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Talk about the core technology of electro-hydraulic servo fatigue testing machine

Reading guide

The testing machine industry is a traditional and new industry. With the application of new materials and the development of new technology, higher quality requirements drive the market to put forward more accurate and higher performance requirements for the testing machine, so as to obtain more authentic and scientific test data and lay a solid foundation for technological development.

LSI focuses on the R & D, manufacturing, sales and service of mechanical test instruments and equipment. Among them, research and development is the core and source of the testing machine. The research and development of high-end testing machine is inseparable from the core technology that keeps pace with the times. Since its establishment, the company has independently innovated and developed a number of high-end products, including electronic universal material testing machine, electro-hydraulic servo fatigue testing machine, multi axis coordinated loading system and various special testing machines.

This time, let’s talk about the core technology of electro-hydraulic servo fatigue testing machine. The force test electro-hydraulic servo fatigue testing machine can be used to test the mechanical properties of all kinds of materials, such as low cycle fatigue, high cycle fatigue, crack growth, fracture toughness and other conventional mechanical tests. At the same time, it can also integrate high and low temperature, humidity, corrosion environment box for coupling tests in complex environments. Such an”all-round” product that can be widely used in aerospace, automobile, ship, military industry, metallurgy, building materials, scientific research institutions, quality inspection and other fields, its core technology can be divided into three points (mechanical structure design of testing machine, measurement and control system, software).

1、 Mechanical structure design of testing machine

1) Coaxiality of loading system

In the test, the conditions of different axes will lead to the bending of the sample in the tensile test, which will cause”early failure” to the sample, and the elastic modulus will also have a large deviation. Through continuous iterative innovation, we have designed the adjustment ring of the centering system, which is used for high-precision fixture, high coaxiality clamping of round samples and flat samples, and the supporting system centering kit can intuitively realize the precise adjustment of six degrees of freedom before, after, left, right and angular direction of sample clamping. The coaxiality of test loading can be greatly improved by cooperating with standard coaxiality measurement samples, strain acquisition system and analysis software.

2) Servo hydraulic cylinder

Servo hydraulic cylinder is the core component of fatigue testing machine, which can directly affect the accuracy, repeatability and efficiency of the test.

Force test applies original advanced sealing technology, professional high-end processing technology, and selects manufacturing materials through a large number of comparative tests, which requires high precision. All parts of each servo hydraulic cylinder are subject to strict quality inspection to ensure that the servo hydraulic cylinder assembled on each testing machine meets the quality requirements of force test.

The advanced sealing technology gives the servo hydraulic cylinder extremely low friction and long service life, ensures the accuracy and reliability to meet the high-end test under the high frequency of 100Hz, and can realize the perfect control of the tension and compression zero crossing test.

3) Oil source hydraulic pressure

Adhering to the principle of safe and efficient operation, we integrate modern design concepts of energy conservation and environmental protection. Hps-he series oil source is a new generation of silent hydraulic power system, which can efficiently provide kinetic energy for hydraulic drive equipment in the laboratory, while allowing users to enjoy a quiet and clean space.

The large flow hydraulic station adopts the parallel design of multiple pump groups, and one or several groups of oil pumps are used as required, which greatly saves energy and effectively reduces the floor space.

The main features are:

a. With”zero voltage” start and high/low voltage switching function;

b. Constant pressure variable system ensures the system pressure and maximizes energy saving;

c. Fully enclosed structure, good sound insulation, heat insulation and environmental protection effect;

d. There is no leakage outside, so as to avoid dirt and oil mixing;

e. The internal layout is optimized, and the medium temperature is kept uniform;

f. Effective ventilation setting ensures good ventilation and ventilation effect;

g. LCD control panel, easy to set parameters and monitor;

h. It can be operated remotely and locally;

i. It can be operated by multiple devices and has the function of opening before closing.

2、 Measurement control system

1) Controller

The controller is the core component of the test machine. On the one hand, it realizes the action control of the test equipment under the command of the upper computer software; On the other hand, it can collect the force, deformation and other possible physical quantity changes on the sample in real time during the test, and the collected physical quantity changes can be fed back to the control according to the control algorithm in the system.

Key performance of controller:

a. System frequency:as the key index of the control system, the system frequency of the controller determines the closed-loop control ability of the controller. The higher the system frequency is, the stronger the ability to perform feedback control is. The faster the action of the testing machine equipment can be supported, and the higher the control accuracy is.

b. Sampling frequency:refers to the effective sampling number of the controller in each time slice. For most controllers, the sampling frequency is consistent with the system frequency. The higher the running speed of the test, the higher the sampling frequency of the controller is required, so as to ensure sufficient sampling data of force, deformation, displacement, etc. during the short-term test.

c. Test frequency:test frequency refers to the number of cycles that can be supported per second when the equipment is used for fatigue test. It is determined by many factors (including the maximum moving speed that the equipment can support, acceleration, smooth switching ability at zero crossing and other factors). The test frequency that the controller can support is the most important link.

d. A/d resolution:refers to the analog-to-digital conversion capability of the control acquisition system. Common a/d chips include 24 bits, 20 bits, 16 bits, etc. different a/d chips have different capabilities, and the resolution they can provide is different.

e. Curve fit:the control can be performed according to the predetermined trajectory function, and the control algorithm of the controller plays a key role.

f. Stability:for fatigue test or long-term creep, relaxation and other tests, many test times are in months, and the stability of the system is very critical.

2) Force measurement system

The force measurement system is an indispensable part of the testing machine. Almost all types of static and dynamic material physical property testing equipment are inseparable from the force measurement. Whether in tensile, compression, impact, shear, peel, fatigue or fracture mechanics tests, the force value is the most important measurement index.

