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This paper reports the implementation of Hardware Inner Loop Simulation HILS on electronic steering system for Steer By Wire SBW technology and the method that is use to identify steering response between conventional steering system and SBW. HILS is a method used to control the SBW system in real time simulation. To run HILS for SBW system, the application XPC Target in MATLAB SIMULINK is used. In the conventional steering system, rack and pinion ratio is introduced in the simulation. The SBW gives close x-trajectory, y-trajectory, yaw angle and lateral acceleration results when compared to the conventional steering system. The sine steer inputs and step steer inputs are used to identify steering response between actual and desired for the experiments.
Keywords: sine steer, step steer, inner loop controller, xpc target
Steer By Wire (SBW) one of electronic steering system type by replacing the mechanical system such as steering column, steering shaft and hydraulic system for conventional and power steering with electronic system to give an advantages for steering system such as safety, fuel economy, less pollution, lager space, design freedom and advance functions. It is an advanced technology that will apply to steering system for next generation future vehicle. The SBW system used to enhance vehicle steering has been undergoing worldwide development as the SBW system has many advantages . An advantage of SBW system is that the application provides more space since the mechanical system is replaced with electronic systems. A SBW system can offer several benefits compared to conventional mechanical steering systems . The limitation of conventional steering system is that some come with power steering which consumes space. Another limitation includes the inability of the engine room compartment to take various suspension system designs that are advance but are bigger in size. Further limitation happens when the road disturbance can give effect to the steering wheel since it is connected mechanically. Kleine S, Niekerk JL  proposed an electronic steering system for SBW system. It can save energy, reduce weight through elimination of the steering wheel column, and reduce noise and vibration through elimination of a hydraulic pump and gear box .
The flexibility to turning the steering feel can reduce the traditional tradeoffs between ride and handling. The desired steering feedback can be obtained without being linked to other design decisions. SBW may enable or enhance vehicle control technologies related to collision avoidance, lane keeping, and enhanced stability control. Despite the continuous cost reduction of electronic sensors, controllers, and actuators, the SBW system cost remains a significant obstacle to application in passenger cars . By developing HILS for electronic steering can generated steering response similar to that in a conventional steering system. HILS is real time simulations that control an actuator for real system . HILS is a method that uses to run the experiment for SBW. In the HILS setup, an electric motor mounted on the pinion of the half car test rig is used as the SBW actuator. The real time running vehicle model data is used to calculate road disturbances and tire forces some of which are feedback to the electronic steering system .
HILS is a technology which replaces real vehicles, engines or other components by simulation based on a mathematical model . This technology can drastically reduce expensive field tests and also without effecting environment. Even failure situations which would normally not be feasible for an experiment can be tested. The mechanism of the HILS system consists of an actuator such as stepper motor for a lateral force on the front tires in a real vehicle. The SBW controllerâ€™s availability was verified through a number of simulations on the HILS system. The SBW fail-safe logic was tested through various simulations of the hazard environment on the HILS system. Consequently, the control logic of the SBW system was developed easily and safely in a laboratory . Steering responsiveness for a SBW system presents unique challenges that are the significant variation of the steering loads experienced. The control design technique can reduce the influence of loading variation such that targets for steering response and control stability can be met . In MATLAB SIMULINK, the steering response of the SBW model is validated with conventional steering system. In the simulation, MATLAB SIMULINK software is used to define the steering response between conventional steering system and SBW system by using sine wave and step as a steering input for sine steer and step steer test. XPC Target is the application in MATLAB SIMULINK to run HILS for both test for sine steer and step steer in experiments to compare the actual and desired value for steering input.
The vehicle model is developed in MATLAB SIMULINK to replace actual vehicle for safety before implementation and utilization in an actual production vehicle . The vehicle model is developed in early stage to using comprehensive vehicle simulation as a safety testing to investigate and identify the model. In the simulation, many tests are done using vehicle model such as double lane change, lane keeping, and slalom test and also for sine steer and step steer without effecting environment. For SBW, vehicle model are important to test the steering control for early stage in simulation for sine steer and step steer test as a steering input to find the output for x-trajectory, y-trajectory, yaw angle and lateral acceleration between conventional steering system and SBW system for both tests.
The equation of motion gathered from  has been used to build the vehicle model in MATLAB SIMULINK. The first equation is longitudinal and lateral acceleration. The equation is derived through handling model as shown in Eq. (1) and Eq. (2). The longitudinal and lateral acceleration is equal to the sum force of every tire and divided with the mass.
To find side slip angle, Î², lateral and longitudinal acceleration can be integrated to get lateral and longitudinal velocity that can be used in the Eq. (3).
The equation for yaw motion can be derived by taking moment at the vehicle centre of gravity (COG), G and the value of b and c is a distance between COG from front and rear tire. The equation of yaw motion is shown in Eq. (4).
