Design of the time-delay measurement method for capturing images of electronic police systems

introduction

The image-based electronic police system is an effective means of traffic supervision at present. Its external hardware equipment usually includes four parts: front-end sensing equipment, controller, capture imaging equipment and back-end transmission equipment. According to the difference of its front-end sensing equipment, the image-based electronic police system can be subdivided into coil type, radar type and infrared type. Its working principle: the front-end sensing equipment detects the arrival (or departure) of the vehicle according to specific detection conditions, and sends the sensing information to the controller. The controller determines whether the capture conditions are met. If so, it starts the corresponding capture imaging equipment (camera, video camera, etc.) to capture, and uploads the capture results and sensing information to the server through the back-end transmission equipment.

The quality of images captured by the image-based electronic police system is the most important factor affecting the overall system performance. The key issue of image quality lies in the focusing of the imaging device, which will directly affect the quality of the captured image. In order to obtain a clearer image and facilitate the identification of motor vehicles, the captured motor vehicle must be within the depth of field of the electronic police system. There is a delay from the detection of the vehicle by the front-end sensing device of the electronic police system to the start of the capture device. Unstable or too large a delay will bring difficulties to image capture. When the delay is unstable, the capture position of vehicles with the same speed will have a large deviation; and too large a delay will cause a large difference in the capture position of vehicles with different speeds, making it difficult for the imaging system to achieve clear imaging. Therefore, measuring and controlling the capture delay of the electronic police is an effective means to ensure system performance. The capture of electronic police requires the process of "sensing → controller → camera (camcorder)", and each link will cause a delay. For example, the capture delay of radar-based electronic police mainly includes: the fixed delay from the illegal vehicle entering (or leaving) the detection area to the radar outputting the speed information; the delay caused by data processing from the controller receiving the speed data to sending the capture command; the delay from the imaging part receiving the capture command to the capture. Among them, the data output delay of the radar can be directly obtained from the device parameters (generally about 10 ms); while the data processing delay of the controller and the delay of the imaging system are usually not directly available and need to be measured specifically. Since there are many links in which the delay occurs, high measurement accuracy is required, and multiple links are difficult to synchronize, it is difficult to measure these delays using existing equipment. Therefore, a method for measuring the capture delay of electronic police with radar capture delay as the object is proposed here.

2 Working Principle

In the radar-based electronic police system, the radar sends the vehicle speed information to the controller through the serial port (RS232 or RS485 bus). When there is no vehicle, the radar does not send data or sends data with a speed of 0; when the radar detects a vehicle, it sends the speed value to the controller. The electronic police controller processes the received data and determines whether to capture the image according to a certain algorithm. In actual operation, in order to avoid and reduce false triggering, the electronic police controller usually adopts some specific processing technologies, which will cause system delay.

Figure 1 shows the principle diagram of the radar-based electronic police capture delay measurement. The timing display is a specially designed electronic device that can display the timing in milliseconds starting from the receipt of the trigger signal.

The actual measurement of the radar-type electronic police capture delay is carried out in the following steps: ① Place the timing display within the shooting range of the camera of the electronic police system; ② Synchronously send the radar detection data to the timing display; ③ Set the trigger circuit of the timing display, and when the first valid data from the radar is received, start the time display and start timing; ④ After the electronic police system receives the radar detection data, it captures the image according to its work flow, and the captured image includes the time image displayed on the timing display; ⑤ Read the time value from the captured image, and add the value to the radar output delay to get the overall capture delay time of the electronic police system.

The key to this measurement method is the design of the timing display. The settings of different interface triggers and different trigger rules are adapted to different types of electronic police systems (such as coil type, radar type, etc.) and capture methods (such as front capture, rear capture, etc.). The following is a detailed introduction to the timing display design suitable for front capture and rear capture for radar type electronic police.

