Keyswitch with hysteresis

In a keyboard there are a plurality of key operated variable capacitor switches acting as differentiators to change the widths of electrical pulses which are then integrated to give signals whose level indicate which switches have been operated. The gain and operating points of amplifiers can be changed to change the transfer function of the electronics and thus increase the mechanical hysteresis of the switches.

BACKGROUND OF THE INVENTION 
The invention pertains to keyboards and more particularly to keyboards 
using key switches to control electrical signals. 
Key switches can be used for many control and selection functions on 
devices such as typewriters, telephone handsets and calculators. The 
keyboards of such devices, since they control electronic signals, try to 
avoid the use of mechanical make and break contact switches because of 
their complexity, unreliability and cost. Thus, these keyboards utilize a 
variety of transducers to perform the switching operation. People have 
tried to use mechanically operated reed switches, Hall effect devices, 
photoelectronic devices and capacitive techniques. However, all of these 
electronic keyboard switches have less mechanical hysteresis than 
conventional "over-center" key switches. 
Mechanical hysteresis is the phenomenom which occurs when a keyboard is 
depressed and an electrical signal is generated at a first point (make 
point) during the downward travel of the key. This signal cannot be 
generated again until the key is released and travels a certain distance 
to a second point (break point) above a first point and then travels 
downward again past the first point. The distance between the first and 
second points is the mechanical hysteresis. However, it is usually 
expressed as percentage of total key travel. For example, if the spacing 
between the first and second points is 0.010 inch and the total possible 
key travel is 0.20 inch then the key is said to have 5% mechanical 
hysteresis. 
Usually, the make and break points are determined by the make and break 
levels of a sensing amplifier. Generally, a sensing amplifier in the form 
of a bistate device is utilized. The bistate device has electronic 
hysteresis so that the device assumes a first state upon receipt of a 
signal having a first level (the make level) and assumes a second state 
when the received signal is at a different (lower) level. The voltage 
difference between the two levels is the electronic or electrical 
hysteresis of the device. These first and second levels determine the 
level of the signals required from the key switch. If the level of the 
signal transferred from the key switch is a monotonic function of key 
switch displacement, then the first and second levels of the sensing 
amplifier fix the make and break points of the key switch. 
It should be apparent that for a sensing device having a given electrical 
or electronic hysteresis one would want a key switch transducer which has 
a great mechanical travel or hysteresis between the generation of the two 
different signal levels recognized. The reason is that the larger the 
mechanical hysteresis the smaller is the probability of emitting more than 
one signal from a single operation of the key switch transducer. 
Of the many key switch devices, it has been found that capacitive switch 
devices (wherein the key acts as a coupling element between two coplanar 
pads) characteristically have the smallest mechanical hysteresis 
properties. In spite of this poor mechanical hysteresis property of 
conventional capacitive switch devices, they afford the greatest economy 
of manufacture since they rely on the positions of conductive elements 
that are easily printed on substrates. Therefore, it is highly desirable 
to utilize capacitive switch devices in multiswitch arrays such as 
keyboards. 
SUMMARY OF THE INVENTION 
An object of a first aspect of the invention is to increase the mechanical 
hysteresis of a transducer which is connected to a sensor having a fixed 
electronic hysteresis. 
Another object of this aspect of the invention is to provide a method of 
increasing the operative mechanical travel of a key device which 
monotonically modulates the amplitude of a signal transmitted to a signal 
sensor having a fixed electronic hysteresis. 
An object of another aspect of the invention is to provide an improved 
method of controlling the transfer of signals by the travel of a key 
switch. 
Another object of this aspect is the utilization of variable capacitors to 
control such transfer. 
Another object of the invention is to provide an improved capacitive key 
switch assemblage to implement the other objects of the invention. 
Briefly, the invention contemplates a method for increasing the mechanical 
hysteresis of a key switch by decreasing the transfer gain associated with 
the signal amplitude modulation of a signal transferred to a signal sensor 
by the key switch and adding a constant direct voltage to the transferred 
signal before receipt by the signal sensor. 
Concurrent with this aspect of the invention there is contemplated the 
method of controlling the transfer of signals by means of the travel of a 
key switch between two points along a path by generating packets of 
pulses, modulating the widths or durations of the pulses in the packets in 
accordance with the instantaneous position of the key switch along the 
path, and generating a signal whose amplitude is a function of the 
durations of the width-modulated pulses. 
