Abstract:
A hydro-pneumatic suspension system includes one or more hydraulic suspension cylinders mounted between the vehicle body and the axle. Each cylinder includes a piston chamber and rod chamber, each of which are connected to an accumulator and which can be connected to a pressure source and a tank by valve systems. The valve system for the rod chamber includes a first solenoid valve and a hydraulically and electrically actuated proportional pressure adjusting valve. The proportional valve is exposed to rod chamber pressure and to tank pressure. The rod chamber pressure can be applied on the proportional valve either directly as control pressure or can be detected with a sensor, and can be used to control the proportional valve. Such a system can influence the dependency of the suspension stiffness on the axle load and can be used to adjust the stiffness to ballast conditions and operating or driving states.

Description:
BACKGROUND 
   The present invention relates to a hydro-pneumatic suspension system for use in a wheel or axle suspension of a motor vehicle, such as a farm or industrial vehicle. 
   Such a suspension system includes at last one hydraulic suspension cylinder connected between the vehicle body and the wheel and/or vehicle axle. The cylinder has a piston chamber and rod chamber, each connected to at least one accumulator. Valve systems can controllably connect the piston chamber and rod chamber to a pressure source and to a tank. Such a suspension system can be a single-wheel suspension or an axle suspension. 
   The design of a hydro-pneumatic suspension system is a particular challenge especially for farm or industrial equipment, such as tractors, because a wide range of wheel or axle loads are possible. The axle load range usually exceeds the pressure ratio possible with diaphragm/membrane type accumulators. In order to be able to adhere to the operating range of the accumulator, the system is therefore pre-loaded. This pre-load acts like an additional axle load, thus allowing a decrease in the ratio between minimal and maximum axle load. 
   The system is rarely pre-loaded by means of mechanical pre-stressing, during which process also a single-acting cylinder can be used. Generally a double-acting hydraulic cylinder is employed, which allows the rod chamber side to be pre-loaded to a specific pressure via an accumulator. 
   The pressure in the rod chamber can either correspond to the maximum system pressure as described in published German patent application DE 43 08 460, or the pressure can be adjusted by means of a pressure regulator to a specific pre-selected value as described in German patent DE 42 42 448. In both methods, as is the case with any hydro-pneumatic system, the spring rate essentially has a quadratic dependency on the axle load in accordance with a given function. For a consistent natural frequency of the tractor body mass in combination with the suspension system, however, a linear function would be ideal. 
   Published German patent applications DE 197 19 076 and DE 197 19 077 describe systems which include a hydraulically actuated pressure control valve in the line leading to the rod chamber, and the control valve spring regulating force is adjusted as a function of the pressure in the line leading to the piston chamber. Thus, the pressure in the rod chamber is supposed to be regulated as a function of the pressure in the piston chamber of the hydraulic cylinder, wherein the additional load in the rod chamber is greatest at low load levels and is reduced at higher load levels down to a minimum value. This is intended to improve, among other things, the driving comfort. 
   Published German patent application DE 41 20 758 describes a hydro-pneumatic vehicle suspension system of the above-mentioned kind in which, a hydraulically actuated control valve is arranged in the feed line to the rod chamber for the purpose of adjusting to a large range of axle load. The control valve is subjected to the pressure in the rod chamber and to the pressure of the piston chamber so that the pressure in the rod chamber is regulated as a function of the pressure in the piston chamber. This is intended to provide a greater stiffness spring in the smaller axle load range. 
   Different ballast conditions on the vehicle can lead to the same axle load. For example, in order to compensate the weight of a heavy plow and apply sufficient load onto the front axle, ballast weights can be mounted on the front of the tractor. This ballast balancing can result in the same axle load on the front axle, such as on a tractor that is equipped neither with a plow nor with front weights. In the known vehicle suspension systems, however, the rigidity cannot react to different ballast conditions and operating or driving states of the vehicle—the rigidity is not dependent on these conditions and solely depends on the respectively present axle load. This can result, when there is a lot of ballast, in a tractor suspension which is too soft with respect to the characteristic pitch frequency, and, when there is no ballast, in a tractor suspension which is too stiff. 
   SUMMARY 
   Accordingly, an object of this invention is to provide a hydro-pneumatic suspension system with a stiffness which can be controlled or adjusted as a function of ballast conditions and operating or driving states of the vehicle and which is not dependent solely on the axle load. 