Roll motion, , and pitch motion, , is derived by considering only the sprung mass since unsprung mass did not effect by roll and pitch motion.
Longitudinal force,, act at each tire is obtained from the tire model and for this vehicle model, Calspan tire model has been used. From literature review, this tire model can provide useful force for use in full vehicle model since it contains interactions between longitudinal and lateral forces from small level through saturation. In order to calculate the lateral and longitudinal force, the tire model needs vertical force,, tyre slip angle, , and tyre slip rates, , for each tire which can be calculated through the equation below.
The accuracy of the vehicle model is validated with CarSim software through double lane change test at the certain speed. The model is consider validated when the graph for lateral acceleration, yaw angle, roll angle, pitch angle, x and y position follow the graph from CarSim software. The validation of vehicle model is important to prove that this model is accurate.
Steering system model
To develop the electronic steering system in the simulation model, the steering system model and vehicle model are combined. The steering system model is developed by following the half car test rig. The half car test rig use an actuator located on the pinion to rotate the pinion clockwise or counterclockwise. The actuator model is inside the steering system model. The relation between rack and pinion is defined by comparing the simulation and experimental result. Then, control structure for steering system model is developed so that the output from the simulation and the experimental result for the steering input is followed. Figure 1 shows SBW model where the steering system model is combined with the vehicle model.
Figure 1: SBW model
To control the pinion rotation, control structure is developed to control the steering system for SBW model. The control structure is developed to control the actuator to rotate pinion clockwise or counterclockwise and to adjust pinion angle. Figure 2 shows the inner loop controller that is using PID as the controller to control the actuator. PID controls the actuator to make actual data follow the desired data.
Inner loop controller
Figure 2: Control structure
The graph from Figure 3 shows that the steering input between desired and actual for sine steer and step steer. By controlling the PID, the actual result can follow the trend of desired. The error between actual and desired were reduce by adjusts the parameter of PID. The input value from Figure 3 shows the graph for sine steer test is 45 degree and 0.5Hz and for step steer is 45 degree.
Figure 3: Steering input for sine steer and step steer in simulation
From the simulation result the output for both test is x trajectory, y-trajectory, yaw angle and lateral acceleration. The outputs are shown in Figure 4 to 12 for sine steer test and Figure 13 to 15 for step steer test. For sine steer test, the result between SBW and conventional steering system closely follow. This proves that this SBW system can be applied as an electronic steering system for automotive application. The input parameters that are use for sine wave is 15 degree, 30 degree and 45 degree and the frequency are 0.1, 0.25 and 0.5 Hz. The graphs for sine steer are shown in figure 4 to 12 such as x-trajectory (a), y-trajectory (b), yaw angle (c), and lateral acceleration.
The results for sine steer test in simulation between conventional steering system and SBW system are shown in Figure 4 to 12. From the results, SBW model follows the trend of conventional steering system model. For 15 degree angle, the SBW model outputs almost closely follow the conventional steering system model output with different frequencies.
Figure 4: Graph of sine steer for 15 degree and 0.1Hz
Figure 5: Graph of sine steer for 15 degree and 0.25Hz
Figure 6: Graph of sine steer for 15 degree and 0.5Hz
For 30 degree input value, SBW outputs still follow the conventional steering system output trend but not very closely. The graphs showed the results for sine steer test 30 degree steering input value in Figure 7, 8 and 9 with different frequencies. The steering input value is set to 30 degree at speed 80kmph with 0.1Hz, 0.25Hz and 0.5Hz different frequencies to achieved 0.2g to 0.3g lateral acceleration .
Figure 7: Graph of sine steer for 30 degree and 0.1Hz
Figure 8: Graph of sine steer for 30 degree and 0.25Hz
Figure 9: Graph of sine steer for 30 degree and 0.5Hz
The steering input value for 45 degree shows that the SBW outputs still follow the trend but the lateral acceleration value more than 0.3g. The steering input value for sine steer test is set to 30 degree and the maximum frequency is 0.5Hz to achieved 0.2g to 0.3g lateral acceleration. The graph for 45 degree as an input value had shown in Figure 10, 11 and 12.
Figure 10: Graph of sine steer for 45 degree and 0.1Hz
Figure 11: Graph of sine steer for 45 degree and 0.25Hz
Figure 12: Graph of sine steer for 45 degree and 0.5Hz
The graphs have shown in figure 13, 14 and 15 for step steer shows the result of x-trajectory, y-trajectory, yaw angle, and lateral acceleration. The results for step steer in simulation between conventional steering system and SBW system. The input parameter is 15 degree, 30 degree and 45 degree. For step steer test, the speed is set 90kmph on constant speed. The steering input value for step steer test is set the achieved the 0.4g peak value for lateral acceleration . The graph shows in Figure 13 for 15 degree as an input value for step steer test. The SBW outputs almost closely follow the trend conventional steering system output.
