BACKGROUND ART Parallel control circuits are used to control processes in a wide range of industries and are provided to use inputs from a process or device being controlled and to provide outputs for control of that process or device. The parallel circuits ensure that if one control circuit fails then the other control circuit can continue to control or the fact that one of the control circuits has failed can be noted.

A problem exists, however, that with parallel control circuits or redundant control circuits operating on the same input signals and providing in theory at least the same output signals there may be a slight and perhaps insignificant time delay between the output signal from one control circuit to another control circuit. If an output control circuit is tracking the control signals and such a difference in output occurs then an error signal may be generated when in fact no error signal is needed as the difference in the outputs may only occur for a fraction of a second or some other insignificant time period.

It is the object of this invention to provide an output compare circuit for a time tolerance control circuit which will prevent such spurious error signals being generated.

DISCLOSURE OF THE INVENTION In one form, therefore, the invention is said to reside in an electric or electronic time tolerance control unit for control devices incorporating two or more parallel control circuits, the time tolerance control unit incorporating an output compare circuit which includes a time tolerance circuit whereby a selected

time delay is provided before an error signal is generated or a specific action is activated if one of the control circuits provides an output signal different than that of another of the control circuits.

Preferably the output compare circuit includes an array of relay switches in series with a tolerance relay and a capacitor charged by current flow through the output compare circuit and in parallel with a relay coil of the tolerance relay, the array of relay switches including pairs of relay switches in series operated by corresponding output signals, the pairs of relay switches providing a continuous current path to the relay coil of the tolerance relay provided the output signals are the same and wherein if the continuous current path is broken by a different corresponding output signals the capacitor provides current to hold the relay coil of the tolerance relay closed for the selected time delay.

In an alternative embodiment the output compare circuit may include an array of relay switches in series with a tolerance relay, a capacitor charged by current flow through the output compare circuit and in parallel with a relay coil of the tolerance relay, the array of relay switches including a relay switch operated by each corresponding output signal, each relay switch having a normally open and a normally closed position and the relay switches providing a continuous current path to the relay coil of the tolerance relay provided the output signals are the same and wherein if the continuous current path is broken by a different corresponding output signals the capacitor provides current to hold the relay coil of the tolerance relay closed for the selected time delay.

By this arrangement the relay switches can be self checking in case the pole of one of the relay switches sticks to a terminal.

A short circuit path may be provided if the time delay is exceeded, the short circuit path including a normally closed relay switch operated by the relay coil of the tolerance relay and held open while current is supplied to the tolerance relay.

In one embodiment there may be a fuse in the short circuit path is fused if the time delay is exceeded.

Each pair of relay switches may be provided by separate relay switches in the same relay assembly operated by their respective magnetic coils or they may be the normally opened and normally closed positions of a single relay switch.

In an alternative embodiment the relay coil of the tolerance relay may be adapted to provide a warning signal or a resetting function of the working process if the relay is opened.

The pairs of relay switches operated by corresponding output signals may include a normally open and a normally closed relay switch in parallel for each corresponding output relay.

The array of relay switches operated by corresponding output signals may include a relay switch with a normally open and a normally closed position whereby the normally opened position is connected to normally opened position of the comparing relay switch and the normally closed position is connected to normally closed position of the comparing relay switch.

The array of relay switches may be provided for only one of the outputs of the control units. Alternatively arrays of paired relays switches may be provided for each of the outputs of the control units and wherein the arrays of relay switches are in series.

In an alternative form the invention may be said to reside in a method of comparing output signals from parallel control circuits to detect a malfunction in one or more of the control circuits, the method including the step of comparing the output signals and if there is a difference allowing a selected time delay before providing a error signal whereby to allow for minor temporal differences in control signals.

Preferably the step of comparing the output signals includes the step of holding a relay coil closed for the selected time delay by providing current from a capacitor in parallel with the relay coil.

In a further form the invention is said to reside in an electronic or electric safety control which includes two or more control circuits and an output compare control circuit which is made up of one or more pairs of magnetic relay poles and a capacitor.

Such an electric or electronic control will if there is a difference or inconsistency in sending out or stopping of the output signals being compared in the output compare circuit beyond a designated time tolerance there will be for instance a locking out procedure whereby a fuse will break.

Alternatively the locking out procedure may stop the power to the control circuits or to the output circuits or other selected circuits.

In a further form the error signal generated may be for the activation of a warning signal or the resetting of the electronic control circuits.

