Patent Publication Number: US-2020282969-A1

Title: Traveling Vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Patent Application No. 2019-040292 filed Mar. 6, 2019, the disclosure of which is hereby incorporated by reference in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to a traveling vehicle capable of driving a pair of left and right driving wheels independently of each other. 
     BACKGROUND ART 
     Conventionally, there has been used a traveling vehicle having a pair of left and right drive wheels that are driven independently of each other by a pair of left and right motors. As such traveling vehicle, there is one in which PI (Proportional-Integral) control is employed for controlling currents running in the motors. The PI control is known, so its detailed explanation is omitted herein. But, PI control effects control by combining the so-called proportional operation and the integral operation. A technique for controlling driving of a vehicle with utilization of PID (Proportional-Integral-Differential) control developed from such PI control is known from e.g. Patent Document 1 identified below. 
     Patent Document 1 discloses a control device for a vehicle which controls a current of a motor for driving wheels of the vehicle by the PID control. This control device includes an integral term correction gain calculation section. The integral term correction gain calculation section calculates an integral term correction gain for correction of an output of an integral term of the PID control in order to be able to stabilize the behavior of the vehicle even in the event of occurrence of “wobbling” in steering. Such integral term correction gain is calculated based on a steering angle calculated from a yaw rate and a steering angle provided by the driver. The yaw rate is obtained from a curvature of the traveling path and a vehicle speed. In particular, the integral term correction gain is calculated based on a differential value obtained by time differentiation of result of comparison between the steering angle calculated from the yaw rate and the driver&#39;s steering angle. And, this control device is configured such that the greater the differential value, the smaller the integral term correction gain. 
     PRIOR ART DOCUMENT 
     Patent Document
     Patent Document 1: JP2017-132406A   

     DISCLOSURE OF THE INVENTION 
     Problem to be Solved by Invention 
     For instance, some traveling vehicles are configured such that a small turning travel is made possible by rotating one of the pair of left and right motors and stopping the other and causing it to spin-turn at its present position. When such small-turn traveling is to be carried out, an operational position of an operational lever for giving a driver&#39;s instruction to one motor is often a position corresponding to a low-speed instruction near the zero speed. In such case, in the technique disclosed in Patent Document 1, if an integral term correction gain is set based on a differential value obtained by the time differentiation of the result of comparison between a steering angle calculated from a yaw rate and a steering angle provided by the driver, there is possibility of the driving wheels may be locked at the zero speed or at an extremely low speed. As a result, a feeling of discomfort or “wrongness” may be provided to the user. 
     In view of the above, there is a need for a traveling vehicle capable of reducing such feeling of discomfort or wrongness to the user. 
     Means for Solving Problem 
     According to a characterizing feature of a traveling vehicle relating to the present invention, the traveling vehicle comprises: 
     a pair of left and right driving wheels; 
     a pair of left and right motors capable of driving the pair of left and right driving wheels independently of each other; 
     an operational amount reception section capable of receiving operational amounts respectively for the pair of left and right motors; 
     a setting section capable of setting a period of a first control method having a predetermined first control parameter and a period of a second control method having a predetermined second control parameter, within a predetermined control cycle for controlling driving of the motors, in accordance with the operational amounts; and a motor driving section capable of driving the respective motors based on result of setting made by the setting section. 
     With the above-described characterizing arrangement, in case the motors are controlled by a control method having set control parameters (a control gain, various limit values, switchover of control mode or algorithm, maps, ON/OFF of energization of motors, etc.), even if the user carries out an acceleration or deceleration, it is possible to improve discontinuity in the control parameters (in particular, a gain) at the time of switchover of the control method. Accordingly, in accordance with the operational amounts received by the operational amount reception section and without giving feeling of discomfort or wrongness to the user, the traveling vehicle is allowed to travel smoothly. 
     Preferably, the setting section progressively decreases a ratio of one of the period of the first control method and the period of the second control method within the control cycle from 100% to 0% and progressively increases a ratio of the other of the period of the first control method and the period of the second control method within the control cycle from 0% to 100%. 
