U.S. patent number 3,764,819 [Application Number 05/234,952] was granted by the patent office on 1973-10-09 for electronic switch actuated by proximity of the human body.
Invention is credited to Harro Muller.
United States Patent |
3,764,819 |
Muller |
October 9, 1973 |
ELECTRONIC SWITCH ACTUATED BY PROXIMITY OF THE HUMAN BODY
Abstract
An improved contact-less electronic switch, adapted to be
actuated by the approach of a human hand, with a capacitive probe
insulated against the chassis and installed in the zone of
approach, and connected to a capacitance detecting circuit which
delivers a switching signal when the capacitance is increased by
the approach of a hand, wherein the improvement comprises that the
switch remains unaffected by the approach of all other substances,
such as metals or insulating materials.
Inventors: |
Muller; Harro (Konigsdorf/Obb,
DT) |
Family
ID: |
62567056 |
Appl.
No.: |
05/234,952 |
Filed: |
March 15, 1972 |
Foreign Application Priority Data
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Mar 16, 1971 [DT] |
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P 21 12 521.3 |
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Current U.S.
Class: |
307/116; 340/562;
340/551; 340/573.1 |
Current CPC
Class: |
H03K
17/955 (20130101); H03K 17/945 (20130101) |
Current International
Class: |
H03K
17/94 (20060101); H03K 17/945 (20060101); H01h
035/00 () |
Field of
Search: |
;307/116 ;200/DIG.1
;340/258C |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Schaefer; Robert K.
Assistant Examiner: Ginsburg; M.
Claims
I claim:
1. In an electronic switching arrangement actuated in a
contact-less manner by the approach of a portion of a human body,
having a capacitive probe which is insulated from a support
chassis, and connected to a capacitance measuring circuit which
delivers a signal when a certain increase of the capacitance is
caused by a predetermined approach of the portion of the human body
into the electric field of the capacitive probe, the improvement
which comprises an inductive probe disposed in the proximity of
said capacitive probe, means whereby said inductive probe forms a
portion of an oscillator circuit and having a magnetic field, the
magnetic field of said inductive probe permeated the electric field
of said capacitive probe, said inductive probe being connected to
an inductive measuring circuit, said inductive measuring circuit
being adjusted to deliver a switching signal indicative of the
presence of a metal object at a predetermined distance from said
support chasses, and chassis, capacitance measuring circuit being
adjusted to deliver a switching signal only when the metal object
is closer to said support chassis than the predetermined distance,
means connected to receive switching signals from said inductive
and capacitance measuring circuits and delivering an output signal
only if the said capacitance measuring circuit supplies a switching
signal and the measuring circuit associated with the inductive
probe does not supply such a switching signal.
2. An arrangement as defined in claim 1, wherein are provided to
adjust the sensitivity of the said capacitance measuring circuit to
deliver a signal on the approach of an electrically conducting
body, but not by the approach of a body of insulating material.
3. An arrangement as defined in claim 1, wherein said capacitive
probe is mounted on said support chassis and said chassis is
grounded and forms the counter-electrode of said capacitive
probe.
4. An arrangement as defined in claim 1, wherein said inductive
probe consists of a coil mounted on the legs of a U-shaped high
frequency iron core and said capacitive probe consists of a metal
plate disposed to lie between the two legs of said iron core but in
spaced relationship thereto.
5. An arrangement as defined in claim 1, wherein said capacitive
probe and said inductive probe are supplied by the same oscillator
and means whereby the damping of the oscillator is used as a
measure of the degree of approach of a metal part.
6. An arrangement as defined in claim 1, wherein means are provided
whereby the initial capacitance of the said capacitive probe is
neutralized.
7. In an electronic switching arrangement comprising a switch
actuated in a contact-less manner by the approach of a portion of a
human body, comprising a coil divided into two partial windings,
means including one of said windings connected to a capacitor to
form an oscillator circuit of an oscillator, the other of said
windings being connected to a capacitive probe and applying an
electrical potential thereto; said capacitive probe being connected
to a capacitance voltage converter circuit; said converter circuit
being connected to a trigger; said trigger having a response
threshold which is above the level of the signal supplied by said
converter circuit in the absence of an object in a zone proximate
to the switch and means whereby said response threshold of said
trigger is exceeded when a portion of a human body is in the zone
proximate to the switch.
8. An arrangement as defined in claim 7, wherein said capacitance
converter circuit comprises a diode pump.
9. An arrangement as defined in claim 7, wherein means are provided
whereby the response threshold of said trigger is adjustable.