The signal transfer controlling method is carried out by a key switch 
assemblage which width modulates the pulses by utilizing a signal 
differentiator having a capacitor whose capacitance is controlled by the 
travel of a key switch.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1a there is shown the transfer characteristic OA of a key switch 
transducer. The transducer has a gain of p volts per inch of travel, for 
example 1 volt per 0.033 inch of travel, that is, for over a given range 
of travel the signal transmitted by the transducer will change at the rate 
of 1 volt per 1/30 inch of travel of the key stem of the switch. Usually, 
the output of the key switch transducer is connected to a signal sensor 
which has a fixed electronic hysteresis EH of m volts as determined by the 
separation of the make and break voltage levels of the sensor. For 
example, the make voltage level can be 2 volts and the break voltage level 
1 volt. The difference between these levels is 1 volt. Therefore, the key 
switch makes at .066 inches and breaks at 0.033 inches of travel with a 
mechanical hysteresis MH1 of 0.033 inches of travel. 
In order to increase the mechanical hysteresis or the displacement between 
make and break, the gain has been decreased to 1 volt per 0.100 inches of 
travel. However, as can be seen the transferred signal level never reaches 
the make level of 2 volts. 
In order to achieve the result it is necessary to add a DC component to the 
transferred signal. Thus in FIG. 1c there is shown the transfer 
characteristic FG having the same gain as characteristic OB of FIG. 1b, 
but with an added DC component of 0.67 volts. Now, the switch makes at 
0.132 inches of travel and breaks at 0.033 inches of travel for a 
mechanical hysteresis of 0.099 inches of travel or three times the 
previous mechanical hysteresis. 
This method of translating a fixed electronic hysteresis to different 
mechanical hysteresis can be used with various key switch assemblages. 
There will first be shown its use in a capacitive keyboard system. 
The system of FIG. 2 includes a processor PRO interfaced via a keyboard 
encoder read-only memory KEM to a keyboard switch matrix KBS. 
The processor PRO can be any conventional data processor which can accept 
serial characters wherein the characters are coded combinations of up to 9 
bits in parallel from pins P5 to P14 under control of a signal on pin P16 
of the memory KEM. In addition, it can send signals to pins P28 and P29 of 
the memory to control the type of encoding such as case shift to be 
employed by the memory. Since the processor PRO forms no part of the 
invention it will be described no further. 
The memory KEM can be one of many off-the-shelf encoders. A typical encoder 
is the MTNS Keyboard Encoder Read Only Memory AY- 5-3600 of General 
Instrument Corporation. The memory KEM has pins corresponding to those of 
the General Instrument AY-5-3600 chip. 
In addition to the pins already recited, pins P1 to P5 are optional 
features not needed for the present invention, pins P15, P27 and P30 
receive operating voltages specified by the manufacturers such as V.sub.DD 
=OV, V.sub.GG =12V, and V.sub.CC =+5V. Connected to pin P31 is a timing 
capacitor having a typical value of 0.01 .mu.F. 
For the invention the remaining pins are important. The pins P32 to P40 are 
connected respectively via lines X8 to X0 to nine row lines of the 
keyboard switch matrix KBS. The memory KEM emits a packet of, say, eight 
pulses from each of the pins P32 to P40 in sequence, i.e., pin P32 first 
emits eight pulses then pin P33, etc. The pins P17 to P26 are connected 
via lines Y0 to Y9 to ten column lines of switch matrix KBS. The pins P17 
to P26 are sequentially connected to a sense amplifier within the chip 
which has an electronic hysteresis. 
The keyboard switch matrix KBS can be a matrix of up to 90 switches. A 
typical matrix is shown in FIG. 3 wherein each XN line is connected via an 
amplifier AXN to a row line of the matrix, and each column line of the 
matrix is connected via a signal processing circuit to one of the YN 
lines. At the "intersection" of each row and column line is a key switch 
KSNN shown as a variable capacitor. 
