GB2204692A

GB2204692A - Inductive switching apparatus

Info

Publication number: GB2204692A
Application number: GB08708498A
Authority: GB
Inventors: Donald Hardy
Original assignee: CHESFORD ELECTRONICS Ltd
Current assignee: CHESFORD ELECTRONICS Ltd
Priority date: 1987-04-09
Filing date: 1987-04-09
Publication date: 1988-11-16
Also published as: GB8708498D0

Abstract

An inductive switching apparatus comprises an integrated oscillator circuit 1, including three Schmitt triggers 2, 3, 4 connected in cascade, and a tuned circuit 5 driven by the oscillator circuit and including an inductive sensor L1 and a capacitor C1. A detector circuit 7 senses the output of the tuned circuit 5 and is responsive to a change in the output to produce a switching signal when a metal member is brought into proximity with the inductive sensor L1. Detector circuit 7 includes Schmitt triggers 8, 9 connected in cascade via a timing circuit to maintain the detector output in a selected state. <IMAGE>

Description

INDUCTIVE SWITCHING APPARATUS The present invention relates to inductive switching apparatus which is responsive to the presence of a metallic material in the vicinity of an inductive sensor to perform some switching function, for example, in machine control applications.

Hitherto, inductive switching apparatus has utilised relays to effect a switching function and has required a 24-volt supply to drive the relays. The international standard for the supply voltage of computers, microprocessors and other electronic logic systems is 5 volts so that, when used with such systems, prior inductive switching apparatus has required a separate, higher voltage supply. Prior inductive switching apparatus has also had a relatively high unit cost.

An object of the present invention is to provide an inductive switching apparatus which is capable of operating with a 5 volt supply and which is simple and can be manufactured at low cost.

The invention consists in an inductive switching apparatus comprising an integrated oscillator circuit, a tuned circuit driven by the oscillator circuit and including an inductive sensor, a detector circuit for detecting an output signal produced by the tuned circuit or the oscillator circuit and responsive to a change in said output signal to produce a switching signal.

Conveniently, the oscillator comprises bistable pulse generators, preferably, three Schmitt triggers, connected in cascade. The oscillator frequency may be trimmed by a resistive component included in the cascade connections. The tuned circuit may have its inductive sensor connected between the input and output of the oscillator and a capacitor connected between the oscillator input and earth.

Such an arrangement in which the oscillator comprises three Schmitt triggers connected in cascade produces a sinusoidal waveform at the output of the tuned circuit (also the input of the oscillator) from the square pulses produced by the Schmitt triggers.

Because of the stable switching points of a Schmitt trigger, the switching apparatus is very temperature stable. Variations are mostly dependent on the tuned circuit components and any resistive component connected between the Schmitt triggers forming the oscillator in order to control the oscillation frequency. All other components virtually have no effect on the operational stability of the apparatus.

To change the sensitivity of the apparatus, that is, the distance from the inductive sensor at which the apparatus can detect metallic material, an inductive sensor having a coil of different diameter may be used.

The larger the diameter of the coil of the inductor, the greater is the sensitivity of the apparatus. The presence of three components, that is, the inductive sensor and capacitor of the tuned circuit and the frequency and amplitude trimming resistor in the oscillator circuit, enables ready adjustment of the sensitivity, metal selectivity or both.

The detector circuit may be connected to the output of the tuned circuit via an a.c. coupling device and a d.c. restorer.

The switching apparatus may be provided with two outputs, one of which is connected directly to the output of the detector circuit whilst the other is connected to the detector output by an inverter circuit. The provision of both positive and negative going outputs provides the ability to make the apparatus more fail-safe. By monitoring both outputs, it is always possible to detect when the apparatus is not functioning. Moreover, with the present invention, the outputs of the apparatus may be coupled directly to a computer, microprocessor or other logic system as the switching apparatus can be operated from a 5-volt supply.

In order that the invention may be more readily understood, reference will now be made to the accompanying drawing which illustrates a circuit diagram of one embodiment.

Referring to the drawing, the inductive switching apparatus comprises an oscillator 1 formed from three Schmitt triggers 2, 3, 4 connected in cascade, and a tuned circuit 5 formed from an inductive sensor L1 interconnecting the input and output of the oscillator and a capacitor C1 connected between the input of the oscillator and earth or zero volts. The oscillator is connected to a 5 volt d.c. source via the supply leads 6.