10. An arrangement as defined in claim 7, wherein means are
provided whereby the regenerative feedback factor of said
oscillator is adjustable.
11. In a electronic switching arrangement including a switch
actuated in a contact-less manner by the approach of a portion of a
human body comprising a high frequency iron pot core, a coil
disposed in said core, said coil being tapped to provide two
partial windings, one of said partial windings being larger than
the other of said partial windings, said one of said partial
windings being arranged in space non-symmetrically with respect to
said other of said partial windings; means including said coil
connected to a capacitor to form an oscillator circuit of an
oscillator; said coil being connected to a capacitance converter
circuit; said converter circuit being connected to a trigger; said
trigger having a response threshold which is above the level of the
signal supplied by said converter circuit in the absence of an
object in a zone proximate to the switch and means whereby said
response threshold of said trigger is exceeded when a portion of a
human body is in the zone proximate to the switch.
12. An arrangement as defined in claim 11, wherein said other of
said partial windings is disposed outside of said one of said
partial windings.
13. An arrangement as defined in claim 11, wherein said other of
said partial windings is disposed inside said one of said partial
windings.
Description
BACKGROUND OF THE INVENTION
Such proximity switches are used extensively in engineering for
initiating electrical switching processes, e.g. in elevators, in a
contact-less manner.
In the case of dangerous machines operated by operators such as,
e.g., paper guillotines, punches, or presses, a so-called two-hand
operation is usual in which the operator must operate
simultaneously two switches located outside the danger zone in
order to initiate the potentially dangerous working stroke of the
machine. The machine must stop immediately when even only one
switch is released. In this manner, the operator is compelled to
keep his hands outside the dangerous zone during the performance of
the working stroke and is, therefore, protected against injury.
Hitherto, the use of known capacitive proximity switches was
impossible with these dangerous machines because the switches could
also be operated, e.g., by placing on them a piece of metal. In
this case, the operator might release his hands during the
dangerous working stroke in order to accelerate the operation,
which is usually paid according to piece rates, by actuating the
switch by a piece of metal, and this is to be avoided under all
circumstances.
SUMMARY OF THE INVENTION
It is, therefore, an object of the invention to provide an
electronic switch suitable for use in the safety circuits of
dangerous machines which responds only to the approach of a human
hand or potentially to the approach of another part of the human
anatomy, but which remains unaffected by the approach of all other
materials, such as metals or insulators.
It is a further object of the invention to provide a switch of the
type hereinbefore described, comprising an inductive probe supplied
by an oscillator and whose magnetic field permeates the electric
field of a capacitive probe mounted in close spatial relationship
to the said capacitive probe and supplied a detection circuit which
delivers a switching signal on the approach of a metal part at a
larger distance than the capacitance measuring circuit, wherein the
two switching signals are logically linked capacitance measuring
circuit delivers a switching signal whilst the measuring circuit
supplied by the inductive probe delivers no such signal. A metal
part brought into the vicinity of such an electronic switch
triggers off first the inductive measuring circuit, so that the
logic interconnection according to the invention makes the release
of the electronic switch immediately impossible. The approach of an
insulating material, e.g. plastic, wood, glass, leather, paper and
the like, does not affect either the inductive detection circuit,
nor the capacitance measuring circuit, so that the logic
interconnection according to the invention prevents a signal from
being delivered also in this case.
If, on the other hand, a human hand approaches the switch, the
inductive protection circuit does not respond owing to the low
capacitance of the approaching object, whilst the capacitance of
the capacitive probe is raised to such an extent that the
capacitance measuring circuit delivers a switching signal. In this
case, the conditions for the release of an approach signal are met.
responds in principle also to the approach of other parts of the
human body; misuse may be avoided simply by mounting the swtich in
such a position that it can be reached only the operators hand.
According to the invention the capacitance measuring circuit is
adjusted to such a low sensitivity that it does not respond to the
approach of insulating material but to the approach of an
electrically conducting body. Thus, the differential effects on the
capacitance of such a probe by insulating materials on the one
hand, and by materials with bad electrically conducting properties
on the other hand are utilized.
It is a further object of the invention to provide an electronic
proximity switch of the type hereinbefore described in which the
capacitive probe is mounted in an insulated and earthed metal
housing forming the counter electrode. In this manner, the
capacitive probe forms a structural unit with the wall of the
housing, thereby producing a compact arrangement without externally
projecting parts which is also easy to clean.