Common force sensors include strain type, pressure type, piezoelectric type and accelerometer.

Key indicators of force measurement system:

a. Zero drift:refers to the change of force value when the sensor is stationary. Generally speaking, the smaller the change range, the more stable the sensor is or the smaller the environmental interference is. However, when the sensor is stationary, it may also be caused by insufficient sensitivity of the sensor system or high a/d resolution.

b. Temperature drift:refers to the force value shift of the sensor when the ambient temperature changes.

c. Nonlinearity:within the range of the sensor, the nonlinearity index is often used for the linearity. The lower the nonlinearity value is, the better the linearity is.

d. In particular, the force sensor of fatigue testing machine has been in the working state of high-frequency alternating. The hysteresis and creep characteristics related to elastomer are very different from ordinary static force sensors. The force sensor of general fatigue machine is far better than the normal load sensor in these aspects.

3) Deformation measurement system

Deformation measurement system is a commonly used measurement component of testing machine. It is generally used to measure the deformation, bending disturbance, crack opening width change, compression height change and deformation caused by impact within the sample gauge distance.

a. The deformation measurement system can be divided into contact extensometer and non-contact extensometer according to whether it contacts the specimen.

Commonly used contact deformation measuring devices include:electronic extensometer, full-automatic extensometer, dial indicator, interference meter, capacitive induction extensometer, electromagnetic induction extensometer, etc

Common non-contact deformation measurement devices include:Video extensometer, laser displacement meter, infrared displacement meter, etc. The non-contact extensometer has the characteristics of non-destructive measurement of samples, and has irreplaceable advantages in the measurement of soft materials, complex environment and large deformation.

b. According to the applicable range of temperature, the deformation measurement system can be divided into; Normal temperature extensometer, low temperature extensometer and high temperature extensometer, etc.

The key indicators of the deformation measurement system are basically similar to those of the force sensor.

c. Strain fatigue has higher requirements for extensometer, which requires higher resolution and response frequency. In particular, high temperature strain fatigue extensometer has high technical difficulty.

4) Displacement measurement system

Slightly different from the force sensor measurement system and deformation measurement system, the displacement measurement system is generally provided with the machine. It is mainly used to capture the movement changes of the equipment beam or actuator. It is an important measurement component for the equipment to achieve accurate movement control.

Displacement meters commonly include rotary encoder fixed on the motor shaft for indirect measurement and LVDT, grating ruler, wire encoder, hysteresis dilatometer, etc. directly used to measure the movement of beam or actuator.

Generally, electronic universal testing machine adopts rotary encoder to realize the indirect measurement of beam displacement. The resolution of displacement is obtained through the indirect calculation of parameters such as the number of lines of the encoder, screw pitch, reduction ratio and so on; The hydraulic press and most fatigue testing machines are mainly direct measurement, and the resolution of displacement is directly reflected in the displacement sensor.

Whether it is indirect measurement or direct or indirect measurement, due to the flexibility of the testing machine frame, the connecting parts of the force sensor and the sample fixture, the deformation often acting on the sample cannot be simply equivalent to the displacement measurement value, and there is a large deviation between them in essence.

3、 Dynamicexpert test software

Dynamicexpert, a multi-purpose dynamic testing software with independent intellectual property rights, is an easy-to-use but professional testing software. It has simple and intuitive scheme editing interface, flexible and convenient curve adjustment function, configurable real-time cyclic data operation function, powerful data storage function, rich test waveform support and fast sample protection function.

a. Test application technology of testing machine:advanced composite test software package, low cycle fatigue test software package, crack growth test package (dead load increase K, decrease K, constant k), fracture toughness test package (KIC, CTOD, j1c test, resistance curve), spectral load test software package, elastomer test software package, etc; In particular, the core algorithm of crack growth real-time automatic reduction of K, elastic dynamic stiffness, elastic stiffness, damping stiffness, loss angle, energy and damping coefficient has been compared and verified with many authoritative customers.

b. High density data storage technology of software:the fatigue test software of force test has realized the functions of high-density data storage and data retrieval and query of all channel data up to 10^8 life test data, and is applying for independent intellectual property rights.

c. Real time data processing technology:high performance calculation and processing of real-time data such as peak and valley values of cyclic load, dynamic modulus, plastic strain, elastic strain, stiffness, loss angle, etc.

d. It is convenient to customize the stability, reliability, expansibility, openness and upgradeability of the software; The test software has passed the durability test of many customers, and has experienced more than 3000 hours at many customer sites. The system does not restart, the software does not get stuck, and there is no abnormal test.

e. Software control:it has realized the functions of automatic phase adjustment and synchronous and coordinated control of loading start and end points; The sample protection module effectively solves the problem of excessive clamping load.

Conclusion

Try hard to know that the core technology of the testing machine is the core technology of the enterprise. China’s testing machine industry wants to take得良性发展,必须注重技术创新,牢牢掌握核心关键技术。

正如文章导读所说,新材料技术的应用和技术在不断发展,这也表明试验机行业绝不能停滞不前。力试重视技术人才的引进和培养,提高企业专利意识,加强与高校、企业的交流合作。近两年的基础研发投入(R&D)为7.87%、10.95%,比肩发达国家的企业基础研发投入,取得多项成就,在国产试验机行业中大放光彩。

我们会继续秉承自主创新精神,加强交流合作、开拓视野,在关键领域、卡脖子的地方下大功夫,集合精锐力量取得更大的突破。

本文作者:力试科仪