fig3 schematically shows the arrangement of the controls provided near the operator &# 39 ; s seat of the crawler - type tractor to be controlled by the device of the present invention . a lever 10 shown in the same figure is supported so as to be operable according to the operation pattern shown in fig4 and a knob 10a thereof is provided with a push switch 11 . a speed - stage command device 12 commands speed stages ( r1 , r2 , r3 , f1 , f2 and f3 ) of the transmission ( not shown ) and is provided with push button switches 12 - 1 , 12 - 2 , 12 - 3 and 12 - 1 &# 39 ;, 12 - 2 &# 39 ;, 12 - 3 &# 39 ; corresponding to respective speed stages with respect to forward or backward travel . these push button switches are of normal - open type . fig5 is a block diagram showing an example of the control device of the present invention for controlling the transmission and steering of the crawler - type tractor . in the same figure , when the lever 10 is operated to the &# 34 ; right turn &# 34 ; position or the &# 34 ; left turn &# 34 ; position of the operation pattern as shown in fig4 a potentiometer 13 outputs a voltage signal s1 of the value corresponding to the operated amount with respect to these directions . furthermore , when the lever 10 is operated from the neutral position in the above pattern to the &# 34 ; forward &# 34 ; position or the &# 34 ; reverse &# 34 ; position , a potentiometer 14 outputs a voltage signal s2 of a value corresponding to the operated amount with respect to these directions , the output signals s1 and s2 of these potentiometers 13 and 14 being converted into digital values by respective analogue - to - digital converters 15 and 16 ( hereafter referred to as a / d converters ). a potentiometer 17 outputs a voltage signal s3 corresponding to the operated amount of a brake pedal 18 ( refer to fig3 ), the output signal s3 being inputted into an a / d converter 19 . a signal generation circuit 20 generates parallel signals sa , sb and sc of logic levels as shown in the following table in response to the operation of the push button switches 12 - 1 , 12 - 2 and 12 - 3 and 12 - 1 &# 39 ;, 12 - 2 &# 39 ; and 12 - 3 &# 39 ; in the speed - stage command device 12 . ______________________________________ sa sb sc______________________________________f 1 12 - 1 1 1 1f 2 12 - 2 1 0 1f 3 12 - 3 1 1 0r 1 12 - 1 &# 39 ; 0 1 1r 2 12 - 2 &# 39 ; 0 0 1r 3 12 - 3 &# 39 ; 0 1 0______________________________________ since this signal generation circuit 20 can be easily constructed by using logic circuits such as an and circuit , or circuit , and inverter , description of the circuit configuration is omitted here . a neutral lock switch 21 operates to output a signal s4 which urges the transmission to come to its neutral position and is located at the side of the speed - stage command device 12 as shown in fig3 . a central processing unit 22 ( hereinafter referred to as cpu ) executes processing which will be described below when the signals s1 , s2 , s3 , sa , sb or sc is received . memories 23f and 23r store respectively the speed stages for forward and backward directions as commanded by the speed - stage command device 12 , and a display 24 displays the memory contents of the memories 23f and 23r . this display 24 is disposed at a suitable place on the front panel before the operator &# 39 ; s seat as shown in fig3 . a driving circuit 25 selects and drives valve driving solenoids 26 - f , 26 - r , 26 - 1 , 26 - 2 and 26 - 3 in the electric - hydraulically operated transmission having a planet gear ( not shown ) in accordance with a speed - stage setting signal coming from the cpu 22 . valves 27 - f and 27 - r respectively operated by the solenoids 26 - f and 26 - r select forward or reverse of the transmission by controlling the individual hydraulic cylinders ( not shown ) corresponding thereto , and valves 27 - 1 , 27 - 2 and 27 - 3 respectively operated by the solenoids 26 - 1 , 26 - 2 and 26 - 3 select a speed stage among lst , 2nd and 3rd speed stages in the forward or backward direction by controlling the corresponding hydraulic cylinders ( not shown ). for example , when the solenoid 26 - f and the solenoid 26 - 1 are energized , the transmission becomes in the forward 1st speed stage . the driving circuit 28 drives the electric - hydraulic clutch . more particularly , the circuit 28 individually drives motors 31l , 31r , 32l and 32r which operate respectively spools of a left clutch valve 29l , a right clutch valve 29r , a left brake valve 30l and a right brake valve 30r in accordance with the instruction of the cpu 22 , whereby the circuit 28 can change the clutch engagement state with respect to the right and left crawler . further , the positions of the valves 29l , 29r , 30l and 30r are respectively detected by sensors 34l , 34r , 35l and 35r such as potentiometers or the like , and the output of these sensors can be fed back to the cpu 22 through an a / d converter 33 . the operations of the driving control device of this embodiment will now be described with reference to the flow chart in fig6 showing a processing sequence of the cpu 22 . firstly , the cpu 22 loads a value 1 showing lst - speed stage into a memory 23f and a memory 23r ( step 100 ). then , it judges whether any of switches 12 - 1 , 12 - 2 and 12 - 3 and 12 - 1 &# 39 ;, 12 - 2 &# 39 ; and 12 - 3 &# 39 ; of the speed - stage command device 12 is pushed ( step 101 ). when the judgment of step 101 is &# 