# Optical Localization and Tracking Method of a Mobile Micro-Conveyor Over a Smart Surface

Saly Malak, Hani Al Hajjar, Erwan Dupont, Muneeb Ullah Khan, Christine

Prelle, Frederic Lamarque

► To cite this version:

Saly Malak, Hani Al Hajjar, Erwan Dupont, Muneeb Ullah Khan, Christine Prelle, et al. Optical Localization and Tracking Method of a Mobile Micro-Conveyor Over a Smart Surface. IEEE Sensors Journal, 2021, 21 (9), pp.10618-10627. 10.1109/JSEN.2021.3062731. hal-03331093

HAL Id: hal-03331093

https://hal.science/hal-03331093v1

Submitted on 7 Apr 2025

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# Optical Localization and Tracking Method of a Mobile Micro-Conveyor Over a Smart Surface

Saly Malak, Hani Al Hajjar, Erwan Dupont, Muneeb Ullah Khan, Christine Prelle, and Frederic Lamarque

**Abstract**—In this paper, a remote optical method to localize and track a mobile micro-conveyor is presented. The principle of the proposed method consists of detecting a laser beam that is reflected by the micro-conveyor and analyzing this optically received signal to obtain the corresponding positions of the micro-conveyor. The main elements of the system are a corner cube retroreflector, a steering mirror, and a beam splitter. The corner cube retroreflector is placed onto the micro-conveyor in order to allow the optical signal to be reflected back in its incident direction. The steering mirror aids in scanning the surface with the laser. The beam splitter is used to guide the reflected laser beam towards a quadrant photodiode, which interprets the optical signal to corresponding output voltages. In this work, a micro-conveyor is successfully localized and tracked with a resolution better than 55 µm, over a surface of 100 mm × 100 mm, with a displacement velocity of 30 mm/s.

**Index Terms**— High resolution tracking, laser diode, MEMS mirror, micro-conveyor, optical localization and tracking, quadrant photodiode.

<figure><img src="image_2.png" /></figure>

## I. INTRODUCTION

POSITIONING and tracking of moving targets is often required in several fields of engineering and science. Many studies have been devoted to localize and track objects or individuals at different scales, ranging from macro to micro scale, depending on the application. Most research studies focus in increasing the localization accuracy within the same working area or to maintain a high localization accuracy while increasing the working area. These two parameters are inversely proportional. The current existing methods that perform localizing and tracking are mainly based on radio waves or optical systems [1], [2]. Radio Frequency (RF) technologies are widely used for localization purposes, because

This work was supported in part by the French Government under the framework of the Labex MS2T through the Program Investments for the Future managed by the National Agency for Research under Grant ANR-11-IDEX-0004-02, in part by the Picardie region under the Microcosm Project, and in part by the Europe/Fonds européen de développement régional (FEDER) with the Euro-peak funds for regional developments at Picardie region of France. The associate editor coordinating the review of this article and approving it for publication was Prof. Hsin-Ying Lee. (Corresponding author: Saly Malak.) The authors are with the Roberval Laboratory, CNRS, Sorbonne Universités, Université de Technologie de Compiègne, 60203 Compiègne, France (e-mail: saly.malak@utc.fr; hani.al-hajjar@utc.fr; erwan.dupont@utc.fr; muneeb-ullah.khan@utc.fr; christine.prelle@utc.fr; frederic.lamarque@utc.fr).

they have large coverage areas and require less hardware compared to other technologies. Wireless Fidelity (Wi-Fi) is commonly used in positioning systems since the infrastructure already exists and no additional hardware is needed. It is a set of wireless communication protocols covered by the standards of the IEEE 802.11, that deliver the communication range up to 1 km instead of formerly 100 m [3]. The principles used for localization are based on measurements of the Received Signal Strength (RSS) or Time of Arrival (TOA) or Angle of Arrival (AOA) between an RF base station and an RF receiver. Usually the mobile target holds the RF receiver. In the study [4], localization is ensured using a single RF receiver and the method is based on analyzing the AOA and TOA of the received multipath signals considering the direct path as a reference. The obtained localization error reached is around 1.5 m, for a fixed position of the receiver and random target positions distributed in a space size of 7.5 × 12 m². The maximum accuracy reported in recent studies was about 23 cm, in an indoor office environment, using an array of antennas and by analyzing the AOA information [5]. Radio Frequency IDentification (RFID) is another technology where the target holds an RFID reader and receives radio signals coming from multiple tags that are distributed in the environment at known positions [6]. Moreover, the position of the target is associated to the detected RFID tags via binary detection method [7]. Other approaches measure the RSS [8] or phase shift [9] of the received signals, or the
combination of the two measurements together [10]. However, the positioning accuracy using the RFID technique has resulted in a low accuracy of few centimeters order for a range up to hundreds of meters. Radio waves have the advantage of penetrating obstacles like walls, buildings, and human bodies. In addition, the components of such methods are free from line-of-sight requirement. However, RF methods suffer from interference, electromagnetic noise, and low accuracy [2]. Alternately, to solve aforementioned problems in RF methods, researchers focused on optical methods, such as Visible Light Communication (VLC) techniques. Apart from its data communication and illuminations application, this technique can also be used for localization purposes [11], [12]. The target mounted with a VLC receiver, for example a photodiode, is localized when receiving signals from two or more Light-Emitting Diodes (LEDs) with known positions [13]. In [14], a single LED is mounted on the ceiling to communicate with several photodiodes that are placed on top of the target. The VLC technique provides millimeter level accuracy within a range of a few meters.

