Patent Publication Number: US-2021169000-A1

Title: Work machine

Description:
TECHNICAL FIELD 
     The present invention relates to a work machine that travels autonomously on the work site while performing a given task on the work site. 
     BACKGROUND ART 
     Work machines such as lawn mowers, snow blowers, etc. have a driving source such as a motor etc., and perform a given task using the rotational driving force of the driving source. In this case, the load acting on the motor during the task varies according to conditions of the work site (the lawn is thick, snow is hard, etc.). Accordingly, problems such as excessive electric power consumption, nonuniform operating performance, etc. arise if the motor is set to rotate at the same speed on work sites with lighter loads and work sites with heavier loads. 
     For this reason, the work machine (electric lawn mower) disclosed in Japanese Laid-Open Patent Publication No. 09-201126 is configured to increase and decrease the rotational speed of the motor in accordance with the load on the motor. That is, it can achieve power saving etc. by operating at lower speeds in areas with lighter loads, and achieve superior lawn mowing performance by operating at higher speeds in areas with heavier loads. 
     Such a work machine can be configured to autonomously travel with a motor, and such a self-propelled machine encounters the problem of nonuniform operating performance when it travels at the same travel speed in areas with lighter loads and areas with heavier loads. Accordingly, it is preferred that the work machine operates at suitably controlled travel speeds during the given task. 
     SUMMARY OF INVENTION 
     The present invention has been devised in order to solve the problems mentioned above, and an object of the present invention is to provide a work machine that can vary travel speed suitably according to the load applied to the work machine from the work site to thereby further facilitate reduction of nonuniform operating performance and reduction of power consumption. 
     In order to achieve the object above, a work machine according to the present invention includes: a main body unit including a work unit configured to perform a given task; a motor configured to generate a rotational driving force for at least causing the main body unit to travel; a travel operation unit configured to provide information concerning a rotational speed of the motor; and a control unit configured to change the rotational speed of the motor indicated by the travel operation unit according to variation in a load on the work unit. 
     In this case, the control unit may be configured to reduce the rotational speed of the motor based on an increase in the load. 
     Further, preferably, the motor includes a travel motor configured to cause the main body unit to travel, and a work motor provided for the work unit and configured to provide a motive power for performing the given task; and the control unit increases the rotational speed of the work motor based on an increase in the load. 
     In addition to the configuration above, the control unit may be configured to change a speed status between a low-speed status for rotating the work motor at a low speed, and a high-speed status for rotating the work motor at a high speed, and the control unit may change the speed status from the low-speed status to the high-speed status based on the load exceeding a first work threshold, and change the speed status from the high-speed status to the low-speed status based on the load falling below a second work threshold that is lower than the first work threshold. 
     Preferably, the control unit includes a travel judgment threshold that is compared with the load when slowing down the travel motor, and the travel judgment threshold is set higher than the first work threshold. 
     Further, the control unit may acquire information about a present status that is divided into “rotational speed being changed” indicating that the rotational speed of the work motor is being changed to a work target rotational speed, and “constant speed rotation” indicating that the rotational speed of the work motor is substantially equal to the work target rotational speed, and the control unit may allow a speed reduction of the travel motor when recognizing the “constant speed rotation”. 
     Here, the control unit may set a reference rotational speed based on information concerning the rotational speed of the motor, and when performing the changing of the rotational speed based on the load, the control unit may change the reference rotational speed to set a target travel rotational speed, based on a correspondence map indicating a speed ratio of the reference rotational speed that corresponds to the load. 
     Then, more preferably, the travel operation unit may be configured to change the reference rotational speed based on an operation performed by a user. 
     According to the present invention, in the work machine, the control unit varies the rotational speed of the travel motor indicated by the travel operation unit, according to variations in the load on the work unit. Thus, for example, when the load is heavy, the amount of work target processed per unit time can be reduced by slowing down the speed. As a result, the work machine can avoid a situation in which an excessive load acts on the work unit and can maintain the work in a preferable manner while preventing unexpected stall of the work unit. That is, the work machine can further facilitate reduction of nonuniform operating performance and reduction of power consumption in the given task on the work site. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a side view illustrating the overall configuration of a lawn mower being a work machine according to an embodiment of the present invention; 
         FIG. 2  is a block diagram illustrating the configuration of a work control unit of the lawn mower; 
         FIG. 3  is a block diagram illustrating the configuration of a travel control unit of the lawn mower; 
         FIG. 4  is a state transition diagram illustrating transitions of a present status of the work unit; 
         FIG. 5  is a graph showing an example of a correspondence map of the travel control unit; 
         FIG. 6  is a first flowchart illustrating a control process performed when a work motor is driven; 
         FIG. 7  is a second flowchart illustrating the control process performed when the work motor is driven; 
         FIG. 8  is a flowchart illustrating a control process performed when a travel motor is driven; and 
         FIG. 9  is a time chart showing a simulation of variations in work target rotational speed and travel target rotational speed based on variation in load. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The present invention will now be described in detail referring to the accompanying drawings in conjunction with preferred embodiments. 