     With the above-described arrangement, in association with the progressive reduction of the ratio of one of the period of the first control method and the period of the second control method within the control cycle, the ratio of the other of the period of the first control method and the period of the second control method within the control cycle is progressively increased. With this, the control cycle is caused to undergo an intermediate (transitional) state between the state of the control cycle consisting entirely of the period of the first control method and the state of the control cycle consisting entirely of the period of the second control method. Therefore, irregularity in the control parameter at the time of switchover of the control method can be alleviated, thus realizing smooth switchover of the control method. Moreover, with this above-described arrangement, change of control parameter is possible in a manner similar to that of the known DUTY control, so the control can be carried out easily. 
     Further, preferably, the setting section sets the period of the first control method within the control cycle to zero when a rotational speed of the motor is less than a predetermined rotational speed and sets the period of the second control method within the control cycle to zero when the rotational speed of the motor is equal to or greater than the predetermined rotational speed. 
     With the above-described arrangement, when the control method is switched over in accordance with a rotational speed, smooth traveling is made possible without giving feeling of discomfort or wrongness to the user with respect to the behavior of the traveling vehicle. 
     Or, preferably, the setting section is configured to set both the period of the first control method and the period of the second control method within the control cycle to values greater than zero, in case the operational amount increase the rotational speed of the motor at the time of reception of this operational amount to be greater than a preset change amount. 
     With the above-described arrangement, when the control method is to be switched over according to an acceleration, smooth traveling is made possible without giving feeling of discomfort or wrongness to the user respecting the behavior of the traveling vehicle. 
     Further, preferably: 
     the first control method is PI control in which an integral gain is a predetermined first value and a proportional gain is a predetermined second value; 
     and the second control method is P control in which the integral gain is zero value and the proportional gain is a predetermined third value. 
     With the above-described arrangement, when the control method is switched over between the PI control and the P control, smooth traveling is made possible without giving feeling of discomfort or wrongness to the user respecting the behavior of the traveling vehicle. 
     Further, preferably, the pair of left and right driving wheels are used also as steerable wheels. 
     With the above-described arrangement, e.g. also in case a turning is made by not rotating one of the pair of left and right driving wheels and driving the other of the pair of left and right driving wheels, it is possible to avoid giving damage to the road surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a traveling vehicle, 
         FIG. 2  is a system diagram showing an electric system and a power system of the traveling vehicle, and 
         FIG. 3  is an explanatory view of periods of P control and periods of PI control set within a control cycle. 
     
    
    
     BEST MODE OF CARRYING OUT THE INVENTION 
     A traveling vehicle relating to the present invention is configured to be able to travel smoothly even when a control method of motors is switched over. Next, a traveling vehicle  1  according to the instant embodiment will be explained. 
       FIG. 1  shows a perspective view of a riding electric grass cutting machine as an example of the traveling vehicle  1  according to the instant embodiment.  FIG. 2  shows a system diagram showing an electric system and a power system of the traveling vehicle  1 . In the instant embodiment, as an example of the traveling vehicle  1 , a riding electric grass cutting machine which travels in a field while carrying out a grass (lawn) cutting work will be explained. 
     An electric power control device  9  controls power used by the traveling vehicle  1  which travels while carrying out a utility work (a grass cutting work). The electric power used by the traveling vehicle  1  includes electric power used for traveling of the traveling vehicle  1 , electric power used for the utility work carried out by the traveling vehicle  1 , and electric power used for a user who uses the traveling vehicle  1 . A specific example of the electric power used for traveling of the traveling vehicle  1  is electric power supplied to motors  20  which respectively drive a pair of left and right driving wheels  3  included in the traveling vehicle  1 . A specific example of the electric power used for a utility work carried out by the traveling vehicle  1  is electric power to be supplied to motors (“mower motors  130   a ,  130   b ,  130   c ” to be described later) which rotate cutter blades used for a grass cutting work. A specific example of the electric power used for a user who uses the traveling vehicle  1  is electric power used for displaying by an instrument panel  99 . The electric power control device  9  is mounted on the traveling vehicle  1  that carries out such work. The electric power control device  9  distributes the respective electric powers appropriately to functional sections for carrying out the respective functions. 