Where separate oscillators are used, the oscillator supplying the
inductive probe has preferably a frequency of a few hundred kc/s,
whilst the oscillator connected to the capacitive probe operates
preferably with frequency in the Mc/s range. In this manner
sufficient currents are produced even with comparatively small
probe capacitances.
It is a further object of the invention to provide an electronic
switch of the type hereinbefore described in which the capacitive
probe is supplied by the same oscillator as the inductive probe,
and the samping of the oscillator is a measure of the degree of
approach of a metal part. Preferably, the oscillator operates at a
frequency of the order of magnitude of a few hundred kc/s. This
construction substantially reduces the complexity of the device and
ensures the necessary logic interconnection simply in that the
oscillator output voltage strongly declines or even collapses on
the approach of a metal part, so that the capacitance measuring
circuit does not receive a sufficient input voltage to enable it to
indicate an increase in the capacitance.
Preferably, the inductive probe is a coil portion of the
oscillator. Also this supports an extremely rational production of
the electronic switch according to the invention.
Preferably, the initial capacitance of the capacitive probe is
neutralized because this substantially increases the response
sensitivity for increases in the capacitance.
To enable the sensitvity of the switch according to the invention
to be adjusted, the response threshold of the trigger, and
preferably also the regenerative coupling factor of the oscillator
are adjustable.
It is a further object of the invention to provide a further
structural and technological simplification of the switch
hereinbefore described, wherein the oscillator coil or a part of
this coil, forming the inductive probe forms simultaneously the
capacitive probe. Thus, the invention utilizes the fact that every
induction coil also has a capacitance against mass.
The part of the coil serving as capacative probe is in this case
substantially larger than the remianing part of the coil. The
remaining part of the coil contributes in this case to the
utilization of the initial capacitance.
Preferably, the part of the coil acting as capacitive probe is
arranged in space non-symmertically relative to the remaining part
that in the case of the approach of an object changing the
capacitance, the changes in the capacitances of the two coil
portions against mass are non-proportional. This is the condition
for detecting changes in the capacitances by measuring
techniques.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be further described by way of example with
reference to embodiments shown in the accompanying drawings in
which:
FIG. 1 is a plan view of the key portion of an electronic switch
arrangement according to the invention;
FIG. 2 is a cross-section along the line II--II in FIG. 1;
FIG. 3 is a block diagram of the basic construction of an
electronic switch according to the invention;
FIG. 4 is a block diagram of a substantially simplified embodiment
of the electronic switch according to the invention;
FIG. 5 is a detailed circuit diagram of the object shown
diagrammatically in FIG. 4;
FIG. 6 is an axial cross-section of a particularly preferred
embodiment of a key portion for an electronic switch according to
the invention;
FIG. 7 is a preferred circuit for evaluating the signals delivered
by the key portion according to FIG. 6.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to FIGS. 1 and 2 of the drawing, a key portion of an
electronic switch according to the invention is mounted in an
earth, or grounded, metal housing 1. A U-shaped core of powdered
iron is arranged in the housing 1 in such a manner that the ends of
the two legs terminate at the level of the upper wall of housing 1.
Induction windings 3, forming the inductive probe, are mounted on
the two legs of the core 2.
Mounted symmetrically between the legs of the core 2 is a metal
plate 4, which forms the capacitive probe. The top of the housing
is covered by an insulating plate 5 on which the capacitive probe 4
may be mounted from the bottom, so that the housing has no
protruding parts.
FIG. 2 shows also in solid lines the lines of the magnetic field,
in dotted lines the lines of the electric field, and, by way of
illustration, a hand 6 approaching the switch.
The earthed, or grounded, metal housing 1 forms the
counter-electrode for the capacitive probe 4, so that approaches
from the side of the housing remain ineffective.
The block diagram of FIG. 3 shows diagrammatically the connection
of the inductive probe 3 and of the capacitive probe 4 forming the
permeating fields. The winding of the probe 3 is the coil portion
of an oscillator 8, operating at a frequency of a few hundred kc/s.
The output voltage of the oscillator 8 is applied to a threshold
means 11 with a threshold which supplies an output signal L during
the normal operation of the oscillator 8 and an output signal O
when the threshold of the oscillator output voltage is not
reached.
The output of the threshold means 11 is applied to one input of an
AND gate 12.
The capacitive probe 4, which is according to the invention
structurally combined with the inductive probe 3, is connected with
the housing 1 as counter-electrode to a circuit responding to
changes in the capacitance, briefly referred to as capacitance
measuring circuit 10; this circuit has also a threshold so that it
delivers under normal conditions an output signal O and supplies an
output L' only when a certain increase in the capacitance of the
probe 4 is present. The output of the circuit 10 is applied to the
other input of the AND gate 12. Preferably, the capacitive probe 4
is supplied by an oscillator with a frequency of several Mc/s.
