GB2177216A - Proximity switch - Google Patents

Abstract

Generally a non-contacting proximity switch has the following functional units combined into an operational system: a proximity sensor (10) (inductive/capacitive), a circuit part (11) (pulse shaper) acting on the sensor signals, an output stage (13) for forming a usable output signal and, if required, a short- circuit protection circuit (12). The outputs from these functional units, are provided to monitoring circuitry (20) which monitors their operation to provide a monitor output if faulty operation is detected. Further monitoring elements integrated into the switch, such as pressure sensors (33), temperature sensors (32) and moisture sensors 31 can also provide inputs to the monitoring circuitry. The monitoring circuitry (20) may be a hard-wired logic circuit or it may be provided by a programmable processor. <IMAGE>

Description

SPECIFICATION Proximity switch Field of Invention The present invention relates to proximity switches and concerns non-contacting proximity switches.

Background of the Invention Non-contacting switches are sensors which have two clearly defined states. They are normally electronic or associated with electronics and the two states are associated with a "low" or "high" logic signal respectively.

Switching from one state to the other is often used for initiating or indicating a new state in associated machinery, so that such sensors are also called initiators. Initiators are e.g.

used in a production line, where they monitor machine parts and/or sort product parts. Frequently a system depends on the reliable operation of the initiators and an unnoticed functional failure of a single initiator can lead to serious problems. Accordingly it is desirable to increase the operational reliability of the initiator towards high MTBF and/or monitor the initiator for undesired/desired operating states and indicate or eliminate the detected failure.

The present invention concerns the monitoring of a switch, and seeks to provide a noncontacting proximity switch indicating its own behaviour and any failure thereof, in a manner which avoids any active intervention or interference in the operation of the switch.

Summary of the invention According to the present invention there is provided a non-contacting proximity switch with the following functional units combined in operational wiring or circuitry: a proximity sensor (inductive/capacitive), a switch part (pulse shaper) preparing the sensor signals, an Out- put stage for forming a useful signal and with or without a short-circuit protection circuit, characterised in that the circuit parts cooperating an operational wiring or connection are connected in a control circuit integrated into the switch to provide an additional control connection.

The signal output circuitry may be as is used in known short-circuit-proof and pole confusion-proof inductive proximity switches.

The monitoring circuity may supply a signal to the indicator output if there is e.g. a shortcircuit or interruption at the initiator output, an incorrect level at the oscillator output, incorrect switching of the trigger stage, (e.g.

Schmitt trigger), interruption of the supply line, interruption of the test line, etc.

A proximity switch embodying the present invention will normally be completely self-monitoring in both signal output states (loaded and unloaded states). The monitoring ability may be reduced during on-off or off-on switching operations. The time conditions (switching speed and frequency) are generally such that this transition represents no loss of monitoring reliability.

A switch embodying the present invention may be completely self-monitoring, and indicate immediately any malfunction to the monitor indicator output in an active manner, without it being required to do so by an external control and without an external electronic control line.

Embodiments of the present invention, given by way of non-limitative example, will now be described with reference to the accompanying drawings, in which: Figure 1 shows a self-monitoring proximity switch embodying the invention; Figure 2 shows a first embodiment of a monitoring circuit for use in the switch of Fig.

1; Figure 3 shows a second embodiment of a monitoring circuit for use in the switch of Fig.

1; and Figure 4 is a diagram of program structure for the operation of the embodiment of Fig. 3.

Detailed description of the drawings.

A normal, short-circuit and pole confusionproof inductive proximity switch generally comprises an oscillator part, a signal shaped circuit, an output stage and a short-circuit protection. These functional groups are combined in Fig. 1 in a functional block 1. The actual initiator switch is the oscillator part 10. A Schmitt trigger 11 shapes the oscillator signal.

An output stage 13 is controlled by the Schmitt trigger. The output stage is connected to a short-circuit protection circuit 12, which in the case of a short-circuit acts as the signal generator to block the Schmitt trigger. This is shown in simplified form with respect to an exemplified embodiment of a known non-contacting proximity switch.

