Abstract:
A suspension system isolates the operator cab or seat from vibrations in the chassis of a vehicle. At least one hydraulic actuator is connected between the cab or seat and the vehicle chassis. An accumulator is coupled to the hydraulic actuator and acts as a hydraulic spring to attenuate high frequency vibrations. An electrically operable, three-position, closed-center control valve selectively connects the hydraulic actuator to a source of pressurized hydraulic fluid or a reservoir. Sensors detect acceleration and displacement of the cab or seat to which a controller responds by operating the valve. That control of the valve actively drives the hydraulic actuator to produce motion that counters the relatively low frequency vibrations from the chassis.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    Not Applicable  
         STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    Not Applicable  
         BACKGROUND OF THE INVENTION  
         [0003]    1. Field of the Invention  
           [0004]    The present invention relates to suspension systems for supporting a component of a vehicle in a manner that isolates that component from vibrations in other sections of the vehicle, such as isolating the operator cab or seat from vibration of the chassis as the vehicle travels over rough terrain; and more particularly to active hydraulic suspension systems in which an actuator is drive to counter the vibration.  
           [0005]    2. Description of the Related Art  
           [0006]    Vibration has an adverse affect on the productivity of work vehicles in which an operator cab is supported on a chassis. Such vehicles include agricultural tractors, construction equipment, and over the road trucks. The vibrations experienced by such vehicles reduce their reliability, increase mechanical fatigue of components, and most importantly produce human fatigue due to motion of the operator&#39;s body. Therefore, it is desirable to minimize vibration of the vehicle cab or seat where the operator sits and of other components of the vehicle.  
           [0007]    The operator of off-highway vehicles is subjected to large amplitude, low-frequency vibration when traveling over rough terrain. Previous vehicle cab suspension systems often performed poorly in the range of vibration frequencies to which the human body is most sensitive, i.e. one to ten Hertz. When subjected to vertical movement, or bounce, the human abdomen resonates at approximately four to eight Hertz and the head and eyes resonate at ten Hertz. The upper torso resonates in response to pitch and roll motion at between one and two Hertz. As a consequence, it is desirable to isolate the cab or seat of the vehicle from these vibration frequencies to improve the operator&#39;s comfort and increase equipment productivity.  
           [0008]    Traditional approaches to vibration isolation employed either a passive or an active suspension system to isolate the vehicle cab or seat along one or more axes to reduce bounce, pitch, and roll of the vehicle. Passive systems typically placed a series of struts between the vehicle chassis and the components to be isolated. Each strut included a parallel arrangement of a spring and a shock absorber to dampen movement. This resulted in good vibration isolation at higher frequencies produced by bumps, potholes and the like. However, performance a lower frequencies, such as encountered by a farm tractor while plowing a field, was relatively poor. The lower frequency vibrations can be in the same range as the natural frequency of the system, thereby actually amplifying the vibration. One approach to decrease that amplification has been to increase the damping ratio ζ given by the expression  
       ζ   =     C     2          m                 K                                 
 
           [0009]    where C is the damping coefficient, m is the mass being isolated, and K is the spring rate. Unfortunately increasing the damping ratio compromises isolation of vibration at the higher frequencies.  
           [0010]    In addition, a two-point passive isolation system, designed to reduce roll vibration (left and right movement) creates an undesirable inertial roll of the cab when the vehicle turns. Generally, if the isolated mass, e.g. the cab, is allowed to roll with a passive suspension, a torsion bar must be added to provide stiffness which resists the inertial roll produced by a vehicle turn. The addition of a torsion bar not only adds cost to the system, it also reduces the effectiveness of other suspension components.  
           [0011]    Active suspensions place a cylinder and piston arrangement between the chassis and the cab or seat of the vehicle to isolate that latter component. The piston divides the cylinder into two internal chambers and an electronic circuit operates valves which control the flow of hydraulic fluid to and from each chamber.  
           [0012]    U.S. Pat. No. 4,887,699 discloses one type of active vibration damper in which the valve is adjusted to control the flow of fluid from one cylinder chamber into the other chamber. The valve is operated in response to one or more motion sensors, so that the fluid flow is proportionally controlled in response to the motion.  
           [0013]    U.S. Pat. No. 3,701,499 describes a type of active isolation system in which a servo valve selectively controls the flow of pressurized hydraulic fluid from a source to one of the cylinder chambers, and drains oil from the other chamber back through a return line to the source. A displacement sensor and an accelerometer are connected to the mass which is to be isolated from the vibration and provide input signals to a control circuit. In response, the control circuit operates the servo valve to determine into which cylinder chamber fluid is supplied, from which cylinder chamber fluid is drained and the rate of those respective flows. This control of the cylinder produces movement of the piston which counters the instantaneous vibration motion.  
