Patent Publication Number: US-9421841-B2

Title: Hydraulic suspension system for work vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 13/687,335, filed on Nov. 28, 2012, which is a divisional of U.S. patent application Ser. No. 12/561,637, filed Sep. 17, 2009, now U.S. Pat. No. 8,406,955, which claims priority to JP 2008-332949 filed on Dec. 26, 2008 and JP 2009-004673 filed on Jan. 13, 2009, all of which are incorporated by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a suspension system for a tractor or another work vehicle. 
     2. Description of the Related Art 
     Among tractors, which are an example of a work vehicle, there are those which comprise a suspension mechanism in the front wheels, such as is disclosed in U.S. Pat. No. 6,145,859, for example. A suspension mechanism generally acts in rising and falling motions in accordance with unevenness in the ground surface, whereby the unevenness in the ground is absorbed and the ride is made more comfortable. 
     Worked ground on which the work vehicle travels has much unevenness in comparison with paved roads, and the work vehicle is often equipped with an implement, which causes the suspension stroke position of the suspension mechanism to deviate upward or downward of the vehicle body from a preset target range (when the suspension stroke position of the suspension mechanism is positioned in the target range, the vehicle height is maintained within the height range), whereby the vehicle height goes beyond the predetermined height range (unusual fluctuation). 
     In this case, it is preferable to minimize unusual fluctuations in the vehicle height of the work vehicle and to maintain the vehicle height within the predetermined range. Therefore, it has been proposed that when the suspension stroke position of the suspension mechanism deviates upward or downward of the vehicle body from the target range as previously described, the vehicle height should be maintained as much as possible within the predetermined height range by a suspension reference position variation mechanism for moving the suspension stroke position of the suspension mechanism to the target range. 
     A suspension system of a work vehicle such as the one previously described has the following problem in cases in which a suspension reference position variation mechanism is provided for varying the reference position of the suspension stroke in relation to the suspension mechanism. Specifically, every time the suspension mechanism operates and the suspension stroke position of the suspension mechanism deviates upward or downward of the vehicle body from the target range, the operative frequency of the suspension reference position variation mechanism is extremely high when the suspension reference position variation mechanism operates, and the suspension reference position variation mechanism has a disadvantage in terms of durability. 
     SUMMARY OF THE INVENTION 
     The present invention is a suspension system of a work vehicle, the object of which is to reduce the operative load of the suspension reference position variation mechanism and to create favorable conditions pertaining to durability. 
     In order to achieve this objective, the hydraulic suspension system according to the present invention comprises a suspension mechanism for a traveling vehicle body, a suspension reference position variation mechanism for varying the reference position of a suspension stroke of the suspension mechanism, a suspension stroke position sensor for detecting the suspension stroke position (rising/lowering displacement), and a control unit for controlling the suspension reference position variation mechanism on the basis of the suspension stroke position so that the stroke of the suspension mechanism moves toward a preset target range. The hydraulic suspension system also comprises a stopped state detector for detecting a state in which operation of the suspension mechanism has stopped, and a control management unit for supplementarily, or preferably preferentially, controlling the suspension reference position variation mechanism. The control management unit controls the suspension reference position variation mechanism so that when the operation of the suspension mechanism is detected to be in a stopped state, the operation of the suspension mechanism moves toward the target range, in preference to control based on the suspension stroke position obtained from the suspension stroke position sensor. 
     For example, when a loader, a plow, or another implement is linked to the work vehicle and the implement is raised to a raised non-operating state, the entire load of the implement is applied to the traveling vehicle body; and when the implement is lowered to a lowered operating state, the load applied to the traveling vehicle body decreases because the implement is supported on the ground. Particularly in cases in which earth has been lifted by a loader, a large load is applied to the traveling vehicle body by the weight of the lifted earth; and in cases in which the earth has been released by the loader, there is no longer the large load that was applied to the traveling vehicle body by the lifted earth. 
     In other words, in cases in which the suspension mechanism operates due to a small load caused by ground surface unevenness or the like, the suspension mechanism undergoes return displacement in a vehicle body lowering direction after being displaced in a vehicle body rising direction, or return displacement in the vehicle body rising direction after being displaced in the vehicle body lowering direction. In cases in which work has been performed by the implement, the large load applied to the traveling vehicle body by whatever is being handled by the implement is either created or lost, whereby the suspension mechanism is greatly displaced in the vehicle body rising direction or the vehicle body lowering direction, and there are instances in which there is no return displacement or the return displacement is small. In other words, when the suspension mechanism has been displaced in the vehicle body rising direction or the vehicle body lowering direction, a so-called state of stopped operation occurs in the suspension mechanism, wherein the suspension mechanism remains displaced in the vehicle body rising direction or the vehicle body lowering direction, and either there is no return displacement thereafter or the return displacement is small. 
     In view of this, the state in which the operation of the suspension mechanism has stopped is detected by a stopped state detector, and based on the detection information, a control management unit controls the reference position variation mechanism in preference to the detection information from the suspension stroke position sensor, so that the reference position of the suspension mechanism moves toward a target range. The terrain clearance of the traveling vehicle body can thereby be maintained in a predetermined height range even when the suspension mechanism undergoes significant operative displacement as a result of the implement being linked or the implement doing work. 
     Moreover, in cases in which displacement caused by a small load of the suspension mechanism continues and no state of stopped operation occurs in the suspension mechanism, the reference position variation mechanism is controlled by the control unit on the basis of the detection information from the suspension stroke position sensor. At this time, deterioration of the reference position variation mechanism is suppressed by making the operating frequency of the reference position variation mechanism relatively low. In cases in which a load is applied by the weight of the implement or by the weight of whatever is being handled by the implement, or the load from whatever is being handled by the implement is lost; the control management unit controls the reference position variation mechanism on the basis of the detection information from the stopped state detector, in preference to the detection information from the suspension stroke position sensor. It is thereby possible to quickly adapt to operative displacement of the suspension mechanism caused by loads resulting from the weight of the implement or the weight of whatever is being handled by the implement, and to move the reference position of the suspension mechanism in the target direction. As a result, the traveling vehicle body and the implement are maintained at an appropriate terrain clearance or a height close thereto. 
