Automobile Inspection Technology

Chapter 4 Introduction (3)
The above laws, regulations and management systems are of great significance to ensure that the vehicles in use in our country have a good technical condition, and are of great significance to the inspection and maintenance of vehicles.

(Section [-]) Basic knowledge of automobile testing equipment
[-]. The basic composition of testing equipment
At present, automobile testing equipment is generally composed of test condition simulation device, sampling device, additional device and measurement system.

1. Test condition simulator
The test condition simulator is used to simulate the test conditions of the car indoors.The test condition simulation device is optional, usually it appears in the form of a mechanical bench, when the testing equipment has a test condition simulation device, it is called a test bench; otherwise it is called a tester (meter).

2. Sampling device
The sampling device is used to transmit the raw measured data generated in the test to the sensor.The sampling device is different due to the nature of the original measured parameters. For example, the sampling device of the exhaust gas analyzer is composed of a sampling probe and related gas circuits, while the sampling device of the chassis dynamometer is a force-measuring lever.

3. Additional devices
Additional devices are devices required to ensure the smooth progress of the test, such as safety protection devices and lifting devices.

4. Measurement system
The measurement system consists of three parts: sensor, signal processing circuit and output unit, which respectively complete the acquisition, conversion, data processing and display output of measurement signals.The sensor converts various non-electrical physical quantities of the measured object (such as sideslip, braking force, light illumination, gas concentration, etc.) into another electrical signal (voltage, current, etc.) that has a definite corresponding relationship with it and is easy to measure .The signal processing circuit and output unit process the output signal of the sensor, extract useful information, and perform calculation and processing according to the requirements of measurement and control functions, and output the result.Among them, the sensor is the key component of the measurement system, and its performance directly affects the measurement accuracy.

[-]. Function and classification of sensors

1. The composition of the sensor
A sensor is generally composed of a sensitive element and a conversion element, where the sensitive element refers to the part of the sensor that can directly sense or respond to the measured, and the conversion element refers to the part of the sensor that can convert the sensing or response of the sensitive element into a sensor that is suitable for transmission or measurement. Power signal part.Sometimes the electrical part and signal conditioning in the sensor is converted into the circuit

Together, the signal conditioning conversion circuit in the sensor and the auxiliary power supply are used as part of the sensor composition, and its composition is shown in Figure 1-1.

2. Classification of sensors
There are many types of sensors, which are generally classified according to the following aspects.

(1) According to the physical quantity to be measured, it can be divided into displacement sensor, speed sensor, acceleration sensor, force sensor, luminous intensity sensor and temperature sensor.

For example, displacement is measured for sideslip detection, and a displacement sensor is used; pressure is measured for axle load, and a force sensor is used for braking deceleration.

According to the working principle of the sensor, it can be divided into resistance strain type, inductive type, capacitive type, piezoelectric type, photoelectric type, magnetoelectric type, pyroelectric type, etc.

(3) According to the nature of the output signal, the sensors can be divided into switch type, analog type and digital type.

The output of the switch type sensor has only 2 values, that is, "1" and "0" or "ON" (ON) and "OFF" (OFF).If the input physical quantity of the sensor reaches a certain value, its output is "0"; when it is below the value, the output is "".For example, the photoelectric switch is used in the automatic vehicle detection line to report whether the vehicle is in place, so as to arrange the computer to automatically start the detection.

The output of the analog sensor is a continuously changing electric quantity corresponding to the transformation of the input physical quantity.The input/output relationship of the sensor may be linear as shown in Figure 1-2(a), or it may be nonlinear as shown in Figure 1-2(b).Linear output signals can be used directly, while non-linear output signals need to be modified appropriately.The output signal of the analog sensor needs to be conditioned by the signal, and then sent to the analog/digital (A/D) conversion, and finally the data is processed by the computer.

Digital sensors are usually used to detect position or speed, and there are two types: counting type and code type.Among them, the number of pulses sent by the counting type as shown in Figure 1-3(a) is proportional to the input volume, and the input volume can be counted by adding a counter, such as the photoelectric sensor of the vehicle speed detection platform, when the drum rotates through a certain angle Send a pulse signal.The code type sensor is also called an encoder. It outputs a digital code as shown in Figure 1-3(b). Each code is equivalent to a certain input value. When the input value is as shown in the figure, the output code is 1010. When the value of the input quantity is such, the output code is 1011.Code "1" is high level, "0" is low level.

Due to the wide variety of sensor principles, what is meaningful in the obtained electrical signal is a certain electrical attribute in the electrical signal, such as polarity, amplitude, phase, and frequency.Due to the limitation of the principle or manufacture of these sensors, the electrical signal output by the sensor is often not suitable for direct use of the signal processing circuit in the measurement system.Such as temperature and electromagnetic interference and signal are too weak.The weak and noisy signal output by the sensor can be processed by various forms (such as power conversion, impedance conversion, isolation shielding, small signal amplification, temperature compensation, filtering and modulation, etc.) A signal adjusted to be suitable for the application of subsequent processing circuits (such as A/D conversion).

[-]. Measuring instruments

At present, the measuring instruments of automobile testing equipment all adopt intelligent instruments and virtual instruments.For detection equipment with few detection parameters and simple measurement process, smart meters are generally used as measurement instruments, such as speedometer test benches, sound level meters, etc.; for detection equipment with many detection parameters and complex measurement processes, virtual instruments are generally used As a measuring instrument, such as an engine comprehensive detector, a four-wheel aligner, etc.

1. Smart Meter
Smart meters are the product of the combination of microprocessors and electronic instruments. They have data storage, calculation and logic judgment capabilities. The work that can only be completed by the wisdom of the human body, that is, has a certain degree of intelligence, so it is called a smart instrument.

