GB2075201A - Electronic proximity switch - Google Patents

Abstract

An electronic proximity switch comprises an oscillator O feeding a demodulator D, which feeds a trigger circuit T which in turn feeds output circuitry R and a hysteresis switch H which completes a feedback loop to the oscillator. A piece of metal moving away from the oscillator 0 causes it to start oscillating, and the oscillation is fed via D, T, and H to increase the loop gain so that the piece of metal must be moved by a significant distance towards the oscillator to damp it sufficiently to stop it oscillating. In addition, there is a feedback from the demodulator D to the oscillator O, which has an inherent high loop gain which is reduced by this feedback when the oscillation amplitude reaches a level somewhat below the switching point of the trigger circuit T. This increases the response speed of the circuit when a piece of metal is removed from the oscillator. <IMAGE>

Description

SPECIFICATION Electronic proximity switch The invention relates to electronic proximity switches.

Electronic proximity switches are frequently used in electrical measuring, control, and regulating circuits instead of mechanically operated electrical switches. In general, they consist of an oscillator which is damped by an approaching piece of metal.

A demodulator and a level switch and/or an output stage are connected to the output of the oscillator.

As long as a piece of metal approaching the proximity switch has not yet reached a predetermined distance, the closed loop gain of the oscillator is greater than 1 and the oscillator oscillates. The closed loop gain is given by the product k.V of the coupling coefficient k and the gain V. As the approaching piece of metal reaches a predetermined distance, the increasing damping of the oscillator results in a decrease of the closed loop gain k.V so that this gain finally reaches the value smaller than 1 and the oscillator ceases oscillating. The output stage of the proximity switch then dependent on the oscillating condition of the oscillator attains a condition which is determined by the kind of circuit; that is, the output stage is rendered either nonconducting or conducting.

To adjust the switching hysteresis, which is the difference between the switching distance when removing the piece of metal and, on the other hand, the switching distance when the piece of metal approaches the proximity switch, it is known from German published patent application 1966 178 to reduce from the output stage the emitter resistance of a transistor of an oscillator stage operating in an emitter-based circuit. Thereby the gain of the stage is increased.

Further, it is known from German published application 2461169 to influence the coupling coefficient of the oscillator by means of the output stage. This is achieved by reducing the feedback factor when the oscillator is damped. This measure, compared with the first mentioned measure, has the advantage that the D.C operating point of the amplifier stage remains unchanged.

By determining a switching hysteresis, a clear point of switch-on and switch-off of the proximity switch is given, so that the piece of metal initiating the switching operation even may undergo some oscillation back and forth without the proximity switch being operated several times. However, it has been found that the switching speed of such proximity switches is limited in principle. The reason for this is that, when removing the damping of the oscillator, the amplitude of the oscillation increases exponentially. Therefore, when the piece of metal is removed, some time is required until the proximity switch really switches. If several quickly moving pieces of metal follow each other, the switching point is no longer solely determined by the switching distance.During a counting process it may happen that the next piece of metal already damps the oscillator before the oscillator has responded to the removal of the preceding metal piece.

Therefore, the object of the present invention is broadly to increase the switching speed of known proximity switches.

Accordingly the invention provides electronic proximity switch, comprising an oscillator and a circuit for influencing the loop gain dependent on the switching condition for achieving a hysteresis, including a further circuit which increases the loop gain k.V when the oscillator is damped so as to achieve an oscillation initiating support and switch off this support shortly before reaching the switching point.

Two embodiments of the invention will now be described, by way of example, with reference to the drawings, in which: Figure 1 is a block diagram of both embodiments; Figure 2 is a circuit diagram of the first embodiment; Figure 3 is a circuit diagram of the second embodiment, which is derived from a proximity switch known from German patent application 1966 178; and Figure 4 is a graphical representation of the operation of the embodiments.

In Figure 1, a known electronic proximity switch consists of an oscillator 0, a demodulator D fed from the output of the oscillator, a trigger circuit Tfed with the rectified voltage of the demodulator, and further circuitry R which may include an inverter, an output stage, a switch-on delay circuit and a power stage and which is of no further interest in this connection.

