Many cars are now equipped with
computer-controlled active suspension systems. In these systems, a
computer-controlled actuator is positioned at the top of each shock absorber or
strut. The actuators rotate a shaft connected to the shock's valve inside the
piston rod. Many of these systems have two models, soft and firm. In the soft
mode, the actuators position the valves so these is less restriction to the
movement of oil. When the computer changes the actuators to the firm mode, the
actuators position the valves so they provide more restriction to oil movement,
which results in a firmer ride.
MagneRide is a semi-active
suspension system that features shocks or struts without electromechanical
valves or small moving parts. Instead of valve-controlled orifices, MagneRide
regulates the flow of fluid by a variable magnetic field produced by a small
electric coil mounted in the shock. The shocks are filled with magneto-rheological
(MR) fluid. MR fluid consists of magnetically soft particles, such as iron,
suspended in a synthetic fluid.
The action of the shock forces the
fluid through an opening. When the shock is in its off state, the fluid is not
magnetized and flows freely through the orifice. When current is sent to the
coil, the fluid becomes magnetized and its viscosity changes instantly.
The material changes from a fluid
state to a semi-solid state that is directly proportional to the magnetic field
applied to it. With little or no electrical current, the iron particles are
randomly distributed, and the fluid passes freely through the piston orifice.
When a strong electrical current is applied to the coil, the resulting magnetic
field aligns the iron particles so that the fluid stiffens and the flow is
resisted. This causes heavy damping. The resulting damping force is proportional
to the viscosity, which is proportional to the strength of the magnetic field.
Sensors monitoring wheel position,
lateral acceleration, vehicle speed, steering wheel angle, and brake pedal angle
are inputs to the control module that sends current to the coil in the shocks.
This system provides extremely quick response time, typically about 5 ms, and
the fluid is capable of reaching 30000 times per second.