Spring-biased push-button switch having a spring-loaded tactile feedback feature
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a push-buttom switch for use on the keyboard of an electronic equipment and, more particularly, to improvements in a push-button switch capable of making the operator detect the action of the contact thereof by the sense of touch. 
     2. Description of the Prior Art 
     Japanese Patent Laid-open Publication No. 60-81719 (U.S. Ser. No. 538,728: U.S. Pat. No. 4,528,431) discloses a push-button switch having a fixed contact point provided on a substrate, and a movable contact point operated by an operating key to open or close the switch, and capable of making the operator detect the action of the movable contact point by the sense of touch. The construction of this known push-button switch will be described hereinafter with reference to FIGS. 13, 14 and 15. 
     A frame 112 having a cylindrical supporting part 111 is attached to a substrate 114. A key 110 is mounted on the cylinderical supporting part 111 for sliding motion along the cylindrical supporting part 111. The key 110 has a stem 116 extended downward through the interior of the cylindrical supporting part 111. The free end of the stem 116 is bifurcated to form a skirt 117, and a projection 126 is formed on the outer surface of the extremity of the skirt 117. The projection 126 engages a shoulder 127 formed on the inner surface of the cylindrical supporting part 111 to limit the upward movement of the key 110. 
     A membrane switch 115 is provided on the substrate 114. An actuator 119 of a swing type is provided in place on the upper surface of the membrane switch 115. When the key 110 is depressed, the actuator 119 closes one of the contacts 120 of the membrane switch 115. 
     A compression coil spring 118 is provided between the key 110 and the actuator 119 with one end thereof seated on a spring seat 121 formed on the stem 116 of the key 110 and the other end thereof fitted on a stud 123 formed integrally with the actuator 119. The compression coil spring 118 is bent slightly in a predetermined direction. The lateral bend of the compression coil spring 118 is limited by the skirt 117 of the stem 116. The actuator 119 has a supporting portion 122 which serves as a pivot in the initial stage of turning motion of the actuator 119, and a projecting portion 124 having a downward convex curved bottom surface. As shown in FIG. 13, a small gap is formed between the projecting portion 124 and the upper surface of the membrane switch 115 at a position corresponding to the contact 120 when the key 110 is not depressed. When the key 110 is depressed to a position shown in FIG. 14, a force acting through the compression coil spring 118 on the actuator 119 increases. 
     FIG. 15 shows the relation between the pressure applied to the key 110 and the displacement of the key 110. When the key 110 is depressed, a force greater than a force acting on the actuator 119 while the actuator 119 is at an idle position acts on the actuator 119, and a clockwise moment resulting from the bending of the compression coil spring 118 exceeds a counterclockwise moment acting on the actuator 119 while the actuator 119 is at the idle position. Consequently, the actuator 119 turns clockwise on the extremity of the supporting portion 122 thereof, so that the projecting portion 124 of the actuator 119 presses the upper surface of the membrane switch 115 at the position corresponding to the contact 120 to close the contact 120 as shown in FIG. 14. In FIG. 15, a point 131 indicates the status of the key 110 when the contact 120 is closed. When the contact 120 is thus closed, the compression coil spring 118 is bent greatly as shown in FIG. 14, which is detected by the opertor by the sense of touch. Upon the detection of such a mode of bending of the compression coil spring 118, the operator releases the key 110. In an initial period after the key 110 has been released, the actuator 119 continues to turn clockwise because the decreasing rate of the pressure acting on the actuator 119 is greater than the decreasing rate of the clockwise moment acting on the actuator 119 and hence the contact 120 remains closed. Since the compression coil spring 118 tends to recoil, the clockwise moment decreases. A point 132 in FIG. 15 indicates the status of the key 110 under such a condition. Shortly, the counterclockwise moment acting on the actuator 119 exceeds the clockwise moment acting on the actuator 119, and then the actuator 119 starts turning counterclockwise to the idle position. A point 133 in FIG. 15 indicates the status of the key 110 under a condition where the key 110 has returned to its initial position and the actuator 119 is at the position shown in FIG. 13. The deviation of the position of the key 110 in the state indicated by the point 133 from the position of the same in a status indicated by a point 130 is zero. However, the force acting on the key 110 in the status indicated by the point 133 is different slightly from that acting on the same in the status indicated by the point 130 due to frictional resistance against the movement of the key 110. 
     As is obvious from FIG. 15, the physical hysteresis in the motion of the key 110 indicates that the switching operation of the contact 120 gives a responsive effect, which enables the operator to detect the switching operation of the contact 120. That is, the switching operation of the contact 120 causes a slight movement of the key 110, which can be detected by the operator by the sense of touch. 
