Patent Publication Number: US-2023150330-A1

Title: Work vehicle and controller for work vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Patent Application No. 2021-186896 filed on Nov. 17, 2021, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates to a work vehicle and a controller for a work vehicle. 
     2. Description of the Related Art 
     Some work vehicles for agricultural use, such as tractors, have models equipped with the function of turning at a smaller turning radius than they would in a usual turn. For example, work vehicles are prevalent that have a front wheel speed increaser which, when the front wheel is steered by a predetermined angle or greater, reduces the turning radius by increasing the rotational speed of the outer front wheel over the rotational speeds of the right and left rear wheels. Such a work vehicle is able to smoothly turn while keeping itself from messing the soil on the field surface. Japanese Laid-Open Patent Publication No. 2017-134471 discloses an example of a work vehicle having such a front wheel speed increaser. 
     On the other hand, work vehicles equipped with a suspension mechanism at the vehicle front for absorbing vibration and shock during travel for improved riding comfort have also become prevalent. The suspension mechanism provided at the vehicle front is called a “front suspension”. In tractors or other work vehicles, the load undergoes large changes. Thus, a hydraulic front suspension is commonly adopted to ensure an adequate suspension stroke. Japanese Laid-Open Patent Publication No. 2012-224338 discloses an example of a work vehicle having a hydraulic front suspension. 
     Japanese Laid-Open Patent Publication No. 2021-17150 discloses a suspension controller for properly preventing a vehicle such as a truck from becoming overturned during a sharp turn. The vehicle disclosed in Japanese Laid-Open Patent Publication No. 2021-17150 includes a pair of suspension members that allow for vehicle height adjustments. Based on the steering angle and steering angular velocity of the vehicle during travel, the suspension controller, when it is estimated that the turning radius of the vehicle immediately afterwards will be smaller than a predetermined value, controls the suspension members so as to increase the vehicle height on the outer side of the vehicle&#39;s turn. Japanese Laid-Open Patent Publication No. 2021-17150 describes that this can properly prevent the vehicle from becoming overturned during a sharp turn. 
     SUMMARY OF THE INVENTION 
     When a work vehicle turns with a small turning radius or a high speed, a strong centrifugal force acts in the outer direction of the turn. As a result of this, the tilt of the work vehicle may increase, possibly resulting in poorer riding comfort or loss of balance. 
     Preferred embodiments of the present invention provide techniques for reducing or preventing tilting when a work vehicle turns with a small turning radius or a high speed to achieve a more stable turn. 
     A work vehicle according to an implementation of the present disclosure includes a vehicle body, running gear to cause the vehicle body to travel, a height adjuster to change a height of a center of gravity of the vehicle body, and a controller configured or programmed to, in accordance with at least one of a turning radius and an angular velocity of the vehicle body during a turn, control the height adjuster to maintain or lower the height of the center of gravity. 
     A controller according to another implementation of the present disclosure controls a work vehicle including a vehicle body, running gear to cause the vehicle body to travel, and a height adjuster to change a height of a center of gravity of the vehicle body. The controller includes one or more processors, and one or more memories storing a computer program to be executed by the one or more processors. The one or more processors is configured or programmed to acquire, during a turn, information concerning at least one of a turning radius and an angular velocity of the vehicle body, and control the height adjuster to maintain or lower the height of the center of gravity in accordance with the information. 
     General or specific aspects of various example preferred embodiments of the present disclosure may be implemented using a device, a system, a method, an integrated circuit, a computer program, a non-transitory computer-readable storage medium, or any combination thereof. The computer-readable storage medium may be inclusive of a volatile storage medium, or a non-volatile storage medium. The device may include a plurality of devices. In the case where the device includes two or more devices, the two or more devices may be disposed within a single apparatus, or divided over two or more separate apparatuses. 
     According to certain preferred embodiments of the present disclosure, tilting of a work vehicle during a turn is reduced or prevented, and the turning stability is improved. 
     The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective view showing an exemplary appearance of a work vehicle according to an illustrative preferred embodiment of the present invention. 
         FIG.  2    is a side view schematically showing the work vehicle and an example of an implement that is linked to the work vehicle. 
         FIG.  3    is a diagram schematically showing a strong centrifugal force at work when the work vehicle is making a small turn. 
         FIG.  4    is a diagram schematically showing a strong centrifugal force at work when the work vehicle is making a small turn. 
         FIG.  5    is a diagram schematically illustrating an effect obtained by lowering the center of gravity of a vehicle body when the turning radius is small. 
         FIG.  6    is a conceptual diagram showing a schematic configuration of a suspension device. 
         FIG.  7    is a side view showing a specific example of the structure of the suspension device. 
         FIG.  8    is a diagram showing a schematic configuration for a hydraulic circuit. 
         FIG.  9    is a block diagram showing an exemplary configuration for a front suspension control system. 
         FIG.  10    is a block diagram showing a hardware configuration of an ECU. 
         FIG.  11    is a schematic diagram showing an example of an operational terminal and operation switches to be provided in the cabin. 
         FIG.  12    is a diagram showing an exemplary relationship between the output voltage of a stroke sensor and the stroke length of the suspension. 
         FIG.  13    is a graph showing an example of change in stroke length over. 
         FIG.  14    is a flowchart showing one example of the operation of the ECU. 
         FIG.  15    is a flowchart showing another example of the operation of the ECU. 
         FIG.  16    is a flowchart showing still another example of the operation of the ECU. 
         FIG.  17    is a block diagram showing an exemplary configuration where the ECU controls the front suspension based on a signal which is output from an IMU. 
         FIG.  18    is a flowchart showing an example of vehicle height control based on an angular velocity of yawing of the work vehicle. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, preferred embodiments of the present disclosure will be described more specifically. Note however that unnecessarily detailed descriptions may be omitted. For example, detailed descriptions on what is well known in the art or redundant descriptions on what is substantially the same configuration may be omitted. This is to avoid lengthy description, and facilitate the understanding of those skilled in the art. The accompanying drawings and the following description, which are provided by the present inventors so that those skilled in the art can sufficiently understand the present disclosure, are not intended to limit the scope of claims. In the following description, component elements having identical or similar functions are denoted by identical reference numerals. 
     The following preferred embodiments are only exemplary, and the techniques of the present disclosure are not limited to the following preferred embodiments. For example, numerical values, shapes, materials, steps, and orders of steps, etc., that are indicated in the following preferred embodiments are only exemplary, and admit of various modifications. Any one implementation may be combined with another so long as it makes technological sense to do so. 
     Hereinafter, preferred embodiments where the work vehicle is a tractor will be described as an example. Without being limited to tractors, the techniques according to the present disclosure are also applicable to other types of agricultural machines, e.g., rice transplanters, combines, vehicles for crop management, and riding lawn mowers. The techniques according to the present disclosure are also applicable to work vehicles for use in non-agricultural applications, e.g., construction vehicles or snowplow vehicles. 
