GB2077501A - Safety electrical switch arrangement - Google Patents

Abstract

An electric joystick control comprises a pair of switch contacts 18,20 for disabling the controlled equipment. In the neutral, undepressed condition of the joystick handle 10, an annular contact 20 on the handle stem 10 locates within an insulating gate 16 and the handle must be depressed against spring bias before it can be pivoted to execute controls, but if the depressing force is removed whilst the handle is pivoted out of its neutral position, then contact 20 will engage contact 18, which is formed on the lower side of gate 16. When the contacts 18,20 close, relays are operated to cause the controlled equipment to be disabled until the controls 18,20 are opened. If the contacts 18,20 become corroded resulting in an increased contact resistance, the controlled equipment is disabled until the contacts 18,20 have been cleaned. <IMAGE>

Description

SPECIFICATION Safety electrical switch arrangement This invention relates to a safety electrical switch arrangement in or for a manually operable electrical control, to provide a "dead mans handle" control, particularly but not solely for a crane.

On desk master controllers in crane cabs (controlling the electrical power to the drives for various crane motions such as hoist, lower, traverse, slew etc.), it is known to provide a "dead mans" feature on manual controls which comprise pivotable or rotatable handles. One example of control handle comprises a pivoted joystick provided with a pushbutton in its head, which push-button must be maintained depressed by the thumb of the operator in order to keep an electric switch actuated to enable the power output of the controller. In another example, the joystick must itself be maintained depressed relative to a rod extending up and within the joystick stem, for the rod to keep an electric switch actuated within the head of the joystick.The joystick with push-button is not ideal to operate, whilst the joystick with electric switch in its head has the inconvenience of an external electric cable leading to the head. Both forms of control handle tend to be complex and expensive to construct.

In accordance with this invention, as seen from one aspect, there is provided an electric control incorporating a safety switch arrangement and comprising a manually pivotable handle which is manually depressible to actuate a pair of contacts of the safety switch arrangement, one said contact being mounted on and projecting radially outwards from the handle stem and the other contact being fixed and arranged to co-operate selectively with said one contact depending whether the handle is depressed or not.In an embodiment to be described herein, the one contact is an annular element mounted concentrically on the stem of a joystick handle and, in a neutral position of the joystick, this annular element engages within a peripheral insulated gate: the joystick must be depressed for movement out of the neutral position, but if it is then relaxed the annular element makes contact with its other contact, which is in the form of peripheral contact provided on the lower side of the insulated gate.

Also in accordance with this invention, as seen from a second aspect, there is provided a safety electrical switch arrangement in orfora manually operable electrical control, comprising an electric circuit having a pair of electrical contacts and arranged to provide a first electrical output signal when the contacts are open, for enabling a control equipment, and arranged to provide a second electrical output signal when the contacts are temporarily closed, for temporarily disabling the control equipment, the circuit being arranged to latch in a condition providing said second electrical output signal when the resistance between said contacts is above a first threshold value but below a second threshold value.Thus, if the contacts become corroded so that the contact resistance is above the first threshold when the contacts are closed (but below the second threshold representing open-contacts), the circuit latches so as to disable the control equipment until the contacts are cleaned.

An embodiment of this invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure lisa perspective partial view from below of a joystick-type electrical control incorporating a safety switch arrangement; Figure 2 is a block circuit diagram of the safety switch arrangement; and Figure 3 is a detailed circuit diagram of part of the safety switch arrangement.

Referring to Figure 1, a joystick-type electrical control comprises a stem 10 extending downwards from a head 12, through an aperture 13 in an upper wall 14 of a desk master controller and into the interior of the controller. The joystick is journalled by means not shown, for manual pivotal movement in any direction and its lower end operates a plurality of electrical contacts according to the position to which the joystick is pivoted: these contacts control the supply of electrical power, for example to the drives for certain crane motions. Preferably a rubber shroud is provided around the joystick stem, being sealed at its upper end to the joystick and at its lower end to the wall 14.

