Patent Publication Number: US-11396295-B2

Title: Work vehicle

Description:
CROSS-REFERENCE 
     This application is a US National Stage Application under 35 U.S.C. § 371 of International Application No. PCT/JP2019/015798 filed Apr. 11, 2019, which claims foreign priority of JP2018-098271 filed May 22, 2018, the disclosures of which are hereby incorporated by reference in their entirety. 
     TECHNICAL FIELD 
     The present invention relates to a work vehicle including a boarding-type driver unit and a foot brake for braking a driving device. 
     BACKGROUND ART 
     Some passenger cars use a driving support device and a driving support method in which emergency brake control is performed when there is a risk of collision with a preceding vehicle (for example, refer to PTL 1). 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Unexamined Patent Publication No. 2017-43193 
     DISCLOSURE OF INVENTION 
     Problems to be Solved by the Invention 
     In recent years, work vehicles, such as tractors, have been automated to enable autonomous driving of work vehicles through the use of satellite positioning systems (navigation satellite system (NSS)), such as a global positioning system (GPS). Advancement in such automation of work vehicles leads to the realization of unmanned work vehicles that are capable of autonomous driving of the unmanned work vehicles. In order to realize such an unmanned work vehicle that is capable of autonomous driving, the work vehicle needs to have an emergency brake function for quickly stopping the work vehicle when an error occurs in the control system related to autonomous driving, such as a speed change control system and a steering control system. 
     Therefore, it is conceivable that the emergency brake function adopted in passenger cars be applied to the work vehicles. However, the emergency brake function adopted in passenger cars is a driving support device or a driving support method such as those described in PTL 1, which are intended for the prevention of collision with a preceding vehicle and the reduction of collision damage. For this reason, it is necessary to develop a unique emergency brake function in order to solve the above problems. 
     Since a work vehicle capable of unmanned autonomous driving is originally configured to be driven by a passenger, the emergency brake function must not hinder the manual driving by the passenger. 
     In view of such a situation, a main object of the present invention is to provide an emergency brake function capable of quickly stopping a work vehicle when an error occurs inside the vehicle. 
     Means for Solving the Problems 
     A first characteristic configuration of the present invention is in a work vehicle including: 
     a boarding-type driver unit; 
     a foot brake that brakes a driving device; 
     an autonomous drive unit that enables autonomous drive of a vehicle; and 
     an electric actuator that switches the foot brake between a braking state for braking the driving device and a released state for releasing the braking, wherein, 
     the driver unit includes a mode selector that enables selection of an autonomous drive mode in which the autonomous drive unit autonomously drives the vehicle, 
     the autonomous drive unit includes a control unit that controls an operation of the electric actuator, and 
     in the autonomous drive mode, if an error is detected inside the vehicle based on detection information from a vehicle state detection device that detects a state of each component in the vehicle, or if an emergency stop command is obtained from a wireless communication device that is capable of wireless communication with the autonomous drive unit, the control unit controls the operation of the electric actuator and switches the foot brake from the released state to the braking state. 
     According to this configuration, if an error occurs inside the vehicle, the electric actuator switches the foot brake to the braking state under the control of the control unit when the control unit detects an error inside the vehicle on the basis of detection information from the vehicle state detection device or when the control unit obtains an emergency stop command sent by an outside administrator through the wireless communication device in response to noticing an error. As a result, the work vehicle can be quickly braked and stopped. 
     In this way, even if an error occurs inside the vehicle in an unmanned drive state in which the tractor autonomously drives without a passenger, the work vehicle can be quickly braked and stopped. 
     Since the foot brake is highly reliable and does not cause problems even when it is frequently used during manual driving by the passenger, it is possible to certainly stop brake and stop the work vehicle in an unmanned state. 
     By using the foot brake for an emergency stop, it is possible to avoid the complication of the structure due to introduction of a new brake dedicated to the emergency stop. 
     In a second characteristic configuration, 
     the driver unit includes an operating tool for brake release, and 
     if the operation tool is manually operated in the braking state of the foot brake by the operation of the electric actuator, the control unit controls the operation of the electric actuator and switches the foot brake from the braking state to the released state. 
     According to this configuration, in an emergency stop state in which the electric actuator switches the foot brake to the braking state under the control of the control unit, the administrator or the like can get into the driver unit and operate the operation tool to release the emergency stop state of the work vehicle at any time. 
     This enables the administrator who has gotten into the driver unit to manually drive the work vehicle, and the work vehicle can be moved to a safe place or a repair shop by manual driving. 
     In a third characteristic configuration, 
     if the autonomous drive mode is deselected when the foot brake is to be switched to the braking state and if the autonomous drive mode is selected by the mode selector after the deselection, the control unit determines whether or not an error has occurred inside the vehicle on the basis of detection information from the vehicle state detection device and permits a transition to the autonomous drive mode when it is determined that no errors have occurred inside the vehicle. 
     According to this configuration, in an emergency stop state of the work vehicle under the control of the control unit, even when the emergency stop under the control of the control unit is released, the work vehicle will not be able to autonomously drive until the control unit determines that there is no error inside the vehicle. 
     This makes it possible to prevent the automatic driving of the work vehicle while an error continues inside the vehicle. 
     A fourth characteristic configuration includes: 
     an electronically controlled engine, 
     wherein, if the control unit controls the operation of the electric actuator and the foot brake is switched from the released state to the braking state, the control unit automatically stops the engine. 
     According to this configuration, in the case where the work vehicle includes a work device driven by power from an engine, the work vehicle can be urgently stopped and the work device can be stopped under the control of the control unit. 
     In this way, the emergency stop of the work vehicle can be more preferably performed along with the stop of the work device. 
     In a fifth characteristic configuration, 
     if the operation of the electric actuator switches the foot brake to the braking state, and if power is turned on again by turning on a key switch provided in the driver unit after the power has been turned off by turning off the key switch in a state in which the engine is automatically stopped, the control unit permits activation of the engine and switches the foot brake form the braking state to the released state by controlling the operation of the actuator. 
     According to this configuration, in an emergency stop state of the work vehicle under the control of the control unit, the emergency stop state of the work vehicle can be released and the engine can be activated simply by the administrator or the like getting into the driver unit and operating the key switch. 
     As a result, the work vehicle can be more readily switched to a state in which it can be manually driven, and the work vehicle can be moved to a safe place or a repair shop more quickly. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating the schematic configuration of an autonomous drive system. 
         FIG. 2  is a block diagram illustrating the schematic configuration of an autonomous drive system. 
         FIG. 3  is a block diagram illustrating the schematic configuration related to a safety brake function. 
         FIG. 4  is a perspective view from the upper left rear side of a main portion illustrating the configuration of a brake system. 
         FIG. 5  is a perspective view from the upper right rear side of a main portion illustrating the configuration of a brake system. 
         FIG. 6  is a perspective view of a main portion of a brake system illustrating the configuration of brake pedals and the vicinity. 
         FIG. 7  is a rear view of a main portion of a brake system illustrating the configuration of brake pedals and the vicinity. 
         FIG. 8  is a perspective view from the upper right front side of a main portion of a brake system illustrating the brake operation structure by an electric motor. 
         FIG. 9  is a perspective view from the upper right rear side of a main portion of a brake system illustrating the brake operation structure by an electric motor. 
         FIG. 10  is a right side view of a main portion of a brake system illustrating the brake operation structure by an electric motor. 
         FIG. 11  is a longitudinal left side cross-sectional view of a main portion of a brake system illustrating foot brakes in a state not operated by an electric motor. 
         FIG. 12  is a longitudinal left side cross-sectional view of a main portion of a brake system illustrating foot brakes in a state operated by an electric motor. 
         FIG. 13  is a graph illustrating the relation between the operation amount by the operation of an electric motor, a braking force, and an operating load. 
         FIG. 14  is a flowchart of emergency stop control. 
         FIG. 15  is a diagram illustrating a home screen on a liquid crystal monitor. 
         FIG. 16  is a diagram illustrating a safety-brake-check selection screen on a liquid crystal monitor. 
         FIG. 17  is a diagram illustrating a safety brake check screen on a liquid crystal monitor in which an initial check button is displayed in an inoperable state. 
         FIG. 18  is a diagram illustrating a safety brake check screen on a liquid crystal monitor in which an initial check button is displayed in an operable state. 
         FIG. 19  is a diagram illustrating a safety brake check screen on a liquid crystal monitor indicating that a check is being performed. 
         FIG. 20  is a diagram illustrating a safety brake check screen on a liquid crystal monitor indicating that a check has failed. 
         FIG. 21  is a diagram illustrating a safety brake check screen on a liquid crystal monitor indicating that a check has been completed. 
         FIG. 22  is a diagram illustrating an autonomous drive start screen on a liquid crystal monitor. 
         FIG. 23  is a diagram illustrating the sequence of an initial check (operation confirmation process). 
         FIG. 24  is a state transition diagram illustrating state transition in an initial check of a safety brake function unit. 
         FIG. 25  is a state transition diagram illustrating state transition in an error detection state of a safety brake function unit. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     An embodiment in which a work vehicle according to the present invention is applied to a tractor will now be described as an example of an embodiment of the present invention, with reference to the drawings. 
     Note that, besides a tractor, the work vehicle according to the present invention may be applied to a passenger work vehicle, such as a riding mower, a riding rice transplanter, a combine, a carrier, a wheel loader, or a snowplow. 
     As illustrated in  FIGS. 1 and 2 , a tractor  1  exemplified by the present embodiment is capable of autonomous driving by an autonomous drive system for work vehicles in a field or the like, which is an example of a work area. The autonomous drive system includes an autonomous drive unit  2  and a mobile communication terminal  3 . The autonomous drive unit  2  is installed in the tractor  1 . The mobile communication terminal  3  is an example of a wireless communication device set up to wirelessly communicate with the autonomous drive unit  2 . The mobile communication terminal  3  may be a tablet-type personal computer or a smart phone including a multi-touch type display unit (for example, a liquid crystal panel)  4  for displaying information regarding autonomous drive. 
     As illustrated in  FIG. 1 , a rear portion of the tractor  1  is coupled to a rotary tiller  6  via a three-point link mechanism  5  such that the rotary tiller  6  can be raised, lowered, and rolled. The rotary tiller  6  is an example of a work device. In this way, the tractor  1  is configured for rotary tilling specifications. 
     Note that, in place of the rotary tiller  6 , the rear portion of the tractor  1  may be coupled to various work devices, such as a plow, a disc harrow, a cultivator, a subsoiler, a seed planter, a spraying device, and a grass cutter. 
     As illustrated in  FIGS. 1 to 3 , the tractor  1  includes drivable and steerable left and right front wheels  10  and drivable left and right rear wheels  11 , which function as a wheel-type driving device; a cabin  13  that defines a boarding-type driver unit  12 ; an electronically controlled diesel engine (hereinafter referred to as “engine”)  14  including a common rail system; a speed change unit  15  that varies the power from the engine  14 ; a fully hydraulic power steering mechanism  16  that steers the left and right front wheels  10 ; a brake system  17  that brakes the left and right rear wheels  11 ; an electronically controlled clutch operation mechanism  19  that enables hydraulic operation of a work clutch that engages/disengages the power transmitted to the rotary tiller  6 ; an electrohydraulically controlled elevating drive mechanism  20  that drives the rotary tiller  6  to raise/lower the rotary tiller  6 ; an electrohydraulically controlled roll-direction drive mechanism  21  that drives the rotary tiller  6  in the roll direction; a vehicle-mounted control system  22  including various control units; a vehicle state detection device  23  that includes various sensors and switches for detecting the various setting statuses and operation states of various parts of the tractor  1 ; and a positioning unit  24  that measures the current position, the current orientation, etc., of the tractor  1 . 
