WO1991010250A1

WO1991010250A1 - Safety circuit

Info

Publication number: WO1991010250A1
Application number: PCT/GB1990/001998
Authority: WO
Inventors: George Jackson
Original assignee: E.J.A. Engineering Company Limited
Priority date: 1989-12-22
Filing date: 1990-12-20
Publication date: 1991-07-11
Also published as: AU7051591A; GB8929104D0

Abstract

An electrically operable power switch (2) is connected between a power supply (3) and a power consuming device (1). A safety circuit (4) is arranged to monitor at least one safety-related condition of the power consuming device (1), the safety circuit controlling the energisation of at least one relay (6) in dependence upon the monitored condition. A first set of contacts (5) controlled by the relay is connected to supply power to the safety circuit, and a second set of contacts (7) controlled by the relay is connected to enable or disable the power switch. The first set of contacts (5) comprises at least two contacts arranged to be switched simultaneously and connected in series such that the supply of power to the safety circuit (4) is maintained except during switching of the contacts. The safety circuit (4) comprises an energy storing circuit arranged to maintain the energisation of the safety circuit for a period of time at least as long as the period of time required for the first set of contacts (5) to switch, and the second set of contacts (7) is arranged so as to disable a power switch (2) if the safety circuit (4) is deenergised.

Description

SAFETY CIRCUIT

The present invention relates to a circuit for supplying electrical power to a power consuming device, and in particular to such a circuit which is disabled in the event of unsafe conditions being detected.

Power supply circuits are known for controlling an electrically operable power switch connected between a power supply and a power consuming device such as a machine tool. A safety circuit is arranged to monitor at least one safety-related condition of the power consuming device, for example the position of a door in an enclosure within which the power consuming device is located. The power consuming device is to be disabled if the door is not closed.

Most safety arrangement of the above type rely upon one or more relays which are switched in dependence upon the monitored safety condition, for example the door position. Unfortunately it can happen that the contacts of a relay become inoperative due to for example welding effects. In the event of this occurring the safety circuit is in effect made inoperative with potentially disastrous results.

With the problem of contact failure in mind, it has been proposed to provide two or more sets of relay contacts connected in series so that if one of the contacts fails the circuit still operates normally. This approach has obvious advantages but is not entirely satisfactory as if one has say two contacts connected in series and one fails in a circuit enabling condition, the circuit will continue to operate reliably but there will be no indication that one of the contacts has failed. Thus as soon as one of the contacts has failed the circuit then operates as if it only had one set of contacts.

Relatively complex arrangements have been provided for monitoring the condition of series connected contacts. For example a separate contact monitoring circuit is provided across the series connected contacts so as to enable detection of failure of any one contact. Unfortunately this approach does require the provision of an extra monitoring circuit which it would obviously be desirable to avoid if possible.

It is an object of the present invention to obviate or mitigate the problems outlined above.

According to the present invention there is provided a circuit for supplying electrical power to a power consuming device, comprising an electrically operable power switch connected between a power supply and a power consuming device, a safety circuit arranged to monitor at least one safety- related condition of the power consuming device, the safety circuit controlling the energisation of at least one relay in dependence upon the said at least one monitored condition, a first set of contacts controlled by the said at least one relay and connected to supply power to the safety circuit, and a second set of contacts controlled by the said at least one relay and connected to enable or disable the power switch, wherein the first set of contacts comprises at least two contacts arranged to be switched simultaneously and connected in series such that the supply of power to the safety circuit is maintained except during switching of the contacts, the safety circuit comprises an energy storing circuit arranged to maintain the energisation of the safety circuit for a period of time at least as long as the period of time required for the first set of contacts to switch, and the second set of contacts is arranged so as to disable a power switch if the safety circuit is de- energised.

In the event of one of the contacts of the first set failing to operate correctly, eg. as a result of welding, the safety circuit will be de-energised when the other contact of the first set is switched. The mains power switch is thus disabled, and the safety circuit cannot be re-energised without direct intervention by an operator. It is theoretically possible for both of the first contacts to fail in the same way simultaneously and if this was to occur energisation of the safety circuit would be maintained. In practice however the probability of such circumstances ever arising is so small that it can be discounted.