Alternatively, vision-based positioning system is another optical detection method, which is often employed for localization tasks. It uses a camera to localize the position of the pixels corresponding to the target's position in the image. By calibrating the camera and the image, the real position of the target can be obtained [15]. Two scenarios can be deployed, either the camera is set in motion and hence needs to be localized [16] or the camera is set in a fixed position and is used to localize the mobile targets within the field of view [17]. In [18], a sub-micrometric positioning resolution is reported using a pattern attached to the robot being filmed by a CCD camera. However, the field of view captured by the camera has a limited dimension. The drawback of the vision-based localization system is that the resolution of positioning is inversely proportional to the field of view. Several cameras are required to obtain localization of high resolution over large area, which in turn makes the system more complex and expensive. In addition, there are systems that provide micrometer-level localization resolution for specific applications in the biomedical context, using methods such as magnetic resonance imaging, X-ray imaging and ultrasonography [19]. In this work, we propose a high resolution optical localization and tracking method of a micro-conveyor moving over a large 2D electromagnetic smart surface [20], in the context of micro-factory. The main advantage of the micro-factory is saving large quantities of resources such as space, energy, material, and time [21]. A supervision system is mandatory for such a micro-conveyance system, first, to verify that the micro-conveyor has not lost its trajectory for any reason such as friction, actuation failure, collision with another micro-conveyor, etc. Secondly, and more importantly to monitor and ensure that the micro-conveyors have reached their final positions with high precision.

The developed remote optical localization method is based on scanning the conveying surface using a laser beam and a precise steering mirror. The mirror reflects the beam towards the conveying surface where the micro-conveyor, equipped

<figure><img src="image_3.png" /><figcaption>Fig. 1. Localization and tracking system architecture.</figcaption></figure>

with a Corner Cube Retroreflector (CCR), is present. The CCR reflects back the laser beam towards the optical detector for analysis.

The paper is organized as follows: section II describes the proposed localization and tracking method. In addition, it presents the system's architecture and the tracking algorithm. The geometrical modeling and dimensioning of our localization method is described in section III. The experimental setup from optical scheme inspection is explained in section IV. Experimental results are discussed in section V to validate the proposed method. In the end, conclusion and perspectives are provided.

## II. PROPOSED LOCALIZATION AND TRACKING METHOD

### A. Scanning and Localization Phase Description

The proposed localization and tracking method of a mobile micro-conveyor is based on the detection and analysis of a light beam reflected from the micro-conveyor. In addition, scanning the conveying surface, where the micro-conveyor moves, is a preliminary step to localize the micro-conveyor. A 2D steering mirror scans the conveying surface by reflecting the laser beam emitted by a fixed laser diode. The mirror continues to rotate until the laser beam reflected from the laser reaches the micro-conveyor. The CCR is fixed onto the surface of the micro-conveyor to reflect the incident laser beam towards a fixed optical Quadrant PhotoDiode (QPD). The CCR is a trihedral prism made up of three adjacent, mutually orthogonal and reflective intersecting surfaces. The reflected beam is parallel to the incident beam and shifted by a distance that depends on the position at which the incident beam touches the entrance surface of the CCR. The maximum value of the shifted distance is equal to the diameter of the CCR (denoted $D_{CCR}$), in the case where the incident beam hits an edge of the CCR, see enhanced view of CCR in Fig. 1.

The micro-conveyor is localized when the laser beam reaches the CCR, placed at the center of the micro-conveyor. In such a scenario, there is no need for triangulation with multiple laser beams to determine the position of the micro-conveyor, since the reflected beam has the same orientation as in the incident direction. Thus, the micro-conveyor's position onto the surface can be calculated from the steering mirror orientation.
Figure 1, illustrates the proposed architecture for scanning and localization system, having the following sequence: First, the light beam from the laser source is reflected by the steering mirror, and is directed towards the conveying surface (steps 1, 2, and 3 or 3'). Each orientation of the steering mirror corresponds to a spot position onto the conveying surface. In a first case, the orientation I of the steering mirror does not correspond to a beam reaching the mobile target (i.e., micro-conveyor), thus beam 3' is not reflected back towards the QPD. Therefore, the steering mirror continues to scan the surface until orientation II, where beam 3 reaches the target. Then, the passive CCR reflects the beam towards the optical receiver (step 4). Reflected light beam 4 is transmitted towards a fixed QPD, through the beam splitter (step 5 and 6). The beam splitter is an optical component used to divide incident light into reflected and transmitted beams with a specific ratio (in our case 50%/50%). Finally, the laser beam 6 reaching QPD is interpreted and the target, i.e., the micro-conveyor, is localized. Upon localization, the tracking phase is launched. The following section describes the proposed tracking method.