     However, the foregoing known push-button switch has drawbacks that actuators of a complicated shape and many parts are necessary to enable the operator to detect the switching operation of the contacts by the sense of touch, the keys and the actuators must be inserted respectively in different directions increasing the steps of assembling work in assembling the push-button switch, and hence the push-button switch is expensive. Furthermore, the substrate must be removed even in replacing a single faulty actuator with a new one for repair, which requires much time and labor. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a push-button switch eliminating the foregoing drawbacks of the conventional push-button switch, comprising a reduced number of parts, facilitating the assembling work, capable of being manufactured at a reduced cost, and facilitating work for repairing the push-button switch in case of malfunction. 
     To achieve the object of the invention, the present invention provides a push-button switch comprising: a substrate; a fixed electrode provided on the substrate; a movable electrode corresponding to the fixed electrode; a key housing having an upright tubular spring retainer formed in the central portion of the bottom wall thereof; a key for operating the movable electrode, slidably fitted on the tubular spring retainer for sliding movement within a range limited by stopper means; and a coil spring both for returning the key and for depressing the movable electrode, integrally having a large coil section seated on the tubular spring retainer of the key housing, a middle extension coil section receiving a stem formed integrally with the key therein, and a small compression coil section connected to the middle extension coil section; characterized in that a portion of one of the small compression coils of the small compression coil section of the coil spring is projected radially outward to form a projecting part, and a protrusion is formed on the inner circumference of the tubular spring retainer of the key housing in a shape so that the projecting part of the small compression coil section can be moved over the protrusion by a force that acts on the projecting part of the small compression coil section in depressing the key and by a force that acts on the projecting part of the small compression coil section when the key is released. 
     In assembling the push-button switch of the present invention, first, the coil spring is put in the tubular spring retainer with the small compression coil section foremost so that the upper large coil section thereof is seated on a shoulder formed in the upper end of the tubular spring retainer. Then, the key is put on the tubular spring retainer with the stem thereof received in the middle extension coil section of the coil spring. Thus, the key housing, the coil spring and the key can be assembled easily by inserting the coil spring and the key in the key housing in the same direction. In this state, the stem of the key is fitted in the middle extension coil section of the coil spring, the lower end of the stem rests on a shoulder between the middle extension coil section and the small comprssion coil section, and the projecting part of the small compression coil section engages the upper surface of the protrusion formed on the inner circumference of the tubular spring retainer. The large coil section of the coil spring is seated in place on the shoulder formed in the upper end of the tubular spring retainer. Although the middle extension coil section is strained slightly to apply an upward force to the key, the upward movement of the key is limited by the stopper means, so that the key is positioned at a predetermined uppermost position. 
     The middle extension coil section of the coil spring is an extension coil spring for urging the key upward to the uppermost position. The small compression coil section of the coil spring is a compression coil spring for depressing the movable electrode. The movement of the projecting part of the small compression coil section over the protrusion formed on the inner circumference of the tubular spring retainer can be detected by the operator by the sense of touch. That is, the key moves downwrd along the tubular spring retainer when depressed, consequently, the middle extension coil section of the coil spring is extended by the stem of the key, and the small compression coil section is compressed. When the compressive force acting on the small compression coil section exceeds a resistance of the protrusion against the projecting part of the small compression coil section, the projecting part of the coil moves downward over the protrusion and the small compression coil section snap extends to depress the movable electrode, so that the switch is closed. The snap movement of the projecting part of the small compression coil section over the protrusion enables the sensual detection of closing of the contact. When the key is released, the key is urged upward by the contractive force of the middle extension coil section and the recoiling force of the small compresson coil section. Since the contractive force of the middle extension coil section is greater than the resistance of the protrusion against the projecting part of the small compression coil section, the projecting part moves upward over the protrusion, so that the key is returned to the uppermost position. 
