Limit switch

Abstract

 A limit switch (1) is described, able to permit the braking control of movable elements (2) at respective end-of-stroke elements therefor; the microswitch comprises a push button (10) which can come into contact with the movable element to be controlled, a rigid casing (11) mounted fixedly to the said element and within which is slidably guided a piston fixed to the said push button, and at least one resiliently compressible plug (13) housed within the said casing, between the said piston and an end wall (12) of the casing, the said plug being made of an electrically conductive elastomeric material the electrical resistance of which varies as a function of the compression state of the material.

Description

[0001] The present invention relates to a proximity microswitch operable to signal when a movable element reaches a predetermined position, in particular, operable to permit braking control of a movable element at an associated end-of-stroke stop element therefor.

[0002] It is known that in very many technical applications it is necessary to detect when a movable element or member reaches a predetermined position, in particular when it reaches an end-of-stroke position at which the movable member must be stopped; for example, the movable element or member may be a lift cabin of a cable lift installation, a carriage of a machine tool, a palett of a conveyor belt etc. For this purpose proximity microswitches are currently used the closure (or opening) of which is controlled by coming into contact with the movable element or member to be detected; the actuation of the microswitch consequently involves the opening (or closure) of an electrical control circuit (for example, for controlling the movement of the movable element or member) or an indicator circuit.

[0003] The described arrangement is not free from disadvantages; in the first place microswitches currently available on the market are all of the movable contact type and are thus expensive and easily damaged or short circuited. Moreover, such switches typically have an "on-off" function, that is of the "open/closed" type such that they are able to function only when contact with the movable member has lasted sufficiently long to displace the movable contacts by an amount such as to cover the opening/closure path of the switch, equal to the clearance between the movable contacts: this, in particular, in the case of end-of-stroke switches involves the fact that the switch itself is subjected in use to a violent shock by the movable member in that this cannot be stopped until it has struck against the end-of-stroke switch (closing it or opening it) with consequent activation for example of braking means for the movable element or member.

[0004] The object of the invention is that of providing a proximity switch which will be free from the described disadvantages, and in particular which will be of low cost and reliable operation.

[0005] The said object is achieved by the invention which relates to a proximity microswitch, in particular operable to permit braking control of a movable element at a respective end-of-stroke stop element therefor, characterised by the fact that it comprises, in combination: a push button which can come into contact with a movable element to be controlled; a rigid casing mounted fixedly to an end-of-stroke element for the movable element and by which casing the said push button is slidably guided; and at least one resiliently compressible plug which is housed within the said casing between the said push button and an end wall of the casing, and which is formed of an electrically conductive elastomeric material the electrical resistance of which is variable in dependence on the compression to which it is subjected.

[0006] For a better understanding of the invention there is now given a non-limitative description of an embodiment, with reference to the attached drawings, in which:-

Figure 1 is a longitudinal section on a larger than real scale of a proximity switch formed according to the invention, a detail of which is illustrated on a further enlarged scale within the circle; and

Figure 2 is a section taken on the line II-II of the microswitch of Figure 1.

[0007] With reference to Figures 1 and 2, a proximity microswitch is generally indicated with the reference numeral 1 which can be actuated by a movable element or member 2 of any known type, for example, a carriage of a machine tool, in particular operable to permit the braking control of the movable element 2 at an associated end-of-stroke stop element 3 therefor, obviously fixed with respect to the element 2 and on which the switch 1 can be fixedly mounted, for example, by means of screws 5.

[0008] The switch 1 of the invention essentially comprises a push button 10 which can come into contact with the movable element 2 to be controlled, a rigid casing 11, for example, defined by a cylindrical tubular element fixedly mounted to the end-of-stroke element 3 by screws 5 at an end wall 12, and a plurality of resiliently compressible plugs or tablets 13 housed as a stack within the casing 11 between the end wall 12 of this latter and the push button 10; in particular, the push button 10 is slidably guided through the casing 11, projecting partially out from this in the direction of the arrow (Figure 1) perpendicularly of the wall 12 and from the end opposite this, in such a way as to be able to compress the plugs or tablets 13 in a direction perpendicular to the respective opposite faces 14 , 15 of these.