     In order to achieve the objectives described above, another hydraulic suspension system according to the present invention comprises an intermediate value calculator for detecting the maximum position and minimum position of the suspension stroke and calculating an intermediate value between the maximum and minimum positions, and a cumulating unit for cumulating the frequency at which the intermediate position deviates from a target range as a cumulative frequency. The control unit controls the operation variation mechanism so that the intermediate position moves in a preset target direction when the cumulative frequency obtained from the cumulating unit exceeds a preset cumulative frequency. The control management unit controls the suspension reference position variation mechanism so that when the operation of the suspension mechanism is detected to be in a stopped state, the operation of the suspension mechanism moves toward the target range, in preference to control based on the cumulative frequency obtained from the cumulating unit. 
     In this configuration, when the reference position deviates from the target range, an immediate correction is not necessarily made by the reference position variation mechanism. In other words, in cases in which displacement caused by a small load continues in the suspension mechanism, the suspension mechanism is either displaced in the vehicle body lowering direction after being displaced in the vehicle body rising direction, or is displaced in the vehicle body rising direction after being displaced in the vehicle body lowering direction, whereby the maximum and minimum positions occur sequentially. The maximum position and minimum position of suspension mechanism displacement are thereby detected. The intermediate position between the maximum and minimum positions is compared with the target range and the frequency at which the intermediate position deviates from the target range is cumulated, and when the cumulative frequency exceeds a set cumulative frequency, the reference position variation mechanism functions so that the intermediate position moves toward the target range. Therefore, even when the suspension mechanism displacement is displaced from the target range in the vehicle body rising direction and the vehicle body lowering direction, and also when the intermediate position is displaced from the target range in the vehicle body rising direction and the vehicle body lowering direction, the reference position variation mechanism does not operate if the frequency at which the intermediate position deviates from the target range does not exceed the set cumulative frequency. 
     A state in which operation of the suspension mechanism has stopped is detected by the stopped state detector, and the control management unit controls the reference position variation mechanism on the basis of the detection results, in preference to control based on the cumulative frequency obtained from the cumulating unit. Thereby, in cases in which the suspension mechanism undergoes significant operative displacement due to the linking of the implement or to the implement doing work, adaptation is quickly made and the reference position of the suspension mechanism is moved in the target direction, whereby the terrain clearance of the traveling vehicle body can be maintained in the predetermined height range. 
     Other characteristics and merits of the present invention should be clear from the descriptions of the embodiments using the following drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an overall side view of an agricultural tractor; 
         FIG. 2  is a side view of the vicinity of a front wheel axle case, a support bracket, and a hydraulic cylinder; 
         FIG. 3  is a drawing showing a hydraulic circuit structure of the hydraulic cylinders; 
         FIG. 4  is a perspective view of the support bracket; 
         FIG. 5  is a diagram showing the state of the suspension stroke position (extended/retracted position) of the hydraulic cylinders; 
         FIG. 6  is a block diagram showing the relationship between the control unit and the pilot valves; 
         FIG. 7  is a function block diagram showing the functions of the control unit; 
         FIG. 8  is a diagram showing the first half of the flow of controlling the hydraulic cylinders; 
         FIG. 9  is a diagram showing the second half of the flow of controlling the hydraulic cylinders; 
         FIG. 10  is an explanatory diagram showing the operative displacement of the hydraulic cylinders; and 
         FIG. 11  is an explanatory diagram showing the change in the operating rate of the hydraulic cylinders. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments of the present invention are described hereinbelow with reference to the accompanying drawings. The characteristics of one embodiment can be combined with the characteristics of another embodiment, and such combinations are included within the scope of the present invention as long as there are no discrepancies. 
     Examples of the present invention are described hereinbelow with reference to the drawings. 
     A tractor, given as an example of a work vehicle, is configured comprising front wheels  1 ,  1  as a pair of left and right travel devices and a pair of left and right rear wheels  2 ,  2 , as shown in  FIG. 1 . 
     A traveling vehicle body  5  of the tractor comprises an engine  4 , a transmission case  3  linked to the rear part of the engine  4 , and a front wheel support frame  5   a  linked to the bottom part of the engine  4  so as to support the front wheels, as shown in  FIG. 1 . The traveling vehicle body  5  comprises lift arms  3   a  mounted in a vertically swingable manner on both transverse sides of the rear part of the transmission case  3 . The right and left front wheels  1  are attached to the traveling vehicle body  5  via a suspension mechanism  100 . The right and left rear wheels  2  are directly attached to the transmission case  3  at the rear part of the vehicle body constituting the traveling vehicle body  5 , without the aid of a suspension mechanism. 
     The tractor is capable of functioning as any of a variety of ridden work vehicles in which various implements are linked in a raisable and lowerable manner to the rear part of the traveling vehicle body  5 . For example, the tractor functions as a ridden tiller in which a plow (not shown) is linked in a raisable and lowerable manner between a raised non-working state and a lowered working state, to the rear part of the traveling vehicle body  5  via the lift arms  3   a . The tractor also functions as a ridden loader in which a front loader (not shown) is linked in a raisable and lowerable manner to the front part of the traveling vehicle body  5 . 
     At the bottom part of the engine  4  disposed at the front part of the vehicle body, a support frame  5  is linked and made to extend forward, a support bracket  6  having a U shape in a side view is supported so as to be capable of swinging vertically about a transverse axis P 1  at the rear part of the support frame  5 , and two hydraulic cylinders  7  (included in the suspension mechanism) are connected between the front part of the support frame  5  and the front part of the support bracket  6 , as shown in  FIGS. 1, 2, and 4 . A front wheel axle case  8  is supported so as to be capable of rolling about a longitudinal axis P 2  of the support bracket  6 , and the right and left front wheels  1  are supported on the right and left sides of the front wheel axle case  8 . 
