Development of multifunctional current-carrying friction and wear test machine based on LabVIEW

In order to study the effect of friction wear behavior on electrical signal transmission, a carrier friction wear test machine was developed which can be analyzed under different test conditions (load, speed and current). The testing machine builds a measurement and control system through LabVIEW platform to realize real-time acquisition and synchronous display of electrical signals, friction coefficient and temperature, and control of load size, and uses wavelet decomposition and arithmetic average to process data and PID control algorithm to revise load in real time. The experimental results show that the stability of the test machine and the accuracy of control system meet the test requirements.


Introduction
Carrier friction wear belongs to special working conditions, which is the friction wear behavior of friction pair in electric or magnetic fields when the current passes through [1] .In recent years, with the rapid development of rail transit systems, power transmission systems and substation, and industrial generators, the requirements of conductivity, abrasion resistance, reliability and service life of friction pairs under current carrying conditions are becoming higher and higher.During carrier friction and abrasion, the surface of the friction pair will generate a lot of friction heat and joule heat due to friction, resulting in a large amount of energy loss, and the wear will also reduce the service life of the friction pair, and may lead to the failure of the entire current-carrying friction and wear system in serious cases.Therefore, it is of great significance to carry out the research on friction wear and tear in order to energy saving and emission reduction and improve the service life of friction pairs.At present, researchers at home and abroad usually use testing machine to simulate the current friction wear phenomenon and discuss the factors affecting the current friction wear phenomenon and its properties.Therefore, in the research of carrier friction wear, the carrier friction wear test machine is an indispensable equipment, which is mainly used in the research of the mechanism of carrier friction wear, the study of lubricating oil performance and the evaluation of friction pair material technology.Although the current bearing friction and wear test machine has many kinds of structures, its function is too simple, its performance is poor, its measuring material and detection range are limited, its use is inconvenient, and it has some limitations [2] .Therefore, it is of great significance to develop the multifunctional carrier friction wear test machine for studying the friction wear mechanism.
Therefore, this paper independently developed a multifunctional current-carrying friction and wear testing machine to study the influence of friction and wear behavior of friction pairs on electrical signal transmission under different test conditions (load, speed, current), and proved the stability of the testing machine and the accuracy of the measurement and control system.

Design of mechanical structure
The mechanical structure of the test machine consists of three parts: signal output structure, clamping structure and signal lossless transmission structure.The main design indicators are shown in Table Table , and the three-dimensional model is shown in Figure 1.
Table 1.Design index of testing machine

Signal output structure design
The signal output structure is the mechanical structure that provides the sine wave signal for the testing machine.During the working process of the testing machine, the strain gauge on the rotating axis is subjected to the load imposed by the weight, and thus deformation occurs, resulting in a sine wave change in the resistance value of the strain gauge, so when the current passes through, the voltage of the strain gauge also produces a sine wave change.

Clamping structure design
The fixture structure is an indispensable component of the test machine, the sample is fixed by the fixture, and the electric cylinder is used to control the size of the applied load.In the test process, the relative position and relative motion accuracy of the friction pair mainly depend on the precision of the rotating shaft, the sample, the fixture, the spring and the electric cylinder, and the fixture plays an important role in lifting and lifting.At the same time, the exact position of the conductive slip ring is determined by installing a scale line on the rotating shaft.

Signal lossless transmission structure design
Signal lossless transmission structure is a kind of mechanical structure that realizes current or signal transmission between two relative rotating mechanisms.During the test, the current or signal is transmitted harmlessly through the liquid metal.

Design of measurement and control systems
Test machine measurement and control system is based on the master machine, data acquisition card, various sensors and the next machine integrated measurement and control system.LabVIEW is used as programming language for the upper computer acquisition interface.LabVIEW is a typical graphical programming language (G language), which combines the high performance and flexibility of graphical programming and the high-end performance and matching function designed for test, measurement and automation control applications, providing the necessary development tools for various applications such as data acquisition, measurement analysis , instrument control and data display [3] .

Hardware design of measuring and controlling system
The model parameters of the measuring and control system hardware are shown in table Table 2

Friction coefficient measurement principle
To measure the friction between the friction force pairs, the friction force f received by the composite carbon brush is converted into the pressure of the friction sensor through the corresponding mechanical structure, and the measurement is carried out.The schematic diagram of the measurement structure is shown in Figure Figure 3.At the same time, in order to ensure the accuracy of friction data measurement, it is necessary to "clear" the friction sensor data before the measurement [4] .Where: ȝ is the friction coefficient; f is the friction force; F is for pressure.In the process of rotation, the composite carbon brush will be subjected to shear force and compression force, resulting in changes in the loading load of the friction pair.Therefore, in order to ensure the accuracy of various data during the test, the stability and constancy of load loading in the contact area of the friction pair should be ensured [5] .For this reason, the PID algorithm is used to revise the load in real time during the test, and the load output is carried out by controlling the electric cylinder by the lower machine.

