Capacitive sensor control system

An electronic control system for controlling the operation of electromotive apparatus utilizes a capacitive sensor for sensing human touch and producing a command signal whose magnitude corresponds to the contact area of the contact. The sensor command signal is applied to an oscillator which produces a series of output width pulses having a frequency corresponding with the magnitude of the sensor command signal. A discriminator detects the trailing edge of the oscillator pulses and produces a series of fixed width pulses and an integrator produces a D.C. voltage signal corresponding to the discriminator output, which voltage signal corresponds to the area in proximity to and thereby stimulated by the human body. A comparator is employed to produce a pulse-width modulated rectangular pulse output whose duty cycle corresponds to the integrator D.C. voltage by comparing the integrator D.C. voltage with a reference triangular waveform. The comparator output is applied via power circuit to drive the electromotive apparatus which may typically comprise one or more D.C. electric motors.

BACKGROUND OF THE INVENTION 
This invention relates to motorized vehicle control systems and more 
particularly to an electromic control system especially suited for 
electrically powered wheelchairs for use by disabled persons with severely 
limited ability to manipulate the limbs, particularly the hands and 
digits. 
It has been found that disabled persons suffering from poor coordination, 
inadequate muscle strength or limited movement of the arms, hands and 
fingers, conditions which typically result from the afflictions of 
muscular dystrophy and multiple sclerosis, are largely unable to exercise 
the necessary degree of control for existing electrically powered 
wheelchairs. Presently available electrically-powered wheelchairs 
typically employ for control purposes either "ON-OFF" switch controls, a 
control stick associated with a plurality of electrical switches, or a 
rather crude proportional control system. Each of these systems suffers 
from serious disadvantages to be presently described. 
The "ON-OFF" switch control system which usually employs either a manually 
actuable push-button switch or a mercury switch mounted on a headband 
results in an erratic and jerking vehicular motion due to the sudden 
starting and stopping of the electric motor which powers the wheelchair. 
Moreover, a sudden start of the wheelchair in one direction will cause the 
rider's head and body to jerk in the reverse direction, which in turn 
creates a sharp false instruction to the motor to reverse direction. Such 
rapid forward and reversing motions of the wheelchair causes it to wheel 
around rapidly out of control of the rider. A high degree of coordination 
of the upper body and neck of the rider is required to avoid or minimize 
the aforementioned loss of vehicle control but most disabled wheelchair 
riders lack the required muscular control to overcome the just-described 
oscillatory motion of the vehicle. Where digitally actuated push-button 
switches are employed, severely disabled riders find it very difficult to 
accurately position their hands and fingers over the various push-buttons, 
usually four in number, and apply sufficient pressure thereto in order to 
achieve the desired vehicular motion. On the other hand, the push-buttons 
may easily be accidentally actuated by an inadvertent brush of the finger, 
causing unwanted vehicular motion. 
In the switch and control stick system, at least four switches and a 
control stick are provided. In order to achieve forward vehicular motion 
the rider moves the control stick forwardly to actuate the two forward 
switches, which concurrently cause the electric drive motor to rotate each 
wheel at equal speed in the forward direction. 
Movement of the control stick to the right will cause actuation of the left 
front switch, causing the motor to rotate the left wheel forwardly, and 
actuation of the right rear switch causing the motor to rotate the right 
wheel in reverse, thus causing the wheelchair to pivot to the right. 
Again, due to the "ON-OFF" nature of the four switches, sudden changes in 
vehicular motion result, particularly if the motor speed controls are set 
to the High Speed position, as is usually the case. Unfortunately, due to 
their muscular disabilities, many such persons confined to a wheelchair 
find themselves unable to exercise a sufficient degree of control over the 
vehicle. Even if the control stick is moved straight ahead, due to 
positional inaccuracies of the two forward switches they will not actuate 
perfectly simultaneously, whereby the wheelchair moves slightly to the 
left or right before moving forwardly. In addition, due to differences in 
friction or electrical wiring of the left and right drive motors, the 
wheelchair will tend to "wander" to the right or left requiring steering 
correction by the rider. 
The use of switch controls imposes a severe reduction of battery life due 
to the fact that maximum power is always applied to the wheel drive motors 
on start-up even if the wheelchair is to be moved a short distance. 
