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CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is a continuation in part of and claims priority to co-pending U.S. patent application Ser. No. 13/774,062, filed on Feb. 22, 2013, which claim priority to then U.S. patent application Ser. No. 12/876,080, filed on Sep. 3, 2010, and issued on May 7, 2013, as U.S. Pat. No. 8,437,921, which claimed priority to U.S. Provisional Patent Application Ser. No. 61/240,158, filed on Sep. 4, 2009. This patent application also claims priority to U.S. Provisional Patent Application Ser. No. 61/727,982, filed on Nov. 19, 2012. 
    
    
     TECHNICAL FIELD 
     The present novel technology relates generally to the field of mechanical engineering, and, more particularly, to a method and apparatus for preventing a back hoe bucket from digging beyond a predetermined depth or grade. 
     BACKGROUND 
     Keeping on grade while digging with a back hoe continues to be a challenge even for the most experienced operators. More so than most digging machines, the extended lever arm of the hoe combined with the downward digging forces applied to produce wiggling and vibration of the hoe arm and bucket. Even experienced operators, having developed a tactile ‘feel’ for how well the bucket is digging and cutting, have difficulty maintaining grade, and the more precisely grade must be maintained, the more difficult and draining the job. While very good operators are able to maintain grade reasonably well even over prolonged digging sessions, the job does take its toll both physically and mentally. 
     Conventional laser alignment and even GPS guided devices have been developed to give the operator more reliable feedback regarding how close the digging bucket is to the desired grade. Such devices provide feedback to the operator that the bucket is too high, too low, or on grade at any given time during the digging operation. However, the operator must still receive and manually respond to the feedback signals (up or down) provided by the devices. Such constant correction of the bucket depth has proven to be physically demanding and exhausting. 
     Thus, there is a need for a system for automatically preventing overdigging and for automatically keeping the excavation on a predetermined grade. The present novel technology addresses this need. 
     SUMMARY 
     The present novel technology relates to a method and apparatus for maintaining a predetermined grade while digging with a back hoe. One object of the present novel technology is to provide an improved means for generating laser lines. Related objects and advantages of the present novel technology will be apparent from the following description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a first embodiment of the present novel technology, a system for automatically maintaining a back hoe bucket on grade during a digging operation. 
         FIG. 2  is a perspective view of a second embodiment of the present novel technology, a system for automatically maintaining a back hoe bucket on grade during a digging operation. 
         FIG. 3  is a side elevation view of a first embodiment back hoe bucket of the resent novel technology. 
         FIG. 4A  is a perspective view of the bucket of  FIG. 2  having the contact member engaged. 
         FIG. 4B  is a perspective view of the bucket of  FIG. 2  having the contact member disengaged. 
         FIG. 5A  is a top plan view of the bucket of  FIG. 2  having the contact member engaged. 
         FIG. 5B  is a top plan view of the bucket of  FIG. 2  having the contact member disengaged. 
         FIG. 6A  is a front elevation view of the bucket of  FIG. 2  having the contact member engaged. 
         FIG. 6B  is a front elevation view of the bucket of  FIG. 2  having the contact member disengaged. 
         FIG. 7  is a schematic diagram of the process of  FIG. 1 . 
         FIG. 8  a perspective view of a first embodiment system including an elongated bucket and interrupt bar assembly as connected to a skid loader. 
         FIG. 9A  is a schematic view of the loader of  FIG. 8  with the interrupt bar positioned away from the cutting edge of the bucket. 
         FIG. 9B  is a schematic view of the loader of  FIG. 8  with the interrupt bar moved toward a deployed position adjacent the cutting edge of the bucket. 
         FIG. 9C  is a schematic view of the loader of  FIG. 8  with the interrupt bar in a deployed position adjacent the cutting edge of the bucket. 
         FIG. 10  is a front perspective view of another embodiment back hoe bucket according to the system of  FIG. 2 . 
         FIG. 11  is a partially cut away side elevation view of the bucket of  FIG. 10 . 
         FIG. 12A  is a rear perspective view of the bucket of  FIG. 10 . 
         FIG. 12B  is a partially cut away rear perspective view of the bucket of  FIG. 10 . 
