Development of three-axis inkjet printer for gear sensors

1. 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
*1
Kyoto 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

2. 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)

3. 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.

4. 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 42
Laser sintering distance [mm]
7
8
9
10
11
12
13
14
Resistance[Ω]
Sample 1
Sample 2
Sample 3
Polynominal 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

5. 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
Parameters Values Unit
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 0.25
Laser power [%] (Max 1.6W)
0
5
10
15
20
25
30
35
Resistance[Ω]
Experimental data
Curve fit (exponential function)
Figure 5. Effects of laser power on resistance
0 5 10 15 20 25
Laser power [%] (Max 1.6W)
150
200
250
300
350
400
450
Linewidth[µm]
Experimental data
Polynominal fit
Figure 6. Laser power influence on line width (Feed 240mm/min)

6. 0 5 10 15 20 25
Laser power [%] (Max 1.6W)
0.4
0.6
0.8
1
1.2
1.4
1.6
Crosssectionarea[µm2
]
×10
4
Experimental data
Polynominal fit
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)
0 100 200 300 400 500 600 700
X axis [µm]
1280
1290
1300
1310
1320
1330
Yaxis[µm]
0 100 200 300 400 500 600 700
X axis [µm]
1270
1280
1290
1300
1310
1320
1330
Yaxis[µm]
(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.

7. 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.
Table 5. Sintering condition for conductive ink
Parameters Values Unit
Feed 120, 240 [mm/minute]
Focus-object distance 40 [mm]
Laser power (Max1.6W) 5 [%]
(a) (b)
0 100 200 300 400 500 600 700
X axis [µm]
1270
1280
1290
1300
1310
1320
Yaxis[µm]
0 100 200 300 400 500 600 700
X axis [µm]
1280
1290
1300
1310
1320
1330
1340
Yaxis[µm]
(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).

8. 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.
Table 6. Sintering condition for conductive ink
Parameters Values Unit
Feed 240 [mm/min]
Focus-object distance 40 [mm]
Laser power (Max1.6W) 5 [%]
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 3
Laser sintering time
20
25
30
35
Resistance[Ω]
Experimental data
Polynominal fit
1 2 3
Laser sintering times
4000
5000
6000
7000
Crosssectionarea[µm2
]
Experimental data
Polynominal fit
(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

9. 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.
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