An Approach to Measure Slips in Friction Contacts Using a Self-Powered Sensor

The accumulation of small tangential slip will occur on the contact surfaces of components in mechanical structures when subjected to cyclic loadings, which will lead to system failure. However, the small tangential slip is difficult to measure directly. In this paper, a method for measuring the contact slip by using a self-powered displacement sensor based on triboelectric nanogenerator (TENG) is proposed, and the accuracy is verified by the finite element analysis (FEA). The contact slips for a flat-on-flat contact configuration are measured by this method. When the test piece is subjected to a cyclic tangential load with a frequency of 10Hz, the average obtained slip displacement of the test piece under single load cycle is less than 10μm. Then the finite element simulation of the contact configuration is carried out using software Abaqus 6.14. The analysis results are close to the test results, which verifies the effectiveness of the measurement method. Moreover, the slips of the flat-on-flat contact configuration under different frequency and amplitude of cyclic tangential load are investigated.


Introduction
Many fastening connection components in mechanical systems are connected by mutual friction of contact surfaces.When a tangential load is applied to the component, a small slip will be generated between the contact surfaces in the tangential direction.If this micro-slip is accumulated, a macroscopic slip will be generated [1], which will lead to loosening of fastening connection and even serious safety accidents.
But this micro slip is difficult to measure directly.At present, the existing displacement measurement methods mainly include mechanical measurement methods, electrical measurement methods and optical measurement methods [2][3][4][5][6][7][8].Among them, the stylus method is the main method in mechanical measurement methods, in which a stylus is used to directly measure the displacement on the surface of an undulating object.It has the advantages of large measurement range, good repeatability and high resolution.However, its measurement speed cannot be too fast because the stylus is easily damaged [9,10].Electrical measurement methods include the resistance method, the capacitance method and the inductance method.For example, the displacement of two parallel plates can be measured by the change of the capacitance based on the inverse relationship between the capacitance and the distance of the parallel plates.Although the accuracy of this method is high, the structure of its equipment is relatively complex, and its stability is not high [11,12].Optical measurement methods developed with laser technology and holography technology, mainly including the laser triangulation, the homodyne interferometry, the heterodyne interferometry, the grating interferometry and the laser speckle measurement.Compared with mechanical measurement methods and electrical measurement methods, 2 optical measurements have obvious advantages, such as high measurement accuracy, fast measurement speed and wide range of application, but have the disadvantages of higher requirements for the environment, more complex measuring instruments, poor anti-interference ability and high measurement cost [13][14][15][16].These methods are usually used to test the overall tangential displacement of the specimen, which includes the deformation of the specimen and is therefore not the same as the slip at the contact interface.
In 2012, Professor Zhonglin Wang of Georgia Institute of Technology first proposed the concept of Triboelectric Nanogenerator (TENG).It can convert mechanical energy into electrical energy through the contact or sliding between two different material parts, and its output voltage, current, and other electrical signal can then be adopted to characterize the corresponding mechanical input signal and can be developed as a self-powered (active) sensor [17][18][19][20][21][22][23][24].In this paper, a self-powered displacement sensor based on the principle of triboelectric nanogenerator is proposed to test contact slips.Compared with traditional measurement methods, this method can directly measure the micro-slip between the contact surfaces and does not require additional power supply.In this sensor, two triboelectric layers of different materials with different electron-binding abilities are fixed on the contact interface.When the two layers separate from the contact state due to the slip of the contact surfaces, the charge on the surface is transferred and generate an electrical signal, and the magnitude of the open-circuit voltage which is linearly related to the separation distance of the triboelectric layers can be adopted to measure the relative slip distance of the contact surfaces.The slip of the flat-on-flat contact in the test bench established in this paper is tested using this method, and the results are consistent with the results of finite element analysis (FEA).

