Design and implementation of the automotive PEPS system

　　The automotive PEPS system is rapidly becoming one of the most representative solutions in vehicle wireless entry applications. The so-called PEPS is the abbreviation of Passive Entry & PassiveStart, which means keyless entry and keyless start system. It uses advanced RFID wireless radio frequency technology and vehicle identity coding recognition system, which has completely changed the development prospects of automotive security and anti-theft applications, and brought users a convenient and comfortable new driving experience.

　　The following article will introduce and analyze the PEPS system, which combines safety and comfort, from the aspects of system functions, working principles, and design and implementation of system solutions.

　　2. Introduction to PEPS system

　　For a car equipped with the PEPS system, the driver can open the door and start the vehicle engine without pressing the remote control button on the smart key or inserting and removing the key from the lock cylinder. The prerequisite is only to carry the smart key with you and press the trigger button or the one-touch start button on the handle. In terms of vehicle anti-theft security, the complex two-way identity authentication process between the smart key and the PEPS base station has also been substantially improved compared to the previous generation of remote key entry (RKE) systems.

　　3. Working principle and certification process of PEPS system

　　From the perspective of system functions, the PEPS system can be divided into two parts: the PE keyless entry part and the PS keyless start part, which represent the two stages before and after the driver enters the vehicle.

　　However, from the perspective of the working principle of the system, the two are extremely similar. In simple terms, both the PE and PS systems use low-frequency antennas to detect the relative position between the smart key and the vehicle body base station (i.e., PEPS ECU, hereinafter referred to as ECU), and establish effective two-way interactive communication between the ECU and the smart key through high and low frequency signals (high frequency 433.92MHz, low frequency 125KHz). Based on the identity authentication result of the ECU on the smart key, it decides whether to open the door lock (PE system) or start the vehicle engine (PS system).

　　In the two-way identity authentication process between the smart key and the ECU, low-frequency signal wake-up and high-frequency signal authentication are not only the key to determining the vehicle's anti-theft safety performance, but also one of the key elements that determine the performance of the PEPS system. The so-called low-frequency signal wake-up, taking the PE system as an example, means that when the driver gives the PEPS system a trigger signal, the ECU will switch from the sleep state to the working state, and send a key wake-up message to the smart key through the low-frequency antenna. When the key receives this message through its own low-frequency antenna, it will verify the message through its own smart chip. If the verification result matches the data stored in the key, the smart key will be awakened; and the high-frequency signal verification means that after the smart key is awakened, its own ID identity code will be sent to the ECU in the form of a high-frequency signal. If the ECU recognizes that this ID number matches the key code of its own system, it will send a verification code to the smart key through a low-frequency signal. The smart key that receives the verification code will encrypt the verification code through a specific jump code algorithm, and send the encryption result back to the ECU through a high-frequency signal. The latter will compare the received encrypted data with its own calculation result. If the two match, it will send the corresponding operation instruction to the BCM through the CAN bus, and the latter will complete the operation of unlocking the door lock or opening the trunk.

    Although the above authentication process is very complicated, thanks to the high-speed computing power of the smart chip, the entire authentication process only takes tens of milliseconds in actual applications, so users will not feel any lag.

　　4. Design of PEPS system

　　The PEPS system designed this time includes the following components: PEPS ECU, BCM, smart key, electronic door handle, low-frequency antenna, one-touch start button, electronic column lock and trunk button. The following is a brief introduction to the basic functions and design principles of each component.

　　4.1 PEPS ECU

　　PEPS ECU is the abbreviation of Electronic Control Unit, which means electronic control unit. It is the core component of the PEPS system. Its function is to identify the smart key. If the key is legal, the corresponding operation instructions will be sent to BCM or EMS through the CAN bus, and the subsequent operations will be performed by the two. If the key is illegal, the ECU will enter sleep mode and reject subsequent operation requests sent by this key.

　　4.2 BCM

　　BCM is the abbreviation of Body Control Module, which means body control unit. It is a control module that integrates body lights, doors, windows and anti-theft functions. In the PEPS system, BCM is connected to the ECU through the CAN bus and controls the doors, windows and lights according to the instructions provided by the latter.

　　4.3 Smart Key

　　When the smart key is working, it will establish high and low frequency two-way communication with the ECU. After passing a series of identity authentication, it can unlock the door or start the engine. During the whole process, the driver does not need to operate the key and only needs to carry it with him, which is also the origin of the name of the keyless system.

