Contact-free operating electronic switchgear

A contact-free operating electronic switchgear, having an oscillator capable of being externally affected, a switching amplifier with at least two operating units disposed downstream of the oscillator, an electronic switch, which can be controlled by the oscillator via the switching amplifier, and a supply circuit for the provision of the supply voltage for the oscillator and for the switching amplifier, is shown and described. In electronic switchgear operating contact-free in accordance with the invention the maximum switching frequency is considerably increased in that a control circuit is provided between the two operating units of the switching amplifier and the oscillator and that the oscillator is controlled by means of the control circuit in such a way that the oscillator also oscillates in the affected state.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a contact-free operating electronic switchgear, 
having an oscillator capable of being externally affected, a switching 
amplifier with at least two operating units disposed downstream of the 
oscillator, an electronic switch, e.g. a transistor, a thyristor or a 
triac, which can be controlled by the oscillator via the switching 
amplifier, and a supply circuit for the provision of the supply voltage 
for the oscillator and for the switching amplifier. 
2. The Prior Art 
Electronic switchgear of the type basically addressed herein is 
contact-free and has been increasingly used for approximately twenty years 
in place of electrical, mechanically activated switchgear with contacts, 
in particular in connection with electrical or electronic measurement, 
open or closed loop control circuits. This is true in particular for 
so-called proximity switches, i.e. for electronic switchgear which 
operates contact-free. An indication is given by means of such proximity 
switches whether an activating element, to which the corresponding 
proximity switch is sensitive, has come sufficiently close to the 
proximity switch. If an activating element, to which the corresponding 
proximity switch is sensitive, has come sufficiently close to the 
proximity switch, the electronic switch is reversed. In a switchgear 
acting as a closing element the non-conducting electronic switch now 
becomes conducting, while in a switchgear acting as an opener the 
conducting electronic switch now inhibits. (By means of a switch-gear of 
the type under discussion it is also possible to indicate whether a 
physical quantity of an actuation medium to which the switchgear is 
sensitive has reached a corresponding value.) The oscillator which be 
externally affected as an essential component of electronic switchgear. 
In regard to the mode of affecting the oscillator, differentiation between 
inductive and capacitive control is made. In connection with an 
electronic, contact-free switchgear with inductive control of the 
oscillator, it is true for the oscillator, as long as a metallic part has 
not yet reached a preselected distance, that K.times.V=1, with K=feedback 
factor and V=amplification factor of the oscillator, i.e. the oscillator 
oscillates. When the respective metallic part has reached the pre-selected 
distance, the increasing damping of the oscillator leads to a reduction of 
the amplification factor V, so that K.times.V&lt;1, i.e. the oscillator 
ceases to oscillate. 
In connection with an electronic, contact-free switchgear with capacitive 
control of the oscillator, it is true for the oscillator, as long as an 
actuating body has not sufficiently increased the capacitance between an 
actuating electrode and a backplate electrode, i.e. has not reached a 
pre-selected distance, that K.times.V&lt;1, i.e. the oscillator does not 
oscillate. When the actuating body has reached the pre-selected distance, 
the increasing capacitance between the actuating electrode and the 
backplate electrode leads to an increase in the feedback factor K, so that 
K.times.V=1, i.e. the oscillator begins to oscillate. In both embodiments, 
inductive proximity switch and capacitive proximity switch, the electronic 
switch, e.g. a transistor, a thyristor or a triac, is controlled depending 
on the different sides of the oscillator. 
In the beginning electronic contact-free switchgear was subject to a number 
of problems, in comparison with electrical mechanically actuated 
switchgear, namely among others "Provision of a Supply Voltage for the 
Oscillator and the Switching Amplifier", "Design of the Oscillator", 
"Resistance to Short Circuits" and "Activating Pulse Prevention". 
Addressing these problems and their solutions (as well as other problems 
and their solutions relating to electronic contact-free switchgear) are, 
for example, German Non-examined or Examined Published Applications or 
Patent Nos. 19 51 137, 19 66 178, 19 66 213, 20 36 840, 21 27 956, 22 03 
039, 22 03 040, 22 03 906, 23 30 233, 23 31 732, 23 56 490, 26 13 423, 26 
16 265, 26 16 773, 26 28 427, 27 11 877, 27 44 785, 29 43 911, 30 04 829, 
30 38 102, 30 38 141, 30 38 692, 31 20 884, 32 05 737, 32 09 673, 32 14 
836, 32 38 396, 33 20 975, 33 26 440, 33 27 328, 33 27 329, 34 20 236, 34 
27 498, 35 19 714, 35 29 827, 36 05 199, 36 05 885 and 36 38 409. 
