GB2207307A

GB2207307A - Fail-safe monitoring circuit

Info

Publication number: GB2207307A
Application number: GB08717304A
Authority: GB
Inventors: John Patterson Sinclair
Original assignee: MSA
Current assignee: MSA
Priority date: 1987-07-22
Filing date: 1987-07-22
Publication date: 1989-01-25
Also published as: GB8717304D0

Abstract

A fail safe circuit (10) for monitoring signals from a sensor (16) includes a microprocessor control (12) which is alternately connected to the sensor (16) and a reference source (18) via an A-D converter (20). In normal operation, a pulse train output from the control (12) is applied to the gate of a MOSFET (VT1) to control a normally energised and open relay (24), which is powered by a voltage source V, via a diode pump circuit (28). Any fault in the sensor (16) or circuitry (10) will result in the relay (24) becoming deenergised so that it closes to activate an alarm or the like. <IMAGE>

Description

"Fail Safe Circuit" The present invention relates to a circuit for use in monitoring signals from a sensor or the like which will fail in a safe manner in the event of a malfunction of the sensor or of the circuitry itself.

A disadvantage associated with conventional control circuitry for monitoring sensor signals is that it is not possible to predict the manner in which solid state components will fail in the event of malfunction; eg. a comparator may fail either "on" or "off". This presents a serious danger in the case of equipment monitoring or contr- olling the operation of unattended machinery-in a hazardous environment, such as a coal mine, where it is essential that the equipment should fail safely to avoid the risk of fire or explosion.

It is an object of the present invention to obviate or mitigate the aforesaid disadvantage.

In accordance with a first aspect of the invention, a fail safe circuit for monitoring a signal source comprises control means and switching means controlled by said control means whereby said control means may be connected to either the signal source or to a reference signal source, wherein said control means measures said signal and said reference signal alternately and generates a control signal in response to the measured values thereof.

Preferably, said reference signal is set outside the normal range of said signal.

Preferably also, said control means is responsive to either said signal or said reference signal falling outside predetermined ranges, or to zero signal from either source, or to the measured values of both sources being equal.

Preferably also, said control means generates said control signal under normal conditions and ceases to do so when any of the above mentioned fault conditions is detected.

Preferably also, the circuit further includes analogue to digital converter means between said switching means and said control means.

According to a second aspect of the invention, a fail safe circuit is provided for energising a relay under the control of a pulse train generated by a signal monitoring circuit via a diode pump circuit wherein a DC voltage source alternately charges a first capacitance connected across said relay and is connected to earth via variable resistance means control Led by said pulse train, and wherein said relay is de-energised if said pulse train stops.

Preferably, said variable resistance means comprises a MOSFET having said pulse train applied to its gate.

According to still another aspect of the invention, a method of monitoring a signal source comprises alternately measuring said signal and a reference signal set outside the normal range of said signal, generating a control signal if said signal and said reference signal lie within predetermined ranges and discontinuing said control signal if either said signal or said reference signal fall outside said predetermined ranges, or if the measured value of either said signal or said reference signal is zero, or if the measured values of both said signal and said reference signal are equal.

Preferably, said control signal is a pulse train.

Preferably also, said control signal serves to maintain a relay in an energised condition.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawing which is a diagram of a circuit embodying the invention.

Referring now to the drawing, a fail safe monitoring circuit, generally designated 10, comprises microprocessor control means 12 which controls a switch 14 whereby it is alternately connected to the output of a sensor 16 and to a reference signal source 18 via an analogue to digital converter 20. The output 22 from the control means 12 controls the operation of a relay 24 to actuate contacts 26 via a diode pump circuit 28.In this example the contacts 26 are closed, when the relay 24 is energised, and when open activate an alarm or the like (not shown) The diode pump circuit 28 comprises a MOSFET, VT1 having the output 22 of the control means 12 connected to its gate and having its other two terminals connected to a first capacitance C1 and earth respectively, a first diode D1 connected between C1 and the relay 24, a second diode D2 connected between the junction 30 of C1 and D1 and earth and a second capacitance C2 connected between the junction 32 of D1 and relay 24 and earth. A D.C.

voltage V is connected to the junction 34 between VT1 and C1 via a resistance R1.

In use, the control means 12 continuously switches the switch 14 between the sensor 16 and the reference source 18 to alternately measure the sensor output signal and the reference signal, and under normal conditions outputs a continuous pulse train to maintain C2 charged and hence relay 24 energised. The reference signal is set outside the normal range of the sensor signal le.vel.

The relay 24 will be de-energised in response to the control means 12 stopping the output pulse train.

This will occur if the sensor signal falls outside normal limits (whether due to an out of limit signal being sensed or to failure of the sensor 16 itself), if the switch 14 fails (in which case the control means will detect no difference between the sensor and reference signals), if the reference signal fails or varies from its expected value, if the analogue to digital converter 20 fails, or if the control means 12 itself fails. Failure of any of the components of the diode pump circuit 28 will also cause the relay 24 to be de-energised. The only possible fail to danger modes are short circuiting of C1 or the contacts 26 being mechanically held closed. These are both low probability events.

The circuit 10 will thus fail safely under any likely fault condition and is therefore suitable for use in hazardous environments.

Claims

1. A fail safe circuit for monitoring a signal source comprising control means and switching means controlled by said control means whereby said control means may be connected to either the signal source or to a reference signal source, wherein said control means measures said signal and said reference signal alternately and generates a control signal in response to the measured values thereof.

2. A circuit as claimed in claim 1 wherein said reference signal is set outside the normal range of said signal.

3. A circuit as claimed in claim 1 or 2 wherein said control means is responsive to either said signal or said reference signal falling outside predetermined ranges, or to zero signal from either source, or to the measured values of both sources being equal.

4. A circuit as claimed in claim 3 wherein said control means generates said control signal under normal conditions and ceases to do so when any of the above mentioned fault conditions is detected.

5. A circuit as claimed in any preceding claim wherein the circuit further includes analogue to digital conventer means between said switching means and said control means.

6. A fail safe circuit for energising a relay under the control of a pulse train generated by a signal monitoring circuit via a diode pump circuit wherein a DC voltage source alternately charges a first capacitance connected across said relay and is connected to earth via variable resistance means controlled by said pulse train, and wherein said relay is de-energised if said pulse train stops.

7. A circuit as claimed in claim 6 wherein said variable resistance means comprises a MOSFET having said pulse train applied to its gate.

8. A method of monitoring a signal source comprising alternately measuring said signal and a reference signal set outside the normal range of said signal, generating a control signal if said signal and said reference signal lie within predetermined ranges and discontinuing said control signal if either said signal or said reference signal fall outside said predetermined ranges, or if the measured value of either said signal or said reference signal is zero, or if the measured values of both said signal and said reference signal are equal.

9. A method as claimed in claim 8 wherein said control signal is a pulse train.

10. A mehtod as claimed in claim 8 or claim 9 wherein said control signal serves to maintain a relay in an energised condition.

11. A fail safe circuit substantially as hereinbefore described with reference to the accompanying drawings.

12. A method of monitoring a signal source substantially as hereinbefore described.