GB2476684A

GB2476684A - A robust foot switch for control of a mooring winch

Info

Publication number: GB2476684A
Application number: GB201000113A
Authority: GB
Inventors: Edward Higgins
Original assignee: STRAINSTALL UK Ltd
Current assignee: STRAINSTALL UK Ltd
Priority date: 2010-01-05
Filing date: 2010-01-05
Publication date: 2011-07-06
Anticipated expiration: 2030-01-05
Also published as: GB2476684B; GB201000113D0

Abstract

A robust foot switch comprises a solid stainless steel rod 104 with a load cell comprising a strain gauge (figures 3-5) mounted in a bore of the rod at a waisted section 12 to detect strain caused by application of foot force to the end of the rod. Deflection of the rod end is slight, and is typically less than 2 mm, so that actuation of the rod is unlikely to be impeded by deposits of coal or ore around the foot switch. The signal output by the strain gauge may be amplified by an amplifier within the bore (9, figure 3). Standard foot switches comprise rubber seals, pivots and materials which are unsuited to a harsh environment.

Description

FOOT SWITCH

This invention relates to a foot switch, that is to a switch which normally rests on the ground and is operable by a foot providing an actuation force, to generate an output electrical switching signal for external apparatus.

Foot switches are used by operators to control electrical equipment. In one example, quick-release mooring hooks are used by large ships when mooring alongside jetties.

A small line on a dedicated winch is used to pull the heavy mooring rope to the hook and to enable the mooring rope end loop to be placed on the hook. The winch is controlled by a foot switch, leaving the operator's hands free. The foot switch has to operate in adverse marine environments with exposure to bad weather, strong sun and salt. The foot switches are treated roughly and often are not maintained, being permanently exposed to the tough conditions. Additional problems are encountered on some jetties, where for example coal or ore gathers around the jetty equipment, introducing aggressive chemicals such as sulphur, and requiring the foot switch to operate in heaps of grit. Standard foot switches for this purpose currently available on the market have rubber seals, pivots and materials which do not stand up well to this aggressive environment. This causes early malfunction and failure. These standard foot switches require displacement of a switch pedal for satisfactory operation: typically, the pedal is flat and extends over a significant area, and it needs to swivel through a substantial angle, such that it needs a vertical gap of 20 mm or 30 mm.

When the switch becomes embedded in a mound of coal dust, the underside of the switch pedal may be obstructed by the material beneath it, rendering the switch inoperable. Also, the heaps of loose material provide a poor surface for the standard foot switch to operate on. The foot switch may lie at the wrong angle, making it difficult to use, and it may be in the wrong' place, positioned to avoid heaps of material, relative to the machine being operated. Thus the switch ergonomics can be detrimental to the ability of the operator to perform his tasks safely and efficiently.

Accordingly, there is a need for a more reliable and easily operable foot switch suitable for such harsh environments.

The present invention provides a foot switch comprising a rod mounted in a housing which is shaped to rest stably on the ground, the rod being a load cell comprising a strain gauge system arranged to sense strain in the rod caused by an actuation force on an outer end of the rod, the inner end of the rod being held rigidly by the housing, and a cable terminal electrically connected to the strain gauge system to provide an electrical output switching signal in response to the sensed strain.

By providing a rod load cell, instead of a pivoted pedal arrangement, very little space is required for the movement of the rod when depressed by the operator's foot. This makes the foot switch of the invention substantially less susceptible to obstruction.

The rod is preferably cylindrical, and this helps further to reduce the likelihood of obstruction, as the rod can displace loose objects and material more easily than a flat pedal.

In order that the invention may be better understood, a preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. Ia is a side elevation of a foot switch embodying the invention; Fig. lb is an end elevation from the right hand side of the foot switch of Fig. Ia; Fig. ic is an underneath plan view of the foot switch of Fig. la; Fig. 2 is an enlarged top plan view of part of the foot switch of Fig. Ia; Fig. 3 is an axial cross-section vertically through part of the foot switch of Fig. 2; Fig. 4 is a circuit diagram of a conventional load cell used in the foot switch of Fig. 2; Fig. 5 is a diagram of one configuration of the load cell in the foot switch of Fig. 2; and Fig. 6 is a diagram of an alternative configuration of the load cell in the foot switch of Fig. 2.