     Note that the engine  14  may be an electronically controlled gasoline engine including an electronic governor. The power steering mechanism  16  may be an electric power steering mechanism including an electric motor. 
     As illustrated in  FIGS. 1 and 3 to 5 , the driver unit  12  includes various operation levers, such as an accelerator lever and a speed change lever; various operation pedals, such as an accelerator pedal  28  and a clutch pedal  29 ; a steering wheel  30  that enables manual steering of the left and right front wheels  10  via the power steering mechanism  16 ; a seat  31  for a passenger; and a multi-touch type liquid crystal monitor  32  serving as a display unit that displays various types of information including information related to autonomous drive and enables an input operation. 
     As illustrated in  FIG. 1 , the cabin  13  is supported in a vibration-proof manner by a front frame  34  disposed on the front side of the tractor  1  and the speed change unit  15  also serving as a rear frame, via anti-vibration rubber or the like. The engine  14  is supported in a vibration-proof manner by the front frame  34  via vibration-proof rubber or the like. The engine  14  is covered by a bonnet  35  provided on the front side of the tractor  1 . 
     As illustrated in  FIG. 2 , the speed change unit  15  includes an electronically controlled continuously variable transmission  36  that varies the power from the engine  14 ; an electrohydraulically controlled forward-reverse switching device  37  that switches the power that has been varied by the continuously variable transmission  36  between forward travel and reverse travel; and a rear wheel differential that allows difference between the left and right rear wheels  11 . The continuously variable transmission  36  is implemented by an integrated hydrostatic mechanical transmission (I-HMT), which is an example of a hydromechanical continuously variable transmission having transmission efficiency higher than that of a hydrostatic transmission (HST). The forward-reverse switching device  37  includes a hydraulic clutch for interrupting forward power, a hydraulic clutch for interrupting reverse power, and a solenoid valve for controlling the flow of oil to the clutches. 
     Note that, in place of the I-HMT, the continuously variable transmission  36  may be implemented by a hydraulic mechanical transmission (HMT), which is an example of a hydromechanical continuously variable transmission, a hydrostatic transmission (HST), a belt-type continuously variable transmission, or the like. In place of the continuously variable transmission  36 , the speed change unit  15  may include an electrohydraulically controlled stepped transmission including multiple hydraulic clutches for speed change and multiple solenoid valves for controlling the flow of oil to the clutches. 
     As illustrated in  FIGS. 4 to 7 , the brake system  17  includes left and right brake pedals  40  and a parking lever  41  provided in the driver unit  12 ; left and right brakes  42  for respectively braking the left and right rear wheels  11 ; left and right first linkage mechanisms  43  that link the left and right brake pedals  40  and the left and right brakes  42 , respectively, in conjunction with each other; a second linkage mechanism  44  for parking that couples the parking lever  41  and the left and right brakes  42  in conjunction with each other; and an electrohydraulically controlled first brake operating device  45  that operates the brake  42  on the inner side of the turn in conjunction with steering of the left and right front wheels  10  by a set angle or more. When a passenger depresses at least one of the left and right the brake pedals  40 , pulls up the parking lever  41  to a braking position, or steers the left and right front wheels  10  with the steering wheel  30  by a set angle or more, the brake system  17  operates the corresponding brake(s)  42  to brake the corresponding rear wheel(s)  11 . Thus, when the left and right brake pedals  40  are depressed at the same time, the left and right brakes  42  function as foot brakes that simultaneously brake the left and right rear wheels  11 . When the parking lever  41  is pulled up, the left and right brakes  42  function as parking brakes that simultaneously brake the left and right rear wheels  11 . When at least one of the brake pedals  40  are depressed or when the left and right front wheels  10  are steered by a set angle or more, the left and right brakes  42  function as side brakes that brake the at least one of the rear wheels  11 . An urging member, such as a compression spring, is disposed inside each of the left and right brakes  42 . The urging member urges each of the brakes  42  to return the rear wheel  11  in a braking state in which the rear wheels  11  is being braked to a released state in which the braking of the rear wheel  11  is released. 
     As illustrated in  FIGS. 2 and 3 , the vehicle-mounted control system  22  includes an engine control unit  22 A that performs control related to the engine  14 ; a speed change control unit  22 B that performs control related to the continuously variable transmission  36 , the forward-reverse switching device  37 , etc.; a steering control unit  22 C that performs control related to the power steering mechanism  16 , the first brake operating device  45 , etc.; a work device control unit  22 D that performs control related to work devices, such as the rotary tiller  6 ; a display control unit  22 E that controls the display operation of the liquid crystal monitor  32 ; an autonomous drive control unit  22 F that performs control related to autonomous drive; and a non-volatile vehicle-mounted storage unit  22 G that sores preliminarily generated target travel routes for autonomous drive. Each of the control units  22 A to  22 F is constructed by an electronic control unit in which a microcontroller controller, etc., are integrated and various control programs. As illustrated in  FIG. 3 , the autonomous drive control unit  22 F is included in the liquid crystal monitor  32  together with the display control unit  22 E. The control units  22 A to  22 F are connected such that they can communicate with each other via a controller area network (CAN). 
     As illustrated in  FIG. 3 , “vehicle state detection device  23 ” is a term collectively referring to various sensors and switches provided in the respective parts of the tractor  1 . The vehicle state detection device  23  includes an accelerator sensor that detects the operation amount of the accelerator lever and the accelerator pedal  28  from idling positions; a speed change sensor that detects the operation amount of the speed change lever from a zero speed position; a reverser sensor that detects the operation position of the reverser lever for switching between forward travel and reverse travel; left and right brake switches  25  (see  FIG. 3 ) that detect whether or not the left and right brake pedals  40  are at depression release positions; left and right brake sensors  26  (see  FIG. 3 ) that detect the operation amount of the left and brake pedals  40  from depression release positions; a rotation sensor that detects the output rotation speed of the engine  14 ; a vehicle speed sensor that detects the vehicle speed of the trucker  1 ; and a steering angle sensor that detects the steering angle of the front wheels  10 . 
     The engine control unit  22 A performs engine speed change control on the basis of detection information from the accelerator sensor and detection information from the rotation sensor. The engine speed change control changes the engine speed from an idling speed to a speed corresponding to the operation amount of the accelerator lever or the accelerator pedal  28 . 
     The speed change control unit  22 B performs speed change control, forward-reverse switching control, brake speed change control, and other control. The speed change control controls the operation of the continuously variable transmission  36  so that the vehicle speed of the tractor  1  varies to a speed corresponding to the operation amount of the speed change lever on the basis of the detection information from the speed change sensor, the detection information from the vehicle speed sensor, etc. The forward-reverse switching control switches the power transmission state of the forward-reverse switching device  37  on the basis of the detection information from the reverser sensor. The brake speed change control controls the operation of the continuously variable transmission  36  so that the vehicle speed of the tractor  1  decreases from a speed corresponding to the operation amount of the speed change lever to a speed corresponding to the depression operation amount of the left and right brake pedals  40  when the left and right brake pedals  40  are simultaneously operated, on the basis of the detection information from each of the brake sensors  26  and the detection information from the vehicle speed sensor. The speed change control includes a deceleration stop process of decelerating the continuously variable transmission  36  to a zero speed state and stopping the driving of the tractor  1  when the speed change lever is operated to a zero speed position. The braking speed change control includes a braking deceleration stop process of decelerating the continuously variable transmission  36  to a zero speed state and thereby stopping the driving of the tractor  1  when the left and right brake pedals  40  are depressed to a depression limit position. 
     As illustrated in  FIGS. 4 to 7 , the left and right brake pedals  40  of the brake system  17  are disposed next to each other in the right front lower portion of the driver unit  12 . The left and right brake pedals  40  are urged by left and right extension springs  46  to return to depression release positions. The left and right brake pedals  40  include boss portions  40 A, pedal arm portions  40 B, and pedal portions  40 C. The boss portions  40 A are supported by a rotary shaft  47  for pedal support that extends in the left-right direction below and forward of the steering wheel  30 . The pedal arm portions  40 B extend rearward and downward from the boss portions  40 A. The pedal portions  40 C are attached to the free ends of the pedal arm portions  40 B. The boss portion  40 A of the right brake pedal  40  rotates relative to the rotary shaft  47 . The right brake pedal  40  includes a linkage arm portion  40 D that extends forward and downward from the boss portion  40 A. The boss portion  40 A of the left brake pedal  40  rotates integrally with a linkage arm  48  via the rotary shaft  47 . The linkage arm  48  is fixed to the left end of the rotary shaft  47 . 
     As illustrated in  FIGS. 4 and 5 , the left and right brakes  42  are included in the speed change unit  15 . Each of the left and right brakes  42  includes an operation shaft  49  and an operation arm  50 . The operation shaft  49  protrudes from the front end of the corresponding brake  42  in a transversely outward direction from the vehicle. The operation arm  50  is fixed to the protruding end of the operation shaft  49 . 
     As illustrated in  FIGS. 4 to 7 , the left and right first linkage mechanisms  43  includes boss members  52 , first linking rods  53 , and second linking rods  54 . The boss members  52  are rotatably supported by left and right fixed shafts  51  that extend in the left-right direction below the rotary shaft  47 . Each of the first linking rods  53  is vertically long and extends between a first arm portion  52 A of the corresponding boss members  52  and the linkage arm portion  40 D of the right brake pedal  40  (or the linkage arm  48 ). The second linking rods  54  are long in the front-back direction and extend between second arm portions  52 B of the boss members  52  and the operation arms  50  of the brakes  42 . The left first linkage mechanism  43  includes the rotary shaft  47  and the linkage arm  48  described above. That is, the left and right first linkage mechanisms  43  are of a rod linkage type in which the left and right brake pedals  40  are respectively linked to the left and right brakes  42  via the first linking rods  53 , the second linking rods  54 , etc. 
     In the brake system  17  having the configuration described above, when only the right brake pedal  40  is depressed, the resulting operating force is transmitted to the operation arm  50  of the right brake  42  via the right first linkage mechanism  43 . As a result, the brake system  17  switches to a right braking state in which the right rear wheel  11  is braked by the right brake  42 . When the depression operation of the right brake pedal  40  is then released, the right braking state switches to a released state. 
     When only the left brake pedal  40  is depressed, the resulting operating force is transmitted to the operation arm  50  of the left brake  42  via the left first linkage mechanism  43 . As a result, the brake system  17  switches to a left braking state in which the left rear wheel  11  is braked by the left brake  42 . When the depression operation of the left brake pedal  40  is then released, the left braking state switches to a released state. 
     When the left and right brake pedals  40  are both depressed, the resulting operating force is transmitted to the operation arms  50  of the left and right brakes  42  via the left and right first linkage mechanisms  43 . As a result, the brake system  17  switches to a braking state in which the left and right rear wheels  11  are respectively braked by the left and right brakes  42 . When the depression operation of the left and right brake pedals  40  is then released, the braking state switches to a released state. 
     In this way, when the passenger manually drives the tractor  1 , the passenger can perform a brake-turn of the tractor  1  to reduce the turning radius of the tractor  1  by depressing the brake pedal  40  on the inner side of the turn while operating the steering wheel  30  in the turning direction. The passenger can depress both the left and right brake pedals  40  to brake and decelerate or brake and stop the tractor  1  while the tractor  1  is kept in a straight-ahead orientation as a result of the braking action of the left and right brakes  42  and the braking speed change control of the speed change control unit  22 B described above. 