     The above and other objects, features and advantages of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a longitudinal sectional view of a push-button switch in a first embodiment according to the present invention, in which the contact is open; 
     FIG. 2 is a longitudinal sectional view of the push-button switch of FIG. 1, in which the contact is closed; 
     FIG. 3 is a front elevation of a coil spring employed in the push-button switch of FIG. 1; 
     FIG. 4 is a graph showing the relation between the displacement of a key and the pressure acting on the key; 
     FIGS. 5(a) to 5(e) are fragmentary sectional views of the push-button switch of FIG. 1, showing the statuses of the coil spring in the push-button switch of FIG. 1, corresponding respectively to points A to F in the graph of FIG. 4; 
     FIG. 6 is a longitudinal sectional view of a push-button switch in a second embodiment according to the present invention; 
     FIGS. 7, 8 and 9 are plan views, taken along the line X-X of FIG. 3, of modifications of the coil spring, respectively having modified projecting parts; 
     FIGS. 10 and 11 are fragmentary sectional perspective views of modifications of the tubular spring retainer, respectively having modified protrusions; 
     FIG. 12 is a front elevation of a coil spring suitable for use in combination with the tubular spring retainer of FIG. 11; 
     FIG. 13 is a longitudinal sectional view of a conventional push-button switch, in which the contact is open; 
     FIG. 14 is a longitudinal sectional view of a conventional push-button switch of FIG. 13, in which the contact is closed; and 
     FIG. 15 is a graph showing the relation between the displacement of the key of the push-button switch of FIG. 13 and the pressure acting on the key. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 1 and 2, there are shown a substrate 5, a fixed contact point 6 provided on the substrate 5, a spacer 7, a flexible member 8 provided over the spacer 7, a movable electrode 9 attached to the lower surface of the flexible member opposite to the fixed electrode 6, a switch holding frame 10, a key 11 having a key head 11d, a stem 11a projecting from the central portion of the lower surface of the key head 11d, and legs 11b extending downward from the key head 11d around the stem 11a and each having a tapered claw 11c at the lower end thereof, a coil spring 12, and a key housing 13. 
     Referring to FIG. 3, the coil spring 12 has an upper large coil section 12a, a middle extension coil section 12b, an upper parallel section 12c, a small compression coil section 12d and a lower parallel section 12e, which are formed integrally in that order. The sections 12c, 12d and 12e together form an end coil portion of the spring 12. A portion of one of the coils of the small compression coil section 12d is deformed radially outward to form a projecting part 12f. 
     The key housing 13 has a bottom wall or base 13a, a tubular spring retainer 13b extending upright from the central portion of the bottom wall 13a and provided with a shoulder 13h in the upper end theeof, a substantially annular guide wall 13c surrounding the tubular spring retainer 13b, a stopping flange 13d inwardly extending from the upper end of the guide wall 13c, a flange 13e extending radially outward from the outer circumference of the guide wall 13c, and locking claws 13f projecting from the outer circumference of the bottom wall 13a. The key housing 13 is retained firmly on the frame 10 by means of the flange 13e and the locking claws 13f clamping the frame 10 therebetween. 
     In assembling the push-button switch, first, the coil spring 12 is put in the tubular spring retainer 13b of the key housing with the lower parallel section 12e foremost to position the coil spring 12 in the tubular spring retainer with the large coil section 12a seated on the shoulder 13h formed in the upper end of the tubular spring retainer 13b. In this state, a small predetermined gap of l is formed between the lower parallel section 12e and the flexible member 8. Then, the key 11 is put on the annular guide wall 13c of the key housing 13 with the tapered claws 11c of the legs 11b thereof on the stopping flange 13d, and then the key 11 is depressed so that the legs 11b are bent elastically to allow the tapered claws 11c of the legs 11b to slip into the interior of the annular guide wall 13c of the key housing 13 over the stopping flange 13d. In this state, the stem 11a of the key 11 is inserted in the middle extension coil section 12b of the coil spring 12 with the lower end thereof in abutment with the upper parallel section 12c formed between the middle extension coil section 12b and the small compression coil section 12d, and the tapered claws 11c of the key 11 engage the stopping flange 13d of the key housing 13. Since the large coil section 12a of the coil spring 12 is seated on the shoulder 13h, the middle extension coil section 12b is extended slightly, and hence the key 11 is urged upward through the stem 11a by the coil spring 12, whereby the tapered claws 11c are pressed against the inner surface of the stopping flange 13d to position the key 11. The projecting part 12f of the small compression coil section 12d of the coil spring 12 is in engagement with the protrusion 13g formed in the inner circumference of the tubular spring retainer 13b, so that the predetermined gap of l is formed between the lower parallel section 12e and the flexible member 8. 
     In operation, when the key 11 of the push-button switch in the status shown in FIG. 1, in which the switch is open, is depressed, the key 11 moves downward along the tubular spring retainer 13b of the key housing 13 by being guided by the annular guide wall 13c of the key housing 13 pushing the upper parallel section 12c of the coil spring 12 with the stem 11a to extend the middle extension coil section 12b of the coil spring 12. 
     In FIG. 4 showing the relation between the pressure applied to the key 11 and the displacement of the key 11, a point A indicates a status in which the key 11 of the push-button switch is not depressed, a point B indicates a status in which the key 11 is depressed to an extent where the projecting part 12f of the coil spring 12 has not yet moved down over the protrusion 13g and hence the small compression coil section 12d of the coil spring 12 is compressed, a point C indicates a status in which the key 11 is depressed to an extent where resistance against the downward movement of the key 11 is increasing progressively, the projecting part 12f of the coil spring 12 is deformed elastically and is almost moving down over the protrusion 13g, a point D indicates a status in which the projecting part 12f of the coil spring 12 has just moved down over the protrusion 13g and hence the small compression coil section 12d of the coil spring 12 has extended, so that the flexible member 8 is depessed to bring the movable contact point 9 into contact with the fixed electrode 6 provided on the substrate 5, whereby the switch is closed, and a point E indicates a status in which the lower ends of the legs 11b of the key 11 are in contact with the upper surface of the bottom wall 13a of the key housing 13. 