In known manner, trigger T operates a hysteresis switch H which influences the oscillator 0 and increases the loop gain k.V when the proximity switch is switched. This results, in known manner, in that the proximity switch, after having been switched, is not switched back by small movements of the metal piece which initiated the switch. Compared with the initiating switching distance, the metal piece rather must be brought closer by a predetermined amount before the proximity switch is operated in opposite sense.

In the present circuit, in addition to the above features, an oscillating starting circuit A is provided which is operated at a predetermined level of the rectified voltage and before reaching the switching point of trigger T. It reduces the loop gain k.V which, at the beginning, when the oscillator is damped, had been increased.

Figure 2 shows a circuit which comprises, in a series relation, an oscillator 0 comprising a main stage H and an oscillating support stage A, a demodulator D, a hysteresis switch H and a trigger T, as well as further circuitry which is not shown.

The oscillator 0 has a main stage H in the form of a Hartley circuit and essentially comprises an amplifier stage in a collector-based configuration together with a resonant circuit. This resonant circuit consists of a transformer L1 and a capacitor C1, connected in parallel. The transmission relation of transformer L1 is chosen such that a coupling coefficient k greater than 1 is achieved. A central tap thereon is connected via a coupling resistor Rk to the emitter of a transistor 03, forming an amplifying stage. The resonant circuit consisting of L1 and C1 is connected on one side to reference potential, and by means of two transistors Q1 and 02, operating as diodes, is connected on the other side to the base of transistor 03, which is also connected to the supply voltage via a constant current source J 101.The collector of transistor Q3 is directly connected to the supply voltage. Constant current source J 101 as well as further constant current sources used in the circuit can be formed, as well as the rest of the circuit, in the form of integrated circuitry and essentially replace high resistance resistors which are difficult to realize as integrated circuits.

The oscillator support circuit A comprises a first transistor 0102, whose emitter is connected to the emitter of oscillator transistor 03 and whose collector is connected to the tap of transducer L1 via a resistor R1. The base of transistor 0102 is connected to the collector of a transistor 0103 which, in turn, is connected to reference potential via constant current source J1. The emitter of transistor 0103 also is connected directly to the supply voltage.

The demodulator D comprises a transistor Q5, operated in C-mode as an emitter follower. It is connected with its base to the emitter of oscillator transistor Q3. The collector of transistor 05 is connected directly to the supply voltage and the emitter of this transistor is connected to reference potential via a constant current source J2. On the other side, the emitter of transistor 05 is connected to the supply voltage and the emitter of this transistor is connected to reference potential via a constant current source J2. On the other side, the emitter of transistor 05 is connected to the bases of two transistors 06 and 09 via a resistor R2.Transistor Q6 operates as the oscillator support circuit and has its collector connected via a constant current source J102 to the supply voltage and to the base of transistor 0103. The emitter of transistor OS is connected to reference voltage via a resistor R3.

Transistor Q9 has its collector connected via a constant current source J103 to the supply voltage and has its emitter connected to reference voltage via a resistor R6. A capacitor C2 is connected between the collector of transistor 09 and reference voltage.

Hysteresis switch H consists of a transistor 011 which has its collector connected to the supply voltage and its emitter connected to the central tap of transducer L1 via a hysteresis resistor Rh. The base electrode of transistor is controlled by the following trigger circuit T.

The trigger circuit T comprises essentially a differential amplifier consisting of two transistors Q12 and Q14, the emitter electrodes of which are commonly connected via a constant current source J3 to reference voltage. The collector electrodes are connected to the supply voltage via a current mirror 0108. The base electrode of transistor 012 is connected via a resistor R8 to capacitor C2 and is connected via the emitter/collector path of transistor 0107 to reference potential. The base electrode of transistor 012 is connected to the supply voltage via the collector/emitter path of a transistor 0106 and a resistor R9. Transistor Q106 has two collectors, the second collector being connected to the base of transistor 011 of the hysteresis switch H.The collector of transistor 012 is connected to the base of transistor Q106 and to the residual circuit via a current mirror 0109 which is connected in series with a further constant current source J4. The base of the second transistor Q14 of the differential amplifier is connected to the supply voltage via a constant current source J 104 and to the reference voltage via two transistors 015 and 016, connected in series and operated as diodes.