     Next, the hydraulic circuit structure of the hydraulic cylinders  7  will be described. 
     The hydraulic cylinders  7  are configured as double-acting cylinders comprising oil chambers  7   a  on the bottom side and oil chambers  7   b  on the piston side, as shown in  FIG. 3 . An oil passage  9  is connected to the oil chambers  7   a  of the hydraulic cylinders  7 , and connected to the oil passage  9  are a gas-filled accumulator  11 , a pair of pilot-operated check valves  13 , and a relief valve  15  for preserving the hydraulic circuit. A pilot-operated switching valve  17  having orifices of three different aperture sizes, “large,” “medium,” and “small,” are provided to the frontal portion of the accumulator  11 , and pilot valves  20  are provided for operating the switching valve  17 . An oil passage  10  is connected to the oil chambers  7   b  of the hydraulic cylinders  7 , and connected to the oil passage  10  are a gas-filled accumulator  12 , a pair of pilot-operated check valves  14 , and a relief valve  16  for preserving the hydraulic circuit. 
     A pilot valve  19  through which actuating oil passes to and from the check valves  13 ,  14  is provided as shown in  FIG. 3 , and the check valves  13 ,  14  are operated by the pilot valve  19  to a shutoff state (a state of shutting off the accumulators  11 ,  12  from the oil chambers  7   a ,  7   b  of the hydraulic cylinders  7 ) and an open state (a state of allowing actuating oil to flow from both accumulators  11 ,  12  to the oil chambers  7   a ,  7   b  of the hydraulic cylinders  7 , and also from the oil chambers  7   a ,  7   b  of the hydraulic cylinders  7  to the accumulators  11 ,  12 ). 
     The actuating oil of a pump  30  is supplied to a control valve  18  (equivalent to a suspension reference position variation mechanism) via a filter  31 , a flow-dividing valve  32 , and a check valve  33 ; and a relief valve  34  is connected between the flow-dividing valve  32  and the check valve  33 , as shown in  FIG. 3 . An oil passage  21  is connected from the control valve  18  to the portion of the oil passage  9  between the oil chambers  7   a  of the hydraulic cylinders  7  and the check valves  13 , and an oil passage  22  is connected from the control valve  18  to the portion of the oil passage  10  between the oil chambers  7   b  of the hydraulic cylinders  7  and the check valves  14 . 
     The control valve  18  is configured to be pilot-operated in a three-position switching system, including a raised position  18 U for supplying actuating oil to the oil passage  21  (the oil chambers  7   a  of the hydraulic cylinders  7 ), a lowered position  18 D for supplying actuating oil to the oil passage  22  (the oil chambers  7   b  of the hydraulic cylinders  7 ), and a neutral position  18 N; and a pilot valve  29  is provided for operating the control valve  18 , as shown in  FIG. 3 . 
     The oil passage  21  is provided with a pilot-operated check valve  23  and an aperture  25 , as shown in  FIG. 3 . The oil passage  22  is provided with a pilot-operated check valve  24 , a check valve  26  (the check valve  24  is near the oil passage  10  and the check valve  26  is near the control valve  18 ), and an aperture  27 ; and a relief valve  28  is connected between the check valve  24  and the check valve  26  (the aperture  27 ). 
     The pilot valves  19 ,  20 ,  29  are electromagnetically operated. The pilot valve  19  and the pilot valves  20 ,  29  are operated by a control unit  35 , described hereinafter, and the check valves  13 ,  14 , the control valve  18 , and the switching valve  17  are operated. 
     Next, the action of the hydraulic cylinders  7  will be described. 
     In cases in which the control valve  18  is operated to the neutral position  18 N and the check valves  13 ,  14  are operated to the open state as shown in  FIG. 3 , when the front wheel axle case  8  and the support bracket  6  begin to swing vertically about the transverse axis P 1  in accordance with unevenness in the ground surface, the hydraulic cylinders  7  reciprocate, actuating oil flows back and forth between the oil chambers  7   a ,  7   b  of the hydraulic cylinders  7  and the accumulators  11 ,  12 , and the hydraulic cylinders  7  function as a suspension mechanism having a spring constant K 1 . 
     In this case, the pressure in the oil chambers  7   b  of the hydraulic cylinders  7  and in the oil passage  10  is maintained at a set pressure MP 1  by the relief valve  28 . The following formula (1) holds true when PH is the pressure in the oil chambers  7   a  of the hydraulic cylinders  7 , AH is the pressurized surface area of the pistons of the oil chambers  7   a  of the hydraulic cylinders  7 , AR is the pressurized surface area of the pistons of the oil chambers  7   b  of the hydraulic cylinders  7  (AR is less than AH in proportion to the pistons), M is the weight on the front part of the vehicle body (the weight on the hydraulic cylinders  7 ), and g is the gravitational acceleration.
 
 M×g=PH×AH−MP 1 ×AR   Formula (1)
 
     Since the set pressure MP 1  of the oil chambers  7   b  of the hydraulic cylinders  7 , the pressurized surface area AH of the pistons of the oil chambers  7   a  of the hydraulic cylinders  7 , and the pressurized surface area AR of the pistons of the oil chambers  7   b  of the hydraulic cylinders  7  are constant, the pressure PH of the oil chambers  7   a  of the hydraulic cylinders  7  is thereby higher than the set pressure MP 1  of the oil chambers  7   b  of the hydraulic cylinders  7 , and the pressure PH varies depending on the weight M on the front part of the vehicle body (the weight on the hydraulic cylinders  7 ). 
