referring to fig1 an agricultural tractor is shown comprising a frame 1 carrying right and left lift arms 3 vertically pivotable by a hydraulic cylinder 2 mounted at a rear portion thereof , and a rotary plow 5 vertically movably connected to the lift arms 3 through a three - point link mechansim 4 . the three - point link mechanism 4 includes a top link 4a and two lower links 4b . the lower links 4b are connected to the lift arms 3 through lift rods 6 . a ultrasonic distance sensor 8 is mounted on a rear end of a support member 7 pivotally connected to extending rearwardly from the rotary plow 5 . the support member 7 is pivotable by a hydraulic cylinder 9 to change its posture . a hand lever 10 is operatively connected to an electromagnetic valve 26 for controlling the hydraulic cylinder 2 . this construction constitutes a position control mechanism for vertically moving the rotary plow 5 to a position corresponding to a position to which the hand lever 10 is shifted . the rotary plow 5 includes a rotor 5a for plowing the ground . a leveling member 12 is pivotally connected to the rotary plow 5 for leveling the ground plowed by the rotor 5a , the leveling member 12 being urged to neutral position by an urging mechanism 11 . the distance sensor 8 is located in a position to measure a vertical distance between the position of the sensor 8 and the ground rearwardly of the leveling member 12 . a meter panel 14 provided forwardly of a driver &# 39 ; s seat 13 includes an indicating unit 15 for indicating an actual plowing depth of the rotary plow 5 . as shown in fig3 the indicating unit 15 comprises a liquid crystal indicator having a bar graph to indicate the actual plowing depth , such that the longer bar represents the greater plowing depth . signals output by the distance sensor 8 are transmitted through an a / d converter 16 to a microprocessor 17 . the microprocessor 17 has an input line for receiving signals from a first switch 18 and an output line for outputting signals to the indicating unit 15 through a decoder 19 . the described construction constitutes a plowing depth detecting system and its operation will be described hereinafter . the rotary plow 5 is lowered while the tractor is standing still , to a level in which a lower end of the rotor 5a contacts the ground . in this state , the microprocessor 17 receives from the distance sensor 8 a reference value corresponding to a unplowed ground go rearwardly of the rotary plow 5 . the indicating unit 15 is adjusted to show &# 34 ; 0 &# 34 ; at this time . then the rotary plow 5 is lowered further by manually operating the lever 10 of the position control mechanism . during the descent , the magnitude of the distance signal periodically transmitted from the distance sensor 8 to the microprocessor 17 gradually reduces with an increasing amount of subsidence ( corresponding to the plowing depth ) of the rotary plow 5 . the microprocessor 17 operates on a deviation from the initial value and sends the deviation to the indicating unit 15 , and this deviation is indicated on the panel 14 as representing the plowing depth . the driver of the tractor stops the descent of the rotary plow 5 when the plowing depth indicated reaches a target value . he then turns on the first switch 18 , causing the microprocessor 17 to store the target plowing depth , and starts the tractor . while the tractor is running , a plowed ground surface leveled by the leveling member 12 is formed rearwardly of the rotary plow 5 . this plowed ground surface is raised by a substantially constant amount r from the unplowed ground go . therefore , when a measurement point of the distance sensor 8 comes to the plowed ground surface gp , the measured distance reduces by the amount of rise r and the indicating unit 15 indicate a plowing depth added with the amount of rise r . in order to correct this error the microprocessor 17 is provided with corrector means . this corrector means is required since the distance sensor 8 has not detected any rise r before the tractor begins to plow as illustrated in fig1 and , accordingly , there is an intitial transient portion of r . this transient portion of r is a function of the distance the tractor initially travels increasing at first and then leveling off . accordingly , this transient disappears and r becomes a constant when the tractor travels a certain distance or travels at a certain speed for a fixed period of time . in operation when the value detected and output by the distance sensor 8 begins to increase rapidly , a timer comes into operation to interrupt the input of the detected value to the microprocessor 17 . the input is resumed when a certain period of time lapses and the measurement point of the distance sensor 8 becomes constant for plowed ground surface gp . upon receipt of a first detection value from the distance sensor 8 corresponding to the plowed ground surface gp , the microprocessor 17 calculates the deviation by subtracting this detection value from the reference value . since this deviation represents the sum of the target plowing depth value and the amount of rise r , the amount of rise r is derived by subtracting the stored target plowing depth value from the deviation and is stored in the microprocessor 17 . thereafter , the microprocessor 17 receives a detection value from the distance sensor 8 , works out a deviation thereof from the reference value , subtracts the amount of rise r stored as the correction value from this deviation , and outputs a resulting value to the indicating unit 15 . the indicating unit 15 shows the input value as the actual plowing depth p . a further embodiment of the plowing depth detecting system will be described next , which is adapted to correct the amount of rise r by using manual corrector means . referring to fig3 a casing 20 of the indicating unit 15 includes a correcting unit 21 which is one example of the manual corrector means and is operable by a dial 21a . the dial 21a is surrounded by a scale 21b divided for every centimeter from 0 to 16 cms . corresponding to target plowing depths . a correction value ( i . e . the amount of rise ) is input to the microprocessor 17 by setting a pointer 21c of the dial 21a at a division of the scale 21b corresponding to a desired plowing depth . this scale 21b is determined on the basis of empirical data of the amount of rise r with respect to the target plowing depth obtained in relation with standard or average soil nature . the correcting unit 21 includes a &# 34 ; 0 &# 34 ; division of the scale to which the amount of correction is set for an