Abstract:
A drive system includes an electric motor having a rotor and a stator. A nut is rigidly connected to the rotor. A spindle is positioned to be non-rotating and has a drive connection with the nut. A piston rod has one longitudinal end fixed to the spindle, with the spindle extending coaxial to the piston rod. The spindle is driven by the nut in response to rotation of the rotor so that the piston rod executes a linear movement controlled by the electric motor.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the priority of German Application DE 20 2005 003 981.0 filed on Mar. 12, 2005, the subject matter of which is incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to a drive system as defined in the preamble to patent claim  1 .  
         [0003]     Drive systems of this type are used specifically for locking objects. In particular, these drive systems can be used for locking manhole covers. The locking of manhole covers represents an essential safety-technical aspect for increasing the safety of streets. The importance of this area has increased considerably in recent years. The drive systems used consequently must meet high requirements with respect to capacity and operational failure, so that a fail-safe operation for locking manhole covers can be achieved even for longer period of times.  
         [0004]     Drive systems of this type can generally be provided with linear-moving elements such as piston rods, which are used for different types of locking processes or other applications.  
       SUMMARY OF THE INVENTION  
       [0005]     It is the object of the present invention to provide a drive system of the aforementioned type which is extremely reliable while simultaneously having a robust and compact design.  
         [0006]     The above and other objects are accomplished according to the invention wherein there is provided a drive system, which in an exemplary embodiment comprises: an electric motor including a rotor and a stator; a spindle nut rigidly connected to the rotor; a spindle positioned to be non-rotating and having a drive connection with the spindle nut; and a piston rod having one longitudinal end fixed to the spindle, the spindle extending coaxial to the piston rod; wherein the spindle is driven by the spindle nut in response to rotation of the rotor so that the piston rod executes a linear movement controlled by the electric motor.  
         [0007]     A particularly robust and compact structural design of the drive system according to the invention is obtained when using the spindle nut, which is connected directly and securely with the rotor and drives the spindle for generating the linear movement of the piston rod. The non-rotating positioning of the spindle ensures, in a simple manner, that the rotational movement of the rotor is converted to a purely translational movement of the piston rod.  
         [0008]     According to one advantageous embodiment, the nut which drives the spindle is a plastic injection-molded part that is injection-molded onto a rotor shaft which is securely connected to the rotor. This type of embodiment can be produced especially easily and cost-effectively.  
         [0009]     According to a different advantageous embodiment of the invention, the piston rod can be displaced between two end positions, wherein limit switches that cooperate with a control cam are provided for controlling the end positions. It is especially advantageous if the control cam, which can be displaced between the limit switches, is used for the non-rotating positioning of the spindle. As a result of this double function of the control cam, the number of parts for the drive system can be reduced, thereby resulting in a reduction of the production costs and, simultaneously, in a compact structural design of the drive system.  
         [0010]     To ensure the non-rotating positioning of the spindle, the control cam is usefully attached to one longitudinal end of the spindle and the exposed end of the control cam is embodied as a fork-shaped receptacle, positioned so as to be displaceable along a guide element.  
         [0011]     According to a particularly simple and useful embodiment of the invention, the guide element is a fastening screw that connects the housing lid to the housing for the drive system. The double function of the fastening screw, in turn, makes it possible to reduce the number of parts for the drive system according to the invention.  
         [0012]     The drive system according to the invention also has a compact and space-saving design with respect to the electrical components. The limit switches are arranged on a circuit board which furthermore contains integrated electrical and electronic components for controlling the drive system, in particular a capacitor for actuating the stator coils. These components are furthermore encapsulated in a plastic outer shell, thus resulting in high electrical insulation protection. In particular, this encapsulation ensures that cables, inserted into the housing by a screw-type conduit fitting, do not extend into the region of the limit switches and the control cam, which could result in undesirable interference with the operation.  
         [0013]     According to one particularly advantageous embodiment, the housing for the drive system according to the invention has a water-tight design, up to a predetermined water pressure. This is achieved in particular with the aid of a suitable sealing mechanism to prevent water from entering the housing. It is furthermore advantageous if a dry gel is affixed at predetermined locations to the housing inside, wherein this gel can absorb and bind water that may be present inside of the housing.  
         [0014]     In particular, the drive system according to the invention can be used advantageously for locking/unlocking objects, especially manhole covers, wherein the linear piston-rod movement is used for the locking and unlocking operations. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     These and other features and advantages of the invention will be further understood from the following detailed description of the exemplary embodiments with reference to the accompanying drawings, which show in:  
         [0016]      FIG. 1 : A three-dimensional representation of an exemplary embodiment of the drive system according to the invention;  
         [0017]      FIG. 2 : A longitudinal section through the drive system shown in  FIG. 1 ; and  
         [0018]      FIG. 3 : A perspective representation of the housing for the drive system shown in  FIG. 1 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]     FIGS.  1  to  3  show an exemplary embodiment of a drive system which in the present case is a locking system, in particular used for manhole covers.  