Preferably, at least two relays are provided, each relay being energised in dependence upon a respective one monitored condition of the power consuming device.

Preferably, the energy storing circuit is a capacitor. The power switch may be a conventional contactor switchable by a control current generated by for example depression of a manual start button, a current being delivered to the contact via the second set of contacts. Two contactors may be provided, each supplied via a respective contact of the second set of contacts. An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 is a schematic block diagram of an embodiment of the present invention; and

Fig. 2 illustrates relay contacts in an arrangement of the type described with reference to Fig. 1.

Referring to Fig. 1, a power consuming device in the form of a machine 1 is supplied with energy via contactors 2 from a mains supply 3. A safety circuit 4 monitors safety related conditions of the machine 1, for example the position of a safety enclosure door. The safety circuit is energised from the mains supply 3 through a first set of contacts 5. Assuming that the safety circuit senses safe operating conditions, a relay 6 is energised switching contacts 5 and switching a second set of contacts 7. The second set of contacts controls the contactors 2. Assuming that a manual control switch 8 has been actuated to indicate that the machine should be powered up, power is delivered through the control switch and the second contacts to the contactors 2, causing the contactors 2 to deliver power to the machine 1. In the event of the relay 6 being de-energised, the second contacts 7 disable the contactors 2, thereby isolating the machine 1 from the power supply. The relay 6 is de- energised if the safety circuit indicates that the machine condition is not safe, eg. a guard door has been opened, or if the safety circuit 4 is de-energised as the result of the failure of the first contacts 5.

The first contacts 5 switch simultaneously with the second contacts 7 and are arranged such that the safety circuit remains energised so long as the first contacts 5 operate correctly. If the first contacts fail however the safety circuit 4 is de- energised, thereby de-energising the relay 6 and causing the second contacts 7 to disable the contactors 2.

The relationship between the contacts 5 and 7 of Fig. 1 will now be described in more detail with reference to Fig. 2.

Energy is supplied to the circuitry via a transformer 9 and a rectifier 10, the supply voltage being stabilised by a capacitor 11 and the supply of power being indicated by a light- emitting diode 12. The DC supply voltage is supplied through a pair of series connected contacts RL1/1 and RL2/1 to a safety circuit comprising an input section made up from a diode 13 and a capacitor 14. The supplied DC voltage is applied to a first safety circuit 15 incorporating a first relay RL1 and a second safety circuit 16 comprising a relay R 2. The relay RL1 is energised if and only if a first safety condition of the machine controlled by the circuitry indicates that the machine is in a safe condition to run. This condition is indicated by the output of a first condition monitor 17. The first condition to be monitored could be an input indicating that a magnetic reed switch mounted on one component of the safety enclosure is close to a permanent magnet mounted on a door of the enclosure which must be closed before the machine can be considered to be in a safe condition. Likewise, the relay RL2 is only energised if a second safety condition is satisfied. The second safety condition is indicated by an input derived from a second condition monitor 18. The second condition monitor could monitor for example the output of a tuned circuit, the tuned circuit only providing a safety-indicating output if an associated tuned circuit is in close proximity to it. Thus both the first and second safety conditions could relate to the position of the door of an enclosure protecting the controlled machine or the two safety conditions could be only indirectly related, for example each safety condition input indicating the closure of a respective door in an enclosure protecting the machine.

The contacts shown in Fig. 2 are illustrated in full lines in the positions which are appropriate to an attempt being made to turn the machine on. Assuming that the first and second safety conditions are met, both relays RL1 and RL2 are energised and the four contacts RL1/1, RL1/2, RL2/1 and RL2/2 switch from the positions shown in full lines to the positions shown in broken lines. Simultaneous switching of the contacts RL1/1 and RL2/1 temporarily disconnects the safety circuit from the power supply but the safety circuit continues to operate normally so long as the voltage across the capacitor 14 is above a predetermined level. The time constant of the charge storage circuit formed by the capacitor 14 is such that the safety circuit remains energised until the contacts RL1/1 and RL2/1 have reached the position shown by broken lines. Thereafter the safety circuit is directly energised from the power supply and the capacitor 14 is recharged.