## B. Tracking Phase Description

When the localized CCR situated on top of the mobile micro-conveyor moves, the QPD can detect a variation in the position of light spot of beam 6 over its active surface.

The tracking of the mobile target is ensured by keeping the light spot centered onto the QPD surface, through controlling the steering mirror orientation. To do so, the proposed tracking method consists of using a light spot, onto the conveying surface, having a diameter ($D_{beam3}$) larger than that of the CCR ($D_{CCR}$). In such a way, the QPD detects the movement of the CCR inside the light spot of beam 3 in the conveying surface. In this work, we used the QPD (reference: QP50-6-TO8) purchased from 'First Sensor' corporation. It generates the following output voltages [22].

$$V_{SUM} = [V_{I} + V_{II} + V_{III} + V_{IV}] \quad (1)$$

$$V_{B-T} = [(V_{III} + V_{IV}) - (V_{I} + V_{II})] \quad (2)$$

$$V_{L-R} = [(V_{II} + V_{III}) - (V_{I} + V_{IV})] \quad (3)$$

where, $V_i$ ($i = I, II, III$, and $IV$) corresponds to voltage generated by each quadrant, which is proportional to the received light intensity onto the corresponding quadrant. $V_{SUM}$ is the value corresponding to the summation of these four voltages. It indicates whether the QPD is exposed to the laser beam or not. A threshold value ($V_{Th}$) is introduced to test the reception of the light beam 6. If $V_{SUM}$ is greater than $V_{Th}$, reception has occurred. The threshold value depends on the electronic noise and the ambient light noise. $V_{L-R}$ is equal to the subtraction of the output voltages of the left and right quadrants. Similarly, $V_{B-T}$ is the difference between the voltages generated by the bottom and top quadrants. Figure 2, shows four different examples of the position of the light spot (beam 6) received at the active surface of the QPD, providing the corresponding analysis of the QPD output signals and the light spot position.

In case 1, the QPD is not illuminated by light beam 6 reflected by the CCR. $V_{SUM}$ has a relatively low value.

<table><caption>Fig. 2. QPD with different light spot positions.</caption><tr><td>QPD</td><td>Case 1: $V_{B-T} \approx V_{L-R} \approx 0$ [$V_{SUM} < V_{Th}$]<br/>No information on the light spot position</td></tr><tr><td>light spot<br/>of beam 6</td><td>Case 2: $V_{B-T} \approx V_{L-R} \approx 0$ [$V_{SUM} > V_{Th}$]<br/>The light spot is near the center of the QPD</td></tr><tr><td>$D_{beam6}$</td><td>Case 3: $V_{B-T} < 0$ &amp; $V_{L-R} \approx 0$ [$V_{SUM} > V_{Th}$]<br/>The light spot shifted to the top</td></tr><tr><td>$V_{L-R}$<br/>$V_{B-T}$</td><td>Case 4: $V_{B-T} \approx 0$ &amp; $V_{L-R} > 0$ [$V_{SUM} > V_{Th}$]<br/>The light spot shifted to the right</td></tr></table>

For the other three cases, the QPD receives light beam 6 and $V_{SUM}$ increases in value. Once $V_{SUM}$ becomes greater than $V_{Th}$, $V_{L-R}$ and $V_{B-T}$ values undergo analysis to determine the light spot center's position. For case 2, $V_{L-R}$ and $V_{B-T}$ values are near zero, which means that the light spot is near the center of the QPD. It is very difficult to reach exact zero value of $V_{L-R}$ and $V_{B-T}$ for a centered position of the light spot due to the electronic and ambient light noises, and the fact that the spot cannot have a flawless symmetric circular shape. In case 3, the reflected light spot shifts from the center to the top which decreases $V_{B-T}$. For a light spot at the middle right, as seen in case 4, $V_{L-R}$ becomes positive and $V_{B-T}$ remains near to zero. The obtained values of $V_{L-R}$ and $V_{B-T}$ give a direct information of the CCR's position inside the light spot of beam 3. Next, the steering mirror is re-oriented in accordance with these values, in such a way to return the light spot of beam 3 at the center of the CCR. As a result, the latter remains centered inside the movable light spot of beam 3 for any new position of the micro-conveyor on the surface.

The ratio between the displacement velocity of the moving micro-conveyor, relative to the response time of the tracking system, defines the distance at which the light spot of beam 3 and 6 will be shifted from the center of the CCR and the QPD, respectively. The faster the micro-conveyor moves, compared to the system tracking speed, the more the light spots will be shifted away from their original centered position. For a velocity of the micro-conveyor greater than a maximum value, noted $V_{max}$, the control system fails to track the micro-conveyor, where $V_{max}$ is written as following:

$$V_{max} = \frac{d_{max}}{t_{R}} \quad (4)$$

where, $t_R$ is the response time of the tracking system. It is the interaction time between the QPD and the steering mirror that ensures the tracking of the micro-conveyor. Parameter $d_{max}$ is the distance traveled by the CCR from its initial centered position to the edge of the light spot of beam 3, [see Fig. 3]. $d_{max}$ is written as function of $D_{beam3}$ and $D_{CCR}$ as follows:

$$d_{max} = \frac{D_{beam3} + D_{CCR}}{2} \quad (5)$$

In conclusion, the maximum value ($V_{max}$) of the detectable micro-conveyor's velocity increases by reducing the $t_R$ value and by increasing the diameter of the light spot illuminating the conveying surface. In the following section, a model for
<figure><img src="image_4.png" /><figcaption>Fig. 3. Position of the CCR inside the optical spot.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 4. Schematic representation of the optical system for a general case where the light spot is at the positive abscissa of the surface.</figcaption></figure>

our system is proposed to dimension the system conveniently to fulfill the aim of our work.

