Non-contact proximity detector to detect the presence of an object

A non-contact capacitive proximity detector includes a sensing plate and detection circuitry electrically coupled to the sensing plate. The detection circuitry comprises an oscillator electrically connected to the sensing plate and signal conditioning circuitry electrically connected to the oscillator. The signal conditioning circuitry includes a coupling capacitor C.sub.1, an envelope demodulator, a first comparator, a delay circuit and a second comparator. When an object is within a predetermined proximity to the sensing plate, the capacitance of the capacitor constituted by the sensing plate and the proximal object is at a level which causes the oscillator to generator an oscillating signal having a sufficient magnitude. The oscillating signal generated by the oscillator is applied to the envelope demodulator by way of the coupling capacitor. The envelope demodulator generates an envelope of the oscillating signal and applies the envelope to he first comparator. The first comparator in turn generates a logic high output which is fed to the delay circuit. The delay circuit in turn passes the logic high output to the second comparator after a predetermined amount of time has elapsed causing the second comparator to generate a logic high output representative of the presence of than object within a predetermined proximity to the sensing plate. When the object is removed from the sensing plate outside the predetermined proximity, the output of the proximity detector changes allowing the proximity detector to serve as a removal detector.

TECHNICAL FIELD 
The present invention relates to proximity detectors and in particular, to 
a non-contact proximity detector which relies upon a capacitive proximity 
effect to detect the presence of an object. 
BACKGROUND ART 
Proximity detectors are known in the art and three basic types of proximity 
detectors exist, namely electromagnetic, optical and ultrasonic. 
Electromagnetic proximity detectors may be subdivided into inductive and 
capacitive types. Most inductive proximity detectors respond to the 
presence of any metal object, although ferrous metals give the best 
response (about two-thirds higher than for non-ferrous metals). 
One common principal of operation for inductive proximity detectors 
involves the use of an eddy current killed oscillator (ECKO). The 
oscillator is in the form of a tuned LC tank circuit amplifier. A sensor 
coil and ferrite core form the inductor. The amplifier provides just 
enough positive feedback to make the circuit oscillate in the RF range. 
When a metal object is positioned near the detector, the RF field causes 
eddy currents to flow in the surface of the metal object. When the metal 
object reaches a predetermined proximity to the detector, (depending on 
the detector sensitivity), the load caused by the eddy currents is enough 
to disable the oscillator. A level detector is connected to the oscillator 
for producing an output signal. Usually, a certain amount of hysteresis 
(differential travel) is built into the detector. The purpose of the 
hysteresis is to prevent chatter when the metal object is positioned right 
at the oscillation threshold point of the oscillator. 
Although these inductive proximity detectors are suitable for detecting the 
presence of a metal object, when using these types of detectors to detect 
the presence of a portion of an individual, such as a hand, the inductive 
detection methods yield non-linearities that result in large errors. 
Because the human hand represents an irregular reflective surface, optical 
and ultrasonic proximity detectors are also unsuitable. 
Capacitive proximity detectors have been used to detect the presence of a 
human hand. In these prior art capacitive proximity detectors, 
differential capacitive arrangements have been implemented wherein the 
centre plate of the capacitive arrangement is formed by the hand to be 
detected. Human tissue exhibits certain electrical properties (such as 
relatively low impedance to surrounding grounds at 100 kHZ), which are 
sufficient to make such arrangements practical. In capacitive arrangements 
of this nature, the centre plate (constituted by the hand) is maintained 
at ground potential and two fixed capacitors are provided in a complex 
bridge configuration. Unfortunately, these prior art capacitive proximity 
detectors are bulky, complex and have significant power requirements. 
It is therefore an object of the present invention to provide a novel 
proximity detector. 
DISCLOSURE OF THE INVENTION 
According to one aspect of the present invention there is provided a 
non-contact proximity detector comprising: 
a sensing plate forming a plate capacitor with said object when said object 
to be sensed is adjacent said plate, said capacitor changing capacitance 
as said object approaches said plate; and 
detection means responsive to said change in capacitance and generating 
output representative of the presence of said object when said object is 
within a predetermined proximity to said sensing plate. 
According to another aspect of the present invention there is provided a 
non-contact proximity detector to detect the presence of a human appendage 
comprising: 
a sensing plate forming a plate capacitor with said appendage when said 
appendage to be sensed is adjacent said plate, said capacitor changing 
capacitance as said appendage approaches said plate; and 
detection means responsive to said change in capacitance and generating 
output representative of the presence of said appendage when said 
appendage is within a predetermined proximity to said sensing plate. 
Preferably, the detection means includes an oscillator responsive to the 
change in capacitance for generating an oscillating signal having a 
magnitude which varies as a function of the capacitance and signal 
conditioning means responsive to the oscillating signal and generating 
output representative of the presence of the object or appendage when the 
magnitude of the oscillating signal is above a threshold value. It is also 
preferred that the signal conditioning means includes delay means to delay 
a change in the output of the signal conditioning means upon a change in 
the position of the object or appendage relative to the sensing plate. 