     As shown in  FIG. 1 , a work machine  10  according to one embodiment of the present invention is configured as a lawn mower  12  that cuts and collects the grass growing on a work site. Hence, the work machine  10  will hereinafter be referred to also as the lawn mower  12 . The configuration of the work machine  10  according to the invention is not limited to the lawn mower  12 , but can be modified suitably to be applied to work machines  10  such as grass cutters, snow blowers, cultivators, etc. 
     The lawn mower  12  includes a main body unit  14 , a travel device  16  mounted in the main body unit  14  to cause the main body unit  14  to travel, and an operation unit  18  through which a user of the lawn mower  12  controls operations of the main body unit  14 . The main body unit  14  contains therein a work unit  20  configured to directly cut grass (to perform a given task), a battery  22  capable of supplying electric power to electric components of the lawn mower  12 , and a control unit  24  configured to perform control to drive the travel device  16  and the work unit  20 . 
     The main body unit  14  further includes a disc-shaped housing  26  formed relatively thick and constituting the external appearance of the main body unit  14 . In the main body unit  14 , the work unit  20  is disposed in a lower position in a center of the housing  26 , and the battery  22  is disposed in a higher position in the housing  26 . 
     The work unit  20  is exposed from the bottom of the main body unit  14  to face the ground of the work site at a certain interval therefrom. The work unit  20  includes a work motor  28 , a blade  30  that rotates with the rotational driving force of the work motor  28  transmitted thereto, a floating mechanism (not shown) that sucks up the grass cut by the blade  30 , and a grass bag  32  for storing the grass floated by the floating mechanism. 
     The work motor  28  is connected to the control unit  24  through a work inverter unit  34  (see  FIG. 2 ), and has an output shaft  28   a  protruding downward. The work inverter unit  34  converts the direct-current power supplied from the battery  22  into three-phase alternating current in accordance with a pulse signal of the control unit  24 , so as to rotate the output shaft  28   a  of the work motor  28 . The work inverter unit  34  varies conditions (rotational speed, torque, etc.) of the work motor  28  by suitably adjusting the phases of the three-phase alternating currents according to the pulse signal (pulse width). 
     The blade  30  of the work unit  20  is fixed to the output shaft  28   a  and extends in a direction perpendicular to the output shaft  28   a  (substantially parallel to the ground of the work site). The blade  30  cuts grass by rotating integrally with the output shaft  28   a . The blade  30  constitutes part of the floating mechanism, and causes an air current as it rotates to suck up the cut grass. Preferably, the blade  30  is configured in such a manner that its height with respect to the work site can be adjusted. 
     The travel device  16  of the lawn mower  12  includes a travel motor  36 , a pair of front wheels  38  and a pair of rear wheels  40  mounted on the sides of the main body unit  14  in the width direction, and a transmission mechanism unit  42  for transmitting the rotational driving force of the travel motor  36  to given wheels (the rear wheels  40  in this embodiment) to rotate the given wheels. 
     The travel motor  36  is disposed at the rear of the work motor  28  in the main body unit  14  and has an output shaft (not shown) projecting toward the transmission mechanism unit  42 . The travel motor  36  is also connected to the control unit  24  through a travel inverter unit  44  (see  FIG. 3 ). The travel inverter unit  44  converts the direct-current power supplied from the battery  22  into three-phase alternating current according to a pulse signal of the control unit  24 , to thereby rotate the output shaft of the travel motor  36 . The travel inverter unit  44  varies conditions (travel speed etc.) of the travel motor  36  by suitably adjusting the phases of the three-phase alternating currents according to the pulse signal (pulse width). 
     The transmission mechanism unit  42  includes gears, pulleys, belts, etc., and suitably converts the rotational driving force of the output shaft of the travel motor  36  to rotate a rear-wheel shaft  40   a.    
     The lawn mower  12  of this embodiment includes two motors  46  (work motor  28  and travel motor  36 ) that generate motive power. The operation (given task) of the work unit  20  is implemented using the work motor  28  and the travel device  16  is traveled using the travel motor  36 . The lawn mower  12  is not limited to this configuration but may employ a configuration in which the given task and traveling are implemented using one motor  46 . In this case, a transmission mechanism (not shown) having a clutch function is preferably connected to the output shaft of the one motor  46  so that the transmission mechanism transmits rotational driving forces that have been converted to rotational speeds suitable respectively for the work unit  20  and the travel device  16 . 
     The operation unit  18  includes: a pair of bars  48  that have their ends on one side connected to the main body unit  14 , extend backward from the ends in an obliquely upward direction, and are connected together at the other ends (upper ends); and an input device  50  provided at that other end side of the bars  48  to allow the user to enter operational information. The input device  50  includes a work operation unit  52  (see  FIG. 2 ) corresponding to the work of mowing lawns (i.e. the work unit  20 ) and a travel operation unit  54  (see  FIG. 3 ) corresponding to traveling of the travel device  16 . 
