an embodiment of this invention will be described hereinafter with reference to the drawings . fig1 shows a rear portion of a tractor acting as a working vehicle . this tractor is used chiefly in agriculture . the tractor includes a lift cylinder 1 ( one example of hydraulic actuators ) in the rear portion thereof for raising and lowering a pair of right and left lift arms 2 . a rotary plow 5 ( one example of working implements ) is connected to the tractor through a three - point link mechanism having a single top link 3 and a pair of right and left lower links 4 . the lower links 4 are suspended from the lift arms 2 through lift rods 6 , whereby the rotary plow 5 is vertically movable by the driving force of lift cylinder 1 . the right lift rod 6 has a double - acting rolling cylinder 7 mounted thereof to be extendible and contractible to cause rolling of the rotary plow 5 . working implements attachable to the three - point link mechanism include , besides the rotary plow 5 , a plowing implement 25 as shown in fig1 and a boom spray 20 for spraying a fertilizer as shown in fig1 . the plowing implement 25 and boom sprayer 20 will be described hereinafter . the description will be made centering on the rotary plow 5 first . as shown in fig2 a hydraulic system is provided for operating the lift cylinder 1 and rolling cylinder 7 . the hydraulic system includes a flow priority valve 10 disposed on an oil line extending from a hydraulic pump p driven by an engine e . the flow priority valve 10 branches the oil line to supply a fixed quantity of control flow to a rolling control valve 11 of the electromagnetically operable type , and to supply a surplus flow to a raising and lowering control valve v of the electromagnetic proportional type . the rolling control valve 11 is switchable by electric signals to a contracting position for contracting the rolling cylinder 7 , an extending position for extending the rolling cylinder 7 , and a neutral position for preventing the extension and contraction . the raising and lowering control valve v includes a raising control valve 12 for supplying pressure oil to the lift cylinder 1 , a raising pilot valve 12 p for operating the raising control valve 12 with pilot pressure , a lowering control valve 13 for draining pressure oil from the lift cylinder 1 , and a lowering pilot valve 13 p for operating the lowering control valve 13 with pilot pressure . the raising pilot valve 12 p and lowering pilot valve 13 p are opened to varied degrees in proportion to currents supplied to respective electromagnetic solenoids 12 s and 13 s , to vary the pilot pressure , thereby adjusting opening degrees of the raising control valve 12 and lowering control valve 13 . thus , the raising and lowering control valve v is an electromagnetic proportional control valve . with this construction , when raising the rotary plow 5 , a current is supplied in an adjusted amount to the electromagnetic solenoid 12 s of raising pilot valve 12 p . the raising pilot valve 12 p is opened in proportion to the current value , to vary the pilot pressure applied from the raising pilot valve 12 p to the raising control valve 12 , whereby the raising control valve 12 is opened to a degree proportional to the current value . as a result , pressure oil is supplied to the lift cylinder 1 in a quantity proportional to the valve opening degree , which determines a rising speed of the rotary plow 5 . similarly , when lowering the rotary plow 5 , a current is supplied in an adjusted amount to the electromagnetic solenoid 13 s of lowering pilot valve 13 p . the lowering pilot valve 13 p is opened in proportion to the current value , to vary the pilot pressure applied from the lowering pilot valve 13 p to the lowering control valve 13 , whereby the lowering control valve 12 is opened to a degree proportional to the current value . as a result , pressure oil is drained from the lift cylinder 1 in a quantity proportional to the valve opening degree , which determines a lowering speed of the rotary plow 5 . as shown in fig3 a controller 16 with a microprocessor has an input system for receiving signals through an analog - to - digital converter 19 from various devices disposed on the vehicle body . these devices include a lever sensor 17 s of the potentiometer type for measuring an operative position of a position lever 17 ( one example of setter for setting a target amount of operation ), a lift arm sensor 2 s of the potentiometer type ( one example of operative position detecting sensor for detecting an operative position of the hydraulic actuator ) for measuring a pivoting amount of the lift arm 2 , a plowing depth setter 18 s of the potentiometer type ( one example of setter for setting a target amount of operation ) operable by a dial 18 for setting a plowing depth of rotary plow 5 , and a cover sensor 5 s of the potentiometer type ( one example of operative position detecting sensor for detecting an operative position of the hydraulic actuator ) for measuring a plowing depth of rotary plow 5 by measuring a pivoting amount of a rear cover 5 a of rotary plow 5 . an electric