     As illustrated in  FIGS. 6 to 12 , the brake system  17  includes a coupling mechanism  55  that switches the left and right brake pedals  40  between a coupled state and a released state. In the coupled state, the left and right brake pedals  40  are coupled. In the released state, the coupled state of the left and right brake pedals  40  is released. The coupling mechanism  55  includes an operation rod  56 , a compression spring  57 , a guide plate  58 . The operation rod  56  is supported by the right brake pedal  40  so as to be movable in the left-right direction. The compression spring  57  urges the left end of the operation rod  56  to cause the operation rod  56  to protrude toward the left brake pedal  40 . The guide plate  58  guides a guided portion  56 A of the operation rod  56 . The guide plate  58  has a J-shaped guide hole  58   a  that guides the guided portion  56 A of the operation rod  56  between a couple position and a release position. The left brake pedal  40  has a through-hole  40 E (see  FIGS. 8, 11, and 12 ) into which the left end of the operation rod  56  is passed when the guided portion  56 A of the operation rod  56  is at the couple position. 
     In the coupling mechanism  55  having the above-described configuration, when the operation rod  56  is operated so that the guided portion  56 A of the operation rod  56  is positioned at the couple position, the left end of the operation rod  56  passes through the through-hole  40 E of the left brake pedal  40 . As a result, the left and right brake pedals  40  switches to a coupled state, and the coupled state is held by the compression spring  57 . In the coupling mechanism  55 , when the operation rod  56  is operated so that the guided portion  56 A of the operation rod  56  is positioned at the release position, the left end of the operation rod  56  is removed from the through-hole  40 E of the left brake pedal  40 . As a result, the left and right brake pedals  40  switches to a released state, and the released state is held by the compression spring  57 . 
     In this way, when the passenger manually drives the tractor  1  in a field, the passenger can operate the operation rod  56  and switch the coupling mechanism  55  to a released state, to perform any necessary brake-turns while driving in the field. When the passenger manually drives the tractor  1  outside the field, the passenger operates the operation rod  56  and switches the coupling mechanism  55  to a coupled state, to prevent the risk of performing unnecessary brake-turns while driving outside the field. 
     In the brake system  17 , the parking lever  41  is disposed to the left of the seat  31  in the driver unit  12 . The parking lever  41  is of a two-position switching type that switches and holds two positions: an upper braking position and a lower release position. At the upper braking position, the left and right brakes  42  are switched to a braking state. At the lower release position, the left and right brakes  42  are switched to a released state. The operation of the parking lever  41  to the braking position is detected by a parking switch included in the vehicle state detection device  23 . 
     As illustrated in  FIGS. 4 to 5 , the second linkage mechanism  44  for parking includes left and right control cables  59 , an equalizer unit  60 , left and right link plates  61 , and left and right linking pins  62 . The equalizer unit  60  couples one of the ends of inner cables of the left and right control cables  59  to the parking lever  41 . The left and right link plates  61  and the left and right linking pins  62 , couple other ends of the left and right inner cables to the operation arms  50  of the left and right brakes  42 . That is, the second linkage mechanism  44  is of a cable linkage type in which the parking lever  41  is linked to the left and right brakes  42  via the left and right control cables  59 , etc. The left and right link plates  61  have long holes through which the linking pins  62  fixed to the left and right operation arms  50  are passed. The long holes function as allowance portions that allow displacement of the left and right operation arms  50  relative to the left and right link plates  61  in conjunction with the depressing operation of the left and right brake pedals  40 . 
     In the brake system  17  having the above-described configuration, when the parking lever  41  is pulled up from the lower release position to the upper braking position and held in the braking position, the resulting operating force is transmitted to the operation arms  50  of the left and right brakes  42  via the second linkage mechanism  44 . As a result, the left and right rear wheels  11  are respectively braked by the left and right brakes  42 , and the braked state switches to a parking brake state in which the braking state is kept. When the parking lever  41  is then pushed down from the upper braking position to the lower release position and held in the release position, the parking brake state is switched to a released state. 
     As illustrated in  FIG. 6 , vertically long holes  53   a  are formed in the lower end portions of the first linking rods  53  in the left and right first linkage mechanisms  43 . Linking pins  63  fixed to the first arm portions  52 A of the boss members  52  are passed through the long holes  53   a . As illustrated in  FIGS. 3 to 5 , in the left and right first linkage mechanisms  43 , when the left and right brakes  42  are switched from the released state to the parking brake state as a result of the parking lever  41  being pulled up, in conjunction with this switching, the left and right second linking rods  54  are pulled rearward, and the second arm portions  52 B of the left and right boss members  52  swing to be displaced rearward. In conjunction with the swing displacement, the first arm portions  52 A of the left and right boss members  52  swing to be displaced upward. At this time, the long holes  53   a  of the first linking rods  53  function as allowance portions that allow the upward swing displacement of the left and right first arm portions  52 A relative to the left and right first linking rods  53 . This avoids a reduction in operability due to the left and right brake pedals  40  interlocking and causing the operation to become heavy when the left and right brakes  42  switch to a parking brake state as a result of the parking lever  41  being pulled up. 
     As illustrated in  FIGS. 4 to 7 , the first brake operating device  45  in the brake system  17  includes a hydraulic unit  64  for automatic brake, a pair of push-pull links  65 , a pair of crank arms  66 , left and right control cables  67 , left and right link plates  68 , and left and right linking pins  69 . The hydraulic unit  64  includes two hydraulic cylinders and two solenoid valves corresponding to the left and right brakes  42 . The push-pull links  65  are pushed and pulled by the hydraulic unit  64 . The crank arms  66  swing around the longitudinal axis in conjunction with the push-pull links  65 . The left and right control cables  67  extend from the crank arms  66  toward the left and right boss members  52 . The left and right link plates  68  and the left and right linking pins  69  couple the front ends of inner cables  67 A of the left and right control cables  67  to third arm portions  52 C of the left and right boss members  52 . Oil from a hydraulic pump driven by power from the engine  14  is supplied to the hydraulic unit  64 . The left and right link plates  68  have long holes  68   a  through which linking pins  69  fixed to the left and right third arm portions  52 C are passed. The long holes  68   a  function as allowance portions that allow depression of the left and right brake pedals  40  or swing displacement of the left and right third arm portions  52 C relative to the left and right link plates  68  caused by a pulling operation of the parking lever  41 . 
     As illustrated in  FIG. 2 , the brake system  17  includes the steering control unit  22 C. When a selection switch for automatic brake included in the driver unit  12  is operated to select an automatic brake mode, the steering control unit  22 C controls the operation of the hydraulic unit  64  of the first brake operating device  45  on the basis of the detection by the steering angle sensor and executes automatic brake control for operating the left and right brakes  42 . Under the automatic brake control, the steering control unit  22 C keeps each of the solenoid valves of the hydraulic unit  64  in a discharge state in which oil is discharged from each hydraulic cylinder while the steering angle of the left and right front wheels  10  is less than a set angle. In this way, the steering control unit  22 C keeps the left and right hydraulic cylinders in a contracted state and the left and right brakes  42  in a released state. When the steering angle of the left and right front wheels  10  reaches the set angle or more, the solenoid valves corresponding to the rear wheel  11  on the inner side of the turn switch to a supply state in which oil is supplied to the hydraulic cylinders so as to extend the hydraulic cylinder corresponding to the rear wheel  11  on the inner side of the turn and switch the brake  42  on inner side of the turn to a braking state. 
     During manual driving in which the automatic brake mode is selected, the brake system  17  having the above-described configuration keeps the left and right brakes  42  in a released state by the first brake operating device  45  while the steering angle of the left and right front wheels  10  based on the turning operation of the steering wheel  30  is smaller than the set angle. As a result, the turning state of the tractor  1  is kept in a normal turning state in which the tractor  1  turns with a turning radius corresponding to the steering angle of the left and right front wheels  10 . When the steering angle of the left and right front wheels  10  based on the turning operation of the steering wheel  30  reaches the set angle or larger, the first brake operating device  45  switches the brake  42  on the inner side of the turn to a braking state. As a result, the turning state of the tractor  1  switches to a brake-turn state in which the tractor  1  turns with a turning radius smaller than the turning radius in the normal turning state. Subsequently, when the steering angle of the left and right front wheels  10  based on the turning operation of the steering wheel  30  falls below the set angle, the first brake operating device  45  switches the left and right brakes  42  to a released state. As a result, the turning state of the tractor  1  switches to the normal turning state described above. 
     That is, when the passenger selects the automatic brake mode during manual driving by the passenger, and during turning in which the left and right front wheels  10  are steered by the set angle or more, the brake system  17  automatically switches the turning state of the tractor  1  between the normal turning state and the brake-turn state on the basis of whether the angle of the left and right front wheels  10  being less than the set angle or larger than or equal to the set angle, without the passenger depressing the brake pedal  40  on the inner side of the turn. As a result, the passenger can readily turn the tractor  1  with a small radius by only turning the steering wheel  30 . 
     As illustrated in  FIG. 6 , the lower ends of the first linking rods  53  of the left and right first linkage mechanisms  43  have long holes  53   a  that function as allowance portions, as described above. As a result, even when the left and right brake pedals  40  are coupled by the coupling mechanism  55 , the first brake operating device  45  can operate the brake  42  on the inner side of the turn when the left and right front wheels  10  are steered by the set angle or more, and switch the turning state of the tractor  1  to a brake-turn state. 
     The positioning unit  24  includes a satellite navigation device and an inertial measurement unit (IMU). The satellite navigation device measures the current position and the current orientation of the tractor  1  by using a global positioning system (GPS), which is an example of a navigation satellite system (NSS). The IMU includes a three-axis gyroscope, a three-direction acceleration sensor, etc., and measures the attitude, the orientation, and the like, of the tractor  1 . Positioning methods using a GPS include a differential GPS (DGPS) (which is a relative positioning method) and a real time kinematic GPS (RTK-GPS) (which is an interference positioning method). In the present embodiment, an RTK-GPS suitable for measuring the position of a moving body is employed. Accordingly, a reference station  73  that enables positioning by the RTK-GPS is installed at a known location in the periphery of the field, as illustrated in  FIG. 1 . 
     As illustrated in  FIGS. 1 and 2 , the tractor  1  and the reference station  73  include GPS antennas  75  and  76 , respectively, and communication modules  77  and  78 , respectively. The GPS antennas  75  and  76  receive radio waves transmitted from GPS satellites  74  (see  FIG. 1 ). The communication modules  77  and  78  enable wireless communication of various data items, including positioning data, between the tractor  1  and the reference station  73 . In this way, the satellite navigation system of the positioning unit  24  can measure the current position and current orientation of the tractor  1  with high accuracy on the basis of the positioning data obtained by the GPS antenna  75  on the tractor side receiving radio waves from the GPS satellite  74  and the positioning data obtained by the GPS antenna  76  on the base station side receiving radio waves from the GPS satellite  74 . The positioning unit  24 , which is provided with a satellite navigation device and an inertial measurement unit, enables highly accurate measurements of the current position, the current orientation, and an attitude angle (yaw angle, roll angle, pitch angle) of the tractor  1 . 
     In the tractor  1 , the inertial measurement unit of the positioning unit  24 , the GPS antenna  75 , and the communication module  77  are included in an antenna unit  79  illustrated in  FIG. 1 . The antenna unit  79  is disposed at the center of the upper portion in the left and right direction on the front side of the cabin  13 . 