   This and other objects are achieved by the present invention, wherein a hydro-pneumatic suspension system includes at least one double-acting hydraulic suspension cylinders mounted between the chassis of a vehicle and its wheel and/or axle. Each cylinder has a cylinder or piston chamber and a rod chamber, each connected to an accumulator. A valve system independently controls communication between the piston chambers, the rod chambers, a pressure source and a tank. A rod chamber valve includes a low leakage first solenoid valve, which is closed in an un-energized position, and an electrically actuatable proportional pressure adjustment valve in series with the solenoid valve. 
   The rod chamber pressure can preferably be applied directly to the proportional valve as a control pressure or it can be detected by using a sensor and be used to control the proportional valve. In a preferred embodiment of the invention, a pressure sensor senses the rod chamber pressure and/or a force sensor senses the axle load. The proportional valve is controlled in response to sensor signals from these sensors. Alternatively or in addition, it is particularly beneficial to actuate the proportional valve hydraulically as a function of the rod chamber pressure. The proportional valve is thereby exposed to rod chamber pressure and to tank pressure. 
   The solenoid valve is preferably connected between the rod chamber and the proportional valve. In its closed position it prevents leakage from the rod chamber to the tank. In its closed position, a check valve is active. Suitable solenoid valves with extremely low leakage are commercially available, such as the SV08-20 and SV08-22 models manufactured by HydraForce, Lincolnshire, Ill., USA. 
   The rod chamber pressure is adjusted by the proportional valve. A certain pressure can be pre-selected via the control current of the magnet controlling the proportional valve, and the rod chamber pressure is then adjusted to the pre-selected pressure. The control current can be adjusted by a control unit as a function of different parameters, such as ballast, driving speed and working conditions. If the suspension system also includes a level control device with a position transmitter, the control unit can also respond to signals from the position transmitter which can represent the magnitude of jolts or shocks transmitted from the ground to the tires. 
   This suspension system allows the rod chamber pre-load of the primary suspension to be modified within a broad range. In particular, the rod chamber, and hence the suspension characteristic, can be adjusted in a wide range within given physical limits so that on one hand a stronger than a quadratic dependency—a superproportional or an “over-proportional” dependency—of the stiffness on the axle load can be dealt with, and e.g. a more linear suspension characteristic can be adjusted. On the other hand, the suspension system can also react to different ballast conditions, speeds and working conditions of the vehicle through a suitable electrical actuation of the proportional valve. The suspension characteristics can therefore be individually adjusted to different driving and operating uses, automatically and in an optimal fashion. It should be emphasized that for the actuation of the proportional valve, no pressure sensor signal detected by the control unit is required, even if in special applications the use of a pressure sensor signal can be useful. 
   The first solenoid valve preferably includes a closed shut-off un-energized position wherein it prevents fluid from flowing out of the rod chamber. Preferably, the first solenoid valve has a shut-off position wherein it prevents fluid from flowing through in both directions. 
   A flow restriction limits the flow between the first solenoid valve and the proportional valve and enables a controlled adjustment of the rod chamber pressure. It is also possible to adjust the rod chamber pressure in a controlled fashion by slowly adjusting the proportional valve and eliminate the flow restriction. A flow restriction may still be beneficial in connection with a load sensing control arrangement. The flow resistance may be a local narrowing area (orifice) or a constriction extending over a longer flow path, such as a throttle. 
   Preferably, the proportional valve is an electromagnetically controlled proportional pressure control valve, which controls communication between the rod chamber, the tank and the pressure source. Suitable proportional pressure control valves are sold, for example, as the TS98-31 model by HydraForce, Lincolnshire, Ill., USA. 
   Preferably, the proportional pressure control valve has an un-energized position wherein it communicates the rod chamber to the tank, and an energized position wherein it communicates the pressure source to the rod chamber. The proportional pressure control valve includes a return spring which urges the proportional pressure control valve to its un-energized position. 
   Preferably, the rod chamber pressure acts upon a control element or spool of the proportional pressure control valve in the same direction as the force of the return spring, and the tank pressure acts upon the spool in the same direction as the electromagnetic force. Thus, increasing pressure on the rod chamber side causes the proportional pressure control to connect the rod chamber to the tank, whereby the tank pressure serves as a reference counterpressure. 
   Alternatively, an electromagnetically controlled proportional pressure limiting valve may be used instead of the proportional pressure control valve. Such a valve optionally establishes a connection between its rod chamber side port and the tank. Suitable proportional pressure limiting valves are available, for example, as the TS08-27 model made by HydraForce, Lincolnshire, Ill., USA. 