system is formed from supplying currents from the controller 16 to the electromagnetic solenoids 12 s and 13 s of raising pilot valve 12 p and lowering pilot valve 13 p . the controller 16 has a microcomputer as a central component thereof , with various functions provided by software , hardware or both . the functions particularly important to this invention are performed by an input signal discriminating device 16 a for determining , from the control input signals received through the above input system , sources and causes of the input signals , and an output signal generating device 16 b for determining starting flow rate curves ( fig4 and 10 ) as control characteristics to be referred to , based on results of discrimination by the input signal discriminating device 16 a , and generating control output signals for controlling the position of the working implement in accordance with the control input signals . in this agricultural tractor , two types of feedback control are carried out . one is a position control for vertically moving the rotary plow 5 until a rough agreement is attained between a target height relative to the vehicle body of the rotary plow 5 set by the lever sensor 17 s and an actual height relative to the vehicle body of the rotary plow 5 measured by the lift arm sensor 2 s . the other is an automatic plowing depth control for vertically moving the rotary plow 5 for attaining a rough agreement between a target plowing depth set by the plowing depth setter 18 and an actual plowing depth of the rotary plow 5 measured by the cover sensor 5 s . the position control will be described next . as shown in fig9 a starting flow rate qs is selected along a characteristic line with a flow rate increasing with time . as shown in fig1 , a deviation flow rate qh is selected along a characteristic line to increase with deviation , based on a set signal of a setting system such as the lever sensor 17 s and a detection signal of a feedback system such as the lift arm sensor 2 s . in carrying out a basic control with these two flow rates , as shown in fig6 which is an enlargement of a control portion of what is shown in fig5 the control is started with the starting flow rate qs which is slightly higher than 0 , while the deviation flow rate qh high due to a large deviation between a value set by the position lever 17 and a detection value by the lift arm sensor 2 s . this is because the control by the controller 16 adopt , as a control output , the lower of the starting flow rate qs and the deviation flow rate qh . fig5 is a graph showing results of a test conducted with the boom sprayer 20 attached as the working implement , in which pressure oil is artificially leaked from the lift cylinder 1 to lower the boom sprayer 20 beyond a dead zone of a set height . that is , the working implement is set to a predetermined height by the position control , and pressure oil was artificially leaked from the lift cylinder 1 . the working implement descends beyond the dead zone , and a corrective control is started at this point of time to return the working implement to the predetermined height . as shown in fig5 and 6 , the control is started upon lapse of 12 . 2 seconds , with the starting flow rate qs lower than the deviation flow rate qh . the lift cylinder 1 is extended by gradually increasing the starting flow rate qs for 0 . 21 seconds to the 12 . 5 second point when the starting flow rate qs exceeds the deviation flow rate qh . thereafter the control is continued with the deviation flow rate qh lower than the starting flow rate qs . both the deviation flow rate qh and starting flow rate qs are decreased gradually . thus , a final raise / lower flow rate qe representing an operating flow rate corresponds to the starting flow rate qs in the first half of operation , and to the deviation flow rate qh in the second half after 0 . 21 seconds . as shown in fig4 the starting flow rate qs used in this control rises in a sharp gradient , i . e . based on a second control characteristic ( a second starting flow rate curve table ). that is , as control characteristics to be referred to in calculating the starting flow rate qs , two types with different increase gradients are provided , which are a first control characteristic with a gentle gradient ( first starting flow rate curve table ) and the second control characteristic with a sharp gradient ( second starting flow rate curve table ). the first characteristic with the gentle gradient is used when raising or lowering the working implement slowly . the second characteristic with the sharp gradient is used when raising or lowering the working implement quickly . the starting flow rate determined with reference to the first control characteristic is denoted by qs ′, and that determined with reference to the second control characteristic is denoted by qs ″. the graphs shown in fig5 and 6 show results of the test conducted with the starting flow rate qs ″ determined with reference to the control characteristic with the sharp gradient . where the boom sprayer 20 is attached as the working implement , the implement could be moved vertically at excessive speed during an operation to correct its height , resulting in an inconvenience of water scattering out of a storage tank 21 of the boom sprayer 20 . fig7 and 8 show results of a test conducted with the starting flow rate qs ′ determined with reference to the control characteristic with the gentle gradient ( first control characteristic ) in place of the starting flow rate qs ″ determined with reference to the control characteristic with the sharp gradient ( second control characteristic ). here , since the starting flow rate qs ′ increases gradually , the flow rate required for the vertical movement decreases from 4 . 