onse to pressure and are scanned on a ro~-column sequence to pro-vide a multi-bit digital signal output for such purposes.
(See, for example, Barry Nright Corporaeion 1984 bulletin ~Sensorflex/Aste~", p. 17, 18). Such'sensors, while two-dimensional, have problems in stability of conductivity over time, require comple~ electronics, and have practical llmits on the size or area that can be monitored in view of the pad resistance involved.
An ob~ect of the present inventlon, accordingly, is to provide a new and improved method of and apparatus for providing such two-dimensional pressure-sensing responses for such applications and others requiring simi-lar responses.
A further ob~ect is to provide novel musical and rhythmic instruments of great flexibility, including ~ -drum-like instruments, resulting from the use of the novel pressure sensing of the ~nvention.
Other and further ob~ects will be e~plained hereinafter and will be more particularly delineated in the appended cla~ms.
: . ~ ' : ' ' , . ', -1;~'775~
In summary, however, from one of its aspects, the invention embraces a capacitive pressure~sensitive sensor having, in combination, a first electrode comprising a thin resilient conductive plastic sheet having a plurality of closely spaced resilient conductive projections protruding from one surface of the sheet and with adjacent regions pressure-deformable by application of pressure thereat from the opposite surface of the sheet, and a second electrode facing and coextensive with the projections and separated from the same by a thin dielectric layer therebetween. Preferred and best mode embodiments and components, including drum instruments and the like, are hereinafter described in detail.
In its method aspect the invention relates to a method of capacitive pressure-sensing, that comprises, dynamically deforming adjacent regions of a conductive resilient plastic two-dimensional array of closely spaced projections in a predetermined direction and in a contour of pressure corresponding to a predetermined pressure pattern extending over one or more regions of the array, with each projection deformed by the pressure thereabove, limiting the contoured deforming of the projections at a fixed~position coe~tensive cooperative capacitive electrode surface separated from the array by a thin dielectric medium, and sensing the dynamic capacitive variations effected by the projections under contoured pressure to generate electrical signals corresponding thereto.
The invention will now be described with reference to the accompanying drawings, Fig. 1 of which is a transverse MLS/bp 4 .
, ' ~.
~775~8 section of a preferred two dimensional capacitive pressure sensor useful for the practice of the invention;
Figs. 2A-2C are experimentally derived variations obtained by drumstick impacting of the sensor of Fig. 1, and Figs. 2D and 2E are outputs for surface pressure-pattern applications thereto;
Figs. 3A-3D are enlarged transverse sectional views of projection configurations useful as electrodes of - 4a -MLS/bp - ~, ' 1'~77~
the sensor of Fig. l;
Fig. 4A is an isomecric view, partially cut awav, of a multi-section drum using effectively a plura-lir,~ of sensor pads or sensing zones for selective and relativelv independent effects;
Fig. 4B is si~ilar to Fig. 4A but employs a single elastomeric pad sensor electrode;
Fig. 4C is a si~ilar view of a bottom section of the drum useful with both of the embodiments of Figs. 4A
and 4B; and Fig. S is a circuit diagram of a preferred sig-nal-processing apparatus for responding to the capacitive variations of the sensors of the instrument of Figs. 4A
and 4B to produce signals that may, for e~ample, be used to control sound generators to generate desired tones and sounds.
Referri~g to Fig. 1, the pressure sen50r, in preferred form, comprises a thln plastic conductive rubber or other resilient elastomeric pad electrode 1, preferably provided with a protective cover layer C, as of Mylar or the like as later more fully discussed, and having a planar surface from one side of which (shown as the bottom surface) curved or otherwise variable thickness or tapered proiections 1' of the same conductive resilient material `' ''' ' '' ' ~ ' ' ~
protrude in a two-dimensional closely spaced preferably uniform array e~tending in close capacitive relationship wieh a coe~tensive two-dimensional conductor elec~rode sur~ace 3, separa~ed from t~e projections 1' bv a thin dielectric layer 2, preferably somewhat resilien~ly defor-mable, also. The electrode surface 3 is shown fixedly disposed on a hard immovable board B, so that pressing of the electrode 1 into mechanical force contact with the lmmovable electrode 3 develops the desired capacitance changes to be measured, and with the electrode 3 limiting the downward depression of the upper elastomeric pad elec-trode. In Figs. 3A-3D, various curved or tapered shapes for the projections 1' are illustrated as substantially hemispherical, as truncated hemispheres with conical or tapered tips, a double conical tip, and a cone with a somewhat rounded tip, respectively.
It has been discovered that when the opposite (upper) surface of the electrode 1 is deformed, as by the finger F in Fig. 1, the curved or tapered projections 1' under the pattern of the finger tip will correspondingly be depressed and de~ormed, compressing their tapered thickness to produce greater capacitive effects, with sub-stantially individual independent projection-deforming selectivity, to simulate the finger contour and the .