A foot switch 100 embodying the present invention has a bored solid metal bar I supported generally horizontally above the flat ground 106 by a housing 101, 102. The bar I is stiff and has no moving parts, in contrast to the pivots, pistons etc. of conventional foot switches. The bar has a main cylindrical portion 104 which tapers inwardly at its outer end so as to present a smoother, flatter surface to the sole of a boot 105 which is used to provide a downward actuation force on the end of the rod to cause the switching action. As shown in Fig. Ia, the operator's foot rests with the heel on the ground 106 and with the ball of the foot providing the actuation force on the rod.

A short inner end 103 of the rod is cylindrical and has a larger diameter than the main portion 104; it is formed with axial bores 3 around its periphery, to accommodate bolts (not shown) which connect the rod rigidly to a vertical support plate 2 which rests stably on the ground 106 in use. The support plate 2 has a circular aperture on the axis of the rod for connection to a cable terminal 102, to which is removably connected a conventional armoured electrical cable 5. A fluid tight seal is formed between the cable and the rod 1, both of which have interior cylindrical cavities, accommodating the wires, circuit components and load cell described below.

The housing for the bar 1 further comprises a pair of triangular plates 101 joined integrally at a ridge which extends longitudinally parallel to the rod and directly over the rod in use. The plates are bent, during manufacture, from a flat piece that is arrow-shaped. The ridge is inclined to the horizontal 106 at an angle of about 30°, to assist in keeping it clear of liquid and debris. As shown in Fig. lb most clearly, the triangular plates 101 are also inclined, typically at about 45° to the horizontal. Each triangular plate 101 has a wing which extends outwardly away from the outer end of the rod, and terminates at a foot 108 which rests stably on the ground 106 in use at a point spaced outwardly from the rod. Thus the housing rests on the ground on its two feet 108 and on its vertical support plate 2, to provide supports on longitudinally opposite sides of the point of application of the vertical actuation force. The tripod support arrangement provides stability for the foot switch in uneven environments, and provides stability for the operation of the switch by the operator's foot.

As shown in Fig. Ia most clearly, the foot 105 is partially accommodated by the housing 101 which provides sufficient clearance for easy placement of the boot over the rod I, even in harsh environments.

The rod 1 is supported within the housing with a substantial vertical spacing between the underside of the rod 104 and the feet 108 as well as the base of the plate 2, so as to provide an air gap between the rod 104 and the ground 106 which is typically between 1110th and 1 or 2 times the diameter of the main portion of the rod 104. This minimises the risk of loose materials obstructing the operation of the foot switch, which risk is low in any event as the displacement required is minimal. In this example, the displacement of the outer end of the rod 1, relative to the housing, need only be less than 0.5 mm, and preferably in the range 0.01 mm to 2mm, more preferably 0.1 mm to 2mm, in any event.

The two triangular plates 101 are interconnected by a horizontal strut 107 parallel to the vertical plate 2 and close to the end of the rod. This stabilises and stiffens the housing. It also serves as a barrier to prevent large objects from entering the space inside the housing and interfering with the operation of the foot switch.

The rod I has a waist portion 12 between the main portion 104 and the inner portion 103, the waist presenting a parttoroidal outer surface as shown most clearly in Figs. 2 and 3. The axial length of this waist portion is approximately the same as that of the inner end portion 103, which is about 118th of the overall length of the rod 1. In other embodiments, the waist is radiused at each end, but is cylindrical over a middle portion.

The rod I has a cylindrical bore 6 of a diameter slightly smaller than the inner diameter of the centre of the waist portion 12.

