Characterization Of Multi Axis Force Sensor

CHARACTERIZATION OF MULTI AXIS FORCE SENSOR

Yung Ting, suprapto, Adytia Nugraha, Hariyanto Gunawan,Yu-Heng Lin

Chung Li, Taiwan, Department of mechanical Engineering, Chung Yuan Christian University

[anonimizat]

Abstract

PVDF is thin and flexible a piezoelectric material, making it easier to created and applied as a force sensor. In this study, PVDF built as a 3 axis force sensors with one layer of PVDF only. Thickness and surface polarization were applied to make this sensor. Piezoelectric constants measured by FTIR, XRD, PFM, and voltage response by polarization of PVDF is measured by Oscilloscope. From the test results showed that the sensor can respond the force properly.

Key words: PVDF, 3-axis sensor, surface polarization

Introduction

PVDF is a typical piezoelectric polymer that has strong piezoelectric and pyroelectric properties. A in the last few decades, PVDF has been widely applied in the fields of sensors and actuators [1][2][3][4]. A Piezoelectric polymers are supplied in the form of thin films that are flexible with large compliance. A accompanying the flexible electronics development rapidly in recent years, in various applications such as flexible tactile sensors [5] and cutting force sensor [6]. A PVDF is widely applied in flexible fields due to its good mechanical strength, electro-chemical stability, and its ability to be cut apart freely in polymer materials [7].

Currently, 3D sensor typically use many layers, each layer is used to sense the force of each axis. Moreover, to measure the cutting force in three different axis requires 3 PVDF sensor that is placed in a certain position [6]. In contrast to the aforementioned sensors, the sensor in this article can sense the force from three-axis axis using one layer only. Article is described the good performance of the force sensor with 3- axis.

In this research, surface polarization method is applied to gain high output voltage and sensitivity. Experimental results of a 3D-sensor made by PVDF show good performance by using Fourier Transformed Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD), and Piezoresponse Force Microscopy (PFM) respectively.

FABRICATION

Stretching

The material film of PVDF manufactured by Pennwalt Corporation (Kynar 740) and thickness of 130 mm  were used. Polymer sheet size were cut (150 x 150 mm) was stretched uniaxially when the clamps move apart from each other at a constant rate (21 mm/hr). Then, in order to obtain the optimum results, all of the sheet PVDF was stretched at different temperature (80, 90, 110, 110 and 120 C) and ratio (1,3,4,5,and 6). To achieve good thermal equalibirium, before strectching the sheet of PVDF to remain preheated inside in chamber until 1 hour, and after stretching to the desired extent, the sheef of PVDF were held until chamber temperature cooled down to ambient temperature ( it take about 1 hours). According [8] the optimum temperature of stretching to changed -phase content at 87 C.

Multi Direction Poling

Domain is one of important characteristic of ferroelectric material like PVDF. A domain is a microscopic regions of a crystal in which the polarization is homogenous. Naturally, these domain are un-alligned, but it can be alligned in a common direction by applying a DC electric voltage in an expected period time.[9].

Figure 1. Planar Polarization (d31)

Figure2. Thickness Polarization (d33)

Domain direction depending on the direction of polarization [10, 11] the direction of polarization used is d33 and d31. The design of both polarizations can be seen from Figure 1 and 2. Fig. 1 and 2 d33 and d31 polarization mode. New poling surface method [12]. was applied order to produce d33 and d31 modes and to avoid cross talk effect, independent electrode pattern is designed and shown in fig.3. There are two steps in the process of poling applying electric field on the (X1, X2, Y1,Y2,G) and (Z,G) to complete the combined planar polarization as well as along the thickness.

Electrode Design

There are 6 of electrodes was placed on the PVDF show in the Fig 3. Polarization can be carried out structured on both d33 and d31 in one layer. Combination of d33 and d31 polarization was used in experiment. Both of polarization method were applied in one layer only. In this experiment, polarization with DC high voltage was applied to PVDF around 3KV with room temperature for 15 minutes. d33 polarization was applied for Z axis sensor, while x and y axis were used d31 polarization. Lamination used to protect PVDF sensor from damage.

Figure. 3 Design of 3D sensor

EXPERIMENTAL AND RESULTS

Material Characterization

The fraction of the -phase of PVDF sample was investigated by Fourier Fourier Transformed Infrared Spectroscopy (FTIR). Fraction of β-phase crystals F(β) in each sample is calculated by [9]

(1)

where Aα and Aβ is determined by absorption bands at 763 and 840 cm-1, respectively.