           [0014]    Although an active isolation system is particularly effective in the low vibration frequency range, there are cost and performance penalties to use this type of system for higher frequency vibration. Because the system has to respond more rapidly at the higher frequencies, the measurement of mass movement and the reaction of the servo valve often introduce a phase lag in the counter motion, unless relatively expensive, very fast-acting components are employed. In addition, fluid must be supplied to the active suspension from the vehicle&#39;s hydraulic system. During prolonged vibrating conditions, such as when an agricultural tractor is plowing a field, the constant draw of the fluid from the tractor&#39;s pump requires that the hydraulic system have increased capabilities.  
         SUMMARY OF THE INVENTION  
         [0015]    An active suspension system for isolating a first body from a second body has at least one hydraulic actuator connected between the first body and the second body. Each hydraulic actuator comprises a cylinder and a piston received within the cylinder thereby dividing the cylinder into a first chamber and a second chamber. The first and second chambers are coupled to a hydraulic circuit node, and an accumulator also is connected to the node. An electrically operable valve selectively connects the node to a source of pressurized hydraulic fluid or to a reservoir. A sensor detects movement of the first body and produces an electrical signal indicating the detected movement. A controller responds to the electrical signal from the sensor by operating the valve to move the piston relative to the cylinder so as to attenuate transmission of movement of the second body with respect to the first body.  
           [0016]    In a first embodiment of the active suspension system, the node is directly connected to the first chamber and to the second chamber. In a second embodiment, a check valve permits fluid to flow only in a direction from the first node to the second node, and a fixed orifice is connected in parallel with the check valve. In a third version of the active suspension system, the fixed orifice is replaced with a variable orifice controlled by the controller.  
           [0017]    The sensor may detect an amount of displacement between of the first body and the second body. Alternatively, the sensor detects inertial motion of the first body. In a preferred embodiment of the active suspension system, both types of sensors provide input signals to the controller. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIGS. 1 and 2 are rear and side views respectively of an agricultural tractor incorporating an active suspension system according to the present invention;  
         [0019]    [0019]FIG. 3 is a diagram of the hydraulic circuit for one of the vibration isolators in the active suspension system;  
         [0020]    [0020]FIG. 4 is a diagram of the hydraulic circuit for a vibration isolator with fixed rebound damping;  
         [0021]    [0021]FIG. 5 graphically depicts the relationship between cylinder velocity and an opposing force produced by a damping orifice in FIG. 4;  
         [0022]    [0022]FIG. 6 is a diagram for the hydraulic circuit of a vibration isolator with variable rebound damping; and  
         [0023]    [0023]FIG. 7 is a schematic representation of another embodiment of the active suspension system for the agricultural tractor. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]    With reference to FIGS. 1 and 2, a vehicle  10 , such as an agricultural tractor, has a cab  12  within which an operator sits on seat  15 . The cab  12  is supported on the chassis  14  of the vehicle by three vibration isolators  16 ,  17  and  18 . The first and second vibration isolators  16  and  17  are attached to the vehicle cab at the rear of the chassis  14 . The third vibration isolator  18  is located at the center of the front of the cab  12 . The three vibration isolators  16 ,  17  and  18  can be located at other positions underneath the cab. Although the present invention is being described in the context of an isolation system which supports the cab  12  of the vehicle  10 , this system also could be employed to isolate only the operator seat  15  from the floor of the cab  12 .  
         [0025]    The vehicle cab  12  is susceptible to motion in several degrees of freedom. Movement in a vertical direction Z is commonly referred to as “bounce”, whereas “roll” is rotation about the X axis of the vehicle  10 , while rotation about the Y axis is referred to as “pitch.” The illustrated three-point active suspension, provided by the three vibration isolators  16 - 18 , addresses motion in these three degrees of freedom. However, one and two point suspension systems which address fewer degrees of freedom can also utilize the present invention.  
