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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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j'l-I EI G LIT PEAK DETERMINESVELOCITY VALUE
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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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