         [0020]      FIG. 1  is a perspective representation of the components of the drive system  1 , which are integrated in a housing  2 .  FIG. 2  shows a longitudinal section through the drive system  1 .  FIG. 3  shows a perspective view of the housing  2  for the drive system  1 .  
         [0021]      FIGS. 1 and 3  in particular illustrate that the housing  2  has a hollow-cylindrical shape. Two mounting rails  3  extend outward from the outer shell of housing  2 , by which the drive system  1  can be mounted on a base that is not shown herein.  
         [0022]     A back end of the housing  2  is closed off with a lid  4 , which is screwed to the housing  2  with several fastening screws  5 . A screw-type conduit fitting  6  is provided on the housing lid  4 , which is used for inserting cables, not shown herein, into the inside of housing  2  to supply power to the drive system  1 .  
         [0023]     A lid  7  closes off the front end of housing  2 , wherein this lid  7  contains a sealing sleeve  8  in the center, inside of which a piston rod  9  is guided.  
         [0024]     For executing locking movements, the piston rod  9  can be displaced in a longitudinal direction. An electric motor with a rotor  10  and a stator  11  is integrated into the housing  2  and is used for displacing the piston rod  9  in the longitudinal direction.  
         [0025]     The front end of piston rod  9  projects through the front end of housing  2 . The exposed end of piston rod  9  is provided with a fastener for attaching the piston rod  9  to the object to be locked, for example, a manhole cover. In the present case, the fastener is a screw  12   a , which is positioned in fork-shaped arms  12   b  of the piston rod  9 .  
         [0026]     A rotor shaft  13  is compression-molded together with the rotor  10  of the electric motor, so that both components are rigidly connected to each other. The rotor shaft  13  encloses a spindle  14 , which extends coaxial to the piston rod  9 . The back end of the piston rod  9 , which is positioned inside the housing  2 , is fixedly connected to the spindle  14 . In principle, the spindle  14  and the piston rod  9  can be produced in one piece. The piston rod  9  and the spindle  14  in the present embodiment are connected with the aid of pins, wherein the spindle  14  is embodied as a trapezoidal spindle.  
         [0027]     The end of the rotor shaft  13 , which faces away from the lid  7  in front, is fixedly connected to a spindle nut  15  that encloses the spindle  14 . The nut  15  for the present embodiment is a plastic injection-molded part which is injection-molded onto the rotor shaft  13 . The nut  15  drives the spindle  14 , thereby causing the piston rod  9  to be displaced in the direction of its longitudinal axis. The spindle  14  is positioned inside the housing  2  to be non-rotating in order to convert the rotational movement of the rotor  10  and the rotor shaft  13 , which drive the nut  15 , to a purely translational movement of the spindle and the piston rod  9 .  
         [0028]     In this exemplary embodiment, the spindle  14  and the piston rod  9 , can be displaced between two end positions, wherein each end position is controlled by a limit switch  16 . The limit switches  16  for the present embodiment are comprise mechanical switches that are activated by a control cam  17 . For the activation, the control cam  17  is moved toward a yoke  16   a  of the respective limit switch  16 .  
         [0029]     The control cam  17  can move back and forth between the limit positions and, for that purpose, is screwed to the back end of spindle  14 . As soon as the control cam  17  engages in one of the limit switches  16 , the electric motor is stopped, thereby preventing the piston rod  9  from moving past the respective end position.  
         [0030]     The control cam  17  additionally ensures the non-rotating positioning of the spindle  14  and the piston rod  9 . The exposed upper end of the control cam  17  is embodied as a fork-shaped receptacle  17   a . This fork-shaped receptacle  17   a  of control cam  17  is guided along one of the fastening screws  5  which serves as a guide element. As a result, the control cam  17  is guided non-rotating and displaceable only in the longitudinal direction along the fastening screw  5 .  
         [0031]     The limit switches  16  are arranged on a circuit board  18  on which the electronic and/or electrical components for controlling the drive system  1  are integrated as well. In particular, a capacitor is soldered onto the circuit board  18 , which capacitor is not shown separately herein and functions to actuate the stator  11  coils.  
         [0032]     A plastic outer shell  19  encapsulates the inside region of the housing, in which the circuit board  18  with limit switches  16  for the control cam  17  is arranged. As a result, it is ensured that no cables which might block the movement of the control cam  17  can extend into this region.  
         [0033]     The invention has been described in detail with respect to preferred embodiments, and it will now be apparent from the foregoing to those skilled in the art, that changes and modifications may be made without departing from the invention in its broader aspects, and the invention, therefore, as defined in the appended claims, is intended to cover all such changes and modifications that fall within the true spirit of the invention.