     The work operation unit  52  includes a work operation switch (not shown) for switching the work unit  20  between driving and stoppage (rotation and stoppage of the work motor  28 ). The lawn mower  12  may be configured in such a manner that the rotational speed of the work motor  28  can be adjusted by an operation performed by the user, and the work operation unit  52  may include a work speed changing switch (not shown). 
     The travel operation unit  54  includes a travel operation switch for switching between rotation and stoppage of the travel motor  36 , and a speed adjusting switch for setting the travel speed of the travel device  16  (neither of which are shown). The speed adjusting switch changes the travel speed stepwise according to an operation performed by the user, like low speed, middle speed, high speed, etc., for example. 
     The control unit  24  is a computer having a processor, memory, input/output interface (none of which are shown), where the processor processes a program stored in the memory as the lawn mower  12  starts, and constructs functional units therein. Specifically, the control unit  24  constructs a work control unit  60  for controlling operations of the work unit  20  and a travel control unit  80  for controlling operations of the travel device  16 , and thereby controls the work unit  20  and the travel device  16  in a mutually linked manner. 
     During operations of the work unit  20 , the work control unit  60  performs a control to increase or decrease the rotational speed (the number of rotations) of the work motor  28  on the basis of the load on the blade  30  (i.e. on the work motor  28 ) applied from the lawn being the target of work. For example, when the load on the work motor  28  is light, it suppresses power consumption of the battery  22  by reducing the rotational speed of the work motor  28 . On the other hand, when the load on the work motor  28  is heavy, it increases the amount of lawn processed (work target) by increasing the rotational speed of the work motor  28 . The work control unit  60  thus enables the work unit  20  to operate with improved energy efficiency. 
     For this purpose, as shown in  FIG. 2 , the work control unit  60  includes therein a work driving judging unit  62 , a work rotational speed setting unit  64 , a work driving instructing unit  66 , a torque acquisition unit  68 , a torque judging unit  70 , and a status monitoring unit  72 , and operates as the work operation switch is turned on. The work control unit  60  constructs a work storage area  74  in the memory of the control unit  24 , and various information used during operations of the work unit  20  is stored in the work storage area  74 . 
     The work driving judging unit  62  obtains, from the work operation unit  52 , a signal indicating whether the work operation switch is on or off according to an operation by the user, and determines to drive or stop the work motor  28 . In the case of on, the work driving judging unit  62  reads a default rotational speed (not shown) of the work motor  28  that is stored in the work storage area  74  and provides it to the work rotational speed setting unit  64 . When the work operation unit  52  is configured to adjust the speed, the work driving judging unit  62  reads a suitable default rotational speed based on a signal indicating the speed, from among multiple default rotational speeds stored in the work storage area  74 . 
     The work rotational speed setting unit  64  is a functional unit configured to set a work target rotational speed, which is the target value of the rotational speed of the work motor  28 , and output the work target rotational speed to the work driving instructing unit  66 . For example, at the time of start, the work rotational speed setting unit  64  sets, as the work target rotational speed, the default rotational speed provided from the work driving judging unit  62 . After the start, the work rotational speed setting unit  64  sets a work target rotational speed stored in the work storage area  74 . 
     That is, the work rotational speed setting unit  64  changes the work target rotational speed on the basis of the result of determination made by the torque judging unit  70  described later, and stores the changed work target rotational speed in the work storage area  74 . The operation of changing the work target rotational speed will be described later in detail. 
     The work driving instructing unit  66  functions as a driver that gives driving instructions (outputs a pulse signal) to the work inverter unit  34 . Further, in this embodiment, the work driving instructing unit  66  also performs a feedback control with respect to the work motor  28  so as to control the rotational driving in accordance with the work target rotational speed. That is, the work driving instructing unit  66  outputs, to the work inverter unit  34 , a driving instruction based on the work target rotational speed, and also detects the actual rotational speed of the work motor  28 . The work driving instructing unit  66  then determines whether the actual rotational speed and the work target rotational speed agree or disagree, and performs control so that the actual rotational speed comes close to the work target rotational speed. 
     Now, as has been mentioned earlier, the work site where grass grows exerts different loads on the work motor  28  depending on various factors such as the density of grass, degree of growth, conditions of the work site, etc. Accordingly, the lawn mower  12  of this embodiment is configured to change the target rotational speed of the work unit  20  on the basis of the load. That is, when a lighter load acts on the work motor  28  (when resistance from the lawn etc. is smaller), the lawn mower  12  stably rotates the work motor  28  according to the work target rotational speed. On the other hand, when a heavier load acts on the work motor  28  (when resistance from the lawn etc. is larger), then it is hard to follow the work target rotational speed and so the lawn mower  12  performs a process of increasing the work target rotational speed itself to enhance the working capacity of the work unit  20 . 