     As shown in  FIG. 1  and  FIG. 2 , the riding electric grass cutting machine includes a machine body  4 , a battery  23 , a driver&#39;s seat  11 , a ROPS frame  12  and a mower unit  13 . The machine body  4  is supported by caster wheels  5  constituting front wheels and driving wheels  3  constituting rear wheels. The battery  23  is disposed at a rear portion of the machine body  4 . The driver&#39;s seat  11  is disposed forwardly of the battery  23 . The ROPS frame  12  is mounted erect from the rear side of the driver&#39;s seat  11 . The mower unit  13  is an example of a utility work unit. This mower unit  13  is suspended from the machine body  4  to be lifted up/down via a lift (elevating/lowering) mechanism in a space beneath the machine body  4  and between the caster wheels  5  and the driving wheels  3 . 
     The driving wheels  3  are driven by a traveling control unit  6 . Operations of this traveling control unit  6  are controlled by a traveling control device  2 . Operations of the mower unit  13  are controlled by a mower control device  7 . Here, it is noted that the caster wheels  5  consist of a left caster wheel  5   a  and a right caster wheel  5   b  and the driving wheels  3  consist of a left driving wheel  3   a  and a right driving wheel  3   b.    
     Forwardly of the driver&#39;s seat  11 , there is provided a floor plate acting as a “footrest” for the driver. From this floor plate, a brake pedal  14  protrudes. On the left and right opposed sides of the driver&#39;s seat  11 , there are disposed a left maneuvering lever  8   a  and a right maneuvering lever  8   b . Further, on the side of the driver&#39;s seat  11 , there is provided an electric operational panel  18 . This electric operational panel  18  includes switch buttons, switch levers or the like of the electric control system. On this electric operational panel  18 , a mower switch for activating the mower unit  13  and the instrument panel  99  described above are disposed. Incidentally, in the following explanation, the above-described left maneuvering lever  8   a  and the right maneuvering lever  8   b  will be generically referred to as “maneuvering lever  8 ” when no distinction therebetween is particularly needed. 
     In the instant embodiment, power sources for the left driving wheel  3   a  and the right driving wheel  3   b  are rotational forces from a left motor  21  and a right motor  22 , respectively. Both the left motor  21  and the right motor  22  are in-wheel motors. Via a left power supply section  41 , electric power is supplied to the left motor  21 . The left power supply section  41  is included in an inverter  10 . Via a right power supply section  42 , electric power is supplied to the right motor  22 . The right power supply section  42  also is included in the inverter  10 . By changing the supplied electric powers respectively, it is possible to change at least one of a rotational speed and a torque. With this, respective rotational speeds (actual speeds) of the left driving wheel  3   a  and the right driving wheel  3   b  can be made different from each other. As a result, due to a rotational speed difference between the left driving wheel  3   a  and the right driving wheel  3   b , the riding electric grass cutting machine makes a turn. 
     The traveling control unit  6  is a functional section for controlling traveling and turning of the riding electric grass cutting machine. In the instant embodiment, the traveling control unit  6  includes the left motor  21 , the right motor  22  and the inverter  10  (in particular, the left power supply section  41  and the right power supply section  42 ) which were mentioned above. The inverter  10  supplies electric power to the left motor  21  and the right motor  22  respectively. Electric power outputted from the inverter  10  corresponds to a speed instruction value (a target value) calculated by the traveling control device  2 . When the actual rotational speed (the actual speed) becomes smaller than a target value due to a traveling load, the electric power outputted from the inverter  10  will be corrected to increase the motor output torque. On the other hand, in the case of e.g. traveling on a downslope, if the actual rotational speed (the actual speed) becomes greater than the target value, the electric power outputted from the inverter  10  will be corrected to decrease the motor output torque. 
     The mower unit  13  includes three rotary blades  131   a ,  131   b ,  131   c . These rotary blades  131   a ,  131   b ,  131   c  respectively use the mower motors  130   a ,  130   b ,  130   c  as their driving power sources. Electric power supplies to the mower motors  130   a ,  130   b ,  130   c  are carried out via a mower power supply section  43 . This mower power supply section  43  too is included in the inverter  10 . The mower power supply section  43  is controlled by the mower control device  7 . This mower control device  7  constitutes a control device  100  together with the traveling control device  2  and the electric power control device  9  which were described above. 
