Ecole doctorale SMAER Sciences Mécaniques, Acoustique, Electronique, Robotique _______________________________________________________________________________________________________________________________________ ED SMAER (ED391) Tour 45-46 Bureau 205- case courrier 270- 4, place Jussieu - 75252 PARIS Cedex 05 ': 01 44 27 40 71 [email protected]Sujet de thèse_20 Thesis subject 2020 Laboratory : Institut des Systèmes Intelligents et de Robotique, CNRS UMR 7222 University: Sorbonne University Title of the thesis: Design and control of active force sensors for meso and micro-scale robotics Thesis supervisor: Mokrane Boudaoud and Stéphane Régnier Email contact: [email protected]Co supervisor:/ Collaborations within the thesis:/ Program affiliation:/ Cotutelle:/ University : / This subject can be published on the doctoral school’s web site: Yes Thesis’s summary (abstract): Active force sensors are key instruments to get around the tradeoff between the sensitivity and the measurement range of conventional passive force sensors. Thanks to their quasi-infinite stiffness in closed loop, active force sensors can be used for the indentation and the mechanical characterization of samples over a large stiffness range without any interference with the mechanical properties of the sensor. The thesis aims at proposing breaking approaches on the design and the control of active force sensors embedded in poly-articulated micro-robotic systems for precise 3D force measurements and characterization at the small scales. The application will mainly be in the biological filed where the force sensor will be used as an instrument for nano-indentation and biophysical characterization as well as for the analysis of physical transformations of living cells for diagnostic purposes. This thesis will require research works in the fields of robotics and control.
Thesis subject 2020 Laboratory : Institut des Systèmes Intelligents et de Robotique, CNRS UMR 7222 University: Sorbonne University Title of the thesis: Design and control of active force sensors for meso and micro-scale robotics Thesis supervisor: Mokrane Boudaoud and Stéphane Régnier Email contact: [email protected] Co supervisor:/ Collaborations within the thesis:/ Program affiliation:/ Cotutelle:/ University : / This subject can be published on the doctoral school’s web site: Yes
Thesis’s summary (abstract): Active force sensors are key instruments to get around the tradeoff between the sensitivity and the measurement range of conventional passive force sensors. Thanks to their quasi-infinite stiffness in closed loop, active force sensors can be used for the indentation and the mechanical characterization of samples over a large stiffness range without any interference with the mechanical properties of the sensor. The thesis aims at proposing breaking approaches on the design and the control of active force sensors embedded in poly-articulated micro-robotic systems for precise 3D force measurements and characterization at the small scales. The application will mainly be in the biological filed where the force sensor will be used as an instrument for nano-indentation and biophysical characterization as well as for the analysis of physical transformations of living cells for diagnostic purposes. This thesis will require research works in the fields of robotics and control.
Force sensing and control is a key capability in robotics for a myriad of applications requiring a direct interaction between the end effectors and the external environment. The most common force sensing technique relies on the measurement of the deformation of a flexible structure, attached to the force sensor probe, with a known stiffness [1]. Such sensors are referred as passive sensors.
Fig.1 : Schematic view of the active force sensor working principle.
The performances of passive sensors in terms of sensitivity and measurement range depend on their mechanical properties. A high sensitivity can be obtained with low stiffness sensors but at the cost of a low measurement range while high stiffness passive sensors allows for a higher measurement range but with a reduced sensitivity [2]. In addition, the sensor stiffness must be very high compared to that of the external environment, which seriously restrict the usability of passive force sensors for the measurement of forces on objects with a large stiffness variation [3]. The alternative active sensor working principle is based on force balancing between an unknown force and a known quantity, as in Roberval balance [4]. Such sensors incorporate an actuator [5] whose objective is to keep the position of the probe at a fixed value when an external force is applied on it. This is achieved thanks to a closed loop control feedback. Therefore, using this principle, the external force can be measured from the control signal of the actuator. This measurement principle overcomes the previously cited drawbacks of passive sensors. The sensitivity and the measurement range of the sensor do not become longer dependent on the properties of the suspensions but to that of the actuator, which offers much more possibilities to reduce the measurement range to sensitivity tradeoff. Moreover, such sensors have a quasi-infinite closed loop stiffness making them suitable for the measurement of forces on objects with a large stiffness variation without interference with the mechanical parameters of the sensor. At ISIR, we have developed two technologies of active force sensors [3][6]. The first one is based on MEMS1 technology with a high resonant frequency (>2kHz) and a measurement range in the µN level
