Abstract:
An agricultural vehicle having a hydraulic system, a cab for the vehicle operator includes a self-levelling cab suspension system having a plurality of suspension units each having a spring, a damper and an accumulator. The accumulator comprises a hydraulic working chamber which is separated by a movable wall from a gas filled chamber serving as a spring. The working chamber is also connected to a working chamber of the damper in a closed hydraulic circuit. In the invention, there is no need for a dedicated oil pump since a hydraulic actuator powered by the vehicle hydraulic system is provided for causing hydraulic fluid to flow in the closed hydraulic circuit between the working chamber of the accumulator and the damper, to vary the height of the cab without fluid from the vehicle hydraulic system entering or leaving the closed hydraulic circuit of the damper and accumulator.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This present application is a divisional application of U.S. patent application Ser. No. 12/712,783 filed Feb. 25, 2010, by Patrizio Turco and Michele Ieluzzi, entitiled “AGRICULTURAL VEHICLE SUSPENSION”, which is hereby incorporated by reference herein in its entirety for all purposes, which claims priority under 35 U.S.C. 119(a)-(e) or 365(b)-(c) to foreign application number TO2009A000139, filed Feb. 26, 2009, by Patrizio Turco and Michele Ieluzzi, entitiled “AGRICULTURAL VEHICLE SUSPENSION”, which is also hereby incorporated by reference herein in its entirety for all purposes. 
     
    
     TECHNOLOGY FIELD 
       [0002]    The present invention relates to an agricultural vehicle having a hydraulic system, a cab for the vehicle operator and a cab suspension system comprising a plurality of suspension units each having a spring, a damper and an accumulator, wherein the accumulator comprises a hydraulic working chamber which is separated by a movable wall from a gas filled chamber serving as a spring, and is connected to at least one working chamber of the damper in a closed hydraulic circuit. 
       BACKGROUND 
       [0003]    It is common to design agricultural vehicles, such as tractors, with so-called structural engines. In such vehicles, the engine, together with the transmission and the rear axle, constitute the rigid chassis of the vehicle. Because there is no suspension between the chassis and the ground, at least at the rear of the vehicle, shocks resulting from unevenness in the terrain are transmitted directly to the driver&#39;s cab. 
         [0004]    To improve the comfort of the driver, it is known to pivot the cab about a transverse axis at its front end and to provide at least one suspension unit, comprising a spring and a damper, between the rear end of the cab and the chassis. This allows a cushioned up and down movement of the cab on the chassis. Such an arrangement is shown in  FIGS. 1 and 2  of the accompanying drawings, in which  FIG. 1  is a schematic representation of a cab that is pivotable at its front end and supported at its rear end on suspension units, and  FIG. 2  is a detailed perspective view of the suspension unit in  FIG. 1 . 
         [0005]    In  FIG. 1 , the front end of the cab  10  is pivoted about an axis  12  that is fixed to the chassis. A suspension unit  14  at the rear of the cab, as better shown in  FIG. 2 , has a flexible strap  16  attached to a bracket  18  fixed to the vehicle chassis at one end and at its other end to a bracket  20  attached to the cab  10 . The flexible strap  16  acts as an anchor which allows the cab  10  to move up and down relative to the chassis while preventing it from moving upwards beyond acceptable limits. 
         [0006]    The suspension unit  14  is shown as having a strut  22  that incorporates a spring  24  and a damper  26 . The spring  24  which supports the weight of the cab, need not be a coil spring, nor need it be concentric with the damper. It may alternatively be a gas spring or a leaf spring mounted separately from the damper.  FIG. 2  also shows that a resilient bump stop  28  is provided to cushion the cab if the suspension ever reaches the end of its permissible travel. 
         [0007]    While such a cab suspension goes some way towards improving the ride quality in the cab, it still suffers from the disadvantage that when the tractor is being driven along an incline, the cab and the driver&#39;s seat, though parallel to the ground, are inclined relative to the horizontal and this causes discomfort to the driver, aside from being disconcerting. Similarly, when the tractor is being driven up or down an incline the cab can assume an extra-pitch angle causing discomfort to the driver. 
         [0008]    It is therefore desirable to provide the cab of an agricultural vehicle with a self-levelling suspension system which maintains an optimal attitude of the cab controlling roll and pitch angles, even if the inclination of the ground over which the vehicle is travelling varies within certain limits. 
         [0009]    In U.S. Pat. No. 6,273,203, there is disclosed a suspension system that employs four hydraulic actuators capable of tilting the cab about mutually inclined axes and a control system for independently controlling each of the hydraulic actuators in dependence upon signals received from sensors, which can be constructed as inclinometers or gyroscopes. In this known system, the fact that all four actuators can be independently controlled makes for a complicated control system. This is because the control algorithm needs to take into account when an actuator is near the bottom or top of its stroke and is therefore unable to bring about the required tilt of the cab. 
