Keywords

1 Introduction

With the development of modern industry, high precision fabrication techniques of sensors and instrumentation tools is gaining more attention and becoming an essential research topic in various domains. Several research studies presented different fabrication techniques of sensors and instrumentation tools, along with different levels of complexity.

The study presented by Jéssica Santos Stefano et al. [1], has illustrated the manufacturing techniques of low-cost disposable electrochemical sensors for the possibility of using consumables as a base material for their construction providing attractive characteristics, such as simplicity, sustainability, and applicability in a single device. The strategies used to manufacture such sensors are screen & stencil printing, laser-scribing, and pencil drawing. These techniques do not require complicated methods nor expensive equipment; however, the fabrication steps must be strongly considered because they influence the performance of the final device. In the field of medicine & healthcare, Sudhanshu Nahato et al. [2] presented a feasibility study towards fabricating custom-designed surgical instruments for knee and hip replacement using metal additive manufacturing. To establish the feasibility, several tests were conducted where the additive manufacturing-built materials were compared with the traditionally manufactured material. Another study presented in [3], illustrated the design of a simple and high-performance flexible strain sensor, based on gold nanoparticles and a polyimide substrate. The fabrication included drop-casting and high temperature annealing. The sensor revealed high linearity with low power consumption. It will be used for detection of human motion and subtle strain.

Several other technologies such as machining techniques are highly recommended, as they provide the desired accurate dimensions, where the accuracy of a workpiece can be improved by surface measurement and compensation machining. The study presented by Zao Zao et al. [4], introduced the design of an on-machine measurement device based on a chromatic confocal sensor, it can inspect workpiece surfaces with larger depths and slopes. The machine tool is equipped with three linear hydrostatic axes (X, Y and Z axes) and two rotational axes, the chromatic confocal sensor is installed on the rotational axis platform.

The objective of this study is to present the high precision machining technique employed to fabricate the reflector part of a 3D fiber-optic linear displacement sensor. This sensor is targeted to measure the linear displacement with high resolution, for an axis performing a simultaneous motion, of rotation and translation at the same time.

2 Sensor Principle

The sensor consists of two fiber-optic probes associated to a highly reflective surface. Each probe has one center emission fiber and four reception fibers placed around the emission fiber. The sensor performance when it is associated to a planar surface has been already analyzed [5, 6, 7]. In classical configuration, the emission fiber placed in the center emits light on a flat reflective surface. The light reflected by the surface is injected in the reception fibers and guided to a PIN photodiode. The voltage output of the sensor is a function of the mirror displacement (Fig. 1). The mirror displacement is millimetric, the emission fiber diameter is approximately 460 µm, the reception fiber diameter is 240 µm and the space between these two is 30 µm.

Fig. 1.
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Fiber-optic sensor.

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When translating the flat mirror perpendicularly to the probe axis, the sensor response curve is as shown in Fig. 2.

Fig. 2.
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Response curve of the fiber-optic displacement sensor.

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As seen from Fig. 2, the sensor response curve consists of four zones. The first one is the dead zone, where the reception fibers cannot collect the reflected light because of the space between the emission and the reception fibers. Zones 2 and 4 are strongly non-linear with poor resolution. Zone 3 is the most interesting working zone because of its high sensitivity and linearity. On the other hand, this zone has a limited measurement range (less than 200 µm). For this reason, and in order to increase the measurement range the displacement direction of the flat mirror can be different from the normal vector orientation of its surface, resulting in the multiplication of the nominal range value by (sin ε)−1 factor, where ε is the inclination angle related to the grating axis [5]. This inclined mirror configuration, has been duplicated to increase more the linear measurement range of the sensor, in this configuration two fiber-optic probes will be needed to avoid the transition between two successive inclined step as shown in the following Fig. 3. The dimensions for the length, height and the angle for this grating is fixed with a MATLAB model, that evaluates the sensor performance as a function of these dimensions.

Fig. 3.
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Long range sensor principle.

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The current study interests in the linear displacement measurement of an axis performing a rotational motion. To satisfy this criteria, the inclined mirror configuration is replaced with 3D cones assembled grating [8], which gives the result illustrated in the following figure (Fig. 4).

Fig. 4.
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3D cones assembled grating configuration.