     As illustrated in  FIG. 2 , the mobile communication terminal  3  includes a terminal control unit  80  and a communication module  81 . The terminal control unit  80  includes an electronic control unit on which a microcontroller, etc., are integrated and various control programs. The communication module  81  enables wireless communication of various data items including the positioning data with the communication module  77  on the tractor side. The terminal control unit  80  includes a display control unit  80 A, a travel route generating unit  80 B, and a non-volatile terminal storage unit  80 C. The display control unit  80 A controls the operation of a display unit  4 . The travel route generating unit  80 B generates a target travel route for autonomous drive. The non-volatile terminal storage unit  80 C stores the target travel route generated by the travel route generating unit  80 B. 
     The target travel route includes various route section, such as multiple work route sections disposed in parallel at regular intervals corresponding to the work width of the tractor  1  and multiple non-work turning route sections connecting the end and start of adjacent work route sections in the order of travel. The target travel route also includes proper engine speed, proper vehicle speed, traveling direction of the tractor  1 , front wheel steering angle at the turning route section, stop position of the tractor  1 , etc., that are set in accordance with the drive mode of the tractor  1  in the various turning route sections. 
     As illustrated in  FIG. 3 , detection information from various sensors and switches included in the vehicle state detection device  23  is input to the autonomous drive control unit  22 F via the speed change control unit  22 B, the steering control unit  22 C, etc. In this way, the autonomous drive control unit  22 F can monitor various setting states of the tractor  1  and operating states of the various parts. 
     When the display unit  4  of the mobile communication terminal  3  is operated by a user, such as the passenger or an administrator outside of the vehicle, and the start of autonomous drive is instructed while the drive mode of the tractor  1  is switched to an autonomous drive mode, the autonomous drive control unit  22 F starts autonomous drive control to cause autonomous drive of the tractor  1  along the target travel route while the positioning unit  24  acquires the current position of the tractor  1 . 
     The autonomous drive control by the autonomous drive control unit  22 F includes an autonomous control process for the engine in which a control command for autonomous drive related to the engine  14  is sent to the engine control unit  22 A; an autonomous control process for speed change in which a control command for autonomous drive related to the continuously variable transmission  36 , the forward-reverse switching device  37 , etc., is sent to the speed change control unit  22 B; an autonomous control process for steering in which a control command for autonomous drive related to steering is sent to the steering control unit  22 C; and an autonomous control process for work in which a control command for autonomous drive related to work devices, such as the rotary tiller  6 , is sent to the work device control unit  22 D. 
     In the autonomous control process for the engine, the autonomous drive control unit  22 F sends, to the engine control unit  22 A, an engine speed change command for instructing a change in engine speed on the basis of the proper engine speed or the like included in the target travel route and an engine stop command for instructing the engine  14  to stop on the basis of the establishment of an engine stop condition. 
     In the autonomous control process for speed change, the autonomous drive control unit  22 F sends, to the speed change control unit  22 B, a speed change operation command for instructing the continuously variable transmission  36  to change speed on the basis of the proper vehicle speed included in the target travel route, a forward-reverse switching command for instructing the forward-reverse switching device  37  to switch between forward travel and reverser travel on the basis of the travel direction of the tractor  1  included in the target travel route, a neutral switching command for instructing the forward-reverse switching device  37  to switch to a neutral state on the basis of the establishment of a travel power cutoff condition. 
     In the autonomous control process for steering, the autonomous drive control unit  22 F sends, to the steering control unit  22 C, a steering command for instructing the steering of the left and right front wheels  10  on the basis of the front wheel steering angles or the like in the target travel route. 
     In the autonomous control process for work, the autonomous drive control unit  22 F sends, to the work device control unit  22 D, a work start command for instructing the rotary tiller  6  to switch to a work state on the basis of the work state point included in the target travel route and a work stop command for instructing the rotary tiller  6  to switch to a non-work state on the basis of the work stop point included in the target travel route. 
     Note that, in regard or the engine stop condition and the travel power cutoff condition described above, the autonomous drive control unit  22 F determines that the engine stop condition and the travel power cutoff condition have been established when an error is detected on the basis of various items of information from the vehicle state detection device  23 , etc. The error is, for example, an error of the speed change control unit  22 B, such as a speed change control failure in which the proper vehicle speed differs from the vehicle speed of the tractor  1 , or an error in the CAN communication with the speed change control unit  22 B and the steering control unit  22 C. 
     The engine control unit  22 A performs automatic engine speed change control, automatic engine stop control, etc., in accordance with various control commands related to the engine  14  sent from the autonomous drive control unit  22 F in the autonomous control process for the engine. The automatic engine speed change control automatically changes the engine speed. The automatic engine stop control automatically stops the engine  14 . 
     The speed change control unit  22 B performs automatic speed change control, automatic forward-reverse switching control, automatic neutral switching control, etc., in accordance with various control commands related to the continuously variable transmission  36 , the forward-reverse switching device  37 , etc., sent from the autonomous drive control unit  22 F in the autonomous control process for speed change. The automatic speed change control automatically controls the operation of the continuously variable transmission  36 . The automatic forward-reverse switching control automatically controls the operation of the forward-reverse switching device  37 . The automatic neutral switching control automatically switches the forward-reverse switching device  37  to a neutral state so that the power transmitted to the left and right front wheels  10  and the left and right rear wheels  11  are cut off. The automatic speed change control includes an automatic deceleration stop process of decelerating the continuously variable transmission  36  to a zero state and stopping the driving of the tractor  1 , for example if the proper vehicle speed included in the target travel route is zero. 
     The steering control unit  22 C performs automatic steering control, automatic brake turning control, etc., in accordance with a steering command sent from the autonomous drive control unit  22 F in the autonomous control process for steering. The automatic steering control controls the operation of the power steering mechanism  16  and steers the left and right front wheels  10 . The automatic brake turning control operates the first brake operating device  45  and operates the brake  42  on the inner side of the turn if the left and right front wheels  10  are steered at a set angle or more. 
     The work device control unit  22 D performs automatic work start control, automatic work stop control, etc., in accordance with various control commands related to the rotary tiller  6  sent from the autonomous drive control unit  22 F in the autonomous control process for work. The automatic work start control controls the operation of the elevating drive mechanism  20  and the clutch operation mechanism  19 , lowers the rotary tiller  6  to a work height, and operates the rotary tiller  6 . The automatic work stop control stops the rotary tiller  6  and raises the rotary tiller  6  to a non-work height. In a work state in which the rotary tiller  6  is lowered to a work height and operated, the work device control unit  22 D performs automatic tillage depth control and automatic roll angle maintenance control. The automatic tillage depth control controls the operation of the elevating drive mechanism  20  and keeps the tillage depth made by the rotary tiller  6  to a set depth on the basis of the detection of a tillage sensor that detects a tillage depth of the rotary tiller  6 . The automatic roll angle maintenance control controls the operation of the roll-direction drive mechanism  21  and keeps the tilt of the rotary tiller  6  in the roll direction to a set orientation (for example, a horizontal orientation) on the basis of a tilt sensor that detects the roll angle of the tractor  1  and detection of an acceleration sensor of an inertia measuring unit. 
     That is, the above-described autonomous drive unit  2  includes a power steering mechanism  16 , a clutch operation mechanism  19 , an elevating drive mechanism  20 , a roll-direction drive mechanism  21 , a vehicle-mounted control system  22 , a vehicle state detection device  23 , a positioning unit  24 , and a communication module  77 . The proper operation of these components enables accurate autonomous drive of the tractor  1  along the target travel route and proper tillage by the rotary tiller  6 . In case of an error in the speed change control unit  22 B of the tractor  1  or an error in the CAN communication, the driving of the tractor  1  can be automatically stopped. 
     As illustrated in  FIGS. 2, 4, 5, and 8 to 11 , the brake system  17  includes an electric second brake operating device  100  that operates the left and right brake pedals  40  coupled by the coupling mechanism  55  so as to operate the left and right brakes  42  as safety brakes. As illustrated in  FIG. 3 , the autonomous drive control unit  22 F includes a safety brake function unit  22 Fa that controls the operation of the second brake operating device  100  so as to cause the left and right brakes  42  to function as safety brakes. 
     As illustrated in  FIGS. 8 to 11 , the second brake operating device  100  is disposed on the right side of the right brake pedal  40  in the driver unit  12 . The second brake operating device  100  includes an operated body  101 , an electric actuator  102 , and an allowance portion  103 . The operated body  101  is coupled to the right brake pedal  40 . The electric actuator  102  operates the operated body  101  in the front-rear direction. The allowance portion  103  allows displacement of the right brake pedal  40 , etc., relative to the electric actuator  102  between the right brake pedal  40  and the electric actuator  102 , in conjunction with the depression of the brake pedal  40 . 
     As illustrated in  FIGS. 8 to 12 , the operated body  101  includes a first member  104 , a second member  105 , a damper  107 , and a link plate  108 . The first member  104  and a second member  105  are coupled to the pedal arm portions  40 B of the right brake pedal  40 . The damper  107  is swingably coupled to the second member  105  in the vertical direction via a coupling pin  106  extending in the left-right direction. The link plate  108  is coupled to the damper  107  so that the position of the link plate  108  is adjustable in the front-back direction. A long hole  108   a  is formed in the link plate  108 . The long hole  108   a  extends in the front-back direction and functions as the allowance portion  103 . The electric actuator  102  is an electric motor including a worm reducer  102 A. The electric motor  102  is switched between a forward rotation operation state, a reverse rotation operation state, and an operation stop state by the control and operation of the steering control unit  22 C. In the forward rotation operation state, forward rotation power is output. In the reverse rotation operation state, reverse rotation power is output. In the operation stop state, the output of rotation power stops. A deceleration gear set  111  and a linking pin  112  are disposed between the operated body  101  and the worm reducer  102 A of the electric motor  102 . The deceleration gear set  111  includes a small-diameter input gear  109  and a large-diameter output gear  110  and further reduce the power from the worm reducer  102 A. The linking pin  112  links the outer circumference of the output gear  110  of the deceleration gear set  111  and the operated body  101  via the allowance portion  103 . The linking pin  112  is fixed to the outer circumference of the output gear  110  while being passed through the long hole  108   a  in the link plate  108 . The linking pin  112  moves from a non-operating position (see  FIGS. 8 to 11 ) to a maximum operation position (see  FIG. 12 ), by the forward rotation power from the electric motor  102 , and moves from the maximum operation position to the non-operating position by the reverse rotation power from the electric motor  102 . At the non-operating position, the right brake pedal  40  (the right brake  42 ) is not operated. At the maximum operation position, the operation amount of the right brake pedal  40  (the right brake  42 ) is maximized. The non-operating position of the linking pin  112  is set so that the linking pin  112  is positioned at the front end of the long hole  101   a  when the right brake pedal  40  is at a depression release position. 
     With the above-described configuration, when the right brake pedal  40  is depressed, the forward movement of the operated body  101  relative to the linking pin  112  caused by the depression operation is allowed by the allowance of the long hole  108   a  (the allowance portion  103 ) of the link plate  108 . As a result, when the right brake pedal  40  or the left and right brake pedals  40  are depressed while the tractor  1  is manually driven by the passenger, the right brake  42  or the left and right brakes  42  can be braked in conjunction with the depression by the action of the allowance portion  103 , without hinderance by the electric motor  102 . 
     By the electric motor  102  switching to the forward rotation operation state in a state in which the left and right brake pedals  40  are coupled by the coupling mechanism  55 , the left and right brake pedals  40  can be moved to a depression limit position, and thereby the left and right brakes  42  can be switched to the braking state. Switching of the electric motor  102  to the reverse rotation operation state moves the left and right brake pedals  40  to the depression release position. As a result, the left and right brakes  42  can be switched to a released state. 