Figure 13: Graph of step steer for 15 degree
Figure 14: Graph of step steer for 30 degree
Figure 15: Graph of step steer for 45 degree
Figure 15 shows the graph of step steer for 45 degree input value. From the graph, the SBW output is follow trend of conventional steering system output. The peak value of lateral acceleration for 45 degree steering input is 0.4g. The steering input value for 45 degree is set to achieved based on  by getting the peak value 0.4g for lateral acceleration.
Hardware inner loop set up
HILS system is method that can give more advantages because the experiment can be run as a real time situation by sending input signal from host pc. To analyze the steering response, HILS system is processed in the main computer and the steering system is replaced by the HILS mechanism. The pinion rotations execute real time control in the HILS system. This section described the hardware design in the HILS system and the steering control. To set up the HILS for experiment, the actuator driver and rotary encoder is connected to DAQ (Data Acquisition) card. An actuator is located on pinion to rotate the pinion clockwise or counterclockwise. The rotation of pinion will be read by rotary encoder. The DAQ card which sends the digital output and receives digital input is connected with target pc. Host pc is connected with target pc using LAN connection to give input signal for hardware driver. AC/DC power supply functions to supply voltage to the actuator and the rotary encoder. Figure 6 shows the experimental setup for HILS. The half car test rig has a stepper motor as an actuator that is connected to the pinion. The rotary encoder reads the pinion rotation. Inner loop controller is developed in MATLAB SIMULINK software to give input signal. The XPC target application where HILS is ran is used to control the pinion rotation.
Figure 16: Experimental set up for Hardware Inner Loop Simulation (HILS)
From the host pc, the input signal goes to the target pc and driver motor reads from DAQ card in digital input. The DAQ card converts from digital input to analog input to run the stepper motor. The pinion rotation was read by rotary encoder as analog output to DAQ card. Then the DAQ card converts it to digital output and display the graph on target pc. The graph for desire, actual and error for steering input are displayed.
Figure 17: Hardware Inner Loop Simulation (HILS) diagram
The experimental results are shown at figure 18, 19, 20 and 21 for sine steer and step steer. The results are comparing between desired and actual for steering input in HILS. For sine steer test, the input parameters are 15 degree, 30 degree, and 45 degree and the frequencies are 0.1Hz, 0.2Hz, and 0.5 Hz. For the step steer tests, 15 degree, 30 degree, and 45 degree are set to be the input parameters. The input parameters for both cases are shown below. From the sine steer tests, the actual result for 15 degree has most noise. As the angle increases, the noise level decreases where the 45 degree angle give the least noise. All of the graphs for sine steer test show that the actual result follows the desired results even for different frequencies.
Figure 18, 19 and 20 shows the experimental results for sine steer test. The steering input value is 15 degree (a), 30 degree (b) and 45 degree (c) are set for sine steer test with different frequencies. Increasing the angle and the frequency value will decrease noise. From the graphs of sine steer test shows that the small angle has most noise and the noise level decrease by increased the steering input values.
Figure 18: Graph of steering input for sine steer for 0.1Hz
Figure 19: Graph of steering input for sine steer for 0.25Hz
Figure 20: Graph of steering input for sine steer for 0.5Hz
Figure 18 shows the experimental results for step steer test. The steering input value is 15 degree (a), 30 degree (b) and 45 degree (c). From the graph of step steer test, the actual steering input results for 15 degree, 30 degree and 45 degree are closely follow the desired steering input. More noise at the low angle and the noise proportional with angle. The noise of actual results is decreased when the steering input is increased.
Figure 21: Graph of steering input for step steer test
The response of steering input was achieved by plotting the graph of SBW system and conventional steering system for sine steer and step steer test. From the simulation results, the steering input value for sine steer test is set to 30 degree. The steering input value for sine steer test is selected to achieve the lateral acceleration value between 0.2g to 0.3g. The output from sine steer test for lateral acceleration is achieved while the frequencies are increased. The steering input value for step steer test is 45 degree. The lateral acceleration value for step steer test is set to 0.4g. The graph from simulation result for lateral acceleration shows that the steering input value is set 45 degree to achieve 0.4g lateral acceleration. The steering input values are taken to run the experiment. From the experimental result, the graph shows the desired and actual steering input for sine steer and step steer test. The actual result for sine steer test shows much noise in low frequencies and proportional with frequencies. The results for step steer test are shown in Figure 2. From the graph for step steer test show most noise at small angle. The noise proportional with angle by increased the angle were reduce noise. The response of steering input for step steer test become slow when the angle are higher. The control structure is developed to analyzed steering feel and failure feedback for the next stage. The analysis of steering feel is done by doing experiments using HILS for SBW system. The steering input from steering rig will control the pinion rotation whether clockwise or counterclockwise.