SMART Series 451 Service Manual page 78

SN_Typ 451_us.book Seite 77 Freitag, 30. November 2007 1:56 13
Function
Function requirement
Steering assistance is activated after engine start
and at a minimum engine speed of 500 rpm.
Steering power assistance
As the driver turns the steering wheel, a steering
torque is produced at the drag link which is regis-
tered by the power steering torque sensor and
forwarded to the power steering control unit.
The steering torque is used as an actuating variable
for the electric assistance force. It is derived from
the rotation of the torsion bar, which is located
between the steering gear input shaft and steering
gear pinion shaft on the same axis.
The power steering torque sensor is located around
the torsion bar. This determines the torque over a
range of +/- 8.1 lb-ft (11 Nm) and at a resolution of
0.037 lb-ft (0.05 Nm).
The power steering control unit uses the steering
torque measurement to determine the level of torque
that needs to be provided to supplement the steering
force.
Based on this information and depending on the
vehicle speed signal from the ESP control unit and the
engine speed signal from the ME-SFI [ME] control
unit, the power steering control unit determines the
current requirement of the power steering motor as
per the programmed characteristic field.
The power steering motor transmits the appropriate
supplementary steering torque to the face of the
hyperbolic gear via the worm shaft.
Steering torque of the electric power steering (EPS)
2 Steering gear pinion shaft
3 Steering gear input shaft
5 Worm shaft
6 Hyperbolic gear
7 Torsion bar
B23 Power steering torque sensor
M19 Power steering motor
N68 Steering assist control unit
Introduction of the smart fortwo, Model Series 451
Steering
P46.10-2934-00
77