     A first limit switch  113  is disposed at the non-operating position of the linking pin  112 . The first limit switch  113  detects the linking pin  112  reaching the non-operating position by the operation of the electric motor  102 . A second limit switch  114  is disposed at the maximum operation position of the linking pin  112 . The second limit switch  114  detects the linking pin  112  reaching the maximum operation position by the operation of the electric motor  102 . When the left and right brake pedals  40  are coupled by the coupling mechanism  55 , the non-operating position of the linking pin  112  is a release position at which the braking by the left and right brakes  42  is released, and the maximum operation position of the linking pin  112  is a braking position at which the braking by the left and right brakes  42  is maximized. In this way, the first limit switch  113  functions as a release switch that detects the linking pin  112  reaching the release position by the operation of the electric motor  102 . The second limit switch  114  functions as a braking switch that detects the linking pin  112  reaching the braking position of the linking pin  112  by the operation of the electric motor  102 . The release switch  113  and the braking switch  114  function as a first operation sensor for detecting the operation of the electric motor  102 . 
     As illustrated in  FIG. 3 , the release switch  113  and the braking switch  114  are included in the vehicle state detection device  23  together with the left and right brake switches  25 , the left and right brake sensors  26 , etc. The vehicle state detection device  23  includes a coupling switch that is turned on when the left and right brake pedals  40  are coupled by the coupling mechanism  55 . 
     When the left and right brake switches  25  detect the left and right brake pedals  40  at the depression release positions, and when the detection values of the left and right brake sensors  26  indicate the depression release positions of the left and right brake pedals  40  while the coupling switch is turned on, the steering control unit  22 C can detect the released state of the left and right brakes  42 . When the left and right brake switches  25  do not detect the left and right brake pedals  40  at the depression release positions, and when the detection values of the left and right brake sensors  26  indicate the maximum depression positions of the left and right brake pedals  40  while the coupling switch is turned on, the steering control unit  22 C can detect the braking state of the left and right brakes  42 . In other words, the left and right brake switches  25  and the left and right brake sensors  26  function as a second operation sensor that detects the operation of the left and right brakes  42  when the left and right brake pedals  40  are coupled by the coupling mechanism  55 . 
     The steering control unit  22 C having the above-described configuration controls the operation of the electric motor  102  on the basis of the detection of the left and right brake switches  25 , the detection values of the left and right brake sensors  26 , the detection of the release switch  113 , and the detection of the braking switch  114 , while the left and right brake pedals  40  are coupled by the coupling mechanism  55 . Through such control, the steering control unit  22 C can certainly switch the left and right brakes  42  from a released state to a braking state, and from a braking state to a released state. 
     As illustrated in  FIGS. 8 to 12 , the second brake operating device  100  includes a housing case  116  that is fixed to a floor plate (an example of a fixing portion of the vehicle)  38  of the driver unit  12 . The housing case  116  has a base plate  117 , a left case body  118 , a first support plate  119 , a second support plate  120 , and a right case body  121 . The base plate  117  is detachably bolted to the floor plate  38 . The left case body  118  is bolted to the floor plate  38  and the base plate  117 . The first support plate  119  is welded to the inner face of the left case body  118 . The second support plate  120  is fixed to the sidewall of the left case body  118  with a predetermined gap provided therebetween. The right case body  121  is bolted to the left case body  118 . In the housing case  116 , a storage space is formed between the left case body  118  and the right case body  121 . 
     In the housing case  116 , three stepped bolts  122  extending from the sidewall of the left case body  118  in the right direction are fixed to the left case body  118 . The second support plate  120  is fixed to the sidewall of the left case body  118  via the stepped bolts  122 . Among the three stepped bolts  122 , the stepped bolt  122  positioned near the center of the sidewall is used as a support shaft  123  to rotatably support the output gear  110 . A guide hole  118   a  for guiding the linking pin  112  between the release position and the braking position is formed in the sidewall of the left case body  118  in an arc shape centered about the support shaft  123 . The electric motor  102  is fixed to the first support plate  119  with three bolts  124 . The input gear  109  that meshes with the output gear  110  is fixed to the output shaft of the worm reducer  102 A of the electric motor  102 . One end portion of the second support plate  120  functions as a first receiving portion  120 A for receiving the linking pin  112  having reached the release position. The other end portion of the second support plate  120  functions a second receiving portion  120 B for receiving the linking pin  112  having reached the braking position. The release switch  113  is fixed to the one end portion of the second support plate  120 , and the braking switch  114  is fixed to the other end portion of the second support plate  120 . 
     In the second brake operating device  100  having the above-described configuration, the electric motor  102 , the deceleration gear set  111 , the release switch  113 , the braking switch  114 , etc., are housed in the housing case  116 . In this way, the housing case  116 , the electric motor  102 , the deceleration gear set  111 , the release switch  113 , the braking switch  114 , etc., can be detachably attached to the floor plate  38  in an integrated state as a drive unit. After the driver unit has been attached, the operated body  101  coupled to the right brake pedal  40  and the output gear  110  of the drive unit are linked via the allowance portion  103  and the linking pin  112  so that the operated body  101  can be operated by the electric motor  102 . In this way, the second brake operating device  100  can be installed to the driver unit  12  so that the left and right brakes  42  can be operated by the second brake operating device  100 . 
     In other words, the second brake operating device  100  can be readily installed to the driver unit  12  without significantly modifying the configuration of the driver unit  12 . In this way, the second brake operating device  100  can be retrofitted to the tractor  1 . As a result, the second brake operating device  100  can be readily installed to the tractor  1 . Also, maintenance, such as replacement of the second brake operating device  100 , can be readily performed when a problem occurs in the second brake operating device  100 . 
     As illustrated in  FIGS. 8 to 10 , the first receiving portion  120 A and the second receiving portion  120 B of the second support plate  120  function as a movement restrictor that restrict the movable range of the linking pin  112  between the release position and the braking position. The movable range of the linking pin  112  is set to a range in which the linking pin  112  moves between the release position and the braking position across an imaginary straight line L 1 . The imaginary straight line L 1  passes through the coupling pin  106  and the support shaft  123 . The coupling pin  106  is the coupling point of the operated body  101  with respect to the right brake pedal  40 . The support shaft  123  is the rotation center of the output gear  110 . 
     With the above-described configuration, when the second brake operating device  100  operates the left and right brakes  42 , the forward rotation power from the electric motor  102  is first transmitted to the output gear  110  to rotate the output gear  110  in the braking direction, and thereby, the linking pin  112  moves from the release position to the braking position while following an arc. At this time, a braking force A of the left and right brakes  42  increases in accordance with the amount of movement of the linking pin  112  passing through a play region including the release position and reaching the braking position, as illustrated in  FIG. 13 . An operating load B applied to the electric motor  102  increases as the linking pin  112  approaches the imaginary straight line L 1  in a first movable range of the linking pin  112  after passing through the play region until crossing the imaginary straight line L 1  because the reaction force from various parts, such as friction plates, generated inside the left and right brakes  42  increases with an increase in the braking force A of the left and right brakes  42 , and the left and right brake pedals  40  are urged to return to the depression release position by the tension of the left and right extension springs  46 . On the contrary, as the linking pin  112  approaches the imaginary straight line L 1 , the angle θ (see  FIG. 11 ) between the line L 2  connecting the linking pin  112  and the rotation center of the output gear  110  and the operated body  101  becomes smaller. As a result, the reaction force from the left and right brakes  42  and the tension from the left and right extension springs  46  applied to the left and right brake pedals  40  do not readily act as forces for returning the linking pin  112  to the release position. Thus, the amount of increase in operating load B applied to the electric motor  102  decreases. Thereafter, when the linking pin  112  crosses the imaginary straight line L 1 , the reaction force from the left and right brakes  42  and the tension of the left and right extension springs  46  shift to assisting the braking operation of the left and right brakes  42  by the electric motor  102 . Therefore, the operating load B applied to the electric motor  102  decreases as the linking pin  112  moves away from the imaginary straight line L 1  and approaches the braking position in a second movable range of the linking pin  112  after crossing the imaginary straight line L 1  until reaching the braking position. With the linking pin  112  reached at the braking position, the linking pin  112  is urged to move in the braking direction by the reaction force from the left and right brakes  42  and the tension of the left and right extension springs  46  while the movement of the linking pin  112  in the braking direction is restricted by the second receiving portion  120 B. In this way, for example, if the worm reducer  102 A of the electric motor  102  is damaged or the output gear  110  is defective when the linking pin  112  reaches the second movable region after crossing the imaginary straight line L 1 , the linking pin  112  moves to and is held at the braking position by the reaction force from the left and right brakes  42  and the tension of the left and right extension springs  46 . As a result, the left and right brakes  42  can be kept in a braking state, and the tractor  1  can be kept in a braking stop state. 
     As a result, as the operating load B applied to the electric motor  102  is reduced during the braking operation of the left and right brakes  42  by the second brake operating device  100 , the tractor  1  can be kept at the braking stop state while the tractor  1  is braked and stopped by the operation of the second brake operating device  100 , regardless of a damage to the worm reducer  102 A of the electric motor  102  and a defect in the output gear  110 . 
     As illustrated in  FIG. 3 , the safety brake function unit  22 Fa monitors the operating state of the components of the tractor  1  and the communication state with the components on the basis of various items of detection information from the vehicle state detection device  23  received via the speed change control unit  22 B, the steering control unit  22 C, etc. In the autonomous drive mode, when the safety brake function unit  22 Fa detects an error inside the vehicle on the basis of detection information from the vehicle state detection device  23  or obtains an emergency stop command from the mobile communication terminal  3  or an emergency stop remote control  90  (see  FIG. 2 ), which is an example of a wireless communication device, the safety brake function unit  22 Fa performs emergency stop control to control the operation of the electric motor  102  and switch the left and right brakes  42  from a released state to a braking state. For this reason, the tractor  1  includes an emergency stop communication antenna  91  for receiving an emergency stop command sent from the emergency stop remote control  90 , as illustrated in  FIG. 2 . 
     An error inside the vehicle may include a drop in the engine speed to or below a set lower limit that may lead to engine stall, a control failure such as the vehicle speed of the tractor  1  deviating from the proper vehicle speed under the automatic speed change control by the speed change control unit  22 B, and a communication failure in the CAN communication due to disconnection, etc. 
     Note that the error inside the vehicle may include a control failure such as a deviation of the current position of the tractor  1  measured by the positioning unit  24  from the target travel route under the automatic steering control by the steering control unit  22 C. 
     Therefore, under the emergency stop control, the safety brake function unit  22 Fa performs a first determination process (step # 1 ), a second determination process (step # 2 ), a third determination step (step # 3 ), and a fourth determination process (step # 4 ), as illustrated in the flowchart in  FIG. 14 . In the first determination process (step # 1 ), the safety brake function unit  22 Fa determiners whether or not the engine speed has dropped to or below a set lower limit that may lead to engine stall on the basis of the detection information from a rotation sensor for engine speed detection included in the vehicle state detection device  23 . In the second determination process (step # 2 ), the safety brake function unit  22 Fa determines whether or not a control failure, such as the vehicle speed of the tractor  1  deviating from the proper vehicle speed, has occurred in the speed change control unit  22 B on the basis of the detection information from a vehicle sensor or the like included in the vehicle state detection device  23 . In the third determination step (step # 3 ), the safety brake function unit  22 Fa determines whether or not a communication failure has occurred in the CAN communication on the basis of the detection information from an error detector for CAN communication included in the vehicle state detection device  23 . In the fourth determination process (step # 4 ), the safety brake function unit  22 Fa determines whether or not an emergency stop command has been obtained from the mobile communication terminal  3  or the emergency stop remote control  90 . When the engine speed decreases in the first determination process (step # 1 ), when a control failure has occurred in the speed change control unit  22 B in the second determination process (step # 2 ), when a communication failure has occurred in the CAN communication in the third determination process (step # 2 ), or when an emergency stop command is obtained in the fourth determination process (step # 4 ), an emergency stop process (step # 5 ) that causes an emergency stop of the tractor  1  and an emergency stop notification process (step # 6 ) that provides a notification about the emergency stop are performed. 
