Page 120 - Handbook of Biomechatronics

116 Naser Mehrabi and John McPhee

3.3 Electric Power Steering (EPS) Control Design
The main responsibility of an EPS system is to reduce the driver physical
effort. As a result, almost all power steering systems have a component in
their logic to magnify driver torque by generating an assist torque propor-
tional to the driver torque. The relation that assigns an EPS assist torque to
each driver steering torque is presented in so-called characteristic curves.
Typically, the steering characteristic curves are multilinear functions of
the driver steering torque at different vehicle speeds. In this research, we
used a bilinear characteristic curve at each given speed as shown in Fig. 8.
This characteristic curve consists of an unassisted zone to avoid the off-
center feeling, a steering assistance zone, and a maximum assist value that
is restricted by maximum motor torque. The bilinear characteristic curves
can be expressed as follows:
8
0
< 0 < T d < T d0
ð
T a ¼ K a T d T d0 Þ T d0 < T d < T d max (30)
T m T d < T d
: max max
max
where T a , T m , and K a , respectively, represent the assist torque, the max-
max
imum torque of the motor, and the assist gain. T d , T d0 , and T d represent
the driver’s steering torque, the driver’s steering torque when the motor
begins to assist, and the driver’s steering torque when the motor assist reaches
the maximum assistance (T max ¼ T m max ), respectively. The coefficient
d K a + T d 0
K a is an adjustable shape factor that represents the rate of assist. Note that
K a reduces as vehicle speed increases. In the high-fidelity integrated

Fig. 8 Bilinear EPS characteristic curve.