Patent Application
20080078658
A specific embodiment of the invention will now be described by way of example only, with reference to the accompanying drawings in which:
a through 2c are perspective views of a safety switch which embodies the invention;
A safety switch according to the present invention is shown in
The safety switch 50 may be provided at a door of equipment/machinery, and arranged such that the door may be opened only when the safety switch has been actuated. For example, the key 53 may be mounted on the door, with the housing 51 being mounted on a door post, such that the door cannot be opened without first removing the key from the housing. The safety switch 50 is arranged to interact with a control system to control the supply of power to the machine, such that power cannot be supplied to the machine once the safety switch has been actuated.
Referring to
Referring to
Although the switches have been described as being operated by a cam, it will be appreciated that any other suitable mechanism may be used. Similarly, the locking claw 54 and arm 55 may be replaced by any other suitable mechanism arranged to prevent the key 53 from being removed from the housing until a verification signal is received.
The set of switches 100 comprises four switches 101-104. First and second switches 101, 102 are normally closed, whereas third and fourth switches 103, 104 are normally open. The set of switches is of the break before make type, such that the first and second switches 101, 102 will open before the third and fourth switches are allowed to close.
When the first and second switches 101, 102 are closed, the circuit generates an output which causes the control system to allow power to be supplied to the machine. When the first and second switches 101, 102 are opened the circuit provides an output which causes the control system to interrupt the supply of power to the machine.
When the first switch 101 is closed it provides a closed circuit between a first terminal 701 of the circuit, via a first door actuated contact 401 and a pair of linked terminals 702, to a second terminal 703 of the circuit. Similarly, when the second switch 102 is closed it provides a closed circuit between a third terminal 704 of the circuit, via a second door actuated contact 402 and a second pair of linked terminals 705, to a fourth terminal 706 of the circuit. The closed circuits are monitored by the control system, which has inputs connected to the first, second., third and fourth terminals 701, 703, 704, 706. The control system may be located adjacent to the power supply for the machine. This may be some distance away from the machine itself.
The control system is shown schematically in
The control system 800 includes an electric power isolation system 804 and a pneumatic (or, alternatively, hydraulic) power isolation system 806. The electric power isolation system 804 receives three-phase power from a three-phase power source (not shown) by way of an electrical input port 822. The three-phase power received at the electrical input port 822 is provided both to an electrical isolation module 824 and a control power module 826. The control power module 826 converts the three-phase power into 120 Volt AC power, which it then provides to an internal control module 828.
The internal control module 828 governs the operation of the electrical isolation module 824 based upon signals that it receives from the safety switch 51. When an open circuit occurs across the first and second terminals 701. 703 and/or the third and fourth terminals 704, 706, the internal control module 828 causes isolation contactors 830 within the electrical isolation module 824 to open so as to disconnect the three-phase power received at the electrical input port 822 from an electrical output port 832.
Subsequently, grounding contactors 834 within the electrical isolation module 824 are further actuated by the internal control module 828 so as to couple the electrical output port 832 to ground. More particularly, in the preferred embodiment, the electrical output port 832 is coupled to an isolated ground port 836 of the electrical power isolation system 804. Also as shown, the electric power isolation system 804 includes a protective earth (PE) input terminal 840, and a PE output terminal 842. The PE input terminal 840 is coupled to a standard earth ground, and the PE output terminal 842 is coupled both to the PE input terminal 840 and in turn to the machine 801 such that the machine has access to the standard earth ground. Thus, by virtue of the operation of the isolation contactors 830 and the grounding contactors 834, the machine 801 is entirely isolated from the three-phase power source and grounded.
Further as shown in
Thus, when an open circuit is detected across the terminals 701, 703, 704, 706, the machine 808, which is coupled to each of the output ports 832 and 850, ceases to receive any electrical or pneumatic power and is isolated from the power sources coupled to the input ports 822 and 844. However, when a closed circuit is established across the terminals 701, 703, 704, 706, the internal control module 828 causes the electrical power isolation system 804 and the pneumatic isolation system 806 to maintain the connections between those power sources and the machine 808.
The control system 800 further comprises a verification module 810, which is arranged to monitor the terminals 701, 703, 704, 706 and to monitor signals output from the internal control module 828.
When the key 53 of the safety switch 50 is moved to the first position (see
The switches 101-104 are linked such that they operate together, with the qualification that those switches which are closed will move to open configurations before those switches which are open are allowed to move to closed configuration. Thus, once the first and second switches 101, 102 have opened, the third and fourth switches 103, 104 are closed. The third switch 103 does not have any effect at this stage, since no power is supplied to a fifth terminal (terminal 707) to which it is connected.
Closing the fourth switch 104 allows a 24 Volt power supply (not illustrated) to supply power through a sixth terminal (terminal 708), via a fuse 301 and a pair of resistors 302.303, to a yellow LED 304. The yellow LED 304, which may be located adjacent to the switch, is thereby illuminated, indicating that a request to turn off the power supply has been sent from the safety switch circuit to the control system 800.