The inductive sensor L1 comprises a coil wound on a plastic former and mounted inside a ferrite pot core.

One end of the sensor is magnetically sealed and the other end is magnetically opened and, in practice, constitutes the sensing end of the sensor. Metallic materials to be detected are advanced towards this end of the sensor and their presence changes the inductance of the coil and also introduces eddy current losses in the oscillator circuitry. The capacitor C1 may be a small conventional ceramic capacitor and, in conjunction with the inductive sensor L1 forms a tuned circuit, the resonant frequency of which is a function of the capacitance and inductance of the two components L1,C1.

The output of the first Schmitt trigger 2 is connected to the input of the second Schmitt trigger 3 via a resistor R1 which is important in that it can be used to trim the frequency of the oscillator and adjust the amplitude of the input to the Schmitt trigger 2.

The Schmitt triggers 2, 3, 4 of the oscillator form an inverter having a very large open-loop gain. On the 5-volt system described, the high switching points of each Schmitt trigger may be approximately +3 volts when going to high level and approximately +2 volts when going to low level.

The output of the tuned circuit 5 is connected to the input of the oscillator 1 and also to the input of a detector circuit 7. It is connected to the latter via a capacitor C2 which a.c. couples the sinusoidal oscillations produced by the tuned circuit to the detector circuit. This capacitor may, for example, be a small ceramic capacitor, the size being of importance here. A resistor R2 connects the detector input to earth or zero volts and is used to d.c. restore the a.c. signal supplied by the capacitor C2 so that at the input of the detector this a.c. signal oscillates relative to zero volts.

The detector circuit comprises two Schmitt triggers 8,9 connected in cascade, the output of the first Schmitt trigger 8 being connected to the input of the second Schmitt trigger 9 by two series connected resistors R3, R4 and a capacitor C3 connecting the input of the second Schmitt trigger 9 to earth. The resistor R4 is connected in parallel with a diode D1 which shunts the resistor R4 in the discharge direction of the capacitor. The Schmitt trigger 8, resistors R3, R4 and capacitor C3 serve to filter the high frequency oscillations supplied from the tuned circuit; which may, for example, be of the order of 200-300 kHz, down to a switching speed of approximately 1 kHz or other desirable switching frequency.

The output of the Schmitt trigger 9 is directly connected to a first output of the apparatus 10, via a line 11, and to a second output 12 via a Schmitt trigger 13.

The Schmitt triggers 2-13 may be formed as a single integrated circuit.

The apparatus operates as follows. As the oscillator 1 comprises three Schmitt triggers, at no time can the input be at the same level as its output.

Hence, the oscillator must always oscillate. With any Schmitt trigger device, the outputs are either high or low and there is no output in between. Therefore, all the outputs of the three Schmitt triggers 2, 3, 4 of the oscillator are square waves. When the Schmitt trigger 4 output goes high, the capacitor C1 will by definition have been discharged, say, below 2 volts to create this condition. Capacitor C1 will then begin to charge via the inductive sensor L1 and, when the capacitor C1 reaches 3 volts, that is, its high level, the Schmitt trigger 4 is set to low (zero volts) and the capacitor C1 discharges via the inductive sensor L1. This procedure, of course, reaches a steady state oscillation and the signal applied to the input of the first Schmitt trigger 2 and hence, the output of the tuned circuit 5, is a sine wave.

The resistor R1 has the following effect on the operation of the oscillator. When the Schmitt trigger 2 input reaches, say, the 3-volt high level, its output goes low and the input to the Schmitt trigger 3 will also go low, but only after a time determined by the resistor R1 and the stray input capacitances of the Schmitt trigger 3, represented by the capacitor CS.

Such a delay, which occurs upon both high and low transitions of the output of the Schmitt trigger 2, controls the operating frequency of the oscillator 1.

It also enahles the input voltage of the Schmitt trigger 2 to rise above and fall below its +2 and +3 volt switching levels. Thus, the amplitude of the oscillations applied to the input of the Schmitt trigger 2 can be regulated by selecting the resistance value of the resistor R1.