Software design of measurement and control system
Testing and control system of the test machine carries out real-time acquisition, synchronous display, threshold alarm and data storage of four signals of temperature, pressure, electrical signal and friction coefficient in the test process through the acquisition card, and uses PID algorithm to carry out real-time revision and size control of the load [6] .The program block diagram is shown in Figure 4. Since the carbon brush will produce a lot of electrical noise in the process of transmitting current or electrical signal, which will cause interference to the collection of electrical signal, the mixed programming technology of LabVIEW and MATLAB is used to denoise the collected electrical signal data by wavelet decomposition.At the same time, three kinds of data such as temperature, pressure and friction are processed by arithmetic mean to improve the test accuracy of the measurement and control system [7] .

Sensor calibration
Since the sensor will change with the different environment during use, it is necessary for the user to conduct on-site calibration before use.The friction sensor and pressure sensor in the testing machine are of the same type, so the pressure sensor is taken as an example to carry out field calibration and obtain the data of load and voltage values, as shown in Table Table 3 Linear fitting was carried out according to the data in Table Table 3, as shown in Figure 5.The fitting formula of load and voltage value was obtained as follows: The load is reversely derived from equation (2), namely: Where: F is the load; V is the voltage value.

PID parameter tuning
At present, about 90-95% of the control circuits in the control field adopt PID control technology, which has simple principle, convenient parameter adjustment, wide application and easy implementation, and is widely used in industrial motor control [8] .The performance of PID control is determined by three parameters: ratio (P), integral (I) and differential (D).The function of ratio (P) is to improve the adjustment accuracy and speed up the response speed of the system.The function of integral (I) is to eliminate the steady-state error of the system.The function of differential (D) is to improve the dynamic performance of the system.Figure 6 is the variation diagram of PID tuning curve of the testing machine.
The load is set to 15N.After adjustment, the final P parameter is set to 1, I parameter to 0, and D parameter to 0.001.

Test section
In order to verify the stability of the testing machine and the accuracy of the measurement and control system [9] , the composite carbon brush/conductive slip ring was taken as the test object to conduct current-carrying friction and wear tests, in which the load was set at 15N, the speed at 200r/min, and the current at 0.05A.The conductive slip ring is mainly composed of a slip ring assembly and a brush assembly, and is a precision transmission device that relies on the brush and the conductive ring to form a sliding contact friction pair to achieve current or signal transmission between two relative rotating mechanisms [10,11] .In this paper, the conductive slip ring is made of brass H62, the size is ĳ16mm×4mm×1mm, and the composite carbon brush is sintered by powder metallurgy process by mixing copper powder and graphite powder with mass ratio of 1:1 [12] .

Test result
The evolution law of electrical signals during the composite carbon brush/conductive slip ring cycle is shown in Figure 7

Conclusion
(1)In order to study the influence of friction and wear behavior on electrical signal transmission, a multifunctional current-carrying friction and wear testing machine was developed which can be tested and analyzed under different test conditions (load, speed, current), and the performance of the testing machine was proved through tests.
(2)The testing machine converts the friction force received by the composite carbon brush into the pressure of the friction sensor through the mechanical structure, and measures it, and in order to ensure the stability of load loading, the PID control algorithm is used to revise the load during the test in real time.
(3)In order to conduct real-time acquisition and synchronous display of electrical signals, friction coefficient and temperature during the test, as well as control of load size, the testing machine measurement and control system is built through LabVIEW, and the collected data is processed by wavelet decomposition and arithmetic mean value, so as to improve the test accuracy of the measurement and control system.

Figure 1 .
Figure 1.3D model of testing machine

3. 2
Principle of signal measurement 3.2.1 Electrical signal measurement principleIn the rotation process, the resistance of the strain gauge changes sine wave with the rotation of the rotation axis, and when the current passes, the corresponding voltage also changes sine wave.Finally, it is collected and stored in real time through the acquisition card.The electrical signal acquisition schematic diagram is shown in Figure Figure2.

Figure 4 .
Figure 4. Program block diagram of testing machine measurement and control system 4. Testing machine debugging

Figure 5 .
Figure 5. Calibration curve fitting of pressure sensor

Figure 6 .
Figure 6.Change diagram of PID tuning curve (a), the change curve of friction coefficient is shown in Figure 7 (b), and the change curve of friction temperature is shown in Figure 7 (c).

Table 3 .
. Pressure sensor calibration table