       FIG.  1    is a perspective view showing an exemplary appearance of a work vehicle according to an illustrative preferred embodiment of the present disclosure. The work vehicle  100  according to the present preferred embodiment is a tractor for use with agricultural work in a field (e.g., an agricultural field, an orchard, or a paddy field).  FIG.  2    is a side view schematically showing the work vehicle  100  and an example of an implement  300  that is linked to the work vehicle  100 . 
     As shown in  FIG.  2   , the work vehicle  100  includes a vehicle body  101 , a prime mover (engine)  102 , a transmission  103 , running gear  104  to cause the vehicle body  101  to travel, and an adjusting device  110  (height adjuster) to change the height of the center of gravity of the vehicle body  101 . A cabin  105  is provided on the vehicle body  101 . The running gear  104  includes four wheels (a pair of front wheels  104 F and a pair of rear wheels  104 R), a wheel axis for rotating the four wheels, and a braking device to apply braking to each wheel. Inside the cabin  105 , a driver&#39;s seat  107 , a steering device  106 , an operational terminal  200 , and switches for manipulation are provided. The work vehicle  100  is able to switch between a four-wheel drive ( 4 W) mode in which all of the front wheels  104 F and the rear wheels  104 R serve as driving wheels, and a two-wheel drive ( 2 W) mode in which only the front wheels  104 F or only the rear wheels  104 R serve as the driving wheels. 
     The prime mover  102  may be a diesel engine, for example. Instead of a diesel engine, an electric motor may be used. The transmission  103  can change the propulsion and moving speed of the work vehicle  100  through a speed changing mechanism. The transmission  103  can also switch between forward travel and backward travel of the work vehicle  100 . 
     The steering device  106  includes a steering wheel, a steering shaft connected to the steering wheel, and a power steering device to assist in the steering by the steering wheel. The front wheels  104 F are the wheels responsible for steering, such that changing their angle of turn (also referred to as a “steering angle”) can cause a change in the traveling direction of the work vehicle  100 . The steering angle of the front wheels  104 F can be changed by manipulating the steering wheel. The power steering device includes a hydraulic device or an electric motor to supply an assisting force for changing the steering angle of the front wheels  104 F. The work vehicle  100  may have an automatic steering function. When automatic steering is performed, under the control of a controller disposed in the work vehicle  100 , the steering angle of the front wheels  104 F may be automatically adjusted by the power of the hydraulic device or electric motor. 
     A linkage device  108  is provided at the rear of the vehicle body  101 . The linkage device  108  may include, e.g., a three-point linkage (also referred to as a “three-point link” or a “three-point hitch”), a PTO (Power Take Off) shaft, a universal joint, and a communication cable. The linkage device  108  allows the implement  300  to be attached to or detached from the work vehicle  100 . The linkage device  108  is able to raise or lower the three-point linkage device with a hydraulic device, for example, thus controlling the position and/or attitude of the implement  300 . Moreover, motive power can be sent from the work vehicle  100  to the implement  300  via the universal joint. While towing the implement  300 , the work vehicle  100  allows the implement  300  to perform a predetermined task. The linkage device may be provided frontward of the vehicle body  101 . In that case, the implement may be connected frontward of the work vehicle  100 . 
     Although the implement  300  shown in  FIG.  2    is a rotary tiller, the implement  300  is not limited to a rotary tiller. For example, any arbitrary implement such as a mower, a seeder, a spreader, a rake implement, a baler, a harvester, a sprayer, or a harrow, may be connected to the work vehicle  100  for use. The work vehicle  100  may travel without the implement  300  being attached thereto. 
     The running gear  104  may include a front wheel speed increaser which causes the outer front wheel  104 F to rotate more rapidly than the inner front wheel  104 F and the right and left rear wheels  104 R during a turn, thus to reduce the turning radius. The front wheel speed increaser may increase the rotational speed(s) of the front wheel(s)  104 F when the steering angle of the front wheels  104 F has reached a predetermined angle or greater as the driver turns the steering wheel to a great extent, for example. By increasing the rotational speed of the outer front wheel  104 F to approximately, e.g., 1.5 to 2.5 times of the rotational speed of the outer rear wheel  104 R, the front wheel speed increaser can reduce the turning radius of the work vehicle  100 . This allows the work vehicle  100  to smoothly turn in a small space, while keeping itself from messing the soil on the field surface. Such a turn may be referred to as a “bi-speed turn” in the present specification. When the front wheel speed increaser increases the rotational speed(s) of the front wheel(s)  104 F, a control of automatically braking the inner rear wheel  104 R may be performed. Braking the inner rear wheel  104 R in addition to increasing the rotational speed(s) of the front wheel(s)  104 F allows the turning radius to be further reduced. By manipulating the operational terminal  200  or the switches within the cabin  105 , the driver is able to set: whether the front wheel speed increasing function is enabled/disabled; how much the rotational speed(s) of the front wheel(s)  104 F is to be increased during a turn; and whether or not the rear wheels  104 R are braked and the degree of such braking during a turn. 
     Although a bi-speed turn can be made within a small circle, a stronger centrifugal force will act in a bi-speed turn than in a turn which is made at the same vehicle speed but not in the manner of a bi-speed turn. A strong centrifugal force may cause the body of the vehicle to tilt during a turn, thus detracting from the riding comfort, or resulting in a loss of balance. 
       FIG.  3    and  FIG.  4    are diagrams schematically showing a strong centrifugal force at work when the work vehicle  100  is making a small turn via bi-speed turn. In  FIG.  3   , a curved arrow (a) in solid line represents an example locus of the work vehicle  100  during a usual turn, while a curved arrow (b) in broken line represents an example locus of the work vehicle  100  during a small turn via bi-speed turn. A straight arrow in solid line represents a centrifugal force during a usual turn, while a straight arrow in broken line represents a centrifugal force during a small turn. An object having a mass m, which undergoes a circular motion with a velocity v and a radius r (angular velocity v/r), receives a centrifugal force with a magnitude of mv 2 /r in the outer direction of the circular motion. Therefore, the turning work vehicle  100  is subject to a centrifugal force which is, in the outer direction of the turn, essentially in proportion to its velocity raised to the second power and its weight, and which is in inverse proportion to the turning radius. Therefore, given the same vehicle speed, a greater centrifugal force than in a usual turn is at work during a small turn. Consequently, as shown in  FIG.  4   , the body of the vehicle is more likely to tilt and become unbalanced during a small turn (b) than during a usual turn (a). In particular, when the center of gravity of the work vehicle  100  is high or when the interval between the right and left wheels (i.e., tread) of the work vehicle  100  is narrow, the body of the vehicle is likely to have a large tilt due to centrifugal force. This problem may occur not only when making a bi-speed turn, but commonly when any turn is made with a small turning radius or a high velocity (i.e., a high angular velocity). 
