the present invention is directed to an agricultural implement having a frame that carries a number of farming related tools , such as discs , tillers , sweeps , or dispensers , whose depth is controlled by a depth control system . as will be described , the depth control system includes a sensor , such as a linear potentiometer , that outputs a signal having characteristics that are a function of the linear displacement of a hydraulic cylinder that raises and lowers the implement frame . as known in the art , the hydraulic cylinder includes an extendable piston or ram that when extended lifts the implement frame and when retracted lowers the implement frame . in one embodiment , the sensor includes an integrated linear position sensor , such as those described in u . s . pat . nos . 7 , 307 , 418 , 7 , 259 , 553 , and 7 , 034 , 527 , the disclosures of which are incorporated herein by reference . in fig1 , an implement 10 is illustrated having a central frame 12 , two wings 14 and 16 pivotally coupled to the central frame , lift actuators 18 and 20 for lifting the wings above the frame , wheel actuators 22 , 24 , 26 , and 28 for raising and lowering wheel sets 30 , 32 , 34 , and 36 , front tool gang 38 fixed to the front of the frame and wings , rear tool gang 40 fixed to the rear of the frame and wings . the implement may optionally have a leveling mechanism for leveling the implement , as described in u . s . pat . no . 7 , 063 , 167 . front tool gang 38 includes inner forward gang tubes 44 and 46 which are bolted to central frame 12 and extend laterally away from the central frame . these gang tubes have pivotal couplings 48 and 50 disposed at their outer ends to which outer forward gang tubes 52 , 54 , respectively , are pivotally connected . rear tool gang 40 includes inner rear gang tubes 56 and 58 which are bolted to central frame 12 and extend laterally away from the central frame . these gang tubes have pivotal couplings 60 and 62 disposed at their outer ends to which outer rearward gang tubes 64 and 66 , respectively , are pivotally connected . a plurality of ground engaging tools , here shown as discs 68 , are fixed to and disposed below each of the gang tubes . like each pair of inner and outer gang tubes themselves , these discs are arranged in a substantially straight line . the gang tubes on each side of the implement are bolted to a wing frame on that side of the implement . outer gang tubes 52 and 64 are coupled to wing frame 70 , and outer gang tubes 54 and 66 are coupled to wing frame 72 . the outer gang tubes are pivotally coupled to the inner gang tubes to permit them to be lifted above and over the central frame to permit the implement to be folded up for clearance when towed over the road . this lifting is provided by lift actuators 18 and 20 , here shown as hydraulic cylinders . lift actuator 18 is coupled between central frame 12 and wing frame 70 to lift wing 14 , and lift actuator 20 is coupled between central frame 14 and wing frame 72 to lift wing 16 . when lift actuators 18 and 20 are retracted , they pull their associated wings 14 and 16 upward and over the top of central frame 12 about pivotal couplings 48 , 60 , and 50 , 62 , respectively . wing 14 includes wing frame 70 , front and rear gang tubes 52 and 64 , respectively , and the ground engaging tools attached to those tubes . wing 16 includes wing frame 72 , front and rear gang tubes 54 and 66 , and the ground engaging tools attached to those tubes . referring to fig2 , central frame 12 includes two fore - and - aft extending members 74 and 76 to which wheel sets 32 and 34 , respectively , are pivotally mounted . side - to - side members 78 and 80 are disposed at the front and rear , respectively , of the frame and are coupled to members 74 and 76 to form a substantially rectangular framework . a tongue 82 is coupled to central frame 12 and allows the implement to be hitched to a tractor in a known manner . a rockshaft 84 extends laterally across central frame 12 and is supported in rotation at each end by bearings 86 and 88 that are mounted on fore - and - aft members 74 and 76 , respectively . bearings 86 and 88 constrain rockshaft 84 to rotate about its longitudinal axis with respect to central frame 12 . four wheel supports 90 , 92 , 94 and 96 extend downward and rearwardly from rockshaft 84 to which they are attached . wheel supports 90 and 92 are disposed on the inside and the outside , respectively , of bearing 86 and member 74 to which bearing 86 is attached . wheel supports 94 and 96 are disposed on the inside and outside , respectively , of bearing 88 and fore - and - aft member 76 to which bearing 86 is attached . thus , when rockshaft 84 rotates , it causes the outer ends of wheel supports 90 , 92 , 94 and 96 to simultaneously and equally raise or lower with respect to central frame 12 . two axles 98 and 100 are provided to which wheel sets 32 and 34 are mounted for rotation . axle 98 is mounted to the outer ends of wheel supports 90 and 92 , and axle 100 is mounted to the outer ends of wheel supports 94 and 96 . wheel set 32 has two wheels that are mounted to opposing ends of axle 98 , and wheel set 34 has two wheels that are mounted to opposing ends of axle 100 . the wheels in each wheel set are disposed on opposite sides of their associated fore - and - aft member , one inside and one outside . wheel actuators 24 and 26 are pivotally coupled to fore - and - aft members 74 and 76 , at one end , and at the other end to brackets 102 and 104 . brackets 102 and 104 are mounted to rockshaft 84 to rotate with rockshaft 84 . when wheel actuators 24 and 26 are retracted , the wheels are raised thereby causing a lowering of the implement and the work units coupled thereto . when actuators 24 and 26 are extended , they push the upper ends of brackets 102 and 104 away from the actuators toward the rear of the implement . the lower ends of brackets 102 and 104 are coupled to rockshaft 84 , which causes rockshaft 84 to rotate clockwise . this clockwise rotation causes wheel supports 90 , 92 , 94 and 96 to also rotate clockwise . as the wheel supports rotate clockwise , the outer ends of the wheels supports and the two wheels sets coupled to the wheel supports also lower . as a result , the wheels pivot about rockshaft 84 as they are lowered thereby lifting the implement . in one embodiment , the actuators 24 and 26 are hydraulic cylinders , with one of the cylinders including an integrated linear position sensor , such as those described in u . s . pat . nos . 