As an embodiment of the invention, the switch 1 is modified by the provision of a monitoring circuit 20 integrated into the switch and shown in a further functional block 2. It is assumed that for providing the monitor output signal a final or driver stage is required somewhere in the circuit. Although not essential to the invention, such an output stage 21 is also indicated in the functional block 2 for switch monitoring. The self-monitoring proximity switch has, as a result of the provision of the monitoring circuit, a second alarm (monitor) output A2, as well as the conventional switch signal output Al.

Monitoring circuit 20 has a number of monitor input lines El, E2, E3, E4, etc and an output A at which an alarm signal appears in the case of a failure of the proximity switch or the unit (not shown) at output Al. The output Al may therefore indicate a faulty internal operating state of the proximity switch, and additionally may indicate a faulty output, indicat ing e.g. a fault in the system controlled by the initiators, e.g. in a production line. Thus, the proximity switch not only monitors its own internal operation, but also its output activity.

In the arrangement of Fig. 1, the output sig nals of the oscillator unit 10 are supplied by one input El. A further input E2 supplies the output signals of the Schmitt trigger 11 to the monitoring circuit 20 for checking the input to the output stage 13, whilst another input E3 connected to the output of the output stage 13 informs the mounting circuit about the switching state of the proximity switch. The output signal of short-circuit protection circuit 12 indicating whether or not there is a shortcircuit is applied to a further input E4. The way in which these monitor input lines El to E4 and their signals are connected and used in the monitoring circuitry 20 will be explained in greater detail with reference to the embodiments of Figs. 2 and 3, but before this certain comments will be made on the various signal significances.

The bistable output signal of a non-contacting proximity switch is the product of a relatively complicated sensor and signal preparation process in analog circuit technology. It has two single output states, "On" and "Off", which are not able to indicate a malfunction. "On" can be both correct or incorrect, whilst "Off" can be correct or incorrect.

Thus, from the outset of a completely satisfactory statement cannot be expected from the output signal of the switch. Nevertheless reliance is still placed on the states "proximity switch On/Off" for switching process-strategic machines or complete plants if one of the two states occurs at the initiator.

On investigating the switch from the functional standpoint, there are so-called undefined states between individual functional parts which bring about a defined initial state On or Off, e.g. the malfunction of the Schmitt trigger in the case of intact residual functions, etc.

This malfunction cannot be detected from the Schmitt trigger output or from the output of the "stuck" initiator in which the malfunction has ocurred and is only detected when there is an overall operational fault. There is always a lack of information indicating incorrect behaviour when a malfunction occurs.

For the automatic checking or automatic monitoring of the switch, its individual functional parts, which are conventionally only interlinked in operational circuitry, must additionally be linked to monitoring circuitry having the function of representing the truth. This link is realized by means of the monitoring circuit 20, its output A or A2 indicating the truth of the switch operating output Al.

The control wiring in the example of Fig. 1 can use the information values El, E2, E3, E4,...En and the combinations thereof such as E1/E2; E1/E3; E1/E4; E2/E3; E2/E4; E3/E4; E1/E2/E3; E1/E2/E4; E2/E3/E4; E1/E2/E3/E4 etc in a commutative manner.

Not every possible information value need be used for representing and testig for reliable switch operation. In different types of operat ing circuitry for a switch, the information avail able for monitoring correct switch operation will also differ. The following two embodi ments show two possible arrangements of monitor wiring links, and two possible monitoring hardware realizations, one being perma nently hard-wired and the other being installed as a programmable circuit.