         [0010]    The Applicants&#39; earlier U.S. Pat. No. 7,198,125 mitigates some of the above disadvantages and provides a vehicle having a chassis and a cab connected to the chassis by means of a support system which comprises two pairs of hydraulic actuators, each pair of actuators being operative to tilt the cab relative to the chassis about a respective one of two mutually inclined axes. The two actuators of each pair are connected to a common pumping element in such a manner that whenever the volume of hydraulic fluid in one of the actuators in a pair is reduced, the volume of hydraulic fluid in the other actuator of the same pair is correspondingly increased. 
         [0011]    A disadvantage of U.S. Pat. No. 6,273,203, U.S. Pat. No. 7,198,125 and other prior art cab suspension systems which use hydraulic dampers as actuators to level the cab stems from the fact that they require a dedicated pumping unit. Such pumping units need furthermore to be fairly substantial as they are required to supply hydraulic fluid to the actuators sufficiently rapidly to counteract changes in the inclination of the chassis. 
       SUMMARY 
       [0012]    The present invention seeks to provide a suspension system for the cab of an agricultural vehicle that dispenses with the need for a dedicated pumping unit. 
         [0013]    An agricultural vehicle has a vehicle hydraulic system, a cab for the vehicle operator and a self-levelling cab suspension system comprising a plurality of suspension units each having a spring, a damper and an accumulator, wherein the accumulator comprises a hydraulic working chamber which is separated by a movable wall from a gas filled chamber serving as a spring, and is connected to at least one working chamber of the damper in a closed hydraulic circuit, characterised in that a hydraulic actuator powered by the vehicle hydraulic system is provided for causing hydraulic fluid to flow in the closed hydraulic circuit between the working chambers of the accumulator and the damper, so as to vary the height of the cab without any fluid from the vehicle hydraulic system entering or leaving the closed hydraulic circuit of the damper and the accumulator. 
         [0014]    The actuator may be formed as a separate unit having two hydraulic working chambers separated by a movable wall, one of the working chambers being connected by an associated changeover valve to a supply and a return line of the vehicle hydraulic system and the other working chamber being connected to the hydraulic working chamber of the accumulator. 
         [0015]    A single such actuator may advantageously be connected to the accumulators of a plurality of suspension units by way of respective isolation valves. 
         [0016]    As an alternative the actuator may be integrated with the accumulator into a single unit having two movable walls defining three variable volume working chambers, consisting of two hydraulic working chambers separated from one another by a gas filled working chamber, one of the hydraulic working chambers being connected to at least one of the working chambers of the damper and the other hydraulic working chamber being connected by a changeover valve to a supply and a return line of the vehicle hydraulic system. 
         [0017]    In both embodiments, the same changeover valve may be shared by a plurality of suspension units by providing isolation valves between the changeover valve and the suspension units. 
         [0018]    It is desirable in the invention that the hydraulic system present in most agricultural vehicle, to serve as oil pressure supply for the gearbox, also referred to as the vehicle hydraulic system, is used to power the cab suspension, thereby obviating the need for a dedicated pump. Such an oil supply does not have the purity needed by the damper and the accumulator of a suspension unit but the invention avoids this problem by maintaining the closed hydraulic circuit of the suspension unit isolated from the less pure oil drawn from the vehicle hydraulic system. 
         [0019]    Another preferable feature of the invention is that the hydraulic working chamber of the accumulator is isolated when the engine is at a standstill so that the level of the cab is maintained even after the engine is switched off. In case of excessive oil venting through the changeover valve toward the return line, or in case of pipes damage, the floating piston inside the actuator reaches its maximum travel and acts to limit cabin suspension stroke. 
         [0020]    According to a further embodiment of the present invention the suspension units are paired in the manner proposed in EP 1 419 956. In this way, the volume of fluid that needs to be displaced by the actuator to change the attitude of the cab is reduced because one of the suspension units will supply at least some of the fluid required by the other. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0021]    The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: 
           [0022]      FIGS. 1 and 2  have already been described above and show a cab suspension in which the front end of the cab is pivoted to the chassis and only the rear end is supported by two suspension units, 
           [0023]      FIG. 3  is a hydraulic circuit diagram of a cab suspension in accordance with a first embodiment of the invention, and 
           [0024]      FIG. 4  is a view similar to that of  FIG. 3  showing an alternative embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    While the invention can be used in a cab suspension as shown in  FIGS. 1 and 2  it is also possible to provide suspension units at the four corners of the cab to control pitch and roll. The hydraulic circuits shown in  FIGS. 3 and 4  each show one pair of suspension units. It is also possible to couple the suspension units in pairs in the manner previously proposed in U.S. Pat. No. 7,198,125. 
