ricardo rodriguez lopez spie paper final ver3

Development of three-axis inkjet printer for gear sensors

Daisuke Iba*1, Ricardo Rodriguez Lopez*1, Takahiro Kamimoto*1, Morimasa Nakamura*1, Nanako Miura*1, Takashi Iizuka*1,

Arata Masuda*1, Ichiro Moriwaki*1, Akira Sone*1

*1Kyoto Institute of Technology, Dept. of Mechanical Engineering, Precision Manufacturing Laboratory, Goshokaidou-cho, Sakyo-ku, Kyoto-shi, Kyoto, JAPAN 6068585

ABSTRACT

The long-term objective of our research is to develop sensor systems for detection of gear failure signs. As a very first step, this paper proposes a new method to create sensors directly printed on gears by a printer and conductive ink, and shows the printing system configuration and the procedure of sensor development. The developing printer system is a laser sintering system consisting of a laser and CNC machinery. The laser is able to synthesize micro conductive patterns, and introduced to the CNC machinery as a tool. In order to synthesize sensors on gears, we first design the micro-circuit pattern on a gear through the use of 3D-CAD, and create a program (G-code) for the CNC machinery by CAM. This paper shows initial experiments with the laser sintering process in order to obtain the optimal parameters for the laser setting. This new method proposed here may provide a new manufacturing process for mechanical parts, which have an additional functionality to detect failure, and possible improvements include creating more economical and sustainable systems.

Key Words : Gear sensor, Failure detection, Printed sensor, Conductive Ink, Three-axis printer, Laser sintering

1. INTRODUCTION Nowadays, every machine is capable of measuring in real time the physical conditions, for instance, acceleration, deformation, temperature, etc., of the mechanism. This measured parameters can be used to modify the desired conditions for the machinery operation or to detect failure for an appropriate maintenance, performance and most important, safety. However, some measurements cannot be done with conventional techniques or with the actual range of sensor devices, due to space limitation, geometric complexity, environment conditions, moving parts, high rotating speed mechanical parts and more. Specifically, gears have the geometric complexity, and rotate at high speed in the gearbox, and hence it appears that it is difficult to measure the condition of gears1. Indeed, methods for achieving data acquisition of gear condition, such as sticking strain gages at the root of gears or installation accelerometers on gears for vibration measurement, have been studied, but these measurements are limited in large size and to application of experiment.

Recently, rapid prototyping tools, by using a wide range of different techniques aided by using CAD software, offers the possibility to quickly fabricate parts, mechanical elements, electronic circuitry, and so on2. This rapid prototyping tools are attracting the attention of many researchers and are rapidly being developed. Moreover, despite the low accuracy or reliability for some applications, the rapid prototyping tools is friendly with the environment and less expensive compared with other manufacturing techniques, thus, this techniques are slowly being implemented in the industry and expected to increase.

Our interest lay on printed electronics, which is recently developed to create electronic circuits or sensors on flat planes by printing conductive inks. This technology has now lower integration density and is not as competitive as other technologies used for electronics manufacturing, such as, vacuum deposition, photolithography and pick-and-place. However, this technology has apparent advantages, such as simple fabrication, the possibility of printing on any shape and flexible substrates, the low energy consumption and extremely low cost. Therefore, there is a growing tendency to research and develop the new printing methods and companies are releasing new formulations of conductive inks.

Our study attempts to create new sensors for mechanical components, which are not under the same constraint as ordinary sensors. We intend to develop a new method to print sensors on the complex gear surface by using conductive ink, for instance, polymer or water based ink with silver nano-particles. However, a typical printing machine is only

capable of printing in two-dimensional plane. For more complex shapes such as gear surfaces, the printing tool should be introduced onto numerically controlled multi-axis machineries.