     An operational amount (a pivot angle) of the left maneuvering lever  8   a  is detected by a left maneuvering angle detection sensor  80   a . An operational amount (a pivot angle) of the right maneuvering lever  8   b  detected by a right maneuvering angle detection sensor  80   b . Further, an operational angle of the brake pedal  14  is detected by a brake detection sensor  80   c . An operation of the mower switch is detected by a mower sensor  80   d . Further, a rotational speed of the left driving wheel  3   a  is detected by a left rear wheel rotation detection sensor  70   a . A rotational speed of the right driving wheel  3   b  is detected by a right rear wheel rotation detection sensor  70   b . Further, rotational speeds of the mower motors  130   a ,  130   b ,  130   c  are detected respectively by rotation sensors  100   a ,  100   b ,  100   c . And, detection results from the respective sensors are transmitted to the control device  100  to be used by this control device  100  when necessary or appropriate. 
     In the traveling control device  2 , based on operational amount of the respective maneuvering levers  8 , electric powers to be supplied respectively to the left motor  21  and the right motor  22  are calculated. Further, in this traveling control device  2 , by a known feedback control technique, the above-describe electric powers are corrected. Namely, the traveling control device  2  calculates required driving torques (to be referred to simply as “necessary torques” hereinafter) required by the left motor  21  and the right motor  22 . Here, the necessary torques means a torque amount required by the left motor  21  or the right motor  22  for the actual speed to reach the target speed in case an actual rotational speed has not arrived at a target rotational speed with a control amount calculated by a target rotational speed set in correspondence with an operational amount by the left maneuvering lever  8   a  or the right maneuvering lever  8   b . The traveling control device  2  derives such necessary torques from the target rotational speeds of the left driving wheel  3   a  and the right driving wheel  3   b  and the actual rotational speeds of the left driving wheel  3   a  and the right driving wheel  3   b . Such target rotational speeds of the left driving wheel  3   a  and the right driving wheel  3   b  are obtained, based on detection results of the left maneuvering angle detection sensor  80   a  and the right maneuvering angle detection sensor  80   b . The actual rotational speeds of the left driving wheel  3   a  and the right driving wheel  3   b  are obtained by the left rear wheel rotation detection sensor  70   a  and the right rear wheel rotation detection sensor  70   b  respectively. Then, based on such calculated necessary torques, the traveling control device  2  corrects the electric power amounts. 
     Next, there will be explained a function (to be referred to as “switchover function” hereinafter) relating to switchover of a control method of the motors  20  relating to the instant embodiment. This switchover function is realized by the respective functional sections of the driving wheels  3 , the motor  20 , an operational amount reception section  51 , a setting section  52  and a motor driving section  53 . In particular, the operational amount reception section  51 , the setting section  52  and the motor driving section  53  are constituted of hardware and/or software including a CPU as the core component thereof, in order to carry out processes relating to the above-described switchover function. 
     The driving wheels  3  are provided in the machine body  4  as described above and consist of the left driving wheel  3   a  and the right driving wheel  3   b , in the instant embodiment. The motor  20  is capable of driving the pair of left and right driving wheels  3  independently of each other. And, this motor  20  consists of the left motor  21  and the right motor  22  in this embodiment, as described above. The left motor  21  inputs a rotational force to the left driving wheel  3   a . The right motor  22  inputs a rotational force to the right driving wheel  3   b.    
     In the instant embodiment, the left driving wheel  3   a  and the right driving wheel  3   b  are configured to be able to input rotational forces of the left motor  21  and the right motor  22 , independently of each other. In accordance with a difference between rotational forces inputted respectively to the left driving wheel  3   a  and the right driving wheel  3   b , the traveling vehicle  1  makes a turn. Accordingly, in the instant embodiment, the pair of left and right driving wheels  3  are used also as steerable wheels. 