34 ; yes &# 34 ;, the cpu 22 , after sounding a buzzer ( not shown ) for a short time ( step 103 ), judges a combination of logic levels of signals sa , sb and sc , which have been outputed from the signal generation circuit 20 in accordance with the relation as shown in the above table ( steps 104 through 109 ). when the signals sa , sb and sc have the logic levels of 1 , 1 and 1 ; 1 , 0 and 1 ; and 1 , 1 and 0 , values 1 , 2 and 3 showing the forward speed stages are loaded respectively into the memories 23f . when the signals sa , sb and sc have the logic levels of 0 , 1 and 1 ; 0 , 0 and 1 ; and 0 , 1 and 0 , values 1 , 2 and 3 showing the reverse ( backward ) speed stages are loaded respectively into the memories 23r ( step 110 through 115 ). for example , when the operator pushes the pushbutton switch 12 - 1 of the above speed - stage command device 12 to command the forward lst speed stage so that signals sa , sb and sc of 1 , 1 and 1 from the signal generation circuit 20 are outputted , the value 1 is loaded into the memory 23f . after displaying the memory contents of the memories 23f and 23r on the display 24 ( step 116 ), the cpu 22 then judges whether or not the lock signal s4 is being produced as output from the neutral lock switch 21 ( step 117 ). when the signal s4 is not being outputted , the cpu 22 judges as to which of the forward position , neutral position , and reverse position of pattern shown in fig4 the lever 10 has been operated to in accordance with the output signal s2 of the potentiometer 14 ( step 118 ). more specifically , when the lever 10 is operated to the &# 34 ; forward &# 34 ; position , to the &# 34 ; neutral &# 34 ; position , or to the &# 34 ; reverse &# 34 ; position , the values of the output signal s2 of the potentiometer 14 become v f , v n and v r , so that the cpu 22 judges the forward or reverse position of the lever 10 on the basis of these signal values . now , if the value of the signal s2 indicates v f , i . e ., if the lever 10 has been operated to the &# 34 ; forward &# 34 ; position , the cpu 22 judges whether or not the knob switch 11 mounted on the lever 10 has been operated , i . e ., whether or not the same switch 11 has been closed ( step 119 ). when this judgement is &# 34 ; no &# 34 ;, the content of the memory 23f is checked ( step 120 ). the speed - stage setting signal actuating the valves 27f and 27 - 3 , if the content of the memory 23f is 3 ; the speed - stage setting signal actuating the valves 27f and 27 - 3 , if the content of the memory 23f is 2 ; and the speed - stage setting signal actuating the valves 27f and 27 - 1 , if the content of the memory is 1 are respectively introduced as input into the driving circuit 25 to selectively actuate these valves . thus , speed stages corresponding to the contents commanded by the above speed - stage command device 12 can be selected . when the speed stage is selected as described above , and the knob switch 11 of the lever 10 is closed while the tractor is being driven at a specific speed , the judgement result of the above step 119 becomes &# 34 ; yes &# 34 ;. in such a case , the cpu 22 sounds the above mentioned buzzer for a short time ( step 122 ). then , when the content of the memory 23f is 3 , the cpu 22 changes this to 2 ; when it is 2 , the cpu 22 changes this to 1 ; and when it is 1 , the cpu 22 changes this to 2 ( step 123 ). thus , the contents changed of the memory 23f are indicated on the display 24 ( step 124 ), and the processes shown in steps 120 and 121 can be executed . in the driving of the tractor , switching operation of the knob switch 11 is performed in the following cases . when the tractor is being driven at a high speed of the 3rd or 2nd speed stage , the operator will often desire to make a gear change to a lower speed to avoid danger , in which case it may be troublesome for the operator to operate the switch of the speed - stage command device 12 . also , when the tractor is being driven at the 1st speed , the operator will sometimes desire to drive at the 2nd speed to raise the work efficiency , in which case it is desirable to change speed stages without operating the switch of the speed - stage command device 12 . the knob switch 11 can be used in such cases so that changes of speed stage can be easily performed . furthermore , when the above step 101 judgement is &# 34 ; no &# 34 ;, the cpu 22 immediately executes the content of step 116 . also , when the step 109 judgement is &# 34 ; no &# 34 ;, the cpu 22 checks whether or not any of the above valves 27 - f , 27 - r , 27 - 1 , 27 - 2 and 27 - 3 is being driven ( step 125 ), and if any valve is being driven , the cpu 22 stops the driving of that valve ( step 126 ) and then rings continuously the buzzer ( step 127 ). also , if no valve is being driven , the cpu 22 immediately executes the content of step 127 . further , when the step 117 judgement result is &# 34 ; yes &# 34 ;, i . e ., when the above neutral lock switch is closed , and when the output voltage of the potentiometer 14 is judged to be v n in step 118 , i . e ., when the lever is positioned at the neutral position as shown in fig4 the cpu 22 judges whether or not any of the valves 27 - f , 27 - r , 27 - 1 , 27 - 2 and 27 - 3 is being driven ( step 128 ). when the result of this judgement is &# 34 ; yes &# 34 ;, the cpu 22 stops the driving of the valve ( step 129 ) and places the transmission in the neutral state . in the case