Land vehicles – Suspension modification enacted during travel – Suspension stiffness for ride comfort
Reexamination Certificate
1999-11-11
2003-01-07
Culbreth, Eric (Department: 3616)
Land vehicles
Suspension modification enacted during travel
Suspension stiffness for ride comfort
C701S037000
Reexamination Certificate
active
06502837
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to computer controlled vehicle suspension systems and methods, and more particularly to vehicle suspension systems and methods in which computer controlled damping forces in compression and rebound directions are used to optimize ride and handling characteristics of the vehicle.
A typical suspension system interlinks wheels and axles of the vehicle with the body and chassis of the vehicle. The suspension system generally includes springs and damping devices. The spring compress and expand to minimize movement of the chassis and body when the wheels encounter perturbation in a roadway surface. Excess movement created by the springs is controlled by the damping devices.
A common damping device, or damper, is a velocity sensitive hydraulic system which uses hydraulic pressure to resist movement of a piston. Piston velocity is a direct function of the speed of the sprung mass with respect to the unsprung mass. As the damper is a velocity sensitive device, the greater the piston velocity, the greater the damping force provided by the damper in a direction opposite the movement of the piston.
The damping force is generally created when the moving piston forces a hydraulic fluid, typically oil, through an orifice or a valve. A flow resistance encountered by the oil results in a set of damping forces, a compression force and a rebound force. The damping forces act to counter and dissipate a stored energy associated with a springs movement that generates a spring-induced force. Varying the fluid flow through the valve or orifice varies the force acting against the spring-induced forces and, therefore, changes the ride and handling characteristics of the vehicle.
The damping forces are passive resistive forces. Accordingly, the respective compression and rebound forces only have effect when the piston is moving. When the velocity of the piston is zero no force is applied.
To improve ride and handling, one type of control system used in suspensions is a constant force type suspension system. Details of a constant force type suspension system are discussed in PCT Application entitled Computer Optimized Adaptive Suspension System and Method Improvements published Feb. 29, 1996, international publication number WO 96/05975 the entirety of the disclosure of which is incorporated herein by reference. In a constant force type suspension system pressures in the compression and rebound chambers of a damper are controlled by one or more pressure regulators to control a compression force and a rebound force. A computer utilizes a control input as a feedback signal to generate a constant compression and/or rebound force at a wheel.
If a vehicle equipped with such a constant force suspension hits a sharp bump the pressure is increased in a compression chamber of the damper. To maintain a constant force the control system allows for a pressure release of the hydraulic fluid from the compression chamber to maintain a constant force. This is done by providing a pressure relief valve on the regulator so that once a preset pressure is reached in the fluid, the valve opens to allow the hydraulic fluid to flow out of the chamber relieving the pressure. However, while a compression force is being applied and regulated it is not possible to regulate force in the rebound direction. Thus, only one chamber at a time is being pressurized, due to the piston movement.
A problem with this design is that there is a lack of force when the direction of travel of the damper is wrong for the application of a control force. For example, if during braking a bump is hit an upward force is exerted on the damper. After the wheel's carrier assembly compresses to the maximum extent possible in one direction, it begins to rebound. During rebound the compression forces cannot be maintained and can only be reestablished once the wheel reaches its lowest position and begins to compress again. This is because the direction of travel has changed, and forces applied to control movement in a direction opposite to the movement are ineffective. Thus, control forces can be momentarily lost during damper movements in an undesired direction when attempting to maintain a constant force. This loss of force is not as serious as what happens when the direction of movement one again engages the control valve and the previously preset pressure returns very abruptly, which results in severe harshness. Mechanical compliance is a way of maintaining control forces during damper movement in an undesired direction.
FIG. 1
is a cross sectional view of a damper provided with mechanical compliance. Compliance is the ability of an object to yield elastically when a force is applied. In suspension systems compliance is provided by springs or rubber bushings in the damper. Compliance provides transitional forces during a period of time in which damper forces would otherwise be lost.
As shown in
FIG. 1
compliance provided by a set of springs or a set of elastic rubber bushings has been incorporated in damper designs. A piston
102
a
is coupled to a damper shaft
104
a.
Resilient rings
106
a
and
108
a
are springs or rubber bushings that are disposed abode and below the piston, such that they elastically couple the shaft and piston. A bolt
110
a
and a washer
112
a
retain an upper resilient ring
106
a
to a damper shaft
104
a.
A lower washer
114
a
rests on a shoulder machined into the damper shaft
104
a
to retain a lower resilient ring
108
a.
The rubber bushings
106
a,
108
a
compress when pushed upon.
Thus, when the damper shaft is forced upward hydraulic oil in compression chamber
116
a
pushes against the piston. Because the piston is not fixed to the shaft the rubber bushings or springs
108
a
are compressed. Accordingly, the movement of the damper shaft does not produce a corresponding equal movement of the piston. The oil in the compression chamber is in contact with the oil in an outer sleeve
118
a
through a port
120
a.
The outer sleeve is in contact with pressure regulator
122
u
through port
120
a.
Therefore, when the pressure in the compression chamber reaches a relief setting, pressure regulator
122
a
opens, allowing the fluid in the compression chamber to flow out of the chamber, maintaining a predetermined force on the fluid. Thus, the compliance provided by the resilient rings or springs
106
a,
108
a,
allows the damper shaft to move upward before the preset relief setting of pressure regulator
122
a
has been reached. Therefore, instantaneous changes of piston position with respect for the chamber are allowed for.
In addition in some applications a loaded vehicle's suspension becomes mushy. The ratio of a DFT of the SNF to one half of the SNF tracks the load change in producing compensating forces. In some SNF applications there was too little damping for small bumps and too much on big bumps. Taking the square root of the DFT SNF damping output provides optimum damping for all bumps. In providing UNF forces a steep force causes a mechanical strain on the system hardware. By taking the square root of the UNF damping force this is eliminated. In the bottoming out and toping out force generation sufficient control was not obtained. By taking position as a displacement and the velocity as an axle speed the responses now provide sufficient control.
After the springs become fully compressed as a result of the force applied from hitting the bump, the springs rebound in the opposite direction. Thus, the wheel and carrier move away from the chassis once the compressive force is removed. The lower rubber bushing
106
a
provides compliance in the rebound direction in a manner analogous to the discussion above.
The compliance provided by the mechanical system does not require the relief valves in the pressure regulators
122
a
and
124
a
to activate quickly when presented with short, abrupt motions. Further, compliance reduces high frequency vibrations and harshness encountered when the vehicle hits a bump by allowing the piston and chamber to move
Hamilton James M.
Woods Lonnie K.
Christie Parker & Hale LLP
Culbreth Eric
Dunn David R.
Kenmar Company Trust
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