Open ECU EPS System, preliminary design using a Hardware-in-the-loop approach

The introduction of mechatronic systems in vehicles opens the possibility to a wide range of vehicle functionality. These systems allow active control of vehicle enabling a new level of driving comfort and safety. The development of these complex technological systems, however, requires a long time to integrate mechanical, electrical and software components. Innovative development strategies and devices are hence crucial to accelerate the integration and deployment of these mechatronic systems. In order to fulfil these needs, this paper presents the process to size a development electronic power steering which aims to support vehicle industry during testing and validation of innovative steering-based control strategies, including autonomous driving, steer-by-wire and all-wheel steering applications. Using an hardware-in-the-loop test bench for a wide range of vehicle maneuvers a stock power steering unit has been characterized taking into account both driver and assistance motor torque contributions.


Introduction
In recent years, technological evolution in the automotive industry has led to a continuous development of the most advanced vehicle features, with particular attention to safety and dynamic behaviour.One of the key areas where a central role has been observed is the active intervention on the steering system thanks to the introduction of electric power steering (EPS).Compared with hydraulic steering systems, the use of EPS systems has brought significant advantages, including greater energy efficiency (lower fuel consumption and reduced CO 2 emissions) and improved steering response and precision.More over they offer increased flexibility regarding system control, allowing for optimized steering response in different driving conditions [1].Apart from providing the necessary assist torque and the aforementioned benefits, electro-actuated steering systems, enable the accurate execution of Advanced Driver Assistance Systems (ADAS) functionalities.These advanced driving assistance technologies leverage the electronic control capability of the steering system to offer features such as Lane-Keeping Assistance (LKA), active blind-spot monitoring, and automatic parking, contributing to driving safety and efficiency.In addition, EPS steering systems are more easily integrable with future autonomous driving systems [2].Despite the growing interest in these topics, the development of customized control strategies and the implementation on existing experimental vehicles have proven to be challenging, as the programming of control units is often restricted to the specific system manufacturer.From these considerations arises the need to equip prototypal steering systems with a fully customizable open ECU control logic, whose operation is controllable, to support research and development activities in the automotive field.This article examines the design process of an Electric Power Steering (EPS) system with an open ECU, starting from the characterization of an existing EPS system installed on a commercial vehicle and its control logic.The examined EPS system is a parallel-axis electric motor system, which is commonly used in modern vehicles.The electric motor is controlled by an ECU that analyses data from sensors detecting the steering wheel angle, steering torque and vehicle speed.Based on this information, it provides the appropriate amount of assistance to facilitate steering manoeuvres.This analysis aims to identify the operating points of the electric servomotor, enabling the sizing and selection of a Brushless DC (BLDC) motor and a suitable drive, along with other electronic components necessary for system implementation.The introduction of an open ECU EPS will expedite the development and calibration stages of advanced control logics based on steering management, both for ADAS systems and autonomous vehicles.To characterize the control logic, it was necessary to identify the transmission ratios and its geometric properties.An experimental test campaign was conducted, integrating the steering system in the Hardware-in-theloop test bench, provided by Meccanica 42 S.r.l.[3], [4], [5].The tests were designed to investigate the EPS response to various driving conditions, including Open-loop and Closed-loop steering maneuvers.The implementation of an open ECU steering system represents a significant step in automotive research, allowing greater flexibility in customizing driving functionalities and opening new opportunities for the development of advanced technologies for road safety and autonomous driving.

