POSITION AND FORCE SENSORSAND

THEIR APPLICATION TO KEYBOARDSAND RELATED CONTROL DEVICES

ROBERT A. MOOGVICE PRESIDENT, NEW PRODUCT RESEARCH

KURZWEIL MUSIC SYSTEMS, INC.

ABSTRACT

With the advent of MIDI, attention is being focussed on the design of touch-sensitive controlinterfaces for electronic music performances. Some recent MIDI keyboard designs employ positionand/or force sensors on each key. The sensors are sensitive and repeatable, yet relatively simpleand inexpensive. Keyboards equipped with these sensors enable the musician to impart continuousexpressive variations to each tone that s/he is producing. Keyboards to be described include theKurzweil MIDIBOARD, which enables the musician to continuously control one musical parameterper key, the Key Concepts Notebender, on which two parameters per key can be continuouslycontrolled, and an experimental multiply-touch-sensitive keyboard on which three parameters perkey can be continuously controlled.

BACKGROUND

The notion of using a clavier (music keyboard) to control musical parameters other thantiming of note onset and ending, is not new. Tracker-action organs date back to the 14th century. Inthese instruments, the keys are connected directly to the pipe air valves through elaborate,carefully-constructed mechanical linkages, a design feature that enables the musician to control thecharacter of each note's attack. The clavichord, another venerable instrument, enables the musicianto stretch a string by pressing harder on the key which is exciting that string. Clavichord playersregularly use this feature to impart pitch bend and vibrato to individual notes. And, of course, thepiano's complex action enables a musician to deliver precisely-metered amounts of kinetic energy toeach string at the onset of each note.

More recently, electric and electronic keyboard instruments have been developed with avariety of touch-sensitive features. The reproducing pianos of the 1920's and 1930's incorporatedmechanisms that recorded, then played back the velocities of each key depression. Early Baldwinorgans circumvented the problem of key contact clicks by incorporating a rheostat in each keyassembly, a design feature that enabled players to determine the attack times of the notes, just as atracker-organ player would do. The SynKet, developed in the early sixties by Paul Ketoff, is aperformance-oriented synthesizer with three small keyboards, each of which has a key bed that

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moves up and down as well as sideways. Motion sensors attached to the Synket's key assembliesare used to produce pitch and loudness inflections. Finally, many of today's popular electronickeyboards incorporate force sensors under the keys to register the total key force exerted by theplayer, and dual contacts on each key to register the key's velocity as it is depressed.

The keyboards that are described in this paper (the Kurzweil MIDIBOARD, the KeYConcepts Notebender, and the Big Briar multiply-touch-sensitive keyboard) provide keyboardplayers with new types of touch sensitivity. Their sensor designs are matched to the mechanicalcharacteristics of the keys themselves. They all incorporate microprocessor-based sensor scanningand signal processing systems which enable the designers tO program a variety of responsecharacteristics (keyboard 'feels') to cover a range of musical applications.

THE MIDIBOARD

The Kurzweil MIDIBOARD is an 88-key master MIDI controller. The keys themselves arewooden key levers of conventional design. A lead weight is placed at the back of each key lever.This mechanical inertia of the key/weight assembly provides the keyboardist with the tactilefeedback necessary to determine the key's attack velocity during rapid playing.

A single variable-capacitance sensor per key measures the key's attack velocity, its releasevelocity, and the downward force on it while it is depressed (sometimes called key afterpressure ).The sensor consists of a half-cylinder-shaped conductive rubber element, and a conductive area ona circuit board, covered with a thin layer of Kaptan. The rubber piece is attached to the key weight,and is connected to a 100 kHz. drive signal buss by means of another piece of conductive rubber,in the shape of a very thin strip (Figure 1). When the key is depressed, the semicylindrical rubberpiece bears against the circuit board, forming a capacitor. The harder the key is pressed, the morethe rubber spreads out across the circuit board, and the higher is the capacitance. The circuit boardpattern is shown in Figure 2, while Figure 3 shows how a key and sensor are positioned withrespect to the circuit board.

