Abstract:
A linear actuator utilizing limit switches to implement reliable and effective travel limit functions. The actuator having a robust construction, lower part count, smaller dimension, and lower assembly cost.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a linear actuator and, more particularly to a linear actuator having travel limit function. 
     2. Description of the Prior Art 
     A typical actuator of this kind comprises a motor, a screw spindle and a screw nut, a driving tube engaged to the screw nut, and an external tube, which protects the driving tube. In many applications, when the driving tube reaches one of the two preset extreme positions, it is often required that the driving tube is stopped moving. 
     One known actuator of this type is as illustrated in FIG. 5. A driving tube  3 , engaged with a screw nut  4 , surrounded by an external tube  2 , and two limit switches: shortening limit switch  61  and extending limit switch  62 , mounted on a circuit board  6 , are located at two extreme positions of the actuator. When driving tube  3  reaches one of the extreme positions, screw nut  4  will trigger one of the limit switches and cut off the motor current, and therefore the driving tube will stop moving outwards or inwards. One of the drawbacks of this solution is that the installation of these limit switches and its control circuit requires a large external tube, and hence increases the dimension of this actuator. Furthermore, assembling limit switches into the external tube is quite time consuming. Finally, for each travel length, a different circuit board is required. This increases part variety, and hence production cost. 
     The second solution, which implements travel limit function, is by counting the rotational turns of screw spindle. A gear train being connected to the screw spindle, for instance, is used for this counting purpose. When the number of turns is larger than a preset value, i. e. the actuator reaches one of the extreme positions, the gear train will turn off the motor electrically or mechanically. Similar to the previous approach, the gear train needs to be altered when actuator stroke is changed. 
     The third solution, which implements travel limit function, is by monitoring the current consumed by motor of the actuator. When the driving tube reaches one of the extreme positions, the driving tube or the screw nut will begin to move against an obstacle, and motor current increases tremendously. When the monitored current is larger than the preset value, a control circuit will turn off the motor. A pitfall of this over-current approach is that before motor current reaches the preset current limit, motor torque might already damage the power transmission components, such as screw spindle and nut. This solution requires precise tuning of the current limit for each actuator, and thus increases manufacturing cost. 
     A further solution, as disclosed in EP 0647799A2, uses internal projections, a spring clip  23  and a bushing  22  as shown in FIG. 6, surrounded by an external tube  2  are used to trigger two opposite oriented limit switches: shortening limit switch  61  and extending limit switch  62 . When driving tube  3  reaches shortening limit position, screw nut  4  will push against spring clip  23  and thereby pull external tube  2 , which will press the trigger button of the shortening limit switch  61 . This will disconnect the motor current and stop driving tube  3 . When driving tube  3  reaches extending limit position, screw nut  4  will push the bushing  22 , and thereby pull external tube  2 , which will release the trigger button of the extending limit switch  62  and stop motor  9 . When driving tube  3  is subjected to a vertical force, external tube  2 , due to assembly clearance, will bent to one side and might miss the two opposite oriented limit switches  61  and  62  as shown in FIG.  7 . This might cause the failure of the travel limit function. Choosing a tighter tolerance will increase friction between external tube  2  and driving tube  3  and indirectly increases power consumption of motor, and also the assembly cost. Furthermore, triggering limit switches solely through projections inside the external tube results in high manufacturing and assembly cost of the external tube. 
     SUMMARY OF THE INVENTION 
     The objectives of this invention are to create a linear actuator completed with travel limit function, which compared to the prior art, is independent of travel length, and has a robust construction, lower part count, much smaller dimension and lower assembly cost. 
     In accordance with these objectives of the invention, this invention uses limit switches to implement reliable and effective travel limit function, which eliminates pitfalls of the over-current approach. 
     To reduce the diameter of the external tube and dimension of the actuator, two limit switches, both mounted on a circuit board, are situated at one end of the external tube, and will be activated by the screw nut and the end of the external tube. This arrangement makes the limit switch module independent of the stroke length, and therefore reduces manufacturing cost of the external tube. 
     To simplify the assembly task of the circuit board into the actuator housing, the housing of actuator is specially designed to have insert slots and a protrusion pad, which help to position the circuit board and guide the limit switches during assembly. 
     Furthermore, the trigger buttons of the limit switches on the circuit board are arranged in a straight line, which parallel to the axis of the external tube, so that their activities are less sensitive to the vertical deformation of the external tube. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above mentioned and other objectives, features and advantages of the present invention will become apparent from the following descriptions read in conjunction with the attached drawings, of which: 
     FIG. 1 shows the driving tube at its shortening limit position 
     FIG. 2 shows the driving tube at its extending limit position 
     FIG. 3 shows the limit switch circuit board situated in the actuator housing 
     FIG. 4 shows the arrangement of trigger buttons for the limit switches on the circuit board 
     FIG. 5 shows the prior art of an actuator with limit switches assembled inside the external tube 
     FIG. 6 shows another prior art of an actuator with limit switches. 