1 Microelectromechanical systems.
Design and Control of Large-Range Nil-Stiffness Electro-Magnetic
Active Force Sensor
Jonathan Cailliez*, Antoine Weill–Duflos*, Mokrane Boudaoud, Stéphane Régnier and Sinan Haliyo
Abstract— Active force sensors are key instruments to get
around the tradeoff between the sensitivity and the measure-
ment range of conventional passive force sensors. Thanks to
their quasi-infinite stiffness in closed loop, active sensors can be
applied for force measurements on samples with a wide range of
stiffness without interference with the mechanical parameters of
the sensor. MEMS (Micro-Electro Mechanical Systems) active
force sensors have been wildly developed in the literature but
they are ill adapted for force measurements at the Newton
level needed in meso-scale robotics. In this article, a novel
structure for a meso-scale active force sensor is proposed for the
measurement of forces from the milli-newton to the newton.This
novel meso-scale sensor is based on a nil-stiffness guidance and
an electromagnetic actuation. This paper deals with its design,
identification, calibration and closed loop control. The sensor
exhibits nil-stiffness characteristic in open loop and an almost
infinite stiffness in closed loop. This allows measuring forces
with a large range of gradients. First experiments shows the
ability of this new sensor architecture to measure low frequency
forces up to 0.8N with a precision of 0.03N and a closed loop
-20dB cutoff frequency of 73.9Hz.
I. INTRODUCTION
Force sensing and control is a key capability in roboticsfor a myriad of applications requiring a direct interactionbetween the end effectors and the external environment.The most common force sensing technique relies on themeasurement of the deformation of a flexible structure,attached to the force sensor probe, with a known stiffness[1]. Such sensors are referred as passive sensors. For highprecision position measurements of the probe, one can usepiezoresistive [2][3], capacitive [4][5] or more exotic strate-gies such as the current flowing trough a MOSFET transistor[6] or an optical trap principle [7] .
The performances of passive sensors in terms of sensitivityand measurement range depend on the mechanical propertiesof the sensor flexible structure. The stiffness is one of themost influent parameter. A high sensitivity can be obtainedwith low stiffness sensors but at the cost of a low measure-ment range while high stiffness passive sensors allows fora higher measurement range but with a reduced sensitivity[8]. In addition, the stiffness of passive sensors interfere inthe force measurement when it is close to that of the object
J. Cailliez, M. Boudaoud, S. Haliyo and S. Régnier are withSorbonne Université, UMR 7222, ISIR, F-75005 Paris, France. A.Weill–Duflos is with McGill University, Centre for Intelligent Machines,3480 Rue University, Montréal, Québec H3A0E9, Canada. E-mail:cailliez(at)isir.upmc.fr, mokrane.boudaoud(at)sorbonne-universite.frantoine.weill-duflos(at)mcgill.ca, sinan.haliyo(at)sorbonne-universite.fr,stephane.regnier(at)sorbonne-universite.fr
*(equal contribution)
mFext
Fcoil
Control Position Sensor
Probe
Actuation Guiding
Fig. 1: Schematic view of the active force sensor workingprinciple.
in contact with the probe. To avoid such an interference,the sensor stiffness must be very high compared to that ofthe external environment. This condition seriously restrictthe usability of passive force sensor for the measurement offorces on objects with a large stiffness variation [9].
The alternative active sensor working principle is basedon force balancing between an unknown force and a knownquantity, as in Roberval balance [10]. Such sensors in-corporate an actuator [11] whose objectif is to keep theposition of the probe at a fixed value when an externalforce is applied on it. This is achieved thanks to a closedloop control feedback. Therefore, using this principle, theexternal force can be measured from the control signalof the actuator. This measurement principle overcome thepreviously cited drawbacks of passif sensors. The sensitivityand the measurement range of the sensor do not becomelonger dependent to the properties of the suspensions butto that of the actuator, which offer much more possibilitiesto reduce the measurement range to sensitivity tradeoff.Moreover, such sensors have a quasi-infinite closed loopstiffness which makes them suitable for the measurementof forces on objects with a large stiffness variation withoutinterference with the mechanical parameters of the sensor.Another important benefit of the active sensing is to avoidthe displacement of the probe during the measurement. Thislinks precisely a measured force to a given position, andallows for example for a complete observation of a distance-dependent force field. With a passive sensor, the probedisplacement has to be taken into account ([14], [3]). Thisapproach works also equally well regardless attractive orrepulsive forces, or even in the case of time-varying fields.
(Fig. 2: left). The second one is a meso-scale active force sensor for the measurement of forces from the milli-newton to the Newton. This novel meso-scale sensor is based on a nil-stiffness guidance and an electromagnetic actuation (Fig.2: right).
Fig.2. Left: MEMS based force sensor and enlarged view of the internal structure of the mechanical part. The movable structures are highlighted in red. Detail of one quarter of the comb drive actuator is presented above the global plan (the whole structure is 30 μm thick). Right : 3D CAD representation of the meso scale active force sensor.