The rod 1 flexes preferentially at the waist portion 12, and the risk of metal fatigue and fracture is minimised by the radiussing, i.e. by the part-toroidal shaping of the waist portion as shown in this particular example. Vertical depression of the end of the bar causes a proportionate strain occurring mainly at the waist portion. The dimensions of the rod are selected, in relation to the Young's Modulus of the rod material, such that a force of about lokgf causes linear movement of the outer end of the rod of about 0.5 mm or less, relative to the housing. As described below, a strain gauge system senses the strain resulting from the applied actuation force. A predetermined strain triggers the generation of a switching signal, for controlling external apparatus (not shown).

The threshold actuation force is preferably in the range of 1 to 2okgf, preferably 5 to l5kgf, and usually about lokgf.

As shown in Figs. 3 to 6, a strain gauge system 7 comprising at least one strain gauge is positioned within the waist portion 12 of the rod 1 so as to sense strain in the rod wall. Various types of strain gauge may be used, but in this example a conventional arrangement of four strain gauges is used. The strain gauges, typically piezoelectric devices, are bonded by adhesive to the inner wall of the rod. As shown in Fig. 4, two of the strain gauges are under compression, and two are under tension, when the vertical actuation force is applied to the end of the rod. Connected in a Wheatstone Bridge arrangement, a power supply is connected across two terminals of the bridge and a signal is supplied from the other two terminals of the bridge. Supply or excitation voltages V+ and V-are supplied to the bridge, and signal voltages S-f and S-are delivered at the other terminals.

Two alternative arrangements of the load cell are illustrated in Figs. 5 and 6. In Fig. 5, strain gauges TI and T2 sense tension in the upper portion of the wall of the rod, and strain gauges Ci and C2 sense compression in the lower portion of the wall, diametrically opposite to strain gauges Ti and T2. In the arrangement of Fig. 6, strain gauges Ci and TI respectively sense compression and tension in the side wall, and strain gauges T2 and C2 respectively sense tension and compression in the opposite side wall. In each case, a pair of wires from each strain gauge is connected to the Wheatstone Bridge arrangement illustrated in Fig. 4.

As shown in Fig. 3, wires 10 connect the Wheatstone Bridge circuit both to the terminal for the external cable 5 and to an internal amplifier circuit 9 accommodated within the bore 6 which is terminated at the outer end of the rod by a cap Ii forming a fluid tight seal. The amplifier circuit 9 amplifies the signal Si-, S-and supplies the amplified signal as an output signal to the terminal for the external cable. External electrical apparatus (not shown) receives the output signal and processes it to control a switching operation. For example, it may have a control circuit for comparing the amplified signal with a predetermined threshold voltage corresponding to a predetermined actuation force applied to the end of the rod, to generate an output electrical switching signal for switching external apparatus. In this way, when the operator depresses the foot switch at or above the predetermined threshold force, the output switching signal switches on or off the external apparatus. The electronic circuitry may be such as to switch the external apparatus alternately on and off.

Alternatively, it could be arranged to switch the external apparatus on only for as long as the force remains above the threshold force.

Preferably, the rod is of marine grade stainless steel, to provide a long life in adverse environments and it is completely sealed against dust and water. All seals are welded, except for the signal cable which is sealed by a compression gland and is also protected within a durable hose 5. The hose end fittings are sealed to the rod to resist long term damp conditions and immersion in puddles or pools of shallow water. The sealing life of the foot switch is enhanced by the fact that the rod is raised above the ground 106, such that the rod is usually in a dry surrounding.

The triangular ridged shape of the housing is distinctive to make it easily recognisable, and also to provide a tripod arrangement which gives great stability and a predictable attitude. The feet 108 have a relatively small area and are pointed, allowing them to penetrate heaps of loose material, to stand securely on firm ground beneath that material. The balance and weight of the switch contributes to its stability. The ridged roof structure of the housing causes debris to be shed by gravity, and it protects the switch element inside from mechanical abuse, from weather and from direct sunshine.

The bar is also protected from objects that would otherwise rest on it and cause it to operate inadvertently. The pointed tail of the triangular roof, on the left hand side of Fig. 1, extends over the cable 5 to provide protection at its most vulnerable point, where it exits the switch element.

