WO1999048207A1

WO1999048207A1 - Proximity switch and actuator using such a switch

Info

Publication number: WO1999048207A1
Application number: PCT/IB1999/000399
Authority: WO
Inventors: Georges Gerber; Jean-François Pfister
Original assignee: Sonceboz S.A.
Priority date: 1998-03-13
Filing date: 1999-03-11
Publication date: 1999-09-23

Abstract

The invention concerns a proximity switch supplying an electric signal which is a function of the presence of a reference element to be detected, in conformity with the NAMUR standard, comprising a sensor associated with an electronic processing circuit. The invention is characterised in that the sensor consists of an optical detector, such as an optical barrier, including a light source and a light-sensitive detector, and the electronic processing circuit includes a FLIP-FLOP circuit whereof the output controls a transistor such that the circuit equivalent resistance varies between a low value and a high value depending on whether the element to be detected is present or not.

Description

PROXIMITY SWITCH AND ACTUATOR IMPLEMENTING SUCH A SWITCH.

The present invention relates to a proximity switch.

Many embodiments of proximity switches are known in the state of the art. American patent US5623963 describes for example a valve position indicator, comprising a proximity switch respecting the so-called NAMUR standard resulting from the work of the "Standards working Group for Instrumentation and Controls" promulgated as a standard under the reference "VDI / VDE 3845 ".

These proximity switches or detectors generally use inductive detectors.

Such a detector comprises in the prior art an analog oscillator provided with an oscillating circuit LC which has an equivalent loss resistance R, and which is excited by a current source. The oscillator is arranged to translate the approach of a metallic object into an analog variation of a quantity characteristic of the oscillation, particularly a variant of the amplitude of the latter resulting from the modification of the resistance of equivalent losses R.

Such switches are for example described in European patent EP0062597.

These sensors have various drawbacks. First, they are sensitive to electromagnetic interference. They are therefore not entirely suitable for use in highly noisy industrial conditions.

Furthermore, the manufacturing cost of such proximity switches is relatively high. The object of the present invention is to provide a proximity switch capable of comply with the NAMUR standard, lower manufacturing cost than inductive sensors and lower electromagnetic sensitivity.

To this end, the invention firstly relates to a proximity switch capable of supplying an electrical signal which is a function of the presence of a reference element to be detected, capable of complying with the NAMUR standard, of the type comprising a sensor associated with a electronic processing circuit, characterized in that the sensor consists of an optical detector, for example an optical barrier or a reflection detector, comprising a light source and a photodetector, and in that the electronic processing circuit comprises a FLIP circuit -FLOP whose output controls a transistor so that the equivalent resistance of the circuit varies between a low value and a high value depending on the presence or not of the reference element to be detected.

Advantageously, the optical barrier comprises a light source supplied by a frequency generator, and in that the processing circuit is synchronized with said frequency generator.

According to a particular variant, the optical barrier comprises a light source supplied by a frequency generator producing a square signal with a frequency of approximately 180 Hz.

According to a preferred embodiment, the equivalent resistance of the circuit varies between a minimum value of approximately 0.9K and a maximum value of approximately 11K depending on the position of the reference element to be detected.

According to a first variant, the optical detector is formed by an optical barrier comprising a light-emitting diode and a phototransistor and by a pin secured to the movable member. According to a second variant, the optical detector is formed by an optical element comprising a light-emitting diode and a phototransistor, the mobile member acting as a reflector. According to a particular application, the invention relates to an electric actuator comprising a member movable in translation, actuated by an endless screw, and an end-of-travel detector constituted by a proximity switch, characterized in that said proximity switch is formed by a sensor the sensor constituted by an optical detector, for example an optical barrier, comprising a light source and a photodetector associated with an electronic processing circuit comprising a FLIP-FLOP circuit the output of which controls a transistor so that the resistance equivalent of the circuit varies between a low value and a high value depending on the presence or not of the reference element to be detected.

Advantageously, the movable member comprises a reference element which is positioned in the optical barrier in the end-of-travel position.

The invention will be better understood on reading the description which follows, referring to the appended drawings corresponding to a nonlimiting exemplary embodiment where:

- Figure 1 shows a front view, partially broken away, of an actuator according to the invention;

- Figure 2 shows an enlarged view of a detail of the actuator; - Figure 3 shows a view along the cutting plane AA of the actuator; Figure 4 shows the block diagram of the sensor; Figures 5 and 6 show the response curves of the proximity switch 1 according to the invention. Figures 1 to 3 respectively show a front view, partially broken away, of an actuator according to the invention, an enlarged view of a detail of one actuator and a view along the sectional plane AA of one actuator.