, :,.
--~775~3 , various forces e~erted by the various partions of the fin-ger tip on the individual projections immediately there-under. ~ith suitable electronics connected at the out~ut te~minals ~ and 5 o~ t~is var-'abLe capacitor l-1'-7-3, as later described, the application and movement of the fin-ger tip will generate capacitive variations that are readily processed into signals that may control the gener-ation of audio tones or sounds, with audible effects pro-portional to the pressure and corresponding to the attac~
or impact of the finger tip and to surface area dynamic pressure pattern of the moving finger tip on the e1ectrode pad surface 1. With the drum cover layer or head C placed over the silicone rubber or other elastomeric pad elec-trode 1, protection against abrasion or soiling of the pad and the static attraction of dist is provided. Addition-ally, the layer C serves as an electrical insulaeor and isolator to prevent body capacitance from infl~encing the system and to prevent introduction of noise. The layer further acts as a "spreader" cover, useful where there may be high local forces (such as the tip of a drumstick) both to limit the compression sec of the pad and mechanically to ampliry the signal by spreading the impact over a lar-ger area of the capacitor.
Referring to the embodiment of Fig. 4A, impact-ing the dru~t head membrane or cover layer C with a dru~
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~775~8 stick, or wire brush, and/or sweeping the stick, brush, fingers, or hands over the membrane, have been found t~us to generate individual capacitive variations over the t-~o-dimensional surface thac c~n be signal-processed in~o sound patterns corresponding to and in substantially pro-portional response to the pressure patterns applied, and preferably in the continuous pressure-sensing manner of the single sensor units described in said patent. Various signal threshoLds for degrees of depression can be est-ablished as described in said patent, and in connection ~ith Fig. 5, for particular tone or sound effects, includ-ing second striking effects during depression and tone variation effects.
Figs~ 2A-2C show expesimentally obtained visuai representations of output signals generat~d by the capaci-tive changes with this two-dimensional shaped resilient capacitive electrode configuration for light, medium and hard drum stick impacts or strikes of the membrane, dis-played on a print-out connected to the electrode, the sig-nal generation being later described in connection with Fig. 5~ The electrode 1-1' was of silicone carbon-loaded elastomeric plastic sheeting, about a tenth of an inch thic~ and of about 60 Durometer, carrying a two-dimen-' ' : ' ~775~3 sional ar~ay of closely spaced shaped projeceions (1~pro~ections per squar~ inch) pro~ruding about 0.06 inch from a web 1 of abo~le 0.035 inch thickness. The other elecrrode 3 was of 1 mll aluminum foil with tne dielect c f ra ~/~ ~7~ ~ ~ ,~ f ~
layer 2 of '~apcon" (DuPon~polyimide plastic), also abou~
1 mil thick.
The surEace pressure pattern effect is shown ~n Figs. 2D and 2E, the former showing the sensing surface output (arbitrary units) in response to area over which the force is applied, and the latter illustrating the out-put as a function of force applied to the sensing sectors.
Returning to the drum-like application of Fig.
4A, an edge clamp 9 may hold the assembly together and with a dress plate 7 ~Figs. 4A and 4C), which may incor-porate a ground plane. If desired, the electronics for the signal processing may be mounted in the underside of the base board B at B', Fig. 4C, as later described.
Separate sectors or regions oE the drum head C
may be provided as ae 6', 6", eec., Fig. 4A, for different and independent effects at such regions or sectors, and with a for~ed metallic "spider" separator 10 between regions. The spider separator is bonded to the drum head cover C with an adhesive layer 8 to provide a structure thae prevents cross-talk between regions.
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1'c:77~8 The basic system configura~ion, ~hen, is a P.C. board, (1) for e~ample (screened on a polyester film c ,na r~
as of ~ylar,~) which contains the sensing hoteom elec-trode(s) 3, means to connec~ t~e drive signal to the elas-tomer upper electrode(s) 1-1', and means to connect ~o the main electronics; (2) a sheet of dielectric 2 which may or may not be adhesive-bonded to or screened onto the P.C.
board; ~3) the upper electrode~s) l of te~tured conductive elastomer as described above; (4) a top cover or drum head C; (5) electronics which provides drive signal to the elastomer electrode(s) l, supplies the inverse of the drive signal to the other side of the sensing capacitor, monitors for changes in capacitance of the sensor area, and converts such changes to useable electronic signals.
There may be multiple electrodes 3 beneath a single elas-tomeric sheet electrode 1-1', Fig. 4B, to produce a number of independent zones, as well, as later more ~ully e~plained.