### III. GEOMETRICAL MODELING OF THE ILLUMINATING SYSTEM

#### A. Diameter of the Light Spot Illuminating the Conveying Surface

Figure 4 is a representation of the system used to calculate the light spot diameter illuminating the conveying surface, the maximum working distance, the system sensitivity, and resolution. Due to 2D symmetry, this study is conducted along one axis and the solution is extended to other axis. Furthermore, we assumed for the modeling that the laser spot is a perfect circle. Top left part shown in Fig. 4 shows the static part composed of the convergent lens and the incident beam light coming from the laser source. On the top right, the actuated, i.e., mobile part, is composed of the steering mirror that reflects the light towards the conveying surface in different directions. The steering mirror has an angle of $\theta_x$ with respect to the parallel plane of the conveying surface. For real application conditions, $\theta_x$ is between 0 degree and 90 degrees, and equal to:

$$ \theta_x = \theta_{x0} \pm \Delta\theta_x \qquad (6) $$

where, $\Delta\theta_x$ is the angle variation of the steering mirror and $\theta_{x0}$ is the initial orientation. Moreover, when the steering mirror rotates by an angle $\Delta\theta_x$, the reflected beam moves by an angle $2\Delta\theta_x$ [see Fig. 4].

The diameter of the spot illuminating the conveying surface ($D_{beam3}$) can be extracted in function of the algebraic distance $OA$ and $OB$, using the geometric model. It can be expressed as follows:

$$ \begin{aligned} D_{beam3} &= \overline{BA} = \overline{OA} - \overline{OB} \\ &= \text{Hei} \times [\cot(2\theta_x - \alpha) - \cot(2\theta_x + \alpha)] \end{aligned} \qquad (7) $$

where, B and A are the two point extremities of the light spot and Hei is the elevation of the steering mirror ($Hei = OF'_1$). The normal projection of the focal point $F'_1$ on the conveying surface corresponds to the point denoted as O. $\alpha$ is the incident aperture angle with respect to the steering mirror and it is written as:

$$ \alpha = \arctan\left(\frac{D_{beamS}}{2 \times O_1 F'_1}\right) \qquad (8) $$

From equations (7) and (8) the spot size obtained on the conveying surface ($D_{beam3}$) depends on the width of the source beam ($D_{beams}$), the focal length value of the convergent lens ($O_1F'$), the elevation of the steering mirror with respect to the conveying surface ($OF'_1 = Hei$), and the angle of rotation of the steering mirror ($\theta_x$). By adjusting these parameters, we can reach the desired beam diameter, which are chosen according to the CCR's dimensions.

#### B. Maximum Coverage Distance

In the real case, we consider the steering mirror has an angular rotation range, where $\Delta\theta_x$ is limited between two constant values:

$$ \Delta\theta_{xmin} < \Delta\theta_x < \Delta\theta_{xmax} \qquad (9) $$

The maximum distance that the spot can scan over the conveying surface is denoted as ($L_{max}$). It is the distance between the maximum position of point in the positive abscissa ($A_{max}$) and the maximum position of point in the negative abscissa ($B_{max}$).

$$ OA_{max} = Hei \times \cot(2\theta_{x0} + 2\Delta\theta_{xmax} - \alpha) \qquad (10) $$

$$ OB_{max} = Hei \times \cot(2\theta_{x0} + 2\Delta\theta_{xmin} + \alpha) \qquad (11) $$

Thus, $L_{max}$ is equal to:

$$ L_{max} = OA_{max} + OB_{max} \qquad (12) $$

#### C. Sensitivity and Limit of Resolution

Spot detection sensitivity (S) is an important parameter since it allows to observe the variation of the light spot position of beam 3 for each respective angular variation of the steering mirror. The spot displacement is denoted by $\overline{x_0x}$, where $x_0$ and $x$ are the center points of the spot for the initial and the considered orientation at a given moment, respectively. $\overline{x_0x}$ is equal to:

$$ \begin{aligned} \overline{x_0x} &= \frac{\overline{OA} + \overline{OB}}{2} - \overline{Ox_0} \\ &= \frac{\text{Hei} \times [\cot(2\theta_x - \alpha) + \cot(2\theta_x + \alpha)]}{2} - \overline{Ox_0} \end{aligned} \qquad (13) $$

where, $\overline{Ox_0}$ depends on the initial orientation of the steering mirror ($\theta_{x0}$).
By deriving the above equation, we obtain:

$$ \frac{d(\overline{x_0x})}{d(\theta_x)} = \frac{Hei}{2} \times \frac{d([\cot(2\theta_x - \alpha) + \cot(2\theta_x + \alpha)])}{d(\theta_x)} \quad (14) $$