         FIG. 13  is an exploded perspective view of a third embodiment of the present novel technology, a kit for converting a standard hoe bucket into a bucket according to the embodiment of claim  1  or  2 . 
         FIG. 14A  is a perspective view of a fourth embodiment of the present novel technology, and elongated bucket having an interrupt plate operationally connected thereto. 
         FIG. 14B  is a perspective view of the embodiment of  FIG. 14A  with the interrupt plate pivoted. 
         FIG. 15  is a block diagram illustrating an example method of maintaining a back hoe bucket on grade during a digging operation. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     For the purposes of promoting an understanding of the principles of the novel technology and presenting its currently understood best mode of operation, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the novel technology is thereby intended, with such alterations and further modifications in the illustrated device and such further applications of the principles of the novel technology as illustrated therein being contemplated as would normally occur to one skilled in the art to which the novel technology relates. 
     A first embodiment of the present novel technology is illustrated in FIGS.  1  and  3 - 9 C, a system  10  for automatically preventing a track hoe bucket, back hoe bucket, loader bucket, skid loader bucket or like bucket or shovel from digging substantially deeper than a predetermined grade depth parameter. While the following example and drawings focus on a hoe bucket, the claimed novel technology is not limited to a hoe system and includes other digging machines, such as front loaders and the like. The system  10  includes a position sensor  15  and a depth sensor  20  operationally connected to a microprocessor  25  and likewise connected in communication with a reference signal  30 . The sensors  15 ,  20  may be separate, or may both be the same (such as a GPS transceiver). Further, some embodiments may only have a depth sensor  20 , while others may only have a position sensor  15 . The reference signal  30  may be from a GPS satellite, a laser, or the like. 
     The microprocessor  25  is also connected to an actuator assembly  37 . The actuator assembly typically  37  includes a pressure source or pump  40 , such as a hydraulic or pneumatic pump  40  is connected in fluidic communication with at least one hydraulic or pneumatic cylinder  45 . The hydraulic cylinder  45  is fixedly, and typically pivotably, connected to a hoe or shovel bucket or blade  50  having a cutting edge or teeth  53 . While actuator assembly  37  is described herein as being of the pressurized piston/cylinder type, actuator assembly  37  may likewise include other types of actuators, such as mechanical, electromechanical, or the like. 
     Bucket  50  is likewise connected to the distal portion of a hoe armature  51 . The hydraulic cylinder  45  is also operationally connected to an interrupt bar  55 , which is likewise pivotably connected to the bucket  50 . The position and depth sensors  15 ,  20  are likewise operationally connected to the bucket  50  such that the depth of the bucket, and the cutting edge  53 , is either directly measured (such as by direct attachment of the sensor(s)  15 ,  20  to the bucket  50 ), or calculated (such as by connection of the sensor(s)  15 ,  20  to a predetermined position on the distal portion of the armature  51  connected to the bucket  50 ). 
     In operation  100 , as schematically illustrated in  FIG. 7 , microprocessor  25  is first programmed with the location and depth parameters of the grade or excavation to be dug  105 . The reference signal  30  is received  110  by the depth sensor  20  and/or microprocessor  25  when the digging machine is in operation, and the depth of the bucket  50  is calculated in substantially real-time. The location of the bucket  50  is also typically calculated from information supplied by the location sensor  15  and received  115  by the microprocessor  25 . In some embodiments, the position sensor  15  may also be used to calculate the orientation of the bucket  50 , such as its degree of pivot relative to a predetermined base orientation, such as teeth down and parallel to the horizontal. The depth, location and orientation information are used to calculate the position of the bucket  50  and this is compared  120  by the microprocessor  25  to the programmed grade information. If the bucket  50  begins exceed  125  programmed grade parameters, such as moving deeper than the programmed grade, an actuation signal  130 , typically a voltage, is generated by the microprocessor  25  and sent to the hydraulic pump  40 , energizing the pump  40  and actuating the cylinder  45  to extend  145  and pivot the interrupt bar  55  into position to engage the ground ahead of the bucket  50 . This operation is shown sequentially in  FIGS. 9A-9C , wherein the interrupt bar  55  connected to a skid loader bucket  50  is moved from a standby position ( FIG. 9A ) into an engaged position ( FIG. 9C ), preventing the bucket  50  from digging into the ground and, typically, slightly lifting the front end of the loader. If the bucket position does not exceed  135  the programmed grade parameters, a null signal  140  is sent to the pump  40 . Engagement of the ground by the interrupt bar  55  prevents the shovel or bucket  50  from penetrating deeper into the ground. The microprocessor  25  may then query the sensors  15 ,  20  for bucket location information, and the cycle starts over. It should be noted that although the process of digging to grade is typically one of vertically removing dirt, the programmed grade may likewise be a substantially horizontal parameter, such as the walls of a dug basement. The microprocessor  25  may likewise combine vertical, horizontal, and/or bucket orientation parameters to govern the excavation of curved and/or complex shape surfaces. 
