The characterization of the motion of robots along several degrees of freedom at the nanoscale is a very challenging task. It requires multi-DoF measurement with very high range-to-resolution ratio, large bandwidth and very small sensors to enable their integration. This lack of sensor strongly limits the understanding of micro-nano systems and robots behavior. To tackle this lock we are working on pattern-based visual measurement and apply them on several typical cases of study.
Pattern-based visual measurement
We develop visual measurement methods that rely on structured patterns to get outstanding performances in term of resolution, range and accuracy. The methods are based on an encryption of a localization code over a pseudo-periodic pattern. The position is obtained by combining fine (and relative) and coarse (and absolute) simultaneous measurements. The fine measurement is performed in the frequency domain using Fourier-like transformations to separate the different directions of modulation of the pattern and to calculate theirs phases. The localization code allows for absolute coordinate transformations of the observed part of the pattern. Multiple degrees-of-freedom can be measured simultaneously using specific patterns .
Illustration of the Vernier-like coding method 
Several patterns and associate algorithms have been developed and reach the following performances:
- Sub-nanometric resolutions
- Millimeters ranges
- Multi-dof absolute measurements
- Non-contact measurements (vision)
Several applications are chosen as case-study using pattern-based visual measurement.
Characterization of micropositioning robots : The underneath Figure (a) shows the XYΘ serial robotic structure used as case study and (b) its behavior at the microscale.
This characterization enables to understand the behavior of the robot, then to derive physical based models. Such models can then be used to compensate imperfections through robot calibration. Above Figure (b) notably displays positioning accuracy of a micropositioning robot without and with compensation showing a significant improvement of the performances [5-7]. The position of the robot end-effector can be modelled with :
where (X,Y,Θ) are joint axis, (ξ,α,xR,yR) are geometric imperfection (perpendicularity error between axis) whereas (fax, fay) are errors intrinsic to axis. Applied on a XYΘ micropositionning robot, we succed to understand the most influent imperfection, quantify their own influence onto positioning accuracy and to improve the whole positioning accuracy by a factor of 35 (96 µm initialy to less than 2 µm after compensating imperfections) .
Characterization of a compliant structure for force sensing :
Pattern-based visual measurement has also been used to characterize the behavior of a compliant structure (widespread at the microscale). It is notably possible to study accurately the free oscillations of a compliant structure. Characterization conducted to the model of the structure and latter to the estimation of forces applied to it. The following set-up has notably been used :
These works notably conducted to a force measurement system with 50 mN of measurement range, 50 nN resolution (thus providing a range/resolution ratio of 106) and 7.8 µN of repeatability .
- Multi-ddl motion characterization by vision
- Micro/nano robot calibration
- High precision visual servoing
2D visual micro-position measurement based on intertwined twin-scale patterns
V. Guelpa, P. Sandoz, M. Asmad Vergara, C. Clévy, N. Le Fort-Piat & Guillaume J. Laurent (2016), Sensors and Actuators A: Physical, 248, pp. 272-280.
Accuracy Quantification and Improvement of Serial Micropositioning Robots for In-Plane Motions
N. Tan, C. Clévy, G. Laurent, P. Sandoz & N. Chaillet (2015), IEEE Transaction on Robotics (T-Ro), 31, pp. 1497-1507..
Vision-Based Microforce Measurement with a Large Range-to-Resolution Ratio using a Twin-Scale Pattern
V. Guelpa, G. J. Laurent, P. Sandoz & C. Clévy (2015), IEEE Transactions on Mechatronics.
Subpixelic Measurement of Large 1D Displacements: Principle, Processing Algorithms, Performances and Software
V. Guelpa, G. J. Laurent, P. Sandoz, J. Galeano Zea & Cédric Clévy (2014), Sensors, 14(3):5056--5073.
Performance analysis and characterisation of micro-nanopositioning systems
N. Tan, C. Clévy and N. Chaillet, Electronics Letters, 50(24), pp.1853,1855, November 2014.
Calibration of Single-axis Nanopositioning Cell Subjected to Thermal Disturbance
N. Tan, C. Clévy and N. Chaillet, IEEE ICRA International Conference on Robotics and Automation, Karlsruhe, Germany, May, 2013.