GB2110386A

GB2110386A - Apparatus for monitoring the state of a multi-state device

Info

Publication number: GB2110386A
Application number: GB08135774A
Authority: GB
Inventors: James Lewis Chisho Livingstone
Original assignee: Solartron Electronic Group Ltd
Current assignee: Gemalto Terminals Ltd
Priority date: 1981-11-26
Filing date: 1981-11-26
Publication date: 1983-06-15
Also published as: GB2110386B

Abstract

An input channel of a process control system for monitoring a status-indicating switch is also connected to another circuit for selectively supplying signals representative of each state of the switch. Successive application of each signal enables the processor to check for a corresponding change in the signal supplied by the input channel, thereby confirming integrity of the channel irrespective of the current state of the switch. <IMAGE>

Description

SPECIFICATION Apparatus for monitoring the state of a multistate device This invention relates to apparatus for monitoring the state of a multi-state device such as a switching device.

Switching devices such as relay contacts are used extensively in, for example, industrial process control systems to provide remote indication of the status of such equipment as valves. It is important that the occurrence of a fault in the circuits used to monitor the states of such switching devices should not cause or permit damage to the plant being controlled by the system. One conventional way of ensuring this is to provide duplicate or redundant monitoring circuits, and compare the outputs of these circuits for any differences. However, this approach is costly in terms of components used, and, in the event of a circuit failure which leaves the output of the circuit stuck in its usual state, cannot detect the failure until the switching device being monitored changes in its own state.

According to one aspect of this invention there is provided apparatus for monitoring the state of a multi-state device, comprising sensing means arranged to sense the state of said device, first signal supply means arranged selectively to supply a first signal corresponding to a first state of said device, second signal supply means arranged selectively to supply a second signal corresponding to a second state of said device, and means for coupling signals from said first and second supply means to said sensing means irrespective of the state of said device, whereby the ability of said sensing means to detect both said states of said device can be tested.

Apparatus in accordance with this invention will now be described, by way of example, with reference to the accompanying drawing, in which: Figure 1 is a circuit diagram of one form of the apparatus; and Figure 2 is a circuit diagram of another form of the apparatus.

The invention is for use in, for example, a process control system, in which a central processor (typically a computer) monitors various parameters indicative of the progress of an industrial process, and selectively operates equipment, such as valves, heaters and pumps, to regulate the process. In such circumstances, information about the position of, for example, a valve may be signalled to the processor by a switch associated with the valve. Typically, the valve may be operated by energisation of a relay having contacts which switch current to the value actuator, and the relay may have another set of contacts signalling the state of energisation of the relay to the processor. Alternatively, the movement of the valve may be arranged to operate directly a microswitch monitored by the processor.

Thus, referring to Figure 1, the processor (not shown) controls an actuator 10 which operates an item of equipment (not shown). The status of this equipment is indicated by the state of a switch 12 connected to a +V Volts supply and operated in conjunction with the equipment, and the state of this switch 12 is monitored in the processor via a filter 14 and an opto-coupler 16 which provides galvanic isolation.

If this monitoring arrangement develops a fault, the processor will of course no longer be able to sense the status of the equipment in question. Furthermore, the monitoring arrangement may fail such that the signal to the processor remains in the condition it normally assumes. Thus, for example, the switch 12 may normally be open, in which case the light-emitting diode in the opto-coupler 16 would normally be de-energised. Failure of the light-emitting diode would not then produce any change in the signal to the processor while the switch 12 remained open. Consequently the fault may not be discovered for some time, or, worse, only when the absence of the expected signal from the optocoupler caused malfunction of the industrial process.

Accordingly, and as shown in Figure 1, the processor can selectively energise two more opto-couplers 1 8 and 20. The first opto-coupler 18 is arranged to turn a transistor 22 on, thereby applying +V volts to the opto-coupler 16 via a diode 24 and simulating the signal from the switch 12 in its closed or on state. Conversely, the second opto-coupler 20 is arranged to turn a transistor 26 on, applying zero volts to the optocoupler 16 via a diode 28 and simulating the signal from the switch 12 in its open or off state.

The diodes 24 and 28 provide a wired-OR connection, allowing the simulated signals from the transistors 22 and 26 to be applied to the opto-coupler 1 6 irrespective of the current state of the switch 12.

Thus the integrity of the arrangement for monitoring the switch 12 can be tested at any time by energising the opto-couplers 18 and 20 in turn, thereby temporarily changing the state of the drive to the opto-coupler 1 6. Failure of the signal from this opto-coupler 1 6 to the processor to change in response to either test signal indicates that a fault has developed.

The same test signals from the transistors 22 and 26 can be applied to other opto-couplers like the device 1 6 in other switch monitoring channels.

Figure 2 shows another way of implementing the invention. Referring to Figure 2, a relay coil 30, having associated contacts 32 for controlling plant equipment, is selectively energised by the central processor via an open-collector buffer/inverter 34. The current state of the relay 30 is monitored by a comparator 36 receiving the output of the buffer/inverter 34 on one input and a reference signal on its other input. The reference signal is normally OV from an electronic switch 38, but can selectively be made +V volts under the control of the processor by tandem operation of the switch 38 and a second electronic switch 40 coupled to the +V volts supply.

In order to check the integrity of the comparator 36, to ensure for example that it has not failed to the state corresponding to the normal state of energisation of the relay 30, the electronic switches 38 and 40 are periodically operated momentarily, thereby changing the reference signal to the comparator 36. The processor simultaneously monitors the comparator output to check for a corresponding change in the signal there. Lack of any such change would indicate a malfunction.

It is also possible to monitor the integrity of the relay coil 30, by momentarily changing the input signal to the buffer/inverter 34. This should cause a corresponding change at the output of the buffer/inverter 34, which will be sensed by the comparator 36 (if the change is made appropriately brief, the inertia of the magnetic field in the relay 30 will prevent any corresponding change in the state of the contacts 32). However, should the relay coil 30 have become open-circuit, the open-collector output of the buffer/inverter 34 will not be able to change in voltage, since it then has no output load (equivalent to a pull-up resistor in logic circuit terms). Accordingly, the brief change in input signal will not be accompanied by any change in the signal from the comparator 36, indicating that a fault has occurred.

Claims

Claim

1. Apparatus for monitoring the state of a multi-state device, comprising sensing means arranged to sense the state of said device, first signal supply means arranged selectively to supply a first signal corresponding to a first state of said device, second signal supply means arranged selectively to supply a second signal corresponding to a second state of said device, and means for coupling signals, from said first and second supply means to said sensing means irrespective of the state of said device, whereby the ability of said sensing means to detect both said states of said device can be tested.