Design of Automobile Digital Instrument Measurement and Control System Based on LabVIEW and PXl

Introduction
In the development process of automotive intelligent digital instruments, the amount of information that needs to be collected by digital instruments is relatively large, and the information parameters of various models are quite different. The existence of these problems has brought difficulties to the actual vehicle testing and parameter calibration of the instrument. In order to quickly and effectively test the various functions of the system during the development process and improve the efficiency of system development, we designed a test system that can simulate and generate various parameter information on the car, quickly conduct comprehensive tests on the designed instrument, save bench or actual vehicle testing time, and reduce testing risks.
System Design
The development of the automotive intelligent digital instrument test system requires that it can simulate and generate various acquisition signal information required by the instrument for different models, and can communicate with the instrument under test through the CAN interface. The test system introduced in this article includes the following main functions: Simulation generation of pulse signals of the speedometer;
simulation generation of pulse signals of the engine tachometer;
simulation generation of vehicle fuel gauge signals;
simulation generation of vehicle water temperature gauge signals;
simulation generation of various body switch signals such as lights, windows, and doors.
The digital instrument has a CAN communication interface. As a CAN node, it can communicate with other nodes on the CAN network on the vehicle.
System hardware design
The hardware system of the digital instrument test system mainly includes the main controller, PXI board, signal junction box, data communication conversion board, power supply and the instrument under test. The PXI modular board equipment provided by NI has the characteristics of small size, fast speed and easy expansion. Therefore, in terms of hardware design, we use PXI boards to generate various signals required by automotive instruments. The odometer and engine tachometer of the automotive digital instrument need to collect digital pulse signals. Different models use different sensors, and the high level of the output pulse signal ranges from 3V to 12V. In order to test the applicability of the signal range of the designed instrument, the PXI-6624 board is used, and the external power supply circuit can generate the digital pulse signal required by the instrument. PXI-6624 is an industrial-grade isolated 32-bit timer/counter: PXI interface board with 8 isolated channels. We use C outer 0 and Counterl as the pulse signal providing channels for the speedometer and tachometer. The fuel gauge and water temperature gauge collect analog signals. PXI-6233 can output 4 10V analog level signals, and PXI-6713 can output 8 10V analog level signals. We choose 2 analog output channels of PXI-6713 as signal supply channels. Since there are many switch signals on the instrument, the interference between them is also relatively large. We choose PXI-8528R to control the switch quantity of the instrument. PXI-6528 is a high-speed isolated digital I/O channel. The input and output channels are independent, which effectively suppresses the interference between signals.
The calibration of instrument parameters and the data communication between the CAN node and other CAN nodes on the vehicle are completed by a data communication conversion card. The main function of this card is to complete the conversion function between serial port signals and CAN signals. The purpose of developing a data communication conversion card is to save costs and consider that most PCs do not have CAN interfaces. Through this board, the characteristic parameters of the controlled instrument, such as the characteristic coefficient of the vehicle, the sensor coefficient of the sensor, the speed ratio of the engine, and some calibration parameters of the instrument, are set. Since the target vehicle model is uncertain, some characteristic parameters of the instrument need to be calibrated after actual vehicle testing, so this board can be used for instrument parameter calibration in the future. The hardware function block diagram of the entire test system is shown in Figure 1.