Figure 1-4 shows a simple block diagram of the intelligent instrument structure.

Intelligent instruments mainly include basic parts such as sensors, amplifiers, A/D conversions, CPUs and displays, keyboards and other necessary input/output control ports, and serial communication interfaces.It uses a microprocessor as the control unit to organically combine the various measurement links of the instrument, overcomes the shortcomings of digital instruments that cannot be separated from human operation, and endows it with certain computer-specific functions, such as most of them are programmable. Therefore, it has data processing and fault diagnosis capabilities.The CPU, the core part of the smart instrument, can not only automatically complete various adjustments of general mathematical instruments, but also complete some additional measurement conditions during the measurement process, such as the maximum or minimum value of the measured parameter within a certain period of time that is required to be measured. .

2. Virtual instrument
A virtual instrument is a measuring instrument composed of a computer and an instrument module as the base, plus control software.In recent years, the outstanding advantages of virtual instruments make it develop very rapidly.

The hardware of the virtual instrument is usually in the PC host, without the traditional front panel of the instrument.Most of the virtual instruments are equipped with a large-screen display, using software to generate various virtual instrument control panels on the screen, with multimedia use prompts, online help information, instructions on how to use the equipment, test operation steps and related precautions , users only need to follow the prompts to perform correct operations.The operator can flexibly use the mouse (or button) to perform test work on various "buttons" on the user-friendly virtual instrument GUI on the computer display screen, and complete the measured data collection, calculation, analysis, judgment, display, Storage, printing, transmission and data management.The test results can be displayed on the screen in the form of numbers, images and curves.In addition, the test results can also be stored in the hard disk in the form of a database, which can be recalled by the user at any time for statistics, query and printout.The printer has a micro-printer and an ink-jet printer, and is used to print out a report of the test results.Compared with smart instruments, virtual instruments further realize data display with software, and can realize data editing, storage, printing of curves and charts, and enhance the ability of data exchange, information synthesis, and automatic analysis and judgment of instruments.

[-]. Measurement error and precision
In the measurement process, the measurement personnel, the measured object, the measurement method, the measurement device and the measurement environment are called measurement conditions. Due to the influence of inaccurate measurement tools, uncertain measurement methods and other factors, the actual measurement results are different from the measured ones. The true value is not the same, and this difference is the measurement error.Measurement errors are inevitable, and there are measurement errors in any measurement process.Measurement errors mainly come from system errors, environmental errors, method errors and human errors.

1. Error and its expression method
It can be seen intuitively from the above formula that the smaller the value of 3 is, the closer the measured value is to the true value and the higher the measurement accuracy.However, this representation method is not suitable for the comparison of measurement accuracy among the measured values.For example, if a certain instrument is used to measure the length of 10m, the absolute error is 0.01mm; while another instrument is used to measure the length of 100m, the absolute error is also 0.01mm. Although the absolute error value of the two measurements is the same, the measured size is different. , so the measurement accuracy of the two instruments is different, and the accuracy of the latter is obviously higher than that of the former.Therefore, if you want to compare the measurement accuracy between the measured values, you need to compare the relative errors between them.

It can be seen from the above formula that the relative error of the second measurement is smaller than that of the first measurement, so the measurement accuracy of the latter instrument is higher than that of the former.

Testing instruments often use the allowable value that the "maximum reference error" cannot exceed as a scale for dividing the tolerance level of the instrument.The reference error 70 is the ratio of the absolute error S to the range L of the indicating instrument, expressed as a percentage, as shown in the following formula. For a certain testing device, the maximum reference error is a fixed value.Common tolerance levels are 0.1, 0.2, 0.5, 1.0, .5, 2.0, 2.5, 5.0.For testing instruments with a tolerance level of 1.0, the maximum quoted error value in use shall not exceed ±1.0%.Therefore, when selecting the measuring range of the testing equipment, try to choose the measuring value close to the full scale to improve the measuring accuracy of the instrument.

2. Measurement Error Classification

(1) System error In the process of multiple measurements of the same measured object, the measurement error remains unchanged or changes according to a certain law, which is called systematic error.Factors such as low precision of the measuring equipment itself, improper measurement methods, improper use methods, and changes in environmental conditions may cause systematic errors.If the dial of the indicating instrument is installed incorrectly
error.

The cause of the system error is clear and has a definite change rule, and corresponding technical measures can be taken to eliminate or weaken it in actual measurement.

Random error The error in which the measurement error changes in an unpredictable way during multiple measurements of the same measured object is called random error.Random errors are mainly caused by the joint influence of many independent small factors in the measurement process.Such as noise interference, slight changes in electromagnetic fields, etc.

(3) Gross error The error that obviously exceeds the specified expectation is called gross error.Gross errors are mainly caused by unreasonable reasons, such as operational errors of measuring personnel, failure of measuring devices and sudden interference from the outside world.Measurement results with gross errors are called outliers, which should be eliminated during error analysis.

3. Accuracy
Accuracy is accuracy, which is a comprehensive reflection of precision and accuracy.The level of precision indicates the size of random error.The random error is large and the precision is low; on the contrary, the precision is high.If the system error is small, the accuracy of the measurement result is high; otherwise, the accuracy is low.Highly accurate measurements mean that both systematic and random errors are small.

thinking and practice
1. What is the concept of car inspection?What is the difference and connection between it and car diagnosis?
2. How many car detection parameters are there?What is its selection principle?
3. What are the testing standards?What parts does it generally consist of?

4. How to determine the best detection cycle?
5. Briefly describe the basic composition and functions of automobile testing equipment.

6. Briefly describe the concepts of absolute error, relative error, systematic error, random error, and precision.

(End of this chapter)