Abstract:
An automatic table leveling system includes a linear accelerometer, a microcontroller, motor drivers, motors and lead screws. The microcontroller reads linear accelerometer sensor information which indicates the table top&#39;s attitude with respect to the gravity vector. Then the microcontroller sends signals to the motor drivers to cause the motors to drive the table legs up or down via a lead screw mechanism. The changed leg lengths adjust the attitude of the table top to make it level.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a system for automatically leveling a portable table when the portable table is set up, moved or located on terrain that may be uneven. 
         [0003]    2. Description of Related Art 
         [0004]    Many types of portable tables exist for use indoors and out. A good example of a special purpose manually leveled portable table is a camera tripod. In this case, the three legs can be adjusted manually in length in order to level the top portion that mounts a camera. U.S. Pat. No. 4,265,027 describes an automatic self-leveling instrument mount. This device relies on a pendulum switch to control motors that drive support arms extending between the table and a frame. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention utilizes a Micro-Electro-Mechanical System (MEMS) linear accelerometer, a microprocessor, motor drivers and motors to automatically control the leg lengths of a table for the purpose of making the table top level. These components provide a relatively low cost solution to the need for camping tables that are level which is quite desirable for certain types of outdoor cooking and other purposes. The same technology can be applied to camera tripods. The linear accelerometer senses the gravity vector to determine a level attitude. The microprocessor reads the accelerometer and interprets any deviation from a level attitude to drive the motors that lengthen or shorten the table&#39;s leg lengths to cause the table top to be level. A first embodiment, the simplest and lowest cost implementation, would have one fixed length leg and two motor driven legs. A second embodiment would have three motor driven legs—allowing the additional benefit of having an adjustable table height. A third embodiment would have one fixed length leg and three motor driven legs—allowing for better stability than the three leg solution by placing the legs at the corners of a square or rectangular table top. Finally, a fourth and preferred embodiment would have four motor driven legs—allowing for better stability and an adjustable table top height. In order to enhance portability, the legs will be manually foldable under the table surface when not in use. An electronic circuit board that contains the linear accelerometer, microprocessor and motor drivers mounts just below the table top and is referenced to the table top such that the linear accelerometer senses the attitude of the table top. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The present invention may be better understood, and its many features and advantages made apparent to those skilled in the art by referencing the attached drawings. 
           [0007]      FIG. 1  is an illustration of the automatic portable self-leveling table in accordance with the preferred embodiment of the present invention. 
           [0008]      FIG. 2  is a block diagram of an electronic system in accordance with the preferred embodiment of the present invention. 
           [0009]      FIG. 3  is a flow chart that illustrates the process performed by the microprocessor in accordance with the preferred embodiment of the present invention. 
           [0010]      FIG. 4  is an illustration of the table leg including motor and lead screw drive. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0011]      FIG. 1  is an illustration of the automatic portable self-leveling table. A table top  100  is supported by four legs  101 . An electronic circuit board  102  is powered by battery pack  103 . It should be understood that other means of powering the circuit board are possible. For example, an AC adapter or solar cells could be used. The circuit board  102  is connected to motors contained inside the legs  101  of the table. It is not essential to contain the motors within the legs except for esthetic reasons. As noted above, this is the preferred embodiment. Other embodiments are also noted above. 
         [0012]      FIG. 2  is a block diagram of an electronic system in accordance with the preferred embodiment of the present invention. The circuit board&#39;s main components are the 3-axis MEMS accelerometer  200  (e.g. a LIS302DL—manufactured by STMicroelectronics), microprocessor  201  (e.g. a MSP430F2232—Manufactured by Texas Instruments) and motor drivers  202 - 205  (e.g. A3950SLP&#39;s—Manufactured by Allegro Microsystems). The battery  103  is shown off board, but could be mounted on board. Motors  207 - 210  connect to the motor drivers. The circuit board is mounted such that the board is in parallel with the table top in both surface axes of the table top. This is for the purpose of simplicity. In fact the relationship between the board and the table top may have any attitude since a difference vector in three dimensions can be known and the three dimensional accelerometer can provide information to accommodate the calculations necessary to perform the table top leveling. 
         [0013]    In addition to the components already defined, there are: switch  206  for turning the power on or off, switch  207  for raising or lowering the table top height and switch  208  for stowing the table legs (causing all legs to retract to minimal length). 
         [0014]    In addition to the functionality already described, each motor driver includes a current sense resistor. The voltage across this resistor is read by an analog to digital converter that is part of the microprocessor  201 . In this manner, the process knows the current in the motors. The purpose of this is to eliminate the need for limit switches (thus reducing cost) which would otherwise be required to indicate that the limit of the range of the leg extension or retraction had been reached. 
         [0015]      FIG. 3  is a flow chart that illustrates the process performed by the system controller. A power on initialization  300  is first performed in which system parameters are set up and the leg positions are driven to the stowed position. In the next process element  301 , all legs are moved to half extension by driving them for a given amount of time having previously determined the drive time empirically. Process element  302  measures the tilt of the table by reading the accelerometer. This reading is the vector of gravity with respect to the circuit board. If the circuit board and table top are not in parallel, the known difference is compensated here. Process element  303  determines if the table top is level by comparing the gravity vector to the vector of the table top. When these vectors do not agree, the process  304  of computing the extension (or retraction) requirements are performed and process  305  extends (retracts) the legs to the computed position. Next, the process goes back to element  302  to again measure the tilt of the table top. If in process  303 , the table is now level, tests are performed on the stow, up and down switches to perform the desired functions as necessary. In the case of the stow switch being found active  306 , all legs are retracted  307  and the processor goes into an idle state  308 . In the case of the up switch being active  309 , the legs are all moved up  310  for as long as the switch stays active. In the case of the down switch being active  311 , the legs are all moved down  312  for as long as the switch stays active. In addition to the process shown, all motors are continuously monitored for the current they are using to determine if they have reached the limit of their range. If they do reach a limit, a flag is set to inhibit driving the motor in that direction until it has been driven in the opposite direction. 
         [0016]      FIG. 4  is an illustration of the table leg including motor and lead screw drive. The top portion  400  of table leg  101  is nominally constructed of square aluminum tubing large enough to house the motor  405  which is fastened to this tubing  400 . The bottom portion  401  of table leg  101  is also nominally constructed of square aluminum tubing that is smaller in size than the top portion such that it can be held stable by the top portion but also slide within the top portion. Other materials besides aluminum may be used. Other shapes of tubing besides square may be used provided that the bottom portion is restricted from rotating within the top portion. A coupler  404  attaches a threaded rod  403  to the shaft of motor  405 . A plate  402  is fixed by welding or other fastening means to bottom portion  401  and includes a mating threaded section to accommodate the threaded rod  403 . An end cap  406  is attached to the end of the threaded rod  403  to provide a stop so that the threaded rod may not escape the bottom portion. Therefore it can be seen that the rotation of the motor causes the threaded rod to rotate and in turn causing the bottom portion of the table leg to extend or retract depending on the direction of the motor&#39;s rotation. Upon extension, if the plate  402  hits the end cap  406 , the motor current will rise to an amount that is detected as the limit. Upon retraction, if the plate  402  hits the coupler  404 , the motor current will rise to an amount that is detected as the limit.