     Specifically, the lawn mower  12  includes an ammeter  76  and a voltmeter  78  for detecting the electric power (dc current and dc voltage) supplied from the battery  22  to the work inverter unit  34 . The ammeter  76  and the voltmeter  78  respectively output, to the work control unit  60 , a current value and a voltage value detected when the work motor  28  is driven. Then, the torque acquisition unit  68  of the work control unit  60  receives the current value and the voltage value from the ammeter  76  and the voltmeter  78  and calculates the torque (i.e. load) acting on the work motor  28  according to a given calculation method. Since the torque on the work motor  28  is substantially linked to the current value, the torque acquisition unit  68  may be configured to acquire only the current value to calculate the torque. 
     Then, based on the torque acquired by the torque acquisition unit  68 , the torque judging unit  70  of the work control unit  60  determines whether to change the rotational speed of the work motor  28 . The torque judging unit  70  reads a work threshold To previously stored in the work storage area  74  and compares the acquired torque and the work threshold To. 
     In this embodiment, when the load is light, the work control unit  60  (work rotational speed setting unit  64 ) sets a low-speed rotation to operate the work motor  28  at a low target rotational speed, and when the load is heavy, it sets a high-speed rotation to operate the work motor  28  at a high target rotational speed. That is, the state of the speed of the work motor  28  includes two states: a low-speed status LS and a high-speed status HS. 
     Also, the work threshold To includes a first work threshold To 1  for determining whether to increase the target rotational speed when the work motor  28  is in the low-speed status LS, and a second work threshold To 2  for determining whether to decrease the target rotational speed when the work motor  28  is in the high-speed status HS. That is, when the work motor  28  is at a low speed, the torque judging unit  70  makes a determination using the first work threshold To 1  between a first pattern in which the work motor  28  is kept at the low-speed rotation and a second pattern in which the work motor  28  is brought to the high-speed rotation. On the other hand, when the work motor  28  is at a high speed, the torque judging unit  70  makes a determination using the second work threshold To 2  between a third pattern in which the work motor  28  is kept at the high-speed rotation and a fourth pattern in which the work motor  28  is brought to the low-speed rotation. 
     For this purpose, the work control unit  60  includes the status monitoring unit  72  configured to monitor the present speed state of the work motor  28 . The status monitoring unit  72  stores the low-speed status LS and high-speed status HS indicating the present speed state (speed status SS) in the work storage area  74 , and reads and sends the speed status SS to the torque judging unit  70  when judging the torque. Then, the torque judging unit  70  suitably reads the first or second work threshold To 1 , To 2  and compares the threshold with the acquired torque. 
     The torque judging unit  70  outputs, to the work rotational speed setting unit  64 , the result of determination from among the first to fourth patterns. On the basis of the result of determination from among the first to fourth patterns, the work rotational speed setting unit  64  maintains or changes (increases or decreases) the work target rotational speed of the work motor  28 . That is, in the case of the first pattern, the work rotational speed setting unit  64  does not change the work target rotational speed (maintains low-speed state). In the case of the second pattern, the work rotational speed setting unit  64  increases the work target rotational speed (changes from low-speed state to high-speed state). In the case of the third pattern, the work rotational speed setting unit  64  does not change the work target rotational speed (maintains high-speed state). In the case of the fourth pattern, the work rotational speed setting unit  64  decreases the work target rotational speed (changes from high-speed state to low-speed state). 
     Returning to  FIG. 1 , the travel control unit  80  of the lawn mower  12  performs a control to increase or decrease the travel speed of the main body unit  14  (the rotational speed of the travel motor  36 ) that is realized by the travel device  16 , on the basis of the load acting on the work motor  28  during the operation of the work unit  20 . For example, when the load on the work motor  28  is light, the travel speed of the travel device  16  is kept at a reference speed (reference rotational speed), whereby the work unit  20  is moved so as to efficiently cut the lawn on the work site. On the other hand, when the load on the work motor  28  is heavy, the travel speed of the travel device  16  is lowered below the reference rotational speed, thus slowing down the movement of the work unit  20  to reduce the amount of lawn processed per unit time. In particular, the lawn mower  12  of this embodiment maintains or increases/decreases the reference rotational speed specified by the user in such a manner that the reference rotational speed is also liked to the rotational speed of the work motor  28 . 
     For this purpose, as shown in  FIG. 3 , the travel control unit  80  includes therein a reference rotational speed acquisition unit  82 , a work torque acquisition unit  84 , a work state acquisition unit  86 , a work content judging unit  88 , a travel speed setting unit  90 , and a travel driving instructing unit  92 , and operates as the travel operation switch is turned on. The travel control unit  80  constructs a travel storage area  94  in the memory of the control unit  24 , and various information used during operations of the travel device  16  is stored in the travel storage area  94 . 
     The reference rotational speed acquisition unit  82  acquires, from the travel operation unit  54 , information about the reference rotational speed of the travel motor  36  that has been set by the user. As mentioned earlier, the travel device  16  can set the travel speed at multiple levels (e.g., low-speed travel, middle-speed travel, high-speed travel) as the user operates the speed adjusting switch. Hence, upon receiving information about the speed level from the travel operation unit  54 , the reference rotational speed acquisition unit  82  refers to a speed reference map (not shown) held in the travel storage area  94 , extracts a rotational speed of the travel motor  36  associated with the speed level, and acquires the extracted rotational speed as the reference rotational speed. 