Total vertical deflection in the system as cur-rently configured is appro~imately 1/16". The force re~uired to deflect an area ls at least roughly propor-tianaL to the signal produced, and it "gives bac~' force in a manner that makes it an effective pressure sensor.
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1~77~8 The system as described can be modified mechanically and electronically to transduce a wider range oE forces and to have a deeper ac~uation distance for applications for which that would be useful, if desired.
The planar nature of the system means that ehe smaller the ratio of activation area to total area of the sensing zone, the smaller the activation signal relative to the "base'-, or resting capacitance of the zone. Since large zones are employed, this base capacitance is lar~e.
~urther, once the rubber projections 1' are fully depressed, no signal increase results from additional force or pressure Because of the limited vertical travel, high-velocity small-area stri~es "top out' quick-ly. The use of the semi-rlgid Mylar cover C for mechani-cal amplification brings additional area of the capacitor into play for both light and heavy strikes of small-area imple-~ents, producing a broader range of differentiable "attacks". The ratio of the area of neighboring capacitor brought into play versus activating implement area is reduced as the activating implement area gets larger, un-til an implement as large as the zone shows no amplifica-t~on effect. In other words, the use of mechanical ampli-fication allows for compressing a broader range of force-- - . - - .
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7~8 -L~-area products (pressure or impac~) into the narro~er range of effective transduc~ion of the sensor/electronics com-binaeion.
Ie is r~or~h noting tha~ the "~eb' of the conduc-tive rubber electrode 1-1' plays a s~milar, although noe ideneical, role, with web thickness adjustable to tailor the system for a specific application.
This construction does reduce the degree of independence of local areas of the surface; but it is this which enables the obtaining of comparable signals from a high velocity small area strike (drumstick) and a large area low velocity strike (a finger). The semi-rigid cover of head layer C perormq a further dynamic unction in the drum. The harder it is hit, the more instantaneously rigid it appears, and the broader the area of the capaci-tor which is affected (again, mechanical amplification).
The cover can vary from none~istent to thin and elastome-ric (protective only), to thin and semi-rigid (thin Mylar), to thicker semi-rigid, to rigid.
The last would be used to make the system area-insensieive for high range low-profile applications such as weighing devices or discrete impact sensorg, or in com-~ blnation ~ith tnothet sengor ln a s~acX to d~rive area-:
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~'~77518 sensitive informa~ion from the top sensor and simuleaneous area-insensicive information from the bottom, or in a stack of many sensors for precise force measuremen~ over longe dis~ances.
The multi-zone or sec~or electronic dru~ instru-ment application of the invention, in its preferred prac-tical configurations, Figs. 4A and 4B, embodies five inde-pendent strike zones 6', 6", 6"-, etc. on its top surface, and five CV (analog Control ~oltage) outputs. It is powered by a 12 volt battery or other d.c. power supply and mounts on a standard tom post via a clamp on the bottom, as later explained. The system is responsive to both steady and i~pulsive forces and with response speed in the tens of microseconds range and a frequency response well into the kilohertz range.
Output is an analog voltage which tracks the changes in capacitance due to striking or pressing the pad; these being scaled to dri~e most existing CV elec-tronic drum '-brains". As before stated, a preferred elec-tronlc circuit for operating with the sensors of Fig. 1, 4A and 4B is shown in Fig. 5, using a bottom section, Fig. 4C, com~on to both of the embodiments of Figs. 4A and 4B. In a practical apparatus, the body of the de~ice is, : :
, ~77~8 for e~ample, a l"-~hic~ par~icle board disc B which has a cavity B' roueed in the bac~ for the electronics. On this is placed a printed circui~ sheet 11, as of a die-cu~
s~ee~ of Mylar, on which is screened a conductive pattern to provide the five bottom elec~rode surfaces 3 for the five zones 6', 6", 6"', etc. The drive signal is connec-ted to the eiastomeric electrodes 1-1', with conductors 4 and 5 for connection of these areas to the electronics E.
The traces travel along a membrane "rail" 11', which wraps around the body to the electronics cavity B'. Over the electrode areas 3 is subsequently screened a urethane-based material which serves as the dielectric layer Z.
This layer is also preferably screened on the tail to pro-vide insulation. Upon the printed circuit sheet are placed five die-cut pieces of the elasto~eric electrode 1-1' and the spider separator 10. The spider separator i5 fastened through the printed circuit sheet into the body with several fasteners F, such as screws. This simul-taneously positions the electrodes 1-1' in position, and electrically connects the pattern 4 to the five electrodes ~ubsequently to provide the drive signal.