     In the present preferred embodiment, in order to reduce or prevent tilting of the body of the vehicle during a small turn and to achieve a stable turn, the work vehicle  100  includes the adjusting device  110  (height adjuster), which changes the height of the center of gravity of the vehicle body  101 . As used herein, “height” means a height (i.e., distance) from the ground surface on which the work vehicle  100  is traveling. The adjusting device  110  may include a suspension device that changes the height of the front of the vehicle body  101 , for example. In the example shown in  FIG.  2   , the adjusting device  110  includes a hydraulic suspension device provided at the lower front of the vehicle body  101 . By controlling such a suspension device, the height of the center of gravity of the vehicle body  101  can be adjusted. Without being limited to a suspension device provided at the front of the vehicle body  101 , the adjusting device  110  may be configured to adjust the center of gravity of the vehicle body  101  by other mechanisms. For example, suspension mechanisms may be provided at both the front and the rear of the vehicle body  101 . Alternatively, the adjusting device  110  may be implemented by a mechanism that raises or lowers a weight provided at a predetermined position (e.g., the bottom, the front, the side, or the rear) of the vehicle body  101 . 
     The adjusting device  110  is controlled by a controller, such as an electronic control unit (ECU), that is included in the work vehicle  100 . In accordance with at least one of the turning radius and the angular velocity of the vehicle body  101  during a turn, the controller is configured or programmed to control the adjusting device  110  to maintain or lower the height of the center of gravity of the vehicle body  101 . For example, the controller controls the adjusting device  110  to lower the center of gravity when a state of a small turn is entered during a turn, i.e., a state where the turning radius is smaller than a reference radius, and if the height of the center of gravity of the vehicle body  101  is higher than a reference height. Even in a state of a small turn, if the height of the center of gravity is equal to or lower than the reference height, the controller does not lower the center of gravity but maintains its height. For example, if at the beginning of the turn the center of gravity of the vehicle body  101  is already close to the lowest height within a controllable range, then the controller maintains that height of the center of gravity. 
       FIG.  5    is a diagram schematically illustrating an effect obtained by lowering the center of gravity of the vehicle body  101  when the turning radius is small (i.e., the angular velocity is high). The left side of  FIG.  5    depicts an example of the work vehicle  100  during a turn where the control for the center of gravity is not performed. The right of  FIG.  5    depicts an example of the work vehicle  100  during a turn where the control for the center of gravity according to the present preferred embodiment is performed. As shown in the right side of  FIG.  5   , by lowering the center of gravity, the tilt of the work vehicle  100  caused by centrifugal force is reduced or prevented. This achieves improvement in the stability of the work vehicle  100  during a small turn. 
     As described above, the running gear  104  may include a front wheel speed increaser. In that case, the running gear  104  can operate in a small turn mode where the rotational speed of the outer front wheel  104 F is made higher than the rotational speed(s) of the two rear wheels  104 R during a turn so that the turning radius becomes smaller than the reference radius. A turn made in the small turn mode corresponds to the aforementioned “bi-speed turn”. The controller may include a first control circuit to control the running gear  104  and a second control circuit to control the adjusting device  110 . When the angle of rotation of the steering wheel or the steering angle of the front wheels  104 F has exceeded a reference angle, the first control circuit causes the running gear  104  to operate in the small turn mode. When the small turn mode is begun while the height of the center of gravity of the vehicle body  101  is higher than the reference height, the second control circuit causes the adjusting device  110  to lower the center of gravity of the vehicle body  101 . With such a configuration, the second control circuit is able to cause the adjusting device  110  to lower the center of gravity of the vehicle body  101  in response to a signal which is output from the first control circuit indicating the start of the small turn mode. Thus, adjustments of the center of gravity can be made without having to separately provide a sensor to detect a state of a small turn. 
     In the small turn mode, the running gear  104  may have the function of automatically braking the inner one of the two rear wheels  104 R. In that case, the first control circuit may be configured to control the presence or absence of braking or intensity of braking on the inner rear wheel. The second control circuit may cause at least one of the amount of lowering the center of gravity and the reference height to vary in accordance with the presence or absence of braking or intensity of braking. When the inner rear wheel  104 R is braked, the turning radius is further reduced such that the centrifugal force is further increased. Therefore, in the case where a small turn is made with the braking of the inner rear wheel  104 R, the condition for lowering the center of gravity may be more relaxed than when a small turn is made without such braking. 
     The controller may perform the aforementioned control for the center of gravity even in the case where a bi-speed turn is not enabled, or in the case where the work vehicle  100  lacks bi-speed turn functionality. The work vehicle  100  may include an angular velocity sensor that is capable of measuring the angular velocity of the vehicle body  101 , e.g., an inertial measurement unit (IMU). In that case, the controller may cause the adjusting device  110  to lower the center of gravity when the angular velocity of a yawing (i.e., rotational motion around an axis in the top-bottom direction of the vehicle) of the vehicle body  101  as measured by the angular velocity sensor has become equal to or greater than a threshold, and if the height of the center of gravity is higher than the reference height. With such a configuration, it is possible to adjust the center of gravity based on the result of measurement by the angular velocity sensor, even in the case where a bi-speed turn is not enabled or where the work vehicle  100  lacks bi-speed turn functionality. 
     The controller may change at least one of the amount of lowering the center of gravity and the reference height in accordance with at least one of the turning radius, the angular velocity, and the weight of the work vehicle. Moreover, the controller may change the reference radius in accordance with at least one of the speed and the weight of the work vehicle  100 . The magnitude of the centrifugal force depends on the turning radius, the speed of the work vehicle  100 , the angular velocity of the work vehicle  100 , and the weight of the work vehicle  100 . Therefore, based on these conditions, the controller may change at least one of the amount of lowering the center of gravity, the reference height, and the reference radius. For example, the controller may increase the amount of lowering the center of gravity or lower the reference height as the turning radius decreases (or the angular velocity increases) or as the weight increases. Moreover, the controller may increase the reference radius as the speed of the work vehicle  100  increases or as the weight increases. 
     The controller may change at least one of the amount of lowering the center of gravity, the reference height, and the reference radius in accordance with the type of the implement  300  linked to the work vehicle  100  or the presence or absence of the implement  300 . Once the implement  300  is attached, the center of gravity of the system combining the work vehicle  100  and the implement  300  may become higher than the center of gravity of the work vehicle  100  alone, etc., which makes tilting more likely. Therefore, when the implement  300  has been attached, the controller may increase the amount of lowering the center of gravity, lower the reference height, or increase the reference radius, relative to when no implement  300  is attached. 