It is essential for the invention that during the approach of a
metal object to the key portion of the electronic switch
arrangement, according to FIGS. 1 and 2 the threshold in the
threshold means 11 is not reached, before the threshold in the
capacitance measuring circuit is exceeded.
Hence, the output 15 of the AND gate 12 delivers an output signal
L" only if an L signal and an L' signal is present at its inputs.
In all other cases, the output 15 carries the output signal O.
The operation of the electronic switch shown and explained in FIGS.
1 to 3 is as follows:
If a metal part is brought into the proximity of the key according
to FIGS. 1 and 2, the oscillator 8 is damped and the output voltage
of the oscillator 8 drops below the threshold of the gate 11, so
that the corresponding input of the AND gate 12 receives the input
signal O. Hence, the output 15 will deliver the signal O
irrespective of the output of the capacitance measuring circuit 10.
In other words, the switch will not be actuated even by approaching
the metal part further to the key.
In the case that an insulating material, such as plastic, wood,
glass, leather, paper or the like is approached to the key, the
output voltage of the oscillator 8 does not drop below the
threshold of the gate 11, so that the corresponding input of the
AND gate 12 delivers the signal L even with the insulating material
in close proximity. Since according to the invention the threshold
of the capacitance measuring circuit 10 is so high that the
comparatively low increase in the capacitance, caused by the
insulating material, cannot lead to the threshold being exceeded,
the second input of the AND gate 12 receives the signal O so that
also in this case the output signal will be O. Also in this case,
the switch does not respond.
If a human hand 6 approaches the switch, the resulting small
damping of the oscillator 8 owing to the correct setting of the
threshold in the threshold means 11 does not cause the oscillator
voltage to drop below this threshold. The output signal of the
threshold means 11 remains, therefore, L. On the other hand, the
approach of the hand 6 causes the capacitance of the probe 4 to
increase to such an extent that the threshold of the capacitance
measuring circuit 10 is exceeded, and the output of the circuit
delivers the signal L'. Since now inputs of the AND gate 12 receive
the signals L and L', the output 15 delivers the signal L" and the
switch according to the invention responds.
The switch may be actuated by human hand 6, even if the hand is
protected by a strong leather glove, such as in the case, e.g., of
processing sheet metal. The two fields of the switch according to
the invention penetrate such a leather glove.
A substantially simplified circuit, operating with only one
oscillator whilst still ensuring the logic interconnection required
according to the invention, is shown in FIG. 4. In this
construction wth winding 3 forming the inductive probe is divided
into two partial windings of which one forms together with a
capacitor 16 the oscillating circuit of an oscillator 17 tuned to a
frequency of several kc/h, whilst the other supplies the voltage
for the capacitive probe 4. The partial winding supplying the
voltage to the probe 4 is connected to a capacitance current or
voltage converter 19, which is followed by a trigger 20 whose
output 21 supplies normally the output signal O, but which supplies
an output signal L" when the output current or the output voltage
of a converter 19 exceeds the threshold of the trigger 20.
The operation of this circuit is as follows:
The capacitance converter 19 operates in its linear range in
accordance with the following relation:
A = k.U..omega..C.
In this formula
A = the output direct current or the output d.c. voltage of the
converter,
k = a constant,
U = the amplitude of the supply a.c. voltage (which is, therefore,
proportional to the output voltage of the oscillator),
.omega.= the circuit frequency,
C = the capacitance of the probe 4 against earth.
As may be seen from the preceding equation, the approach of a
smaller or larger metal component to the key containing the probes
3 and 4 will give rise to an increase in the capacitance, but
simultaneously the oscillator damping will cause the voltage U to
drop. In other words, the sensitivity of the converter 19 is
strongly reduced, so that the threshold of the trigger 20 cannot be
exceeded even with a strong increase in the capacitance. In the
presence of very large metal parts, the oscillation of the
oscillator may even collapse completely, so that the output of the
converter 19 no longer presents an output signal.
Thus the logic connection required for the approach of a metal part
is assured.
If an insulating part is brought near the key 3, 4, the resulting
small rise in the capacitance is insufficient to overcome the
threshold in the trigger 20.
Only if a hand is placed near, as shown in FIG. 2, the capacitance
of the probe 4 is increased to such an extent that the threshold in
the trigger 20 is overcome with small damping of the oscillator,
and the output 21 delivers the signal L".