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The following paragraphs will show the machining technique employed to fabricate the cones assembled grating.

3 High Precision Machining Technique

The manufacturing process has been done using a high precision turning machine along with single crystal diamond tool; and that allows to get sub-micrometric dimensional precision, along with a surface roughness of several nanometers. The performance of the surface quality are influenced by the quality of the cutting tool. However, the single crystal diamond tool can only be used in machining for certain materials; due to the chemical reaction, which occurs between the carbon of the diamond with the carbide substances (Fe, Ti, etc.). For that reason, aluminum alloy 2017 was chosen to make the 3D cones assembled grating. When a ductile material like the aluminum is machined with a diamond tool, the lubrification facilitates the cutting process, and used to eliminate the chip, so that it won’t damage the machined surface. The geometric characteristics of the machined object define the relative movement between the tool and the sample to machine. Therefore, the high precision machining process of the cones assembled grating, which is the key element of the fiber-optic displacement sensor fabricated in this research work, was done in Roberval laboratory, because this lab is equipped with a high precision turning machine, which allows to have a micrometric precision for each step of the conical grating, in addition to a high surface quality (20–5 nm roughness).

3.1 The High Precision Turning Machine

The high precision turning machine is a prototype with two perpendicular displacement axes and one spindle with magnetic bearing. It has been designed at the end of 1980’s by the European society of Propulsion (SEP) and was targeted to produce aspherical surfaces in various domains. It was transferred to Roberval laboratory in 1994 and fixed in an air-conditioned room at ±21 ℃ (Fig. 5), this machine was fixed on a concrete floor slab isolated from the main slab [9].

Fig. 5.
figure 5

The high precision turning machine [10].

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3.2 The Machining Strategy of the Cones Assembled Grating

Firstly, the cones assembled grating have been geometrically modelled on MATLAB. The model aims to get the sensor performance, in terms of resolution and measurement range as a function of the cones’ assembled grating dimensions mentioned in the following figure (Fig. 6).

Fig. 6.
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The geometric dimensions of the conical step.

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Where:

  • l: the step length (µm) was fixed to 1573 µm.

  • hp: the step segment (µm) was fixed to 194 µm.

  • ɛ: the step angle (°) was fixed to 4.62°.

  • γ: the angle at the bottom of the step (°) to 130°.

Each step on the cones’ assembled grating has been machined with several cuts at several different depths, to get the optimal geometric profile (Fig. 7).

Fig. 7.
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Geometric profile of each step for machining.

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Where:

  • X: Axe of the tool holder slide of the machine.

  • Z: Axe of the spindle holder slide of the machine.

For the prototype considered, 10 steps were fabricated for the cones assembled grating. Every step is machined with 7 successive cuts. The last finishing cut allow to get a high surface quality (Fig. 8). The depth of the first six cuts was fixed to 18 µm, 10 µm for the seventh cut with a speed (Va = 500 µm/s). For the last cut, the depth was fixed to 5 µm, along with a speed of (Va = 50 µm/s), to insure high surface roughness. Firstly, the tool follows the trajectory from point A to point B, then point B to point C.

Fig. 8.
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Machining strategy.

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The first step of the conical grating fabrication consisted of turning a cylinder, whose diameter is 55 mm, firstly, with a carbide tool in order for it to be centered on the spindle axis, and in a second time with a single-crystal diamond tool, with a radius of curvature (Rc = 2 mm), this tool allows to get a polished-mirror surface. The following figure shows the tool orientation with respect to the spindle, which is the axis of rotation. As seen from the following figure, the initial workpiece is a cylinder with a diameter of 55 mm; this cylinder has been straightened and turned (Fig. 9).

Fig. 9.
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Fabricated cones assembled grating.

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The following figure presents the fabricated cones assembled grating on the cylinder (Fig. 10).

Fig. 10.
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Fabricated cones assembled grating.

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The roughness obtained on the cones’ assembled grating is nanometric and high micrometric dimensional precision has been also obtained.

4 Conclusion

This research paper presents the machining strategy used to fabricate the element reflector for a fiber-optic linear displacement sensor. This sensor will be used to measure the displacement of an axis in rotation, the reflector element is a conical grating made from alluminium alloy.