Pattern-based vision

Objectives

The motion characterization along several degrees of freedom at the nanoscale is mandatory to the understanding of the behavior of micro-nano systems and robots. It requires measurement systems with very high range-to-resolution ratio and large bandwidth. With this aim in view, we propose a vision-based approach and several applications to typical cases of study.

The proposed visual measurement methods 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 [1].

Several patterns and associate algorithms have been developed and reach the following performances:

  • Sub-nanometric resolutions
  • Millimeters ranges (with absolute positioning)
  • Multi-dof measurements

Microrobots calibration

These pattern-based methods enable to understand the behavior of microrobots, then to derive physical based models. Such models can then be used to compensate imperfections through robot calibration. The above figure shows an XYΘ serial robotic structure and its behavior at the microscale. We can notably see the positioning accuracy of a micropositioning robot without and with compensation showing a significant improvement of the performances [5-7]. This allows to understand the most influent imperfections, quantify their own influences 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).

Microforce 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.

Contact

Guillaume Laurent

Involved people

Cédric Clévy, Guillaume Laurent, Patrick Sandoz

Selected publications

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.