3 Timing display design

3.1 Hardware Circuit Design

Figure 2 shows the hardware design principle circuit of the timing display. It includes: ① Main control device (AT1): The widely used AT89S51 single-chip microcomputer is used. It is the core of the entire timing display and is used to read serial port data, trigger judgment and control the timing circuit; ② Crystal oscillator (X1): 12 MHz active crystal oscillator (1 machine cycle includes 12 clock cycles, so 1 machine cycle is 1μs, which can accurately generate 1 ms timing interruption); ③ Latches (U1, U2, U3): 74LS573 is used, and 74LS373 can also be used. Since the display result is captured by a camera, the display data must be locked, and the I/O port of the single-chip microcomputer AT89S51 is limited. The latch is used to expand the parallel output. If 3 parallel output ports need to be expanded, 3 select signals (P2.0, P2.1, P2.2) are required; ④ Digital tube (DS1, DS2, DS3): 7-segment common anode digital tube is used to display the timing time. Since the delay to be measured is about several hundred milliseconds, using three digital tubes can meet the system requirements; ⑤Serial interface (MAX232): Since the level logic of the RS232 signal is inconsistent with the level logic of the serial port of the microcontroller, level conversion is required. This system uses MAX232 devices.

The P0 port of the AT89S51 microcontroller is connected to the latch as a data bit, and the microcontroller serial port is converted by the MAX232 device (RXlD, TXD in Figure 2) and connected to the electronic police system (serial port of the radar). [page]

3.2 Software Programming

The software design mainly includes two parts: trigger judgment and timing display.

3.2.1 Trigger judgment

Determine when to start the timing circuit. The microcontroller serial port receives the radar speed data through the buffer SBUF, receives a bit of data, clears the RI flag, and then receives the next data. Different capture methods (capture the front or rear of the vehicle) will result in different trigger conditions. The trigger judgment is shown in Table 1.

When taking a rear-end snapshot, a delay is added during the trigger judgment, and the timing circuit starts counting from 10.

3.2.2 Timing display

The timing and display control are performed in milliseconds. The key is to coordinate the static display control required for shooting, that is, the 3 digits light up at the same time. The specific process is: set the initial value of the microcontroller timer to generate a 1 ms timing interrupt. When the 1ms timing time is reached, the corresponding digit count is increased by 1, and the corresponding enable bit is selected (the ones digit corresponds to P2.0, the tens digit corresponds to P2.1, and the hundreds digit corresponds to P2.2). The corresponding digit is converted into a display code by looking up the table. When the timing time reaches 999 ms, the timing display stops. Figure 3 shows the timing display control program flow of the ones digit.

Figure 3 Flowchart of the control program for the timing display of the units digit

Note: The time display interval of the digital tube should match the shooting speed of the camera, that is, the exposure time should be less than the time resolution. For example, in order to obtain a time-delay measurement resolution of 1 ms, the shutter speed of the electronic police should be controlled below 1/1000 s; if the shutter speed is 1/500 s, the time resolution can only reach 2 ms. If the two do not match, the exposure time is greater than the time resolution, resulting in overlapping time displays and the inability to read the correct time value.

4 Experimental Results

The timing display is applied to the radar-type electronic police system, and a camera with an external trigger function is used to capture the camera, and the delay test is carried out. The delay of the front capture and the rear capture is measured respectively, and 50 data are measured in each group. In the actual measurement, the maximum delay of the electronic police system when capturing the front of the car is 12 ms, and the minimum delay is 6 ms. The delay is very small and the delay amount is stable. When the delay of the rear capture is measured experimentally (the delay of the controller to judge the departure of the vehicle is set to 500 ms), it is found that the maximum delay is 757 ms and the minimum is 748 ms. Although the delay is relatively stable, the total delay time is relatively long. It is basically unusable in areas with high vehicle speeds.

5 Conclusion

The results show that the measurement method designed by the timing display can conveniently and effectively measure the capture delay of the radar-type electronic police system. For different video capture systems, the delay measurement can be achieved by simply changing the corresponding timing display input trigger mode. Therefore, this method has good adaptability, scalability and reference value.