Fig. 2 shows a first embodiment for wiring connections for the monitor input signals referred to in conjunction with Fig. 1 on the monitor lines El to E4 (or up to En in the case of further monitoring elements, such as e.g. a thermal sensor 32 and/or a pressure sensor 33 and/or a moisture or humidity sensor 34 as shown in Fig. 1). The output is passed to a window comparator 22 with two windows for permitted L/H ranges for level monitoring of the oscillator signal. However, the level information provides no details of the oscillator dynamics, whose changing states still have to be detected. This takes place in a circuit comprising a monostable 25 acting as a delay and a XOR gate 26. Signals for the two oscillator levels (H and L) should appear alternatively at the output 22A of window comparator 22.The small memory formed by the monostable 25 delay checks whether the level remains high or low and the output 25A of the memory (monostable 25), representing the past state of the oscillator, is the same as the state still being provided at 22A. If the "past" output at 25A is the same as the "present" output at 22A, it is assumed that the oscillator has "stuck" in one level. The function of a subsequent XOR circuit is to establish this. It carries out the comparison, providing at its output 26A a signal 1 for "oscillator oscillates".

In the operational wiring the oscillator signal is supplied to the Schmitt trigger 11, which transforms the oscillator signal into a clearly defined square-wave signal. These signals could be evaluated by use of the monitor lines E1/E2. However, as the Schmitt trigger signal can be directly or inversely represented by the output stage 13 it is possible. as stated, to use another information element, namely E2 for the Schmitt trigger signal and E3 for the amplified Schmitt trigger signal. A signal inversion takes place during the amplification in this example and if this does not take place, the output stage is not working. An XOR circuit 23 is used in the monitor circuit for connection to the monitor input lines E2 and E3 and its output 23A provides the signal 2*XOR3* for "Schmitt trigger signal amplified".

However, this information still provides no details of the dynamics. For example, in the case of a short-circuit, by means of the load "stuck" at the initiator, the Schmitt trigger is stopped by the short-circuit protection circuit 12 (cf. Fig. 1) and the expected inverse state is static, but not incorrect. A similar memory circuit as discussed in conjunction with the oscillator control evaluates a signal from the output of the short-circuit protection circuit 12 to the input E4 of the control circuit 20. E4 leads to a monstable 24, whose second state is kept stable for a fixed time. This dynamic state appears at the output 24A of monostable 24 as signal 4*, which in the case of a short-circuit merely indicates one stable state of the multivibrator.

The signals of outputs 23A, 24A, 26A can be combined to give a signal for "switch function O.K." or "fault". In the case of short-circuit monitoring, this not only relates to the switch alone, but also to its initial state, which can also indicate a fault e.g. in the production line. The signals 1* for "oscil lator running", 2*XOR3* for "Schmitt trigger signal amplified" and 4* for "no short-circuit" are combined in a NAND circuit 27, for output signal A, which only indicates correct operation in the presence of all three correct input signals. The other state of signal A indicates a fault, but without identifying it. However, the system could be modified to identify the fault by providing the individual signals 23A, 24A and 26A as well as the combination signal A.

As an alternative to the discretely constructed hard-wired embodiment of Fig. 2, Fig.

3 shows an "intelligent" embodiment which uses processors and storage or memory means.

On the basis of the same operational wiring for the switch signal as shown in Fig. 1, the monitoring circuiting 20 according to Fig. 3 has the monitor input lines El to E4, which lead to an evaluation circuit which in this case is a digital circuit 30. This circuit, which is represented as a single functional block, has processor and storage means, as well as loadable or resident software. Whereas in the embodiment of Fig. 2 the logic links were realized with discrete switching means, this is not the case with the active microprocessor construction. All static and dynamic states are processed by the processor so that, apart from the unavoidable hardware, the monitoring operation is software-dependent and is consequently much more flexible and able to take account of more different input states, with only insignificantly greater circuitry expenditure.The output of a bus driver 35 leads to an alarm bus receiving, e.g. from the evaluation or digital circuit 30, serial data words, e.g. 8 bit data words, which can indicate the origin of the fault.

The diagram of Fig. 4 shows the structure of a program for the micro-processor, according to which a proximity switch equipped with a microprocessor can be self-monitoring. Initially, i.e. after switching on the operational circuit, an initializing routine is performed and is called INIT. It e.g. comprises erasing the processor's memory, setting registers, resetting the alarm output, outputting error word 0, etc., i.e. the production of a basic state. A subroutine can perform a self-test in order to define the basic State.