     The spring constant K 1  of the hydraulic cylinders  7  is determined according to the pressures PH, MP 1  of the oil chambers  7   a ,  7   b  of the hydraulic cylinders  7 . The spring constant K 1  increases along with an increase in the pressure PH of the oil chambers  7   a  of the hydraulic cylinders  7 , and decreases along with a decrease in the pressure PH of the oil chambers  7   a  of the hydraulic cylinders  7 . Therefore, the spring constant K 1  of the hydraulic cylinders  7  is determined according to the weight M on the front part of the vehicle body (the weight on the hydraulic cylinders  7 ), and the spring constant K 1  increases along with an increase in the weight M on the front part of the vehicle body (the weight on the hydraulic cylinders  7 ) and decreases along with a decrease in the weight M on the front part of the vehicle body (the weight on the hydraulic cylinders  7 ). 
     When the control valve  18  is operated to the raised position  18 U and the check valves  13 ,  14  are operated to the shutoff position as shown in  FIG. 3 , actuating oil is supplied to the oil chambers  7   a  of the hydraulic cylinders  7  from the control valve  18 , and actuating oil is drained from the oil chambers  7   b  of the hydraulic cylinders  7  via the check valve  24  (operated to the open state by the pilot actuating oil of the control valve  18 ) and the relief valve  28 . In this case, the pressure of the oil chambers  7   b  of the hydraulic cylinders  7  and the oil passage  10  is maintained at the set pressure MP 1  by the relief valve  28 . 
     The hydraulic cylinders  7  thereby extend and the front part of the vehicle body rises (equivalent to a state in which the action of the hydraulic cylinders  7  (the suspension mechanism) has changed to the vehicle body rising direction). When the control valve  18  is then operated to the neutral position  18 N and the check valves  13 ,  14  are operated to the open state, the hydraulic cylinders  7  function as a suspension mechanism as previously described while the hydraulic cylinders  7  remain in an extended state. 
     When the control valve  18  is operated to the lowered position  18 D and the check valves  13 ,  14  are operated to the shutoff position as shown in  FIG. 3 , actuating oil is supplied from the control valve  18  to the oil chambers  7   b  of the hydraulic cylinders  7 , and actuating oil is drained from the oil chambers  7   a  of the hydraulic cylinders  7  via the check valve  23  (operated to the open state by the pilot pressure of the control valve  18 ), the aperture  25 , and the control valve  18 . In this case, the pressure of the oil chambers  7   b  of the hydraulic cylinders  7  and the oil passage  10  is maintained at the set pressure MP 1  by the relief valve  28 . 
     The hydraulic cylinders  7  thereby retract and the front part of the vehicle body lowers. When the control valve  18  is then operated to the neutral position  18 N and the check valves  13 ,  14  are operated to the open state, the hydraulic cylinders  7  act as a suspension mechanism as previously described while the hydraulic cylinders  7  remain in a retracted state. 
     A pressure sensor  36  is provided for detecting the pressure of the oil chambers  7   a  of the hydraulic cylinders  7  as shown in  FIGS. 3 and 6 , a detection value of the pressure sensor  36  is inputted to the control unit  35 , and the weight M on the front part of the vehicle body (the weight on the hydraulic cylinders  7 ) is calculated based on the detection value of the pressure sensor  36 . 
     When the implement mounted on the front part of the vehicle body causes the weight M on the front part of the vehicle body (the weight on the hydraulic cylinders  7 ) to increase (for example, a state in which earth is lifted (loaded) or loads are stacked up by a front loader), the spring constant K 1  of the hydraulic cylinders  7  increases, the switching valve  17  is therefore accordingly operated toward an aperture (toward the orifice having a “small” diameter), and the damping force of the hydraulic cylinders  7  increases. 
     When the implement mounted on the front part of the vehicle body causes the weight M on the front part of the vehicle body (the weight on the hydraulic cylinders  7 ) to decrease (for example, a state in which earth is released or loads are dropped by the front loader), the spring constant K 1  of the hydraulic cylinders  7  decreases, the switching valve  17  is therefore accordingly operated toward an aperture (toward the orifice having a “large” diameter), and the damping force of the hydraulic cylinders  7  decreases. 
     As can be seen from the graph of  FIG. 5 , the maximum value A 1  is a value corresponding to a point at which the suspension stroke position of the hydraulic cylinders  7  is displaced in the vehicle body lowering direction after being displaced in the vehicle body rising direction (the point at which the hydraulic cylinders  7  switch from extending to retracting). The minimum position A 2  is a value corresponding to the point at which the suspension stroke position of the hydraulic cylinders  7  is displaced in the vehicle body rising direction after being displaced in the vehicle body lowering direction (the point at which the hydraulic cylinders  7  switch from retracting to extending). 
     In this case, the value corresponding to the suspension stroke position (extended position) of the hydraulic cylinders  7  from the elapsed time point of the previous control cycle T 12  to the elapsed time point of the current control cycle T 12  (refer to the time point T 2  in  FIG. 5 ) is stored as a value corresponding to the new suspension stroke position (extended position) of the hydraulic cylinders  7 , and the values which correspond to the hydraulic cylinder  7  suspension stroke positions (extended positions) that are older than the time point T 1 , which is older than the time point T 2  by a set time period T 11 , are deleted. With each elapse of the control cycle T 12 , some of the values corresponding to suspension stroke positions (extended positions) of the hydraulic cylinders  7  stored in the control device are renewed. 
     A suspension stroke position sensor  37  is provided for detecting the suspension stroke position (extended position) of the hydraulic cylinders  7  as shown in  FIG. 6 , and a detection value of the suspension stroke position sensor  37  is inputted to the control unit  35  and stored in the control unit  35  as a value corresponding to the suspension stroke position (extended position) of the hydraulic cylinders  7 . In this case, the suspension stroke position (extended position) of the hydraulic cylinders  7  is detected by attaching a reciprocating suspension stroke position sensor  37  directly to the hydraulic cylinders  7  and detecting the suspension stroke position (extended position) of the hydraulic cylinders  7 , or by attaching a rotary suspension stroke position sensor  37  to a position on the transverse axis P 1  shown in  FIG. 2  and detecting the angle of the support bracket  6  in relation to the support frame  5 . 