The magnetic relays and capacitors may be provided by an integrated circuit which acts as a number of relays and a capacitor.

Such control units may be used in a number of industrial processes and devices such as burners or furnaces and temperature controllers, timers or manufacturing processes. Alternatively the apparatus according to this invention may be used in vehicles, in ships and the like.

The control circuits may be relay assemblies whereby outputs are switched on in the selected manner depending upon selected inputs or they may be programmable logic control circuits or similar devices.

In a preferred form of the invention the output compare circuit may include a matrix of relays whereby the number of relay circuits is determined by the number of control circuits and the number of output control signals. For instance if there is two control circuits and three output controls then there may be six relays.

Alternatively if one of the output circuits is for a dominant or critical function then the time tolerance control arrangement of the present invention may be placed on that output only.

BRIEF DESCRIPTION OF THE DRAWINGS This then generally describes the invention but to assist with understanding reference will now be made to the accompanying drawings which show a preferred embodiment of the invention.

In the drawings: Figure 1 shows a first embodiment of a time tolerance control unit according to the present invention; Figure 2 shows an alternative embodiment of a time tolerance control unit according to this invention; Figure 3 shows an alternative embodiment of the output compare circuit of the invention; Figure 4 shows an alternative embodiment of the output compare circuit of the invention; and Figure 5 shows an further alternative embodiment of the output compare circuit of the invention BEST MODE FOR CARRYING OUT THE INVENTION In Figure 1 of the drawings there is illustrated a first embodiment of a time tolerance control unit of the present invention within a control unit. The item of equipment or process which is being controlled is not illustrated.

The control unit 1 has a number of inputs 2,3 and 4 from the device or process being controlled and a number of outputs 6,7 and 8 to control the process or unit. It will be realised that there may be a different number of inputs or outputs depending upon the parameters being monitored and the parts of the process or device to be controlled. Within the control unit 1 there is a first control circuit 10 and a second control circuit 20. These control circuits 10 and 20 are operated in parallel. The same inputs are supplied into each control circuit and provided no problem exists with each control circuit then the same output from each control circuit should be provided. Power to the control unit is supplied by power supply 30. Power is supplied through a fuse 31 to a start switch 32. Power is also supplied continuously to an output compare circuit 40. When start switch 32 is depressed then power is supplied to the coil of a magnetic relay 33 which in turn causes the relay switch poles 33A of the relay 33 to close and hold on power supply to the relay 33 and the control circuits 10 and 20. It is to be noted that the electrical supply switched

by relay 33 and relay switch 33A may be transformed in terms of voltage or current before being supplied to the control circuits 10 and 20.

The control circuits 10 or 20 may be of a similar form or of a different form but they both should be set to have the same process or program to send out or to stop sending out an output signal to the outputs 6,7 and 8. In this embodiment in fact the output signals 11,12 and 13 from control circuit 10 are passed to the magnetic coils of relays 14,15 and 16 respectively. Similarly the output signals 21,22 and 23 from control circuit 20 are passed to magnetic coils of relays 24,25 and 26. A first set of switches 14A, 15A and 16A activated by relay coils 14,15 and 16 are provided to switch power to outputs 6,7 and 8 respectively. A second set of output switches 24A, 25A and 26A are provided to correspond to the magnetic coils 24,25 and 26 respectively and placed in series with the switches 14A, 15A and 16A respectively. By this means a power from power line 45 must therefore be switched by the relay switches 14A and 24A positioned in series to provide an output on line 6 and similarly relay poles 15A and 25A and 16A and 26A must both be switched on at the same time to provide respective outputs 7 and 8.

It is to be noted that the relay poles 14A, 15A and 16A and 24A, 25A and 26A are normally open and only closed when their respective relay coils 14,15,16 and 24,25 and 26 are switched. In other embodiments relays may be normally closed and opened as part of the controlling process.

It will be realised therefore that if one of the control circuits 10 or 20 fails to operate then power to outputs 6,7 and 8 will not be supplied because one or other of the relay poles 14A and 24A or 15A and 25A or 16A and 26A will not be closed. Problems in control of the device or process may then occur.

There may be situations, however, where one of the control circuits 10 and 20 may operate at a very short time such as a fraction of a second after the other of the control circuit and this time difference may be acceptable or unacceptable.

To solve this problem therefore the output compare circuit 40 of the present invention is provided.