Then, we attempted to print conductive ink on the surface of mechanical components covered by a polyimide layer by using a multi-axis printer with a DoD-headprinter (Drop on Demand, typical piezoelectric headprinter) 3. The developed printer consisted of a CNC machine (Original Mind KitMillBT100), which was a 3-axis CNC with a resolution of 0.78 µm and 15 mm/s feeding speed (F900), and an inkjet head from Cluster Technology (PIJ-25NSET) with a 25 µm nozzle diameter. The inkjet head was controlled by a wave builder (PIJD-1SET), which is the printer head controller, and generated drops with from 10 pl to 34pl by setting voltage, frequency and the wave shape parameters. This system successfully printed out conductive patterns. However, accessibility of the inkjet head to tooth roots of gear was not sufficient. Because this printing system could basically print on plane surfaces or curved faces as long as the head printer could access the printing area. The head size was about 5-10-10mm and it was not small enough to access the root of gear.

On the other hand, a different printing method using the laser sintering technology has been proposed4, which was available for printing electronic circuits. It appears that the laser sintering covers our needs, because of the long focal distance of the laser and no need to approach the laser head to the root of gears.

The objective of this paper is to uncover the fundamental printing condition by the laser sintering on a polyimide layer for developing a multi-axis gear sensor printer. In this paper, conductive patterns is printed out on a polyimide layer by using the laser sintering method, and the printed pattern properties of the conductive ink is evaluated. Initially, we will start with an explanation of the process to make the conductive patterns, and reveal the laser sintering condition for printing patterns, such as laser power, feeding speed and focus distance.

2. LASER SINTERING 2.1 Laser sintering machine

For laser sintering of conductive inks, we use an open source laser-cutting machine from smartDIYs as shown in Fig. 1. This machine is a two-axis CNC machine for XY plane, which originally is used as cutting machine for thin materials. The laser power is 1.6W, and the wave length is 445nm, which is suitable for the laser sintering of conductive ink due to the laser wave range4. This machine is composed by 3 basic control modules; Arduino as control unit and communication with the computer for parameters setting, the motor driver for X-axis and Y-axis stepper motors and the laser power unit. The software used is a very simple program where feeding speed and laser power can be set, and by using DXF or SVG files extensions any 2D pattern can be sintered. The advantages of this system is an open source system, and therefore, the system is easy to customize. It means that this laser is easy to install into other CNC machinery, which has higher accuracy for gear sensor printing applications.

Figure 1. Laser sintering machine (Smart Laser Mini)

2.2 Laser sintering process

In this section, we shall first show the fabrication procedure of the test samples (circuit test patterns). Figure 2 shows the schematic view of the fabrication procedure with polyimide spray.

Firstly, in order to isolate the base materials, we cover the base materials by the PI layers before conductive ink printing. For the development of the samples, we use two different PI layers, as shown in Table 1, an adhesive PI tape (Chukoh API-114A FR) and a PI spray (FCJ FC-114). The objective of this paper is also to uncover the effects depending on the different PI layer.

Table 1. Polyimide properties

Polyimide Thickness (µm) Ra (µm) Rz (µm)

Chukoh API-114A FR 20 1.167 3.77

FCJ FC-114 7.0 0.967 2.90

The later PI layer (FCJ FC-114) is manufactured by direct spraying onto the mechanical part with a spray distance about 20-30cm. After rotation of the sprayed sample by spin-coating process, a homogeneous surface without irregularities can be obtained. Right after spraying, this polyimide layer has to be cured by putting the mechanical part into the oven for 20 minutes at 200°C. After cooling down, the following process for laser sintering can be carried out.

The catalog specifications of the conductive ink we used in this study (NPS-J, Harima) are shown in Table 2. The characteristic resistivity value 3 µΩ·cm is obtained by after curing process specified in the catalog, 210-220°C x 60 min. Before the application of the ink, the polyimide surface must be cleansed with acetone to remove traces of grease. Consequently, the conductive ink is sprayed on the surface of the polyimide layer, and we rotate the sprayed surface by spin-coating process at 1400 Rpm to spread the ink on the surface. After this, a pre-curing process is carried out on a hot plate at 100°C during 1 minute. After that, the laser sintering process can be carried out to obtain a conductive trace with an average thickness of 40 µm. In our experiments different conditions for laser sintering are considered: feeding speed, laser focus, laser power and multiple sintering process.