In presently available proportional control systems, which avoid some of 
the above-described problems in "ON-OFF" switching systems, the motor 
speed, i.e., wheel velocity is determined by the magnitude of the 
displacement of the control stick from its rest position. Motor speed 
control is achieved by varying the motor current by means of a variable 
resistor which is heavy, bulky and is very wasteful of battery-supplied 
electrical power due to the large amount of electrical power and resultant 
heat which must be dissipated. Where such motor speed control is 
accomplished by means of transistor or gate controlled rectifier switching 
circuitry, the required relays and other control circuitry greatly 
increases both the size and cost of the overall control system. 
It is therefore an object of the present invention to provide an electronic 
control system for controlling electromotive apparatus which avoids the 
above-described deficiencies in existing control systems. 
It is another object of the present invention to provide an electronic 
control system of the character described which is reliably operative in a 
severe mechanical and electrical environment. 
It is a further object of the present invention to provide an electronic 
control system of the character described which is remotely operated by 
human touch. 
It is yet another object of the present invention to provide an electronic 
control system of the character described which is particularly adapted 
for use by disabled persons in driving and steering a wheelchair. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, there is provided an electronic 
control system for controlling the operation of electromotive means, such 
as an electric motor, comprising electrically capacitive sensing means for 
detecting proximity thereto of a portion of a human body, oscillator means 
operative to produce a series of output pulses having a frequency in 
accordance with the capacitance input thereto and discriminator means for 
detecting the trailing edges of said oscillator pulses and producing a 
series of fixed width pulses in response thereto. Also provided are 
integrator means operative to produce a D.C. voltage in accordance with 
said discriminator pulses, reference waveform generating means for 
producing a triangular reference waveform and comparator means operative 
to compare said integrator D.C. voltage and said triangular reference 
waveform and to produce a pulsed D.C. output having a pulse width 
determined by the difference between said compared D.C. voltage and said 
triangular waveform, and power circuit means for applying said pulsed D.C. 
output to said electromotive means. 
Additional objects and advantages of the invention will become apparent 
during the course of the following specification when taken in connection 
with the accompanying drawings in which:

DETAILED DESCRIPTION OF THE INVENTION 
With reference to the drawings, there is shown in FIG. 1 a control box 10 
housing the control system of the present invention, hingedly mounted 
along one edge thereof to the upper bar 12 mounting one of the arm rests 
14 of the wheelchair 16. As shown in FIG. 2, control box 10 is pivotable 
from a vertical inoperative position represented by dotted outline 10a or 
to a horizontal operative position shown in solid outline 10 in FIGS. 1 
and 2. 
Mounted at the upper surface of control box 10 is a sensor surface 18 
divided into four quadrant sensor areas RF, LF, RR and LR corresponding to 
Right-Forward, Left-Forward, Right-Rear and Left-Rear travel directions 
respectively as shown in FIG. 2. Thus, for example, if the user wishes to 
cause the chair to travel forwardly in the right direction he would touch 
the RF sensor area, and if he wishes to travel in the left-rear direction 
he would touch the LR sensor area, and so forth. If he desires to travel 
in the straight ahead or straight rearward direction he would touch both 
RF and LF sensors simultaneously, respectively. The magnitude of the areas 
touched in any one of the four sensors RF, LF, RR or LR respectively 
determines the magnitude of the velocity vector for the particular 
direction corresponding to the sensor being stimulated. Thus, the user can 
control the degree of steering to the left or right directions by 
correspondingly varying the area being stimulated in the left quadrant 
sensors LF or LR and right quadrant sensors RF or RR respectively. By 
varying the exposure applied to the sensor quadrants by the palm and/or 
fingers of the hand, for example, and hence the area being stimulated in a 
particular sensor area, the user may achieve fine speed control in the 
desired direction. It should be noted, however, that the sensor areas RF, 
RR, LF and LR respectively are responsive only to changes in stimulated 
area rather than directly to contact pressure. Sensing areas RF, RR, LF 
and LR may assume virtually any geometrical shape which is appropriate for 
the particular application, such as cylindrical, flat or spherical shapes. 
The sensors may be connected to the remaining control circuitry by means 
of subminiature coaxial cables, and may, therefore, be mounted at any 
convenient location remote from the remaining control circuitry. 