     The center position of the action of the hydraulic cylinders  7  shown in  FIG. 5  is set by the control unit  35 , and when the suspension stroke position (extended position) of the hydraulic cylinders  7  is in the center position, the vehicle body is substantially parallel to the ground surface (substantially horizontal). The control unit  35  sets a target range H 1  having a range of a certain extent in the vehicle body rising direction or the vehicle body lowering direction in relation to the center position. 
       FIG. 7  shows functional units installed in the control unit  35 , particularly functional units relating to the present invention. Examples of such functional units include a sensor signal processor  51 , a maximum value calculator  52 , a minimum value calculator  53 , an intermediate value calculator  54 , a target range setting unit  55 , a suspension reference position determination unit  56 , an oil pressure controlled amount calculator  57 , a stopped state detector  58 , and a control management unit  60  for managing these functional units. The control management unit  60  comprises a cumulating unit  61 , and a cumulative number setting unit  62 . 
     The sensor signal processor  51  receives detection signals from the pressure sensor  36 , the operative position sensor (suspension stroke position sensor)  37 , and an operation detection sensor  38  for detecting the operated amounts of operative tools (not shown); and converts the signals into data (values) that will be handled in the control unit  35 . The maximum value calculator  52  calculates a maximum value corresponding to the maximum position of the suspension stroke on the basis of the data delivered from the sensor signal processor  51 . The minimum value calculator  53  calculates a minimum value corresponding to the minimum position of the suspension stroke on the basis of the data delivered from the sensor signal processor  51 . The intermediate value calculator  54  calculates an intermediate value from the maximum value and the minimum value corresponding to the minimum position. The intermediate value is commonly the mean value of the maximum value and the minimum value, but various other options can be used as the intermediate value. For example, the intermediate value may be the mean value of a plurality of the aforementioned mean values, or the intermediate value may be the mean value of a plurality of the aforementioned mean values. Furthermore, the mean value is commonly calculated as an arithmetic mean value, but depending on the suspension design, the mean value can also be a weighted mean value. 
     The target range setting unit  55  sets a target range for comparison with the intermediate value. The suspension reference position determination unit  56  determines a suspension reference position as a reference position of the suspension stroke (suspension action reference position) of the suspension mechanism (hydraulic cylinders)  7 , and, based on this determined suspension reference position, the oil pressure controlled amount calculator  57  calculates a controlled amount for the control valve (equivalent to a suspension reference position variation mechanism)  18  so that the suspension reference position is achieved. 
     The stopped state detector  58  detects a state of stoppage in the action of the hydraulic cylinders  7  on the basis of information from the sensor signal processor  51  and other units. When a state of stoppage in the action of the suspension mechanism is detected by the stopped state detector  58 , the control management unit  60  controls the control valve (equivalent to a suspension reference position variation mechanism)  18  via the pilot valve  29  so that the action of the suspension mechanism  7  moves toward the target range, in preference to control based on the suspension stroke position obtained from the suspension stroke position sensor  37 . 
     Next, the flow of control relating to the suspension mechanism (hydraulic cylinders)  7  will be described using  FIGS. 8 and 9 . 
     A first determining number NDS, a second determining number NDL, a first determining number NUS, and a second determining number NUL are set in the cumulative number setting unit  62  of the control management unit  60 , and initially the second determining number NDL is set as a lowering determining frequency ND 1  while the second determining number NUL is set as a rising determining frequency NU 1  (step S 1 ). In this case, the first determining number NDS is set to a lower value than the second determining number NDL, and the first determining number NUS is set to a lower value than the second determining number NUL. A cumulative frequency N is set in the cumulative number setting unit  62  of the control management unit  60 . 
     First, the cumulative frequency N is set to “0” (step S 2 ). The control valve  18  is operated to the neutral position  18 N and the check valves  13 ,  14  are operated to the open state (the hydraulic cylinders  7  acting as a suspension mechanism) (step S 3 ). In this state, counting of the control cycle T 12  is initiated (step S 4 ), the suspension stroke position (extended position) of the hydraulic cylinders  7  is detected, and a value corresponding to this position is stored (step S 5 ). 
     When the control cycle T 12  elapses (step S 6 ) (refer to time point T 2  in  FIG. 5 ), a maximum value A 1  corresponding to the maximum position of the action of the hydraulic cylinders  7  and a minimum value A 2  corresponding to the minimum position are detected (step S 7 ) from the values corresponding to all of the suspension stroke positions (extended positions) of the hydraulic cylinders  7  older than the time point T 2  by a set time T 11  (refer to the span from time point T 2  to time point T 1  in  FIG. 5 ), and an intermediate value B 1  between the maximum and minimum values A 1 , A 2  (an intermediate value between the maximum and minimum values A 1 , A 2 ) is detected (step S 8 ). 
     In steps S 6  and S 7 , when the set time period T 11  is set to the extent of being slightly longer than one resonance frequency cycle of the hydraulic cylinders  7  (suspension mechanism), one maximum value A 1  and one minimum value A 2  are detected during the set time period T 11 , and in this case, an intermediate value B 1  is detected from one maximum and minimum value A 1 , A 2  each (step S 8 ). 
     In steps S 6  and S 7 , when the set time period T 11  is set to be somewhat long, a plurality of maximum values A 1  and a plurality of minimum values A 2  are detected during the set time period T 11 . In this case, the greatest maximum value A 1  is detected among the plurality of maximum values A 1 , and smallest minimum value A 2  is detected among the plurality of minimum values A 2 , and an intermediate value B 1  is detected from the greatest maximum value A 1  and the smallest minimum value A 2  (step S 8 ). 
     When the intermediate value B 1  is detected the intermediate value B 1  and the target range H 1  are compared (step S 9 ). When the intermediate value B 1  deviates in the vehicle body lowering direction from the target range H 1 , “1” is subtracted from the cumulative frequency N as an abnormal frequency (step S 10 ), and when the intermediate value B 1  deviates in the vehicle body rising direction from the target range H 1 , “1” is added to the cumulative frequency N (step S 11 ). When the intermediate value B 1  enters the target range H 1 , nothing is added to or subtracted from the cumulative frequency N. 