An insulated gate 16 is provided around the edge of the aperture 13 and is provided on its lower peripheral edge with an electrical contact 18. An annular contact element 20 is mounted concentrically on the joystick stem 10 and is connected to earth through the stem, as shown. The joystick is depressible lengthwise of its stem against a spring bias and is shown in its depressed condition: in a neutral position and non-depressed condition, the contact element 20 engages within the insulated gate 16 and in this neutral position no electrical power is supplied to the crane drives. Thus, in order to energise and control the cranes drives, the joystick must be depressed to disengage the contact element 20 from within the insulated gate, whereafter the joystick may be pivoted to desired positions.However, if the manual force depressing the joystick is relaxed whilst the joystick is in any position other than its neutral position, the contact element 20 will abut the contact 18to complete an electrical circuit which is arranged to disable the power to the crane drive.

Accordingly, a "dead mans handle" safety feature is provided by means of a simple and reliable construction, with contacts 18 and 20 forming the contacts of the safety switch arrangement.

Referring to Figure 2, the circuit of the safety switch arrangement is supplied with electrical power for the overall crane or other equipment at 30, and an isolating power unit 32 provides a low voltage (e.g. 12 volts) D.C. supply at 34 for a sensing and monitoring circuit 36 incorporating the safety switching feature. If the crane power supply is A.C., then the power unit comprises a step-down isolating transformer followed by a rectifier, whilst if the crane power supply is D.C., the power unit comprises a D.C. - A.C. converter followed by an isolating transformer and rectifier.

The circuit 36 is controlled by the two contacts 18, 20 of the safety switching arrangement and operates a change-over indicated at 38 to connect a signal voltage, applied to an input 40, eitherto an output 42 in a "contacts-closed" condition of circuit 36 orto an output 44 in a "contacts-open" condition. A signal appearing at output 42 is used to disable the electrical power to the crane drives.

Referring to Figure 3, the circuit 36 comprises a capacitor C1 connected across supply terminals L1, L2 and the series connection of a resistor R1 and a zener diode ZD1 connected in parallel with capacitor C1. Three series connections are connected in parallel with zener ZD1, respectively comprising a resistor R2, a transistor T1 and a relay RL1, resistors R3, R4 and a switch S1 (comprises of contacts 18,20), and a resistor R5, a transistor T2 and a relay RL2. The junction point A between the three resistors R2,R3,R5 is connected through contacts RL21,RL11 of relays RL2,RL1 and then through relays RL3, RL4 in parallel to earthed terminal L2. Contacts RL31 and RL41 of relays RL3,RL4 are both connected to provide a change-over of signal input 40 between signal outputs 42 and 44.The junction point A is connected through a change-over (comprising contacts RL32 and RL42 of relays RL3,RL4) and a resistor R7 to the junction between relay RL2 and the collector of transistor T2, which junction is also connectable to junction pointAthrough resistor R7 and contacts RL12 and RL21 of relays RL1 and RL2. The junction between relay RL1 and the collector of transistor T1 is connectable to junction point A through a resistor R6 and relay contacts RL11.

With the "dead mans handle" contacts of switch S1 open, both transistors T1, T2 are non-conductive and both relays RL1 and RL2 are de-energised, so that their contacts RL11, RL12 and RL21 assume the conditions shown. Both relays RL3 and RL4 are therefore energised through contacts RL21 and RL11 and their contacts RL31, RL32, RL41 and RL42 take up the conditions shown. Thus signal input 40 is connected to signal output 44 and this output is used to provide a no-fault indication.

Should the "dead mans handle" contacts of switch S1 close, then both transistors T1,T2 are rendered conductive to energise both relays RL1,RL2, so that both contacts RL21 and RL11, interrupt the current to relays RL3 and RL4, which are therefore deenergised. Contacts RL31 and RL41 are changed over to connect input 40 to output 42, which output is used to disable the power to the crane drives as a consequence of the joystick no longer being depressed.

Resistors R2 and R5 are proportioned so that, should the contacts 18,20 of switch S1 become corroded such that the contact resistance is above a first threshold value (for example 4.5 kQ) when they are closed, then transistor T1 remains nonconductive, whilst transistor T2 is rendered conductive. Consequently, contact RL21 changes from the condition shown, whilst contacts RL11, and RL12 remain unchanged, so that relay RL2 is latched in its energised state through contacts RL21,RL12 and resistor R7. Relays RL3 and RL4 are de-energised so that input 40 is connected to output 42 to disable the crane drives.Because relay RL2 is latched energised by its own contact RL21, this contact ensures that relys RL3,RL4 cannot become energised until the contacts 18,20 of switch S1 are cleaned to return the contact resistance below the 4.5 kQ threshold, whereby transistor T, will be rendered conductive upon closure of switch S1. For the purpose of defining the "open contacts" condition, both transistors are non-conductive if the contact resistance is above a threshold of 7.5 kQ, for example.