The function of the simplified circuit of FIG. 4 corresponds,
therefore, to that of the basic circuit explained with reference to
FIG. 3.
A detailed circuit diagram is shown in FIG. 5.
The oscillator 17 consists substantially of a transistor 22, the
collector of which is connected with the winding 3, forming an
inductive coupling winding 23 of the inductive probe. The
oscillating circuit determining the frequency is formed by the
winding 3 and the capacitor 16.
The regenerative feedback is effected by a further winding 25 and a
variable resistor 26 serving for adjusting the regenerative
feedback factor. A capacitor 27 mounted between resistor 26 and the
emitter produces the galvanic decoupling.
Resistors 28, 29 and 30 adjust the working point of the transistor
22, whilst a capacitor 31 connects the base of the transistor 22,
operating in a base circuit, to earth. The arrangement is supplied
by a d.c. voltage source, shown at +/-.
The winding 3 is divided into two partial coils by a tap 24. The
capacitive probe 4 is connected to the lower end of the upper
partial coil, whilst the upper end of the upper partial coil is
earthed through a trimming capacitor 38.
The capacitance converter 19 is realized by a known diode pump
consisting of diodes 32, 33, a resistor 35, and a capacitor 36. By
means of a capacitor 37, the base of the diode is earthed. The
diode pump is connected to the tap 24 of the coil 3.
The capacitor 38 is so chosen and adjustable that the initial
capacitance of the probe 4 can be neutralized.
Adjacent to the diode pump is a filter member consisting of a
resistor 39 and a capacitor 40. The d.c. voltage produced in this
manner is applied to the non-inverting input of an operational
amplifier 41 which is fed back by means of resistors 42, 43, and 44
in such a way that it behaves like a trigger. The inverting input
of the operational amplifier 41 is connected to the tap of a
potentiometer 46 through a resistor 45 which is necessary for
reasons of compensating the offset. According to the invention, the
adjustment of the potentiometer 46 can be used for adjusting the
threshold of the trigger formed by the operational amplifier
41.
Zener diodes 47, 48 and a series resistor 49 in the voltage lead
serve to stabilize the supply voltages of the circuit. The output
of the trigger is connected to a voltage divider consisting of
resistors 50, 51 through which the base of a switching transistor
52 is controlled. This transistor controls directly a relay 53. A
diode 54 in the collector circuit of the transistor 52 serves to
chop inductive voltage peaks.
The capacitance converter may also realize by a circuit other than
a diode pump, e.g. the output voltage of the capacitive probe can
be rectified as a function of the phase.
The embodiment of FIGS. 6 and 7 shows a further important
possibility of simplifying the electronic switch according to the
invention, wherein the intrinsic capacitance of the winding 3 is
used at least partially for forming the capacitive probe 4. The
fact that the intrinsic capacitance of the winding 3 in FIG. 6 is
used partially, is indicated in the drawing by marking the relevant
part 3a of the winding additionally by a dotted line loading to
reference numeral 4.
The winding 3 is arranged in the FIG. 6 embodiment in a high
frequency iron pot core 7, in which a tap is provided between the
outer partial winding 3b and the larger inner partial winding
3a.
It is an essential feature that the larger partial winding 3a,
serving as capacitive probe 4, is arranged in space
non-symmetrically to the remainder of the winding 3b, so that the
changes in the intrinsic capacitances of the partial windings 3a
and 3b, caused by approaching objects, are non-proportional,
because otherwise the capacitance converter 19 connected to the
windings would deliver no signal in the case of changes in the
capacitance.
Even more favourable than the arrangement shown in FIG. 6 is the
configuration of the partial winding 3b as a disk-shaped winding on
the bottom of the pot core 7.
A preferred, particularly simple circuit for connecting the switch
of FIG. 6 is shown in FIG. 7. Here again, the capacitor 16 forms
with the winding 3 the oscillating circuit of an oscillator, not
shown. The transistor 22, the diode 13, the capacitor 14 and the
output resistor 9 form a capacitance converter 19. Between the
voltage terminals +/-, there is a capacitor 57, forming a high
frequency short circuit. In this circuit, a change of the
capacitance of the partial winding 3a relative to earth brings
about a change of the current through the resistor 9.
The lower end of the partial winding 3b is connected to earth
through a trimmer capacitor 58, which is constructed and adjustable
in such a manner that the initial capacitance of the coil 3 can be
neutralized. The output 59 of the circuit may be applied to a
trigger 20 as shown in FIGS. 4 and 5.
* * * * *