     The operational amount reception section  51  is capable of receiving the operational amounts respectively for the pair of left and right motors  20 . In the instant embodiment, as described hereinbefore, on the left and right opposed sides of the driver&#39;s seat  11 , there are disposed the left maneuvering lever  8   a  and the right maneuvering lever  8   b . The left maneuvering lever  8   a  and the right maneuvering lever  8   b  are operated by a user. The maneuvering lever  8  consisting of the left maneuvering lever  8   a  and the right maneuvering lever  8   b  corresponds to the operational amount reception section  51 . And, the maneuvering levers  8  receive operational amounts for the pair of left and right motors  20 . 
     The setting section  52  can set a period of the first control method and a period of the second control method within a predetermined control cycle T for controlling driving of the motors  20 , in accordance with the operational amounts. The first control method includes a predetermined first control parameter. The second control method includes a predetermined second control parameter. The “operational amounts” mean operational amounts received by the above-described operational amount reception section  51  and the operational amounts will be transmitted from the operational amount reception section  51  to the setting section  52 . 
     Here, in this instant embodiment, the motors  20  are controlled by the known PWM (Pulse Width Modulation) control technique. Therefore, the “predetermined control cycle T for controlling driving of the motors  20 ” corresponds to the control cycle T in the PWM control, namely, the reciprocal of the driving frequency value of the PWM control. For instance, 100 milliseconds may be employed as the control cycle T. 
     Further, in the instant embodiment, driving of the motors  20  can be controlled with switching over of two different control methods within one control cycle. One of such two control methods is the first control method including the predetermined first control parameter. The other of such two control methods is the second control method including the predetermined second control parameter. In the instant embodiment, PI control is employed as the first control method including the predetermined first control parameter. And, in the first control method, the integral gain is a predetermined first value and the proportional gain is a predetermined second value. As the second control method including the predetermined second control parameter, P control is employed. In the second control method, the integral gain is zero value and the proportional gain is a predetermined third value. Therefore, in the instant embodiment, the setting section  52  sets a period for effecting the PI control and a period for effecting the P control, within the control cycle T in the PWM control. Here, some nonlimiting specific examples of the control parameters used in the instant embodiment are control gains, various kinds of limit values, switchover of control mode or algorithm, maps, ON/OFF of motor energization, etc. 
       FIG. 3  shows one example of the predetermined control cycle T set by the setting section  52 . Such control cycle T in the PWM control is a fixed value (a constant) according to the control frequency. The setting section  52 , as shown in  FIG. 3 , sets, in this control cycle T, a P control period (tp) and a PI control period (tpi). 
     Specifically, in the case of (A) in  FIG. 3 , the P control period (tp) is set to a period of 100% in the control cycle T and the PI control period (tpi) is set to 0% in the control cycle T, respectively. In the case of (B) in  FIG. 3 , the P control period (tp) is set to a period of 75% in the control cycle T and the PI control period (tpi) is set to 25% in the control cycle T, respectively. Further, in the case of (C) in  FIG. 3 , the P control period (tp) is set to a period of 50% in the control cycle T and the PI control period (tpi) is set to 50% in the control cycle T, respectively. Further, in the case of (D) in  FIG. 3 , the P control period (tp) is set to a period of 25% in the control cycle T and the PI control period (tpi) is set to 75% in the control cycle T, respectively. Still further, in the case of (E) in  FIG. 3 , the P control period (tp) is set to a period of 0% in the control cycle T and the PI control period (tpi) is set to 100% in the control cycle T, respectively. Needless to say,  FIG. 3  shows just one example. The ratio between the P control period (tp) and the PI control period (tpi) in the control cycle T may be any other ratio than those shown in  FIG. 3 . 
     For instance, the setting section  52  will set the PI control period (tpi) within the control cycle T to zero if the rotational speed of the motor  20  is less than a predetermined rotational speed or will set the P control period (tp) within the control cycle T to zero if the rotational speed of the motor  20  is equal to or greater than the predetermined rotational speed. Here, the “rotational speed” of the motor  20  refers to the number of rotations of the left motor  21  or the right motor  22  within a predetermined period, which may be 1000 rpm, for example. 