The operation of this circuit is as follows: At first, the condition is considered in which the oscillator is completely damped by a piece of metal inserted into the magnetic field of transducer L1. If such a piece of metal is brought close enough to the proximity switch, any oscillations of the oscillator are interrupted. The oscillator transistor Q3, however, is rendered conducting for DC since, via diodes Q1 and Q2, a positive potential is applied to the base in relation to the emitter. Simultaneously, transistor Q102 is conducting and transistor Q103 is nonconducting.By means of the parallel circuit of resistor R1 and resistor Rk, a relatively small total resistance is achieved and a relatively high degree of coupling is present between the resonant circuit L1, C1 and the oscillator transistor 03. The loop gain in total is very high, so that the oscillator amplitude increases rapidly if the piece of metal is removed.

When the oscillator is damped and the proximity switch is not operated, the hysteresis switch H, however, is switched on by transistor 011 being conducting and thereby switching hysteresis resistor Rh in parallel to the oscillator. Transistor 011 is conducting for the following reasons. By means of the two transistors Q15 and 016, operating as diodes, and by transistor 0107, capacitor C2 is charged to about 1.8 V when the trigger is not switched. At the base electrode of transistor 014, there is a potential of about 1.2 V. At the base of transistor Q12, however, a potential of 1.8 V or even higher is present. This means that transistor 012 in the differential amplifier is conducting and it switches, by means of its collector potential, transistor Q106 to the current conducting condition.Thereby, transistor Q11 is switched to condution by means of the potential atthe collector of transistor 0106.

Thereby the condition is reached that all three resistors Rk,R1 and Rh are switched into the circuit.

The insertion of hysteresis resistor Rh in addition to the coupling resistor Rk is known in the art. In the present arrangement, however, by connecting resistor R1 in parallel to coupling resistor Rk, the loop gain is increased when the oscillator is damped.

Therewith, the proximity switch can respond more rapidly when the piece of metal is removed, since the oscillator starts oscillating in a faster manner and already with a small switching distance has a substantial amplitude.

Now it wil be considered what happens when a damping piece of metal is removed. The quickly increasing oscillating amplitude at the emitter of the oscillator transistor Q3 is rectified by the demodulator transistor 05, operating in the C-mode. It is connected via resistor R2 to the bases of transistors Q6 and Q9. By appropriate selection of the two constant current sources J 102 and J 103, and by using different values for the emitter resistors R3 and R6, transistor Q6 reduces its collector potential quicker then transistor Q9. Thus transistor Q1 03 in the oscillating support circuit A is switched before trigger circuit T switches.When transistor Q1 03 is rendered conducting, transistor Q102 is switched off and - without considering hysteresis resistor Rh only coupling resistor Rk is effective. Because of the reduced coupling coefficient k, the loop gain is thereby reduced.

When the oscillator AC output increases further, transistor Q9 also switches, whereby capacitor C2 is discharged. This capacitor previously was connected to a voltage of about 1.8 V. When the voltage across capacitor C2 is reduced so far that the potential at the base of transistor Q12 falls below 1.5 V, the differential amplifier is switched into the opposite state and transistor Q1 4 is rendered conducting. Now the base of transistor Q106 is no longer negative with respect to its emitter so that this transistor Q1 06 and the following transistor Q11 of the hysteresis switch are switched off. This switching off of transistor Q11 and therewith the removal of hysteresis resistor Rh from the circuit leads to again increasing the loop gain.

Referring now to Figure 3, the switching circuit is known as such from German published patent application 1966178 in so far as components are concerned which are labelled with numbers. Only componentsTs1, Rp, Ts2 and Rs are added for present purposes.

In this basic structure, the proximity switch of Figure 3 consists of an oscillator 6 which is damped by a piece of metal (not shown). Further, there is an electronic switch 7, operated by oscillator 6, and further there is a supply circuit 8 for generating the supply voltage for oscillator 6. A switching amplifier 9 is provided between oscillator 6 and electronic switch 7 and, dependent on the conditions of oscillator 6, switches the electronic switch 7 on or off. The oscillator of this inductive proximity switch has an oscillator transistor 10, operating as an emitter-based circuit. In the collector circuit of oscillator transistor 10 there is provided a resonant circuit 11, consisting of a coil 12 and a capacitor 13. A resistor 14 is provided in the emitter circuit of oscillator transistor 10.