         [0026]    The two suspension units  114  in  FIG. 3  are identical with one another and only one of them will therefore now be described. Each suspension unit comprises an adjustable damper  126 , such as for example a variable stiffness damper, a coil spring  124  surrounding the damper  126 , a hydro-gas accumulator  128  and an isolation valve  130 . The two suspension units  114  are connected to a common actuator  132  which in turn is connected by way of a changeover valve  134  to the supply and return lines P and T of the vehicle hydraulic system  136 . 
         [0027]    The damper  126  can be, for example, a twin tube damper having an inner tube  126   a  and an outer tube  126   b . A piston  126   c  divides the inner tube  126   a  into two variable volume working chambers  126   d  and  126   e  that communicate with one another through a throttle valve  126   f  in the piston  126   c . The working chamber  126   e  above the piston  126   c  communicates through an orifice  126   g  with the space  126   h  between the two tubes  126   a  and  126   b . A supply port  126   k  communicates with the working chamber  126   d  through a fixed throttle valve  126   j  and with the space  126   h  between the two tubes  126   a  and  126   b  through an electrically variable throttle valve  126   i  that controls the stiffness or damping ratio of the damper  126 . 
         [0028]    The supply port  126   k  is connected to the hydraulic working chamber  128   a  of the associated accumulator  128 . The accumulator  128  has a gas filled working chamber  128   b  separated from the chamber  128   a  by a movable wall  128   c  that is shown as being a floating piston. As an alternative, the movable wall  128   c  can be formed by a resilient diaphragm. 
         [0029]    Each accumulator  128  is connected by a respective shut-off or isolation valve  130  to the common actuator  132 . The actuator  132  has a first hydraulic working chamber  132   a  connected via the shut-off valve  130  in circuit with the hydraulic working chambers of the accumulator  128  and the damper  126 . The actuator  132  has a second hydraulic working chamber  132   b , separated from the working chamber  132   a  by a movable wall  132   c  that is shown as being a floating piston and connected via the changeover valve  134  to the supply P and return T lines of the vehicle hydraulic system  136 . The vehicle hydraulic system  136  can provide hydraulic fluid under pressure but not of a quality that is suitable for circulating in the closed hydraulic circuit of the damper  126  and the accumulator  128 . As an alternative, the movable wall  132   c  can be formed by a resilient diaphragm. 
         [0030]    When the vehicle is driven over smooth level ground, the weight of the cab  10  is supported by the coil springs  124  and the gas springs of the suspension units  114  and there is no movement of the pistons  126   c.    
         [0031]    If the vehicle is driven over an uneven but level terrain, the cab  10  moves vertically on the coil springs  124  and the gas springs of the accumulators  128  but the movements are damped by the damper  126 . Supposing for example the piston  126   c  moves downwards on account of the wheel encountering a bump on the ground. The volume of the working chamber  126   d  will be reduced and will displace some hydraulic fluid through the throttle valve  126   f  into the upper working chamber  126   e . As the piston  126   c  is a differential piston (on account of the cross sectional area of the piston rod), the reduction in volume of the working chamber  126   d  will be greater than the increase in volume of the working chamber  126   e . The surplus hydraulic fluid will therefore flow through the space  126   h  between the two tubes  126   a  and  126   b  of the damper  126  and via the variable throttle  126   i  and the port  126   k  into the working chamber  128   a  of the accumulator. 
         [0032]    The flow of the hydraulic fluid is thus opposed by the air spring of the accumulator  128  and will encounter resistance while flowing through the throttle valves  126   f  and  126   i  to provide the desired damping. Upwards movement of the piston  126   c  will likewise be assisted by the air spring of the accumulator  128  but resisted by the flow through the throttle valves  126   i  and  126   j.    
         [0033]    The accumulator  128  accommodates the changes in the total volume of the hydraulic fluid contained in the damper  126  as the piston rod moves up and down and the throttles control the degree of damping. By making the throttle valve  126   i  variable, for example electrically, the force and/or the stiffness of the damper  126  can be adjusted by a control system to suit different driving conditions. 
         [0034]    As is known, one way valves may be incorporated into the damper to vary the degree of damping in dependence upon the direction of movement of the piston  126   c.    
         [0035]    The suspension system as described so far can be controlled in a known manner in dependence on the output signal of various sensors responsive to such parameters as speed and acceleration to optimise the operator comfort under different driving conditions. For example, the force and/or the stiffness of the dampers may be changed when driving on metalled roads. Furthermore, the stiffness or the damping ratio of the dampers may be increased as they reach the end of their travel to avoid bottoming of the suspension. 