[0009] In the specific example illustrated the push button 10 is fixed to a piston 16 slidably housed and guided within the casing 11 and the plugs 13 are disposed in a stack between the end wall 12 of the casing 11 and the piston 16 in such a way as to be able to be compressed by the approach of the piston 16 to the wall 12 in the direction of the arrow (Figure 1) after the movable element 2 has come into contact with the push button 10 which causes displacement of this latter, again in the direction of the arrow, from the position illustrated in solid outline to that illustrated in broken outline, which corresponds to the end-of-stroke position of the element 2.

[0010] According to the principal characteristic of the invention, each plug 13 is made of an electrically conductive elastomeric material the resistance of which can vary in dependence on the state of compression to which it is subjected: such a material and a method for manufacturing it are described in Italian patents numbers 1206890, 1210777 and 1211401, in the name of the same Applicant, the contents of which are incorporated herein by reference as far as necessary. As is seen in Figure 1 on an enlarged scale within the circle, such material has the characteristic of comprising a resiliently deformable elastomeric matrix 24 which is non-conductive and subjected to a triaxial precompressioned state and provided with a plurality of open cells which are not visible in the drawing in that they are occupied by particles of electrically conductive material 26 which are uniformly distributed throughout the matrix 14 in such a way as to form chains of particles closely contacting one another throughout the matrix itself.

[0011] In this way, when the material which forms the plugs 13 is subjected to compression stress, more of the particles 16, some of which in any case are already in contact with one another even when the plugs 13 are not compressed, thereby always ensuring a certain electrical conductivity thereof, are pressed into contact with one another more closely and in greater number, consequentially increasing the electrical conductivity of each plug 13 accordingly to a predetermined law, which is characteristic of the material from which it is made.

[0012] According to a further characteristic of the invention the proximity switch 1 further includes means for continuously detecting the overall electrical resistance of the plugs 13 which, are disposed electrically in series and which are therefore equivalent to a single plug 13 of overall thickness equal to the sum of the thicknesses of all the plugs 13 present (in a possible variant not illustrated for simplicity the plurality of plugs 13 can be replaced by a single plug 13). These means comprise, for example, a known electrical resistance measurement device 20 which can emit an electrical signal 21 as a function of the resistance detected, and transmit it to suitable control means (for example, for controlling the movement of the element 2) or a known indicator or signalling device not illustrated for simplicity.

[0013] In order more easily to permit electrical connection between the plug or plugs 13 and the device 20 the faces 14, 15 of each plug 13 are provided with respective, flexible contact elements 23, for example, constituted by metal meshes embedded in the plugs 13 by partial embedding in the matrix 24; the contact elements 23 of the opposite faces, respectively 14 of the first and 15 of the last of the plugs 13 disposed in series within the casing 11, respectively facing towards the push button 10 and towards the wall 12, are then electrically connected to respective electrical conductor wires 22 of opposite polarity connected to the measurement device 20.

[0014] For the purpose of encouraging deformation by compression of the plugs 13 as well as to prevent possible relative rotations between these within the casing 11, each respective plug 13 is provided radially with a plurality of projecting longitudinal ribs 30 in the shape of lobes which are housed with radial clearance within respective longitudinal grooves 31 of the casing 11 formed internally thereof.

[0015] In use, when the movable element 2 arrives close to the end stop stroke 3 it begins to cooperate with the push button 10 causing it to retract towards the position illustrated in broken outline; this retraction movement of the push button 10 starts to cause a compression of the plugs 13 which is shown on an enlarged scale and purely by way of example in Figure 1 by the different thickness of the plugs 13; this compression causes on the one hand the application of a resilient reaction force R on the push button 10 opposite to the direction of movement of the push button 10 itself, and, by virtue of the particular material with which the plugs 13 are made, a variation in the overall electrical resistance thereof, a variation which is immediately detected by the device 20, which therefore emits a suitable signal 21: this signal 21 can be used, for example, to disactivate motor means driving the element 2 and/or to activate braking means therefor: consequently the element 2 slows whilst the push button 10 continues its stroke increasing the compression of the element 13 with consequent further reduction in the electrical resistance measured by the device 20: this progressive reduction can possibly be monitored by means of the variation of the signal 21 emitted by the device 20 to follow the slowing of the element 2 and, possibly suitably to control it. When the push button 10 reaches the position shown in broken outline, the element 2 is by now stationary and therefore, any impact between this and the end stop 3 is prevented. An effective stroke of the push button 10, sufficiently long to allow braking on the element 2 is guaranteed by the possibility of the plugs 13 becoming reduced in thickness, that is in a direction perpendicular to their faces 14, 15, deforming (enlarging) on the other hand in a radial sense, in correspondence with the lobes 30. Once the element 2 is removed from contact with the push button 10 this is returned to the initial position by the resilient restoration of the plugs 13 which return to the undeformed state.