In the case of a wheelchair application, the sensing surface is usually 
flat and has a two-dimensional configuration which is determined by the 
user's requirements. Thus, although sensing areas RF, RR, LF and LR are 
shown as squares in FIG. 2, in the case of a user with poor coordination 
these sensing areas may advantageously assume the shape of a Maltese 
Cross. In such configuration, maximum forward and reverse functions could 
be achieved as easily as with square sensor areas, but it would be 
difficult to switch from forward to a fast reverse direction, for example. 
Any desired geometrical configuration for the sensing areas could be 
achieved merely by placing a mask having the desired sensing area geometry 
over the sensors. 
The structure and operation of the electronic circuitry which comprises the 
proportional control system of the present invention which achieves the 
above-described wheelchair travel control wall now be described in detail. 
Referring to FIGS. 3 and 4, there are shown four control circuit channels 
B, C, D and E respectively connected to sensors RF, RR, LF and LR. The 
outputs of channels B and C are connected to the input terminals of Motor 
R which drives the right wheel 20 of wheelchair 16 and the outputs of 
channels D and E are connected to the input terminals of Motor L which 
drives the right wheel 22 of wheelchair 16. Since each of the four 
channels, B, C, D and E are similar in structure and operation, only 
channel B will be described herein. Sensor RF, like remaining sensors RR, 
LF and LR, comprises a metal plate or etched printed circuit board 24 
having an overlying protective insulating layer 26 forming a capacitor 
between the metal sensing plate 24 and the operator's body, and also 
serves the purpose of eliminating shock and static hazard to the user. As 
shown in FIG. 2A, the sensor metal plate 24 senses body capacitance of the 
user when he touches insulating cover layer 26. It has been found that the 
capacitance of the human body is substantially uniform and lies in the 
range of 22 to 34 picofarads. Adjacent sensors, such as LF and RF, or LF 
and LR are interlaced so that a smooth transition in wheelchair direction 
and velocity is achieved when the user sweeps his hand, for example, from 
one sensor area to an adjacent one. 
Referring to FIG. 3, in operation, assume that the user is about to apply 
his hand to sensor RF which detects the user's body capacity. The 
capacitance detected by sensor RF is coupled through a coaxial cable 32 to 
the input of free-running sensor oscillator U3 which poduces a square wave 
output 34 having a frequency of about 150 KHZ when no body capacity is 
applied to sensor RF such as by the application of the user's hand 
thereto. Oscillator U5 may typically comprise an NE-557 type timer circuit 
which constitutes dual NE-555 type circuits. The output frequency of 
oscillator U3 at output lead 28 is determined by resistors R1 and R2, and 
the total capacitance applied to Junction 30 comprising the sum of the 
capacitance of sensor RF, the capacitance coupled to sensor RF by the 
user's body, and the capacitance of the coaxial cable 32. The "initial" 
frequency, i.e., with no capacity coupled to sensor RF, of the output 
waveform 34 appearing on output 28 shown in FIG. 6, which is quite stable 
and not critical is normally 150 KHZ. The duty cycle of oscillator output 
waveform 34, which is set by the ratio of resistors R1 to R2 is 50% 
thereby providing a square waveform. It is noted, however, that a 50% duty 
cycle for waveform 34 is not mandatory since the circuitry following the 
output of oscillator U3 only detects the trailing edge 34' of waveform 
34. Since the supply voltage, as indicated, is +5 volts, the output level 
of waveform 34 is approximately 0 to +5 volts. 
Since the frequency of output waveform 34 is a function of the total 
capacitance applied to junction 30, the addition of the user's body 
capacitance coupled to sensor RF will cause the frequency of waveform 34 
to decrease by an amount depending on the magnitude of the coupled 
capacitance and the initial frequency of waveform 34; the higher the 
initial frequency, the greater the frequency change for a given coupled 
capacity. It is understood that if the difference in initial frequency of 
the various channels B, C, D and E is too great to achieve substantially 
equal sensitivity for all channels, then the various coaxial cables 32 may 
be trimmed or trimmer capacitors (not shown) may be added thereto. 