         [0036]    Because the piston  126   c  is a differential piston, each of the dampers  126  is also capable of functioning as a hydraulic jack. If hydraulic fluid is somehow introduced into the lower working chamber  126   d  of the damper  126 , the cab will be raised and conversely if fluid is drained from the working chamber  126   d , the cab will be lowered. The damper  126  can thus be used to adjust the attitude of the cab when the vehicle is driven on an inclined surface in order to maintain the cab level, both about the pitch axis and the roll axis of the cab. 
         [0037]    Supposing now that the two suspension units  114  in  FIG. 3  are mounted one at the back and the other at the front of the cab and the vehicle starts to travel down an incline. To level the cab, one of the pistons  126   c  has to be raised and the other lowered. In the prior art, this was achieved by pumping hydraulic fluid from one suspension unit to the other using a dedicated pumping unit. However, as earlier mentioned, the pumping unit needed to be substantial and, in order to dispense with the need for such a pumping unit, the invention uses in its place the actuator  132 , which is powered by the vehicle hydraulic system. 
         [0038]    To lower the suspension unit on the left in  FIG. 3 , the left isolation valve  130  is opened and the changeover valve  134  connects the working chamber  132   b  of the actuator  132  to the return line T of the vehicle hydraulic system  136 . The piston  132   c  now moves upwards under the difference in pressure between the working chambers  132   a  and  132   b  to increase the volume of the working chamber  132   a  and drains fluid from the left damper  126  and accumulator  128 . 
         [0039]    To correct the attitude of the cab in the same sense, it is necessary to raise the damper  126  on the right of the drawing. This is achieved by closing the left shut-off valve  130 , opening the right shut-off valve  130  and setting the changeover valve  134  to connect the upper working chamber  132   b  of the actuator  132  to the pressure line P of the vehicle hydraulic system  136 . The higher pressure in the working chamber  132   b  now forces the piston  132   c  downwards to pump fluid into the right accumulator  128  and damper  126  to raise the cab. 
         [0040]    It is important to note that by using an actuator  132  powered by the vehicle hydraulic system  136 , the embodiment of the invention in  FIG. 3  allows the individual suspension units to be raised and lowered without the hydraulic fluid drawn from the vehicle hydraulic system ever mixing with that circulating in the suspension unit  114 . In this way, the need for a pumping unit is avoided by using equipment already present in the vehicle, without detriment to the cab suspension nor to the vehicle hydraulic system  136 . 
         [0041]    In the system described above there is only used a single actuator  132  to control both suspension units  114 . It is clear that as an alternative it is also possible to associate a single actuator  132  with each suspension unit  114 . 
         [0042]    The system of  FIG. 4  achieves the same objectives as that of  FIG. 3  but uses a different hydraulic circuit configuration. Items serving the same function have been allocated the same reference numerals in the “200” series instead of the “100” series and will not be described a second time. For example the suspension unit  214  corresponds to the suspension unit  114 , the spring  224  corresponds to the spring  114  and the damper  226  corresponds to the damper  126  of the embodiment shown in  FIG. 3 . 
         [0043]    In  FIG. 4 , the two chamber hydro-gas accumulator  128  is replaced by an accumulator  228  having three chambers  228   a ,  228   b  and  232   b  separated by two movable walls  228   c  and  232   c , that are shown as being floating pistons. As an alternative, the movable walls  228   c  and  232   c  can be formed by resilient diaphragms. Working chambers  228   a  and  228   b  correspond to working chamber  128   a  and  128   b , one working chamber  228   a  being filled with hydraulic fluid and the other working chamber  228   b  being filled with gas, for example air, and acting as a gas spring. Working chamber  232   b  and movable wall  232   c  act as an actuator  232  replacing chamber  132   b  and movable wall  132   c  of the actuator  132  of embodiment in  FIG. 3  that is formed as a separate unit. Each working chamber  232   b  is connected by a respective shut-off valve  230  and a common changeover valve  234  to the vehicle hydraulic system  236 . Working chamber  228   a  is connected to working chamber  226   d  of the damper  226  through valve  226   j  and to space  226   h  of the damper  226  through controlled valve  226   i.    
         [0044]    If it is desired to raise the piston of the damper  226  of the suspension unit  214  on the right in  FIG. 4 , the right shut-off valve  230  is opened and the changeover valve  234  connects the working chamber  232   b  to the high pressure line P of the vehicle hydraulic system  236 . The volume of the working chamber  232   b  is therefore expanded and acts through the gas spring formed by the working chamber  228   b  on the chamber  228   a  to cause hydraulic fluid to be transferred from the accumulator  228  to the damper  226 .