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-77~8 A spacer ring 12 $s placed around the periphery of the assembly~ with a die-cuC adhesive film 8 placed over the spider separaeor, and the head C is placed onto the assembly, follo~ed by the dress ring 9, which is not qet swaged over on the bottom. The assembly is iltverted, the dress plate 7 is installed, and the dress ring is swaged to its final configuration. On an access plate 13 are installed the five output iacks and the power jack J
and the two potentiometers P, for all of which, terminals are later identified in the circuit of Fig. 5. These are connected to the electronics E mounted on the bottom B' of the access plate. The membrane tail is then connected to the electronics and the access plate is fastened to the body with the tom cla~tp 14 fastened in pOsitiolt to com-plete the assembly.
If the single elastomeric pad version of Fig. 4B
is used, the die-cut elastomeric electrodes, the splder, and the adhesive film disappear and are replaced by a sin-gle molded pad on which are defined five zones of elec-trode 1-l' separated by segments of solid conductive rubber 1~. Pasteners are driven through these solid sec-tions, through the printed circuit sheet, and into the body simultaneously to lock the assembly in posicion and , .
- , - ' ': ' ' ".: ' .
-: . - ' - :-, ~ z77~8 connect the conductor 4 to ~he electrodes 1-1'.
In the application of the invention to single zone sensors, the invention provides considerable novelty in t~at it can (1~ produce similar signals from similar inputs at different points on the surrace, (2) simulta-neously transduce the resultant of area and pressure at all points on the surface, and (3) provide continuous output proportional to either static or dynamic pressure patterns on its surface. Nhat it cannot distinguish is (1) the location on its surface of a pressure input, (2) the force being applied at any specified point on its surface, or (3) whether the area-pressure pattern is a large area/low force or a s~all area/large force. In order to develop this in-formation, it is necessary to use multiple second elec-trodes, as later described.
Output is an analog voltage which tracks the changes in capacitance due to scriking or pressing ehe pad;
these being scaled to drive most e~isting CV electronic drum "brains." As before stated, a preferred electronic circuit for operating with the sensors of Figs. 1, 4A and 4B i9 shown ln Fig. 5, using a source of high frequency AC
~oltage and ~easuring the degree of AC current flow. The degree of flow is given by the equation: I-2 EFC, ~ .
,: ' ' . ' :
,: ' ' , .: ' ' ~ . , .
.
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1'~775~8 whera I is the current flow in amperes, E is the applied AC (assumed sine ~ave) voltage, F is its frequenc~J, and C
is ehe capacitance of ehe sensor 1-1'-2-3 in Farads.
Typical values or these variables in the drum applicat~on of the invention are as follows:
E=8 volts F~lOOKHz C=300pF
I-l.OmA
Thus the magnitude of current flow represents the instan-taneous amount of capacitance which, in turn~ reflects the instantaneous product of force and area applied to the sensor. There are several methods for "subtracting out"
the "base capacitance" that exists when no force is applied. The preferred method is to app'y an equal but 180 degrees out-of-phase voltage through a fixed capacitor equal to the base capacitance and connec~ the combination to the YenYor output. At rest, the two capacitive currents cancel giving zero net current. When pressure increases the current through the sensor, the net current increa5es away ~rom ~ero, giving a usable output.
At the top of Fig. 5 a push-pull sine wave power oscillator is shown consisting of two transistors Tl, T2, ' ' - , , :' , : .: ' .
:, : ' . -- , ~'~775~8 ne~ork resistors Rl-~s, a center-tapped choke coil CT and a parallel capacitor C'. The co~bina~ion of the coil CL
inductance (250 microhenrv) and the capacitance C' (.01 micro~arad) produces a resonan~ tank circuit with a reso-nant frequency of approximately lOOKHz. The base-to-col-lector resistors R3 and Rs (22K ohm) provide feedback necessary to start and sustain oscillation, while the base-to-e~itter resistors R2 and R4 (4.7 ~ohm) li~it over-drive on the transistor bases. The series resistor Rl (470 ohms) simulates a current source which improves the oscillator's nearly perfect (approximately 1% dlstortion) sine wave. Since the center tap of the coil CT is grounded, the ends of the coil provide precisely out-of-phase sine waves of equal amplitude to the remaining cir-; cuitry. The oscillator output, labelled "Drive Out" goes to the common plate of the sensors (the conductive rubber pad 1-1' of Fig. 4B~ for example) while the opposite oscillator output goes to the signal processing circuitry now to be explained.
The remaining circuitry consists of five similar circuits for the ive sensor pads or sensor sectors, the clrcuit for sensor (pad) #l (say sector 6', for example,) being illustratively desctibed. The pressure sensor is .