Figure 2. Experimental samples fabrication

Table 2. Conductive Ink (NJP-S) properties

Solvent Washing fluid Conductivity (µΩ·cm) Viscosity (mPa · s) Silver particle size (nm)

Tetradecane Toluen 3 5 to 10 3 to 7 (60 wt%)

2.3 Measurement of test samples

For each laser sintering condition, three test samples are sintered and measured three times. The measured values are: resistance, width, thickness and cross-sectional area of the sintered line. The resistance is measured by using a conventional multi-meter. The other values and images taken from the test samples are carried out or measured by using an optical microscope at 10x resolution and the 3D images are reproduced by a 3D measurement software. From any cross-sectional area of the sintered trace, the width value is measured at the lowest peripheral point close to the polyimide surface, and the thickness is measured between the maximum and minimum peaks from the cross-sectional area curvature.

3. LASER SINTERING ON PI TAPE EXPERIMENT AND RESULTS 3.1 Laser sintering on PI tape varying Z-axis distance

As the laser beam is passing thru a lens, the distance from the lens to the surface may be related to the line thickness of sintered line. In order to uncover this relation, we carried out laser sintering tests with the different distance. For this experiment, the variation of Z-distance is from 38 mm to 42 mm, and the feeding speed and laser power are set as constant; F-240 and 10% of 1.6 W, as shown in Tabel 3. The test pattern is the straight line, and the length is 10mm.The polyimide surface used is the adhesive PI tape (Chukoh API-114A FR).

Figure 3 shows the results of the resistance against the laser beam sintering distance. The lowest resistance 7.8 Ω was obtained at 40 mm distance, apparently showing the best conductive trace. The approximate curve indicates the closer to 40mm has the best conductivity.

Figure 4(a) shows a visual observation with an optical microscope x10. The distance is 38mm and it appears that a clear line is obtained. Figure 4(b) shows a better contour and wider line sintered by 40mm distance, however, the center is apparently scorched.

Table 3. Sintering condition for Laser sintering on PI tape varying Z-axis distance.

Parameters Values Unit

Feed 240 [mm/minute]

Focus-object distance 38, 38.5, 39, 39.5, 40, 40.5, 41, 41.5, 42 [mm]

Laser power (Max1.6W) 10 [%]

38 38.5 39 39.5 40 40.5 41 41.5 42Laser sintering distance [mm]

7

8

9

10

11

12

13

14

Res

ista

nce

[Ω]

Sample 1Sample 2Sample 3Polynominal fit

Figure 3. Laser beam focus distance at F-240 and 10% Laser power

Figure 4. Optical microscope x10 image of the conductive trace at laser beam focus distance at F-240 and 10% Laser power

(a) 38mm:left (b) 40mm:right

3.2 Laser sintering results on PI tape at varying laser power

The purpose of the following test is to show the influence on the conductivity by varying the laser beam power value. In this test, the resistance and line width of the sintered circuits are measured for various values of the laser power (from 1% to 25% of 1.6W). Table 4 shows the sintering condition of the test.

Table 4. Sintering condition for conductive ink


Feed 240 [mm/minute]

Focus-object distance 40 [mm]

Laser power (Max1.6W) 1, 3, 5, 10, 15, 20, 25 [%]

Figure 5 shows the effects of the laser power on the resistance. As shown in Fig. 5, low laser power can sinter high resistance lines, and the resistance is inversely proportional to the laser power, e.g. the high resistance variation occurs from 1% to 3% laser power, on the other hand, from 5% to 25%, the conductivity value slightly goes up. As can be seen in the Fig. 6 and 7, 1% laser power can sinter a narrow line width and smaller cross-section area than the line sintered with 15% laser power, moreover, it appears that the relation between the laser power and the line width or cross-section area shows a linear property. However, the comparison of resistance with cross-section area does not have the linear property as it should be. At 1% laser power the resistance deviation between samples is higher, on the other hand, from 3% to 25% the relation between the resistance and cross-section area is linear. The sintering line by 1% laser power seems to be inappropriate process.