Since at any instant in time the memory KEM is concerned with the state of 
only one of the switches KSNN, i.e., only one of the lines XN is receiving 
a packet of pulses and one of the lines YN is connected to the sense 
amplifier in memory KEM, FIG. 4 shows a typical switch KS89 and its 
sampling circuitry which can be called a key switch assemblage. The 
assemblage includes: a source of packets of electrical pulses in the form 
of amplifier AX8 connected to line X8 which periodically receives bursts 
or packets of eight pulses from memory KEM; an electrical signal 
differentiator means DM including variable capacitor KS89 and resistor 
RY91; a threshold amplifier AY9 which can be an open collector comparator 
of the type AM50032 made by Advanced Mirco Devices (AMD) 901 Thompson 
Place, Sunnyvale, Calif. having a (+)--input connected to the output of 
differentiator means DM and a (-)--input connected threshold level voltage 
source VCL; and sample and hold integrator SHI including an integrator 
capacitor C9 connected via a diode D91 to the output of amplifier AY and 
charging resistor RY93 connected to variable voltage source VG and via a 
diode D92 to variable voltage source VL. The variable sources are shown by 
way of example as potentiometers connected across batteries with the taps 
of the potentiometers connected to filter capacitors. 
While the variable capacitor KS89 can take many forms FIG. 5 shows an 
especially desirable configuration. The variable capacitor KS89 is shown 
having an electrical circuit including a substrate 10 of insulative 
material. The pads are "printed" pads 12 and 14 of conductive material. 
The pads are spaced from each other by a gap 16. The pad is connected via 
a signal terminal 18 to pulse source PS of FIG. 4 and the pad 14 is 
connected via a signal terminal 20 to threshold amplifier AY9 of FIG. 4. 
Normally packets of pulses are fed to terminal 18 from source PS and are 
sensed for at terminal 20 by amplifier AY9. The passage of the pulses from 
terminal 18 and pad 12 to pad 14 and terminal 20 is controlled by 
capacitive coupling member. 
The capacitive coupling member includes a switch stem 22 having a 
transverse portion 24 at an end opposite the pads 12 and 14. Fixed to the 
ends of portion 24 is a ribbon 26. The ribbon has a substrate 2 or backing 
of resilient plastic material such as MYLAR on which has been deposited a 
layer of metallic material such as aluminum or silver. The metallic 
material is then covered with a coating of insulating material. The member 
can be positioned opposite the pads by means of support 28 through which 
passes switch or key stem 22. The top end of the stem is provided with a 
key cap 30. Normally the key stem is held in the retracted position as 
shown. One can use a spring such as a compression spring 32 or interacting 
magnets. 
In addition, there can be provided a bumper 31 of elastic material which 
acts as a stop for downward travel of the switch stem 22. 
The operation of the key assemblage will now be described with the aid of 
the waveforms of FIG. 6. Periodically, a packet of eight pulses as shown 
in waveform (a) or (a') is emitted from amplifier AX8 to terminal 18. The 
variable capacitor KS89 and the resistor RY91 act as a conventional RC- 
differentiator wherein the leading edge of the pulse is assumed to be 
transmitted undisturbed while the remainder of the pulse is shaped 
according to the relationships: 
##EQU1## 
Where .tau. is approximate rise time of input waveform; E is steady peak 
voltage of input waveform; t is time; R is the resistance of resistor 
RY91; and C is the capacitive of the variable capacitor KS89. For RC 
&gt;&gt;.tau., both equations reduce to the classical approximation 
EQU e .perspectiveto.E exp (-t/RC) 
if the capacitance is low when key stem 22 (FIG. 5) is in a slightly 
depressed position then the waveform (b) of FIG. 6 is fed to amplifier 
AY9. The amplifier only passes signals above the threshold level TL and 
clips all signals above its saturation level SL. If the output of 
amplifier AY9 were not connected to capacitor C9 its output would be that 
shown in waveform (c). On the other hand when the variable capacitor is 
set to its maximum value , i.e., when key stem is in its fully propelled 
position then waveform (b') is fed to the (+)--input of amplifier AY9 
whose output, if not connected to capacitor C9, would be waveform (c'). 
Note the pulses emitted by amplifier AY9 have a width dependent on the 
instantaneous capacitance of variable capacitor KS89. The instantaneous 
capacitance and therefore the width of the pulses varies with the travel 
of key stem 22. The width of the pulses varies between the limits shown in 
waveforms (c) and (c'). When the output of amplifier AY9 is connected to 
the integrator SHI these waveforms are modified to waveforms (d) and (d') 
respectively due to the integration action of capacitor C9 which causes 
the voltage at terminal T1 to rise according to waveforms (e) and (e') 
respectively as each pulse adds another quantum of charge onto the 
capacitor. 