From the INIT routine, program operation follows a continuous loop representing a state comparable with the first embodiment, in which the analog connections are connected in parallel with "continuously present" monitor input wiring. Thus, LOOP is equivalent to the simultaneous polling performed in the first embodiment.

Program section 40 reads in the data on monitor input lines El to En, to make available to the processor the values of data items El, E2, E3, E4,. . En and the values of combinations of items, such as: E1/E2; E1/E3; E1/E4; E2/E3; E2/E4 E3/E4; E1/E2/E3; E1/E2/E4; E2/E3/E4; E1/E2/E3/E4 etc. Reference should be made to the description of the embodiment of Fig. 2. Analog input values selected by the program sequence are compared with the conditions which should be fulfilled by the particular values in the following program section 50.

A level or state comparison filters out the faulty or incorrect states. However, unlike in the first embodiment, when individual information input values are combined to test whether a further condition is satisfied, the component values remain available in memory, instead of disappearing in the combination. This makes it possible to identify the origin of the fault, so that only can the faulty state be indicated on the bus output 34, but also the "address" of the fault origin.

Program section 60 finally outputs a fault or error word, e.g. (F7, F6, F5, F4, F3, F2, F1, FO)=(10010001), i.e. the fault state is generated by errors F7, F4, FO. F7 to FO are defined by convention and are linked with the aforementioned monitor input values El to En and their combinations.

An additional determination is possible in program section 40 if e.g. the analog input Ei is compared with a threshold 1 for one state and a threshold 2 for the other state. This makes it possible to also detect the state threshold 1 Ei threshold 2, which is defined as a transition state U, i.e. the state in which the proximity switch switches. If the transition state has just occurred, then there is no fault. However, if the previously detected state indicates that the transition state U is maintained, then there is a fault. Thus the previous correct state must be used in order to perform the fault Yes/No decision. The transition state of the proximity switch was also discussed in conjuction with the embodiment of Fig. 2.

Claims (12)

1. A non-contacting proximity switch hav ing signal output circuitry for providing an out put signal indicating the state of the switch and monitoring circuity which monitors in use the operation of components of the signal output circuitry and provides a monitor indicator output to indicate when incorrect operation of any part of the signal output circuitry is detected.

2. A switch according to claim 1, in which the signal output circuitry comprises an oscillator circuit acting as a proximity sensor, a pulse shaping circuit for shaping the pulses from the oscillator circuit, and an output stage for providing the said output signal, the outputs of each of the proximity sensor, the pulse shaping circuit and the output stage being provided as inputs to the monitoring circuitry.

3. A switch according to claim 2, in which the signal output circuitry also has a shortcircuit protection circuit, the output of which is provided as an input to the monitoring circuitry.

4. A switch according to any of claims 1 to 3, in which the monitoring circuitry comprises hard-wired logic circuitry.

5. A switch according to claim 4 when dependant on claim 3, in which the monitoring circuitry comprises a window comparator the output of which is provided to a first XOR circuit both via a first multivibrator and otherwise to provide a first signal giving information derived from the operation of the oscillator circuit, a second XOR circuit for providing a second signal giving information derived from the operation of the pulse shaper circuit and output stage; a second multi-vibrator providing a third signal giving information derived from the operation of the short-circuit protection circuit; and a combination circuit for combining the said first, second and third signals.

6. A switch according to any one of claims 1 to 3, in which the monitoring circuitry comprises a processor and a memory means.

7. A switch according to any one of the preceding claims, in which the monitor indicator output comprises a signal indicating in which part of the signal output circuitry incorrect operation has been detected.

8. A switch according to any one of the preceding claims, in which one or more further elements are used to provide further inputs to the monitoring circuitry.

9. A switch according to claim 8, in which the said further element or elements comprises a pressure sensor.

10. A switch according to claim 8 or claim 9, in which the said further element or elements comprises a temperature sensor.

11. A switch according to any one of claims 8 to 10, in which the said further element or elements comprise a moisture sensor.

12. A non-contacting proximity switch substantially as herein described with the reference to Fig. 1, Fig. 2, or Fig. 3 and Fig. 4 of the drawings.