     Next, before the continuation of the controlling of the hydraulic cylinders  7  is described, the displacement of the hydraulic cylinders  7  will be additionally described based on  FIGS. 10 and 11 . 
       FIG. 10  is an explanatory diagram showing the operational displacement of the hydraulic cylinders  7 .  FIG. 11  is an explanatory diagram showing the change in operating rate of the hydraulic cylinders  7 . The operative displacement shown in range A of  FIG. 10  is the operative displacement in the hydraulic cylinders  7  resulting from unevenness in the ground surface, and the operating rate shown in range A of  FIG. 11  is the operating range corresponding to the operative displacement shown in range A of  FIG. 11 . The operating rate V 1  shown in  FIG. 11  is the operating range occurring after the operative displacement S 1  has occurred. 
     In other words, in cases in which operative displacement in the vehicle body rising direction has occurred in the hydraulic cylinders  7  due to unevenness in the ground surface, return displacement occurs in the vehicle body lowering direction after the operative displacement, and the operation returns to the target range H 1 . In cases in which operative displacement in the vehicle body lowering direction has occurred in the hydraulic cylinders  7  due to unevenness in the ground surface, return displacement in the vehicle body rising direction occurs after the operative displacement and the operation returns to the target range H 1 . Therefore, in cases in which operative displacement has occurred in the hydraulic cylinders  7  due to unevenness in the ground surface, even if the operation of the hydraulic cylinders  7  deviates from the target range H 1 , the operating rate occurring thereafter causes the operation to return to the target range H 1 . The operating rate that causes the operation of the hydraulic cylinders  7  to return to the target range H 1  after deviating from the target range H 1  occurs within approximately one-quarter of the characteristic resonance period of the hydraulic cylinders  7 . 
     The operative displacements S 2  and S 3  in range B of  FIG. 10  are operative displacements in which earth has been lifted by a linked front dozer. The operating rate shown in range B of  FIG. 11  is the operating rate corresponding to the operative displacements S 2  and S 3  shown in range B of  FIG. 10 . The operating rate V 2  shown in  FIG. 11  represents a hypothetical operating rate corresponding to the operative displacement S 2 . 
     In other words, in cases in which an operative displacement in the vehicle body rising direction has occurred in the hydraulic cylinders  7  as a result of the weight of earth lifted by a front dozer, return displacement in the vehicle body lowering direction does not occur. Even if return displacement does occur, the return displacement is small, and the operation does not return to the target range H 1 . Specifically, in cases in which earth is lifted up by a front loader and operative displacement in the vehicle body rising direction has occurred in the hydraulic cylinders  7  as a result of the weight of the lifted earth, the operation of the hydraulic cylinders  7  stops in a position outside of the target range H 1 , there is no return displacement back to the target range H 1 , and the operating rate shown by the hypothetical operating rate V 2  does not occur. 
     In cases in which earth is released by the front dozer, the loss of the load from the earth weight applied to the traveling vehicle body until the earth was released causes the operative displacement occurring in the hydraulic cylinders  7  to be operative displacement in the vehicle body lowering direction. Therefore, the case of releasing earth differs from the case of lifting earth in terms of the displacement direction of operative displacement, but is similar to the case of lifting earth with respect to the fact that the operation of the hydraulic cylinders  7  stops at a position outside of the target range H 1 , return displacement back to the target range H 1  does not occur, and an operating rate for returning to the target range H 1  does not occur. 
     The stopped state detector  58  detects states of stoppage in the operation of the hydraulic cylinders  7  on the basis of detection information from the position sensor  37 . 
     Specifically, operation data of the hydraulic cylinders  7  as detected by the position sensor  37  is differentially processed and the operating rate of the hydraulic cylinders  7  is calculated. After the operation of the hydraulic cylinders  7  has deviated from the target range H 1 , it is determined whether or not there is an operating rate for returning the displacement of the operation of the hydraulic cylinders  7  back to the target range H 1 , and the value of the return displacement operating rate is determined in the case that there is a return displacement operating rate. In cases in which it is determined that a return displacement operating rate has not occurred from the time the operation of the hydraulic cylinders  7  deviates from the target range H 1  until the set measurement time elapses, and in cases in which a return displacement operating rate has occurred and it is judged that the value of the occurring return displacement operating rate is less than the set reference value, the hydraulic cylinders  7  are detected to have stopped operating after the operation of the hydraulic cylinders  7  has deviated from the target range H 1 . A time period greater than one-quarter of the characteristic resonance period of the hydraulic cylinders  7  is set as the set measurement time. 
     When the intermediate value B 1  has been detected, the intermediate value B 1  and the target range H 1  have been compared, and the cumulative frequency N has been added to or subtracted from (steps S 4  to S 11 ), the control unit  35  determines for each instance thereof whether or not the stopped state detector  58  has detected an state of stopped operation (step S 12 ). In cases in which it has been determined that the stopped state detector  58  has detected a state of stopped operation, the control management unit  60  measures the elapsed time t after following the detection of the state of stopped operation. 
     The control management unit  60  sets a lowering setting cumulative frequency ND 1  (equivalent to the set cumulative frequency) and a rising setting cumulative frequency NU 1  (equivalent to the set cumulative frequency) relative to the cumulative number setting unit  62 . 
     A first reference number NDS, a second reference number NDL, a first reference number NUS, and a second reference number NUL are set in the control unit  35 . The control management unit  60  first sets the second reference number NDL as the lowering setting cumulative frequency ND 1  and the second reference number NUL as the rising setting cumulative frequency NU 1  in the cumulative number setting unit  62  (step S 1 ). In this case, the first reference number NDS is set to a lower value than the second reference number NDL, and the first reference number NUS is set to a lower value than the second reference number NUL. 