     As in the example shown in  FIG.  2   , when the adjusting device  110  includes a suspension device, the controller may control the suspension device to change the height of the center of gravity. Hereinafter, an exemplary configuration and an exemplary control for the suspension device will be described. 
       FIG.  6    is a conceptual diagram showing a schematic configuration of the suspension device. By hydraulic action, this suspension device is able to change the height of a front wheel axis frame  111  that is provided at the front of the vehicle body  101 . As an example of the controller,  FIG.  6    illustrates an ECU  510 . Without being limited to the illustrated position, the ECU  510  may be disposed at any arbitrary position. The running gear in this example includes two supports  109  for respectively supporting the two front wheels  104 F. The suspension device includes: two hydraulic suspension cylinders  441  which are provided near the right and left front wheels  104 F; and a hydraulic circuit  400  that is connected to the two hydraulic suspension cylinders  441 . In  FIG.  6   , the front wheel  104 F, the support  109 , and the suspension cylinder  441  on the left side are illustrated. The support  109  is mounted to the front wheel axis frame  111  at one end thereof, so as to be capable of swinging up and down around a spindle  113 . The suspension cylinder  441  interconnects a portion of the support  109  and the front wheel axis frame  111 . The hydraulic circuit  400  adjusts the amount and pressure of hydraulic oil to be supplied to each suspension cylinder  441 . By controlling the hydraulic circuit pn 400 , the ECU  510  controls the extension/retraction action of the suspension cylinders  441 . 
     On the front wheel axis frame  111  and the support  109 , a stroke sensor  442  is mounted to detect an extended or retracted state of the suspension cylinder  441 . The stroke sensor  442  shown in  FIG.  6    includes a rotary displacement potentiometer. The stroke sensor  442  outputs a signal which is in accordance with the stroke length of the suspension cylinder  441 . Based on the signal which is output from the stroke sensor  442 , the ECU  510  calculates the stroke length of the suspension cylinder  441 . By controlling each control valve in the hydraulic circuit  400  based on the results of calculation, the ECU  510  is able to adjust the stroke length to a desired length. 
       FIG.  7    is a side view showing a more specific example of the structure of the suspension device. The rear end of the support  109  is supported by the spindle  113 , which is located below the front wheel axis frame  111  and which extends along the right-left direction with respect to the vehicle&#39;s traveling direction. The support  109  is supported so as to be capable of swinging around the spindle  113 . The front wheel axis  114  is located frontward and upward from the support  109 . To the front wheel axis  114 , a transmission system to transmit a motive force for traveling via a universal joint and the like is connected. The front wheels  104 F are mounted on the front wheel axis  114 . 
     Between the front ends of the right and left supports  109  and the two positions at the front of the front wheel axis frame  111 , two hydraulic suspension cylinders  441  are respectively connected. The two suspension cylinders  441  are controlled by the ECU  510  to extend or retract in conjunction with the up and down movements of the front wheel  104 F. To each suspension cylinder  441 , hydraulic oil is supplied from the hydraulic circuit  400 . As the ECU  510  controls supply and discharge of the hydraulic oil, the suspension cylinders  441  function as springs. As a result of this, shocks during travel are absorbed so as to provide an improved riding comfort. 
       FIG.  8    is a diagram showing an exemplary schematic configuration for the hydraulic circuit  400 . The suspension cylinders  441  are disposed in such an attitude that piston rods protrude downward therefrom. Each suspension cylinder  441  includes a headside oil chamber  441   a  and a rod-side oil chamber  441 b. A first oil channel  413  is connected to the upper (head-side) oil chamber  441   a . A second oil channel  414  is connected to the lower (rod-side) oil chamber  441 b. 
     To the first oil channel  413 , a head-side accumulator  406  is connected via a pilot-operated double check valve  411 . In an oil channel between the head-side accumulator  406  and the double check valve  411 , a pilot-operated variable orifice  409  is provided. To an oil channel between the double check valve  411  and the first oil channel  413 , a pressure sensor  412  is connected. A rod-side accumulator  407  is connected to the second oil channel  414 . 
     A gas, e.g., nitrogen, is sealed inside the accumulators  406  and  407 . The gas compresses as pressure increases in the oil channel connected thereto, and the gas expands as the pressure in the oil channel decreases. Through this action, a spring property is imparted to each suspension cylinder  441 . 
     The variable orifice  409  has three positions associated with different orifice diameters. Two switching control valves  424  and  425  are provided, which allow the variable orifice  409  to be manipulated through control of a pilot pressure. By adjusting the flow of hydraulic oil to be supplied and discharged to and from the head-side accumulator  406 , the variable orifice  409  is able to vary the stiffness (corresponding to the spring modulus) of the suspension. As the ECU  510  drives solenoids  404  and  405  in the switching control valves  424  and  425 , the aperture of the variable orifice  409  can be switched in three steps of large, medium, and small. This allows the damping force of the suspension cylinders  441  to be switched in three steps. If the ECU  510  turns both solenoids  404  and  405  OFF, an aperture at the “large” position is inserted in the oil channel, whereby the damping force becomes minimum. If the ECU  510  turns the solenoid  404  ON, an aperture at the “medium” (left) position is inserted in the oil channel, whereby the damping force becomes intermediate. If the ECU  510  turns the solenoid  405  ON, an aperture at the “small” (right) position is inserted in the oil channel so that the damping force becomes maximum. 
     The hydraulic circuit  400  further includes an open/close control valve  423  to cause a pilot pressure to act on the double check valve  411 . Through pilot pressure control by the open/close control valve  423 , the double check valve  411  is switched between a closed state and an open state. By driving a solenoid  403  in the open/close control valve  423 , the ECU  510  causes a pilot pressure to act on the double check valve  411  so as to open the double check valve  411 . When the double check valve  411  is opened, hydraulic oil flows between the head-side oil chamber  441   a  and the head-side accumulator  406 , thus enabling suspension functionality. In the present specification, this state may be referred to as “suspension-ON”. Conversely, when the double check valve  411  is in a closed state, the flow of hydraulic oil between the head-side oil chamber  441   a  and the head-side accumulator  406  is blocked. This keeps the suspension fixed. In the present specification, this state may be referred to as “suspension locking”. 
     The hydraulic oil from the hydraulic pump  420 , which is driven by the prime mover  102  (engine), is supplied to the pilot-operated main control valve  421 . To the oil channel between the hydraulic pump  420  and the main control valve  421 , a relief valve  426  is connected. When the pressure of the hydraulic oil reaches an upper limit value that was previously set, the relief valve  426  is opened to allow a portion of the hydraulic oil to return to the tank  430 , so as to reduce or prevent an excessive pressure increase of the hydraulic oil. 