   The proportional pressure limiting valve has an energized position wherein it communicates its rod chamber side port to its tank-side port, and has an un-energized position wherein it blocks this communication. The proportional pressure limiting valve preferably includes a return spring which urges it to its blocking position. 
   When using a proportional pressure limiting valve it is especially beneficial that the tank-side pressure acts upon the control element or spool of the proportional pressure control valve in the same direction as the force of the return spring, and that the rod chamber side pressure acts upon the spool in the same direction as the electromagnetic force. Thus, in response to increasing pressure on the rod chamber side, the proportional pressure control valve tends to connect said side to the tank, whereby the tank pressure serves as a reference counterpressure. 
   The proportional pressure limiting valve controls only the communication between the rod chamber side and the tank. In order to be able to apply also pressure from the pressure source to the rod chamber side it is particularly beneficial if a pressure line that is connected to the pressure source branches off the line running between the solenoid valve and the proportional pressure limiting valve. 
   A pressure line flow restriction in the pressure line limits the pressure on the rod chamber side to a desired value. This enables a steady, limited fluid flow from the pressure source to the rod chamber side. However, the proportional pressure limiting valve limits the pressure on the rod chamber side because it opens when an electronically regulated rod chamber side pressure is exceeded. 
   A second solenoid valve can be arranged in the pressure line, in addition or alternatively to the pressure line flow restriction. The second solenoid valve preferably includes a closed or non-return position which prevents fluid from flowing in from the pressure source so that little or no leakage occurs from the pressure source to the rod chamber side. If a proportional valve is used as the solenoid valve, the pressure line flow restriction can be eliminated. 
   The pressure source is preferably a hydraulic pump, such as a hydraulic pump that is already available for other hydraulic parts of the vehicle. The pump may include a load sensing function, in which the required system pressure is used to actuate the pump. Such a hydraulic pump requires hydraulic fluid only upon demand and otherwise assumes a low-power standby operational state. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a hydraulic circuit diagram of a hydro-pneumatic vehicle suspension with a first suspension system pursuant to the invention; 
       FIG. 2  is a hydraulic circuit diagram of an alternative second suspension system pursuant to the invention; and 
       FIG. 3  is a hydraulic circuit diagram of a third alternate suspension system pursuant to the invention. 
   

   DETAILED DESCRIPTION 
   The vehicle suspension system of  FIG. 1  is for the suspension of a steerable, pendulous tractor front axle (not shown). The system includes two cylinders  10 ,  12 . The two piston chambers  14 ,  16  of the hydraulic cylinders  10 ,  12  and the two rod chambers  18 ,  20  of the two hydraulic cylinders  10 ,  12  are connected by hydraulic lines  22 ,  24  to each other and to a corresponding hydraulic accumulator  26 ,  28 . 
   The piston chamber hydraulic line  22  is connected via a first flow restriction  30  and a first low leakage switching valve  32  to a fluid reservoir or tank  34 . In response to electric signals, the first switching valve  32  can switch between a flow position and a blocking position, ensuring that no fluid can unintentionally escape from the piston chamber side of the suspension circuit to the tank  34 . The hydraulic line  22  on the piston chamber side is moreover connected via a non-return valve  36 , a second flow restriction  38  and a second low leakage switching valve  40  to a pressure source, which is a controlled load sensing pump  42 . The second switching valve  40  can be switched in response to electric signals between a flow position and a blocking position, ensuring that no fluid can unintentionally flow from the hydraulic pump  42  to the suspension circuit. The non-return valve  36  blocks the fluid flow in the opposite way as the second switching valve  40  in order to prevent leakage when the pressure in the piston chamber side is greater than the pressure in the supply line  44 . 