     The interrupt bar  55  is typically an elongated member made of a structural material, such as steel. The interrupt bar  55  is more typically rounded or generally cylindrical. The interrupt bar  55  is generally U-shaped, having an elongated and generally rounded middle portion  70  and parallel connection members  75  extending from either end of the middle portion at generally right angles from the axis of the middle portion  70 . The middle portion  70  and connection members  75  may define a unitary piece (see  FIGS. 10-12B ), or may be connected together as separate pieces. 
       FIG. 2  illustrates one specific configuration of the system  10  wherein a single hydraulic cylinder  45  is used to pivot the interrupt bar  55 , while  FIGS. 3-9C  illustrate a configuration wherein a pair of cylinders  45  are used. The cylinders  45  are illustrated as positioned in the interior of the bucket  50 , but may likewise be positioned adjacent the exterior of the bucket  50 . 
       FIGS. 10-12B  illustrate a variation of the bucket  50  illustrated in  FIG. 2  and discussed above, wherein the interrupt bar  55  and piston-cylinder actuator  45  are enclosed in a recess  200  formed in the bucket  50 . In this embodiment, the recess  200  is defined by inner bucket wall  205  and outer bucket wall  201  which create the double-walled bottom portion or recess  200 . The actuator  45  is positioned in the recess  200  and is fixedly mounted to the bucket  50  at one end and to the interrupt bar  55  at the other. Energization of the actuator  45  advances the interrupt bar  55  out of the recess  200  to a position adjacent the cutting edge  53 , where it is interposed between the bucket  50  and the ground. Bottom wall  210  acts to protect the actuator  45  from clogging by dirt and debris, as well as from impact damage and the like. 
     In other embodiments, the grade predetermination function of the microprocessor may be replaced by a mechanical grade indicator, such as a string, line or surface, and the microprocessor voltage or signal generation function may be replaced mechanically, such as by a contact switch or control armature or member. 
     In one embodiment, as shown in  FIG. 13 , a kit  250  is provided for retrofitting existing buckets. The kit  250  includes an interrupt bar  55  operationally connected to a piston actuator  45  and connectable to and/or slidingly disposed in a housing  210 . The housing  210  is structurally connectable to a bucket, such as by bolting, welding, or the like, to define a bottom wall  210 . One or more s sensors  15 ,  20  are typically connected to, and more typically disposed within, the housing  210  and are likewise operationally connectable to a controller  25  (as shown in previous FIGs.). The piston actuator  45  is connectable to a hydraulic pressure source. 
     In another embodiment, as shown in  FIGS. 14A and 14B , a system  310  is shown wherein hydraulic cylinders  345  are connected to a bucket  350  and may be energized to pivot an interrupt plate  355  pivotably connected thereto, urging the plate  355  into engagement with the ground to maintain controlled contact of the bucket  350  with the ground and ensure a maximum depth of cut. The cylinders  345  are illustrated as positioned in the exterior top portion of the bucket  350 . The bucket  350  is illustrated as a wide bucket having an aspect ratio similar to that of a loader or dozer bucket or blade, but may have any convenient shape. 