In connection with electronic switchgear, which can be connected via an 
external conductor with one terminal of a supply voltage source and only 
via another external conductor with a connection of a consumer, the 
provision of the supply voltage or supply current for the presence 
indicator and for the switching amplifier is not without problems, because 
the supply voltage or the supply current must be provided in the 
conducting state as well as in the inhibited state of the switchgear. 
It is of no consequence whether the provision of a supply voltage or the 
provision of a supply current is addressed. Here, provision represents 
derivation from the voltage drop occurring at the switchgear, or from the 
operating current conducted via the switchgear (conducting state), or from 
the operating voltage present at the switchgear or from the residual 
current flowing across the switchgear (inhibited state). Therefore it is 
of no consequence whether the provision of a supply voltage or a supply 
current is addressed, because the oscillator and the switching amplifier 
of course require a supply voltage and a supply current. 
Based on its operation as switchgear, practically no voltage drop should 
occur in the switchgear herein discussed in the conductive state and 
practically no residual current should flow in the inhibited state. 
However since, if no voltage drop is allowed to occur in the conductive 
state in switchgear with only two external conductors, no supply voltage 
for the oscillator and the switching amplifier could be obtained and, if 
no residual current is allowed to flow in the inhibited state, no supply 
current could be obtained, it is true for all electronic switchgear with 
only two external conductors that in the conductive state a voltage drop 
occurs and in the inhibited state a residual current flows. 
It follows from what has been stated above that the voltage drop and the 
residual current should be as small as possible, even though in electronic 
switchgear with only two external conductors a voltage drop occurs in the 
conductive state and a residual current flows in the inhibited state in a 
way which is unintentional but necessary for the operation. 
In the beginning it was stated that, among others, a switching amplifier, 
placed downstream from the oscillator, and an electronic switch are 
associated with the electronic switchgear on which the invention is based, 
and that the electronic switch can be controlled via the switching 
amplifier by the oscillator. The term switching amplifier is to be 
understood in a general way and encompasses the entire circuit between the 
signal output of the oscillator and the control input of the electronic 
switch, thus the entire signal transmission path between the oscillator 
and the electronic switch. In the electronic switchgear on which the 
invention is based (see, for example, German Patent No. DE-PS 30 04 829) 
the oscillator can be externally affected, i.e. damped by a metal part and 
the electronic switch is controlled depending on whether the oscillator 
oscillates or not. In practice the evaluation of the oscillating behavior 
of the oscillator is performed by a demodulator (first operating unit of 
the switching amplifier) and by a Schmitt trigger (second operating unit 
of the switching amplifier). The demodulator changes the oscillator 
voltage, i.e. a signal A.C. voltage, to an analog signal D.C. voltage and 
the Schmitt trigger changes a signal D.C. voltage which is analogous to 
the oscillator voltage to a digital output signal at its output with 
which, if required via a further operating unit of the switching amplifier 
or via a plurality of further operating units of the switching amplifier, 
the electronic switch is controlled. If the signal D.C. voltage at the 
signal input of the Schmitt trigger is above a comparison voltage applied 
to a comparison input, the output signal logically is, for example, 1 , if 
the signal voltage lies below the comparison voltage, the output signal 
logically is 0. 
Oscillators require a certain run-up time in order to pass from the 
non-oscillating state to the oscillating state. This run-up time directly 
determines the maximum switching frequency of an electronic switchgear 
having an oscillator which can be affected in the manner described. The 
minimum time intervall between two actuation events of the oscillator must 
not be smaller than the run-up time of the oscillator; if the time 
intervall between two actuation events of the oscillator is smaller than 
the run-up time of the oscillator, the oscillator remains in the 
nonoscillating state. 
Electronic switchgear of the type on which the invention is based is also 
used for counting tasks, so that the maximum switching frequency of such 
an electronic switchgear is of considerable importance. 
SUMMARY OF THE INVENTION 
It is therefore an object of the invention to recite electronic switchgear 
of the type discussed having a considerably higher switching frequency 
than the electronic switchgear on which the invention is based. 
The electronic switchgear by means of which the object previously derived 
and described in detail has been attained is first and most generally 
characterized in that a control circuit is provided between the two 
operating units of the switching amplifier and the oscillator and that by 
means of the control circuit the oscillator is affected in such a way that 
it also oscillates in the affected state. Thus, while in the state of the 
art the oscillator does not oscillate in the affected state, i.e. if it is 
damped by a metal part, in the electronic switchgear according to the 
invention the oscillation of the oscillator is maintained during the 
affected state by means of the control circuit. The result of this is that 
the oscillator does not need to run up again between two actuation events, 
thus the run-up time does not influence the maximum switching frequency of 
the electronic switchgear in accordance with the invention. There are a 
number of different possibilities to realize the general idea of the 
invention previously described and to design and improve the electronic 
switching gear of the invention which is to be described below my way of 
example. 