The output of oscillator U3, namely waveform 34 is applied to one-shot 
multivibrator circuit U5 which acts as a discriminator. One-shot circuit 
U5, which is also known as a "delay-flop" or "single pulser" may comprise 
standard integrated circuit ("IC") type SN74123. It is understood that 
other IC types, such as Nos. 9316, 74121 or 74122 may be used for one-shot 
circuit U5. 
The duration of output waveform 36 of one-shot circuit U5 shown in FIG. 6 
is fixed by resistors R3 and R4 and capacitor C1. Variable resistor R4 
serves as the main channel adjustment and is set at a value so that at the 
initial frequency, the output at pin 13 of circuit U5 is constantly high, 
i.e., at about +5 volts which corresponds to a 100% duty cycle. Any 
decrease in the frequency output of oscillator U3 due to the body capacity 
coupled to sensor RF causes a corresponding decrease in the duty cycle of 
output waveform 36 of circuit U5 since the time duration of pulses 36 is 
fixed. It is noted that the waveform (not shown) appearing at output pin 4 
of circuit U5 is the complement of waveform 36. As pointed out above and 
shown by waveforms 34 and 36 in FIG. 6 circuit U5 is triggered by the 
trailing edge 34' of the output from oscillator U3. 
Referring to FIG. 6, it is seen that addition of the body capacitance to 
sensor RF, causes an increase in the pulse width of pulses 34 and 
reduction in the duty cycle of waveform 36. Waveforms 34 and 36 for a 50% 
sensed capacitance are designated by the numerals 34a and 36a and for 66% 
sensed capacity by the numerals 34b and 36b respectively in FIG. 6. Thus, 
for no coupled capacity and at the initial frequency of oscillator U3, the 
one-shot waveform 36 is at almost 100% duty cycle; when about one-half the 
sensed capacitance is applied to sensor RF, the one-shot waveform 36a duty 
cycle is reduced to about 66%, and when the maximum body capacitance is 
added to the sensor the one-shot waveform 36b duty cycle is reduced to its 
minimum of about 50%. 
Operational amplifier (Op Amp) U7, to which one-shot waveform 36 is applied 
at input lead 38 and its complementary waveform at input lead 40 serves as 
a balanced integrator comprising a balanced amplifier and integrator, The 
integrator time constant which is determined by R7 and C2 is sufficiently 
large to integrate the one-shot pulses of waveform 36 to a D.C. level 
which is a function of the gain of operational amplifier U7 and the duty 
cycle of waveform 36. In this way, operational amplifier U7 detects the 
change in duty cycle of waveform 36 caused by the changes in capacitance 
coupled to the sensor. 
Initially, the gain of Op Amp U7 may initially be set to about 3 by the 
ratio of R7/R5=R8/R6, however, this gain may be increased if an increase 
in sensitivity is found to be necessary. Due to the presence of capacitor 
C2, one side of Op Amp U7 is unbalanced by acting as an integrator whereby 
gain as well as a D.C. output level signal is produced at output lead 42. 
It is noted that the +5 volt supply voltage for Op Amp U7 is single ended 
because the output voltage for the circuits following Op Amp U7 should be 
a zero volt minimum corresponding to the initial frequency of oscillator 
U3 and a maximum of +4 volts for maximum sensed capacitance coupled to 
sensor RF. The change in D.C. output voltage at integrator output lead 42, 
which is illustrated by line 44 in FIG. 8 rises linearly with a decrease 
in duty cycle of one-shot waveform 36 and thus linearly with the increase 
in body capacitance coupled to sensor RF. 
Channel adjustment may be accomplished by selective variation of resistor 
R4 so that the D.C. level at lead 42 is +0.5 volts. As capacitance is 
added to sensor RF, the duty cycle of waveform 36 will decrease and, at 
the minimum design frequency of oscillator U3, the output 44 at lead 42 
will be approximately +4 volts. 
Oscillator U1, which is similar to oscillator U3 except for its output 
frequency, shown in FIG. 5, provides a square wave output 46 at an output 
frequency of approximately 80 HZ and may comprise a type NE555 integrated 
circuit. Resistors R11, R12 and capacitor C3 determine the frequency of 
oscillator output waveform 46 shown in FIG. 7, while the ratio of 
resistors R11 to R12 determines the duty cycle thereof. Resistor R11 is 
chosen to have as small a resistance as possible in order to provide an 
almost perfect square wave 46 having a 50% duty cycle. In order to 
generate a signal with a sufficiently large amplitude for the circuits 
following, the supply voltage as shown is +8 volts D.C. 