     The work torque acquisition unit  84  acquires the torque (load) on the work motor  28  from the work control unit  60 . As explained earlier, the work control unit  60  calculates the torque acting on the work motor  28  during operation and sends the information about the torque to the travel control unit  80  in response to a request from the work torque acquisition unit  84 . 
     Further, the work state acquisition unit  86  acquires a present status PS of the work motor  28  from the work control unit  60 . The present status PS differs from the speed status SS (low-speed status LS, high-speed status HS) that is information indicating the target of the speed of the work motor  28 , and the present status PS is information indicating the operation that the work motor  28  is performing when the information is sent to the work state acquisition unit  86 . 
     As shown in  FIG. 4 , for example, the present status PS includes “accelerating” indicating that the rotation of the work motor  28  is being accelerated toward the work target rotational speed (default rotational speed) as the work motor  28  at a standstill is started. The present status PS further includes “constant speed rotation” indicating that the work motor  28  has reached the work target rotational speed and the speed is kept constant (feedback control only is being performed). The present status PS further includes “target rotational speed being changed” indicating that the work target rotational speed has been changed from the constant speed rotation and the speed of the work motor  28  is being increased or decreased. In addition, this embodiment is configured to limit situations where a rapid increase in the rotational speed continues for a certain time period during acceleration of the work motor  28 , in order to suppress power consumption of the battery  22 . Accordingly, the present status PS further includes “increase being restrained” indicating that the increase in the rotational speed is being restrained. 
     That is, the present status PS includes a status PS 0  indicating “standstill”, a status PS 1  indicating “accelerating”, a status PS 2  indicating “constant speed rotation”, a status PS 3  indicating “target rotational speed being changed”, and a status PS 4  indicating “increase being restrained”. The present status varies from PS 0  to PS 1  when the work motor  28  is started and the rotational speed is raised toward the target rotational speed. The present status varies from PS 1  to PS 2  when the target rotational speed of the work motor  28 , an instructed rotational speed given to the work inverter unit  34 , and the actual rotational speed of the work motor  28  are equal (when condition 1 holds). The present status varies from PS 2  to PS 3  when the target rotational speed of the work motor  28  is changed. On the other hand, the present status varies from PS 3  to PS 2  when the condition 1 above holds. 
     Also, the work control unit  60  includes a timer (not shown) for measuring the time during which the rotational speed of the work motor  28  is being raised. The present status varies from PS 1  to PS 4  when the measured time has exceeded a rotational speed judgment time (when condition 2 holds). The present status varies from PS 4  to PS 3  when the target rotational speed of the work motor  28  is changed. On the other hand, the present status varies from PS 3  to PS 4  when the condition 2 holds. When the work motor  28  is stopped, the work control unit  60  sets the target rotational speed of the work motor  28  to zero, and the present status varies from PS 2  or PS 4  to PS 0 . 
     While the work motor  28  is being driven, the work control unit  60  (status monitoring unit  72 ) shown in  FIG. 3  monitors the present status PS, as well as the speed status SS. Then, with suitable timing, the work control unit  60  sends the monitored information (present status PS) to the work state acquisition unit  86  of the travel control unit  80 . 
     That is, when the lawn mower  12  changes the travel speed according to the load on the work motor  28 , changing the speed while the rotational speed of the work motor  28  is being changed will cause the operation to vary violently. For example, when the work motor  28  is set at higher speed to cut a sufficient amount of lawn, slowing down the travel speed will immediately reduce the load, causing the work motor  28  to return to lower speed and the travel speed of the travel device  16  to go back to the previous speed, which will increase the load again (the same operation may be repeated). Accordingly, the operation of changing the travel speed is not performed while the target rotational speed of the work motor  28  is being changed (during acceleration, during deceleration: during “target rotational speed being changed”), but is performed while the rotational speed of the work motor  28  is stable. As a result, when the load on the work motor  28  is heavy, the control unit  24  can deal with the heavy load by bringing the work motor  28  to the high-speed status HS, and then reduce the travel speed of the travel device  16  if the load still remains heavy thereafter. 
     The work content judging unit  88  of the travel control unit  80  is a functional unit that judges the states of the work unit  20  described above. On the basis of the present status PS acquired by the work state acquisition unit  86 , the work content judging unit  88  first judges whether it is the right time to change the speed of the travel device  16 . That is, the work content judging unit  88  determines it is not the right time to change the travel speed when the present status PS is PS 0 , PS 1 , PS 3 , and allows change of the travel speed when the present status PS is PS 2 , PS 4 . 