Once every 1-1/2 milliseconds, a scanning circuit on the board samples the 100 kHz voltagefrom each key sensor. Typical sensor outputs are shown in Figure 4. When the key is depressedand the rubber sensor element first strikes the circuit board, the weight's kinetic energy is rapidlydissipated. This results in a brief peak in the sensor output. After ten milliseconds or so, the sensoroutput returns to a low level. From there on until the key is released, the sensor output follows theplayer's force on the key. As the player releases the key, the sensor output drops to zero. TheMIDIBOARD's operating system analyzes the sensor output in real time. It derives a value forMIDI attack velocity from the height of the initial peak; if it does not detect a peak within a certaintime, it derives the velocity from the sensor's maximum output during that time. The MIDI releasevelocity is calculated from the average slope of the release portion of the sensor output. By lookingfor, and detecting, small peaks after the initial attack peak, the MIDIBOARD's operating system isable to produce relrigger signals, thus enabling the player to retrigger notes rapidly without actuallylifting his finger from the key.

The operating system uses the settings of five 'keyboard response' sliders on theinstrument's front panel, plus the contents of several lookup tables stored in memory, to tailor thetransfer function that relates the raw sensor outputs to the MIDI velocity and pressure values.Figure 5 shows the keyboard response sliders. The ATTACK VELOCITY slider determines howfast a key must be depressed in order to produce the maximum MIDI attack velocity; the RELEASEVELOCITY slider does the same for key release. The TOUCH slider determines the sensor'sthreshold: the output value above which the note is on. The PRESSURE SENSITIVITY slider

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determines how hard the key must be pressed after it is down, to produce a given MIDI pressureoutput value. Finally, the RETRIGGER THRESHOLD determines how large a peak in thesensor's post-attack output is necessary to restart the note. The user perceives these five sliders as ameans of tailoring the keyboard's 'feel', even though none of them actually affects the keyboard'smechanical parameters.

This particular sensor configuration has several desirable features. First, only one sensor isneeded to gather data from which MIDI note-on velocity, note-off velocity, polyphonicafterpressure, and channel pressure messages may accurately be computed in real time. Second,the design is inherently repeatable, and is stable over time. In fact, the biggest system variable is thethickness of the Kaptan insulation on the sensor circuit board, and this can be compensated for byvarying the value of a gain-determining resistor on each of a keyboard's six sensor circuit boards.Third, the sensor does not interfere at all with the key's travel, and contributes only a slight elasticfeel once the key is down. Fourth, the frequency at which the system operates (100kHz.) allowsthe sensor to be rapidly scanned to produce outputs which, for musical purposes, are essentiallycontinuous. And finally, the sensor system is inherently inexpensive, and easy to work with intypical electronic assembly environments.

THE NOTEBENDER

The NOTEBENDER is an example of the integration of a sophisticated, carefully designedmechanical system, working in conjunction with simple but appropriate sensors. TheNOTEBENDER is a keyboard on which the key top surfaces can move toward and away from theplayer, as well as up and down. Figure 6, which is taken from U.S.Patent #4,498,365, is anexploded schematic view of one key's mechanism. The key itself pivots on a rod (marked 'upperpivot', which is attached to another pivot (the 'lower pivot'). This allows the key to movehorizontally as well as pivot from the rear. The key is also supported by the rocker, which itselfpivots on a leaf spring. When the player depresses the key, the leaf spring bears against aconductive rubber sensor element, similar to that used in the MIDIBOARD. This is how theplayer's downward force on the key is detected.

The actual key surface (not shown in the drawing) has a special molded matte surface so thatthe player's finger tends to grip the surface rather than slide on it. As the player pushes the keyforward or backward, the rocker rotates, while the rocker pivot remains stationary. Thus, eventhough the top of the key moves linearly, nothing actually slides or otherwise produces asignificant frictional force. When the player stops pushing the key surface, it returns to its normalmid position because of the restoring force of the coil spring, and because of the shape of therocker.