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1'~775~8 connected externally between the terminals labelled "Drive Outpu~" and 'Pad l In". Capacieive current propor~iona~
to the sensor's capacitance thus flo~s into the "Pad 1 In"
terminal. A~ t~e same time, capac'tive cur;en~ o~ oppo-site phase from the opposite side of the oscillator flows into "Pad 1 In' through a series resistance capacitance networ~ combination in which the resistor value is fixed and the capacitor (C") value can be varied over a limited range. In practice, the capacitor is ad~usted so that its value e~uals the sensar's base capacitance, as before explained. The resistor effects more complete cancella-tion of the two currents by accounting for the finite resistance of the conductive rubber pad 1-1'. Perfect balance is achieved only when both C" and the resistance are matched. In practice, the resistance is only a s~all portion of the total impetance, so exact resistance match is not overly important t20% resistance mismatch has little effect).
As pressure is applied to the sensor, the net current into the "Pad 1 In" ter~inal increases away from zero. This current flo~ develops a small AC voltage across the resistor ~" (4.7g). The AC voltage is recti-fied by a diode D (lN270 ger3anium) and the resul~ing DC
voltage is held an a .OlOF capacitor, sa labelled. The .
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~ ~7~8 ~o--germanium diode 3 is used eo avoid the threshold efrect of silicon diodes due to their relativel~ high (0.6 Volts) forward voltage drop. During times of greater pressure, the positive DC voltage de~Jeloped ac oss the .OluF ca?aci-tor is higher. During times OI lesser pressure, the charge of the capacitor leaks away slowly through the diode D over a period of several milliseconds. In this manner, the .OlOF capacitor tends to hold the value of pressure peaks momentarily. The relatively small capaci-tor voltage (generally under a volt) is increased six-fold by an operational amplifier A (L~ 358) and ~eedback net-work Rf and Rf' (lOOK and 22X ohms, respectively, for example). The ampliSier output voltage is finally applied to the "Pad l Out' terminal through a lR ohm protective resistor Ro~ This voltage (and those of the other 4 channels) is then routed to a syntheslzer which responds in a desirable manner to changes in the voltage leveL, as is well known.
In actual use, it is desirable to be able to ad-~ust the circuit sensitivity and response to pressure.
Overall sensitivity o the sensors 15 altered by changing the output voltage of the oscillator Tl-T2, which is accomplished by changing the oscillator's power supply voltage. This is shown accompLished by externally -' - . ' . ' ' . , , .
775~8 cannecting a potentiometer Pl (lKohm) to the 'Sens.Xi", Sens.Wipe", and 'Sens.Low' terminals. The 470-ohm resis-tor connected to '-Sens.Low'- li~its the adjustment to a 3 eo-l range. A threshold e~fect can also be had bv var,v-ing the DC volta~e a~ the "Thresh.l7ipe" terminal. ~hen this voltage is zero, the final output voltage is a faith-ful six-times copy of the rectified AC voltage appearing across the .OlO~F filtering capacitor. As it is made posi-tive, the output voltage (which cannot be negative) will not increase from zero until the recitified voltage increases past a threshold related to the voltage at the "Thresh.Wipe" terminal (bottom left of Fig. 5). This is also accomplished e~ternally by connecting a lR ohm poten-tiometer P2 to the three "Thresh." terminals. The 15K
resistor connected to "Thresh.~i" limits the threshold ad-justment to a useful range.
Summarizing the operation of Fig. 5, therefore, the oqcillator signal (lOOKHz) i9 connecced to all ehe conductive rubber electrodes through "Drive Out.'- The amplitude of that '-drive'- signal iq controlled by poten-tiometer Pl connected to the ehree terminals '-Sens. Hi, 'Low, and Wipe(r)". The second electrode(s) 3 for each of the five sensing zones is connected to one of five ,, "' ' ' ' ~ , :
--. .. - : . . .
- ' : ' ' ' ' ': '' ,' ' ' ~ ' ~775~8 duplicate circuics ehrough the inpucs labelled Pad 1"
through 'Pad 5.' These circuits measure the AC capaci-tive current across each sensor by converting it to an AC
voltage ac-oss the 4.7~ resistor. This AC voltage is con-verced to a DC voltage by the diode D, then is amplified and sent to the output jacks through ehe '-~ad Out' termi-nals. Each of these circuits receives the inverse drive signal; each variable capacitor is adjusted until the two drive signals cancel and the capacitive current (and thus the voltage output of each resting system) is as close to zero as possible. "The smallest signal which will produce a response may be controlled by ad~usting the 'Threshhold' potentiometer."
When pressure is applied to a sensor zone, the capacitance is changed, the capacitive current increases, and the DC voltage on the output rises. When the pressure . i5 removed, the output returns to zero. A rapid strike produces a "pulse" with a rapid rise and fall, Figs. 2A-C, and slow pressure simply produces a proportional slow in-crease in the voltage of the ouCput. This type of analog output, called CV in the music industry, as before stated, is connected to a sound generator which accepts the CV in-put, with the level of control of sound depending entirelY
on the capabilities of the sound generator.