     Then, when the present status PS is PS 2 , PS 4 , the work content judging unit  88  reads a travel judgment threshold Tr stored in the travel storage area  94  and compares the acquired torque and the travel judgment threshold Tr. The travel judgment threshold Tr has a higher value than the work threshold To (first work threshold To 1 ). Hence, when the load on the work motor  28  is heavy, a process of bringing the work motor  28  to the high-speed status HS can be performed in light- and middle-load regions, and further, a process of slowing down the travel motor  36  can be performed in a heavy-load region. 
     Then, when the torque on the work motor  28  is lower than the travel judgment threshold Tr, the work content judging unit  88  outputs, to the travel speed setting unit  90 , an instruction to maintain the travel speed. On the other hand, if the torque on the work motor  28  is higher than the travel judgment threshold Tr, then the work content judging unit  88  outputs, to the travel speed setting unit  90 , an instruction to reduce the torque on the work motor  28  and the travel speed. 
     Basically (in the absence of the instruction to reduce the travel speed from the work content judging unit  88 ), the travel speed setting unit  90  sets, as the travel target rotational speed of the travel motor  36 , the reference rotational speed of the travel motor  36  acquired by the reference rotational speed acquisition unit  82 . When the instruction to reduce the travel speed is issued from the work content judging unit  88 , the travel speed setting unit  90  then reads a correspondence map  96  previously stored in the travel storage area  94  and performs a process of reducing the reference rotational speed on the basis of the correspondence map  96 . 
     As shown in  FIG. 5 , the correspondence map  96  can be represented as a graph in which the torque on the work motor  28  and a speed ratio of the travel motor  36  are associated with each other, for example. That is, when the torque is below or at a first value A 1  in the correspondence map  96 , the travel target rotational speed is set to 100% (kept at the reference rotational speed). When the torque is in the range from the first value A 1  to a second value A 2  in the correspondence map  96 , the travel target rotational speed is set by linearly reducing the reference rotational speed at a reduction ratio of from 100% to 50%. When the torque is larger than the second value A 2 , the reference rotational speed lowered to 50% is set as the travel target rotational speed. 
     Having set the travel target rotational speed based on the reference rotational speed and correspondence map  96 , the travel speed setting unit  90  outputs the setting data to the travel driving instructing unit  92 . The travel driving instructing unit  92  functions as a driver that gives a driving instruction (outputs a pulse signal) to the travel inverter unit  44 . Further, in this embodiment, the travel driving instructing unit  92  performs a feedback control with respect to the travel motor  36  so as to control the rotational driving in accordance with the travel target rotational speed. That is, the travel driving instructing unit  92  detects the actual rotational speed of the travel motor  36 , in addition to outputting the driving instruction based on the travel target rotational speed to the travel inverter unit  44 . Then whether the actual rotational speed and the travel target rotational speed agree or disagree is determined, and control is performed so that the actual rotational speed comes close to the travel target rotational speed (so that they agree with each other substantially). 
     The lawn mower  12  (work machine  10 ) of this embodiment is basically constructed as described above. Next, operations thereof will be described. 
     When the user starts the work machine  10 , the work machine  10  performs the given task by causing the work unit  20  to generate a rotational driving force, and autonomously travels on the work site by the travel device  16 . In the work unit  20 , the work motor  28  operates rotationally in a suitable manner under the control of the work control unit  60 , thereby cutting and collecting the grass on the work site. 
     As shown in  FIG. 6 , when the control unit  24  starts, the work driving judging unit  62  of the work control unit  60  determines whether the work operation switch is on (step S 10 ). Then, if the work operation switch is on (step S 10 : YES), the work rotational speed setting unit  64  of the work control unit  60  performs a process of acquiring the work target rotational speed of the work motor  28  (step S 11 ). 
     In the process of acquiring the target rotational speed, if it is at the start of the work motor  28 , a default rotational speed previously stored in the work storage area  74  is set as the target rotational speed. On the other hand, in the course of operation of the work motor  28 , the target rotational speed (information of the speed status SS) that was set last time is read from the work storage area  74  and set as the target rotational speed. 
     The work driving instructing unit  66  performs the feedback control to cause the actual rotational speed of the work motor  28  to follow the work target rotational speed set by the work rotational speed setting unit  64  (step S 12 ). Then, while the work motor  28  is driven, the torque acquisition unit  68  of the work control unit  60  acquires the current value and the voltage value of the work motor  28  and calculates the torque (step S 13 ). The calculated torque is output to the torque judging unit  70 . 
     Further, the status monitoring unit  72  of the work control unit  60  monitors the low-speed status LS or high-speed status HS of the work motor  28  while the work motor  28  is driven, and provides the torque judging unit  70  with information of the speed status SS to which the work motor  28  is set at present. The torque judging unit  70  judges the speed status SS (step S 14 ) and reads one of the first and second work thresholds To 1 , To 2 . 
     That is, when the speed status SS is judged as the low-speed status LS (step S 14 : YES), the torque judging unit  70  reads out the first work threshold To 1  and compares the first work threshold To 1  with the torque (step S 15 ). Then, if the first work threshold To 1  is larger than the torque (step S 15 : YES), the torque judging unit  70  chooses to keep the low-speed status LS of the work motor  28 , and the work rotational speed setting unit  64  stores the present work target rotational speed (low-speed status LS) in the work storage area  74  (step S 16 ). Accordingly, in step S 11  in the next processing flow, the work control unit  60  reads this work target rotational speed. 