In the present circuit, a series circuit consisting of resistor Rp and a switching transistorTsl is connected in parallel to resistor 14. In the base circuit of oscillator transistor 10, a feedback inductance 15 is provided which is connected between the base of oscillator transistor 10 and the junction of two resistors 16 and 17, forming together a voltage divider 18. A by-pass capacitor 19 is connected in parallel to resistor 16.

The oscillator voltage is derived from the collector of oscillator 10 by means of a coupling capacitor 20.

It is then rectified by means of a transistor 21, operated as a diode, and it is then smoothed by means of the following smoothing transistor 22, connected as an emitter follower stage having a smoothing capacitor 23 in its emitter circuit.

When the oscillator 16 is oscillating, the gain V of the oscillator is increased by means of the rectified oscillator voltage in relation to the gain V of the non-oscillating oscillator. This is achieved in that the gain V of the oscillator 6 can be changed by changing the relation of the collector resistance of the oscillator transistor 10 with respect to its emitter resistance. In detail, connected in parallel to the emitter resistor 14 of oscillator transistor 10 is a serial circuit consisting of an adjustable auxiliary resistor 24, a diode and a collector/emitter path of a control transistor 26. The base of control transistor 26 is controlled by the rectified oscillator voltage.

In the present circuit, a further transistor Ts2 is provided which has its emitter connected to reference voltage and its collector connected via a resistor Rs to the supply voltage. The base of this transistor Ts2 is directly connected to the smoothing capacitor 23, whereas the base of control transistor 26 is controlled from the charged smoothing capacitor 23 via a voltage divider. The collector of transistor Ts2 is connected to the base of transistor Ts1.

Finally, the collector of control transistor 26 is connected to the mains voltage via a collector resistor 27 and a bridge rectifier 28.

The operation of the circuit of Figure 3 will now be described. As long as a piece of metal (not shown) is still remote from the oscillator 6 of the inductive proximity switch, then at a predetermined distance the damping of the oscillator 6 is so small that the oscillator oscilltes. The rectified oscillating voltage controls the base of control transistor 26 in such a manner that this transistor is rendered conducting.

The series circuit consisting of auxiliary resistor 24, diode 25 and the conducting collector/emitter path of control transistor 26 is thereby connected in parallel to the emitter resistor 14 of oscillator transistor 10.

Thus the effective emitter resistance of oscillator transistor 10 is smaller than the emitter resistor 14 and the gain V of oscillator 6 is higher, compared with the condition when the emitter resistor 14 alone forms the effective emitter resistance of oscillator transistor 10.

Because of transistor Ts2 being conducting when the oscillator oscillates, the transistor Ts1 is switched off, and resistor Rp is not connected in parallel to emitter resistor 14. This circuit therefore does not influence the gain V when the oscillator is not damped.

If now the piece of metal comes closer to the oscillator than a predetermined distance, the damping of oscillator 6 becomes so high that the rectified voltage is no longer sufficient to render conducting the control transistor 26. Now emitter resistor 14 alone forms the effective emitter resistance of oscillator transistor 10, and the gain V of oscillator 6 is smaller than in the case when the oscillator is not damped. If the oscillator is damped still more, the rectified voltage is no longer sufficient to keep transistor Rs2 conducting. At this point, transistor Ts1 is switched on and resistor Rp is connected in parallel to emitter resistor 14. The result of this is that the gain V again is increased with an accordingly damped oscillator 6, so that in the case of a rapid removal of the piece of metal the proximity switch can switch rapidly since it is brought to a predeter mined minimum amplitude rapidly. Thus in the case of a rapid movement of the piece of metal, the proximity switch switches almost without delay at a predetermined distance of the piece of metal.

In both the embodiments shown, the hysteresis provided in well known manner is not influenced by the oscillating initiating support since this oscillating initiating support circuit A is switched off before the proximity switch reaches its switching point. Since the oscillation initiating support and the hysteresis are switched on and off at different degrees of damping for the oscillator, the circuit may also be used for providing a proximity switch with different switching levels. This may be used for instance for generating a pre-indication before a final switching off. Further, the oscillating initiating support circuit may also be used as an operation monitoring circuit for the oscillator Figure 4 is a diagram of the function of the circuit units. The amplitude U of the oscillator voltage is shown in dependence on the distance S.Curve I shows the increase of the oscillator amplitude in relation to the switching distance for the case that only the coupling resistor Rk and the hysteresis resistor Rh (Figure 2) or the emitter resistor (Figure 3) are present. Curve II shows the oscillator amplitude in relation to the switching distance in the case when the hysteresis resistor Rh (Figure 2) is removed from the circuit or the auxiliary resistor 24 (Figure 3) is connected in parallel to emitter resistor 14. Curve Ill finally shows the oscillator amplitude for the case where resistor R1 is connected in parallel to coupling resistor Rk (Figure 2) or where resistor Rp is connected in parallel to emitter resistor 14 (Figure 3).