Once the control system 800 has isolated the machine from the power supply (the manner in which this is done is described above), this is communicated from the control module 828 to the verification module 810. The verification module 810 then sends a verification signal to the fifth terminal 707 (i.e. it applies a voltage to the fifth terminal). This will only occur if the verification module 810 has not detected an error. There may be a time delay between the verification module 810 receiving the signal from the control module and sending the verification signal to the fifth terminal 707.
The verification signal passes via a fuse 801, the closed third switch 103, and a pair of linked terminals 710 to a solenoid 201. The solenoid 201 is energised, and on being energised actuates the solenoid actuated monitoring contacts 200. First and third monitoring contacts 202, 204 move from closed configurations to open configurations. The second monitoring contact 203 moves from an open configuration to a closed configuration.
The effect of closing the second monitoring contact 203 is to allow the verification signal to pass to the second status LED 500 via a resistor 501. The second status LED 500, which is preferably green and is located adjacent to the safety switch, is illuminated. This indicates that the machine has been isolated from the power supply.
The claw 54, described above in relation to
In some instances the machine may be sufficiently large that for practical purposes more than one door is required. Where a plurality of doors is provided, each is provided with its own safety switch 50 and safety switch circuit. When the key associated with a given safety switch is moved to the first position., as shown
Access to the machine is possible at other locations by moving the key of the safety switch at that location to the first position. This causes the third switch 103 to close, thereby causing the solenoid 201 to be actuated, and allowing the key to be fully removed from the safety switch by a user. The user may then access the machine via the door controlled by that safety switch. An advantage of this arrangement is that the verification signal from the control panel does not cause all of the safety switches connected to the control panel to unlock. Instead, the only switches that are unlocked are those at which an access request has been made (i.e. by moving the key to the first position).
In some instances the verification module may be configured to incur a delay between receiving an output from the control module 828 indicating that power to the machine 801 has been interrupted, and the verification signal being sent to the fifth terminal 707. This may be necessary if for example it takes a few seconds for the machine 801 to come to a complete stop after it has been isolated from the power supply. The user will know that a request to isolate the machine from the power supply has been sent, since this is indicated by the yellow LED. It should be understood that it is not possible to remove the key 53 from the safety switch housing 50 until the green LED is illuminated. The user will also be informed that upon a request for entry there must be a sequence of yellow LED illuminated then green LED illuminated. The user will further understand to enter only when the correct sequence has occurred.
In some instances damage might allow the door of the machine to be opened without the safety switch 50 being operated. For example, in one arrangement the key 53 may be provided on a chain which is secured to a door of the machine 801. This means that the door can only be opened when the key 53 has been removed from the safety switch housing 51 thereby actuating the switches 101-104 as described above. However, the chain which connects the key 53 to the door ma) be broken, thereby allowing the door to be opened without removing the key 53 from the housing 51. The set of door actuated contacts 400 is included in the safety circuit to ensure that access to machine 801 cannot occur whilst the machine is in operation.
The set of door actuated contacts 400 are reed switches which are actuated by a door of the machine 801. When the door is closed, the first and second door actuated contacts 401, 402 remain closed. If the door is opened without first operating the safety switch, then the door actuated contacts 401, 402 move to open configurations, thereby causing open circuits across the terminals 701, 703 and 704, 706. The control system thus isolates the machine from the power supply 122. An auxiliary contact 403, which may also be a reed switch, is also actuated by the door and causes power to be supplied to an LED 404 via a relay (not illustrated). The illuminated LED shows that the door is open. The LED may be for example replaced by an illuminated sign which indicates that the door is open.
It will be appreciated that the set of door actuated contacts 400 are actuated even if the switch has been correctly operated. This will have no effect at the terminals 701. 703 and 704,706, since an open circuit already exists across them. It will however cause the LED to be illuminated to show that the door is open. In addition, the set of door actuated contacts ensures that power cannot be returned to the machine 801 until the door is closed.
The first and third monitoring contacts 202, 204 are provided to ensure that if the claw of the switch becomes stuck in the retracted position, it is not possible to accidentally open the door by removing the key from the switch, without first causing the power supply to be switched off. In effect, the first and third monitoring contacts 202, 204 monitor the position of the solenoid 400.
The verification signal provided from the verification module 810 to the fifth terminal 707 is electrically isolated from other parts of the safety circuit, and is connected to a different ground. This is to ensure that if a spurious cross fault occurs between for example the fourth terminal 706 and the fifth terminal 707, this will not cause the safety circuit to malfunction.
Although the connections from the switch to the supply panel have been described as being from specific terminals it will be appreciated that, depending upon the control system that is used, connections may be made to different terminals. This is the reason for the pairs of linked terminals 702, 702, 710.
Although the set of door actuated contacts 400 have been described as being reed switches, it will be appreciated that any other suitable switches may be used.