(2)

This equation was used to measure the FTIR results.

Fig.4 FTIR spectrums of stretched pure PVDF (a) No poling (b).Poling with temperature of stretching (80C)

Fig.4 shows the results from Fourier transform infrared spectroscopy (FTIR). Infrared spectra were achieved using a FTIR spectrophotometer in the wave number range of 700–1000 cm-1 for no poling and poling PVDF. The β-phase content is of prime importance in these applications so that increasing β-phase content of the polymer has always been of great concern in this field. From result of FTIR evaluation of the change in fraction of -phase in any various PVDF film sample with configuration of stretching temperature and poling voltage. As seen in figure 4, the absorption peaks occur at 841.78 cm-1 for -phase and 877.45 cm-1 for -phase respectively. The absorption coefficient K and K for the -phase and -phase are 6.1×104 and 7.7×104 cm2/mole respectively. As known, amount of β-phase fraction 89% is good enough to produce high electrical response.

Figure 5. Measurement of and  using X-ray Diffraction (XRD).

Figure 5 shows the series of X-ray Diffraction (XRD) pattern for PVDF stretched film temperature 80C. For condition poling d31, the peaks at around 2 = 18.3 correspond to -phase PVDF and 2 = 20.72 correspond to -phase PVDF, respectively. The intensity for d31 is higher than of both poling d33 and no poling modes. The decrease of the smaller peaks also suggests a conversion from to -phase. These results, agree well with the value of 2 for the -phase and -phase in the another researcher [13].

To get detail information about direction of polarization, it is components have been detected using lateral and vertical PFM modes (LPFM and VPFM, respectively) by PFM SPA400. Fig. 6. shows topographic image sample PVDF with stretched ratio 5.5 and temperature 80C and thick film 27 m.

Figures 6 (a), (b), and (c) show un-poled PVDF, thickness d33 poling and surface d31 poling respectively. Figures 6(a) and (b) use vertical PFM, scanned and 6(c) used lateral PFM scan mode. AC bias voltage amplitude 15V and frequency 5 KHz were applied. ………the domain direction is not uniform, but after polled by d33 and d31 poling, domain direction inclined in the same direction. it can be seen from the darkness and the brightness of PFM result.

Sensor Measurement

Frequency response and input/output measurement are carried out to verify the performance of 3D sensor. A PVDF sample is hung by two light springs at the top corners, which can avoid from any external effect influential to frequency response measurement. A impulse hammer PCB piezotronics 086E80 system is used for frequency response testing.  The frequency response of each axis is shown in Figure 7.

Figure 7. Frequency Response 3D Sensor

Figure 8. Voltage Response (a) X- axis (b)Y- axis (c) Z- axis

A 3D force sensor (PCB 208C01) is used to measure the input/output response. For the measurement along the z axis, force is applied to press the PVDF sample, which is sensed by the force sensor implemented below the PVDF sample.  For the measurement along the X and Y axis, force is applied to pull the PVDF sample, which is tightly attached to a linear slider. The slider will then press the force sensor. The output of the PVDF and input force measured by the force sensor are both presented with voltage signal shown in Figure 8.

Fig. 8 Force sensor testing in X and Y direction

Fig.9. (a), (b), and (c) display the response of PVDF when it was force in x, y, and z direction respectively. It’s clear the signal from X output is bigger than Y and Z when PVDF was given force in X direction, and Y signal is higher than the other output of PVDF when PVDF was given force in Y direction. Moreover X and Y signal aren’t lower than Z signal when horizontal force was applied.

CONCLUSION

Characterization of multi-axis force sensor is presented. The content of -phase is affected by poling process. This is indicated by an increase in -phase phase when polled before and after polled which was showed by FTIR and XRD result. Therefore, poling d31 and d33 is needed to increase the response of PVDF. It’s work d33 and d31 polarization for 1 layer PVDF, PFM image shows the domain change was happened when it’s polarized in both direction. The caracterization of sensor shows the 3D PVDF sensor have a good performance. It can detect force from all axis which given to this sensor. Large electricity output from this sensor prove this sensor have good performance.

ACKONOWLEDGMENT

This research was supported by ……………….+

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