In a specific embodiment, it is preferred that the signal conditioning 
means includes an envelope demodulator to generate an envelope of the 
oscillating signal, a first comparator receiving the envelope and 
generating output when the magnitude of the envelope is above the 
threshold value, a delay circuit receiving the output of the first 
comparator and passing the output to a second comparator after a 
predetermined time has elapsed. The second comparator generates the output 
representative of the presence of the object or appendage in response to 
the output received from the delay circuit. 
The present invention provides advantages in that the design of the 
proximity detector eliminates the need for a complex bridge configuration 
that is required in prior art capacitive proximity detectors. Also, the 
present proximity detector has a compact design and consumes very little 
power as compared to conventional proximity detectors.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring now to FIG. 1 a non-contact capacitive proximity detector is 
shown and is generally indicated by reference numeral 8. The proximity 
detector 8 includes a sensing plate 10 which may be encased in a 
dielectric, and detection circuitry 12 electrically coupled to the sensing 
plate 10. The detection circuitry includes an oscillator 14 electrically 
connected to the sensing plate 10 and signal conditioning circuitry 16 
electrically connected to the oscillator 14. The signal conditioning 
circuitry 16 includes a coupling capacitor C.sub.1, an envelope 
demodulator 18, a first comparator 20, a delay circuit 22 and a second 
comparator 24. 
The oscillator 14 is in the form of a Hartley's LC oscillator and includes 
capacitors C.sub.2, C.sub.3 and C.sub.4, inductors L.sub.1 and L.sub.2, 
resistors R.sub.1, R.sub.2 and R.sub.3 as well transistor Q.sub.1. 
As can be seen, capacitor C.sub.2 is connected between a positive voltage 
source Vcc and ground. Resistors R.sub.1 and R.sub.2 which form a voltage 
divider also extend between the voltage source Vcc and ground. The tap 
from the voltage divider extends to the base of transistor Q.sub.1 as well 
as to the sensing plate 10. The emitter of transistor Q.sub.1 leads to 
ground by way of resistor R.sub.3. Capacitor C.sub.3 is arranged in 
parallel with the two inductors L.sub.1 and L.sub.2 which themselves are 
arranged in series. This parallel circuit constituted by capacitor C.sub.3 
and inductors L.sub.1 and L.sub.2 extends between the voltage source Vcc 
and the collector of transistor Q.sub.1. Capacitor C.sub.4 is connected 
between the inductors L.sub.1 and L.sub.2 and the emitter of transistor 
Q.sub.1. The output lead 28 of the oscillator 14 extends from the 
collector of transistor Q.sub.1 to one terminal of the coupling capacitor 
C.sub.1. 
Capacitor C.sub.3 is in the form of a variable trimer capacitor to allow 
the tuning and sensitivity of the oscillator 14 to be adjusted. An 
additional capacitor can be included in the event that capacitor C.sub.3 
fails to allow the tuning and sensitivity of oscillator 14 to be adjusted 
as desired. In this particular embodiment, the gain of transistor Q.sub.1 
is selected to be large enough to ensure that the oscillating signal 
output of the oscillator 14 is sufficient when an object to be detected is 
within 3 to 5 cm of the sensing plate 10. The loop gain of the oscillator 
is also set sufficiently high so that the oscillating signal output 
produced by the oscillator closely approximates a pure sinewave. 
The envelope demodulator 18 is constituted by resistor R.sub.4, Shottky 
diode D.sub.1 and capacitor C.sub.5. Specifically, resistor R.sub.4 is 
connected between capacitor C.sub.1 and ground. The anode of diode D.sub.1 
is also connected to capacitor C.sub.1. The cathode of diode D.sub.1 leads 
to the inverting terminal of comparator 20 which is in the form of an 
operational amplifier (op-amp). Capacitor C.sub.5 extends between the 
cathode of diode D.sub.1 and ground. The non-inverting terminal of 
comparator 20 taps a voltage divider constituted by resistors R.sub.5 and 
R.sub.6. The values of the resistors forming the voltage divider determine 
the set threshold of the comparator 20. The output of the comparator 20 
leads to the delay circuit 22 which is constituted by an RC network. 
Specifically, the RC network includes resistor R.sub.7 having one of its 
terminals connected to the output of comparator 20 and the other of its 
terminals connected to the non-inverting terminal of comparator 24 by way 
of resistor R.sub.8. Capacitor C.sub.6 which also forms part of the RC 
network is connected between resistors R.sub.7 and R.sub.8 and ground. 
Comparator 24 is also in the form of an op-amp and has its inverting 
terminal connected to a voltage divider constituted by resistors R.sub.9 
and R.sub.10. A feedback loop constituted by resistor R.sub.11 extends 
between the output and non-inverting terminals of comparator 24. 
The operation of the non-contact proximity detector 8 will now be 
described. The sensing plate 10 forms one half of a plate capacitor. When 
an object is within a predetermined proximity to the sensing plate 10, the 
object and sensing plate arrangement form a plate capacitor whose 
capacitance is given by: 
EQU C=.epsilon.(A/d), where: 
.epsilon. is the dielectric permittivity of air; 
A is the area of the sensing plate 10; and 
d is the distance of the hand from the sensing plate. 