, .
.. . : ~ - -..
~ . :
. , ~, ' ~
: .. ~ :
1'~7'7~8 _~3_ ~ he pri~ary target sound generaeors are CV elec-tronic drum 'brains", and these show differene respOnses based on the characteristics of their input circuitry If the inputs to the ''brain" are AC coupled, for instance, then only sharp stri~es twhere the DC output simulates AC) will result in sound generation. If, however, the 'brain"
inputs are DC coupled, any signal which exceeds a particu-lar voltage threshold uill produce a sound. It is on these systems that the drum of the invention produces spe-cial effects, since, unlike conventional piezoelectric controllers, the systems of the invention sustains a vol-tage proportional to pressure. MainCaining pressure on a pad holds the output voltage above the threshold voltage of the "brain", and continuous sound or repetitive trig-gering of sounds may occur. If pitch is modified by the voltage amplitude of the input signal, then fluctuation of the pressure on a pad produces corresponding changes oP
the pitch of the sound.
As before stated, earlier electronic drum con-trollers (drum pads) use piezoelectric crystals as the transducers. While the trangducer of the present inven-tion generates continuous signals relative to an absoluee baseline, the piezo transducers generate transient signals - ' ~ ' .
., - .' :~ -.-- ' ' ' ' ', ~':' -' '' '' '- ~
~:77~i'18 _~4_ proporeional to rate o~ change. They generaee a voltage when physically discorted, and the more rapidly and drama-tically they are "bent", the higher the voltaae genera-ted. ~owever, as soon as the dis~orsin~ s~ops, even i they are held in a ben~ position, they cease to generate a voltage, and the output drops to zero. It is for this reason that they, unlike the present invention, are un-able to provide continuing control based on pressure following the intitial strike. Further, since they oper-ate on rate of change, slow distortion does not generaee a useable signal. For these reasons, they are especially appropriate as transducers for applications where only a trigger signal is required, and this signal is to be generated by significant impact, but they are not particu-larly appropriate for keyboard-like controllers "when con-tinuing control of sound is desired."
While, therefore, the tevice of ~he invention when struck with a drumstick produces an output wavefor~
resembling that produced by conventional eleceronic drum controllers which use a piezoelectric crystal as a trans-ducer, ùnlike p~ezoelectric systems, the system of the invention continues to produce signals proportional to residual pressure, allowing continued conerol of the sound generating device afeer the initial strike. Purther, it effectively transduces less abrupt dynamic forces which .
..
: :
~2775l8 _~5_ ~ould be inadequato to produce a useful signal from a piezoeleceric syste~.
The controller of the invention also works wi~h s~nt~esize-s r~hich produce o~her than rhv~hm sounds and are set up to use CV (Control Voltage) dnputs. With these, the range of potential effects multiplies, since the voltage of the inpue may be programmed to control a variety of musical parameters.
The circuit of Fig. 5 is completely analog. To incorporate digital signal processars, each output, Pither before or after a~plification, is put through an ADC (Ana-log-to-Digital Convereer). A microprocessor (or other well-known digital signal processing circuitry) monitors the resulting digital repre~entations of variations of the pad capacitance over time and constructs corresponding digital control signals according to pre-programmed rules of logic (software).
It is al90 possible to modify the system as des-cribed to output digital conerol signals according to ~IDI
t~usical Instrument Digital Interface) or other communica-tions protocols. This is acco~plished by processing each of the discrete circuit output~ through an ADC to produce digital representations of the variations of sensor capa-citance over ti~e. A microprocessor or other digital - . : . .
' ~
77~i~8 ~? 6-signaL processing circuitry monitors these digital repre-sentaeions and conseructs corresponding di~ital control signals according to pre-programmed rules of logic. Addi-tional control deqices (switc~es, slide potentiometers, displays, etc.) and appropriate hardware and software mav be incorporated to allow users to modify the aforemen-tioned pre-programmed rules of logic. Other protocols are possible for communications with computers and robots.
Techniques for doing this are well known to those skilled in this art.
Other iterations are also possible including different outer shapes, different modes of construction, different shapes of strike zones, different numbers of strike zones, versions deviating from strictly flat con-struction, and versions optimized for playing with the hands (e.g.-congas) rather than with stlcks or mallets.
An another e~ample in the musica~ instrument field, the "sandwich" electrode 1-1'-2-3 discussed above may be incorporated into a guitar pickguard with two or three small sensitive zones which may be struck or strummed to generate CV signals for control of drum mach-ines or driving ~IDI converters. The electronics may be placed in a cavity under the pickguard.