2 liters to 2 . 5 liters , and it takes from 0 . 21 seconds to 1 . 2 to supply this quantity . thus , height is corrected by vertically moving the working implement slowly . next , based on the test results , the position control will be described in which a selection is made between the two starting flow rates qs ′ and qs ″. as shown in fig5 and the flow chart of fig1 , when a deviation occurs between a set value ( signal value ) lo of position lever sensor 17 s and a detection value ( signal value ) ls of lift arm sensor 2 s , whether the position lever 17 has been operated is determined from a change rate of the signal value lo of lever sensor 17 s (# 1 -# 5 ). after determining whether the signal value of lever sensor 17 s has changed , a selection is made between the two starting flow rates qs ′ and qs ″. that is , a change in the signal value of lever sensor 17 s indicates that the driver has operated the position lever 17 . when the position lever 17 has been operated , a selection is made of the starting flow rate qs ″ determined with reference to the control characteristic with the sharp gradient (# 7 ). a change in the signal value of lift arm sensor 2 s may be found while an operation of the position lever 17 is not confirmed . this indicates a leak of pressure oil from the lift cylinder 1 as noted hereinbefore . then , a control is carried out with the starting flow rate qs ′ determined with reference to the control characteristic with the gentle gradient (# 6 ). after selecting the control characteristic to be referred to , i . e . the starting flow rate qs ′ or qs ″, the starting flow rate is compared with the deviation flow rate qh and a control is started with the lower one of the flow rates . for the flow rate qs ( qs ′ or qs ″) or qh selected , a target current i for driving the raising and lowering control valve v is derived , and a duty ration d required for producing the target current i is set . the control valve v is operated to drive the lift cylinder 1 . the control mode based on the selection of the starting flow rate determined by employing one of the two control characteristics noted above , i . e . the starting flow rate qs ′ or qs ″ is applicable to a draft control and an automatic plowing depth control in a plowing operation , the latter being effected by a forcible raising and lowering switch 14 . in the draft control , the plowing implement 25 is raised or lowered to attain an agreement between a value set by a draft setter 8 of the potentiometer type disposed on a driving platform and detection value of a draft sensor 9 disposed on a lower link 4 to detect an operational resistance acting on the plowing implement 25 . the draft control , which requires an overload to be avoided , does not use the starting flow rate qs ′ with the gentle gradient , but uses the starting flow rate qs ″ with the sharp gradient . in the automatic plowing depth control , the forcible raising and lowering switch 14 i s operated to raise and lower the rotary plow 5 repeatedly . an upper limit setter 15 is provided to set an upper limit for the rotary plow 5 . thus , when the upper limit is changed by operating the upper limit setter 15 , the working implement is moved vertically by using the starting flow rate qs ″ with the sharp gradient to correct the upper limit . the boom sprayer 20 will be described next . as shown in fig1 , the boom sprayer 20 includes a storage tank 21 for storing a fertilizer , and a spraying pump 24 for feeding the fertilizer from the storage tank 21 to a spraying boom 22 and spraying the fertilizer through a spray nozzle 23 . the position control for the boom sprayer 20 is carried out according to the flow chart of fig1 . 1 . this invention is not limited to the above embodiment , but is applicable to an automatic plowing depth control and a rolling control , for example . the invention is applicable also to a draft control for raising and lowering a working implement such as a plow . 2 . in the foregoing embodiment , pressure oil is supplied to the lift cylinder 1 by employing the lower of the starting flow rate qs and deviation flow rate qh . the control may be carried out with the starting flow rate qs only . 3 . the sensor for detecting an operative position of the hydraulic actuator such as lift cylinders 1 relies on the movement of lift arms 2 driven by the actuator . instead , a stroke sensor or the like may be used , which detects the movement of the hydraulic actuator itself .