   This valve arrangement serves level control purposes. To reduce the level position the first switching valve  32  is energized, its spool switches into the “open” position and hydraulic fluid flows from the piston chamber side hydraulic line  22  in a controlled fashion via the first flow restriction  30  to the tank  34 . In order to adjust the axle upward, the second switching valve  40  is opened and fluid flows initially from the supply line  44  via the second flow restriction  38  into a load sensing line  46  and reports the demand for pressure to the control port  43  of the hydraulic pump  42  via a shuttle valve  48 . Pump  42  increases the pressure until the non-return valve  36  opens and fluid flows into the piston chamber side hydraulic line  22 . The second flow restriction  38  on one hand limits the volume flow, but on the other hand also generates a pressure drop so that the pressure in the piston chamber side is always reported to the load sensing system. This prevents the pump  42  from adjusting to a maximum flow condition, but the pump  42  always provides a pressure that is for example,  30  bar above the piston chamber pressure level. Parallel to this a volume flow also always flows via a third flow restriction  50  back to the tank  34  in the case of an open second switching valve  40 . This additional cross-section provided by the third flow restriction  50  is required in order to relieve the load sensing pressure toward the tank  34  in the case of a closed second switching valve  40 . 
   It should be emphasized that instead of the first switching valve  32  and the first flow restriction  30  or the second switching valve  40  and the second flow restriction  38 , alternatively a proportionally controlled, low leakage 2 position/2-way valve can be used, which can be opened by a specific amount under current control. 
   The pressure in the piston chamber side of the suspension circuit, namely the hydraulic line  22 , is limited by a pressure limiting valve  52  to a certain maximum pressure, which is usually determined by the accumulators. A drain valve  54  relieves pressure from the piston chamber side in case service is required. 
   The rod chamber pressure can be adjusted by this suspension system. As shown in  FIG. 1 , the suspension system includes an electrically adjustable proportional pressure control valve  60 , a flow restriction  62  and an electrically switchable solenoid valve  64 , connected in series. The solenoid valve  64  is connected to the rod chamber line  24  and the proportional pressure control valve  60  enables an optional connection to the pump  42  or to the tank  34 . The spool of the proportional pressure control valve  60  is exposed on one hand via a control line  65  to the pressure of its rod chamber port  66 , and on the other hand via a control line  67  to the pressure of the tank line  68 , specifically such that the rod chamber side pressure counteracts the force of the solenoid  70  jointly with the force of a return spring  69 . 
   When energized the solenoid valve  64  opens a flow opening against the force of a return spring  63 , and when de-energized said opening is closed such that a fluid outflow from the rod chamber side hydraulic line  24  is prevented reliably. The solenoid valve  64  is for example a low leakage 2-position/2-way valve. However it can also be an electromagnetically controlled proportional valve, wherein the flow restriction  62  may be eliminated. 
   Between the solenoid valve  64  and the flow restriction  62  a load sensing line  72  branches off, which is connected to the shuttle valve  48 . The shuttle valve  48  conducts the greater one of the pressures of the two load sensing lines  46  and  72  on to the pump  42 . 
   It is possible to pre-select a certain fluid pressure, which is then adjusted in the rod chamber side hydraulic line  24 , via the control current of the solenoid  70 . The low leakage solenoid valve  64  ensures that with a shut-off proportional pressure control valve  60  as little leakage as possible occurs from the rod chamber side of the suspension circuit to the tank  34 . Since the proportional pressure control valve  60  connects the rod chambers to the tank  34  in a non-energized state, the closed solenoid valve  64  also makes it possible to decrease the load sensing pressure towards the tank  34  via the proportional pressure control valve  60  without fluid flowing out of the rod chamber side hydraulic line  24 . The rod chamber pressure is controlled by the flow restriction  62  as well as possibly by slowly adjusting the proportional pressure control valve  60  (in the latter case it could be possible to eliminate the flow restriction). Here as well, the elimination of the load sensing pressure behind the flow restriction  62  (i.e. between the flow restriction  62  and the solenoid valve  64 ) ensures that the pump  42  is adjusted to provide only to a pressure of  30  bar above the rod chamber pressure level. A drain valve  74  is used to relieve pressure from the rod chamber. 
   Instead of solenoid valve  64  it is also possible to use a pilot opened check valve. For example, the pressure from the piston side control circuit (e.g. line  46 ) can be connected to its pilot line, causing the check valve to open every time the solenoid valve  40  opens. 
   The two switching valves  32 ,  40 , the proportional pressure control valve  60  and the solenoid valve  64  are controlled and actuated by an electric control unit  76 . The control unit  76  receives signals from a position sensor (not shown), which are used for level control purposes by means of the switching valves  32 ,  40 . For adjusting the rod chamber side pressure by means of the proportional pressure control valve  60 , the control unit  76  also receives signals from a vehicle speed sensor (not shown) and a tractive force sensor (not shown). The stiffness can thus be adjusted automatically as a function of the vehicle speed and/or as a function of whether a device is attached to or mounted on the vehicle, which can be determined from the draft force sensor signal. The control unit  76  can, if useful, also receive and evaluate the signals of a rear and/or front power take-off shaft (not shown) or other vehicle sub-assemblies (not shown). The ballast state of the vehicle can be specified for example based on a switch through an operator. It is likewise detected by the control unit and evaluated for the purpose of adjusting the proportional pressure control valve  60 . The aforementioned and additional signals can be made available to the control unit  76  for example via a CAN bus (not shown). 