..
~'~775~3 Another iteration of this product allows the use of one or ~ore 'roving pads 1-1' 2-3 which may be placed on the surface of the guitar in a selected location such as under the right arm or on the plaver's hand or o~her part or his body with an appropriate fastening mechanism and which uses ehe installed electronics to perform a function similar to that of the captive pads in the pick-guard. Electronics may be modified, ~urthermore, to pro-duce either ~IDI signals or otherwise digitally encoded information which may subsequently be used to control ~IDI
music devices, guitar effects, stage appliances, etc.
Differently shaped actuation pads 1-1', differ-ent nu~bers of actuation pads, pad locations on other parts of a guitar, and unctionally similar systems for mounting independently or on other instruments are also clearly useable.
If it is ~lesired to render the system more insensitive so that absoluce pressure or impact is trans-duced, a rigid layer may be applied above the resilient pad electrode 1-1'. The drum-like instrument, moreover, may function as a keyboard with effects such as those described in said paeent--holding the signal by holding the pressure on the head and controlling pitch or tone - : :
.
i,~775~8 variation by wobbling the pressure, etc.
As before stated, the inve~tion ~ay be also used for other purposes than instruments, including providing visual or picture presentations of pressure variations and patterns as on a printout or cathode ray tube; and ic is useful more generally as input sensors for telefactoring, force monitors for purposes such as closing valves and the like, and contact monitors for mobile vehicles, among oeher applicaeions.
Further modifications will also occur to those skilled in this art, and such are considered to fall with-in the spirit ant scope of the invention as defined in the appended claims.
, . . - - .
.
. : . . : .
.
Claims (36)
1. A capacitive pressure-sensitive sensor having, in combination, first electrode means compris-ing a thin resilient conductive plastic sheet having a plurality or closely spaced resilient conductive projections protruding from one sur-face of the sheet and with adjacent regions pressure-deformable by application of pressure at the opposite surface of the sheet, and a second electrode means facing and coextensive with the projections and separated from the same by a thin dielectric layer therebetween.
Z. A capacitive sensor as claimed in claim 1 and in which the plurality of projections are disposed in a two-dimensional array of closely spaced projections.
3. A capacitive sensor as claimed in claim 2 and in which the projections, upon deformation, are limited in depression by the presence of the second electrode means which is mounted to be immovable.
4. A capacitive sensor as claimed in claim 3 and in which said projections are substantially uni-formly distributed over said array and have curved surfaces deformable when pressed against said second electrode means with the dielectric layer in-between.
5. Apparatus as claimed in claim 4 and in which each electrode means of said sensor is connec-ted to electronics for sensing the capacitive variations being produced by the pressure deformation of the first electrode means and producing signals corresponding to the same.
6. Apparatus as claimed in claim 5 and in which said electronics produces signals in response to sensor capacitance changes caused by impacts on said opposite surface of the first electrode means.
7. Apparatus as claimed in claim 5 and in which said electronics produces signals in response to capacitance changes caused by pressure-area-patterns applied on said opposite surface of the first electrode means.
8. Apparatus as claimed in claim 5 and in which means is provided for converting the produced signals into audio representations of the pressure deformations.
9. Apparatus as claimed in claim 8 and in which the audio representations are tones and sounds generated by drum-like impacting and sweeping over the said opposite surface of the first electrode means.
10. Apparatus as claimed in claim 9 and in which the pressure is applied to said opposite surface through a drum head layer mounted thereover.
11. Apparatus as claimed in claim 9 and in which further similar sensor regions are provided ad-jacent to the first-named sensor to produce multi-zone independent drum-like effects.
12. Apparatus as claimed in claim 5 and in which means is provided for converting the produced signals into visual representations of the pressure deformations.
13. Apparatus as claimed in claim 2 and in which the projections are of variable thickness such as somewhat tapered.
14. Apparatus as claimed in claim 13 and in which the thickness of said sheet is of the order of tenths of an inch, the projections are discri-buced in the order of a hundred per square inch and are of the order of hundredths of an inch, and the second electrode means and dielectric layer each of the order of mils.
15. A capacitive sensor as claimed in claim 1 and in which said second electrode means comprises a plurality of adjacent sector electrodes cooper-ative with a single common first resilient electrode means.
16. A capacitive sensor as claimed in claim 1 and in which said first resilient electrode means com-prises a plurality of separate sector resilient electrodes.
17. A capacitive sensor as claimed in claim 16 and which separator means is disposed between the sector resilient electrodes.
18. A capacitive sensor as claimed in claim 16 and in which a semi-rigid cover layer is disposed over said first resilient electrode means.