Thus, the sensitivity (S) is equal to:

$$ S = \frac{d(\theta_x)}{d(\overline{x_0x})} = \frac{1}{Hei \times [2 + \cot^2(2\theta_x - \alpha) + \cot^2(2\theta_x + \alpha)]} \quad (15) $$

The resolution over the conveying surface can be deduced using (15) and the angular resolution of the steering mirror (Resθx). Its equation is represented as follows:

$$ Res_{surface} = \frac{Res_{\theta_x}}{S} \quad (16) $$

A high sensitivity and resolution ensures an accurate detection of the displacement of the micro-conveyor. From the equations (15) and (16), we can state that the sensitivity and resolution is not constant over the conveying surface and depends on the variable Δθx of the steering mirror. However, for small values of Δθx, it gives the best sensitivity and resolution. The more the angle of the steering mirror increases, the more the sensitivity and resolution degrade.

## IV. EXPERIMENTAL SETUP

In order to validate our developed sensor, we have adopted the smart surface, developed at the Robertal laboratory [23]. The smart surface has a dimension of 150 mm × 150 mm. The conveyance is based on the electromagnetic actuation principle. The interaction between magnet arrays integrated into the micro-conveyor and electrical wires placed into the conveying surface, being fed by supplied currents, makes the micro-conveyor moves thanks to Lorentz force [24]. Among possible steering mirror, we have chosen to scan the surface with a 2D electrostatic MicroElectroMechanical System (MEMS) mirror to benefit from its high operating speed, its high accuracy, and its small chip size. The MEMS mirror developed at Mirrorcle Technologies Inc., is about a millimeter in diameter. It can operate over a wide bandwidth from dc to several kHz [25]. This micro-mirror is driven based on the Bias-Differential Quad-channel (BDQ) principle. Moreover, it rotates in two orthogonal axes, by applying four analog voltages (VX+, VX-, VY+, and VY-) to its electrostatic comb-drive actuators based on the following formulas:

$$ V_{X\pm} = V_{bias} \pm \frac{V_{diffx}}{2} \quad (17) $$

$$ V_{Y\pm} = V_{bias} \mp \frac{V_{diffy}}{2} \quad (18) $$

where, Vbias is the specific voltage value corresponding to the rest position of the mirror (i.e., Δθx = Δθy = 0°) that is provided in its datasheet. The differential voltages of x- and y-axis (Vdiffx and Vdiffy) are proportional to Δθx and Δθy, respectively:

$$ \Delta\theta_{x(y)} = K_{x(y)} \times V_{diffx(y)} \quad (19) $$

where, Kx, Ky are the linearity factors. In this work, a MEMS mirror of 1700 μm diameter, selected to perform the scanning task. It has a maximum value of Δθx and Δθy of 5.21 degrees and 5.33 degrees, respectively.

A PIC microcontroller communicates with the MEMS mirror driver and sends analog voltages (VX+, VX-, VY+, and VY-) via the serial peripheral interface (SPI) protocol. More precisely, in order to work in a user interface, a LabVIEW code associated with a National Instrument Data Acquisition control system (NI-DAQ) is implemented in addition with the PIC microcontroller, which receives the commands generated by the DAQ card. Then, the analog-to-digital converter (ADC) of the PIC microcontroller converts these commands into the desired voltages required by the MEMS driver. By sending a series of consecutive voltage pairs (VX+, VX-) and (VY+, VY-), different scanning patterns (spiral or rectangular shapes) can be generated and projected onto the conveying surface. A spiral pattern is chosen for its smoothness to cover the scanning surface. Such a pattern is more suited to the dynamic behavior of the micro-mirror where the high frequencies of the angular variation are avoided compared to the case of the rectangular scanning.

Figure 5 shows the developed optical localization and tracking system.

As mentioned above, the laser beam trajectory starts with a laser beam emitted by a collimated laser diode (light beam 1) directed towards the (50%/50%) beam splitter. The useful light is transmitted towards the MEMS mirror (light beam 2). The reflected light, by the beam splitter, is considered lost. The lost laser beam is blocked by a beam absorber, for security reasons, in order to avoid disturbance in the working environment, see Fig.5. Next, the MEMS mirror directs the light towards the conveying surface (light beam 3). However, due to the fact that the MEMS mirror cannot be implemented on top of the conveying surface easily, a fixed mirror is used to direct the light towards the conveying plane. There exist a geometric relation between the conveying surface and the plane parallel to the MEMS mirror. This can be calculated by applying holography method. It is a projectivity mapping from one plane to another, where any point in the conveying plane can be mapped and corresponds to a point of the conveying surface and vice versa, [26]. In figure 5, the light beam reaches the CCR, which means that a reflection will occur (light beam 4) and be directed towards the QPD. The focal length and position of lens 1 was chosen accordingly to the desired diameter of the light spot projected onto the conveying surface depending on the diameter of the CCR. A second lens 2 is placed in front of the QPD in order to obtain a value of Dbeam6 that fits the QPD's active surface. Due to the configuration of our system, lens 2 should be divergent since it provides the desired beam width at short distance with the QPD. To make the system more compact and resistant to ambient light, the QPD is enclosed in a casing to attenuate the ambient light. In figure 5, this casing is not placed in order to provide a better view of the QPD. As seen in the figure, a second light spot is observed onto the surface with a fixed position and low power intensity. In fact, there is a fixed integrated glass window in front of the MEMS mirror
<figure><img src="image_6.png" /><figcaption>Fig. 5. Photography of the control system.</figcaption></figure>