0 0.05 0.1 0.15 0.2 0.25Laser power [%] (Max 1.6W)

0

5

10

15

20

25

30

35

Res

ista

nce

[Ω]

Experimental dataCurve fit (exponential function)

Figure 5. Effects of laser power on resistance

0 5 10 15 20 25Laser power [%] (Max 1.6W)

150

200

250

300

350

400

450

Line

wid

th [µ

m]

Experimental dataPolynominal fit

Figure 6. Laser power influence on line width (Feed 240mm/min)

0 5 10 15 20 25Laser power [%] (Max 1.6W)

0.4

0.6

0.8

1

1.2

1.4

1.6

Cro

ss s

ectio

n ar

ea [µ

m2 ]

×104


Figure 7. Laser power influence on cross-section area (Feed 240mm/min)

Figure 8 (a) and (b) show the images of the optical microscope at 10x, and fig. 8(c)(d) show the minimum cross-sectional curvature of the images (a) and (b). It can be seen that the scorched area is obtained near the center of line and the silver nanoparticles are scattered in the periphery of the trace. Apparently the trace with the scorched area shows better properties (low resistance) and the PI layer is not compromised.

(a) (b)

0 100 200 300 400 500 600 700X axis [µm]

1280

1290

1300

1310

1320

1330

Y ax

is [µ

m]

0 100 200 300 400 500 600 700X axis [µm]

1270

1280

1290

1300

1310

1320

1330

Y ax

is [µ

m]

(c) (d)

Figure 8. Optical microscope image x10 (a) 1% laser power, (b) 15% laser power, minimum cross-section (c) 1% laser power, (d) 15% laser power.

3.3 Laser sintering results on PI tape at varying feeding speed

The purpose of the following test is to show the influence on the conductivity by varying feed-speed of the laser. In this test, the resistance and line width of the sintered circuits are measured for two feed-speeds; 120 and 240 mm/min at 10% laser power. Table 5 shows the condition of the test.

Figure 9 (a) and (b) show the difference of the laser sintering results at varying feeding speed; 120 and 240 mm/min. Figure 9 (a) has 253 µm line width and shows the clear trace with better contour than (b), which has 195 µm line width. The layer thickness remains the same value; 42 µm. Figure 9 (c)(d) show the cross-section of the line (a) and (b), respectively. This figure says that the lower speed can make the deeper scorched area and scatter the material to the periphery. In addition, the sprayed conductive ink is exposed to the heat of the laser more time; therefore, more ink is sintered at the periphery. In the case of the slow feeding, the width of the trace line became 50 µm wider than that of fast feeding.



Feed 120, 240 [mm/minute]



(a) (b)

0 100 200 300 400 500 600 700X axis [µm]

1270

1280

1290

1300

1310

1320

Y ax

is [µ

m]

0 100 200 300 400 500 600 700X axis [µm]

1280

1290

1300

1310

1320

1330

1340

Y ax

is [µ

m]

(c) (d)

Figure 9. Optical microscope image x10 (a) 120mm/min and (b) 240mm/min. (c) and (d) are the cross-section view of the most scorched section of the sintered trace from (a) and (b).

3.4 Laser multi-sintering of conductive ink on PI tape

Our aim here is to uncover the effect of multi-sintering for printing of the conductive ink. Table 6 shows the condition of the test. We repeated the laser sintering on the same trace three times.



Feed 240 [mm/min]



Sintering times 1, 2, 3 Cycles

Figure 10 shows the influence of multi-sintering on the same trace. As a result of the first laser sintering, the highest resistance value is obtained; 29.5 Ω. By second sintering, it appears that the conductive ink in the periphery was also cured, and the line width was wider than that of the first one, moreover, the conductivity was increased. However, the third laser sintering could not bring any big difference. The effect of the third sintering was quite similar as the second sintering, but the resistance value is slightly increased.