where the tractor , for example , is to be parked , the switch 21 is closed , whereby even when the lever 10 is positioned at any other position than the neutral position as shown in fig4 the transmission is forced to be locked in the neutral state . furthermore , when the step 120 judgement result is &# 34 ; no &# 34 ;, i . e ., when the content of the memory 23 is a value other than 1 , 2 , or 3 for some reason , the cpu 22 stops valves in driven state among the valves 27 - f , 27 - r , 27 - 1 , 27 - 2 and 27 - 3 ( step 130 ) and continuously rings the buzzer thereafter ( step 131 ). also , when the value of the output signal s2 of the potentiometer is judged to be 0 in step 119 , i . e ., when the lever 10 is judged to have been operated to the &# 34 ; reverse &# 34 ; position as shown in fig4 the cpu 22 executes a judgement process having the same content as that of step 119 ( step 132 ). when the judgement result of step 132 is &# 34 ; no &# 34 ;, the cpu 22 checks the content of the memory 23r ( step 133 ). when its content is 3 , 2 and 1 , the cpu 22 actuates respectively the valves 27 - r and 27 - 3 , the valves 27 - r and and 27 - 2 , and the valves 27 - r and 27 - 1 ( step 134 ). thus , when the lever 10 has been operated to the &# 34 ; reverse &# 34 ; position , the speed stage corresponding to the reverse speed stage stored in the memory 23r is selected in the transmission . further , when the step 132 judgement result is &# 34 ; yes &# 34 ;, the cpu 22 executes a process conforming to the process content of step 123 with respect to the memory 23r ( step 136 ) after it has executed the same process as that shown in the above step 122 ( step 135 ) and further executes the same process as step 124 ( step 137 ). also , when the contents of the memory 23r are judged to be other than 3 , 2 and 1 in step 133 , the cpu 22 executes successively the same process as that indicated in step 130 and step 131 ( step 138 , 139 ). next , the steering operation will be described . the cpu 22 processing contents for the steering operation are not shown in fig6 . when the lever 10 is operated by the operator to the &# 34 ; right turn &# 34 ; or &# 34 ; left turn &# 34 ; direction of the pattern shown in fig4 a turning instruction signal s1 of values corresponding to the operating direction and the operating amount is produced as output from the potentiometer 13 . the cpu 22 thereby judges , on the basis of the signal s1 , the direction , right or left , in which the lever 10 has been actuated and at the same time detects the operation amount of the lever . the cpu 22 further introduces as input driving signals corresponding to the operation amount for the respective direction through the driving circuit 28 to the motor 31l and motor 31r . the right and left clutches in the crawler - type tractor are normally in a connected state . when the spool of either of the aforedescribed valves 29l and 29r is actuated beyond a prescribed amount , the clutch connected to the valve actuated by this spool is placed in a disconnected state through a hydraulic cylinder ( not shown ). accordingly , when the lever 10 is operated , for example , to the right turning direction beyond a prescribed amount , the right clutch assumes a disconnecting state to disconnect the power transmission to the right crawler , whereupon the tractor turns to the right . when the right clutch assumes the disconnected state , the right crawler undergoes free rotation , which obstructs quick turning operation . accordingly , in this example , the clutches and the brakes are intercoupled to operate interrelatedly . that is , when the right clutch , for example , moves to the disconnected state , the motor 32r drives in response to the output signal s1 of the potentiometer 13 , whereby the right brake is applied to prevent free rotation of the right crawler . furthermore , when the brake pedal 18 shown in fig3 is depressed , the cpu 22 actuates simultaneously the right and left brake valves 30r and 30l in response to the output signal s3 of the potentiometer 17 , so that the tractor slows down or stops . in the example of practice described above , the position of the lever 10 in the forward or reverse ( backward ) direction is determined by one of the high ( v f ), medium ( v n , and low ( v r ) values of the output signal s2 of the potentiometer 14 , but limit switches which are closed or opened when the lever 10 is actuated to a respective position for forward , neutral or reverse can be , substituted , of course , for the above potentiometer . also , the example of practice carries out programming control by using a cpu , but the operation described above can also be obtained by a combination of logic circuits . further , in the embodiment described above , the speed - stage command device 12 is provided with the push button switches 12 - 1 , 12 - 2 and 12 - 3 for forward and the push button switches 12 - 1 &# 39 ;, 12 - 2 &# 39 ; and 12 - 3 &# 39 ; for reverse , but since forward or reverse is selected by the actuation position of the lever 10 , as indicated by the step 118 of fig6 the speed - stage command device can be constituted by three push button switches by changing a part of the process flow of the same figure . however , when the numbers of speed stages respectively for forward and reverse are different , it is necessary to provide exclusive push switches for each of forward and reverse as in the above described embodiment .