System software design
The instrument test system software is designed using NI's LabVIEW 8.20 platform. This system uses LabVIEW's graphical programming language to establish the front panel human-machine interface and program flowchart in a very intuitive way. The front panel is visible to the user, similar to the operation panel of traditional instruments. The tool template is used to add input controllers and output indicators from the control template. The types of controllers and indicators can be selected. The program flowchart is the core that supports the virtual instrument to realize its functions. The design of the program flowchart involves the design of nodes, data ports and connections. The connection represents the direction of data, and the node is a function, VI subroutine, structure or code interface. This test system takes into account the needs of the overall function test and module function test of the instrument. The whole system mainly includes interface modules and various function test modules. According to the signal type, the instrument function test is divided into: speedometer test module, engine tachometer test module, fuel meter test module, water temperature meter test module, switch quantity test module, CAN communication test module and parameter setting module. The overall function block diagram of the software of the automotive instrument test system is shown in Figure 2.
Interface module
On the left side of the test platform are the switch buttons for various module function tests, which can switch to the test items of a single functional module. The main interface on the right simulates the display interface of a car dashboard, such as the speedometer, tachometer, water temperature gauge, fuel gauge, mileage indicator, and various alarm and switch signals. During the test experiment, the staff can observe the overall function of the instrument test through the main interface, as shown in Figure 3. [page]

Module test design
The speedometer test requires the prior knowledge of the characteristic parameters of the target vehicle model, such as the vehicle characteristic coefficient, the sensor coefficient of the speed sensor, etc., and then downloads the characteristic parameters to the instrument under test through the data communication card (can bus signal), generates a pulse signal according to the test requirements, and the amplitude and frequency of the signal can be adjusted manually/automatically. The speed signal has an overspeed alarm prompt function. According to the set overspeed threshold value, when it exceeds the threshold value, the overspeed alarm light on the front panel of the main interface flashes to prompt. The test process can also be performed manually/automatically, and the test results are archived for future reference. The software test state transition diagram is shown in Figure 4.

The design of the speedometer test module adopts the state machine design mode, which is mainly divided into start, parameter acquisition, manual/automatic selection, acquisition (manual), check time (automatic), output signal and stop. The acquisition of parameters mainly obtains the parameter values ​​of the characteristic coefficient and sensor coefficient on the front panel. Usually, these two values ​​need to be modified online when the instrument parameters are calibrated. Check time refers to outputting the specified signal according to the time specified by the program. In this system, the step-like speed change trend of the \'V\' mode is adopted to test the instrument, as shown in Figure 5.

The engine tachometer test module is similar to the speedometer test module, the difference is that its characteristic parameters are different. According to the specific vehicle model, the engine speed ratio is downloaded to the instrument under test through the data communication card (CAN bus signal), and then it is tested.
The test of the fuel meter requires the pre-setting of the fuel test range and fuel threshold alarm value of the target vehicle model, and the parameter value is downloaded to the instrument under test through the data communication card (CAN bus signal), and then the test is started according to the test requirements. According to the set fuel threshold value, when it is lower than the threshold value, the fuel alarm light on the front panel of the main interface flashes to prompt. The test process can be performed manually/automatically. The test of the fuel meter adopts the design mode of the state machine, which is mainly divided into the states of start, obtain parameters, manual/automatic, acquisition, check alarm, output signal, etc. The test of the water temperature meter is the same as the fuel meter, and no specific explanation is given here.
CAN communication test module
Before all module tests, the parameters of the module need to be initialized first, such as setting parameters such as characteristic coefficient, sensor coefficient, engine speed ratio, overspeed threshold, fuel threshold, water temperature threshold and measurement range. The data communication uses the CAN protocol. Considering the cost, we operate the serial port on LabVIEW and then output the CAN signal through the data conversion board. The CAN signal directly communicates with the instrument under test. Therefore, a simple CAN communication protocol needs to be defined. The test system is a node on the CAN network. The node ID number can be set according to the needs. The data area consists of command words, data length, data, and check bits. Figure 6 and Table 1 are the simple CAN communication protocol for instrument parameter setting.

Conclusion
The use of NI series PxI boards and the flexible and convenient LabVIEW software platform enabled us to build a test platform that integrates automotive digital instrument product development, testing, and evaluation in a short period of time. Through the test of actual instruments, the results show that this test system can quickly and accurately complete various functional tests of the tested instruments, and the system is scalable and can be easily transplanted to the test solutions of other products, which has accumulated testing experience for our subsequent research and development of automotive electronic products.