Therefore, it can be seen that, with complete damping of the oscillator and subsequent removal of the piece of metal, the oscillator starts oscillating to a predetermined amplitude IV already at a small distance S of the metal piece along line Ill. When the amplitude reaches a predetermined value, resistor R1 or Rp is removed from the circuit as explained above so that now the oscillating amplitude increases along line Ito the switching point S1 of the trigger. At this instant, the hysteresis switch becomes effective, which means hysteresis resistor Rh is removed from the circuit according to Figure 2 or the auxiliary resistor 24 is connected in parallel to emitter resistor 14 in Figure 3. The oscillating amplitude jumps from switching point S1 to curve II and then rises along line II. If now the metal piece again is brought closer to the proximity switch, the oscillating amplitude of the oscillator is damped according to line II, and the proximity switch switches at switching point S2. Subsequently, line IV determines the attenuation of the oscillating amplitude and further to the complete damping line Ill is applicable.

Claims (10)

1. Electronic proximity switch, comprising an oscillator and a circuit for influencing the loop gain dependent on the switching condition for achieving a hysteresis, including a further circuit which increases the loop gain k.V when the oscillator is damped so as to achieve an oscillation initiating support and switch off this support shortly before reaching the switching point.

2. Proximity switch according to Claim 1, wherein the further circuit influences the gain factor V of the oscillator.

3. Proximity switch according to Claim 1, wherein the further circuit influences the coupling coefficient k of the oscillator.

4. Proximity switch according to Claim 2, comprising an oscillator transistor, operating an emitterbased circuitry, the emitter resistor of which determines the gain factor, wherein a series circuit consisting of a resistor and a switch is connected in parallel to the emitter resistor, such that the switch is closed when the oscillator is damped and is opened by the rectified oscillator voltage before the switching point of the proximity switch is reached.

5. Proximity switch according to Claim 3, com- prising an oscillator transistor, operated as an emitter-follower which, via a coupling resistor, is connected with its emitter to a resonant circuit for influencing the coupling coefficient k, wherein the series circuit, consisting of a resistor and a switch, is connected in parallel to the coupling resistor, such that the switch is closed when the oscillator is damped, and is opened by the rectified oscillator voltage before the switching point of the proximity switch is reached.

6. Proximity switch according to Claim 4, comprising a demodulator connected to the output of the oscillator, and wherein the rectified voltage via a voltage divider controls an output transistor, the base electrode of a transistor is connected to a higher potential of the voltage divider with said transistor being connected between the supply voltage lines, and the collector of this transistor is connected to the base of a further transistor acting as a switch.

7. Proximity switch according to Claim 5, comprising a demodulator connected to the output of the oscillator, and wherein the rectified voltage switches a trigger, the demodulated voltage is applied to the base electrode of a transistor the collector of which is connected to the base of a further transistor and said further transistor is connected with its collector to the base of a transistor used as a switch.

8. Proximity switch according to Claim 7, wherein the demodulated voltage is applied to the base electrode of a further transistor and a capacitor is connected between the collector of this transistor and a reference voltage, with said capacitor being charged to a predetermined voltage level when the oscillator is damped and being discharged through said transistor until reaching the switching level of the trigger circuit when the oscillator is not damped.

9. Proximity switch according to Claim 8, wherein the two transistors receiving the demodulated oscillator voltage are rendered conducting and nonconducting at different instants by means of either constant current sources in their collector circuits delivering different currents or by different emitter resistors, such that the transistor which switches off the oscillating support circuit is rendered conducting before the transistor, which switches the trigger circuit and is rendered non-conducting after this transistor.

10. Proximity switch substantially as herein described and illustrated.