When the contacts RL1/2 and RL2/2 move to the position shown by broken lines, energy is supplied to the coils of contactors (not shown) which are connected to terminals 19, 20 and 21, 22. The protected machine is then fully energised and can operate normally. If one or both of the first and second safety conditions are no longer met, for example as the result of the opening of a door of an enclosure protecting the machine, one or both of the contacts RL1/2 and RL2/2 switches back to the position shown in full lines. As the contactors connected to the terminals 19 to 22 are connected in series with each other and with the machine the de-energisation of any one of the contactors causes the machine to be disabled.

The contacts RL1/1 and RL1/2 are physically linked together so that they must move together between the positions shown in broken lines and the positions shown in full lines. The same is true of the contact pair RL2/1 and RL2/2. If one of the contacts fails as a result for example of mechanical failure or welding of the contacts in the machine enabling position, then if the machine is turned off and then an attempt is made to restart the machine, this is prevented as the safety circuit is not energised. For example, if the contact RL1/2 is welded in the position shown by broken lines, the contact RL1/1 is also held in the position shown by broken lines. If an attempt is made to start the machine energy cannot reach the safety circuit and therefore the relays RLl and RL2 remain de-energised. The contact RL2/2 cannot therefore switch to the position shown in broken line and therefore the machine cannot be started. As another example, if the contact RL1/1 is welded in the position shown by full line, the safety circuit can be energised. However, although the relays RLl and RL2 can then be energised when this occurs the contact RL2/1 switches to the position shown in broken line, and the safety circuit is de-energised after a short period determined by the storage capacity of the capacitor 14. The machine cannot therefore be energised. The circuitry is thus inherently fail safe.

In the illustrated arrangement, a switch 23 is provided in the circuit connected to the terminals 19 to 22. In the position shown in full lines, two series connected contactors are connected to the terminal pairs 19, 20 and 21, 22. If the device is intended to be used to control a single contactor, the contacts of the switch 23 are moved to the position shown by broken lines and the single contactor is connected between terminals 19 and 22. The single contactor will thus still only be energised if both contacts RL1/2 and RL2/2 are switched to the positions shown by broken lines.

In the described circuit, two independent relay energising safety circuits 15 and 16 are provided. A single relay could be provided however driving all of the contacts RL1/1 RL1/2, RL2/1, RL2/2. The single relay could be responsive to only one or a series of safety condition inputs. Such a system is still inherently safe as the safety circuit is de-energised if the contacts RL1/1 and RL2/1 do not move simultaneously between the positions shown in the time taken for the capacitor 14 to discharge below a predetermined voltage level.

Claims

CLAIMS:

1. A circuit for supplying electrical power to a power consuming device, comprising an electrically operable power switch connected between a power supply and a power consuming device, a safety circuit arranged to monitor at least one safety- related condition of the power consuming device, the safety circuit controlling the energisation of at least one relay in dependence upon the said at least one monitored condition, a first set of contacts controlled by the said at least one relay and connected to supply power to the safety circuit, and a second set of contacts controlled by the said at least one relay and connected to enable or disable the power switch, wherein the first set of contacts comprises at least two contacts arranged to be switched simultaneously and connected in series such that the supply of power to the safety circuit is maintained except during switching of the contacts, the safety circuit comprises an energy storing circuit arranged to maintain the energisation of the safety circuit for a period of time at least as long as the period of time required for the first set of contacts to switch, and the second set of contacts is arranged so as to disable a power switch if the safety circuit is de- energised.

2. A circuit according to claim 1, wherein at least two relays are provided, each relay being energised in dependence upon a respective one monitored condition of the power consuming device.

3. A circuit according to claim 1 or 2, wherein the energy storing circuit is a capacitor.

4. A circuit according to any preceding claim, wherein the power switch is a contactor switchable by a control current generated by depression of a manual start button, a current being delivered to the contact or via the second set of contacts.

^~ 5. A circuit according to claim 4, wherein two contactors are provided, each being supplied via a respective contact of the second set of contacts.