Methodology
For the first part of the activity, a complete disassembly of the steering rack was performed.Mechanical measurements were collected using a caliber and a coordinate-measuring machine (CMM).These dimensions were used for the evaluation of basic properties of the steering rack, such as the rack-pinion gear ratio.After this phase, we were able to reproduce the geometry of every needed component using typical solid modelling software; this phase is required for detailed analysis (kinematic or stress analyses).In future, the whole mechatronic part of the rack (electric motor, belt, electronic board) will be redesigned and integrated with the existing components such as the case, the rack and the gear.The knowledge of these measurements was hence mandatory for the aim of this investigation, future activities included.Similarly, this information is necessary to implement a simple, yet effective, numerical model to calculate the forces acting on the system.To validate this model and to evaluate the EPS assist curves of this steering system, we decided to select a specific set of key manoeuvres and adopt a semi-experimental Hardware-in-the-Loop method.This test campaign was performed using an advanced Hardware-in-the-Loop simulator that integrates the entire steering system of a real vehicle in a static driving simulator.The layout of this HiL simulator is briefly explained in the scheme in Figure 1.In this simulator, two electric motors are used to reproduce the forces acting on the tie rods in every step of the real-time simulation; for each motor, one rocker is used to replicate the kinematic movement of the tie-rod as in the real vehicle.The hardware installed includes the entire steering system from the steering wheel up to the tie rod.The vehicle is modelled using the VI-CarRealTime model.All the signals needed for the functionality of the Electric Power Steering (EPS) are provided using the Controller Area Network (CAN) protocol, just like in the real vehicle.Data used for the steering performance characterization are extracted directly from the CAN network of the steering unit.The most relevant signals are the steering column torque and steering wheel angle.The steering column torque signal is sensed at the EPS torque sensor; the signal has a quantization of 8mN m and its value is sampled every 10ms.The steering wheel angle signal is measured at the EPS sensor; the signal has a resolution of 0.1deg and its value is sampled every 10ms.In this test bench, the stock tie rods have been modified to integrate two load cells.The transducer is certified for 0.2% Full Scale (FS) accuracy.These measurements are used for the test bench control logic and these data were used for the evaluation of this study.In addition, the steering rack position is quantified using a linear potentiometer.Figure 2 shows the simulator described and the more relevant components for the aim of this activity.

Maneuvers and test procedure
In literature the assistance torque is defined as the sum of the contribution of the essential functions[1], [6].
• Power-assistance: It aims to support the driver reducing the steering effort, especially during parking and low speed manoeuvre.• Friction compensation: Reduces the effect of friction in the steering system, to provide a more responsive behaviour.• Inertia compensation: Reduces the effect of EPS motor inertia to improve the dynamic performance of the steering.• Active damping: Regulates system damping to improve stability and steering feel [7].This function prevents steering wheel overshoot and unwanted oscillation around the center position during release.• Active return: Generate an adequate and satisfactory runback response, particularly at low vehicle speed.
Therefore specifics, open and closed-loop manoeuvres were chosen to highlight all the aforementioned functions effects.These manoeuvres were executed in a wide vehicle speed range, from 30 km/h up to 180 km/h.In Open-Loop maneuvers the driver provides vehicle inputs independently from vehicle response.On the contrary during Closed Loop maneuvers, the driver continuously adjusts inputs based on the feedback received from the vehicle.Each manoeuvre performed is described as follow: All the aforementioned manoeuvres were performed with an initial vehicle speed of 30km/h and then repeated again by increasing the initial speed by 30km/h until the limit value of 180km/h was reached.Lower speed values were not investigated because they are irrelevant for the automobile handling evaluation.Higher speeds, on the flip side, are difficult to perform with this vehicle due to its limited power output.Results evaluation and significance will hence not be trustworthy.During the execution of the tests throttle position was adjusted to maintain the vehicle speed with a tolerance of 5%.
Prior to the execution of each test, a straight section was driven while monitoring the yaw rate of the vehicle so that this was at a value of 0 ±0.5rad/s.