Basically it could be imagined to regulate the oscillator voltage in the 
electronic switchgear according to the invention, i.e. to assure by means 
of a control circuit expanded into a regulating circuit that the 
oscillator voltage remains the same in the non-affected state of the 
oscillator as in the affected state of the oscillator. It would then be 
necessary to derive from a correcting variable required for such 
regulation whether the oscillator is affected or not affected. 
However, the possibility for realizing the teaching of the invention 
described above has the disadvantage that a relatively large amount of 
energy must be supplied to the oscillator in the affected state, i.e. a 
large power supply is required for the oscillator. But a large power 
supply for the oscillator results in an undesirably large voltage drop 
and/or an undesirably large residual current in an electronic switchgear 
with only two external conductors. Advantageously the control circuit 
between the two operating units of the switching amplifier and the 
oscillator in the electronic switchgear according to the invention is 
designed such that by its influence on the oscillator the signal D.C. 
voltage between the two operating units of the switching amplifier still 
has differentiated values which can be evaluated. 
As known from the state of the art, it is also recommended with the 
electronic switchgear according to the invention to provide the first 
operating unit of the switching amplifier as a demodulator, the 
demodulator having a reference voltage source, a differential amplifier 
and a demodulator condenser connected directly or indirectly to the signal 
output of the differential amplifier. Furthermore it is then possible, 
again as known from the state of the art, for the demodulator to have a 
switching transistor, a load resistor and a constant current network and 
to charge the demodulator condenser with a constant charge current via the 
constant current network and to discharge it with a constant charge 
current. However, it is also possible to connect to the signal output ofo 
the differential amplifier a constant current charge generator and a 
constant current discharge generator and to charge the demodulator 
condenser with a constant charge current via the constant current charge 
generator and to discharge it with a constant discharge current via the 
constant current discharge generator. 
In the electronic switchgear according to the invention the second 
operating unit of the switching amplifier is preferably in the form of a 
Schmitt trigger, as already realized in the state of the art, the Schmitt 
trigger having two comparison inputs, a control input and a signal output 
and the control input of the Schmitt trigger being connected to the 
demodulator condenser.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
The electronic switchgear 1, shown in FIG. 1 by means of a block circuit 
diagram, operates contact-free, i.e. it reacts to, for example, an 
approaching metallic element, not shown and is connected via an external 
conductor 2 with a terminal 3 of an operating voltage source 4 and via 
another external conductor 5 to a connector 6 of a consumer 7, the other 
connector 8 of the consumer 7 being connected to the other terminal 9 of 
the operating voltage source 4. In other words, the switchgear 1 shown is 
connected in a known manner via a total of only two external conductors 2, 
5 to the operating voltage source 4 on the one hand and, on the other, to 
the consumer 7. 
As shown in FIG. 1, the switchgear 1 consists in its basic structure of an 
oscillator 10 which can be externally affected, a switching amplifier 11 
having at least two operating units and placed downstream of the 
oscillator 10, an electronic switch 12, for example a thyristor, 
controllable by the oscillator 10 via the switching amplifier 11, and a 
supply circuit 13 for the provision of the supply voltage for the 
oscillator 10 and the switching amplifier 11. Furthermore, a rectifier 
bridge 14 is provided at the input because the operating voltage source 4 
is an AC voltage source. 
As shown in FIG. 2, a control circuit 15 is provided between the two 
operating units of the switching amplifier 11 and the oscillator 10 and by 
means of the control circuit 15 the oscillator 10 is affected in such a 
way that the oscillator 10 also oscillates in the affected state. Thus, 
while in the state of the art the oscillator 10 does not oscillate in the 
affected state, i.e. if it is damped by a metal part, in the electronic 
switchgear 1 according to the invention the oscillation of the oscillator 
10 is maintained during the affected state by means of the control circuit 
15. The result of this is that the oscillator 10 does not need to run up 
again between tow actuation events, thus the run-up time does not 
influence the maximum switching freuqency of the electronic switchgear 1 
in accordance with the invention. 