Principle of self-powered sensor
The principle of a self-powered sensor based on a vertical contact-separation triboelectric nanogenerator can be illustrated as Fig. 1, which is a conductor-to-dielectric model.When the conductor and dielectric layers are separated after contact, it is assumed that the charge derived from the contact electrification effect is uniformly distributed on the dielectric surface with the density of -σ if the surface size of dielectric layers and the metal electrodes is much larger than their separation distance.Setting the separation distance between the two dielectrics as x(t), the charge transfer between the two electrodes (Metal 1 and Metal 2) due to electrostatic induction is Q and the area of the two electrodes as S, the electric field intensity within the dielectric is where r is the relative permittivity of the dielectric, and 0 is the vacuum permittivity.
In the air gap, the electric field distribution is The voltage between the two metal electrodes is (3) In the open-circuit state, there is no charge transfer between the two electrodes, i.e., Q = 0, and the voltage between the two electrodes is 0 () It can be seen from the Equation ( 4) that the magnitude of the open-circuit voltage is linearly related to the separation distance of the triboelectric layers, so this kind of TENG can be used as a self-powered displacement sensor to measure the relative slip between two contact surfaces without an external power supply.

Experimental test of contact slip 3.1 Test bench
The experiment is conducted using a flat-on-flat contact configuration as shown in Fig. 2 in this paper, which includes the force application piece, the upper test piece made of Qsn7-0.2, the lower test piece made of 42CrMo, the power amplifier, the vibration exciter, the excitation force sensor, the preload sensor, the Keithley 6514 electrometer, the 6500 data acquisition system and the triboelectric nanogenerator.In the configuration, the conductor and dielectric layer of the triboelectric nanogenerator are fixed on the upper test piece and the lower test piece respectively and are in contact initially, the upper test piece is subjected to uniform pressure generated by a force application piece and is in contact with the lower test piece, and the electromagnetic exciter is pressed on the midpoint of the left side of the upper test piece to apply cyclic loads.When the upper test piece slides relative to the lower test piece, the triboelectric nanogenerator will generate an electrical signal due to separation of the triboelectric layers which can be collected by the 6514 electrometer and the 6500 data acquisition system.The triboelectric nanogenerator is of vertical contact-separation type as shown in Fig. 3 with the size of each triboelectric layer is 25mm10mm.One triboelectric layer is made of copper film with a thickness of 0.06mm which is also adopted as an electrode, and the other triboelectric layer is made of PTFE film with a thickness of 0.06mm bonded with a copper foil as another electrode.Both layers are bonded on the acrylic substrates of 2mm in thickness.The conducting wires are used to connected the two electrodes to the 6514 electrometer and the 6500 data acquisition system.

Sensor calibration
The two triboelectric layers of the triboelectric nanogenerator are separated at a displacement of 0.8mm for eight times, and the obtained open circuit voltage results are shown in Fig. 4. It can be seen from the Fig. 4 that the average voltage is 2.65V when the two triboelectric layers are separated by 0.8mm.It can be known from Equation ( 4) that the open circuit voltage has a linear relationship with the separation distance of the triboelectric layers, so the voltage V (V) can be calculated by the separation distance x (mm) as

Test results
Controlling the exciting frequency of the vibration exciter as 10Hz, and the exciting force applied to the upper test piece is gradually increased as shown in Fig. 5.The 6500 data acquisition system is used to capture the voltage generated by the slippage of the upper test piece, and the voltage signal is converted into a displacement-time curve as shown in Fig. 6.It can be seen that the upper test piece begins to slip when the time is 99 s.At this time, the excitation force amplitude acting on the upper test piece is 14.88N, which overcomes the maximum static friction force.This force magnitude maintains for a period of time, 99s to 99.5s, then begins to decrease.
In Fig. 6, it is shown that the whole sliding process lasts for nearly 20s.The whole displacement is 44.19μm at the beginning of slip from 99s to 99.5s.During this period, five cyclic slips of the upper test piece occurred under the cyclic tangential load, and the average slip displacement in one loading cycle is 8. 838μm.