     The main control valve  421  is a  4 -port  3 -position directional control valve including a position-to-raise where hydraulic oil is supplied to the first oil channel  413  so as to extend the cylinders  441  and raise the vehicle height; a position-to-lower where hydraulic oil is supplied to the second oil channel  414  so as to retract the cylinders  441  and lower the vehicle height; and a neutral position where hydraulic oil is neither supplied nor discharged to/from the cylinders  441 . Two operable valves  431  and  432  are provided in order to manipulate the main control valve  421  by allowing a pilot pressure to act thereon. To the first oil channel  413 , a pilot-operated check valve  415  and a throttle  417  are connected. To the second oil channel  414 , a pilot-operated check valve  416 , a check valve  418  that opens or closes with hydraulic oil pressure, and a throttle  419  are connected. A relief valve  427  is connected to the oil channel between the check valve  418  and the check valve  416 . 
     As the ECU  510  drives solenoids  401  and  402  in the operable valves  431  and  432 , the raising and lowering of the cylinders  441  can be controlled. If the ECU  510  turns the solenoid  401  ON, the pilot pressure places the main control valve  421  in the position-to-raise such that hydraulic oil is supplied from the pump  420  to the head-side oil chamber  441   a . As a result of this, the cylinders  441  extend, thus raising the center of gravity of the vehicle body  101 . On the other hand, if the ECU  510  turns the solenoid  402  ON, the pilot pressure places the main control valve  421  in the position-to-lower such that hydraulic oil is supplied from the pump  420  to the rod-side oil chamber  441 b. As a result of this, the cylinders  441  are retracted, thus lowering the center of gravity of the vehicle body  101 . If the ECU  510  turns both solenoids  401  and  402  OFF, the main control valve  421  is switched to the neutral position. In this state, the cylinders  441  are isolated from the pump  420  and the tank  430  by the check valves  415  and  416 . In this state, if the ECU  510  turns the solenoid  403  ON, the accumulators  406  and  407  and the cylinders  441  are coupled, so that the cylinders  441  act as springs extending or retracting in conjunction with up and down movements of the front wheel  104 F. As the oil channel between the head-side oil chamber  441   a  and the head-side accumulator  406  is narrowed by the variable orifice  409 , the moving speed of the hydraulic oil is regulated, so that the cylinders  441  function as dampers. 
       FIG.  9    is a block diagram showing an exemplary configuration for a front suspension control system  500  according to the present preferred embodiment. The control system  500  includes a front suspension ECU  510 , an engine ECU  520 , and a main ECU  530 . The front suspension ECU  510  controls the operation of the front suspension. The engine ECU  520  controls the operation of the engine. The main ECU  530  controls the overall operation of the work vehicle  100 . 
     The ECUs  510 ,  520  and  530  may communicate with one another according to a vehicle bus standard such as CAN (Controller Area Network). Although the ECUs  510 ,  520  and  530  are illustrated as individual corresponding blocks in  FIG.  9   , each of these functions may be distributed among a plurality of ECUs. Alternatively, an onboard computer that integrates some or all of the functions of the ECUs  510 ,  520  and  530  may be provided. The control system  500  may include ECUs other than the ECUs  510 ,  520  and  530 , and any number of ECUs may be provided in accordance with functionality. Each ECU includes a control circuit including one or more processors and one or more memories. The processor(s) operate by executing a computer program(s) stored in the memory(s). In the present preferred embodiment, the main ECU  530  is configured or programmed to perform the function of the aforementioned first control circuit, whereas the front suspension ECU  510  is configured or programmed to perform the function of the aforementioned second control circuit. Therefore, in the present preferred embodiment, a combination of the front suspension ECU  510  and the main ECU  530  has the functions of the aforementioned “controller”. 
     The front suspension ECU  510  is connected to a plurality of switches  461  to  465 , the pressure sensor  412 , the stroke sensor  442 , and the solenoids  401  to  405 . The plurality of switches  461  to  465  are provided in the cabin  105  of the work vehicle  100 . The plurality of switches  461  to  465  include a suspension-auto switch (SW)  461 , a suspension-OFF switch  462 , a suspension-soft switch  463 , a suspension-hard switch  464 , and a suspension-manual switch  465 . 
     The main ECU  530  is connected to an electronic meter  160 , a buzzer  170 , and an angle-of-turn sensor  190 . In accordance with a command from the main ECU  530 , the electronic meter  160  displays the operating status of the work vehicle  100 . The electronic meter  160  displays the operating status of the front suspension, the bi-speed turn operating status, and so on, for example. The buzzer  170  generates an alarm sound in accordance with a command from the main ECU  530 . The buzzer  170  may generate an alarm sound while the user is manually adjusting the vehicle height, for example. The angle-of-turn sensor  190  measures the angle of turn (steering angle) of the front wheels  104 F or the angle of rotation of the steering wheel, and outputs a signal indicating the result of measurement to the main ECU  530 . 
     During travel of the work vehicle  100 , the engine ECU  520  consecutively sends information representing the revolutions of the engine per unit time to the front suspension ECU  510 . The information of engine revolutions is used in the relief-stopping control described below. 
     During travel of the work vehicle  100 , the main ECU  530  consecutively sends to the front suspension ECU  510  information representing: the state of a shuttle lever (forward travel/backward travel); the speed of the work vehicle  100  (vehicle speed); the temperature of the hydraulic oil (oil temperature) in the hydraulic circuit  400 ; the states of braking (brakes) on the right and left wheels; and the bi-speed turn operating status. Upon detecting that the steering angle of the front wheels  104 F or the angle of rotation of the steering wheel has exceeded the reference angle based on the signal which is output from the angle-of-turn sensor  190 , the main ECU  530  causes the running gear  104  to operate in the small turn mode. As a result, a bi-speed turn is made. The main ECU  530  sends information indicating that a bi-speed turn is to be made to the front suspension ECU  510 . The main ECU  530  may control the braking devices to apply braking to the inner rear wheel  104 R or both of the right and left rear wheels  104 R during a bi-speed turn. These controls are to be made in accordance with user-designated settings concerning bi-speed turns. 
       FIG.  10    is a block diagram showing a hardware configuration of the front suspension ECU  510 . The front suspension ECU  510  includes a processor  511 , a ROM  512 , a RAM  513 , a storage device  514 , and a communicator  515 . These component elements are connected so as to be capable of communicating with one another via a bus. 
     The processor  511  is a semiconductor integrated circuit including a central processing unit (CPU), for example. The processor  511  may be implemented as a microprocessor or a microcontroller. Alternatively, the processor  511  may be implemented as an FPGA (Field Programmable Gate Array), a GPU (Graphics Processing Unit), an ASIC (Application Specific Integrated Circuit), or an ASSP (Application Specific Standard Product) incorporating a CPU; or a combination of two or more circuits selected from among such circuits. The processor  511  executes a computer program in which instructions for performing at least one process are stated, this being stored in the ROM  512 , and performs a desired process. 
