What
it is?
Electrically powered steering uses an electric motor to
drive either the power steering hydraulic pump or the steering
linkage directly. The power steering function is therefore
independent of engine speed, resulting in significant energy
savings.
How
it works :
 Conventional
power steering systems use an engine accessory belt to drive
the pump, providing pressurized fluid that operates a piston
in the power steering gear or actuator to assist the driver.
In electro-hydraulic steering, one electrically powered
steering concept uses a high efficiency pump driven by an
electric motor. Pump speed is regulated by an electric controller
to vary pump pressure and flow, providing steering efforts
tailored for different driving situations. The pump can
be run at low speed or shut off to provide energy savings
during straight ahead driving (which is most of the time
in most world markets).
Direct electric steering uses an electric motor attached
to the steering rack via a gear mechanism (no pump or fluid).
A variety of motor types and gear drives is possible. A
microprocessor controls steering dynamics and driver effort.
Inputs include vehicle speed and steering, wheel torque,
angular position and turning rate.
Working
In Detail:
A
"steering sensor" is located on the input shaft
where it enters the gearbox housing. The steering sensor is
actually two sensors in one: a "torque sensor" that
converts steering torque input and its direction into voltage
signals, and a "rotation sensor" that converts the
rotation speed and direction into voltage signals. An "interface"
circuit that shares the same housing converts the signals
from the torque sensor and rotation sensor into signals the
control electronics can process.
Inputs from the steering sensor are digested by a microprocessor
control unit that also monitors input from the vehicle's speed
sensor. The sensor inputs are then compared to determine how
much power assist is required according to a preprogrammed
"force map" in the control unit's memory. The control
unit then sends out the appropriate command to the "power
unit" which then supplies the electric motor with current.
The motor pushes the rack to the right or left depending on
which way the voltage flows (reversing the current reverses
the direction the motor spins). Increasing the current to
the motor increases the amount of power assist.
The system has three operating modes: a "normal"
control mode in which left or right power assist is provided
in response to input from the steering torque and rotation
sensor's inputs; a "return" control mode which is
used to assist steering return after completing a turn; and
a "damper" control mode that changes with vehicle
speed to improve road feel and dampen kickback.
If the steering wheel is turned and held in the full-lock
position and steering assist reaches a maximum, the control
unit reduces current to the electric motor to prevent an overload
situation that might damage the motor. The control unit is
also designed to protect the motor against voltage surges
from a faulty alternator or charging problem.
The electronic steering control unit is capable of self-diagnosing
faults by monitoring the system's inputs and outputs, and
the driving current of the electric motor. If a problem occurs,
the control unit turns the system off by actuating a fail-safe
relay in the power unit. This eliminates all power assist,
causing the system to revert back to manual steering. A dash
EPS warning light is also illuminated to alert the driver.
To diagnose the problem, a technician jumps the terminals
on the service check connector and reads out the trouble codes.
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