The role of smart wireless communications in promoting automotive safety applications

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The role of smart wireless communications in promoting automotive safety applications [Copy link]

The automotive electronics industry continues to see a growing demand for electronic components, and applications based on automotive safety protection are also developing. Automotive safety is extremely important to automakers due to the influence of car drivers and government agencies, as well as the desire of automakers to increase their market share. Intelligent wireless communication technology evolved from remote keyless entry applications. The advent of cheap smart transponders marks the emergence of intelligent wireless communication systems as a new development trend. In existing remote keyless entry (RKE) systems, car drivers open/lock the car door by pressing a button that transmits a wireless signal. With the new car access system, drivers can easily perform the above operations by carrying only a transponder. This new car entry system is called passive keyless entry (PKE).

PKE systems can automatically perform user identification without human intervention. This security protection system requires the addition of intelligent mechanisms to ensure the reliability of operation in any application scenario, and the reliability of the hands-free operation system mainly depends on the performance of the analog detection circuit in the transponder. This article will explore the key issues and solutions in the design of a typical PKE system.
The following will first introduce what smart wireless communication is and introduce several simple application examples. Then, two important functions in smart wireless communication will be explained: programmable digital wake-up filter and amplitude sensitivity and modulation depth. Finally, a design example of a smart passive keyless entry (PKE) system will be shown.

What is intelligent wireless communication? The keyless access control system currently in widespread use consists of two units. One is the base station unit, which consists of a single-chip microcomputer and a high-frequency receiver. The other is a portable transmitter unit, which consists of a high-frequency transmitter and a single-chip microcomputer. When the user starts the transmitter by pressing a button, the transmitter will encrypt the code and send the signal to the base station. But this is not an intelligent wireless communication system. An intelligent wireless communication system should have the following characteristics: 1. Automatic operation, the user does not need to press any button, the system can detect and send signals by itself; 2. 100% independent; 3. It can learn and adapt by itself in different environments, and can eliminate noise and work normally in the presence of noise.
There are many requirements for building such an intelligent system. Some of the simplest requirements and corresponding solutions are as follows: 1. Small size and low cost; the solution is to use an intelligent single-chip microcomputer (MCU), which integrates digital and analog front-end circuits.

2. Economical two-way communication; the solution is to use 125kHz for base station commands and UHF for responses.

3. The communication distance is greater than 2 meters; the solution is that the transponder has high input sensitivity to detect base station commands.

4. Working in a noisy environment (because there is a lot of noise and interference in a general environment); the solution is high input modulation depth sensitivity.

5. Eliminate antenna directivity; the solution is to use three orthogonally placed LF antennas on the transponder board.

6. Long battery life; the solution is to use a wake-up filter.

7. Data security; the solution is to use encryption/decryption algorithms.

Figure 1 is an example of an intelligent passive keyless entry (PKE) system. It is different from the currently commonly used keyless entry systems. In addition to the single-chip microcomputer and high-frequency transmitter, the base station on the left also has a low-frequency transmitter/receiver; the base station sends a low-frequency command of 125kHz, and the intelligent receiver on the right processes the signal after receiving it. If the signal meets certain requirements, high or low frequency can be used as a response. There are three directional (X, Y, Z) antennas in the intelligent receiver, which can ensure that signals from any direction can be received. In addition, the user does not need to press any button, and the intelligent receiver can automatically receive and send signals.

The schematic diagram of the PKE transponder is shown in Figure 2. The MCU is PIC16F639. The 639 consists of another single-chip microcomputer 636 and the analog front end MCP2030. The 636 has Microchip's patented KEELOQ encryption and decryption method. If this encryption method is not required, the MCP2030 can also be combined with other single-chip microcomputers.
PKE Application Examples

PKE can be used in a variety of applications such as intelligent vehicle access systems, engine anti-theft locking systems, and tire pressure monitoring systems (TPMS). The following uses the tire pressure monitoring system as an example to explain its use in detail. As shown in Figure 3, the tire pressure monitoring system consists of three units. The lower left is a sensor unit installed on the tire, which consists of an intelligent single-chip microcomputer, a tire pressure sensor, and a high-frequency transmitter. The upper right is a base station, which consists of a single-chip microcomputer and a high-frequency receiver. The lower right is a low-frequency trigger. It is placed in the car body very close to the tire. When in use, every 3 to 4 seconds, the low-frequency trigger sends a start command to the sensor unit in the tire. The MCU in the sensor unit receives the signal. When the signal meets the requirements, it will ask the tire pressure sensor to measure the tire pressure, which is then sent to the base station by the high-frequency transmitter.