     The ROM  512  may be a writable memory (e.g., a PROM), a rewritable memory (e.g., a flash memory), or a read-only memory, for example. The ROM  512  stores a program to control the operation of the processor  511 . The RAM  513  provides a work area for a control program stored in the ROM  512  to be laid out once at boot time. Each of the ROM  512  and the RAM  513  does not need to be a single storage medium, but may be an aggregation of multiple storage media. The storage device  514 , which may be, e.g. a magnetic storage device or a semiconductor storage device, stores data generated through computation by the processor  511 . An example of a magnetic storage device is a hard disk drive (HDD). An example of a semiconductor storage device is a solid state drive (SSD). 
     The communicator  515  is a communication module to perform communications with the engine ECU  520  and the main ECU  530 . The communicator  515  performs communication in accordance with CAN or other vehicle bus standards, for example. 
     The engine ECU  520  and the main ECU  530  also have a similar hardware configuration to the configuration shown in  FIG.  10   . Therefore, descriptions concerning the hardware configuration of the engine ECU  520  and the main ECU  530  will be omitted. 
       FIG.  11    is a schematic diagram showing an example of the operational terminal  200  and operation switches  210  to be provided in the cabin  105 . In the cabin  105 , switches (including levers)  210 , which are a multitude of switches that are manipulable to the user, are disposed. The operation switches  210  may include, for example, a switch to select the gear shift as to a main gear shift or a range gear shift, a switch (shuttle lever) to switch between forward travel and backward travel, a switch to switch between a bi-speed turn enabled state and a bi-speed turn disabled state, a switch to switch between the presence or absence of braking and the degree of braking on the rear wheels  104 R during a bi-speed turn, a switch to switch the mode of the front suspension, a switch to adjust the damping force of the front suspension, a switch to raise or lower the implement  300 , and so on. Among these switches, switches  461 ,  462  and  465  to switch the mode of the front suspension, and switches  463  and  464  to adjust the damping force of the front suspension are shown in  FIG.  9   . 
     The suspension-auto switch  461  is a switch to enable the automatic control on the front suspension by the ECU  510 . The suspension-OFF switch  462  is a switch to disable the automatic control on the front suspension. The suspension-manual switch  465  is a switch to enable the function of manually setting the front suspension. The suspension-soft switch  463  is a switch to reduce the damping force of the front suspension. The suspension-hard switch  464  is a switch to increase the damping force of the front suspension. The front suspension ECU  510  operates based on signals which are output from these switches. 
     Hereinafter, specific examples of the operation of the front suspension ECU  510  will be described. The front suspension ECU  510  according to the present preferred embodiment is able to perform controls (1) to (8) below, for example. Note that controls (1) to (8) below are mere examples, and the ECU  510  may be configured to perform only some of controls (1) to (8) below. 
     (1) Automatic Vehicle Height Control 
     When the suspension-auto switch  461  is set ON, the ECU  510  performs an automatic vehicle height control. The ECU  510  prevents the suspension cylinders  441  from being extended all out or retracted all in because of fluctuations in the load on the front wheels  104 F of the work vehicle  100 , and controls each cylinder  441  to maintain a state of extending or retracting always near midway of the stroke on average. Specifically, the ECU  510  calculates a stroke length based on a signal which is output from the stroke sensor  442 , and controls the suspension based on the calculated stroke length. For example, if the calculated stroke length is longer than the midway stroke length, the ECU  510  turns the suspension lowering solenoid  402  ON to retract the cylinders  441  and lower the vehicle height. Conversely, if the calculated stroke length is shorter than the midway stroke length the ECU  510  turns the suspension raising solenoid  401  ON to extend the cylinders  441  and raise the vehicle height. As a result, the cylinders  441  are always allowed to make a relatively large move in the extending direction or in the retracting direction, thus maximizing the suspension&#39;s effect of vibration reduction. 
     Now, with reference to  FIG.  12    and  FIG.  13   , an exemplary method of calculating the stroke length of the suspension will be described. 
       FIG.  12    is a diagram showing an exemplary relationship between the output voltage of the stroke sensor  442  and the stroke length of the suspension. The suspension cylinders  441  in this example has a difference in length of about  50  mm between the most extended state and the most retracted state. In  FIG.  12   , the amount of extension of the cylinder  441  is expressed as a stroke length against the length in the most retracted state. The ECU  510  calculates the stroke length from the output voltage of the stroke sensor  442 , based on the data (e.g., a table or a mathematical function) recorded in the storage device representing the relationship shown in  FIG.  12   . The ECU  510  may calculate the stroke length as often as about  20  times in one second, for example. 
       FIG.  13    is a graph showing an example of change in stroke length over time. The ECU  510  stores stroke lengths over a period of, e.g., about  2  seconds in the past (about  40  data points) in a memory, and defines an intermediate value between the maximum value and the minimum value in this period as a current stroke length. Regarding a target range which is supposed to be the neighborhood of the center movable range of stroke length, for any current stroke length that is outside the target range, the ECU  510  controls it to be closer into the target range. 
     (2) Damping Force Switching Control 
     (i) In order to ensure that a proper amount of damping is always attained irrespective of an increase or decrease in the load on the front wheels  104 F, the ECU  510  performs a control of switching the aperture size of the variable orifice  409  to regulate the flow rate of hydraulic oil going in and out of the cylinder  441 . Specifically, in accordance with the pressure of hydraulic oil as measured by the pressure sensor  412 , the ECU  510  switches the aperture size of the variable orifice  409 . This solves problems such as difficulties to stop wobbling of the body of the vehicle under a large load, or the excessive stiffness of the suspension preventing vibrations from being absorbed in the presence of a light load, and makes it possible to constantly maintain a high suspension performance. 
     (ii) Through the user&#39;s manipulations of the switches  463  and  464 , stiffness (i.e., damping force) of the suspension can be changed in three steps of “hard”, “normal”, and “soft”. When the user turns the suspension-soft switch  463  ON, the ECU  510  turns both of the medium damping force-selecting solenoid  404  and the large damping force-selecting solenoid  405  OFF to increase the flow rate through the variable orifice  409 . This results in the stiffness of the suspension being “soft”. When the user turns neither the suspension-soft switch  463  nor the suspension-hard switch  464  ON, the ECU  510  turns the medium damping force-selecting solenoid  404  ON and the large damping force-selecting solenoid  405  OFF, so that the flow rate through the variable orifice  409  is intermediate. This results in the stiffness of the suspension being “normal”. When the user turns the suspension-hard switch  464  ON, the ECU  510  turns the medium damping force-selecting solenoid  404  OFF and the large damping force-selecting solenoid  405  ON to make the flow rate through the variable orifice  409  small. This results in the stiffness of the suspension being “hard”. With this function, regardless of how the work vehicle  100  is equipped or how the work vehicle  100  is supposed to work, the body of the vehicle will always achieve stable behavior to the feeling of the user. 