for protection purposes, which causes this noisy reflection. In order not to be disturbed by this glass reflection, the fixed mirror is oriented in such a way to place the noise spot's position at the edge of the surface, where the micro-conveyor cannot reach.

Three main factors cause significant loss of optical power in such a configuration. First factor results from the beam splitter, where half of beam 1 and half of beam 5 are lost since they are not used in our system. Secondly, a portion of beam 3 is lost due to the fact that the diameter of CCR is smaller than $D_{beam3}$. Third, the small diameter of the MEMS, which acts as a diaphragm for beam 2 and beam 4, resulting in the loss of a portion of the optical power of beam 2 and beam 4, as seen in Fig. 5. As a result, a high power laser diode is mandatory for such system. In this work, we adopted the red laser diode L660P120 from Thorlabs. Its output optical power is regulated using the TLD001 controller by sending the desired operating current to the laser diode. The maximum optical power of the laser is 120 mW. However, for safety reasons, we worked with a lower initial optical power source that is sufficient to receive enough optical power on the QPD. In our case, an initial

<figure><img src="image_7.png" /><figcaption>Fig. 6. Quadrants output voltages during scanning, localization and tracking phases.</figcaption></figure>

power source of 60 mW resulted in 21.2 mW of optical power received over the smart surface. This value is measured by an optical power meter placed in front of the mobile light spot received over the conveying surface. The noise spot shown in Fig. 5 has an optical power of 0.8 mW. After the reflection provided by the CCR, the optical power received on the QPD is equal to 0.3 mW. This value is able to generate a significant value $V_{SUM}$ during the localization and tracking phases.

## V. EXPERIMENTAL RESULTS

The experiment starts with the scanning phase, where the MEMS mirror draws a spiral pattern over the conveying surface, to search the micro-conveyor. In parallel, the control system, composed by the PIC microcontroller and LabVIEW interface, continuously monitors the output signal generated from the QPD, to find the position where $V_{SUM}$ is greater than $V_{Th}$. Once this occurs, the localization of the micro-conveyor is reached and the system switches from surface scanning mode to micro-conveyor tracking mode.

Fig. 6, illustrates the QPD output voltages as a function of time. At time $t < t'$, the system runs during the scanning phase. At time $t = t'$, the value of $V_{SUM}$ becomes greater than the threshold value ($V_{Th}$ = 0.15 V, chosen experimentally), indicating that the QPD has received the light beam reflected by the CCR. For $t > t'$, the system exits the scanning mode to start the tracking of the micro-conveyor in a closed loop control. A proportional corrector $kp$, is implemented in the closed loop to correct the $V_{B-T}$ and $V_{L-R}$ values. The oscillation of these values around zero [see Fig. 6] indicates that the micro-conveyor is correctly tracked. For a small value of $kp$, the oscillation becomes less significant in amplitude, however, the tracking system becomes slower, and vice versa. Our goal is to speed up as much as possible the correction time of the control system. However, for a high $kp$ value, the oscillations around zero increases [see Fig. 6]. This is caused by high angular step corrections realized by the MEMS mirror. In this scenario, the reflected laser beam by the CCR may arrives outside the active surface of the QPD leading to non-detection. As a result, the trajectory of the micro-conveyor
<figure><img src="image_8.png" /><figcaption>Fig. 7. (a) The micro-conveyor equipped with CCR at the center of the smart surface, (b) micro-conveyor equipped with CCR at the center of the smart surface with light illuminating the CCR, (c) micro-conveyor with $\Delta y$ displacement in y direction and optical tracking and (d) micro-conveyor with $\Delta x$ displacement in x direction and optical tracking.</figcaption></figure>

may be lost. In this work, $kp$ value is chosen experimentally to be equal to 0.6 for an optimal speed with acceptable oscillation in amplitude.

Figure 7 illustrates the tracking of the micro-conveyor moving over the smart surface in x- and y- directions. The motion along these axes of the micro-conveyor is generated independently using LabVIEW.

Fig. 7(a) shows the micro-conveyor placed at the center of the surface, before localization for time $t < t'$. Fig. 7(b) shows that the laser beam illuminates the CCR placed at the center of the micro-conveyor indicating that the micro-conveyor is localized at time $t = t'$. Next, the micro-conveyor starts to perform linear movements along the x- and y-axis with a motion speed of 16 mm/s. Figures 7(c) and 7(d) show two positions of the micro-conveyor over the smart surface, where the micro-conveyor carries out displacements of $\Delta y$ and $\Delta x$ from the center, for $t > t'$.

