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Page 4 of 15 Cui et al. Complex Eng Syst 2023;3:3 I http://dx.doi.org/10.20517/ces.2022.57

2.2. Steering performance indexes
Considering the performance requirements of the EHPS system, steering road feel, steering sensibility, and
steering energy loss are taken as the evaluation indexes. The quantitative formulas of the three evaluation
indexes are derived as follows.

2.2.1 Steering road feel
In this paper, steering road feel is defined as a transfer function from steering resistance torque to steering
wheel torque , which reflects the efficiency of transmitting torque fluctuations from the road surface to the
driver. Besides, the steering wheel angle is defined as 0 to reduce a degree of freedom and facilitate the
analysis of the steering system.

It is assumed that the torque sensor can be simplified as a torsion bar spring, and the measured value of the
torque sensor could be computed by

= − = − (2)

where is torque obtained by torque sensor.

According to the current control strategy, the current can be given by

(3)
=
The torque provided by the motor can be given by

= − (4)
Where is the torque coefficient of the motor, is power gain coefficient.

According to (1)-(4), steering road feel can be computed as follows.

( )
( ) = = (5)
2
( ) 1 + 1 + 1
where

2
 1 = m + 2


  2 2 2
2
1 = + 2 + 2 2

 2 1


 1 = +
Generally, the effective road information frequency domain range is 0-40 Hz. Thus, steering road feel is mea-
sured by its average frequency power within this range.
¹
1 0 2
= | ( )| (6)
2 0 0
2.2.2 Steering sensibility
Steering sensitivity reflects the response speed of a vehicle to steering action and has an important impact on
vehicle safety at high speed.
The vehicle three degree of freedom differential equation can be described as [17]

 ¤ − = + + +
¥


+ − ℎ ¥ = + + + (7)
¤

 − + ℎ − ¤ = +
¤
¤
¥