     In the emergency stop process, the safety brake function unit  22 Fa sends an engine stop command to the engine control unit  22 A and sends a safety brake operation command for operating the left and right brakes  42  as safety brakes to the steering control unit  22 C. In the emergency stop process, the safety brake function unit  22 Fa deselects the autonomous drive mode and causes a transition of the drive mode from the autonomous drive mode to the manual drive mode. In the emergency stop notification process, the safety brake function unit  22 Fa operates a notification device  83  (see  FIG. 2 ), such as an indicator light for emergency stop, which is provided in the tractor  1 , and sends an emergency stop notification command to the mobile communication terminal  3 . 
     The engine control unit  22 A performs the above-described automatic engine stop control in response to the engine stop command from the autonomous drive control unit  22 F and automatically stops the engine  14 . 
     The steering control unit  22 C brakes and stops the tractor  1  by operating the electric motor  102  and switching the left and right brakes  42  from a released state to a braking state in response to the safety brake operation command from the safety brake function unit  22 Fa. 
     The mobile communication terminal  3  performs the emergency stop notification process of switching the display screen of the display unit  4  to an emergency stop notification screen in response to the emergency stop notification command from the autonomous drive control unit  22 F. 
     With this configuration, the left and right brake pedals  40  are coupled with the coupling mechanism  55  when the tractor  1  is autonomously driven, so as to automatically stop the engine  14  and automatically operate the left and right brakes  42  as safety brakes, thereby to brake and stop the tractor  1  in case an error occurs inside the vehicle, such as a decrease in the engine speed to or below a set lower limit, a control failure in the speed change control unit  22 B, or a communication failure in the CAN communication. 
     As a result, in case any of the above-mentioned errors occurs inside the vehicle in the autonomous drive mode, the tractor  1  can be quickly braked and stopped and kept in the braking stop state, even when the tractor  1  is in an unmanned drive state in which the tractor  1  is autonomously driven. When the rotary tiller  6  is operating, the engine  14  can be stopped, and the operation of the rotary tiller  6  can also be stopped. 
     Unlike the electrohydraulically controlled first brake operating device  45 , which operates the left and right brakes  42  by the oil from the hydraulic pump driven by the engine power, the electric motor  102  can operate the left and right brakes  42  and keep a braking state, even after the engine  14  has stopped and the hydraulic pressure has dropped. In this way, the tractor  1  can be kept in the braking stop state even after the engine control unit  22 A has stopped the engine  14  in response to an engine stop command from the autonomous drive control unit  22 F. 
     As a result, it is possible to avoid the risk of the tractor  1  unexpectedly descending from an emergency stop position of the tractor  1  located on a slope tilting in the travel direction of the tractor  1 . 
     After the emergency stop process described above has been performed, the safety brake function unit  22 Fa performs the emergency stop release process if the power is turned off by operating to turn off a key switch (an example of an operating tool)  84  (see  FIG. 2 ) provided in the driver unit  12  and then turned on again by turning on the key switch  84 . In the emergency stop release process, the safety brake function unit  22 Fa sends an engine stop release command to the engine control unit  22 A and sends a safety brake release command to the steering control unit  22 C. The engine control unit  22 A permits the activation of the engine  14  in response to the engine stop release command from the safety brake function unit  22 Fa. The steering control unit  22 C releases the braking stop of the tractor  1  by operating the electric motor  102  and switching the left and right brakes  42  from the braking state to a released state in response to the safety brake release command from the safety brake function unit  22 Fa. 
     In this way, when the tractor  1  is urgently stopped under the control of the safety brake function unit  22 Fa, the tractor  1  can be kept in the emergency stop state until the power that was turned off is turned on again by the operation of the key switch  84 . When the power is turned on again by the operation of the key switch  84 , the activation of the engine  14  is permitted, and the braking stop of the tractor  1  is released. In this way, the passenger can activate the engine  14 , and the tractor  1  can be manually driven by the passenger. As a result, the tractor  1  can be moved to a safe place or a repair shop by manual driving. 
     As illustrated in  FIG. 15 , a home screen  32 A of the liquid crystal monitor  32  displays a mode selection button (mode selector)  32   a  that enables selection of the autonomous drive mode and the like. When the autonomous drive mode is selected through operation of the mode selection button  32   a , the autonomous drive control unit  22 F performs a condition establishment determination process of determining whether or not all conditions for the transition of the drive mode of the tractor  1  from the manual drive mode to the autonomous drive mode are established. If all of the conditions are established, the autonomous drive control unit  22 F causes the drive mode of the tractor  1  to transition from the manual drive mode to the autonomous drive mode, and notifies the mobile communication terminal  3  about the transition of the drive mode of the tractor  1  to the autonomous drive mode so as to enable the start of autonomous drive by operation of the mobile communication terminal  3 . If all of the conditions are not established, the autonomous drive control unit  22 F continues the condition establishment determination process until all of the conditions are established or the cancel button displayed on the liquid crystal monitor  32  is operated. 
     The conditions for the transition of the drive mode to the autonomous drive mode includes the completion of various setting operations required for the autonomous drive of the tractor  1 , such as setting the engine speed for autonomous drive through an operation of the accelerator lever and setting the vehicle speed for autonomous drive through an operation of the speed change lever, as well as the confirmation of a normal operation during an initial check (operation confirmation process) for checking whether or not the left and right brakes  42  operate normally as safety brakes. In other words, in order to transition the driving mode to the autonomous drive mode, it is necessary to perform an initial check in advance and confirm that the left and right brakes  42  operate normally as the safety brakes. 
     An initial check is included in the control operation of the safety brake function unit  22 Fa. When the normal operation has been confirmed through an initial check, the safety brake function unit  22 Fa has caused a transition to an initial check completed state (operation checked state: “Check_OK” in  FIG. 24 ). A valid period is set for the initial check completed state. In this embodiment, the valid period for the initial check completed state is set to within the same day. Therefore, the safety brake function unit  22 Fa stores the transition date in the vehicle-mounted storage unit  22 G at the time of the transition to the initial check completed state. 
     Note that the valid period for the initial check completed state may be set to various settings: for example, several days from the transition date, several hours from the transition time, or the time of driving a set distance from the distance at the transition. 
     The execution of an initial check is premised on the establishment of all of first to ninth conditions described below (conditions for starting the operation check process). 
     First condition: the steering control unit  22 C controlling the operation of the electric motor  102  that causes the left and right brakes  42  to function as safety brakes is normal. 
     Second condition: the speed change control unit  22 B that monitors the detection information from the left and right brake switches  25 , the left and right brake sensors  26 , etc., is normal. 
     Third condition: the CAN communication with the steering control unit  22 C is normal. 
     Fourth condition: the CAN communication with the speed change control unit  22 B is normal. 
     Fifth condition: the left and right brake pedals  40  are detected to be at the depression release positions on the basis of the detection information received from the left and right brake switches  25  and the left and right brake sensors  26  via the speed change control unit  22 B. 
     Sixth condition: the reverser lever is detected to be in a neutral position on the basis of the detection information received from the reverser sensor via the speed change control unit  22 B. 
     Seventh condition: the parking lever  41  is detected to not be at the braking position on the basis of the detection information received from the parking switch via the speed change control unit  22 B. 
     Eighth condition: the left and right brake pedals  40  are detected to be coupled by the coupling mechanism  55  on the basis of the detection information received from the coupling switch via the steering control unit  22 C. 
     Ninth condition: the vehicle speed is detected to be zero on the basis of the detection information received from the vehicle speed sensor via the speed change control unit  22 B. 
     The state transitions of the safety brake function unit  22 Fa during an initial check will be described below with reference to  FIGS. 15 to 25 . 
     The safety brake function unit  22 Fa enters an initial state (“Start” in  FIG. 24 ) when the power is turned on by turning on the key switch  84  (see  FIG. 2 ). In the initial state, a period elapse determination process is performed to determine whether or not the initial check completed state has the valid period. 
     In the initial state, if the release switch  113  has detected the linking pin  112  operated by the electric motor  102  at the release position, and if the braking switch  114  has detected the linking pin  112  at the braking position, the safety brake function unit  22 Fa determines that an error has occurred in any one of the electric motor  102 , the release switch  113 , and the braking switch  114  and causes a transition to an error detection state (“Err_Detection” in  FIG. 24 ). 
     In the initial state, if the initial check completed state has continued for more than the valid period while the release switch  113  has not detected the linking pin  112  at the release position, the safety brake function unit  22 Fa causes a transition to a brake release return operation state (“Rev_Start” in  FIG. 24 ) and instructs the steering control unit  22 C to release the left and right brakes  42  by the operation of the electric motor  102 . In this way, the steering control unit  22 C releases the left and right brakes  42  by the operation of the electric motor  102 . If the safety brake function unit  22 Fa has been operating the left and right brakes  42  as safety brakes while the key has been previously turned on, the left and right brakes  42  can be returned to a released state through the release operation. 
     If the autonomous drive mode is selected through the operation of the mode selection button  32   a  on the home screen  32 A illustrated in  FIG. 15  in the initial state, the safety brake function unit  22 Fa performs a pre-condition determination process of determining whether or not the first to ninth conditions are all established. 
     If any one of the conditions 1 to 9 is not established in the pre-condition determination process, the safety brake function unit  22 Fa causes a transition to a pre-condition establishment standby state (“Check_PreStandby” in  FIG. 24 ) and causes a transition of the display screen of the liquid crystal monitor  32  to a safety brake check selection screen  32 B illustrated in  FIG. 16 . 
     If all of the conditions 1 to 9 are established in the pre-condition determination process, the safety brake function unit  22 Fa causes a transition to an initial check standby state (“Check_Standby” in  FIG. 24 ) and causes a transition of the display screen of the liquid crystal monitor  32  to the safety brake check selection screen  32 B illustrated in  FIG. 16 . 
     If the release switch  113  has not detected the linking pin  112  at the release position within the valid period of the initial check completed state, in the initial state, the safety brake function unit  22 Fa causes a transition to a checked brake release return operation state (“OK_Rev” in  FIG. 24 ) and instructs the steering control unit  22 C to release the left and right brakes  42  by the operation of the electric motor  102 . In this way, the steering control unit  22 C releases the left and right brakes  42  by the operation of the electric motor  102 . If the safety brake function unit  22 Fa has been operating the left and right brakes  42  as safety brakes while the key has been previously turned on, the left and right brakes  42  can be returned to a released state through the release operation. 
     In the period elapse determination process, if within the valid period of the initial check completed state and if the release switch  113  has detected the linking pin  112  at the release position, the safety brake function unit  22 Fa causes a transition to the initial check completed state and causes a transition of the display screen of the liquid crystal monitor  32  to an autonomous drive start screen  32 D illustrated in  FIG. 22  to permit the transition of the drive mode from the manual drive mode to the autonomous drive mode. 