The actuator comprises a rotary step-by-step motor (1) driving a worm screw driving a linearly movable member.

A printed circuit (2) supports a component (3) forming an optical barrier, as well as the electronic components of the processing circuit. This component is for example an encapsulated optical fork of the KODENSHI brand, reference SG211.

The movable member (4) includes a reference pin (5) which is inserted in the light barrier at the end of the stroke.

Figure 4 shows the block diagram of the sensor. The circuit comprises on the one hand a stage (10) for supplying a light-emitting diode (11) and on the other hand a stage (20) for processing the signal supplied by the phototransistor (21).

The supply stage (10) is an oscillator producing square signals with a frequency of approximately 180 Hz. The pulses have a duration of approximately 120 μs. The supply stage (10) comprises two NAND gates (12, 13) and a capacitor (14) and a diode (50) defining in a known manner the period of the signal.

The supply of the light-emitting diode (11) by a periodic signal has several advantages.

On the one hand, it reduces the consumption of the circuit, and increases the life of the light-emitting diode (11). It also makes it possible to synchronize the operation of the signal processing stage (20) supplied by the phototransistor (21) and thus eliminate certain disturbances linked to ambient lighting, the frequency of which generally corresponds to the frequency of the electrical network, ie 50 Hz in Europe and 60 Hz in the United States.

The output of the oscillator drives a transistor (15).

The stage (20) for processing the signal supplied by the phototransistor (21) consists of a FLIP-FLOP circuit formed by two NAND gates (21, 22). The output of the FLIP-FLOP circuit controls a transistor (23).

The equivalent resistance of the circuit, measured across the external resistance (30) varies according to the on / off state. The circuit has a resistance of the ON circuit is approximately 0.9K, and an equivalent resistance OFF 11K. The equivalent resistance of the assembly comprising the load resistance of 4.7K is therefore approximately 5.6K and 15.7K respectively. The ON / OFF characteristics can comply with NAMUR standards, as shown in Figures 5 and 6 showing the response curves of the proximity switch according to the invention.

FIG. 5 represents the current measured in the OFF position, corresponding to the pin (5) placed in the light barrier, as a function of the supply voltage.

FIG. 6 represents the current measured at the terminals of the external resistor (30), measured in the ON position, corresponding to the pin (5) placed outside the optical fork, as a function of the supply voltage.

The invention is described in the foregoing by way of nonlimiting example. Those skilled in the art will be able to produce different variants without departing from the scope of the invention.

Claims

R E V E N D I C A T I O N S

1 - Proximity switch capable of supplying an electrical signal which is a function of the presence of a reference element to be detected, capable of complying with the NAMUR standard, of the type comprising a sensor associated with an electronic processing circuit, characterized in that the sensor consists of an optical detector, for example an optical barrier, comprising a light source and a photodetector, and in that the electronic processing circuit comprises a FLIP-FLOP circuit whose output controls a transistor so that the equivalent resistance of the circuit varies between a low value and a high value depending on the presence or not of the reference element to be detected.

2 - Proximity switch according to claim 1 characterized in that the optical barrier comprises a light source supplied by a frequency generator, and in that the processing circuit is synchronized with said frequency generator.

3 - Proximity switch according to claim 2 characterized in that the optical barrier comprises a light source supplied by a frequency generator producing a periodic signal with a frequency of about 180 Hz.

4 - Proximity switch according to at least one of the preceding claims, characterized in that the equivalent resistance of the circuit varies between a minimum value of approximately 0.9K and a maximum value of approximately 11K depending on the position of the reference element to be detected. 5 - Proximity switch according to at least one of the preceding claims, characterized in that the optical detector is formed by an optical barrier comprising a light-emitting diode and a phototransistor.

6 - Proximity switch according to one at least of the preceding claims characterized in that the optical detector is formed by an optical reflection element comprising a light emitting diode and a phototransistor.

7 - Electric sensor comprising a member movable in translation, actuated by an endless screw, and a limit switch detector constituted by a proximity switch, characterized in that said proximity switch is formed by a sensor, the sensor constituted by a optical detector, for example an optical barrier, comprising a light source and a photodetector associated with an electronic processing circuit comprising a FLIP-FLOP circuit whose output controls a transistor so that the equivalent resistance of the circuit varies between a low value and a value high depending on the presence or not of the reference element to be detected.

8 - An electrical sensor according to claim 1 characterized in that one movable member comprises a reference element from being positioned in the light barrier in the end position.