         [0026]    [0026]FIG. 3 illustrates the hydraulic circuit  20  for one of the vibration isolators, for example the first isolator  16 , with the understanding that identical circuits are provided for the other two vibration isolators  17  and  18 . As shown, the first vibration isolator  16  has a hydraulic actuator  25  which comprises a hydraulic cylinder  21  pivotally connected to the cab of the vehicle and a piston  22  with a rod  23  pivotally attached to the vehicle chassis  14 . However, the connections can be reversed in other installations of the vibration isolator. The piston  22  divides the cylinder  21  into a head chamber  24  and a rod chamber  26 . The fluid ports of the head and rod chambers  24  and  26  are connected to a common conduit  28 , which forms a node of the hydraulic circuit  20 . A gas charged accumulator  30  also is connected to the common conduit  28  and thus is directly connected to the head chamber  24  of the cylinder. The term “directly connected” means that associated two components are connected by a conduit without any intervening elements, such as a valve, an orifice ,or other device that restricts of controls the flow of fluid beyond the restriction inherent in the conduit.  
         [0027]    A three-position, closed-center control valve  32  selectively connects the common conduit  28  to either a pump supply line  34  or a tank return line  35 . The control valve  32  is operated by a solenoid which receives electric current from a controller  40 . The pump supply line  34  carries pressurized fluid from a pump  36 , which is driven by the engine of the vehicle  10 . The tank return line  35  carries fluid to the tank  38  of the vehicle&#39;s hydraulic system. The pump supply line  34  and the tank return line  35  also are connected to the other two vibration isolators  17  and  18  and to other hydraulic circuits on the vehicle  10 .  
         [0028]    A displacement sensor  42  is connected between the cab  12  and the chassis  14  adjacent the cylinder  21  and produces an electrical signal which indicates the relative displacement (Zrel) between the cab and the chassis. That relative displacement signal is applied as an input to the controller  40 . In addition, an accelerometer  44  is physically mounted on the cab  12  to provide another input signal to the controller  40  which indicates the acceleration of the cab with respect to the ground  45  on which the vehicle  10  is traveling. A velocity sensor could be used in place of the accelerometer  44 .  
         [0029]    The controller  40  is a conventional microcomputer based device and has a memory which stores a software program for execution by the microcomputer to operated the vibration isolator  16 . The memory also stores data used and produced by execution of that software program. Additional circuits are provided for interfacing the microcomputer to the sensors  42  and  44  and the solenoid of valve  32 . Although a separate controller  40  is shown for the first vibration isolator  16 , it should be understood that a single controller can be employed to govern the operation of all three vibration isolators  16 ,  17  and  18 .  
         [0030]    The present active suspension system utilizes active controls in series with a hydraulic spring formed by the accumulator  30 . The hydraulic spring acts as a mechanical filter for the active controls by attenuating higher frequency vibrations, while the active hydraulic portion responds to the lower frequency vibrations which are most noticeable to the vehicle operator. As a consequence, the operation of the active control is relegated to frequencies near the natural frequency of the system (e.g. less than approximately three Hertz).  
         [0031]    The controller  40  receives the signal from sensor  42  corresponding to the relative displacement Zrel of the cab  12  with respect to the chassis  14  and a signal from accelerometer  44  corresponding to the acceleration of the vehicle cab  12  with respect to the ground  45 . From those input signals indicating instantaneous motion of the cab  12  resulting from the chassis vibration, the controller determines movement of the piston  22  that is required to cancel that instantaneous motion. Next the controller  40  ascertains the direction and amount of fluid flow required to produce that desired canceling movement of the piston  22  and then derives the magnitude of electric current to apply to the control valve  32  to produce that fluid flow. That electric current magnitude is a function of the desired fluid flow and the characteristics of the particular control valve  32 . The position and degree to which the control valve  32  is opened are respectively based on the direction and magnitude of the vibrational motion. When movement of the cab is not occurring, the control valve  32  is closed.  
         [0032]    The fluid flow required from the pump  36  is minimized by requiring gravity to vent hydraulic fluid. Because fluid from the pump  36  is not required when gravity moves the cab  12  downward, thereby venting more fluid from the head chamber  24  that is required to fill the expanding rod chamber  26 , hydraulic power consumption is greatly reduced from that of a traditional active suspension system. The gravity lowering configuration of the hydraulic cylinder  21  also reduces the number of control valves from two required in previous active systems to a single valve.  
         [0033]    The present active suspension system&#39;s use of gravity as the downward force to counteract vibration induced upward movement of the cab is limited to attenuating positive inertial acceleration of the cab mass (in direction of arrow  47 ) that is less than 1 g. Positive acceleration greater than 1 g results in an uncontrolled positive velocity of the cab (upward motion) with the system in FIG. 3. A solution to prevent the uncontrolled positive velocity is to employ regeneration with rebound damping. The embodiment of the active suspension system  50  in FIG. 4 is similar to that shown in FIG. 3 with identical components being identified with the same reference numerals. However, the second system  50  includes a fixed rebound damping orifice  52  between the two cylinder chambers  24  and  26  which provides a damping force to counter upward cab motion. A check valve  54  is connected in parallel with the rebound damping orifice  52  to allow flow there through only from the head chamber  24  to the rod chamber  26 .  