A challenging work consists on embedding active force sensors in poly-articulated robots for precise 3D force measurements and characterization at the small scales. For instance when used for nano-indentation mapping, the robotic system scans the probe along a surface to be analyzed. The force measurements could be distorted due to orthogonal scan configuration and sensor probe geometries especially when the sample has steep sidewalls. This issue will require controlling the XYZ translation motions of the force sensor as well as degrees of freedom in rotation [7].
2- Objectives
There are several targeted objectives in the thesis.
The first one is related to the active sensor design and control. The specifications will be mainly related to the measurement bandwidth, the linearity and the probe geometry to be compatible with high-resolution high-speed nano-indentation mapping and biophysical characterization of biological samples. The second objective consists in designing a polyarticulated micro-robotic system that will carry the force sensor and will allow scanning the sensor probe in the three dimensional space with rotation capabilities to deal with steep sidewalls of samples. The third objective is related to the robot motion control for precise 3D scanning of the sensor probe on the sample.
The final objective is the validation of the proof of concept of 3D force measurement and characterization of biological cells using active force sensors.
Fig. 2 MEMS based forcesensor and enlarged view of theinternal structure of themechanical part. The movablestructures are highlighted in red.Detail of one quarter of thecomb drive actuator is presentedabove the global plan (the wholestructure is 30 µm thick)
MEMS
Printedcircuit board
Mobile part
50µm
3.5µm
Fixed part
three design architectures, the folded flexure design is101
the best candidate when dealing with unidirectional force102
measurement in a wide linear operating range [16]. It has103
been monolithically fabricated on a silicon on insulator104
(SOI) wafer of 30 µm thickness. Wire bonding has been105
used to connect the contact pads of the MEMS to a printed106
circuit board (Fig. 2).107
The conception has been performed in our lab using 108
a CAD software. However, as no cleanroom facilities 109
are available in the lab, the MEMS were realized thanks 110
to the help of the RENATECH platform and the IEMN 111
lab (Institut d’electronique de microelectronique et de 112
nanothechnologie). RENATECH is a french platform of 113
nanofabrication. 114
Most of active force sensors reported in the literature arebased on the MEMS (Micro Electro Mechanical Systems)technology [8][9][12][13]. The comb drive actuator is thestandard actuation mechanism for active MEMS force sen-sors. They allow force measurements at the kHz frequencyrange but their measurement range is limited to the µNrange. Therefore, such sensors are ill adapted for meso-scalerobotics requiring force measurement at the Newton level.Note that here, the term meso-scale is referred to 1-100 mmin contrast to micro-scale (1-100 µm) and nano-scale (1-100nm).
This paper introduces a novel prototype of meso-scaleactive force sensor. The chalenge is to design an active forcesensor using an actuation principle allowing high amplitudeforce measurement (Newton level), a resolution of few tensof µN with a quasi-infinite stiffness in closed loop and awide linear operating range. The design strives to eliminateparasitic forces by getting rid of the internal stiffness of thesensor in open loop as well as avoiding most losses like dryfriction. The sensor (Fig. 1) is based on an electro-magneticactuation, an air bushing system with a guiding system tohold the sensor probe and an optical sensor for the probeposition measurement. This sensor offers the advantage ofactive sensing and can be embedded on robotic systemsrequiring precise low frequency force measurements at theNewton level.
The paper is structured as follows. Section II introducesthe sensor architecture and its main features as well as itstheoretical sensitivity calculation. Modeling and identifica-tion of the sensor are presented in section III. The closedloop control and calibration methodologies as well as theexperimental dynamic force measurements are presented insection IV. Section V ends the paper.
II. DESCRIPTION OF THE SENSOR
The sensor is built around a mobile probe carried on acylindrical air bushing, with a magnetic voice coil actuatorand an optical position sensor. A controller is designed tobalance the external force applied to the probe and to keepit at fixed position in steady state. A schematic view of theactive force sensor working principle is shown in Fig. 1.A 3D CAD view of the sensor is presented in Fig. 2. Thedesigned sensor can be observed in Fig. 10.
A. Sensor’s components
1) Guiding mechanism: In order to minimize the pertur-bations on the motion of the probe due to the air movementsand to achieve a smooth gliding, a porous air bushing isincluded in the sensor architecture. The model S300601 fromNewway Air bearing1 is chosen for its relatively small size.A stainless steel shaft is machined according to the speci-fications of the bearing, with a diameter of 6.337mm anda tolerance of �5/� 14 µm. When the bushing provides a60PSI pressure input, a 2 µm to 10 µm air gap is established
1https://www.newwayairbearings.com
Optical position sensor
Magnetic actuator
Air bushingProbe
Fig. 2: 3D CAD representation of the active sensor.
around the shaft as long as a 12N radial load is not exceededon the sensor probe.