The frequency of the signals supplied by the oscillator is important because, at different frequencies, different metals have different effects on the sensitivity of the inductive sensor L1. When a metal member or object is brought into close proximity with the inductive sensor L1, the inductance of its coil changes and losses due to eddy current occur. As a result, the frequency of the oscillator increases and the amplitude of the sinusoidal signal appearing at the output of the tuned circuit 5 or the input of the Schmitt trigger 2 decreases. It is this decrease in amplitude which is detected by the detector circuit 7, although in other embodiments the detector circuit may equally be arranged to be responsive to changes in frequency.

The sinusoidal signal at the output of the tuned circuit 5 is applied to the input of the Schmitt trigger 8 of the detector circuit via the a.c. coupler C2 and the d.c. restorer R2. The resulting sinusoidal oscillations applied to the input of the Schmitt trigger 8 therefore oscillate about earth or zero voltage. The amplitude of this oscillation is set by adjusting the resistor R2 so that it is normally greater than +3 volts, the upper threshold of the Schmitt trigger 8. The output of this Schmitt trigger switches at the frequency of the oscillator. On each output transition, the capacitor C3 is partially discharged by the diode D1 and the resistor R3. Several pulses of the high frequency oscillation are required completely to discharge the capacitor C3.When the output of the Schmitt trigger 8 is at its high level, the capacitor C3 tends to charge through the resistors R3, R4. The charging time is arranged to be approximately ten times the discharge time and this is achieved by making the value of the resistor R4 equal to ten times the resistance of R3. The time constant of the circuit R3, R4 and C3 may, for example, be a 1 ms.

The input of the Schmitt trigger 9 responds to the charge on the capacitor C3. When the amplitude of the sinusoidal signal applied to the input of the Schmitt trigger 8 decreases upon the inductive sensor L1 detecting the presence of a metal member or part, the output of the Schmitt trigger 8 remains high, the capacitor C3 charges and the output of the Schmitt trigger 9 thereupon goes low to produce a switching signal. The latter is fed directly to the first output 10 of the apparatus and is fed to the second output 12 in inverted form, by reason of the action of the Schmitt trigger 13, so that the apparatus has both positive and negative going outputs.

Whilst a particular embodiment has been described it will be understood that modifications can be made without departing from the scope of the invention. For example, the apparatus may incorporate suitable decoupling and overvoltage protection arrangements.

Claims

1. An inductive switching apparatus comprising an integrated oscillator circuit, a tuned circuit driven by the oscillator circuit and including an inductive sensor, and a detector circuit for detecting an output signal produced by the tuned circuit or the oscillator circuit and responsive to a change in said output signal to produce a switching signal.

2. Apparatus as claimed in claim 1, wherein the tuned circuit has its inductive sensor connected between the input and output of the oscillator circuit and includes a capacitor connected between the oscillator input and earth.

3. Apparatus as claimed in claim 1 or 2, wherein the oscillator comprises a plurality of bistable circuits connected in cascade.

4. Apparatus as claimed in claim 3, wherein the dscillator comprises three bistable circuits connected in cascade.

5. Apparatus as claimed in claim 3 or 4, wherein a resistive component is included in the cascade connections between the bistable circuits for trimming the oscillator frequency.

6. Apparatus as claimed in any one of the preceding claims, wherein the detector circuit is connected to the output of the tuned circuit via an a.c. coupling device and a d.c. restorer.

7. Apparatus as claimed in claim 6, wherein the detector circuit comprises two bistable circuits connected in cascade via a timing circuit which maintains the detector output bistable circuit in a selected state, and wherein the detector circuit is responsive to a change in the output signal applied to its input bistable circuit, upon the inductive sensor detecting the presence of a metal member or part, to enable its output bistable circuit to switch to the other state thereof and produce the switching signal.

8. Apparatus as claimed in any one of the preceding claims which has two switching outputs, said detector circuit being directly connected to one of said switching outputs and being connected to the other of said outputs by an inverter circuit.

9. Apparatus as claimed in claim 3 or 7 or any one of the preceding claims as appendant thereto, wherein the bistable circuits are Schmitt triggers.

10. Inductive switching apparatus constructed and adapted to operate substantially as hereinbefore described with reference to the accompanying drawing.