         [0034]    When the vehicle cab  12  is subjected to downward vibrational movement, the second suspension system  50  operates in the same manner as the previously described system  20 , wherein the check valve  54  allows hydraulic fluid being forced from the head chamber  24  to flow relatively unimpeded into the rod chamber  26 . Upward vibrations less than 1 g still are counteracted by gravity providing a downward force on the cab. During an upward vibrational motion, the check valve  54  is closed and the fixed orifice  52  creates a pressure differential between the head and rod chambers  24  and  26 . The pressure in the rod chamber  26  now is greater than in the head chamber  24 , thus creating a negative net force which opposes the force driving the cab acceleration. Therefore, the parallel arrangement of the check valve  54  and fixed orifice  52  creates a relationship between vibration velocity and that opposing force, wherein the opposing force is relatively constant for negative velocities and rises exponentially for positive velocities as shown in FIG. 5.  
         [0035]    With reference to FIG. 6, another vibration isolation circuit  56  can provide variable rebound damping by replacing the fixed orifice  52  with a variable damping orifice  58  the size of which is determined by an electrical signal from the controller  40 . The control of variable damping orifice  58  is a function of the accelerometer signal and the relative displacement of the cab provided by sensor  42 .  
         [0036]    In addition, the actuator  25  can be protected from extreme extension by closing the damping orifice  58  as the piston  22  approaches the end of its stroke within the cylinder. The relative displacement signal produced by sensor  42  indicates the position of the piston  22  within cylinder  21  and thus is used by the controller  40  to determine when the piston is approaching the end of its stroke. To prevent extreme compression of the actuator  25 . the controller  40  operates the control valve  32  to stop venting the fluid to the tank  38  and to convey pressurized fluid from the supply line  34  to the actuator.  
         [0037]    Although the individual subsystems shown in FIGS. 3, 4, and  6  can be used for each of the vibration isolators  16 ,  17  and  18 , a single controller and a common set of cab motion detectors can be employed as shown in FIG. 7. In this active suspension system  60 , the hydraulic circuit for each vibration isolator is the same as that shown in one of those prior figures. However, instead of a separate accelerometer for each vibration isolator  16 - 18 , this system  60  employs a single accelerometer  62  centrally located on the operator cab  12  to sense vertical acceleration (i.e. bounce). A pair of gyroscopes  64  and  66  also are mounted on the cab  12  to sense pitch and roll angular motion of the vehicle cab  12 . The accelerometer  62  and the two gyroscopes  64  and  66  provide electrical input signals to a common controller  68  that operates the three-position, closed-center control valve  32  in each vibration isolator  16 ,  17  and  18 . Each vibration isolator still includes a displacement sensor  42  which measures the relative displacement between the cab and the chassis at the respective vibration isolator. The signals from the displacement sensor  42  also are applied to the controller  68 .  
         [0038]    The controller  68  responds to the signals from the set of sensors  42  and  62 - 66  and determines how to drive the hydraulic actuator  25  (the piston and cylinder arrangement) in the three vibration isolators  16 - 18  to counter the vibration detected by the sensors. For example, if only bounce occurs, then all the vibration isolators  16 ,  17  and  18  are driven in the same direction. On the other hand, when only roll is sensed, only the first and second vibration isolators  16  and  17  at the rear of the tractor (FIG. 1) are driven to counter that roll vibration. Furthermore, to counteract pitch of the tractor  12 , the two rear vibration isolators  16  and  17  are driven in one direction while the front vibration isolator  18  is driven in the opposite direction. It should be understood that travel over rough terrain likely produced all three types of vibration concurrently and thus the controller responds simultaneously to signals from all the sensors.  
         [0039]    In this final active suspension system  60 , a single controller  68  and shared motion sensors  62 - 66  are employed to operate the vibration isolators  16 ,  17  and  18  to damp vibration in three degrees of freedom.  
         [0040]    The foregoing description was primarily directed to a preferred embodiment of the invention. Although some attention was given to various alternatives within the scope of the invention, it is anticipated that one skilled in the art will likely realize additional alternatives that are now apparent from disclosure of embodiments of the invention. Accordingly, the scope of the invention should be determined from the following claims and not limited by the above disclosure.