　　4.4 Electronic door handle

　　The electronic door handle has a built-in low-frequency antenna and a micro-motion (or induction) switch. The antenna is used to detect the key position, and the switch is used to wake up the PEPS system and switch it from sleep mode to working mode.

　　4.5 Low frequency antenna

　　The low-frequency antenna can send a 125KHz low-frequency signal to a spherical space with a radius of 1.5m centered on itself, which is used to detect the relative position between the smart key and each low-frequency antenna, and transmit the measured key coordinates to the ECU. The latter will determine whether to perform subsequent operations such as opening the door and starting the vehicle engine based on the current coordinate value of the key.

　　4.6 One-touch start button

　　By pressing the button and coordinating with the current working state of the brake (automatic gear) or clutch (manual gear), the ignition device is switched cyclically between the four gears of ACC, ON, START and OFF. The one-touch start button can save the driver from inserting the key into the ignition lock cylinder and then twisting the key to start the engine, greatly simplifying the driver's operation.

    4.7 Electronic Pillar Lock

　　The electronic column lock uses a built-in small motor to drive the extension and retraction of the lock tongue to achieve the locking/unlocking function of the steering column. Since the small motor that controls the movement of the lock tongue is controlled by the PEPS system, the electronic column lock is safer and more reliable than the traditional mechanical column lock.

　　4.8 Trunk button

　　Press the trunk button, and the low-frequency antenna installed on the rear bumper will detect the current position of the smart key. If the unlocking conditions are met, the PEPS ECU will command the BCM to unlock the trunk, simplifying the driver's operation of inserting and removing the key to unlock.

　　5. Implementation of regional detection and key positioning technology of PEPS system

　　The PEPS system has three detection and judgment areas, namely the gray outside area, the red inside area and the gray-white main driver area.

　　There are three parts in the gray exterior area, which are divided into the main driver's seat, the co-driver's seat and the trunk detection area. When the driver enters these areas with the smart key and gives a trigger signal, the ECU will establish high- and low-frequency two-way communication with the smart key, determine the current position of the smart key through the field strength detection of the low-frequency signal, and then verify the key identity through the high-frequency signal fed back by the key to decide whether to unlock the door or the trunk; the red interior detection area is the focus and difficulty of the entire PEPS system design, because the PEPS system needs to accurately determine whether the smart key is in the car to determine whether the door is locked correctly and whether the engine can be started. Both are top priorities closely related to driving safety, so the performance of this area will directly affect the performance of the PEPS system; and in most mid- and high-end models, the PEPS system will also detect the gray-white main driver's seat area to redundantly determine whether the key is valid and whether there is someone in the main driver's seat to avoid safety hazards such as children's misoperation.

　　In summary, we can find that in the automobile PEPS system, regional detection is a very important technology that is different from previous automobile security technologies. The detection accuracy of the key position has become one of the important parameters to measure the quality of a PEPS system. At present, there are two main technical solutions on the market to improve the detection accuracy of the key position. The first is to make a fuzzy judgment on the position of the smart key by adjusting the sensitivity of the low-frequency signal. Its characteristics are limited accuracy but easy implementation; the second is to calculate the relative distance between the smart key and the low-frequency antenna in the car according to the strength of the low-frequency signal, and then accurately locate the specific position of the smart key through the cross-coverage of multiple low-frequency antennas. This is called RSSI (Received Signal Strength Indication) technology. The PEPS system designed this time adopts the second method mentioned above, so two low-frequency antennas are built into the car to cross-locate the exact position of the key.

　　6. Conclusion

　　At present, the PEPS system is still in its infancy in the domestic automobile industry. Compared with foreign first-line brands such as BMW and Mercedes-Benz, there is still a certain gap in the PEPS system independently developed by my country. With the development of the Chinese auto market and the continuous improvement of people's requirements for vehicles, a high-quality PEPS system is bound to become one of the key factors that determine whether a vehicle is hot-selling. Therefore, today when the PEPS system is not yet popular, developing a set of independent brand PEPS systems is being regarded as an imperative strategic policy by more and more domestic vehicle manufacturers. By studying and analyzing the working principle and workflow of the automotive PEPS system, a solid foundation will be laid for the independent development of the PEPS system, and it will also provide the possibility for optimizing the automotive PEPS system.