Research and design of automotive wireless sensors

Radio frequency (RF) technology is commonly used in car driving and vehicle diagnostics. According to international standards, all vehicle technology applications must be thoroughly tested, and these tests must be based on reasonable empirical experiments involving sensor data collection. Therefore, in the automotive industry, the development of wireless sensor networks has been developed along with the development of typical sensors and RF devices. For the automotive test environment, wireless sensors have three advantages: the first is small size, wireless sensors do not require cable ports; the second is time saving, wireless sensors save time to connect all sensors to power and data cables, so wireless sensors can be deployed more quickly and easily moved, which not only improves the spatial resolution of sensor data but also improves the fault tolerance of the sensor network; the third is that during the driving test, the number of data cables that can be safely inserted into the cockpit together with the sensors is limited.

The specific structure of this paper is as follows: In the second part, the hardware design of the wireless sensor is given; in the third part, the wireless sensor network and communication protocol are given; in the fourth part, the usage cycle of the wireless sensor network is mainly discussed; finally, the overall design is summarized.

1 Wireless sensor structure hardware design

1.1 Wireless sensor structure diagram

The wireless sensor architecture contains a power supply unit, a sensing module, a wireless communication module, and a microcontroller. The hardware design structure is shown in Figure 1.

Figure 1 Structure diagram of wireless sensor

1.2 Power supply unit

The typical operating voltage range of wireless sensors is 3.0 V to 3.6 V. This power supply design incorporates the TLFA274 regulator (as shown in Figure 2), which can convert voltages from 3 V to 40 V, such as commonly used small batteries or car batteries, bringing great convenience to users.

Figure 2 Power module hardware circuit design

1.3 Sensing Module

To minimize power consumption, the sensing module (see Figure 3) is powered on before measurement and turned off immediately afterwards (the rapid on-off switching has no effect on the device).

Figure 3: Sensor module hardware circuit design

1.4 Wireless Communication Module

Due to the differences between different brands of cars, the operating environment of radio frequency technology (RF) is uncertain. The wireless communication module must meet two basic design criteria: first, multi-band communication, choose low-power radio frequency (RF) chips, namely 433/868/915 MHz band transceiver nRF905 and 2.4 GHz band nRF2401 transceiver; second, two optional antennas (as shown in Figure 4), PCB (simple, low power, but difficult to adjust) and external antenna (as opposed to PCB). There is a switch function between different RF bands and antennas to ensure smooth communication, especially in vehicles with thick metal plate baffles between the engine compartment and the cockpit.

Figure 4 Schematic diagram of wireless sensor RF antenna

1.5 Microcontroller

Atmel ATmega88 was chosen as the microcontroller chip because of its low power consumption and fast switching between standby mode and active mode, so the wireless sensor can complete the measurement and transmission of data in a very short time; it has a built-in power-off detection circuit that can be used to remind the user whether the sensor network battery needs to be replaced; it also has a built-in A/D converter that can convert the sensor's analog signal into a digital signal with a 10-bit digital value (zero represents 0 V, 210-1 represents the power supply voltage and the maximum possible voltage).

The above only illustrates the design of a single wireless sensor. However, in an actual automotive test environment, you must also consider how many sensors are working at the same time and how much data needs to be transmitted.

2 Wireless Sensor Networks

The wireless sensor data communication will use the time division multiple access communication protocol. Using time division multiple access (TDMA) means that only useful data can be transmitted and received in the communication mode (higher power), so the wireless sensor is in real-time communication mode at all other times. We assume that a wireless sensor (master) acts as a constant data receiver (star topology network), and the wireless sensor is powered by a car battery (so there is no need to consider the power supply problem of the master wireless sensor). In addition to playing the role of the central processor, when all the slave wireless sensors start to transmit data, the master wireless sensor will synchronously return a special request message to the slave wireless sensors.

The two transceivers have a 240-bit RF data packet, using fifteen 16-bit packets (3 bits for the 5-channel A/D converter, 10 bits for the sensing data, and 3 bits for the remainder). The 10 ms window time is enough for the receiver to process a single data packet, and the minimum TDMA also requires a 5 ms interval.

Next, we calculate the maximum number of wireless sensors that this network can carry, that is, the value that a TDMA network can support.

Definition: NP = data bit number in the data packet = 240; MB = measurement size (number of bits) = 16; SIl = sampling time interval (user-defined unit: seconds); SN = number of sensors (user-defined unit: seconds); TT = minimum time required for each transceiver when using TDMA protocol = 5ms; MT = minimum time required for MCU to process each data packet = 10ms.

Based on the above definitions, the variables that can be calculated from the following data are:

TA = average transmission time interval; TM = average window time for each master sensor to transmit and process under TDMA protocol (unit: milliseconds);

The sensor will send a data packet every 15 samples at the sampling time interval rate, and the average transmission time interval is: TA=SP*SI.

If each TA sensor transmits a data packet, then each sensor can obtain a time window of time division multiplexing. In order to accurately calculate the size of the time slot, the interval time must be divided by the number of nodes in the network. 1000 is converted to milliseconds as follows:

The maximum number of wireless sensors that a wireless sensor network can support depends on the sampling time interval (SI). Once the appropriate time interval is selected (based on the rate of change of the input), the number of nodes and the number of TDMA time slots are determined using formula (1).

Using the temperature sensor as the sensor, if a sampling interval of 1 s is required, the maximum number of nodes that the master node can handle is 1 400 (10.714 ms TM) and the master node has reached 10 ms, as shown in Figure 5(b).

(a)

(b)

Figure 5 Comparison of two sampling intervals and the number of signal sources of temperature sensors

In this section, we study the transmission protocol for wireless sensors and show that the design can achieve relatively high spatial resolution (greater than a car with 1,000 nodes). However, if the sensors consume power too quickly, thousands of batteries will need to be replaced regularly, which actually limits the spatial resolution. Therefore, it is necessary to consider the life cycle of the wireless sensor network.

3 Life cycle of sensor networks

The three main power consumption modules of wireless sensors are: sensing module, wireless communication module and microcontroller module, as shown in Table 1. These three modules have two main operation modes: shutdown mode and working mode. The power consumption of each module and the total consumption are analyzed in these two modes.

In addition, using a standard 500 mAh battery, the current required by the supply cannot be reduced, and 20% of the theoretical amount must be deducted. Therefore, to calculate the average life (hours), the calculation formula is:

BS = battery power for one hour = 500 mAh; AO = sensor current in working mode = 453.581 μA;

AOF = sensor current in shutdown mode = 17.479 μA; TO = time rate (%) of sensor in working mode.

4 Conclusion

This article introduces the hardware design of a single wireless sensor and the wireless sensor network transmission protocol in detail, which can quickly and easily collect a large amount of real data in the automotive test environment. This design is relatively flexible and can use a variety of bandwidths, antennas, power supplies, and sensor types. The use of wireless transmission to test the conditions of various parts of the vehicle greatly improves the impact of the test equipment on the real environment and makes the test data more realistic. However, in the actual test environment, a wireless star network topology is often used, with multiple RF transmitters, which may cause intermodulation distortion. The wireless signal may propagate multiple times around the vehicle (reflection, diffraction, etc.), causing noise. These problems will require further experiments and debugging. Due to the large power consumption of the external oscillator, a relatively accurate internal oscillator will be used internally; due to the high clock error rate, resynchronization must be performed regularly to achieve the expected communication performance.