The foot switch is of simple and durable construction, and if maintenance or repair is required, then the switch element, i.e. the bar 1, may be replaced by removing the bolts from the bores 3.

In this embodiment, the rod is generally horizontal, but this is not essential, provided the rod is actuable by foot pressure to undergo strain. For example, a lever arrangement could be used to convey torque to the rod.

Claims

CLAIMS1. A foot switch comprising a rod mounted in a housing which is shaped to rest stably on the ground, the rod being a load cell comprising a strain gauge system arranged to sense strain in the rod caused by an actuation force on an outer end of the rod, the inner end of the rod being held rigidly by the housing, and a cable terminal electrically connected to the strain gauge system to provide an electrical output switching signal in response to the sensed strain.
2. A foot switch according to claim 1, in which the rod extends generally horizontally above the ground when the housing rests on flat ground.
3. A foot switch according to claim I or 2, in which the strain gauge system comprises four strain gauges electrically connected in a Wheatstone Bridge circuit.
4. A foot switch according to claim 1, 2 or 3, comprising an amplifier circuit in the rod arranged to amplify a signal from the strain gauge system to provide the output switching signal.
5. A foot switch according to any preceding claim, in which the cable terminal is connected to power supply wires and signal wires connected to the strain gauge system.
6. A foot switch according to any preceding claim, in which the rod is of solid metal with an axial bore.
7. A foot switch according to claim 6, in which the strain gauge system comprises at least one strain gauge attached to the axial bore.
8. A foot switch according to any preceding claim, in which the rod is formed with a waist portion between its outer and inner ends, the waist portion being narrower than the remainder of the rod so as to undergo greater strain in response to the actuation force.
9. A foot switch according to claim 8, in which the strain gauge system is positioned at the waist portion.
10. A switching system comprising a foot switch according to any preceding claim, and a control circuit connected to receive the output switching signal and responsive to the sensed strain exceeding a predetermined threshold to output a control signal for switching external apparatus, in which the predetermined threshold corresponds to an actuation force in the range of I to 20 kgf.
11. A switching system according to claim 10, in which the predetermined threshold corresponds to an actuation force in the range of I to 20 kgf.
12. A switching system according to claim 11, in which the predetermined threshold corresponds to an actuation force in the range of 5 toI5 kgf.
13. A foot switch or switching system according to any preceding claim, in which the predetermined threshold corresponds to a linear depression of the outer end of the rod relative to the housing of 0.01mm to 2mm.
14. A foot switch or switching system according to claim 13, in which the predetermined threshold corresponds to a linear depression of the outer end of the rod relative to the housing of less than 0.5mm.
15. A foot switch or switching system according to any preceding claim, in which the rod is sealed to be fluid tight to protect the strain gauge system.
16. A foot switch or switching system according to any preceding claim, in which the housing comprises a support plate connected to the inner end of the rod and arranged to rest on the ground in use.
17. A foot switch or switching system according to any preceding claim, in which the housing comprises a cover over the rod and arranged to rest on the ground in use.
18. A foot switch or switching system according to claim 17, in which the cover comprises a pair of triangular plates joined at a ridge over the rod and aligned with the axis of the rod, the plates being arranged to rest on the ground adjacent the outer end of the rod in use.
19. A foot switch or switching system according to any preceding claim, in which the housing has portions which rest on the ground in use both at the inner end of the rod and outwardly of the outer end of the rod, such that in use the actuation force is applied between those portions of the housing.
20. A foot switch or switching system according to any preceding claim, in which the housing is arranged in relation to the rod such as to provide an air gap between the rod and the ground in use.
21. A foot switch or switching system according to claim 20, in which the air gap is between 0.1 and 2.0 times the diameter of the rod.
22. A foot switch or switching system according to any preceding claim, comprising a cable electrically connected to the cable terminal and sealed fluid tight to the rod.
23. A foot switch or switching system, in which the rod is cylindrical.
24. A ground-standing foot switch substantially as described herein with reference to the accompanying drawings.