     In a state in which the second reference number NDL has been set as the lowering setting cumulative frequency ND 1  (equivalent to the operating frequency decreasing) and the second reference number NUL has been set as the rising setting cumulative frequency NU 1  (equivalent to the operating frequency decreasing), and also when it has been determined that the implement-rising operation has been performed, the control management unit  60  sets the first reference number NDS as the lowering setting cumulative frequency ND 1  (equivalent to the operating frequency increasing) and the first reference number NUS as the rising setting cumulative frequency NU 1  (equivalent to the operating frequency increasing) (step S 14 ). 
     A designated elapsed time t 0  following the detection of the state of stopped operation by the stopped state detector  58  is set. For this designated elapsed time t 0 , approximately 10 seconds is preferred as the time needed in order to make the adjustments for returning the operation of the hydraulic cylinders  7  from the stopped state of operation back to the target range H 1 . 
     The first reference number NDS is maintained as the lowering setting cumulative frequency ND 1  and the first reference number NUS is maintained as the rising setting cumulative frequency NU 1  until the designated elapsed time t 0  following the detection of the state of stopped operation by the stopped state detector  58  reaches the designated elapsed time t 0 . When the elapsed time t following the detection of the state of stopped operation by the stopped state detector  58  reaches the designated elapsed time t 0 , the original second reference number NDL is restored as the lowering setting cumulative frequency ND 1 , and the original second reference number NUL is restored as the rising setting cumulative frequency NU 1 . 
     When the lowering setting cumulative frequency ND 1  and the rising setting cumulative frequency NU 1  are set as previously described, the cumulative frequency N is compared with the lowering setting cumulative frequency ND 1  and the rising setting cumulative frequency NU 1 , and it is determined that the cumulative frequency N has reached (or fallen below) the lowering setting cumulative frequency ND 1  (step S 16 ); the front part of the vehicle body lowers, it is determined that the vehicle body is in a forward decline in relation to the ground surface, the control valve  18  is controlled to the raised position  18 U, and the check valves  13 ,  14  are controlled to the operating state (step S 17 ). 
     The hydraulic cylinders  7  thereby extend and raise the front part of the vehicle body in a state in which the pressure in the oil chambers  7   b  of the hydraulic cylinders  7  and the oil passage  10  is maintained at the set pressure MP 1  by the relief valve  28 . When the hydraulic cylinders  7  extend in proportion to the difference between the intermediate value B 1  and the target range H 1  (when the intermediate value B 1  enters the target range H 1 ), the process advances to step S 2 , the cumulative frequency N is set to “0,” and the control valve  18  is operated to the neutral position  18 N, returning to a state in which the check valves  13 ,  14  have been operated to the open state (a state in which the hydraulic cylinders  7  act as a suspension mechanism). 
     When it is determined that the cumulative frequency N has reached (or exceeded) the rising setting cumulative frequency NU 1  (step S 19 ), it is judged that the front part of the vehicle body has risen and the vehicle body is in a forward incline in relation to the ground surface, the control valve  18  is controlled to the lowered position  18 D, and the check valves  13 ,  14  are controlled to the operating state (step S 19 ). 
     The hydraulic cylinders  7  thereby retract and lower the front part of the vehicle body in a state in which the pressure in the oil chambers  7   b  of the hydraulic cylinders  7  and the oil passage  10  is maintained at the set pressure MP 1  by the relief valve  28 . When the hydraulic cylinders  7  retract in proportion to the difference between the intermediate value B 1  and the target range H 1  (when the intermediate value B 1  enters the target range H 1 ), the process advances to step S 2 , the cumulative frequency N is set to “0,” and the control valve  18  is operated to the neutral position  18 N, returning to a state in which the check valves  13 ,  14  have been operated to the open state (a state in which the hydraulic cylinders  7  act as a suspension mechanism). 
     Thus, if the cumulative frequency N does not reach (or fall below) the lowering setting cumulative frequency ND 1  (step S 16 ) and the rising cumulative frequency NU does not reach (or exceed) the rising setting cumulative frequency NU 1  (step S 18 ) even if steps S 4  through S 11  are repeated, a state is maintained in which the control valve  18  is operated to the neutral position  18 N and the check valves  13 ,  14  are operated to the open position (a state in which the hydraulic cylinders  7  act as a suspension mechanism). 
     Therefore, in cases in which operative displacement has occurred in the hydraulic cylinders  7  such that the operation of the hydraulic cylinders  7  has deviated from the target range H 1  in the vehicle body rising direction and the vehicle body lowering direction and the operation of the hydraulic cylinders  7  will be returned to the target range H 1 , the control management unit  60  uses the normally used lower-operational frequency second reference number NDL and second reference number NUL as the cumulative frequency for the cumulating unit  61 , whereby the control valve  18  is controlled on the basis of the detection information from the suspension stroke position sensor  37 , and the hydraulic cylinders  7  move toward the target range in a state in which the frequency of operating the control valve  18  has been reduced so that the operation of the hydraulic cylinders  7  moves toward the target range (steps S 1  to  12  and  16  to  19 ). 
     In cases in which operative displacement has occurred in the hydraulic cylinders  7  such that the operation of the hydraulic cylinders  7  has deviated from the target range H 1  in the vehicle body rising direction and the vehicle body lowering direction, and the operation of the hydraulic cylinders  7  will not return to the target range H 1 , such as is the case when earth is lifted or released by the front loader; the control management unit  60  uses the higher-operational frequency first reference number NDS and first reference number NUS as the cumulative frequency of the cumulating unit  61  in preference to the normally used lower-frequency second reference number NDL and second reference number NUL, on the basis of the detection information from the stopped state detector  58 . The control valve  18  is thereby controlled via the control unit  35  in preference to the detection information from the suspension stroke position sensor  37 , and suspension control is performed for moving the operation of the hydraulic cylinders  7  toward the target range in a state in which the frequency of operating the control valve  18  has been increased so that the operation of the hydraulic cylinders  7  moves toward the target range (steps S 1  to S 19 ). 