   A pressure sensor  78  is connected to the rod chamber hydraulic line  24 , the signals of which are evaluated by the control unit  76  and used to adjust the proportional pressure control valve  60 . When using the pressure sensor  78  it is also possible to use a proportional pressure control valve that is actuated in a purely electric (not hydraulic) fashion so that both control lines  65  and  67  in  FIG. 1  are eliminated. 
     FIG. 2  shows an alternative suspension system. Instead of the proportional pressure control valve  60  shown in  FIG. 1 , the system of  FIG. 2  includes an electrically switchable proportional pressure limiting valve  80  in series with a solenoid valve  82  and which connects the rod chamber side hydraulic line  24  to the tank line  68 . The proportional pressure limiting valve  80  is located on the side of the tank line  68  and the switching valve  82  on the side of the rod chamber side hydraulic line  24 . 
   The proportional pressure limiting valve  80  has a spool which exposed via a control line  84  to the pressure of its rod chamber side port  86 , and which is exposed via a control line  88  to the pressure of the tank line  68 , so that the rod chamber pressure acts against the force of a return spring  90  in the same direction as the force of the solenoid  92 . The solenoid valve  82  is energized to open against the force of the return spring  83 , and is closed when not energized so that a fluid outflow from the rod chamber hydraulic line  24  as well as a fluid flow in the opposite direction are prevented reliably. Between the solenoid valve  82  and the proportional pressure limiting valve  80 , a pressure line  94  branches off and is connected to the supply line. Pressure line  94  includes a flow restriction  96 . The two valves  80 ,  82  shown in  FIG. 2  are controlled in accordance with  FIG. 1  by a control unit  76 . 
   A certain fluid pressure, which is then adjusted in the rod chamber side hydraulic line  24  (rod chamber side of the suspension circuit), can be pre-selected by means of the control current of the solenoid  92  of the proportional pressure limiting valve  80 . The low leakage bidirectional poppet type solenoid valve  82  ensures that as little leakage as possible occurs from the rod chamber side of the suspension circuit to the tank  34  and vice versa from the supply line  44  to the rod chamber side of the suspension circuit, when proportional pressure limiting valve  80  is shut-off. When the proportional pressure limiting valve  80  is closed, its port  86  is subjected to the pump pressure. When it opens, the pump pressure that is present at port  86  is decreased towards the tank  34  via the tank line  68 . The pressure line flow restriction  96  hereby restricts the fluid inflow from the pump  42  so that the pressure at port  86  decreases in accordance with the level to which the proportional pressure limiting valve  80  is opened. With an opened solenoid valve  82 , the pressure in the rod chamber side hydraulic line  24  can thus be adjusted to a desired value. 
     FIG. 3  shows another modified embodiment of the suspension system. In  FIGS. 2 and 3  equivalent components were assigned the same reference numbers. The suspension system illustrated in  FIG. 3  provides a load sensing signal. 
   The system of  FIG. 3  includes in the pressure line  94  the aforementioned pressure line flow restriction  96 , and in addition a second solenoid valve  97  in series therewith. However, the function of the flow restriction  96  may be integrated into the solenoid valve  97  (proportional actuation). The second solenoid valve  97  has a closed position which prevents a fluid inflow from the pressure source  42 . Moreover, port  86  is connected to the tank line  68  by a control line  98 , which contains a flow restriction  99 . 
   The second solenoid valve  97  allows the fluid flow coming from the pump  42  to be interrupted so that with a closed proportional pressure limiting valve  80  the pump pressure is no longer present at port  86 . It is rather decreased towards the tank pressure via the throttled control line  98 . Hence, the pressure at port  86  can be used as a load sensing pressure and is for this purpose connected to the load sensing line  72  (shown in more detail in  FIG. 1 ). 
   While the present invention has been described in conjunction with a specific embodiment, it is understood that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, this invention is intended to embrace all such alternatives, modifications and variations which fall within the spirit and scope of the appended claims.