Based on the above, it should be apparent that the nearer the object is to 
the sensing plate 10, the larger the capacitance of the capacitor 
constituted by the object and sensing plate arrangement becomes. When the 
capacitance reaches a predetermined value (which in this embodiment occurs 
when the object is within 3 to 5 cm of the sensing plate 10), oscillator 
14 generates an oscillating signal having a frequency in the vicinity of 
4.5 MHZ. The magnitude of the oscillating signal is dependant on the 
proximity of the object to the sensing plate and increases as the object 
approaches the sensing plate. The oscillating signal appearing on the 
output lead 28 of oscillator 14 is applied to the coupling capacitor 
C.sub.1 which allows the AC oscillating signal to pass. The envelope 
demodulator 18 receives the AC oscillating signal from the coupling 
capacitor and generates output representing the positive envelope of the 
oscillating signal. To achieve this operation, the RC time constant of the 
envelope demodulator 18 is chosen to be small enough so that when the 
envelope decreases in magnitude, the voltage across the capacitor C.sub.5 
can fall fast enough to keep in step with the envelope but not so small so 
as to introduce excessive ripple. 
The envelope output by the envelope demodulator 18 is filtered and then 
applied to the inverting terminal of comparator 20. When the magnitude of 
the envelope exceeds the set threshold of the comparator as determined by 
the voltage divider, the comparator 20 generates a logic high output. The 
logic high output of the comparator is then applied to the RC network 
which produces a delay before the logic high output is applied to the 
non-inverting terminal of comparator 24. In particular, capacitor C.sub.6 
of the RC network must become charged before the logic high is applied to 
the comparator. 
When the comparator 24 receives the logic high from the RC network, the 
comparator generates a logic high output to signify that an object is 
within a predetermined proximity to the sensing plate 10. 
When the object is outside of the predetermined proximity to the sensing 
plate 10 either causing the oscillator to turn off or causing the 
oscillator 14 to generate an oscillating signal having a magnitude below 
the set threshold of the comparator 20, the output of the comparator 20 
drops to a logic low level. When this occurs, capacitor C.sub.6 of the RC 
network begins to discharge. As capacitor C.sub.6 discharges the logic 
high applied to the comparator 24 is removed causing the comparator 24 to 
generate a logic low output signifying the absence of an object close to 
the sensing plate 10. 
Because the proximity detector 8 changes its output from a logic high when 
an object is within a predetermined proximity to the sensing plate 10 to a 
logic low when the object is removed from the sensing plate, the proximity 
detector 8 serves to function as a removal detector providing an 
indication when an object has been removed from the removal detector. 
Thus, the proximity detector 8 can be used in an alarm circuit to detect 
when an object such as valuable art, valuable automobiles etc. is removed 
from its intended position. 
In this case, the sensing plate 10 would be positioned relative to the 
object spaced from but within the predetermined proximity to the object. 
As long as the object remains within the predetermined proximity, the 
proximity detector 8 generates output signifying a non-alarm condition. In 
the event that the object is removed from the sensing plate outside the 
predetermined proximity, the output of the proximity detector changes 
signifying an alarm condition. This change in output can be used to set 
off an alarm system. 
The proximity detector may also be used to detect people. In a hospital 
environment, the proximity detector can be used to detect the presence of 
patients or the position of IV bags. When it is desired to monitor the 
position of patients, the sensing plate 10 can be attached to the hand of 
the patient (or other convenient location) to be monitored. In this case, 
the sensing plate 10 is encased in a dielectric to space the sensing plate 
10 from the patient. As long as the sensing plate remains attached to the 
patient, the output of the proximity detector signifies a non-alarm 
condition but upon removal of the sensing plate, the output of the 
proximity detector changes signifying an alarm condition. 
As should be apparent, the proximity detector 8 can be used to detect the 
presence of virtually any object where sensitivity non-contact detection 
of proximal objects at ground potential is required provided the object to 
be detected and the sensing plate form a plate capacity arrangement. 
As one of skill in the art will appreciate, when the proximity detector 8 
is to detect the presence of a hand, the capacitance of the capacitor 
constituted by the hand and sensing plate arrangement is small and is in 
the order of 10 pf. Since stray capacitance can overshadow the capacitance 
of the hand and sensing plate capacitor, the Hartley's LC oscillator is 
used since it is more sensitive than conventional RC oscillators. It 
should also be apparent that the sensitivity of the proximity detector 8 
can be increased by increasing the area of the sensing plate 10. 
The RC network prevents a change in output of the proximity detector 8 when 
a hand is placed near the sensing plate 10 for a duration insufficient to 
allow the capacitor C.sub.6 of the RC network to charge or when a hand is 
removed from the sensing plate for a time insufficient to allow the 
capacitor C.sub.6 to discharge. Thus, the delay circuit avoids the 
generation of false alarm output signals. 
It should be appreciated that various modifications may be made to the 
present invention without departing from the scope thereof as defined by 
the appended claims.