Design and implementation of a handheld automobile tachometer measurement and verification device

A handheld automobile engine tachometer is an instrument used to detect the instantaneous speed of an automobile engine. It generally includes two types of tachometers: a gasoline engine tachometer for measuring the speed of a gasoline engine and a diesel engine tachometer for measuring the speed of a diesel engine. The handheld automobile engine tachometer has the characteristics of small size, light weight, and easy operation. It is widely used in automobile and engine production, research, maintenance, and supervision and inspection departments. Since the detection principle and use method of this type of engine tachometer are quite different from those of traditional tachometers. The existing speed measurement standard device of the metrology department cannot calibrate its measurement accuracy. At present, the more commonly used calibration method is to compare it with other types of tachometers such as magnetoelectric or photoelectric. However, this comparison needs to be carried out on a special engine test bench. This method not only cannot meet the metrological requirements of the engine tachometer in terms of measurement range, measurement accuracy and stability, but it is also difficult for general metrology departments to have such conditions. Therefore, it is necessary to develop a calibration device to solve the metrological calibration problem of the handheld engine tachometer.
 
1 Measurement principle of handheld automobile engine tachometer
 
1.1 Measurement principle of gasoline engine tachometer
Automobile gasoline engine belongs to spark-ignition engine. Every time it is ignited, the high-voltage discharge will cause the magnetic flux around the ignition coil and high-voltage ignition wire in the engine ignition system to change. The electromagnetic coil in the gasoline engine tachometer sensor can sense this change and convert the change of magnetic flux into a current pulse signal with the same frequency as the engine ignition frequency. There are many ways to install the gasoline engine tachometer sensor: some are clamped on the high-voltage ignition wire. Some use a suction cup to suck the sensor on the ignition coil: there is also a non-contact type, as long as the tachometer sensor is close to the engine ignition system. It can sense the ignition signal. The gasoline engine tachometer can obtain the engine speed value by calculating the ignition pulse signal frequency value collected by the gasoline engine tachometer sensor with the stroke value and cylinder value of the measured engine. The relationship between the gasoline engine speed value and the ignition pulse frequency is as follows:
 
N=3Okf/m
 
In the formula, IV is the speed ignition pulse frequency displayed by the tachometer; is the number of engine strokes: m is the number of engine cylinders.
 
1.2 Measurement principle of diesel engine tachometer
The diesel engine tachometer clamps its sensor on the high-pressure oil pipe of the first cylinder of the diesel engine with a certain preload. Each time the engine injects fuel, a high pressure will be generated in the high-pressure oil pipe, and the high-pressure oil pipe will expand slightly under the action of the oil pressure. The piezoelectric sensing element in the squeeze sensor is generated. Piezoelectric charges are generated at both ends of the sensing element. The injection pressure signal is converted into an electrical pulse signal, and the frequency value of this electrical pulse signal is equal to the injection frequency value of the first cylinder of the engine. The diesel engine tachometer can obtain the engine speed value by calculating the pulse signal frequency value collected by the sensor together with the measured engine stroke number. Since the current automobile diesel engines are all 4-stroke engines, the handheld automobile diesel engine tachometers used are also designed for 4-stroke diesel engines. For this type of diesel engine tachometer, the conversion relationship between the speed of the diesel engine and the injection frequency is:
 
N=120f
 
, where IV is the speed displayed by the tachometer; F is the injection frequency.

 
2. Measurement principle and composition of the calibration device
 
2.1 Overall design scheme
Due to the particularity of the detection principle of the above-mentioned handheld engine tachometer, in order to calibrate it, we should abandon the traditional speed measurement ideas such as using mechanical standard speed devices. Our plan is to design a low-voltage signal generator to provide a low-voltage pulse signal with accurate and stable frequency. The frequency of this signal is equal to the gasoline engine ignition pulse frequency or diesel engine injection frequency corresponding to a certain standard speed. Then use this low-voltage pulse signal to drive the ignition device or pressure pulse generator used to simulate the signal required by the tachometer sensor. Generate the ignition signal and pressure pulse signal corresponding to the standard speed.
 
Based on the above ideas, we have developed a standard device for calibrating handheld engine tachometers. The standard device consists of three parts: a low-voltage pulse signal generator, a high-voltage ignition generator for gasoline engine tachometer calibration, and a vibration pulse generator for diesel engine tachometer calibration (see Figure 1).