7 , 307 , 418 , 7 , 259 , 553 , and 7 , 034 , 527 , the disclosures of which are incorporated herein by reference . it is contemplated however that both cylinders may include a position sensor . the depth control system 106 is schematically illustrated in fig3 and controls the flow of hydraulic fluid to and from depth control hydraulic cylinder 108 . piston 110 is extendable and retractable from cylinder 108 and has an integrated linear potentiometer that provides a signal to a controller 114 of the tractor t . as will be described , the controller 114 selectively energizes a raise solenoid 116 and a lower solenoid 118 . hydraulic fluid is supplied to the cylinder 108 through supply port 120 and is returned through a return port 122 . the solenoids 116 and 118 are fluidly connected to a fluid reservoir 124 that includes a pump 126 . when raise solenoid 116 is energized , hydraulic fluid is supplied to the cylinder 108 along a fluid path between reservoir 124 and cylinder 108 , thereby causing an extension of piston 110 that is coupled to bracket 102 , fig2 . as the piston is extended , the bracket 102 rotates rearwardly or in a counterclockwise direction thereby causing the implement to lift . conversely , when solenoid 118 is energized , a fluid path is open between the cylinder 108 and the reservoir 124 resulting in fluid being drawing from the cylinder 108 . this causes a retraction of the piston 110 and thus a lowering of the implement . the controller 114 selectively energizes the solenoids 116 , 118 to maintain the depth of the implement at an operator selected level , which is selected using appropriate operator controls 128 within the operator cab of the tractor . the operator cab may also include various displays 130 to provide feedback regarding operation of the depth control system and other systems of the implement or tractor . the feedback provided by the integrated potentiometer 112 is used by the controller 114 to derive a relative depth of the implement 10 . more particularly , and referring now to fig4 , the operator manually sets the implement to a desired depth . the depth can be set remotely using controls within the operator cab of the tractor if so equipped or at the implement itself . once the depth has been selected , the operator activates a set - depth control which is detected by the controller at block 132 . responsive thereto , the controller 114 reads and stores the output of the potentiometer 112 integrally formed with the cylinder 108 . the output of the potentiometer provides a baseline voltage that is stored in memory at block 134 . as the implement is towed along the field , the controller 114 iteratively reads the output of the potentiometer 112 . any leakage of hydraulic fluid as the implement is being towed , which can cause a change in the depth of the implement , is detected by a change in the voltage output of the potentiometer 112 . that is , as the piston 110 is retracted and extended as a result of unintended changes in the flow of hydraulic fluid to and from the cylinder 108 , the output of the potentiometer 112 will also change . those changes are detected by the controller at block 136 and compared to the baseline voltage at block 138 . if the position of the piston 110 has changed , which would result in a change in the depth of the implement , the voltage output of the potentiometer 112 will differ from the baseline voltage . thus , if the voltage is different , the controller selectively energizes one of the solenoids at block 140 to either extend or retract the piston 110 until the output voltage of the potentiometer 112 equals , within some tolerance , the baseline voltage . if the voltage substantially equals the baseline voltage , the controller 114 returns to block 136 with continued monitoring of the potentiometer . in the above described embodiment , one of the wheel actuators includes a cylinder with an integrated potentiometer . thus , changes in depth readings are measured at that cylinder and any changes in hydraulic flow to maintain the depth of the implement at the operator selected level are made in that and the other wheel actuators . it is also contemplated however that each wheel actuator may have a cylinder with an integrated potentiometer and that the hydraulic pressure in the cylinders can be independently controlled to independently vary the position of the wheel actuators . in a preferred embodiment , the position sensor is integrally formed with the hydraulic cylinder and its piston , but it is understood that other types of sensors could be used to directly measure the position of the piston and cylinder relative to one another . moreover , sensors that measure voltage changes as a function of the displacement of a cylinder is representative and as such sensors that measure other types of parameters , such as sound , current , force , and the like , may be used and are considered within the scope of the invention . additionally , it is recognized that the output of the sensor could be provided to the controller in a wired or wireless transmission . many changes and will modifications could be made to the invention without departing from the spirit thereof . the scope of these changes will become apparent from the appended claims .
an agricultural implement has a depth control system in which a position sensor directly measures linear translation of a hydraulic cylinder that lifts and lowers the implement frame to set and adjust the depth of the implement frame . the position sensor may be positioned adjacent to or integrally formed with the hydraulic cylinder , and provides a voltage to a controller remote from the implement . the controller automatically adjusts the flow of hydraulic fluid to and from the hydraulic cylinder to maintain the depth of the implement frame at an operator - selected level .