We can see that the light beam successfully followed the conveyor while translating in the x- or y-axis.

The corresponding variation of the angles of the MEMS mirror, $\Delta\theta_x$ and $\Delta\theta_y$, as a function of time, are presented in Fig. 8. For $t < t'$, the MEMS mirror receives ($VX+$, $VX-$) and ($VY+$, $VY-$) voltage values as spiral coordinates in order to scan the conveying surface. Due to the linearity between applied voltages and the MEMS angles (refer to (19)), the mirror rotates to create a spiral-scanning pattern.

Figure 8(a) corresponds to the tracking in horizontal displacement along the x-axis of the micro-conveyor. In this example, $\Delta\theta_x$ has a constant value while the value of $\Delta\theta_y$ varies. For $t = t''$, the micro-conveyor stops moving, resulting in a constant value for both $\Delta\theta_x$ and $\Delta\theta_y$. In Fig. 8(b), the localization and tracking process is repeated for the micro-conveyor moving vertically along the y-axis. The oblique variation of $\Delta\theta_y$, even with a linear translation in the y-axis,

<figure><img src="image_9.png" /><figcaption>Fig. 8. Variation of $\Delta\theta_x$ and $\Delta\theta_y$ as a function of time during the scanning and tracking phases for (a) horizontal and (b) vertical movement of the micro-conveyor.</figcaption></figure>

is due to an angle deviation between the conveying surface and the plane of the MEMS mirror caused by the usage of the fixed mirror.

Next, the tracking system is tested for a coupled movement of the micro-conveyor moving in a complex trajectory [see Fig. 9]. The micro-conveyor was moved manually to mimic a complex motion trajectory. An example of a complex trajectory is presented in Fig. 9(a). Fig. 9(b) shows the replica of the micro-conveyor's trajectory as plotted in Fig. 9(a) but in the MEMS mirror plane.

Such experiment validates the system's capability to track the micro-conveyor's movement over a complex path.

An approximated calculation of the tracking system sensitivity and resolution can be obtained using (15) and (16) from section III-A, knowing that the employed MEMS mirror has an angular resolution of 0.6 millidegrees. The values of $\alpha$, $\theta_{x0}$, and $\text{Hei}$ were calculated and measured experimentally to be equal to 0.71 degrees, 67.20 degrees, and 286.64 mm,
<figure><img src="image_10.png" /></figure>

A: Rest position

B: Central position

C: Farthest position

<figure><img src="image_11.png" /><figcaption>Fig. 9. (a) Random trajectory of the micro-conveyor being tracked and (b) the corresponding variation of the MEMS angles.</figcaption></figure>

<table><caption>TABLE I
SYSTEM SENSITIVITY AND RESOLUTION FOR THREE DIFFERENT VALUES OF MEMS ANGLES $\Delta\theta_x$, FOR $\theta_{x0} = 67.2$ DEGREES</caption><thead><tr><th>Position</th><th>A</th><th>B</th><th>C</th></tr></thead><tbody><tr><td>$\Delta\theta_x$ [degrees]</td><td>0</td><td>2.11</td><td>3.59</td></tr><tr><td>Sensitivity [degrees/m]</td><td>49.84</td><td>41.25</td><td>35.52</td></tr><tr><td>Geometrical resolution [µm]</td><td>11.46</td><td>13.88</td><td>16.01</td></tr><tr><td>Real resolution - 12 bits [µm]</td><td>39.53</td><td>47.77</td><td>55.47</td></tr><tr><td>Real resolution - 16 bits [µm]</td><td>14.97</td><td>18.10</td><td>21.01</td></tr></tbody></table>

respectively. Using (12), in our experimental configuration, the calculated maximum range of the micro-conveyor over which it can be detected by the MEMS mirror is 168 mm. In this work, we use only a working space of 100 mm because the center of the micro-conveyor cannot reach the other areas, as shown in Fig. 9(a).

The sensitivity and resolution of the system depends on the angular variation of the MEMS mirror ($\Delta\theta_x$). Table I shows the value of sensitivity and resolution of the tracking system, for three different positions (point A, B, and C) over the conveying surface, [see Fig. 9(a)].

Point A, corresponds to the rest position of the MEMS mirror, i.e., ($\Delta\theta_x = 0$). This configuration gives the highest sensitivity and resolution equal to 49.84 degrees/m and 11.46 µm, respectively. Point B corresponds to the central position, where the micro-conveyor is at the center of the conveying surface. In this case, the MEMS mirror is rotated with an angle of $\Delta\theta_x = 2.11$ degrees. For such an orientation the sensitivity and resolution decreases to 41.25 degrees/m and 13.88 µm, respectively. Point C is the limit and farthest position that the micro-conveyor can reach on the conveying surface, where $\Delta\theta_x = 3.59$ degrees. The lowest sensitivity and resolution is obtained at this point for values of 35.52 degrees/m and 16.01 µm, respectively.