Thus, the player can move the key surface in two independent dimensions: up and down,and back and forth. There is little frictional force to interfere with smooth, natural motion in eitheraxis. Furthermore, the sensors are very nearly transparent to the player. The up-down force sensoris perceived as a slight elasticity, once the player depresses a key all the way. The shape of thecurved surface of the rubber sensor determines exactly how elastic the key feels once it is fullydepressed. The in-out motion sensor is a small, centertapped coil. A ferfite core attached to thepivot arm fides back and forth in the coil when the player moves the key surface back and forth.100kHz excitation signals of equal amplitude and opposite phase are applied to the two ends of thecoil. The magnitude and phase of the signal appearing at the coirs center-tap tells, with a highdegree of linearity, where the slug is positioned.

Figure 7 shows how a performer moves the keys in and out as he plays. Figure 8 is an

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overall view of the Notebender.

The Notebender is currently configured to control some continuously-variable parameters ofthe Rhodes Chroma synthesizer. The designers plan to expand the operating system to includestandard MIDI message outputs.

MULTIPLY-TOUCH-SENSITIVE KEYBOARD

The multiply-touch-sensitive keyboard, designed by Big Briar, enables the musician tocontrol three independent axes of motion per key. The keys themselves are conventional woodorgan keys. The top of each key is covered with an electrically resistive film which is coated with athin urethane insulating layer. The resistive f'tlm and urethane layer are on an exopy-glass substrate,the underside of which carries a conductive guard pattern. Figure 9 shows the top and bottom sidesof the epoxy-glass substrate.

The four comers of each key's resistive film are all excited with the same 100kHz signal.When the player places his finger on the key surface, the capacitive coupling between finger andresistive film causes a small current to flow. The position of the player's finger determines therelationships among the currents supplied by the four comers. These comer currents are scannedabout two hundred times per second. From these the keyboard's operating system computes theleft-right and front-back coordinates of the finger on the key. The urethane surface coating istextured (by the addition of very fine sand) to enable the player to smoothly slide his fingers on thekeys while he is playing.

The up-down position of each key is also continuously measured. A portion of the undersideof each key forms a capacitor with the conductive areas on a series of circuit boards beneath thekeys. As a key is depressed, the capacitive coupling increases. The magnitude of the output of eachcapacitor tells the vertical position of the corresponding key. The keyboard's operating systemlinearizes the signal change. It also computes each key's attack and release velocity by examiningthe rate at which the signal increases, then decreases. Figure 10 is a side view of an assembledkeyboard. Circuitry that scans the keytop resistive films is seen at the left of the photo, while thecircuit boards that detect the up-down key position are under the keys.

All three outputs from each active key,- left-to-right position of the finger, front-to-backposition of the finger, and up-down position of the key itself, - are scanned by a simple, dedicated8-bit microcomputer internal to the keyboard, and delivered once every five milliseconds or so. Theraw key sensor data is available as a parallel data stream. Software to convert the data stream toMIDI runs on a personal-size computer that is external to the keyboard. The musical parametersunder the control of a given multiply-touch-sensitive keyboard are determined by the capabilities ofthe tone-producing devices to which it is connected, and the operating software that relates thekeyboard's ouput to the tone-producers' control inputs.

ACKNOWLEDGEMENT

Development of the multiply-touch-sensitive keyboards began in 1972, with a researchcontract between the Indiana University School of Music, and Moog Music, Inc. The immediatepredecessor of the current design was described at the 1982 International Computer MusicConference in Venice.

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Fig. 1: The MIDIBOARD sensor assembly. The semicylindrical sensor sitson a lead weight, which is fastened to the end of the key.

Fig. 2: The sensor circuit board pattern. The pattern itself is covered with athin insulating layer.

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Fig. 3: MIDIBOARD sensor board in place over the rubber sensors.

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Fig. 4: Typical sensor outputs for a) soft srriice and b) hard strike.

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Fig. 5: MIDIBOARD key response sliders.

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Fig. 7: Side view of the Notebender Keyboard.

Fig. 8: Overall view of the Notebender (Courtesy of the Berklee School ofMusic).

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Fig. 9: Epoxy-glass keytop substrates. From top to bottom: Guard side andresistive side for black key; guard side and resistive side for a white key.

Fig. 10: Keyboard with keys and scanning electronics in place. Top surfacescanning circuitry is at the left. Vertical key position detection/scanningcircuitry is visible slightly right of center.

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