In the exemplary embodiment shown in FIG. 2 the first operating unit of the 
switching amplifier 11 is in the form of a demodulator 16, the demodulator 
16 having a reference voltage source, not shown in detail, a differential 
amplifier 17, a demodulator condenser 19 connected to the signal output 18 
of the differential amplifier 17, a constant current charge generator 20 
connected to the signal output 18 of the differential amplifier 17 and a 
constant current discharge generator 21 connected to the signal output 18 
of the differential amplifier 17. The demodulator condenser 19 is charge 
with a constant charge current via the constant current charge generator 
20 and is discharged with a constant discharge current via the constant 
current discharge generator 21. Preferably the constant charge current is 
greater, in particular by a factor of 2 of 3, than the constant discharge 
current. Particular reference is made to German Patent No. DE-PS 30 04 
829, in particular to column 4, lines 17 et. seq.; the contents of the 
disclosure of German Patent No. DE-PS 30 04 829 are hereby specifically 
incorporated by reference and made part of the disclosure in relation to 
and in connection with the electronic switchgear according to the 
invention. 
As further shown in FIG. 2, the second operating unit of the switching 
amplifier 11 is in the form of a Schmitt trigger 22, the Schmitt trigger 
22 having two comparison inputs 23, 24, a control input 25 and a signal 
output 26, and the control input 25 of the Schmitt trigger 22 being 
connected to the demodulator condenser 19. Furthermore a voltage divider 
27 is provided, having three voltage divider resistors 28, 29, 30 and 
being connected to the supply voltage. A comparison input 23 of the 
Schmitt trigger 22 is connected to the connecting point 32 of the voltage 
divider resistor 28, 29 located near the positive terminal 31 of the 
supply voltage, while the other comparison input 24 of the Schmitt trigger 
22 is connected to the connecting point 34 of the voltage divider 
resistors 28, 29 nearest the negative terminal 33 of the supply voltage. 
In the exemplary embodiment shown, the control circuit 15 has an analog 
amplifier 35 with a reference input 36, a control input 37 and a control 
output 38. The reference input 36 is connected to a reference voltage, the 
control input 37 to the demodulator condenser 19 and the control output 38 
to the oscillator 10. 
Furthermore, in the exemplary embodiment shown in FIG. 2 the voltage 
divider 27 has a fourth voltage divider resistor 39 and is connected via 
the fourth voltage divider resistor 39 to the negative terminal 33 of the 
supply voltage. The reference input 36 of the analog amplifier 35 of the 
control circuit 15 is connected to the connecting point 40 between the 
third voltage divider resistor 30 and the fourth voltage divider resistor 
39. FIG. 2 only shows, and this in a very simplified form, the oscillator 
10, the switching amplifier 11 with the two operating units, demodulator 
16 and Schmitt trigger 22, and the control circuit 15 of the electronic 
switchgear 1 shown in FIG. 1. In particular with respect to the oscillator 
10, no details can be seen in FIG. 2, in particular it cannot be seen from 
FIG. 2 that the oscillator 10 has one or more oscillator transistors. As a 
part of the oscillator 10 are only shown an oscillating circuit coil 41, 
an oscillating circuit condenser 42 and an emitter resistor 43 located in 
the emitter circuit of the oscillator transistor, not further shown. In 
this exemplary embodiment the teaching of the invention, according to 
which the oscillator 10 is acted on by the control circuit 15 such that 
the oscillator 10 also oscillates in the affected state, is realized in 
that, by means of the control circuit 15, a control resistor 44 can be 
switched in parallel with the emitter resistor 43 of the oscillator 
transistor. This is accomplished in that the control circuit 15 has 
control transistor 45, the base 46 of the control transistor 45 is 
connected to the control output 38 of the analog amplifier 35 and the 
control resistor 44 can be switched in parallel with the emitter resistor 
43 of the oscillator transistor in a more or less effective manner by 
means of the collector emitter path of the control transistor 45. By "more 
or less effective" is meant that the current flowing via the control 
resistor 44 and therefore the influence on the amplifying factor of the 
oscillator 10 depends on the analog amplifier 35, i.e. it depends on the 
reference voltage at the reference input 36 of the analog amplifier 35 and 
on the signal D.C. voltage at the control input 37 of the analog amplifier 
35, i.e. on the signal D.C. voltage at the demodulator condenser 19. 
Furthermore FIG. 2 shows a preferred embodiment of the electronic 
switchgear shown in FIG. 1 to the extent that a hysteresis resistor 47 and 
a hysteresis transistor 48 are provided. The hysteresis resistor 47 is in 
series with the emitter resistor 41 of the oscillator transistor. The base 
49 of the hysteresis transistor 48 is connected to the signal output 26 of 
the Schmitt trigger 22 and the collector emitter path of the hysteresis 
transistor 48 is switched parallel with the hysteresis resistor 47. For 
details of this reference is made to German Patent No. DE-PS 19 66 178; 
the contents of the disclosure of German Patent No. DE-PS 19 66 178 are 
hereby also incorporated by reference with respect to the electronic 
switchgear 1 according to the invention. 
The exemplary embodiment described above has been given by way of example 
only and other variants and improvements are possible within the scope of 
the invention.