The square wave output 46 is applied via output lead 48 to integrator U2 to 
produce a triangular waveform 50 at output lead 52 of integrator U2. The 
time constant of integrator U2, which is determined by resistor R14 and 
capacitor C4, is chosen so as to make waveform 50 substantially 
triangular, with R15 selected so that the peaks of waveform 50 are not 
clipped either at the top or bottom thereof, the peak-to-peak voltage of 
waveform 50 being about 0 to +7 volts. It is noted that the +8 volt D.C. 
single ended supply to integrator U2 shown in FIG. 5 is required because 
the output of oscillator U1 is about 0 to +8 volts and the output of 
integrator U2 must be approximately 0 to +7 volts. 
Reference triangular waveform 50 is applied to the divider network 
comprising resistors R15 and R16 and diode CR1 to ground to cut the 
amplitude of waveform 50 by about 50% and clamp its lower peaks to about 
0.7 volts above ground. This triangular waveform at junction 53 is applied 
to the + terminals of each of comparators U4 in channels B, C, D and E via 
lead 54 as shown in FIG. 3. 
Comparator U4, which may comprise a Type LM339 integrated circuit, is 
operative to compare the output of balanced integrator U7 at lead 42, 
which is a function of the body capacitance coupled to sensor RF, with the 
reference triangular wave output 50 from integrator U2 on lead 54. Thus, 
D.C. output line 42 pulse-width modulates reference waveform 50 to thereby 
control the D.C. voltage at output lead 56 to be applied to D.C. wheel 
drive motors L and R. Thus, a higher channel voltage output on lead 42 
will produce a wider pulse width of the comparator pulse output 43 on lead 
56. For this reason, as indicated above, resistor R4 is adjusted to a 
quiescent voltage of +0.5 volts, for zero coupled capacitance on sensor 
RF, while the lowest point on triangular waveform 50 at junction 53 is 
+0.7 volts, which will produce a zero output on comparator output lead 56, 
as shown in FIG. 8. If the channel output 44a at lead 42 rises about +0.7 
volts (due to body capacitance coupled to sensor RF), comparator U4 will 
produce a pulse output 43a at a constant rate of 80 HZ, as shown in FIG. 
8. As the channel voltage 44b rises (due to increased coupled capacitance) 
the pulse width of the comparator 43b will increase, as shown in FIG. 8, 
for a constant frequency of 80 HZ, whereby the average power (which is 
directly proportional to pulse width) delivered to the motor will increase 
accordingly. For a channel voltage of +4.2 volts, it will exceed the 
magnitude of triangular waveform 50 whereby constant full power will be 
applied to motor R through power output circuit 60. 
The supply voltage of comparator U4 is +12 volts as indicated in FIG. 4 
which illustrates the power circuit 60 which serves as the driving circuit 
for the motors. In circuit 60, resistors R10 and R9 supply current to 
Darlington-connected output transistors 62, 64 and 66 which drive motor R 
at output lead 68. Since comparator U4 has an open collector output, R10 
serves as the pull-up resistor, while resistor R9 also serves to limit 
comparator U4's current to a safe value and to reduce any inductive 
voltage spikes which may be fed back from motor R and/or power circuit 60 
to the preceding circuitry. 
It is understood that although the control system of the present invention 
has been described in connection with its use for controlling vehicles 
such as wheelchairs, it is to be understood that its use is not limited 
thereto. In addition to its application for use in other vehicles such as 
electric golf carts, the control system of the present invention may be 
employed in industrial applications, such as the control of mechanical 
arms in nuclear applications, electric and electro-hydraulic units such as 
cranes. Moreover, since the sensing areas are made of printed circuit 
materials, they can be etched to produce responses other than linear, such 
as logarithmic, or virtually any other desired response function, a 
feature which is particularly valuable in servo-feedback applications. 
While preferred embodiments of the invention have been shown and described 
herein it is obvious that numerous omissions, changes and additions may be 
made in such embodiments without departing from the spirit and scope of 
the invention.