     If an initial check is selected through an operation of a check selection button  32   b  on the safety brake check selection screen  32 B in the pre-condition establishment standby state, the safety brake function unit  22 Fa causes the display screen of the liquid crystal monitor  32  to transition to a safety brake check screen  32 C illustrated in  FIG. 17 . At this time, an initial check button  32   c  on the safety brake check screen  32 C is grayed out, as illustrated in  FIG. 17 , to indicate that the initial check cannot be started through the operation of the initial check button  32   c.    
     In the pre-condition establishment standby state, if the release switch  113  has detected the linking pin  112  at the release position, and if the braking switch  114  has detected the linking pin  112  at the braking position, the safety brake function unit  22 Fa determines that an error has occurred in one of the electric motor  102 , the release switch  113 , and the braking switch  114  and causes a transition to the error detection state. 
     If the release switch  113  has not detected the linking pin  112  at the release position in the pre-condition establishment standby state, the safety brake function unit  22 Fa causes a transition to the brake release return operation state, and instructs the steering control unit  22 C to release the left and right brakes  42  by the operation of the electric motor  102 . 
     In the pre-condition establishment standby state, if the release switch  113  has detected the linking pin  112  at the release position by the operation of the electric motor  102  and if the first to ninth conditions are all established while the braking switch  114  has not detected the linking pin  112  at the braking position by the operation of the electric motor  102 , the safety brake function unit  22 Fa causes a transition to the initial check standby state and causes the display state of the initial check button  32   c  on the safety brake check screen  32 C to transition to a normal display state illustrated in  FIG. 18 , to indicate that the initial check can be selected through operation of the initial check button  32   c.    
     In the pre-condition establishment standby state, if the release switch  113  has detected the linking pin  112  at the release position and none of the first to ninth conditions are established while the braking switch  114  has not detected the linking pin  112  at the braking position, the safety brake function unit  22 Fa keeps the pre-condition establishment standby state and causes the display screen of the liquid crystal monitor  32  to transition to the safety brake check screen  32 C illustrated in  FIG. 17 . At this time, an initial check button  32   c  is grayed out as illustrated in  FIG. 17  on the safety brake check screen  32 C, to indicate that the initial check cannot be selected by the operation of the initial check button  32   c.    
     In the initial check standby state, if the release switch  113  has detected the linking pin  112  at the release position and the braking switch  114  has detected the linking pin  112  at the braking position, in the initial state, the safety brake function unit  22 Fa determines that an error has occurred in any of the electric motor  102 , the release switch  113 , and the braking switch  114  and causes a transition to the error detection state. 
     If the release switch  113  has not detected the linking pin  112  at the release position in the initial check standby state, the safety brake function unit  22 Fa causes a transition to the brake release return operation state and instructs the steering control unit  22 C to release the left and right brakes  42  by the operation of the electric motor  102 . 
     In the initial check standby state, if any of the first to ninth conditions is no longer established due to, for example, manual operation of the left and right brake pedals  40  or the reverser lever while the release switch  113  has detected the linking pin  112  at the release position, and the braking switch  114  has not detected the linking pin  112  at the braking position, the safety brake function unit  22 Fa causes a transition to the pre-condition establishment standby state and causes the display state of the initial check button  32   c  on the safety brake check screen  32 C to transition to a grayout state illustrated in  FIG. 17 , to indicate that the initial check cannot be selected through operation of the initial check button  32   c.    
     If the initial check button  32   c  is operated in the initial check standby state, the safety brake function unit  22 Fa causes a transition to a braking operation check state, (“Check” in  FIG. 24 ) and causes the safety brake check screen  32 C illustrated in  FIG. 19  to indicate that an initial check is in progress. 
     The safety brake function unit  22 Fa instructs the steering control unit  22 C to brake the left and right brakes  42  by operating the electric motor  102  in the braking operation check state. In this way, the steering control unit  22 C brakes the left and right brakes  42  by the operation of the electric motor  102 . 
     In the braking operation check state, if the release switch  113  has detected the linking pin  112  at the release position and the braking switch  114  has detected the linking pin  112  at the braking position, or if the braking switch  114  does not detect the linking pin  112  at the braking position even after a predetermined braking operation check time, the safety brake function unit  22 Fa determines that an error has occurred in any one of the electric motor  102 , the release switch  113 , and the braking switch  114 , and causes a transition to the error detection state. 
     In the braking operation check state, if any one of the first to ninth conditions is no longer established due to, for example, manual operation of the left and right brake pedals  40  or the reverser lever, the safety brake function unit  22 Fa causes a transition to an initial check failure state (“Check_NG” in  FIG. 24 ) and displays on the safety brake check screen  32 C that the initial check has failed, as illustrated in  FIG. 20 . When a predetermined display time elapses after the transition to the initial check failure state, the safety brake function unit  22 Fa causes a transition to the initial state and causes the display screen of the liquid crystal monitor  32  to transition to the safety brake check selection screen  32 B illustrated in  FIG. 16 . Then, if the check selection button  32   b  on the safety brake check selection screen  32 B illustrated in  FIG. 16  is operated, the safety brake function unit  22 Fa causes a transition to the pre-condition establishment standby state and causes the display screen of the liquid crystal monitor  32  to transition to the safety brake check screen  32 C illustrated in  FIG. 17 . 
     In the braking operation check state, if the release switch  113  no longer detects the linking pin  112  at the release position, and if the left and right brake switches  25  have detected a shift of the left and right brake pedals  40  from the depression release position after the braking switch  114  has detect the linking pin  112  at the braking position, and if the left and right brake sensors  26  have detected the left and right brake pedals  40  at the maximum depression positions, the safety brake function unit  22 Fa determines that the left and right brake pedals  40  have been operated to the maximum depression positions, and the left and right brakes  42  have switched to a braking state. Then, on the basis of the determination, the safety brake function unit  22 Fa causes a transition to a release operation check standby state (“Check_Rev_Wait” in  FIG. 24 ), and instructs the steering control unit  22 C to stop the braking operation of the left and right brakes  42  by the operation of the electric motor  102 . In this way, the steering control unit  22 C ends the braking operation of the left and right brakes  42  by the operation of the electric motor  102 . 
     In the release operation check standby state, if the release switch  113  has detected the linking pin  112  at the release position, or if the braking switch  114  has not detected the linking pin  112  at the braking position, the safety brake function unit  22 Fa determines that an error has occurred in any one of the electric motor  102 , the release switch  113 , and the braking switch  114 , and causes a transition to the error detection state. 
     If any one of the first to ninth conditions is no longer established, for example, as a result of manual operation of the left and right brake pedals  40  or the reverser lever, in the release operation check standby state, the safety brake function unit  22 Fa causes a transition to the initial check failure state, and displays on the safety brake check screen  32 C that the initial check has failed, as illustrated in  FIG. 20 . When a predetermined display time elapses after the transition to the initial check failure state, the safety brake function unit  22 Fa causes a transition to the initial state and causes the display screen of the liquid crystal monitor  32  to transition to the safety brake check selection screen  32 B illustrated in  FIG. 16 . Then, if the check selection button  32   b  on the safety brake check selection screen  32 B illustrated in  FIG. 16  is operated, the safety brake function unit  22 Fa causes a transition to the pre-condition establishment standby state and causes the display screen of the liquid crystal monitor  32  to transition to the safety brake check screen  32 C illustrated in  FIG. 17 . 
     In the release operation check standby state, if the release switch  113  no longer detects the linking pin  112  at the release position and the braking switch  114  has detected the linking pin  112  at the braking position, or if the left and right brake switches  25  have detected a shift of the left and right brake pedals  40  from the depression release position and a predetermined standby time has elapses after the left and right brake sensors  26  have detected the left and right brake pedals  40  at the maximum depression positions, the safety brake function unit  22 Fa causes a transition to a release operation check state (“Check_Rev in  FIG. 24 ). 
     The safety brake function unit  22 Fa instructs the steering control unit  22 C to release the left and right brakes  42  by operating the electric motor  102  in the release operation check state. In this way, the steering control unit  22 C releases the left and right brakes  42  by the operation of the electric motor  102 . 
     In the release operation check state, if the release switch  113  has detected the linking pin  112  at the release position and the braking switch  114  has detected the linking pin  112  at the braking position, or if the release switch  113  has not detected the linking pin  112  at the release position even after a predetermined release operation check time, the safety brake function unit  22 Fa determines that an error has occurred in any one of the electric motor  102 , the release switch  113 , and the braking switch  114 , and causes a transition to the error detection state. 
     In the release operation check state, if any of the first to ninth conditions is no longer established, for example, due to manual operation of the left and right brake pedals  40  or the reverser lever, the safety brake function unit  22 Fa causes a transition to the initial check failure state and displays on the safety brake check screen  32 C that the initial check has failed, as illustrated in  FIG. 20 . When a predetermined display time elapses after the transition to the initial check failure state, the safety brake function unit  22 Fa causes a transition from the initial check failure state to the initial state and causes the display screen of the liquid crystal monitor  32  to transition to the safety brake check selection screen  32 B illustrated in  FIG. 16 . Then, if the check selection button  32   b  on the safety brake check selection screen  32 B illustrated in  FIG. 16  is operated, the safety brake function unit  22 Fa causes a transition to the pre-condition establishment standby state and causes the display screen of the liquid crystal monitor  32  to transition to the safety brake check screen  32 C illustrated in  FIG. 17 . 
     In the release operation check state, if the release switch  113  has detected the linking pin  112  at the release position, and the left and right brake switches  25  and the left and right brake sensors  26  have detected the left and right brake pedals  40  at the depression release position while the braking switch  114  has not detected the linking pin  112  at the braking position, the safety brake function unit  22 Fa determines that the left and right brake pedals  40  have been operated to the depression release position, and the left and right brakes  42  have switched to the released state. Then, on the basis of the determination, the steering control unit  22 C is instructed to stop the release operation of the left and right brakes  42  by the operation of the electric motor  102 . In this way, the steering control unit  22 C ends the braking operation of the left and right brakes  42  by the operation of the electric motor  102 . The safety brake function unit  22 Fa then causes a transition to an initial check successful state (“Check_Comp” in  FIG. 24 ) and displays on the safety brake check screen  32 C that the initial check has been completed, as illustrated in  FIG. 21 . 
     The safety brake function unit  22 Fa keeps the display of the completion of the initial check on the safety brake check screen  32 C for a predetermined time in the initial check successful state. When the predetermined time elapses, the safety brake function unit  22 Fa causes a transition to an initial check completed state (operation confirmed state) (“Check_OK” in  FIG. 24 ), causes the display screen of the liquid crystal monitor  32  to transition to the autonomous drive start screen  32 D illustrated in  FIG. 22 , and permits the transition of the drive mode from the manual drive mode to the autonomous drive mode. 
     When various setting operations necessary for the autonomous drive of the tractor  1 , such as the setting of the engine speed for autonomous drive through an operation of the accelerator bar and the setting of the vehicle speed for autonomous drive through an operation of the speed change lever, are performed after the safety brake function unit  22 Fa has caused a transition to the initial check completed state, all conditions for the transition of the drive mode to the autonomous drive mode are established. In a state in which all conditions are established, if an autonomous drive start button  32   d  displayed on the autonomous drive start screen  32 D illustrated in  FIG. 22  is operated, the autonomous drive control unit  22 F causes the drive mode of the tractor  1  to transition from the manual drive mode to the autonomous drive mode, notifies the mobile communication terminal  3  about the transition of the drive mode of the tractor  1  to the autonomous drive mode, and enables the autonomous drive to be started by an operation of the mobile communication terminal  3 . 
     In other words, when the tractor  1  is autonomously driven after the drive mode has transitioned to the autonomous drive mode, the left and right brakes  42  are preliminarily confirmed to operate normally as safety brakes through confirmation of the operation in advance. In this way, in case any of the above-described errors occurs inside the vehicle in the autonomous drive mode, the left and right brakes  42  can be operated normally as safety brakes, and the tractor  1  can be certainly braked and stopped. 