     On the other hand, when the first work threshold To 1  is equal to or smaller than the torque (step S 15 : NO), it chooses to change from the low-speed status LS to high-speed status HS, and the work rotational speed setting unit  64  stores the work target rotational speed of the high-speed status HS in the work storage area  74  (step S 17 ). Accordingly, in step S 11  in the next processing flow, the work control unit  60  reads the work target rotational speed that has been changed to the high-speed status HS. 
     Meanwhile, as shown in  FIG. 7 , when the torque judging unit  70  judges the speed status SS as the high-speed status HS (step S 14 : NO), it reads out the second work threshold To 2  and compares the second work threshold To 2  with the torque (step S 18 ). Then, if the second work threshold To 2  is larger than the torque (step S 18 : YES), the torque judging unit  70  chooses to change from the high-speed status HS to low-speed status LS, and the work rotational speed setting unit  64  stores the work target rotational speed of the low-speed status LS in the work storage area  74  (step S 19 ). 
     On the other hand, when the second work threshold To 2  is equal to or smaller than the torque (step S 18 : NO), the torque judging unit  70  chooses to keep the high-speed status HS of the work motor  28 , and the work rotational speed setting unit  64  stores the present work target rotational speed (high-speed status HS) in the work storage area  74  (step S 20 ). 
     Then, returning to  FIG. 6 , after the processes of steps S 16 , S 17 , S 19 , S 20 , the work control unit  60  returns to start and repeats the same processing thereafter. 
     When the work operation switch is not on or switched from on to off (step S 10 : No), the work control unit  60  chooses a process of stopping the rotation of the work motor  28  (step S 21 ). In the rotation stopping process, the work rotational speed setting unit  64  changes the target rotational speed of the work motor  28  to zero, whereby the work driving instructing unit  66  stops the rotation of the work motor  28  (step S 22 ). Accordingly, after the processing of step S 22 , the process returns to start as in the cases of after steps S 16 , S 17 , S 19 , S 20 , but the setting of “target rotational speed=0” is maintained unless the work operation switch is on. That is, the work control unit  60  can stop the rotational driving of the work unit  20 . 
     Further, the work unit  20  rotationally drives the travel motor  36  in a suitable manner under the control of the travel control unit  80 , so as to move the main body unit  14 . As shown in  FIG. 8 , when the control unit  24  starts, the travel control unit  80  determines whether the travel operation switch is on (step S 30 ). 
     If the travel operation switch is on (step S 30 : YES), the reference rotational speed acquisition unit  82  of the travel control unit  80  extracts a reference rotational speed on the basis of the speed that the user has set from the speed adjusting switch (step S 31 ). Also, the work torque acquisition unit  84  acquires the torque from the work control unit  60  (step S 32 ), and the work state acquisition unit  86  acquires the present status PS from the work control unit  60  (step S 33 ). 
     Then, the work content judging unit  88  judges whether or not the acquired present status PS is status PS 2  or PS 4  (step S 34 ). If the present status PS is status PS 2  or PS 4  (step S 34 : YES), the process moves to step S 35 . If the present status PS is status PS 0 , PS 1 , or PS 3  (step S 34 : NO), the process moves to step S 38  skipping steps S 35  to S 37 . 
     At step S 35 , the work content judging unit  88  reads the travel judgment threshold Tr from the travel storage area  94  and compares the threshold with the acquired torque. That is, the work content judging unit  88  judges whether or not the torque is equal to or larger than the travel judgment threshold Tr. If the torque is equal to or larger than the travel judgment threshold Tr, the process moves to step S 36 . If the torque is smaller than the travel judgment threshold Tr, the process moves to step S 38  skipping steps S 36 , S 37 . 
     At step S 36 , the travel speed setting unit  90  reads the correspondence map  96  from the travel storage area  94  and extracts the speed ratio of the reference rotational speed on the basis of the correspondence map  96  and the torque. The travel speed setting unit  90  then multiplies the reference rotational speed by the extracted speed ratio to set the travel target rotational speed of the travel motor  36  (step S 37 ). 
     Then, the travel driving instructing unit  92  performs the feedback control to cause the actual rotational speed of the travel motor  36  to follow the travel target rotational speed set by the travel speed setting unit  90  (step S 38 ). After step S 38 , the process returns to start and the same processing is repeated thereafter. 
     When the work operation switch is not on or is switched from on to off (step S 30 : No), the travel control unit  80  chooses a process of stopping the rotation of the travel motor  36  (step S 39 ). In the rotation stopping process, the travel speed setting unit  90  changes the travel target rotational speed of the travel motor  36  to zero, whereby the travel driving instructing unit  92  stops the rotation of the work motor  28  (step S 40 ). Accordingly, as in step S 38 , the process returns to start after the processing of step S 40 , but the setting of “travel target rotational speed=0” is maintained unless the travel operation switch is on. That is, the travel control unit  80  can stop the travel of the travel device  16 . 