operation in a paddy field in view of the fact that a rise of ground surface gp will not be formed when operating in a paddy field . in the plowing depth detecting system including this manual corrector means , the vertical distance between the distance sensor 8 and the lower end of the rotor 5a is stored in the microprocessor 17 as the reference value . the operator shifts the lever 10 of the position control mechanism 10 to lower the rotary plow 5 to a position corresponding approximately to the target plowing depth value . the tractor is started after adjustment is effected by means of the dial of the correcting unit 21 . the microprocessor 17 subtracts from the reference value a detection value received from the distance sensor 8 and a correction value selected by means of the dial , and outputs a resulting value to the indicating unit 15 . the indicating unit 15 shows this value as the actual plowing depth . the described two corrector means may both be provided along with a changeover switch . then the plowing depth detection value may be obtained with greater accuracy be utilizing the arithmetic correcting means in the first embodiment for operations on a relatively flat and smooth ground and the manual correcting means for operations on a relatively rough and uneven ground . while the amount of rise r is considered as the correction value in the foregoing two embodiments , it is necessary to consider a different correction value for a different operational mode . as shown in fig5 a and 5b , for example , there is a difference in the vertical distance ( or the shortest distance ) from the distance sensor 8 to the lower end of the rotary plow 5 between the case of an operation in a dry field ( fig5 a ) and the case of an operation in a paddy field ( fig5 b ) even if the plowing depth x is the same . this is due to a difference in the posture ( θ 1 , θ 2 ) of the link mechanism 4 relative to the tractor frame . therefore , although the plowing depth is the same , a greater detection value is given from the distance sensor 8 to the microprocessor 17 in the case shown in fig5 b than in the case shown in fig5 a . as a result , the value shown by the indicating unit 15 is smaller than the actual plowing depth x . where an ultrasonic sensor is employed as the distance sensor 8 , a sound wave reflected from the field surface and traveling by the shortest distance is detected and the distance is measured by converting a time lapse from emission of the sound wave to when the sound pressure level of the received sound wave reaches a predetermined value ( i . e . threshold level ) as shown in fig6 a and 6b . however , since in the case shown in fig6 b the direction of center line for the emission of the sound wave from the ultrasonic sensor varies with a vertical movement of the link mechanism , the sound wave reflected from the ground and traveling by the shortest distance has a low sound pressure level due to the deviation from the center line of emission and it takes time for the sound pressure level to reach the predetermined value . consequently , the distance is measured to be longer than the actual distance and a value smaller than the actual plowing depth is indicated by the indicating unit 15 . in order to correct this error , an angle sensor 22 for detecting an angle θ of the lift arms 3 with respect to the tractor frame is provided along with means to converting the angle θ detected by the angle sensor 22 into a correction value . this converting means converts the angle θ input from the angle sensor 22 into the correction value on the basis of data in a table stored in the microprocessor memory . as shown in fig7 this table shows the correction value as a function of the detected angle θ , and the correction value corresponds to a distance converted beforehand from an amount of relative movement of the distance sensor based on the detected angle θ and / or an amount of response delay due to a variation of the ultrasonic sensor 8 with respect to the direction of center line for emission . for a plowing operation in a paddy field which requires such correction means , the dial 21a of the correcting unit 21 is set at the &# 34 ; 0 &# 34 ; division since raised ground surfaces are not formed . therefore , while the tractor is running , the microprocessor 17 subtracts from the reference value the value detected by the distance sensor 8 and the correction value provided by the converting means and outputs a resulting value to the indicating unit 15 . the indicating unit 15 shows this value as the actual plowing depth . where automatic plowing depth control is effected on the basis of the actual plowing depth detected by the above plowing depth detecting system , a changeover switch for starting and stopping the automatic plowing depth control , a setter switch for setting a plowing depth value , a plowing depth setting device 23 and comparator means 24 are provided in addition to the elements constituting the plowing depth detecting system . when the changeover switch is turned off and the automatic control is stopped , the operator effects the plowing depth control by means of the hand lever 10 while observing the value indicated by the indicating unit 15 . he turns on the setter switch when the indicated value reaches a desired value , whereby this value is set to the plowing depth setting device 23 as a selected value of plowing depth . when the changeover switch is thereafter turned on for the automatic control operation , the microprocessor 17 compares the actual plowing depth value and the selected plowing depth value and derives a deviation . a signal comprising this value and a control gain is transmitted through a driver 25 to the electromagnetic value 26 for operating the hydraulic cylinder 2 . in response to the input signal the electromagnetic valve operates the hydraulic cylinder 2 to raise or lower the rotary plow 5 thereby to maintain the actual plowing depth to the selected plowing depth .
there is disclosed a plowing depth detecting system for a rotary plow adapted to detect an actual plowing depth on the basis of distance data with respect to the ground received from a distance sensor attached to the rotary plow which is vertically movably connected to a vehicle through a link mechanism . the distance sensor provides a microprocessor with distance data with respect to the ground rearwardly of the rotary plow . the microprocessor determines a value of actual plowing depth of the rotary plow on the basis of this distance data and a correction value according to a working condition of the rotary plow .