     (3) Anti-Dive Control 
     During a braking manipulation, in order to prevent a sudden deceleration G-force from causing the suspension to be completely retracted instantaneously, the ECU  510  performs a control of switching the aperture size of the variable orifice  409 , which regulates the flow rate of hydraulic oil. For example, the ECU  510  calculates an acceleration based on temporal change in the traveling speed of the work vehicle  100 , and adjusts the aperture size of the variable orifice  409  in accordance with the magnitude of a G-force that is estimated from the acceleration. This enhances the behavioral stability of the body of the vehicle. 
     (4) Automatic Suspension Locking Control 
     (i) When manipulating a pallet fork, a front loader, etc., ease of manipulation will be degraded if the change in the vehicle height at the front of the work vehicle  100  is too large. Therefore, when the vehicle speed equals a certain value or lower, the ECU  510  automatically immobilizes the extension/retraction action of the suspension cylinders  441 , i.e., locks the suspension (suspension locking), to prevent further change in the vehicle height. Once travel is begun and the vehicle speed increases, the ECU  510  automatically disengages suspension locking, thus enabling the suspension&#39;s effect of vibration reduction. In one implementation, upon detecting a switch from backward travel to forward travel based on the state of the shuttle lever, the ECU  510  may abstain from locking the suspension (i.e., the suspension remains enabled) for a certain period of time after switching, even if the vehicle speed becomes equal to or lower than the certain value. This allows for alleviating the shock when switching from backward travel to forward travel. 
     (ii) In order to reduce fluctuations in plowing depth during plow work, when the three-point link is in a lowered state (e.g., the lower link being level or lower), the ECU  510  automatically locks the suspension. When the three-point link is raised, the ECU  510  automatically unlocks the suspension. This allows for reducing vibrations during a turning operation or during a move. 
     (iii) During draft control, if the towing load becomes higher than a threshold, the ECU  510  automatically locks the suspension. When a certain period of time (e.g., about 3 seconds) has passed since the load became equal to or lower than the threshold, or if the lift arm is located near the upper end of the controllable range (e.g., the lift arm being in a range of 10 degrees from the upper end), the ECU  510  unlocks the suspension. 
     (5) Manual Suspension Locking 
     When the user turns the suspension-OFF switch  462  ON, the ECU  510  turns the suspension unlocking solenoid  403  OFF to immobilize the extension/retraction action of the suspension cylinders  441 . This disables suspension functionality, and reduces change in vehicle height, which will be effective in situations where implement stability is important, such as during plowing work in soft terrains. 
     (6) Manual Up and Down Operation 
     An operator is able to extend or retract the suspension cylinders  441  through manipulations of switches. In the manual operation mode, automatic vehicle height control is not at work, and therefore the height of the front of the work vehicle  100  can be fixed to any arbitrary height. 
     (7) Raising Restriction (Relief-Stopping Control) 
     When the load on the front wheels  104 F exceeds the tolerable range, the suspension can no longer be raised, and the relief valves  426  and  427  of the hydraulic circuit  400  keep operating. In order to avoid this state, and prevent an excessive load from acting on the hydraulic pump  420 , the ECU  510  inhibits any raising output of the cylinders  441  when the load on the front wheels  104 F exceeds the tolerable range. In the case where the pump  420  has a low performance (e.g., the oil temperature is high and the engine revolutions are low), the raising output of the cylinders  441  is inhibited to prevent the flow rate of hydraulic oil from becoming less than is needed for the power steering device. 
     (8) Vehicle Height Control during a Bi-Speed Turn 
     When a bi-speed turn is being made, the ECU  510  automatically retracts the suspension cylinders  441 , and fixes the vehicle height in a low state. Once the bi-speed turn is finished, the ECU  510  restores the original state of the suspension cylinders  441 , and restarts automatic control of the suspension. Through this control, tilt of the vehicle during a bi-speed turn is reduced or prevented, and the turning stability can be enhanced. 
     Hereinafter, some example methods of controlling the vehicle height when making a bi-speed turn will be described. 
       FIG.  14    is a flowchart showing an example of vehicle height control operation executed by the ECU  510  when a bi-speed turn is made. In the example shown in  FIG.  14   , during travel of the work vehicle  100 , based on information representing the bi-speed turn operating status which is sent from the main ECU  530 , the ECU  510  determines whether a bi-speed turn has been begun or not (step S 101 ). Upon determining that a bi-speed turn has been begun, the ECU  510  determines whether automatic vehicle height control is ON or not (step S 102 ). If automatic vehicle height control is OFF, control returns to step S 101 . If automatic vehicle height control is ON, the ECU  510  retracts the suspension cylinders  441 , and fixes the vehicle height in a lowered state (step S 103 ). The ECU  510  may fix the suspension cylinders  441  in the most retracted state, or adjust the amount of lowering the vehicle height in accordance with conditions such as the vehicle speed or turning radius. Based on information representing the bi-speed turn operating status which is sent from the main ECU  530 , the ECU  510  determines whether the bi-speed turn has been finished or not (step S 104 ). When the bi-speed turn is finished, the ECU  510  restarts automatic vehicle height control (step S 105 ). 
     Through the above operation, if a bi-speed turn is made while automatic vehicle height control is being performed, the ECU  510  automatically lowers the vehicle height (to, e.g., the lowest state). As a result, the turning stability can be improved. 
       FIG.  15    is a flowchart showing another example of the operation of the ECU  510 . The flowchart shown in  FIG.  15    is obtained by replacing steps  5102  and  5105  in the flowchart of  FIG.  14    with steps  5112  and  5115 , respectively. In the example of  FIG.  15   , when a bi-speed turn is begun, the ECU  510  determines whether it is possible to further lower the vehicle height by retracting the suspension cylinders  441  (step S 112 ). For example, if automatic vehicle height control is ON, or automatic vehicle height control is OFF but the vehicle height is not in the lowest state, it is deemed possible to further lower the vehicle height. If it is possible to further lower the vehicle height, similarly to the earlier example, the ECU  510  retracts the suspension cylinders  441 , and fixes the vehicle height in a lowered state (step S 103 ). In this example, too, the ECU  510  may fix the suspension cylinders  441  in the most retracted state, or adjust the amount of lowering the vehicle height in accordance with conditions such as the vehicle speed or turning radius. Thereafter, when the bi-speed turn is finished, the ECU  510  restores the state of the suspension before the turn (step S 115 ). 