     When the status of the push-button switch changes from that indicated by the point C to that indicated by the point D, namely, when the projecting part 12f of the coil spring 12 moves down over the protrusion 13g, the pressure necessary for depressing the key 11 decreases from a pressure at the point C to a pressure at the point D, so that the operator is able to detect the closing of the switch by the sense of touch. When the key 11 is released, the middle extension coil section 12b of the coil spring 12 is allowed to contract and the small compression coil section 12d of the coil spring 12 is allowed to extend, so that the key 11 is pushed upward by the contractive force of the middle extension coil section 12b and the recoiling force of the small compression coil section 12d, and the pressure acting on the flexible member 8 decreases. Consequently, the movable electrode 9 is separated from the fixed electrode 6 to open the switch. 
     As the key 11 is moved further upward, the projecting part 12f of the coil spring 12 engages the lower surface of the protrusion 13g. Since the contractive force of the middle extension coil section 12b is greater than a force necessary to move the projecting part 12b up over the protrusion 13g, the projecting part 12b moves up over the protrusion 13g. Consequently, the status changes from that indicated by the point E through those indicated by the points D and F to that indicated by the point A, namely, the initial status. 
     The contact of the push-button switch is thus operated and the operator is able to detect the closing and opening of the switch by the sense of touch through the sensuous detection of the variation of the resistance against the depression of the key 11 which occurs when the projecting part 12f moves over the protrusion 13g. 
     Although the push-button switch described hereinbefore is of a so-called membrane type having a flexible membrane (the flexible member 8) and a movable electrode (the movable electrode 9), the present invention is applicable also to a push-button switch of a capacitance type as shown in FIG. 6 having fixed electrode 15 provided on a substrate 25, and a movable electrode 16 attached to the lower parallel section 12e of a coil spring 12 similar to that of the first embodiment. 
     The lower portion of the coil spring in FIG. 3, namely the projection part 12f of the coil spring 12 may be modified as illustrated in FIGS. 7 to 9. FIGS. 7 to 9 are plan views taken along X-X of FIG. 3 of the modified coil springs (upper portions thereof are same as that shown in FIG. 3 so that the lower portions thereof are taken along X-X of FIG. 3). The projecting part 12f of the small compression coil section 12d of the coil spring 12 may be an eccentric circular coil having a diameter greater than that of the rest of the coils of the small compression coil section 12d (FIG. 7), an elliptic coil (FIG. 8), a coil having a partial projecting part or a coil having any other suitable shape (FIG. 9). 
     Furthermore, the protrusion 13g formed on the inner circumference of the tubular spring retainer 13b of the key housing may be an annular protrusion (FIG. 10) or a local protrusion (FIG. 11). When the protrusion 13g is a local protrusion, a positioning slot 13i is formed in the upper end of the tubular spring retainer 13b and the coil spring 12 is put in the tubular spring retainer 13b with a stopping projection 12g formed at the free end of the coils of the large coil section 12a thereof engaging the slot 13i to fix the circumferential position of the coil spring 12 relative to the tubular spring retainer 13b so that the projection 12f of the coil spring 12 coincides with the protrusion 13g. 
     As is apparent from the foregoing description, according to the present invention, a mechanism for giving a sensuous stimulus indicating the operation of a push-button switch can be constructed by a single coil spring and a protrusion formed on the inner circumference of a tubular spring retainer for retaining the coil spring without requiring any additional parts including an actuator having a complicated shape which is essential to the conventional push-button switch. Accordingly, the push-button switch of the present invention comprises a reduced number of parts, has a simple construction, enables the insertion of the coil spring and the key in the same direction in assembling the push-button switch, thus facilitating the assembling process, and reduces the manufacturing cost of the push-button switch remarkably. 
     Furthermore, since the coil spring is a single member integrally having a plurality of functional sections, and the key and the coil spring are inserted in the key housing in the same direction, the key and the coil spring can be changed very simply for repair, which facilitates the maintenance of the push-button switch effectively. 
     Although the invention has been described in its preferred forms with a certain degree of particularity, obviously many changes and variations are possible therein. It is therefore to be understood that the invention may be practiced otherwise than specifically described herein without departing from the scope and spirit thereof.