Programmable digital wake-up filter

The programmable digital wake-up filter is a very important function. Its purpose is to reduce the operating current (extend battery life), that is, to keep the digital part in low current mode (sleep) unless the analog part finds a valid input signal. The method to achieve the above purpose is that the transponder only searches for input signals with a predefined header (waveform) and ignores all other input signals without a predefined header.

Figure 4 shows the wake-up filter input signal waveform and output digital. The first section is the stabilization time required by the analog front end, with a gap. The second section is the predefined header of the programmable wake-up filter, which is completely set by the user. Here it is set to 2 milliseconds high and 2 milliseconds low. When the input signal meets this requirement, we will see the digital output from the microcontroller.

5 is the output when the wake-up filter is enabled. Below is the input signal, and above is the adjusted output signal. The first section below is the required stabilization time, with a gap. Our predefined programmable wake-up filter waveform is 2 milliseconds high and 2 milliseconds low. When the input signal meets this requirement, there is an output signal above. When the input signal does not meet the preset wake-up filter timing requirements, there is no output.
Generally, we recommend using a wake-up filter. Because when noise enters, only noise that meets the preset waveform can wake up the digital circuit. If the wake-up filter is not used, any noise signal can wake up, resulting in unnecessary current consumption. Amplitude sensitivity and modulation depth

Amplitude sensitivity and modulation depth are another important function. Figure 6 shows an RF pulse in the near field. Sometimes, when the base station and the transponder are very close, the signal generally swings back and forth from the highest to the lowest, but sometimes the signal does not have enough time to swing from the highest to the lowest. At this time, the highest and lowest values of the signal are very close, which makes it difficult to process the signal.
Figure 7 shows an RF pulse in a noisy environment. The upper figure is the trigger level, and in the lower figure you can see that the lowest value of the noise has been raised to be very close to the highest value. This makes it difficult to process the signal.

How to determine the modulation depth? There are two ways. One is to use the (highest value - lowest value) / highest value of the pulse signal. The other is to use (highest value - lowest value) / (highest value + lowest value). We use the former because it is easier for users to understand.

Figure 8 is an example of modulation input detection. The top one is 100% modulation input, where the highest and lowest values are very distinct. The bottom one is 25% modulation input, where the highest and lowest values are very close.

Smart Passive Keyless Entry (PKE) System Example

Figure 9 shows a smart PKE transponder with no battery and backup battery circuit. In some cases, such as when the TV set has poor contact, the magnetic coil can be used to temporarily power the battery so that the transponder can still work without battery power.

The system has the following requirements: In the transponder, there are LF antenna, UHF transmitter, external backup battery circuit (optional) and intelligent MCU and MCU firmware. In the base station, there are LF transmitter, UHF receiver, antenna and MCU and MCU firmware.

The parameters of two-way communication distance include: in the transponder, there are antenna tuning and Q, antenna positioning (using three-dimensional antenna), receiving sensitivity, modulation depth of the input signal, and data rate; in the base station, there are output power and receiving sensitivity.

In antenna design, the commonly used frequency for the low-frequency part is 125kHz; LC resonant circuit is used; the antenna type is an air-core coil or a ferrite core (1-10 mH); LC resonant frequency = carrier frequency of the base station; range: less than 1 meter for passive tags and less than 5 meters for active tags. High-frequency part, UHF: 315-960 MHz; dipole antenna is used, etched on the PCB; range: 5 meters for passive tags and 100 meters for active tags.

There is a special formula for magnetic flux and antenna induced voltage. There are many factors in determining the induced voltage, such as the number of coil turns, the surface area of the receiver coil, frequency, quality factor, etc. The relationship between the antenna induced voltage and distance is: when the base station and the receiver are very close, the signal voltage is 200V; when the distance is 3 meters, the voltage signal can only reach 5mV peak-to-peak. Therefore, the signal input sensitivity is very critical and important here.

Currently, Microchip has provided a PKE demonstration board, which can be found on the Microchip website www.microchip.com.

in conclusion

Reliable automatic operation requires the following: intelligent two-way communication, low system cost, high LF input sensitivity, low power consumption (achieved by controlling battery usage), and secure data encryption and decryption (Microchip uses patented KEELOQ encryption and decryption). You can build a system using an intelligent microcontroller that provides a solution to meet the above requirements.