       FIG. 15  is a block diagram  1500  illustrating am example method of maintaining a back hoe bucket on grade during a digging operation. In some implementations, the system  10  receives initial configuration information ( 1510 ) concerning the length of the segments of the boom assembly (the entire actuated arm assembly to which the bucket is distally attached), dimensions of the bucket  50 , dimensions of the hydraulic or pneumatic cylinders  45 , chassis assembly, and the like. In some implementations, the system  10  is also configured by operating the system to move to various set points. The set points include the boom fully extend, the boom at a 90 degree angle, and the like. In some implementations, the system is preconfigured with the configuration information for industry common back hoes and back hoe assemblies. In these implementations, a user can select his back hoe from the list of preconfigured back hoes rather enter in the configuration information. The configuration information is stored in a non-volatile memory. 
     If the system  10  has already been configured, the user begins by zeroing the system  10  ( 1520 ). Zeroing the system  10  is performed by maneuvering the bucket to just touch the ground. In some implementations, the system&#39;s sensors will automatically zero the system  10  ( 1520 ). For example, a laser range finder may automatically determine the zero point for the system  10 . The user then enters the desired digging depth ( 1530 ). Alternatively, if a trench or hole or the like is already partially existing, the user can simply maneuver the bucket to rest upon the bottom of the trench or hole or the like to enter the desired digging depth ( 1530 ). In some implementations, the system  10  will emit a sound, in addition to automatically leveling the bucket, when the bucket has reached the desired digging depth or grade. 
     The system  10  then receives the remainder of the parameters of the excavation from the user ( 1540 ). For example, some excavations may require the sides of the excavation have a slope different than perpendicular to the bottom of the hole or trench. In such a case, the user can enter a desired slope. Alternatively, the user can have the system  10  register and duplicate an existing slope by maneuver the bucket to press flat against an already existing sloped partial excavation. For example, a water retention pond may already be partially dug. A user can have the system  10  register and duplicate the existing slope of the partially dug water retention pond by maneuvering the bucket such that the back of the bucket is pressed against the existing side of the retention pond. Note that a user can also simply enter in the entire profile of an excavation (length, width, slope, and depth of the excavation) rather than enter the data in a piecemeal manner. The use of a profile also enables the user to register a stop point in the excavation and begin from that point at a later time. For example, a user could register a stop point, break for lunch, and then begin from that stop point after lunch. Additionally while presented singularly, multiple values can be entered into the system  10  for more complex excavations. For example, a terraced retention pond with multiple plateaus and multiple angled sloped sides could also be entered and excavated by the system  10 . 
     During the excavation, the system will automatically tilt the bucket ( 1550 ) accordingly as the excavation nears the desired grade or depth. For example, the system  10  will tilt the bucket upwards as the dug depth nears the desired grade. In this way, the system  10  prevents digging the user from digging too deep. In some implementations, the angle of the bucket&#39;s teeth with respect to the horizon when the bucket is near grade is modeled by the system  10  as the angle=(gain*2.5)*(inches above grade) or angle=−(gain*2.5)*(inches below grade), where the maximum value of inches is 24 and gain is a user configured parameter. The angle is continuously adjusted as the bucket moves. 
     Upon reaching the desired grade, the system  10  will emit a tone ( 1560 ) that instructs the operator that the desired grade has been achieved. The system  10  will also emit a second tone in the event that a grade greater than the desired grade has been reached. The second tone functions to instruct the operator to stop digging and to possibly put back some of the removed dirt back into the area that was being dug. 
     While the novel technology has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character. It is understood that the embodiments have been shown and described in the foregoing specification in satisfaction of the best mode and enablement requirements. It is understood that one of ordinary skill in the art could readily make a nigh-infinite number of insubstantial changes and modifications to the above-described embodiments and that it would be impractical to attempt to describe all such embodiment variations in the present specification. Accordingly, it is understood that all changes and modifications that come within the spirit of the novel technology are desired to be protected.

Summary:
A kit for modifying a digging bucket for controlling the depth cut, including an elongated member operationally connectable to a hoe bucket defining a cutting edge, an actuator operationally connectable to the elongated member, an electronic controller operationally connectable to the actuator, and a position sensor operationally connectable to the cutting edge and operationally connectable to the electronic controller. The actuator may be energized to pivot the elongated member to a position adjacent the cutting edge for engaging ground. Positioning of the elongated member adjacent the cutting edge prevents the cutting edge from digging ground.