Abstract:
An improved electrical measuring tape that automatically extends and retracts the tap. Switches control a battery powered motor that drives a wheel around which the tape is wound. Induction dots near both ends of the tape cause a short-circuit to stop extension or retraction.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an electrical measuring tape and more specifically to a improved structure for users to measure lengths of objects more conveniently and effectively. 
     DESCRIPTION OF THE PRIOR ART 
     Traditional measuring tapes are designed such that the users have to pull the protruding metal blades of the tapes to hook on sides of object, which is obviously very inconvenient for the users. The users must stabilize the protruding metal blades with one hand while pulling the tapes with their other hand of the objects to be measured don&#39;t have any suitable projections for the blades to hook on. Two people may therefore be required to measure large objects or areas. 
     PURPOSE OF THE INVENTION 
     It is the purpose of this invention to provide a novel device by which an electrical measuring tape can be lengthened or shortened automatically to obtain the most convenient and effective measuring. 
     SUMMARY OF THE INVENTION 
     The present invention comprises a bottom shell, a left top shell, a right top shell, a measuring tape device, a motor, a set of batteries and an inch-unit actuating device, wherein: 
     The bottom shell has a flat circular stable ring with a rod protruding from an inner surface of the bottom shell at a slightly off-center position. The rod is attached to a gear wheel. There is a metal blade on the upper side of an opening which is located at the front edge of the bottom shell. The opening allows a measuring tape to pass through. A spring and a blockout device are located at the bottom of the opening. The blockout device has the function of blocking electrical current. The left top shell has a space for locating the motor and is shaped to attach to the bottom shell. The right top shell is made to be easily opened to change to batteries. 
     The measuring tape device comprises a measuring tape and a wheel around which the measuring tape is wound. The wheel has a hole in the center with gear teeth. The wheel is located in the flat circular stable ring inside the bottom shell. The diameter of the wheel is just a bit smaller than the diameter of the stable ring. The gear teeth of the hole are attached to the gear wheel which is actuated by the motor. 
     Each end of the measuring tape has a protruding induction dot on it. A protruding metal blade head is bend downward at the first end of the measuring tape. 
     The batteries provide electrical current for the rotation of the motor through the inch-unit actuating device. The lengthening and shortening motions of the measuring tape can thus be effectively controlled. A cylindrical protrusion on the center of the motor is attached to a teeth wheel and thus can actuate the gear wheel in the bottom shell. The right top shell provides space for location of the set of batteries. 
     The inch-unit actuating device comprises a positive and negative device, a crystal and a resistor. The inch-unit actuating device is also connected to the motor and two switches by wires, and thus can control the operation of the motor. 
     The switches can be operated to actuate the motor and thus cause the wheel to turn clockwise or counter clockwise to control lengthening or shortening of the measuring tape. The protruding induction dots on each of the measuring tape can actuate transient periods of short-circuiting of the motor to prevent excessive lengthening and shortening of the measuring tape. When the protruding metal blade head touches the blockout device inside the opening, it will short-circuit operating of the motor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded perspective view of an improved structure of an Electrical Measuring Tape; 
     FIG. 2 is a top view; 
     FIG. 3 is a sectional front view of the tape; and 
     FIG. 4 is perspective view of the tape. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An electrical measuring tape comprises a bottom shell 11, a left top shell 12, a right top shell 13, a measuring tape device 14, a motor is, a set of batteries 16 and a inch-unit actuating device 17. The bottom shell 11 has a space inside for location B of a flat circular stable ring 111. A rod 112 is located just off-center on an inner surface of the stable ring 111. A gear wheel 113 is attached to the rod 112. An opening 114 for the tape is located at a lower front area of the shell 11. A metal blade 115 with a spring biases the tape from above. A spring 117 and a blockout device 118 are located on a lower side of the opening 114. The blockout device 118 enable short-circuiting of the electrical current. Two switches 171A and 171B are situated above the stable ring 111, and protrude outside the bottom shell 11. There is a dividing piece 119 to divide the bottom shell 11 into left and right compartments. The inch-unit actuating device 17 and batteries 16 are in the right compartment of the bottom shell 11. The bottom edge of the right compartment of the bottom shell 11 is curved for easy holding by users. The edge of the bottom shell 11 includes a trough 11c to snap together with a protruding edge 124 of the left top shell 12 and the right top shell 13. The left top shell 12 has a space 121 to receive the motor 15. The bottom edge of the right top shell 13 is curved to snap together with the right compartment of the bottom shell 11. The batteries 16 can be easily changed by pulling off the right top shell 13. 
     The measuring tape device 14 comprises a measuring tape 141 wrapped around a wheel 142. The wheel 142 has a hole 145 in the center with a gear teeth 145a around its inner circumference to mesh with the gear wheel 113. The wheel 142 is located in the stable ring 111. Referring to FIG. 4, near both ends of the measuring tape there are protruding induction dots 141a, spaces 5 or 10 cm apart at the ends of the measuring tape 141. The induction dots 141a cause a transient short-circuit of the motor 15, thus slowing tape speed. 
     The set of batteries 16 provides electric current to operate the motor 15, which is controlled by the switches 171A and 171B. One switch causes the motor 15 to turn clockwise and the other causes the motor 15 to turn counter-clockwise. A cylindrical protrusion 151 on the center of the motor 15 is attached to a teeth wheel 152 and thus can actuate the gear wheel 113 to rotate the wheel 142, which moves the measuring tape 141. The gear ratio of the teeth wheel 152 and gear wheel 113 slows the turning speed of the device. 
     The inch-unit actuating device 17 comprises a positive and negative device, a crystal and a resistor. The inch-unit actuating device 17 is connected to the motor 15 and the two switches 171A and 171B by wires 172, and thus can control the direction of rotation of the motor 15. 
     According to the improved structure of an electrical measuring tape explained above, we assume that pressing switch 171A controls clockwise rotation of the motor 15 and pressing switch 171B controls counter-clockwise turning of the motor 15. When the user presses switch 171A, the motor 15 turns clockwise and thus actuates the teeth wheel 152 to move the gear wheel 113 and turn the wheel 142 to extend the measuring tape 141. The inch-unit actuating device 17 is actuated until one of the protruding induction dots on the measuring tape 141 touches the metal blade 115 inside the opening 114 in the bottom shell 11. The inch-unit actuate device then gives out a signal magnified by the crystal and thus stops the motor 15 from turning. If the user wants to measuring tape 141 to be extended or retracted, he simply releases the pressing switch and presses the switches 171A and 171B again to lengthen or shorten the measuring tape 141. 
     The measuring tape 141 moves until the protruding induction lot 141A touches the metal blade 115, the motor 15 then being short-circuited for a transient period. Afterwards, if the user releases the pressing switch and presses it again, the measuring tape 141 is moved again until the protruding metal blade head 143 touches the blockout device 115 inside the opening 114. This short-circuits the motor 15 and thus the measuring tape device 14 is not able to operate. Therefore, when using the present invention, the user has to pull the protruding metal blade head 143 just a bit away from the blockout device 118 in order to actuate the motor 15 by pressing the switch 171A to extend the measuring tape 141. Conversely, the protruding induction dot near the other end of the measuring tape 141 has the same function as the first protruding induction dot, that is to short-circuit the motor 15 to stop further movement of the measuring tape 141. 
     It should be understood that although specific embodiments of the invention have been described herein in detail, such description is for purposes of illustration only and modifications may be made thereto by those skilled in the art within the scope of the invention.