Within the output compare circuit 40 are relay switches 14B and 14C in

parallel corresponding to relay coil 14 and relay switch 14A. Switch poles 14B are normally closed and switch poles 14C are normally opened. Switch poles 14B and 14C may be provided by separate relay switches in the same relay assembly operated by magnetic coil 14 or they may be the normally opened and normally closed positions of a single relay switch. Similarly switch poles 24B and 24C relate to magnetic pole 24 and relay switch 24A.

Relay switch poles 15B and 15C are related to magnetic coil 15 and relay poles 25B and 25C relate to magnetic coil 25. Once again each pair may be provided by separate relay switches in the same relay assembly operated by their respective magnetic coils or they may be the normally opened and normally closed positions of a single relay switch. There is a connection 41 joining the line between relay switch 24B and relay switch 15B and the line between relay switch 24C and relay switch 15C.

Within the output compare circuit 40 a magnetic coil 50 of a relay is connected through the normally closed relays 14B, 24B, 15B and 25B to the fuse 31 so that power is supplied continuously to the relay 50. The relay coil 50 has a capacitor 52 in parallel with it.

The operation of the device according to this invention is as follows: In general the time tolerance circuit shown in Figure 1 will allow continuous electrical supply to relay coil 50 and capacitor 52 but if a fault occurs in control circuit 10 or control circuit 20 the comparison poles within the output compare circuit 40 will not operate simultaneously and power will not be supplied to relay coil 50. The capacitor 52 will supply power to the relay coil 50 for a selected time to hold the power supply to the control unit 1 on if there is a less than significant fault and the relevant relay switch activates within that time.

It will be seen that in a normal operation electrical current can flow from the fuse 31 to the relay coil 50 through relay 14B and 24B, and 15B and 16B, or through 14C, 24C, 15C and 25C. The bridge 41 also allows flow through for instance 14B and 24B, and 15C and 25C so that 14B and 24B may activate at a certain control position but 15B and 25B do not. Hence depending on the control system and the switching of the outputs, supply can also flow through the relays 14B, 24B and then cross to relay poles 15C and 25C on the bridge 41. Similarly supply could come through the poles 14C, 24C and then switch to the poles 15B and 25B on the bridge 41. In all situations, therefore, supply

can be provided to the relay coil 50. If, however, there is discrepancy between 14B and 24B which will also engender a discrepancy between 24C and 14C or between 15B and 25B, which will also engender a discrepancy between 15C and 25C, then power supply to the coil 50 will be interrupted because there is no continuous electrical supply path between the fuse 31 and the relay 50. The power stored in the capacitor 52, however, will continue to hold the magnetic coil of the relay 50 closed so long as there is still charge in the capacitor 52. The electrical energy accumulated inside capacitor 52 will continue to discharge for a selected time depending upon the selected capacitance of the capacitor 52 and hence the capacitor 52 is termed the time tolerance capacitor.

If at the end of the time tolerance the switchings have not been reversed to their normal action then coil 50 will cause its corresponding switch poles 50A which are connected across the power supply after the switch to change from a normally closed to an open position which will in effect short circuit the system and blow the fuse 31 and which will stop the electrical power supply to the control circuits 10 and 20 or to the output or other end circuits in order to lock out the whole process being controlled.

It may be considered that in this embodiment that the outputs 11 and 12 and 21 and 22 are the critical outputs these need to be monitored.

It will be realised that this embodiment has been discussed with magnetic relays and mechanical switch poles but the device could be embodied using electronic circuitry such as switching transistors to carry out the various functions and comparison arrangements.

The embodiment shown in Figure 2 has a slightly different operation but those components with the same function as the corresponding components in Figure 1 are given the same numbering.

In this embodiment power is continuously supplied from power supply 30 to the coil 50 through the fuse 31 provided the various relays within the output compare circuit 40 are in their normal operating positions. Power to the coil 50 operates relay poles 50A which enables relay coil 61 to be activated which closes normally open relay 61 A so that power is supplied to relay coil 65 and to the control circuits 10 and 20. Once power is supplied to the relay coil 65

then relay switch poles 65A which are normally opened are closed which continuously supplies power to the circuit so the switch 60 does not have to be held closed.