[0016] From what has been described the advantages associated with the invention will become apparent; the switch 1 is in fact provided with no movable contacts in that the electrical part is constituted by the plugs 13: these are made of elastomeric material and, therefore, are insensitive to shock; such shocks, however, can be entirely avoided, contrary to normal switches, in that the switch 1 is able to emit a signal indicating that contact has been made long before the push button 10 reaches the end-of-stroke position, in that the emission of this signal depends not on the closure of a contact controlled after achievement by the push button 10 of the end-of-stroke position, but by detection of the variation of an electrical quantity, such as the resistance of the plugs 13 of which the switch 1 is composed, which quantity thus commences to change simultaneously with the commencement of contact between the element 2 and the push button 10. Since the electrically conductive parts are all embedded in an elastomeric matrix such as the matrix 24 and it being possible, moreover, to enclose the plugs 13 and elements 23 within a non-conductive skin, the switch 1 according to the invention has the further advantage of being insensitive to possible short circuits caused by water, and therefore can function perfectly well even in the presence of condensation due to moisture or even to total immersion in a liquid.

[0017] In a possible variant not illustrated for simplicity the same advantages can be achieved also by inserting between one of the electrically conductive elements 26 and the associated elastomeric conductive plug 12 a screen made from an electrically non-conductive sheet provided uniformly with through holes via which the plug 13, upon deforming resiliently under the action of a compression stress applied to it, can contact the conductive element 23, as described in Italian patent no 1210778 by the same Applicant the contents of which are incorporated here by reference as far as necessary. In this case, before contact between the push button 10 and the element 2 takes place, the electrical resistance measured by the device 20 will be close to infinity (a situation equivalent to open contacts), whilst as the push button 10 comes into cooperation with the element 2 consequently deforming the elements 13, it passes to a finite and relative small value (the material of the plugs 13 being conductive), and subsequently this reduces further, with consequent achievement of a condition equivalent to closed contacts, which can be easily detected or, possibly, even utilised directly to control the braking of the element 2 by connecting the switch 1 directly to the electrical power circuit driving the said motor means (or brakes) of the element 2 itself.

Claims

1. A proximity microswitch, in particular operable to permit braking control of any movable element at an associated end-of-stroke element therefor, characterised by the fact that it comprises in combination:-
   a push button which can come into contact with a movable element to be controlled;
   a rigid casing fixedly mounted to an end-of-stroke element for the movable element and by which casing the said push button is slidably guided; and at least one resiliently compressible plug which is housed within the said casing, between the said push button and an end wall of the casing, and made of an electrically conductive elastomeric material the resistance of which is variable as a function of its state of compression.

2. A microswitch according to Claim 1, characterised by the fact that it further includes means for continuously detecting the electrical resistance of the said plug to produce an electrical signal dependent on the detected resistance.

3. A microswitch according to Claim 1 or Claim 2, characterised by the fact that the said push button is fixedly mounted to a piston slidably housed within the said casing.

4. A microswitch according to Claim 3, characterised by the fact that it includes a plurality of said resiliently compressible plugs made of electrically conductive elastomeric material the resistance of which varies as a function of the compression state; the said plugs being disposed in a stack between the said end wall of the casing and the said piston fixed to the push button in such a way as to be compressed upon approach of the piston to the end wall.

5. A microswitch according to Claim 4, characterised by the fact that associated opposite transverse faces of the said plugs disposed perpendicularly of the direction of movement of the said push button are provided with respective flexible contact elements, for example, metal meshes; the contact elements of the opposite faces of the first and last of the said plugs, respectively disposed facing the said push button and the said end wall of the casing being electrically connected to respective electrically conductive wires of opposite polarity connected to an electrical resistance measurement device.

6. A microswitch according to any preceding claim, characterised by the fact that each respective said plug is provided with a plurality of radially projecting longitudinally extending lobe shape ribs which are housed with radial clearance within respective longitudinal grooves of the said casing formed internally thereof.

Drawing