Figure 1 Block diagram of the calibration device
 
2.2 Low-voltage pulse signal generator
The working principle of the low-voltage pulse signal generator is shown in Figure 2. Its function is to generate a corresponding low-voltage pulse signal according to the input tachometer related information and the set speed value. The frequency of the signal is equal to the high-voltage ignition pulse frequency or injection frequency corresponding to the set speed value. To achieve this goal. We designed a device with the MCS-51 series single-chip microcomputer system as the main circuit. The timing interrupt function of the single-chip microcomputer is used to generate a voltage pulse signal. When the system is working, the dial and buttons on the device panel are first used to set the tachometer type (gasoline, diesel engine), engine cylinder number, stroke number, speed and other information. When the single-chip microcomputer system is working, it first reads this information. And transforms it into the corresponding interrupt delay time through calculation. At the same time, the system starts timing. And outputs a pulse after the delay time is reached. Then repeat the "read input information-timing-output pulse" process. So reciprocating cycle. A pulse signal of the corresponding frequency is generated. The pulse signal output by the single-chip microcomputer system is sent to the high-voltage ignition generator and the vibration pulse generator respectively through the drive isolation circuit. [page]


Figure 2 Low voltage pulse signal generator
 
2.3 High voltage ignition generator The high
voltage ignition generator is used to generate the high voltage ignition signal required for the gasoline engine tachometer calibration. It completely simulates the actual working conditions of the gasoline engine ignition system. It is composed of an ignition amplifier, a high voltage ignition coil and a spark plug (see Figure 3). The pulse signal from the low voltage pulse signal generator is first sent to the input end of the ignition module. The drive switch circuit inside the ignition module causes the primary coil of the ignition coil to generate an on-off current. Then, a high voltage of several thousand volts is generated in the secondary coil and discharged through the spark plug to generate a high voltage discharge signal. All components that constitute the high voltage ignition generator are all finished automotive electrical components.
 
This makes the generated ignition pulse close to the actual situation on the one hand, and it is also convenient for the installation of various gasoline engine tachometer sensors on the other hand.


Figure 3 High voltage ignition generator
 
2.4 Vibration pulse generator
calibration The standard frequency injection pressure pulse signal required for the diesel engine tachometer is difficult to obtain through simple methods or devices. We can only find a way to replace the injection pressure pulse signal with other signals. Through experiments. We found that the diesel engine tachometer sensor was fixed on a metal rod and the metal rod was knocked. The vibration of the metal rod can also squeeze the piezoelectric element in the sensor. Thus, an electrical pulse signal synchronized with the knocking signal is generated. Therefore, we can use mechanical vibration to knock on the metal rod to simulate the effect of diesel engine injection pressure, so that the diesel engine tachometer sensor is induced. To this end, we designed a vibration pulse generator. It consists of a small power amplifier, an exciter, a fixed bracket, etc. (see Figure 4). The low-voltage pulse signal from the low-voltage pulse signal generator drives the exciter to generate mechanical vibration through the power amplifier. The metal hammer head installed on the exciter knocks the metal rod that fixes the diesel engine tachometer sensor at the same frequency as the input voltage pulse. The metal rod generates a pulse vibration that simulates the injection pressure to make the sensor induced. The purpose of calibrating the diesel engine tachometer is achieved.


Figure 4 Diesel engine speed signal generator
 
3 Error analysis
The high-voltage ignition generator and pulse vibration generator in this metrological standard device have a low operating frequency. Therefore, as long as the installation and debugging are correct, there will be no misoperation. The error caused by this can be ignored. The main factor affecting the calibration accuracy and stability is the frequency error of the voltage pulse signal generated by the single-chip microcomputer system. Since this metrological standard device uses the single-chip microcomputer timing interrupt method to generate pulse signals, the error size mainly depends on the conversion error of converting input information into delay time. As long as the program design is reasonable, the calibration error caused by this can be completely controlled below 0.1%.
 
4 Conclusion
This calibration device is suitable for calibrating various handheld automobile engine tachometers, with a calibration range of 100-9900r/min and an accuracy better than 0.1%. After a period of use, the calibration device has achieved the expected effect. It has the following main features:
 
(1) It takes into account the calibration of both gasoline and diesel engine tachometers, with a wide range of applications, high accuracy, and easy operation. It is reliable in operation;
(2) It is small in size, simple in structure, low in cost, and easy to manufacture;
(3) It has certain promotion value and can solve the problem of calibrating handheld engine tachometers in metrology departments.