     Here, the setting section  52  sets the PI control period (tpi) and the P control period (tp) within the control cycle T for the left motor  21  and sets the PI control period (tpi) and the P control period (tp) within the control cycle T for the right motor  22 , independently of each other. Therefore, in case the rotational speed of the left motor  21  is less than 1000 rpm, the setting section  52  will set the PI control period (tpi) for the left motor  21  to zero as shown in  FIG. 3  (A). Further, in case the rotational speed of the left motor  21  is equal to or greater than 1000 rpm, the setting section  52  will set the P control period (tp) for the left motor  21  to zero, as shown in  FIG. 3  (E). Similarly, in case the rotational speed of the right motor  22  is less than 1000 rpm, the setting section  52  will set the PI control period (tpi) for the right motor  22  to zero as shown in  FIG. 3  (A). Further, in case the rotational speed of the right motor  22  is equal to or greater than 1000 rpm, the setting section  52  will set the P control period (tp) for the right motor  22  to zero, as shown in  FIG. 3  (E). 
     In case switchover from the control state shown in  FIG. 3  (A) to the control state shown in  FIG. 3  (E) is to be effected, namely, when the control method of driving the left motor  21  and the right motor  22  is switched from the P control to the PI control at some point, the traveling vehicle  1  will make a sudden acceleration, thus possibly giving damage to the road surface. Further, damage may be given to the road surface at the time of zero-turn also. Then, advantageously when the control method for driving the left motor  21  and the right motor  22  respectively is switched from the P control to the PI control, the setting section  52  progressively reduces the ratio of the P control period (tp) within the control cycle T from 100% to 0% and also progressively increases the ratio of the PI control period (tpi) from 0% to 100%. Namely, in case the control method of driving the left motor  21  and the right motor  22  respectively is switched from the P control to the PI control, advantageously, the setting section  52  will switch over from the control state shown in  FIG. 3  (A) to the control state shown in  FIG. 3  (E) through transition of control states shown in  FIG. 3  (B) through (D). 
     The motor driving section  53  is capable of driving the motors  20  based on setting results made by the setting section  52 . Here, the setting results made by the setting section  52  are the control methods set by the setting section  52  for driving the motors  20 . Therefore, to the motor driving section  53 , control methods set by the setting section  52  for driving the motors  20  will be transmitted. The left power supply section  41  and the right power supply section  42  described above correspond to such motor driving section  53 . With the above-described arrangements, the traveling vehicle  1  can travel smoothly, and also it is possible to prevent the driving wheels  3  to be locked at zero speed or an extremely low speed. So, damage to the road surface can be avoided. 
     Other Embodiments 
     In the foregoing embodiment, the explanation was made with citing an electric grass cutting machine as an example of the traveling vehicle  1 . However, the traveling vehicle  1  may be a tractor for carrying out a plowing work in a field for example, or may be a rice planting machine also. Still further, it may be a combine for carrying out a culm harvesting work in a field or may be a seed sowing machine for sowing seeds. Still further, it may be a riding management machine for carrying out e.g. a chemical agent spraying work, etc. Namely, specific examples of the traveling vehicle  1  include a grass cutting machine, a tractor, a rice planting machine, a combine, a seed sowing machine, a riding management machine, etc. Needless to say, the present invention is applicable also to a traveling vehicle  1  used for carrying out works different from the above in a field. Moreover, the present invention is applicable not only to a riding traveling vehicle  1 , but also to a traveling vehicle  1  that can be remotely controlled. 
     In the foregoing embodiment, it was explained that the first control method is the PI control and the second control method is the P control. However, the first control method and the second control method may be other control methods. Specifically, as the first control method and the second control method, the fuzzy control or neuro control may be employed, and the PID control may be employed also. Still further, the first control method and the second control method may be constituted of a same control method, with control parameters thereof being made different from each other. 
     In the foregoing embodiment, it was explained that the setting section  52  progressively reduces the ratio of the P control period (tp) within the control cycle T from 100% to 0% and progressively increases the ratio of the PI control period (tpi) from 0% to 100%. However, it is also possible to arrange such that the setting section  52  progressively reduces the ratio of the PI control period (tpi) within the control cycle T from 100% to 0% and progressively increases the ratio of the P control period (tp) from 0% to 100%. 
     Still further, it is also possible to arrange such that the setting section  52  does not change the ratio of the P control period (tp) and the ratio of the PI control period (tpi) within the control cycle T as fixed values, but changes the control cycle T instead. 