     As it is apparent from the description above, the initial check (operation confirmation process) includes a first operation confirmation process and a second operation confirmation process. The first operation confirmation process confirms whether or not the electric motor  102  is operating normally on the basis of the detection information from the release switch (first operation sensor)  113  and the braking switch (first operation sensor)  114  that detect the operation of the electric motor  102 . The second operation confirmation process confirms whether or not the left and right brakes  42  are operating normally on the basis of the detection information from the left and right brake switches (second operation sensor)  25  and the left and right brake sensors (second operation sensor)  26  that detect the operation of the left and right brakes  42 . 
     In this way, the operation of the electric motor  102  that causes the left and right brakes  42  to operate as the safety brakes and the operation of the left and right brakes  42  functioning as safety brakes are confirmed separately in the initial check (operation confirmation process) for checking whether or not the left and right brakes  42  operate normally as safety brakes. As a result, the initial check can be performed with high accuracy, and the reliability of the initial check can be enhanced. 
     When an initial check is to be performed, the safety brake function unit  22 Fa first performs the pre-condition determination process, and starts the initial check when all conditions for starting the initial check, which are necessary for starting the initial check, are established, as described above. In this way, it possible to avoid inconveniences, such as a decrease in the reliability of the initial check caused by a decrease in the load applied to the electric motor  102  during the initial check due to the left and right brake pedals  40  being depressed or the left and right brake pedals  40  being decoupled, or the incompletion of the initial check due to errors in the speed change control unit  22 B, the CAN communication, etc. 
     As described above, the valid period is set for the initial check completed state. Therefore, the safety brake function unit  22 Fa performs the period elapse determination process described above in the initial check completed state. If the valid period elapses in the period elapse determination process, the safety brake function unit  22 Fa causes a transition from the initial check completed state to the initial state, as illustrated in  FIG. 24 . 
     At this time, if the drive mode is the autonomous drive mode, the safety brake function unit  22 Fa causes the drive mode to transition from the autonomous drive mode to the manual drive mode, and prohibits a transition to the autonomous drive mode. The safety brake function unit  22 Fa causes the display screen of the liquid crystal monitor  32  to transition to the safety brake check selection screen  32 B illustrated in  FIG. 16 . If the check selection button  32   b  on the safety brake check selection screen  32 B illustrated in  FIG. 16  is operated, the safety brake function unit  22 Fa performs the initial check with the above-described state transition, and causes the display screen of the liquid crystal monitor  32  to transition to the autonomous drive start screen  32 D illustrated in  FIG. 22  after a transition to the initial check completed state and permits the transition of the drive mode from the manual drive mode to the autonomous drive mode. 
     In this way, an initial check is performed regularly, for example, when the tractor  1  is autonomously driven for several days. For this reason, it is possible to effectively suppress the risk of the left and right brakes  42  not operating normally as safety brakes in case any of the errors described above or any other errors occur inside the vehicle. 
     Note that, when the valid period of the initial check completed state elapses in the period elapse determination process, and the drive mode is already set to the autonomous drive mode, the safety brake function unit  22 Fa may enable the initial check completed state while the user keeps the autonomous drive mode, and disable the initial check completed state when the user ends the autonomous drive mode, to cause a transition from the initial check completed state to the initial state. 
     On the other hand, when the drive mode is the manual drive mode, the safety brake function unit  22 Fa indicates that the valid period of the initial check completed state has elapsed, on the display screen of the liquid crystal monitor  32  or the like. 
     In this way, when the tractor  1  is manually driven by the passenger, it is possible to inform the passenger in advance that an initial check is required for the transition of the drive mode to the autonomous drive mode. It is also possible to prevent a decrease in work efficiency due to periodical initial checks being performed during manual drive in which the left and right brakes  42  are not operated as safety brakes. 
     If any of the errors described above occur inside the vehicle in the initial check completed state, the safety brake function unit  22 Fa causes a transition to the safety brake operation state (“Brake_Move” in  FIG. 24 ), and instructs the steering control unit  22 C to brake the left and right brakes  42  by the operation the electric motor  102 . In this way, the steering control unit  22 C brakes the left and right brakes  42  by the operation of the electric motor  102 . 
     In the initial check completed state, if the release switch  113  has detected the linking pin  112  at the release position, and the braking switch  114  has detected the linking pin  112  at the braking position, the safety brake function unit  22 Fa determines that an error has occurred in one of the electric motor  102 , the release switch  113 , and the braking switch  114 , and causes a transition to the error detection state. 
     In the initial check completed state, if within the valid period of the initial check completed state, and the release switch  113  has not detected the linking pin  112  at the release position while no error has occurred inside the vehicle, the safety brake function unit  22 Fa causes a transition to the checked brake release return operation state (“OK_Rev” in  FIG. 24 ), and instructs the steering control unit  22 C to release the left and right brakes  42  by the operation of the electric motor  102 . In this way, the steering control unit  22 C releases the left and right brakes  42  by the operation of the electric motor  102 . 
     In the checked brake release return operation state, if the release switch  113  has not detected the linking pin  112  at the release position even after a predetermined time for releasing the braking has elapsed, the safety brake function unit  22 Fa determines that an error has occurred in any of the electric motor  102 , the release switch  113 , and the braking switch  114  and causes a transition to the error detection state. 
     In the checked brake release return operation state, if the release switch  113  has detected the linking pin  112  at the release position within the predetermined time for releasing the braking, the safety brake function unit  22 Fa causes a transition to the initial check completed state (“Check_OK” in  FIG. 24 ), causes the display screen of the liquid crystal monitor  32  to transition to the autonomous drive start screen  32 D illustrated in  FIG. 22 , and permits a transition of the drive mode from the manual drive mode to the autonomous drive mode. 
     In the brake release return operation state (“Rev_Start” in  FIG. 24 ), if the release switch  113  has detected the linking pin  112  at the release position and the braking switch  114  has detected the linking pin  112  at the braking position, or if the release switch  113  has not detected the linking pin  112  at the release position even after the predetermined time for releasing the braking has elapsed, the safety brake function unit  22 Fa determines that an error has occurred in any one of the electric motor  102 , the release switch  113 , and the braking switch  114 , and causes a transition to the error detection state. 
     In the brake release return operation state, if the release switch  113  has detected the linking pin  112  at the release position, and if the braking switch  114  has not detected the linking pin  112  at the braking position, the safety brake function unit  22 Fa causes a transition to the initial state (“Start” in  FIG. 24 ). 
     In the safety brake operation state (“Brake_Move” in  FIG. 24 ), if the release switch  113  detects the linking pin  112  at the release position even after a predetermined time for brake operation has elapsed, or if the braking switch  114  has not detected the linking pin  112  at the braking position, the safety brake function unit  22 Fa determines that an error has occurred in any one of the electric motor  102 , the release switch  113 , and the braking switch  114 , and causes a transition to the error detection state. 
     In the safety brake operation state, if the release switch  113  no longer detect the linking pin  112  at the release position, and if the predetermined time for brake operation has elapsed while the braking switch  114  has detected the linking pin  112  at the braking position, the safety brake function unit  22 Fa determines that the left and right brake pedals  40  have been operated to the maximum depression position and the left and right brakes  42  have switched to a braking state. Then, on the basis of the determination, the safety brake function unit  22 Fa causes a transition to a safety brake operation hold state (“Brake_Stop” in  FIG. 24 ), and instructs the steering control unit  22 C to stop the braking operation of the left and right brakes  42  by the operation of the electric motor  102 . In this way, the steering control unit  22 C ends the braking operation of the left and right brakes  42  by the operation of the electric motor  102 . As a result, the left and right brakes  42  can be held in an operated state as safety brakes, and the tractor  1  can be kept in the braking stop state. 
     As illustrated in  FIG. 25 , in the error detection state, the safety brake function unit  22 Fa causes a transition to an electric motor error state (“Err_Actuator” in  FIG. 25 ), an ON detection error state (Err_Position_ON in  FIG. 25 ), or an OFF detection error state (“Err_Position_Off” in  FIG. 25 ) in accordance with the detection states of the release switch  113  and the braking switch  114 . In any of the error states, if the transition is not from the safety brake operation state, the state transitions to a brake released state for error (“Err_Rev” in  FIG. 25 ), and the steering control unit  22 C is instructed to release the left and right brakes  42  by the operation of the electric motor  102 . In this way, the steering control unit  22 C releases the left and right brakes  42  by the operation of the electric motor  102 . Then, the left and right brakes  42  can be returned to the released state by the release operation, and the tractor  1  can be manually driven by the passenger. 
     If a predetermined time for a release operation elapses in the brake released state for error, the safety brake function unit  22 Fa causes a transition to an error state (“Err_Actuator” in  FIG. 25 ) and keeps this state. 
     In any one of the electric motor error state, the ON detection error state, and the OFF detection error state, the safety brake function unit  22 Fa causes a transition to the error state (“Err_Actuator” in  FIG. 25 ) if the transition is from the safety brake operation state, and keeps this state. In this way, the left and right brakes  42  can be kept in an operated state as safety brake in the above-stated error state, and the tractor  1  can be kept in a braking stop state. 
     OTHER EMBODIMENTS 
     Other embodiments of the present invention will now be described. 
     The configurations of the respective embodiments described below are not necessarily applied independently, but may be applied under combination with the configurations of the other embodiments. 
     (1) Another typical embodiment regarding the configuration of the work vehicle is as follows. 
     For example, the work vehicle may be configured in a semi-crawler specification with left and right crawlers as driving devices  10  and  11  in place of the left and right front wheels  10  and the left and right rear wheels  11 . 
     For example, the work vehicle may be configured in a full-crawler specification with left and right crawlers as driving devices  10  and  11  in place of the left and right front wheels  10  and the left and right rear wheels  11 . 
     For example, the work vehicle may be configured in an electric specification with an electric motor in place of the engine  14 . 
     For example, the work vehicle may be configured in a hybrid specification with the engine  14  and an electric motor. 
     For example, the work vehicle may be configured with a protective frame extending above tractor  1 , in place of the cabin  13 . 
     (2) Another typical embodiment regarding the configuration of the electric actuator  102  is as follows. 
     For example, the electric actuator  102  may be left and right electric motors  102  that individually operate the left and right brake pedals  40 . 
     For example, the electric actuator  102  may be a single electric cylinder that operates the left and right brake pedals  40  coupled by the coupling mechanism  55 . 
     For example, the electric actuator  102  may be left and right electric cylinders that individually operate the left and right brake pedals  40 . 
     (3) The foot brake  42  may be a single foot brake operated by a single brake pedal provided in the driver unit  12 . 
     (4) The operation tool  84  for brake release may be, for example, an operation button displayed on the liquid crystal monitor  32 . 
     INDUSTRIAL APPLICABILITY 
     The present invention may be applied to a work vehicle, such as a tractor, a riding mower, a riding rice transplanter, a combine, a carrier, a wheel loader, or a snowplow. 
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
         
           
               2  autonomous drive unit 
               3  wireless communication device (mobile communication terminal) 
               10  driving device (front wheel) 
               11  driving device (rear wheel) 
               12  driver unit 
               14  engine 
               22 Fa control unit (safety brake function unit) 
               23  vehicle condition detection device 
               32   a  mode selector (mode selection button) 
               42  foot brake (brake) 
               84  operation tool (key switch) 
               90  wireless communication device (emergency stop remote control) 
               102  electric actuator (electric motor)