These values are defined for an angular resolution of the MEMS mirror of 0.6 millidegrees, however, there exist quantification error due to the use of NI-DAQ card and ADC embedded in the PIC microcontroller. The Digital to Analog Converter (DAC) embedded in the DAQ card has a 16-bit resolution and the ADC has a 12-bit resolution. The angular resolution is therefore affected by the latter having the lowest bit resolution and degrades to 1.97 millidegrees. Thus, the tracking system resolution decreases to 39.53 µm, 47.77 µm, and 55.47 µm for the points A, B, and C, respectively. This can be improved by using an ADC with 16-bit resolution, making the angular resolution of the MEMS equal to 0.74 millidegrees and leads to an improvement of the system resolution to 14.97 µm, 18.10 µm and 21.01 µm at points A, B, and C, respectively. The effect of quantization cannot be improved beyond these values since the MEMS driver has a 16-bit resolution.

The angular resolution of the MEMS mirror is measured experimentally to verify the actual sensitivity of the system, by examining the QPD sensitivity. In open loop, the MEMS mirror angle is increased by small steps of 0.8 millidegrees, while the micro-conveyor is at a fixed position, in order to monitor the corresponding output of the QPD. The angular pitch is selected to be lower than the theoretical angular resolution. Fig. 10 illustrates the variation of the mean values of $V_{B-T}$ as a function of the angular variation of the MEMS mirror ($\Delta\theta_x$). For each angular position, an average and a standard deviation value of 300 measurements of $V_{B-T}$ were taken. This figure is an example for the micro-conveyor being fixed at the center of the conveying surface (position B) and the MEMS rotates in the positive direction. Same study can be realized for the $V_{L-R}$ variation as a function of $\Delta\theta_y$ and for any position of the micro-conveyor over the conveying surface. The experiment begins by lighting the micro-conveyor with the laser beam, in such a way that the CCR reflects light beam close to the center of the QPD's active surface, i.e., $V_{B-T}$ and $V_{L-R}$ are near to zero and $V_{SUM}$ is greater than $V_{Th}$. When the MEMS mirror rotates in the positive direction of $\Delta\theta_x$, the reflected beam is moved down the QPD surface, this is explained by an increase in $V_{B-T}$ values. In the case of closed-loop control, we focus on the zone close to the center of the QPD, i.e., for small variations of $\Delta\theta_x$ less than 5 millidegrees, it is called control zone see figure Fig. 10. As the value $\Delta\theta_x$ increases, the standard deviation of $V_{B-T}$ measurements increases, indicating that the
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<figure><img src="image_12.png" /></figure>

**Saly Malak** received the Ph.D. degree in mechanical engineering with specialization in mechatronic, photonics and systems control from Université de Technologie de Compiègne, France, in 2019. She is working as a Researcher with the Roberval Laboratory, Department of Mechanical Engineering, Université de Technologie de Compiègne. Her research interest includes photonics, micro-sensors, design and control of mechatronic systems, and optical wireless communications.

<figure><img src="image_13.png" /></figure>

**Hani Al Hajjar** received the Ph.D. degree in photonics and telecommunications from Telecom Bretagne (Institut Mines-Telecom), Brest, France, in 2013. Since 2015, he has been an Associate Professor of Mechatronics and Communications Between Systems with the Roberval Laboratory, Université de Technologie de Compiègne, Compiègne, France. He is a member of the Laboratory of Excellence Control of Technological Systems of Systems (labex MS2T). His research interests include optical wireless communications, indoor access networks, high resolution optical localization and tracking, micro-mechatronic systems, opto-mechatronics, optical wireless actuation, and signal processing.

<figure><img src="image_14.png" /></figure>

**Erwan Dupont** received the Ph.D. degree in mechanical engineering from Université de Technologie de Compiègne, France, in 2015. He is an Associate Professor of Mechanical Engineering and Mechatronics with the Roberval laboratory, Université de Technologie de Compiègne. His research interest focuses on micro-sensors, optical sensors, and mechatronic systems.

<figure><img src="image_15.png" /></figure>

**Muneeb Ullah Khan** received the double Ph.D. degrees in mechanical engineering with the specialization in microsystem design from Université de Technologie de Compiègne, France, and the Institute for Microtechnology, Technische Universität Braunschweig, Germany, in 2014. He is working as a Research Engineer with the Roberval Laboratory, Department of Mechanical and Mechatronics Engineering, Université de Technologie de Compiègne. His research interests include micro-mechatronic system designs, micro-actuators, micro-sensors, and nanofabrication technology.

<figure><img src="image_16.png" /></figure>

**Christine Prelle** received the Ph.D. degree in industrial automatic control from Institut National des Sciences Appliquées Lyon, France, in 1997. She is currently a Professor of Control Engineering and Mechatronics with the Roberval Laboratory, Université de Technologie de Compiègne, France. She is also in-charge of the Doctoral School of Science for Engineers, Université de Technologie de Compiègne. Her research interest includes micromechatronics and control.

<figure><img src="image_17.png" /></figure>

**Frederic Lamarque** received the Ph.D. degree in electronics from Université Paris-Sud (now part of Paris-Saclay), France, in 1998. Since 2014, he has been a Full Professor of Sensors and Instrumentation with Université de Technologie de Compiègne (UTC). His research main topic focuses on design of microsensors and microactuators with a specific interest in photonics integration into micro-mechatronic systems.