Discussion
The analysis of the data collected during the test manoeuvres described before allows to extrapolate the assistance map of the EPS and individuate the motor operative map.The steering system has been modelled with the help of the one degree of freedom model as the one presented by Thomas Weiskircher et Al [8], [9].The schematic representation of the single DOF model is shown in figure 3.
A more complex five degrees of freedom steering model, such as the one proposed by Harror et Al, [1] needs a detailed characterization of the steering system and all its components properties such as stiffness and damping coefficients.The single DOF model only requires the knowledge of the steering system kinematic properties, which have previously been identified, and its equivalent inertia.This dynamic model is depicted by equations 1, while a brief description of each equation parameter is provided in table 1.The non-linear friction behaviour is represented using the LuGre Friction model [10].This model is one of the most popular approaches to treat friction in modelling and compensation algorithms thanks to its capacity to describe both the Stribeck effect and the stick-slip motion.The identification of the four static and two dynamic LuGre friction parameters has been carried out using dedicated manoeuvres as the one suggested by C. Canudas De Wit and P. Lischinsky [11].LuGre model properly predicts friction phenomena in steering systems as it can accurately describe its dependence not only from steer velocity, but also from the applied torque [12]. .
The assistance map extrapolation has been carried out using the approach suggested by Pfeffer et Al.[13], which enable to define the steering assistance torque with two parameters only per vehicle velocity.This analytical approach allows to efficiently determine a harmonious increase of the assistance torque in relation to the steering wheel torque, avoiding the obsolete and time-consuming point-by-point definition of the assistance map.This method is based on the definition of the assistance torque ratio, equation 3, which defines the support provided by the EPS motor to the driver to balance the steering torque generated by the tire action in steady state corners.To enable proper steering feel and smooth boost curves, the assistance ratio should linearly increase with lateral acceleration as shown in equation 4. Given the kinematic properties of the steering system and analysing the moments acting on the steering axis, it is possible to express the required assistance torque as function of the applied steering torque as in equation 5. Considering this set of equation 3, 4, 5 to characterize the assistance boost curve it is sufficient to define D a and K a .All equation parameter are synthetically described in table 1. .The extrapolated assistance map is shown in figure 4a, where the steering torque, vehicle velocity and EPS assistance torque have been normalized with their respective maximum values.The assistance torque is a function of driver torque and vehicle velocity.In particular, at low vehicle speed, the EPS system supports the driver with high assistance torque while it decreases at high velocity to transmit a more authentic feeling to the driver who can perceive the vehicle state.The total rack force is the result of the contribution of the driver action and the assistance motor torque multiplied by their respective transmission ratio.Figure 4b, shows how in recent vehicles the generation of the rack forces is mostly demanded by the assistance actuator.Figure 6, shows the operative zones of the BLDC motor, where torque and Velocity are normalized against their respective maximum values.For the EPS motor, the high-frequency sine manoeuvre proves to be the most demanding in terms of speed and power.Being a closed-loop test, the track drive is the most representative of real driving conditions, as shown in the figure motor velocity is significantly lower if compared to the one reached during the high-frequency sine manoeuvre.However, high torque levels are observed during these driving conditions, particularly during high-speed corners.The working points of the low-frequency sine and ramp/step manoeuvre are substantially included in those of track drive and high-frequency sine.The above considerations permit the selection of the appropriate commercially available BLDC motor capable of operating in the highlighted area.

Conclusion
The possibility to actively control the steering system has played a central role both in the development of steering-based ADAS systems and in the control of autonomous driving vehicles.The introduction of these complex technologies has led to changes in the development process of vehicle functions, making fundamental the use of Hardware-in-the-loop simulators combined with development devices.In this paper is proposed a novel testing method to determine the realistic working conditions of an electric power assist motor, in terms of maximum torque and velocity.Through the execution of specific manoeuvres the validation of a simple and effective 1 DOF steering model was carried out along with the development of a basic steering assistance system control plant.With the aim to accelerate testing procedures, a steering actuator with customizable control logic will be developed starting from these findings.To support vehicle industry in all the steering research fields, this development device could be coupled not only with classical assistive EPS systems but also with innovative actuation steering systems such as Steer-by-wire, all-wheel steering and autonomous driving.

Figure 2 .
Figure 2. HiL static simulator: layout and main components

Figure 4 .
Figure 4. Assistance and driver effort

( a )
Model and Measured assistance torque (b) Error distribution

Table 1 .
Equation parameter 7 (a) EPS assistance torque map (b) Rack force contribution from EPS and Driver torque