Design of steer-by-wire software architecture for lateral control requirements of autonomous driving

Road feel collaboration technology involves the control of rack power assistance, such as the DP-EPS power assistance controller, which has a hand feel controller above it. In this process, some method is needed to achieve interaction, so that the road feel and hand feel can be experienced through signals, and the fluctuation of the road surface can be felt. The expected road feel time is estimated from the driver, the vehicle model to the vehicle state, and the expected feedback torque is obtained using the dynamic model and related compensation to control the road feel motor and the feedback control of the road feel motor. The overall road feel control is performed at these two levels.

Technical requirements of road sense simulator system

In the closed loop of road sense feedback and angle control, including the road sense cooperative controller and the power-assist controller, the relationship between them is estimated through the displacement angle related to the motor torque and the rack force, which not only shows the magnitude of the torque and the magnitude of the rack force, but also reflects the influence of fluctuations in the three frequency bands of low, medium and high. Substitute the rack force estimation into the control algorithm, and finally obtain the relevant calculation result - the relationship of the variable steering ratio to achieve road sense and coordinated control. According to relevant requirements, such as road sense, direction, variable transmission ratio, active return, steady-state power-assist control, inertia compensation, friction compensation, overheating, overload, overpressure, emergency steering, zero adjustment, angle control, terminal protection, road sense feedback, etc., its functional safety level is higher than EPS and RWA. Other relevant indicators include: responsiveness, resolution, delay time, feedback cycle, response accuracy, overshoot, rack force estimation accuracy, which are all requirements of the road sense simulator, and the overall R&D cycle will not be lower than EPS or RWA.

Rack force filter

The rack force filter enters the filter and road sense torque MAP through the rack force estimator, observer, and private CAN. The friction torque, resistance torque, inertia torque, and active return torque are calculated through the steering wheel angular velocity and vehicle speed to obtain the desired feedback torque, which is then executed by the road sense motor. 5. Test and evaluation of wire control steering In terms of key components of wire control steering and their testing and evaluation, there are currently enterprise standards at home and abroad, which are not very detailed.

Handling stability evaluation index

There are two main standards. Although different companies have different standards, they come from two sources. One is based on steering-related handling stability index requirements, or comes from autonomous driving, such as lane keeping, APA, etc., or the requirements of the steering system at the driving assistance system level. Therefore, the evaluation indicators are mainly autonomous driving command requirements and handling stability.

Main indicators of the drive-by-wire chassis

From the perspective of the chassis, power, economy, and passability are the main indicators related to steer-by-wire and brake-by-wire. Of course, the indicators vary from vehicle manufacturer to vehicle manufacturer, as do the starting levels, some autonomous driving requirements and algorithms, and the selected actuators. These indicators are different, but they are basically related requirements.

Chassis domain integrated design test conditions

The relevant standards for handling stability evaluation indicators are based on integrated test conditions. The more typical ones that are closely related to steering are steady-state turning, steering ease, steering return, angular step and angular pulse, as well as vehicle time domain and frequency domain tests, serpentine tests, and center area tests. These are all tests that are often performed in dynamics or motion control.

SBW and EPS torque step input signal response test

For a system, relevant research is done around the time domain and frequency domain, and the two have different cases and output requirements. For example, when doing a step, we need to look at the actual responsiveness and the requirements for the body stability index, especially the response of the front wheel angle. For example, when the steering wheel of the road feel simulator is turned to a certain angle, the angle of the tire response can be discussed. SBW and traditional EPS include those with and without intermediate shafts, and it depends on whether the requirements for the system response index are good or bad. On the one hand, adding an intermediate shaft will have some degrees of freedom in the spatial gap, and the use of wire-controlled steering road feel simulation will have algorithm lag, rack force estimation is required, and then various filtering is performed to get the results, and then transmitted to the tires through the bus, which requires some actual simulation and testing.

System sinusoidal signal response test

From the sinusoidal signal input, we can see the bandwidth and whether the time interval or hysteresis within the bandwidth is designed reasonably. Of course, different design algorithms, such as the commonly used Kalman filter estimation, may affect the time interval. How to shorten the time interval and make the system stable needs to be considered in the actual calibration process and should also be paid attention to in the simulation process.

Verification and evaluation of double lane shifting conditions

Steering is a circular motion, and relevant tests need to be done on the double lane-changing conditions. The EPA status, front wheel angle status, front and rear wheel angle status, and four-wheel independent status all need to be tested on the double lane-changing conditions, and relevant comparisons must be made with the specifications to obtain the results of the entire system test.

Wire control component test platform

These are the relevant testing components and platforms developed by our research institute.

Wire-controlled chassis HIL bench test content

The newly built drive-by-wire chassis platform can realize functional safety related tests such as drive-by-wire steering, drive-by-wire braking, and chassis domain. The entire test scenario is set up based on the functional safety of intelligent driving, and some remote interfaces are also opened. For example, customers in Changchun, Shenzhen or Chongqing can access through the remote client, do relevant tests, and feedback data on the client. The current test objects include brake steering and chassis domain related tests. The above five parts are all very important and can be discussed in more detail.