     Through the processes above, the lawn mower  12  can suitably deal with the load applied to the work motor  28  from the work site through the operations of both the work unit  20  and the travel device  16 . A simulation based on the processing flow above was performed on the target rotational speed of the work motor  28  and the target rotational speed of the travel motor  36  with varying load.  FIG. 9  shows the results. 
     In  FIG. 9 , the horizontal axis shows time and the waveforms on the vertical axis show the torque on the work motor  28 , the work target rotational speed of the work motor  28 , and the travel target rotational speed of the travel motor  36  in this order. When a high torque acts on the work motor  28  before time t 1 , the work target rotational speed of the work motor  28  starts rising (moves to the high-speed status HS). At time t 2  at which the increase in speed of the work motor  28  is once restrained (the “increase being retrained” in  FIG. 4 : status PS 4 ), the travel target rotational speed of the travel motor  36  decreases. 
     Then, when the torque starts falling at time t 3 , the travel motor  36  increases the travel target rotational speed of the travel motor  36  in response to the variation in the torque. Further, when the torque falls below the second work threshold To 2  at time t 4 , the work target rotational speed of the work motor  28  also falls (moves to the low-speed status LS). From time t 5  to time t 6 , when the torque rises only for a short period, the work target rotational speed of the work motor  28  rises but the speed of the travel motor  36  is not reduced. This is because the torque will decrease while the work target rotational speed of the work motor  28  is rising. 
     At time t 7 , when the torque rises again and keeps rising for a certain period, the work target rotational speed rises and the travel target rotational speed decreases. Further, from time t 8  to t 9 , when the torque varies up and down without falling to the second work threshold To 2 , the travel target rotational speed varies following the torque, with the work target rotational speed remaining approximately constant. 
     As described so far, according to the work machine  10  (lawn mower  12 ) of this embodiment, the control unit  24  varies the rotational speed (reference rotational speed) of the travel motor  36  (motor  46 ) indicated by the travel operation unit  54 , according to variations in the load (torque) on the work unit  20 . Thus, for example, when the load is heavy, the amount of lawn, which is the target of work, processed per unit time can be reduced by slowing down the speed. As a result, the lawn mower  12  can avoid a situation in which an excessive load acts on the work motor  28  (clogging with grass etc.) and can maintain the work in a preferable manner while preventing unexpected stall of the work motor  28 . That is, the lawn mower  12  can further facilitate reduction of nonuniform operating performance and reduction of power consumption in the operation of mowing lawn on the work site. 
     In particular, the control unit  24  reduces the rotational speed of the travel motor  36  (motor  46 ) in response to increasing load. This allows the work unit  20  to travel slower and so more reliably perform the work in a smaller work area. 
     In this case, the lawn mower  12  having the separate work motor  28  and travel motor  36  can control the individual motors more finely. Then, with the configuration in which the rotational speed of the work motor  28  is increased in response to increasing load, the control unit  24  can increase the amount of lawn processed per unit time when the load is heavy, enabling the work motor  28  to operate under more energy efficient conditions. 
     Furthermore, the control unit  24  changes the speed status from the low-speed status LS to high-speed status HS on the basis of the first work threshold To 1 , and changes the speed status from the high-speed status HS to low-speed status LS on the basis of the second work threshold To 2  that is lower than the first work threshold To 1 . This allows the lawn mower  12  to maintain the low-speed status LS and high-speed status HS even when the load varies to some degree. 
     Furthermore, since the travel judgment threshold Tr is higher than the first work threshold To 1 , the lawn mower  12  can first deal with the load by rotating the work motor  28  at high speed, and then slow down the main body unit  14  if the load still remains heavy, to thereby reduce the load on the work unit  20 . This allows the lawn mower  12  to perform the work more efficiently. 
     Moreover, by acquiring the present status PS of the work motor  28  and then slowing down the travel motor  36  during constant speed rotation, the control unit  24  can implement the transition of the work motor  28  to the high-speed status HS and the speed reduction of the travel motor  36  at different points of time. This prevents unstable operations of the work motor  28  and the travel motor  36  that would be caused by variations in the load. 
     Further, the control unit  24  can keep the work unit  20  working without considerably reducing the amount of processing of the work unit  20 , by setting the reference rotational speed to a target travel rotational speed that is suitable for the load by referring to the correspondence map  96 . 
     Furthermore, since the lawn mower  12  can vary the reference rotational speed on the basis of an operation performed by the user, the lawn can be suitably mowed according to the user&#39;s taste and requirements. Moreover, since the target travel rotational speed varies automatically depending on the load, the lawn mower  12  can operate suitably even in places with heavy loads without leaving the lawn unmown, for example. 
     The present invention is not limited to the embodiment described above and can be modified in various manners according to the essence and gist of the present invention.