     In the foregoing embodiment, it was explained that if the rotational speed of the left motor  21  is less than 1000 rpm for instance, the setting section  52  sets the PI control period (tpi) of the left motor  21  to zero as shown in  FIG. 3  (A) and sets the P control period (tp) of the left motor  21  to zero as shown in  FIG. 3  (E) when the rotational speed of the left motor  21  is equal to or greater than 1000 rpm and explained also that if the rotational speed of the right motor  22  is less than 1000 rpm for instance, the setting section  52  sets the PI control period (tpi) of the right motor  22  to zero as shown in  FIG. 3  (A) and sets the P control period (tp) of the right motor  22  to zero as shown in  FIG. 3  (E) when the rotational speed of the right motor  22  is equal to or greater than 1000 rpm. However, this 1000 rpm is only one example, it may be any other rotational speed. Still further, it is also possible to arrange such that the conditions shown  FIG. 3  (A) to (E) are switched over sequentially in accordance with the rotational speed of the motor  20 . In such case, in case the rotational speed of the motor  20  is low, the setting will be made as shown in  FIG. 3  (A) and the setting will be made as shown in  FIG. 3  (E) in case the rotational speed of the motor  20  is high. 
     In the foregoing embodiment, it was explained that the setting section  52  sets the period of the first control method within the control cycle T to zero in case the rotational speed of the motor  20  is less than a predetermined rotational speed and sets the period of the second control method within the control cycle T to zero in case the rotational speed of the motor  20  is equal to or greater than the predetermined rotational speed. However, it is also possible to arrange such that in case an operational amount increases a rotational speed of the motor  20  at the time of reception of this operational amount to be greater than a preset change amount, the setting section  52  sets both the first control method period and the second control method period to values greater than zero. 
     Namely, preferably, in case there is no instruction for accelerating the traveling vehicle  1 , the motor  20  may be driven by the control method of  FIG. 3  (A), whereas, in case there is an instruction for accelerating the traveling vehicle  1 , the motor  20  will be driven by the control method of  FIG. 3  (E). In such case, advantageously, at the time of switchover from  FIG. 3  (A) to  FIG. 3  (E), also, the switchover should be made from  FIG. 3  (A) to  FIG. 3  (E), via the transition from  FIGS. 3  (B) through (D). Or, advantageously, in case there is no instruction for accelerating the traveling vehicle  1 , the motor  20  will be driven by the control method of  FIG. 3  (E), whereas, in case an instruction for accelerating the traveling vehicle  1  is received, the motor  20  will be driven by the control method of  FIG. 3  (A). In such case, advantageously, at the time of switchover from  FIG. 3  (E) to  FIG. 3  (A), also, the switchover should be made from  FIG. 3  (E) to  FIG. 3  (A), via the transition from  FIGS. 3  (D), (C), (B). Incidentally, the “rotational speed of the motor  20  at the time of reception of an operational amount” corresponds to a rotational speed of the motor  20  immediately before the reception of the operational amount. Advantageously, the switchover of  FIGS. 3  (B) through (D) can be made with using acceleration as a threshold value. 
     In the foregoing embodiment, it was explained that the pair of left and right driving wheels  3  are employed as steerable wheels. However, the pair of left and right driving wheels  3  may not be employed as steerable wheels. 
     In the foregoing embodiment, it was disclosed that in  FIG. 3  (B) through (D), when the P control period (tp) and the PI control period (tpi) are to be set within the control cycle T, the P control period (tp) is set first within the control cycle T and then the PI control period (tpi) is set. However, when the P control period (tp) and the PI control period (tpi) are to be set within the control cycle T, the PI control period (tpi) may be set first within the control cycle T and then the P control period (tp) may be set. 
     INDUSTRIAL APPLICABILITY 
     The present invention is applicable to a traveling vehicle capable of driving a pair of left and right driving wheels independently of each other. 
     DESCRIPTION OF REFERENCE SIGNS 
     
         
         
           
               1 : traveling vehicle 
               3 : driving wheel 
               20 : motor 
               51 : operational amount reception section 
               52 : setting section 
               53 : motor driving section 
             T: control cycle