25. A method of operating a foot switch according to any preceding claim, comprising depressing the rod with foot pressure to generate the switching signal.Amended claims have been filed as follows:-CLAIMS1. A foot switch comprising a rod mounted in a housing which is shaped to rest stably on the ground, the rod being a load cell comprising a strain gauge system arranged to sense strain in the rod caused by an actuation force on an outer end of the rod, the inner end of the rod being held rigidly by the housing, and a cable terminal electrically connected to the strain gauge system to provide an electrical output switching signal in response to the sensed strain.2. A foot switch according to claim 1, in which the rod extends generally horizontally above the ground when the housing rests on flat ground.3. A foot switch according to claim I or 2, in which the strain gauge system comprises four strain gauges electrically connected in a Wheatstone Bridge circuit.4. A foot switch according to claim 1, 2 or 3, comprising an amplifier circuit in the rod arranged to amplify a signal from the strain gauge system to provide the output switching signal.5. A foot switch according to any preceding claim, in which the cable terminal is connected to power supply wires and signal wires connected to the strain gauge system.6. A foot switch according to any preceding claim, in which the rod is of solid metal with an axial bore.7. A foot switch according to claim 6, in which the strain gauge system comprises at least one strain gauge attached to the axial bore.8. A foot switch according to any preceding claim, in which the rod is formed with a waist portion between its outer and inner ends, the waist portion being narrower than the remainder of the rod so as to undergo greater strain in response to the actuation force.9. A foot switch according to claim 8, in which the strain gauge system is positioned at the waist portion.10. A switching system comprising a foot switch according to any preceding claim, and a control circuit connected to receive the output switching signal and responsive to the sensed strain exceeding a predetermined threshold to output a control signal for switching external apparatus.11. A switching system according to claim 10, in which the predetermined threshold corresponds to an actuation force in the range of 1 to 20 kgf.12. A switching system according to claim 11, in which the predetermined threshold corresponds to an actuation force in the range of 5 tol5 kgf.13. A foot switch or switching system according to any preceding claim, in which the predetermined threshold corresponds to a linear depression of the outer end of the rod relative to the housing of 0.01mm to 2mm.14. A foot switch or switching system according to claim 13, in which the predetermined threshold corresponds to a linear depression of the outer end of the rod relative to the housing of less than 0.5mm.15. A foot switch or switching system according to any preceding claim, in which the rod is sealed to be fluid tight to protect the strain gauge system.16. A foot switch or switching system according to any preceding claim, in which the housing comprises a support plate connected to the inner end of the rod and arranged to rest on the ground in use. * * **.S:: 30 17. A foot switch or switching system according to any preceding claim, in which the housing comprises a cover over the rod and arranged to rest on the ground in use.18. A foot switch or switching system according to claim 17, in which the cover comprises a pair of triangular plates joined at a ridge over the rod and aligned with the S...S S.S S *axis of the rod, the plates being arranged to rest on the ground adjacent the outer end of the rod in use.19. A foot switch or switching system according to any preceding claim, in which the housing has portions which rest on the ground in use both at the inner end of the rod and outwardly of the outer end of the rod, such that in use the actuation force is applied between those portions of the housing.20. A foot switch or switching system according to any preceding claim, in which the housing is arranged in relation to the rod such as to provide an air gap between the rod and the ground in use.21. A foot switch or switching system according to claim 20, in which the air gap is between 0.1 and 2.0 times the diameter of the rod.22. A foot switch or switching system according to any preceding claim, comprising a cable electrically connected to the cable terminal and sealed fluid tight to the rod.23. A foot switch or switching system according to any preceding claim, in which the rod is cylindrical.24. A ground-standing foOt switch substantially as described herein with reference to the accompanying drawings.25. A method of operating a foot switch according to any preceding claim, comprising depressing the rod with foot pressure to generate the switching signal. * S S...IS..... * S S. * S * S.. *.S *S..S *.SSSS.....