19. A capacitive sensor as claimed in claim 15 and in which a semi-rigid cover layer is disposed over said first resilient electrode means.
20. A capacitive sensor as claimed in claim 19 and in which said single resilient electrode means is of conductive elastomeric rubber-like material defined into sectors separated by seg-menes of solid conductive rubber.
21. A capacitive sensor as claimed in claim 17 and in which said resilient electrode means is of conductive elastomeric rubber-like material.
22. A drum-like instrument having, in combination, thin resilient conductive plastic electrode means having a plurality of closely spaced resilient conductive projections protruding from the inner surface thereof and with adja-cent regions pressure-deformable by application of pressure at the outer surface thereof, a semi-rigid drum cover disposed over the outer surface of the resilient electrode means for receiving the pressure and conveying the same to the resilient electrode means, a second electrode means facing and coextensive with the projections and separated from the same by a thin dielectric layer therebetween, means con-nected with the electrode means for applying ac voltage or current thereto and sensing changes in capacitance caused by the pressure deforma-tion.
23. A drum-like instrument as claimed in claim 22 and in which said second electrode means com-prises a plurality of adjacent sector elec-trodes cooperative with said thin resilient electrode means.
24. A drum-like instrument as claimed in claim 23 and in which said thin resilient electrode means comprises a single resilient conductive sheet.
25. A drum-like instrument as claimed in claim 22 and in which said thin resilient electrode means comprises a plurality of separate sector resilient sheet electrodes.
26. A drum-like instrument as claimed in claim 25 and in which means is disposed between the sec-tor resilient electrodes to prevent cross-talk.
27. A drum-like instruments having, in combination, sensor means comprising thin resilient conduc-tive elastomeric electrode means having adja-cene regions pressure-deformable by application of pressure at the outer surface thereof, a semi rigid drum cover disposed over the said outer surface for receiving the pressure and conveying the same to the resilient electrode means, means connected with the electrode means for applying voltage or current thereto and sensing changes in impedance caused by the pressure deformation, and means for producing signals corresponding to the sensed changes in impedance and generating sounds in response to said signals representing pressure applied to said drum cover.
28. A drum-like instrument as claimed in claim 27 and in which said second electrode means com-prises a plurality of adjacent sector elec-trodes cooperative with said thin resilient electrode means.
29. A drum-like instrument as claimed in claim 28 and in which said thin resilient electrode means comprises a single resilient conductive sheet.
30. A drum-like instrument as claimed in claim 27 and in which said thin resilient electrode means comprises a plurality of separate sector resilient sheet electrodes.
31. A drum-Like instrument as claimed in claim 30 and in which separator means is disposed be-tween the sector resilient electrodes.
37. A method or capacitive pressure-sensing, that comprises, dynamically deforming adjacent regions of a conductive resilient plastic two-dimensional array of closely spaced projections in a predetermined direction and in a contour of pressure corresponding to a predetermined pressure pattern extending over one or more regions of the array, with each projection deformed by the pressure thereabove, limiting the contoured deforming of the projections at a fixed-position coextensive cooperative capaci-tive electrode surface separated from the array by a chin dielectric medium, and sensing the dynamic capacitive variations effected by the protections under contoured pressure to gener-ate electrical signals corresponding thereto.
33. A method as claimed in claim 32 and in which the signals are converted into audio tones and sounds during the deforming.
34. A method as claimed in claim 32 and in which the signals are converted into visual representa-tions during the deforming.
35. A drum-like instrument having, in combination, sensor means having adjacent regions pressure-de-formable by application of pressure thereto, a semi-rigid pressure-flexible smooth and continuous drum cover surface disposed over the said sensor means for receiving the pressure and conveying the same to the sensor means, means connected with the sensor means for sensing changes in impedance caused by the pressure deformation, and means for producing signals corresponding to the sensed changes in impedance and generating sounds in response to said signals representing pressure applied to said drum cover.
36. A drum-like instrument as claimed in claim 35 and in which said sensor means comprises a plurality of adjacent sensors cooperative with corresponding adjacent zones of said drum cover.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000538866A CA1277518C (en) | 1987-06-04 | 1987-06-04 | Capacitive pressure-sensing method and apparatus |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000538866A CA1277518C (en) | 1987-06-04 | 1987-06-04 | Capacitive pressure-sensing method and apparatus |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1277518C true CA1277518C (en) | 1990-12-11 |
Family
ID=4135823
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000538866A Expired - Lifetime CA1277518C (en) | 1987-06-04 | 1987-06-04 | Capacitive pressure-sensing method and apparatus |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1277518C (en) |
-
1987
- 1987-06-04 CA CA000538866A patent/CA1277518C/en not_active Expired - Lifetime
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