(a) (b) (c)

1 2 3Laser sintering time

20

25

30

35

Res

ista

nce

[Ω]


1 2 3Laser sintering times

4000

5000

6000

7000

Cro

ss s

ectio

n ar

ea [µ

m2 ]


(d) (e)

Figure 10. Multiple sintering at 1% Laser power, 240mm/min (a) first laser sintering, (b) second laser sintering, (c) third laser sintering, (d) resistance values and (e) cross-section area (µm²)

4. LASER SINTERING OF CONDUCTIVE INK ON SPRAYED PI LAYER

In this section, we show the results of the laser sintering of the conductive ink on sprayed PI layer (FCJ FC-114) with same conditions as the PI tape. The results say that the printed patterns did not have any conductivity. The trace had very small thickness (around 5 µm), plenty of unsintered regions (see Fig. 11). Only some sections of the whole trace had

conductivity, but it was a very high resistance and the deviation of the resistance between samples was large. Even though the PI tape and the sprayed PI layer showed similarities on roughness, the sintering results differ from each other. Apparently the conductive trace had no adhesion to this type of sprayed PI layer, probably due to some chemical incompatibility or low laser power for sintering.

Figure 11. Optical microscope 10x trace sintered on sprayed PI layer

5. CONCLUSION

The purpose of this study is the development of gear sensor system for health monitoring and failure detection. In this paper the effects of laser sintering of conductive ink under different conditions was uncovered, in order to create a smart sensor directly printed on gears by using the conductive ink. We attempted to print the conductive ink on two polyimide layers, which are on steel plates, and changed the Z-axis distance from the PI layer to the laser, the laser power and feeding speed for the laser sintering. Our test results say that the Z-axis parameter is not sensitive for the laser sintering in our system, and it appears that the laser power has an optimal point for the sintering. The high power can make a high conductive line, but wider and scorched lines resulting from the high power can be also obtained. In order to choose the appropriate laser power, we should evaluate the printed line on various criteria, such as width, cross-section, resistance ratio, and so on. In addition, according to the comparison of the sprayed PI layer with the PI tape under same laser sintering condition, we should find another sintering condition depending on the printing layer. Indeed, in order to obtain good conditions of the laser sintering of conductive ink for achieving low resistance and narrow lines on gear surfaces, we have to keep on conducting experimental research about this laser sintering. However, we are confident about the bright future of making the sensor system on gear surface by printing of the conductive ink, because of non-contact printing method, good accessibility to the root of gear tooth, expandability to multi-axis printers, and easy processing compared with another printing method, such as inkjet printers, dispenser type printers we attempted.

ACKNOWLEGEMENTS

The authors gratefully acknowledge the support by Machine Tool Engineering Foundation,A-218.

REFERENCES

[1] P. J. Dempsey et al, “Investigation of Current Methods to Identify Helicopter Gear Health”, Aerospace conference 2007 IEEE, (2007), pp.1-13.

[2] D. V. Mahindru and P. Mahendru, “Review of Rapid Prototyping-Technology for the Future”, Global Journal of Computer Science and Technology, Graphics & Vision, Vol. 13, Issue 4, Version 1.0, (2013), pp. 26-38.

[3] Ricardo Rodriguez Lopez, Daisuke Iba, Takahiro Kamimoto, Kazutoshi Yoshioka, Morimasa Nakamura, Ichiro Moriwaki, “Development of three-axis inkjet printer for gear sensors (Property evaluation of patterns by conductive ink printed on polyimide layer)”，Japan Society of Mechanical Engineering Annual Meeting 2015,, S1120505, (2015), pp.1-5.

[4] K. Yamazaki and K. Maekawa, “Laser Sintering Characteristics of Silver Nanoparticle Paste for Electronics Packaging”, Journal of Japan Laser Processing Society, Vol. 19, No. 3 (2012), pp. 206-211.