The system will continue to operate in this fashion until there is a discrepancy in the control circuit 10 or control circuit 20 which will cause the pairs of relays 14B and 14C, 24B and 24C, or 15B, 15C, 25B and 25C to stop power supply to the relay coil 50 and unless the power is resupplied within the time tolerance provided by the capacitor 52 which holds the relay coil 50 closed for a selected period of time then relay switch poles 50A are opened and power is no longer supplied to relay coil 61. This will open switch 61 A so that power is no longer supplied to the control circuits and a relay coil 65 and hence switch 65A is also opened. The relay coil 65 may also shift a pole of a safety relay to activate a warning signal or to reset the process or device being controlled.

Figure 3 shows an embodiment of the time tolerance unit 70 in which all of the outputs from the parallel control circuits 10 and 20 are compared. Hence there are relay switches 14B and 14C corresponding to relay 14, relay switches 24B and 24C corresponding to relay 24, relay switches 15B and 15C corresponding to relay 15, relay switches 25B and 25C corresponding to relay 25, relay switches 16B and 16C corresponding to relay 16 and relay switches 26B and 26C corresponding to relay 26. Bridges 71 and 72 are provided between the sets of pairs.

Once again each pair of relay switches may be provided by separate relay switches in the same relay assembly operated by their respective magnetic coils or they may be the normally opened and normally closed positions of a single relay switch.

If any one of the outputs of the first control circuit differs from its corresponding output of the second control circuit then power will not be supplied to the relay coil 50. It may be considered that in this embodiment all of the outputs are critical outputs and all need to be monitored.

In Figure 4 it is considered that there is only one critical output to be controlled and hence in the time tolerance control unit 80 there are only relay switches 15B and 15C corresponding to relay 15 and relay switches 25B and 25C

corresponding to relay 25. Once again each pair of relay switches may be provided by separate relay switches in the same relay assembly operated by their respective magnetic coils or they may be the normally opened and normally closed positions of a single relay switch. The relay switches 15B and 25B are in series and relay switches 15C and 25C are in series and relay switches 15B and 25B are in parallel with relay switches 15C and 25C. If the output 12 of the first control circuit differs from its corresponding output 22 of the second control circuit (see Figure 1) then power will not be supplied to the relay coil 50. It may be considered that only outputs 12 and 22 are critical outputs and need to be monitored.

FIG 5 shows an alternative embodiment of the output compare circuit of the present invention. In this embodiment the time tolerance control unit 90 has an array of relay switches 14,24,15 and 25 in series with the relay coil 50. the capacitor 52 is in parallel with the relay coil 50. Each relay switch 14D&E, 24D&E, 15D&E and 25D&E is a single relay switch which has a normally closed position 14D, 24D, 15D and 25D and a normally open position 14E, 24E, 15E and 25E respectively. Each relay switch 14D&E, 24D&E, 15D&E and 25D&E is operated by its respective relay coil 14,24,15 and 25 (see FIG 1). If the output signals 11 and 21 (see FIG 1) are the same then relay coils 14 and 24 will act the same and relay switches 14D and 24D will stay closed and current can flow through to the relay coil 50 provided relay coils 15 and 25 are also working in unison. It will be seen that if any one of the coils is not activated or is activated at a different time from its corresponding coil then current will not flow to the relay coil 50.

The time tolerance control unit of the present invention therefore generally is a device which monitors output signals from a program or from a working process in accordance with input signals and an analysis of those input signals in the control unit. The control unit has at least two control circuits which sends out or stops sending out their output signals simultaneously with one another. At least one of the output signals from each control circuit is compared in an output compare circuit. If there is any time difference from each comparing output signal beyond the design time tolerance there is a locking out of the working process, an activation of a device to send out a warning signal or a resetting of the working process. The apparatus has the advantage that it significantly reduces the chances of damage occurring to property and or loss of human life and it makes the operator of machinery or

equipment fitted with the device aware of misoperation or faults immediately before further fault, damage or disaster can occur. It can reduce waste and production manufacturing processes if a fault occurs.

Although the general description of the preferred embodiments has been specifically stated in certain terms, these should not be construed as limiting the scope of the invention but as merely providing illustrations or preferred embodiments of the invention. For instance, the beyond time tolerance detected by the output compare circuit can activate a warning signal such as a pilot lamp buzzer, etc, the magnetic relays can be replaced by electronic devices such as an operational amplifier specific integrated circuits or the like and some details of the circuit can be changed such as the output control circuit only being activated after power has been supplied to the circuit.

The output relay switch poles for each output may be in parallel rather than series.

Throughout this specification and the claims that follow unless the context requires otherwise, the words'comprise'and'include'and variations such as 'comprising'and'including'will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.