Finite element model
The finite element model of the flat-on-flat contact configuration in the experiment is developed to simulate the slip displacements using Abaqus 6.14.In order to simplify the finite element analysis and calculation process, the two-dimensional plane models of the force application piece, the upper test piece and the lower test piece are established and assembled together as shown in Fig. 7.The sizes of the upper test piece and the lower test piece are 100mm2mm and 100mm4mm respectively, and the size of the force application piece is 100mm1mm with the mass of 7.85Kg.The CPE4R plane strain element is used to mesh the parts.The element size is 0.50mm, and the mesh model is shown in Fig. 8.The bottom of the lower test piece is fixed, and a cyclic tangential pressure with a frequency of 10 Hz as shown in Fig. 10 is applied to the left side of the upper test piece (see Fig. 9).A frictional contact is applied between the two specimens with a Coulomb friction model.

Finite element simulation results
The dynamic analysis is carried out to obtain the slips at the interface as shown in Fig. 11.The slip displacement of the middle node of the upper test piece is shown in Fig. 12.It can be seen that during 5 load cycles within 0.5s, and the total slip distance of the upper test piece is 44.84μm, and the average slip distance per cycle is 8.968μm, which is close to test value.

Slip results under different load frequencies
The slip displacements of the flat-on-flat contact configuration under different load frequencies are measured in this study.The slip results are shown in Fig. 13 when the frequency of cyclic tangential load is 7Hz, 10Hz, 15Hz and 20Hz respectively.The displacements of the upper test piece within 0.5s at the initial stage of slipping at each frequency are shown in Table 1.It can be seen that when the amplitude of the cyclic load is the same, the higher the frequency of the cyclic load is, the smaller the slipping displacement.

Slip results under different load amplitudes
The load frequency is kept at 10Hz, and the amplitude of exciting force is gradually increased from 14.88N to 15.14N, then to 15.59N, and the slips of the upper test piece obtained are shown in Fig. 14.
The displacements of the upper test piece within 0.5s at the initial stage of slipping under different load amplitudes is shown in Table 2.It can be seen that when the frequency of the cyclic load is the same, the larger the amplitude of the cyclic load is, the larger the slipping displacement., the open-circuit voltage generated by the contact-separation mode TENG is linearly related to the separation distance of the triboelectric layer.Therefore, this TENG can be used as a self-powered displacement sensor to measure the relative slip between two contact surfaces without an external power supply.In this paper, the self-powered sensor is fabricated and adopted in the flat-on-flat contact configuration.After calibration, the slip of contact interface when the load frequency is 10Hz is measured according to the output open circuit voltage of the TENG, which is in good agreement with the finite element simulation results.
The slip displacements of the flat-on-flat contact configuration under different load frequencies and different load amplitudes are measured in this study.The results under different load frequencies show that when the cyclic load is the same, the higher the frequency of the cyclic load is, the smaller the slipping displacement.The results under different load amplitudes show that when the frequency of the cyclic load is the same, the larger the cyclic load is, the larger the slipping displacement.
In the future, this approach of measuring slips in friction contacts using self-powered sensors can be extended to industrial production to detect micro-slip between contacting components.Due to its characteristics of not requiring additional power supply and high accuracy, it has a broad commercial prospect.

Figure 4 .
Figure 4. Voltage of the sensor at a separation distance of 0.8mm.

Figure 13 .
Figure 13.Slips results at different load frequencies.

Figure 14 .
Figure 14.Slips of the upper test piece under different load amplitudes.

Table 1 .
Displacements at the initial stage of slipping under different load frequencies

Table 2 .
Displacements at the initial stage of slipping under different load amplitudes A self-powered displacement sensor based on the principle of triboelectric nanogenerator is presented and implemented for a flat-on-flat contact configuration to measure contact slip.The sensor is a conductor-to-dielectric triboelectric nanogenerator composed of a 0.06mm copper film layer and a 0.06mm PTFE film layer.When the surface dimensions of dielectric layer and the metal electrode is