The instantaneous voltage across the capacitor C9 follows the equation: 
EQU Eo = VG + (VL-VG) exp (-nT/RC) 
where VL is the amplitude of the added DC voltage; VG is the charging 
voltage; n is the number of pulses; T is the width of a pulse; R is the 
resistance of charging resistor RY93; and C is the capacitance of the 
integrator capacitor C9. Note when nT is zero, i.e., before a key is 
depressed and the capacitor has been discharged, Eo=VL which is the 
quiescent DC voltage added to any output signal. Note also that the slope 
of the tops of the pulses in waveforms (d) and (d') and, therefore, the 
slope of the risers (f) and (f') is a function of the charging voltage VG. 
Since the slope of the risers determine the amplitude of the output signal 
it is seen that by varying the voltage VG the transfer gain between 
terminal 18 and line Y9 can be varied. 
At the end of the eighth pulse of each packet the input impedance to sense 
amplifier is switched to a very low value such that the capacitor C9 is 
discharged to the voltage VL. 
It will be seen from waveforms (e) and (e') that when the voltage on line 
Y9 exceeds the level V2 the sense amplifier registered a make condition 
and when the voltage on the line Y9 is below the level V1 the sense 
amplifier registered a break condition where the difference between levels 
V1 and V2 is the electronic hysteresis EH of the sense amplifier. 
There has thus been shown a method of controlling the transfer of signals 
by a capacitor key switch by differentiating a train of high frequency 
pulses with the variable capacitance of the key switch being used as a 
differentiating capacitor, applying the resulting output waveform to a 
voltage comparator so that the output of the comparator is also a train of 
pulses whose widths are a function of the variable capacitance, and 
applying the output of the comparator to a sample-and-hold integrating 
circuit such that the peak amplitude of the final output signal is a 
function of the width of the comparator output pulses which, in turn, is a 
function of the amplitude of the differentiated voltage at the input of 
the comparator which, in turn, is a function of the magnitude of the 
variable capacitance which, in turn, is a function of the downward travel 
of the key switch. 
Note that the combination of pulse source PS and differentiator means DM 
can be also considered a signal transducer wherein the mechanical movement 
of the key stem is transduced into pulses whose width is a function of key 
stem travel. 
In FIG. 7 there is another key switch assemblage having a transfer 
characteristic which can be varied so that a given electrical hysteresis 
is translated to a greater than usual mechanical hysteresis. In this case 
a fixed DC voltage is connected to one end of a potentiometer RT whose 
other end is connected to ground. The tap P1 of the potentiometer which is 
connected to a key stem (not shown) is connected via resistor RI to 
(-)--input of operational amplifier OA whose positive input is connected 
to voltage source VL. The output of the amplifier is connected via 
variable resistor RF to the (-)--input. The output voltage e.sub.o of the 
amplifier OA which is fed to a sensing amplifier with electronic 
hysteresis (not shown) is equal to 
EQU e.sub.o = - (RF/RI) x e + VL; 
where e is the voltage at tap P1, RF and RI are the resistances of the 
feedback and input resistors RF and RI respectively, and VL is a variable 
reference voltage. 
Note when the key is not depressed e is effectively zero and e.sub.o = VL 
the DC component added to the variable signal. Note also that by varying 
resistors RF the gain is varied. 
Therefore, there has also been shown a method of translating a fixed amount 
of electronic hysteresis of a terminating sensing amplifier into a 
significant amount of mechanical hysteresis by adjusting both the gain and 
D.C. position of the final output signal relative to the actual voltage 
location of the two fixed electronic hysteresis levels of the sensing 
amplifier. 
While the sensing of key switch travel has been described by 
width-modulating pulses in accordance with key switch travel and 
thereafter integrating the width-modulated pulses to obtain an 
amplitude-modulated signal which is related to key switch travel, it 
should be apparent that this aspect of the invention contemplates sampling 
the width-modulated pulses per se for pulses having greater than a given 
duration to indicate the key switch has travelled greater than a given 
distance. 
There will now be obvious to those skilled in the art many modifications 
such as different voltage levels, pulse trains, and mechanical details 
which satisfy many or all of the objects of the invention, but which do 
not depart from the spirit thereof as defined by the appended claims.