     FIG. 7 shows the displacement of external tube in the actuator as shown in FIG. 6 subjected to a vertical load. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows a preferred embodiment of the invention where a motor  9  provides power for the actuator. Motor power is transmitted through a worm  8  and a worm wheel  81  to a screw shaft  5 . Rotary movement of the screw shaft  5  is further transformed to linear movement by screw nut  4 . When driving tube  3 , engaged to screw nut  4 , reaches the shortening limit position, screw nut  4  will press the trigger button of shortening limit switch  61  and disconnect the current of the motor  9 , which will stop the driving tube  3 . At this shortening limit position, driving tube  3  can only be driven outwards. As shown in FIG. 1, the trigger button of shortening and extending limit switches  61 ,  62  are both pressed at this shortening limit position. 
     When driving tube  3  moving outwards, and reaches its extending limit position, as shown in FIG. 2, screw nut  4  will push against an end cap  22  of external tube  2  and force this external tube  2  to move along. A spring clip  21 , which fixed on a groove of external tube  2 , will press on a compressing spring  7  located in the chamber of the actuator housing  1 . The external tube will keep moving outwards until its right end leaves the trigger button of extending limit switch  62  and disconnects the current of motor  9  to stop the driving tube. At this extending limit position, the driving tube can only be driven inwards. As shown in FIG. 2, limit switches  61  and  62  are deactivated. When the actuator is operated in shortening mode, compressing spring  7  will push external tube  2  backwards so that the right end of the external tube contacts again with limit switch  62 . This will cause the driving tube again in a position where it can be operated either in outwards or inwards directions. During this circumstance, extending limit switch  62  is activated, and shortening limit switch  61  is deactivated. 
     To implement the above stated travel limit function, the control logic of limit switches is particularly designed in such a way that: 
     When shortening limit switch  61  is not activated by screw nut  4  and extending limit switch  62  is activated by the right end of external tube  2 , driving tube  3  is able to move outwards or inwards. 
     When shortening limit switch  61  is triggered by screw nut  4  and extending limit switch  62  is activated by external tube  2 , driving tube  3  is only able to move outwards. When both the limit switches are not triggered, driving tube  3  is only able to move inwards. 
     When shortening limit switch  61  is triggered by screw nut  4  and extending limit switch  62  is not activated by external tube  2 , the entire mechanism of actuator might fail, for instance, the compressing spring  7  lost its elasticity. In this case, driving tube  3  will not be driven by motor  9  to avoid further damage caused to actuator. 
     In this preferred embodiment, as shown in FIG. 3, both limit switches  61  and  62  are mounted on circuit board  6 . Between limit switch  61  and circuit board  6  is a spacer  6 , so that the position of limit switch  61  is slightly higher than the position of limit switch  62 . This is to ensure that the trigger of limit switch  61  by screw nut  4  does not interfere the trigger of limit switch  62  by the external tube. 
     To simplify the assembly task of circuit board  6  into actuator housing  1 , the housing of actuator is specially designed to have insert slots  12 ,  13  and a protrusion pad  11 , which help to position the circuit board and guide the limit switches during assembly. When circuit board  6  is inserted into slots  12  and  13 , protrusion pad  11  will separate limit switches  61  and  62 , but still remains contact with circuit board  6 . This structure ensures that the trigger of limit switches by screw nut  4  and external tube  2  will not remove circuit board  6  from its original position, so that the travel limit function could be secured and guaranteed. 
     Trigger buttons of the limit switches on circuit board  6  are purposely arranged in a straight line, as shown in FIG. 4, which is parallel to the axis of the external tube. With this particular arrangement, the activities of these limit switches, if compared to existing solutions, are less sensitive to vertical deformation of the external tube as well as to the assembly tolerance between the limit switches, the screw nut and the external tube. 
     In this preferred embodiment, limit switches are used to implement travel limit function. This overcomes drawback of motor over-current approach in which travel limit function is not reliable. Since the two limit switches and its control circuit are positioned outside the external tube, dimension of the external tube and the actuator can be drastically reduced. The housing is designed to have two insert slots and a protrusion pad so that limit switches module can be easily inserted into the housing, which reduces assembly cost and time. When the travel limit positions are changed, only dimension of the screw shaft and external tube should be altered, the limit switches, and its control circuit need not be modified as what stated in the prior art. 
     Having described the preferred embodiment of this invention with reference to the attached drawings, it is understood that this invention is not only limited to the mentioned embodiments, and various changes and modifications may be affected therein by one skilled in the art without departing from the scope or spirit of the invention as defined in the appended claims.