     With the operation shown in  FIG.  15   , irrespective of whether automatic vehicle height control is ON or not, when a bi-speed turn is made, the ECU  510  automatically lowers the vehicle height (to, e.g., the lowest state). As a result, even when the suspension is locked with the vehicle height being high, tilting of the body of the vehicle during a bi-speed turn is reduced or prevented, and the turning stability can be improved. 
       FIG.  16    is a flowchart showing still another example of the operation of the ECU  510 . The flowchart shown in  FIG.  16    is obtained by replacing steps  5112  and  5103  in the flowchart of  FIG.  15    with steps  5122  and  5123 , respectively. In the example of  FIG.  16   , when a bi-speed turn is begun, the ECU  510  determines whether the stroke length of the suspension is equal to or higher than a threshold or not (step S 122 ). The threshold may be, for example, a lower limit value of stroke length that is tolerated for a stable bi-speed turn to be made. The ECU  510  may determine the threshold in accordance with the speed of the work vehicle  100  or turning radius. The stroke length is determined based on a signal which is output from the stroke sensor  442 . When the stroke length is below the threshold, control returns to step S 101 . If the stroke length is equal to or higher than the threshold, the ECU  510  retracts the suspension cylinders  441 , and fixes the stroke length below the threshold (step S 103 ). In this example, too, the ECU  510  may fix the suspension cylinders  441  in the most retracted state, or adjust the amount of lowering the vehicle height in accordance with conditions such as the vehicle speed or turning radius. Thereafter, when the bi-speed turn is finished, the ECU  510  restores the state of the suspension before the turn (step  5115 ). 
     With the operation shown in  FIG.  16   , vehicle height adjustments are made only when the stroke length of the suspension cylinders  441  is equal to or higher than the threshold. The threshold can be set in accordance with conditions such as the vehicle speed or turning radius. As a result, the condition for lowering the center of gravity of the vehicle body  101  can be flexibly set in accordance with the vehicle speed, the turning radius, or the like. 
     In the above examples, the main ECU  530  causes the running gear  104  to operate in the small turn mode when the angle of rotation of the steering wheel or the steering angle of the front wheels has exceeded a reference angle. The front suspension ECU  510  causes the suspension device to lower the center of gravity of the vehicle body  101  when the small turn mode is begun while the height of the center of gravity of the vehicle body is higher than the reference height. The main ECU  530  may control the braking devices to apply braking to the inner one of the two rear wheels  104 R in the small turn mode. The main ECU  530  may control the presence or absence of braking and the intensity of braking on the inner rear wheel  104 R in accordance with switch manipulations by the user. In accordance with the presence or absence of braking or intensity of braking on the rear wheels  104 R, the front suspension ECU  510  may change the amount of lowering the center of gravity or the condition for lowering the center of gravity. For example, in the case where a bi-speed turn is made with the braking of the rear wheels  104 R, the amount of lowering the center of gravity may be increased or the condition for lowering the center of gravity may be more relaxed than in the case where a bi-speed turn is made without braking the rear wheels  104 R. 
     In the examples shown  FIG.  14    to  FIG.  16   , a control to lower the vehicle height is performed only when making a bi-speed turn. However, a similar control may be applied also when not making a bi-speed turn. For example, the ECU  510  may calculate an angular velocity of yawing during a turn, based on a signal which is output from a sensor that can measure angular velocity (e.g., an IMU included in the work vehicle  100 ), and perform a similar control when the angular velocity becomes equal to or higher than a threshold. 
       FIG.  17    is a block diagram showing an exemplary configuration where the ECU  510  controls the front suspension based on an angular velocity of yawing. In the example shown in  FIG.  17   , the main ECU  530  relies on a signal which is output not from the angle-of-turn sensor  190  but from the angular velocity sensor  192  to determine an angular velocity of yawing during a turn, and sends information representing this angular velocity to the front suspension ECU  510 . Based on the information representing the angular velocity, the front suspension ECU  510  determines whether the work vehicle  100  is in a state of a small turn or not. 
       FIG.  18    is a flowchart showing an example of vehicle height control based on the angular velocity of yawing of the work vehicle  100 . The flowchart shown in  FIG.  18    is obtained by replacing step S 101  in the flowchart shown in  FIG.  14    with step S 141 . In the example of  FIG.  18   , the ECU  510  determines whether an angular velocity of yawing as measured by the angular velocity sensor is equal to or higher than a threshold or not. The threshold may be set to a different value depending on the speed of the work vehicle  100 , presence or absence of the implement  300 , or the like. For example, the threshold may be made smaller as the speed increases. Alternatively, the threshold may be made smaller when the implement  300  is attached than when the implement  300  is not attached. If the angular velocity is equal to or higher than the threshold, control proceeds to step S 102 . The subsequent operation is similar to the operation illustrated in  FIG.  14   . 
     With the control shown in  FIG.  18   , regardless of the bi-speed turn operating status, the vehicle height is lowered whenever the angular velocity during a turn is high, whereby the turning stability can be enhanced. Therefore, even in a work vehicle that lacks bi-speed turn functionality, for example, the traveling stability during a rapid turn can be improved. It may be not only in  FIG.  14   , but also in the example of  FIG.  15    or  FIG.  16   , that the operation of step S 101  can be replaced by the operation of step  5141  shown in  FIG.  18   . A similar control may be performed based on an angular velocity of rolling, rather than yawing, of the work vehicle  100 . 
     The ECU  510  may perform a similar control based on a signal which is output from a sensor that measures the centrifugal acceleration of the work vehicle  100  during a turn. For example, the ECU  510  may perform a control of lowering the center of gravity of the vehicle body  101  when a centrifugal acceleration as measured by a sensor (such as a gyroscope that is included in an IMU provided in the work vehicle  100 ) is higher than a threshold. Such a control allows the turning stability to be improved without calculating an angular velocity of yawing or rolling. 
     Instead of the control of extending or retracting the suspension in each of the above examples, other methods may also be used to control the height of the center of gravity of the vehicle body  101 . For example, the height of the center of gravity may be controlled by using a mechanism to raise or lower a weight that is provided at a predetermined position (e.g., the bottom or the front) of the work vehicle  100 . 
     The controller to perform vehicle height control during a small turn in the above preferred embodiment can be mounted on a work vehicle lacking such functionality as an add-on. Such a controller may be manufactured and sold independently from the work vehicle. A computer program for use in such a controller may also be manufactured and sold independently from the work vehicle. The computer program may be provided in a form stored in a computer-readable, non-transitory storage medium, for example. The computer program may also be provided through downloading via telecommunication lines (e.g., the Internet). 
     The techniques according to the present disclosure are applicable to work vehicles for use in agricultural applications, e.g., tractors, rice transplanters, combines, harvesters, vehicles for crop management, vegetable transplanters, and riding mowers. The techniques according to the present disclosure are also applicable to work vehicles for use in non-agricultural applications, e.g., construction vehicles or snowplow vehicles. 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.