2) Actuation: A voice coil actuator is selected. Such anactuator can provide very high accelerations and a highpositioning accuracy when suitably controlled. It can achievea settling time of two milliseconds or less. It has alsolinear electro-mechanic characteristics. In the sensor, themoving (i.e. probe) and fixed parts of the voice-coil arenot in contact and no wiring is required. The force outputis independent of the probe position hence it is a pureforce generator. The selected actuator is a NCC01-04-001-1X from H2W technologies2. It provides a maximum forceof 0.8N in short bursts and 0.27N continuously. The forceconstant of the voice coil is 0.45NA
�1. It is driven by aOPA548 power amplifier from Texas instruments. It has beenchosen because of it’s tunable current limitation and thermalshutdown protection.
3) Position sensor: In order to measure the position of theprobe, a micro-epsilon ILD1420-10 optical sensor3 is chosen.It is based on an optical triangulation with a resolution of0.5 µm at 4 kHz sampling rate.
4) Data acquisition and control system: Characterization,identification and control algorithms are programmed usingMatlab/Simulink software. The implementation of the controlalgorithms and the data communication with the sensor aremade through a DSpace DS1003 controller board at 10 kHz
sampling frequency.
B. Theoretical sensitivity
When an external force Fext is applied on the sensor probein the horizontal direction (Fig. 1), the dynamic equation ofthe probe motion can be expressed as follows:
m⇥ Xprobe = Fcoil � Fext � Ff (1)
Where: m and Xprobe are the mass and the position ofthe probe respectively. Fcoil is the force generated by theactuator. Fext is the external force applied on the probe. Ff
This system has the potential to produce a strong scientific impact for the analysis of muscular cells with myopathy disease. Partners from the institute of Myology at la Pitié-Salpêtrière hospital in Paris that are working with our team in an ANR project can provide muscular cells and the expertise in the interpretation of the characterization results.
3- Environment
The Institute for Intelligent Systems and Robotics (ISIR) is a multidisciplinary research laboratory that brings together researchers and academics from different disciplines of engineering sciences, information and life sciences. The ISIR is a joint research laboratory (UMR7222) which belongs to Sorbonne University and the Centre National de la Recherche Scientifique (CNRS). The “multiscale interactions” team in ISIR has a profound experience in microrobotics including multiphysics modeling, micro sensors, control of robots at the small scales and haptics. Research activities of this team have been published in several well-known international conferences and selected journals and have been rewarded many times. The division was involved in several research projects such as the European Research Council (ERC) PoC "RELAX".
4- Required qualifications
- Automatic control
- Robotics/mechatronics
- Programming (advanced skills in C++, Matlab)
5- Application
Applications should include a detailed CV, a motivation letter and two referees (name, institution, email address). The documents must be sent in a zipped format to [email protected]
References
[1] H. Zang, X. Zhang, B. Zhu, and S. Fatikow, “Recent advances in non- contact force sensors used for micro/nano manipulation,” Sensors and Actuators A: Physical, vol. 296, pp. 155 – 177, 2019.
[2] M. Bulut Coskun, S. Moore, S. R. Moheimani, A. Neild, and T. Alan, “Zero displacement microelectromechanical force sensor using feed- back control,” Applied Physics Letters, vol. 104, no. 15, p. 153502, 2014.
[3] J. Cailliez, M. Boudaoud, A. Mohand-Ousaid, A. Weill–Duflos, S. Haliyo, and S. Régnier, “Modeling and experimental characteri- zation of an active mems based force sensor,” Journal of Micro-Bio Robotics, vol. 15, no. 1, pp. 53–64, 2019.
[4] B. Kisch, Scales and weights; a historical outline. Yale Univ. ISBN-10: 0300006306, 1976.
[5] A. M. Ousaid, S. Haliyo, S. Régnier, and V. Hayward, “Micro-force sensor by active control of a comb-drive,” in 2013 IEEE/ASME International Conference on Advanced Intelligent Mechatronics , 2013.
[6] J. Cailliez*, A. Weill--Duflos*, M. Boudaoud, S. Régnier, D. S. Haliyo. Design and Control of Large-Range Nil-Stiffness Electro-Magnetic Active Force Sensor. IEEE International Conference on Robotics and Automation (ICRA), Paris, France, 2020. (Accepted) * equal contribution
[7] H. Xie, D. Hussain, F. Yang, L. Sun. Development of Three-Dimensional Atomic Force Microscope for Sidewall Structures Imaging With Controllable Scanning Density. IEEE/ASME Trans on Mechatronics, vol 21, 2016.