     In steps S 4  to S 8  of  FIG. 8 , in cases in which a long set time period T 11  is set and the configuration is designed so that a plurality of maximum values A 1  and a plurality of minimum values A 2  are detected, the intermediate value B 1  of step S 8  in  FIG. 8  may be detected in the following manner. 
     (1) These pluralities of maximum values A 1  and minimum values A 2  are divided into multiple groups of maximum values and minimum values A 1 , A 2  in which one maximum value A 1  and one minimum value A 2  constitute one group, and an intermediate value B 1  is detected in each group, whereby a plurality of intermediate values B 1  are detected, and the mean value of these intermediate values B 1  is used as the intermediate value B 1  in step S 8  of  FIG. 8 . 
     (2) The mean value of the maximum values A 1  is detected from the plurality of maximum values A 1 , the mean value of the minimum values A 2  is detected from the plurality of minimum values A 2 , and an intermediate value B 1  is detected from the mean values of the maximum and minimum values A 1 , A 2  and is used as the intermediate value B 1  of step S 8  in  FIG. 8 . 
     Additional Embodiments 
     A First Additional Embodiment differing somewhat from the embodiment described above is described hereinbelow. 
     The hydraulic suspension system according to this Additional Embodiment is composed of the following constituent elements: 
     (a) a suspension mechanism for a traveling vehicle body; 
     (b) a suspension reference position variation mechanism for varying the reference position of the suspension stroke of the suspension mechanism; 
     (c) a suspension stroke position sensor for detecting the suspension stroke position; 
     (d) a control unit for controlling the suspension reference position variation mechanism on the basis of the suspension stroke position so that the stroke of the suspension mechanism moves toward a preset target range; 
     (e) an operation detector for detecting the raising/lowering operation of the implement; and 
     (f) a control management unit for increasing the control frequency for the suspension reference position variation mechanism to be greater than the frequency in cases in which the implement is not raised or lowered, on the basis of raising/lowering operation information generated by the operation detector. 
     In cases in which the implement raising operation has been performed in the hydraulic suspension system according to this First Additional Embodiment, the frequency with which the control valve  18  acts as a suspension reference position variation mechanism is increased by the operation of the control management unit  58  so as to be greater than the frequency in cases of no raising/lowering operation. Thereby, in cases in which the implement is not raised or lowered, the operating frequency of the suspension reference position variation mechanism decreases so as to be less than cases in which a raising/lowering operation is performed, and deterioration of the suspension reference position variation mechanism is prevented. At the same time, the operation of the suspension mechanism is moved toward the target range, whereby the terrain clearance of the traveling vehicle body can be adjusted so as to be within a predetermined height range. In cases in which an implement raising/lowering operation has been performed, the operating frequency of the control valve  18  as a suspension reference position variation mechanism is increased so as to be greater than in cases of no raising/lowering operation. It is thereby possible to quickly adapt to operative displacement in the suspension mechanism accompanying raising or lowering of the implement, to perform an operation for moving the operation of the suspension mechanism to the target range, and to quickly make adjustments so that the terrain clearance of the traveling vehicle body conforms to a predetermined height range. 
     Therefore, it is possible to absorb unevenness in the terrain through the action of the suspension mechanism and make the ride more comfortable, and also to work in an easily workable manner wherein the terrain clearance of the traveling vehicle body and implement is in a predetermined height or substantially in the predetermined height, both in cases in which the implement has been raised or lowered and in cases in which the implement has not been raised or lowered. Moreover, in cases in which the implement is not raised or lowered, the operating frequency of the suspension reference position variation mechanism is reduced to be less than in cases in which the implement is raised or lowered, deterioration in the operation variation means is suppressed, and the operation variation means can be provided with superior durability. In cases in which the implement has been raised or lowered, it is possible to quickly adapt to operative displacement of the suspension mechanism caused by raising or lowering the implement, to move the operation of the suspension mechanism toward the target range, to quickly adjust the traveling vehicle body and implement to an appropriate terrain clearance or to a height close thereto, and to work in an easily workable manner. 
     Furthermore, the following is a description of a Second Additional Embodiment resembling the First Additional Embodiment, whereby the same effects as those described above can be obtained. 
     The hydraulic suspension system according to this Additional Embodiment is composed of the following constituent elements: 
     (a) a suspension mechanism for a traveling vehicle body; 
     (b) a suspension reference position variation mechanism for varying the reference position of the suspension stroke of the suspension mechanism; 
     (X) an intermediate value calculator for detecting the maximum position and minimum position of the suspension stroke and calculating an intermediate value between the maximum and minimum positions; 
     (Y) a cumulating unit for cumulating the frequency at which the intermediate position deviates from a target range as a cumulative frequency; 
     (d′) a control unit for controlling the operation variation mechanism so that the intermediate position moves in a preset target direction when the cumulative frequency obtained from the cumulating unit exceeds a preset cumulative frequency; 
     (e) an operation detector for detecting the raising/lowering operation of the implement; and 
     (f′) a control management unit for varying the set cumulative frequency set in advance on the basis of raising/lowering operation information generated by the operation detector; so that when an operation of raising or lowering the implement is performed, the set cumulative frequency decreases from the time the raising/lowering operation is performed until a designated elapsed time has elapsed, and the preset cumulative frequency increases after the designated elapsed time has elapsed since the raising/lowering operation was performed. 
     In the hydraulic suspension system according to this Second Additional Embodiment, when an operation for raising or lowering the implement is performed, the set cumulative frequency decreases from the time the raising/lowering operation is performed until the designated elapsed time elapses. Furthermore, since the set cumulative frequency increases after the designated elapsed time has elapsed since the raising/lowering operation was performed, the operating frequency of the control valve  18  as a suspension reference position variation mechanism is higher in cases in which an operation for raising or lowering the implement is performed than in cases in which the implement is not raised or lowered. It is thereby possible to quickly adapt to operative displacement of the suspension mechanism accompanying the raising or lowering of the implement, to move the operation of the suspension mechanism to the target range, and to quickly make adjustments so that the ground clearance of the vehicle body conforms to the predetermined height range.