The circuit of Figure 3 incorporates a number of other safety features. If the power to the circuit fails, or if their is any internal fault so that relays RL3 and RL4 are de-energised, then contacts RL31 and RL41 change to provide a signal at output 42 disabling the crane drives. Should relay RL1 become energised but not relay RL2, then relay RL1 becomes latched energised by its own contact RL11 through resistor R6 and relays RL3 and RL4 are de-energised. Should either relay RL3 or RL4 fail, the change-over formed by contacts RL32 and RL42 established a circuit to latch relay RL2 in energised condition, thus deenergising both relays RL3 and RL4.

Although the circuits of Figures 2 and 3 have been described for use with the control of Figure 1, they may instead be used with other controls still to provide the safety switching function. It will be appreciated that since only a low voltage ever appears across the sensing contacts, these contacts could comprise exposed metal elements.

In a modification of the circuit of Figure 2, the relay contacts RL41 are omitted from the output section shown, which thus comprises terminals 40,42,44 and contacts RL31, and a second, similr, output section is provided, comprising contacts only RL41, similarly connected to three terminals. A no-volt relay is then connected across terminals 40,42 of each output section and arranged so that both are required to be energised in order to enable the crane drives (and so that the crane drives are disabled if either relay is de-energised).

Claims (12)

1. An electric control incorporating a safety switch arrangement, and comprising a manually pivotable handle which is manually depressible to actuate a pair of contacts of the safety switch arrangement, one said contact being mounted on and projecting radially outwards from the handle stem and the other contact being fixed and arranged to co-operate selectively with said one contact depending whether the handle is depressed or not.

2. An electric control as claimed in claim 1, in which the handle comprises a joystick handle and the one contact comprises a generally annular element mounted concentrically on the handle stem, arranged such that in a neutral and undepressed position of the joystick handle the annular element engages within a peripheral insulated gate and out of contact with said other contact.

3. An electric control as claimed in claim 2, in which said other contact comprises a peripheral contact provided on the insulated gate and arranged such that the one contact will close therewith if the depressive force on the joystick handle is relaxed once it is other than in its neutral position.

4. A safety electrical switch arrangement in or for a manually operable electric control such as that claimed in any preceding claim, comprising an electric circuit incorporating the or a pair of electrical contacts and arranged to provide a first electrical output signal when those contacts are open, for enabling a control equipment, and arranged to provide a second electrical output signal when those contacts are temporarily closed, for temporarily disabling the control equipment, the circuit being arranged to latch in a condition providing said second electrical output signal when the resistance between the contacts is above a first threshold value but below a second threshold value.

5. A safety electrical switch arrangement as claimed in claim 4, in which the pair of contacts are connected in circuit with first and second normally non-conductive transistor switches controlling respective first and second relays, and arranged for both transistor switches to be rendered conductive and energise both said relays if the contacts close and their contact resistance is below said first threshold value, but for only the second transistor switch to be rendered conductive and only the second relay energised if the contacts close and their contact resistance is above the first threshold value but below the second threshold value.

6. A safety electrical switch arrangement as claimed in claim 5, in which the first and second relays have respective relay contacts connected in series on a current path to a relay means which is energised causes provision of said first, enabling electrical output signal and which if de-energised causes provision of said second, disabling electrical output signal.

7. A safety electrical switch arrangement as claimed in claim 6, in which said relay means comprises third and fourth relays in parallel, both of which are required to be energised for provision of said first, enabling electrical output signal.

8. A safety electrical switch arrangement as claimed in claim 7, in which a change-over means is provided, comprising relay contacts of said third and fourth relays, and arranged so that if either one of said third and fourth relays fails, then said second relay is self-latched energised.

9. A safety electrical switch arrangement as claimed in any one of claims 5 to 8, in which said second relay is arranged for self-latching in the event that it is energised without said first relay being energised.

10. A safety electrical switch arrangement as claimed in any one of claims 5 to 9, in which said first relay is arranged for self-latching in the event that it is energised without said second relay being energised.

11. An electric control incorporating a safety switch arrangement and substantially as herein described with reference to the accompanying drawings.

12. Asafetyelectrical switch arrangement substantially as herein described with reference to Figure 2 of the accompanying drawings.