Simultaneous multi degree of freedom (DoF) measurement system

Mechatronic motion systems form the basis of a large variety of production systems like machine tools, wafer scanners in semiconductor integrated circuit production and robotic applications as well as for the associated measurement equipment like coordinate measurement machines (CMMs) and atomic force microscopes (AFMs). Some of them require precision and accuracy in several, if not all translational and rotational degrees of freedom (DoF). The use of multiple DoF positioning systems becomes more widespread over measurement ranges from nanometres to metres in applications from nanometrology, industrial metrology to mechatronics and robotics [1].
Optical interferometry is established in high-precision engineering applications with commercial instrumentation. It serves for the need for non-contact measurements and large measurement ranges in sensitive metrology stages, e.g. AFM and CMM [2]. However, state-of-the-art equipment typically has a single probing direction only. Thus the measurement of multiple DoF either requires multiple individual measurement systems or a complicated distribution of several sensors for the simultaneous detection of 3D translation, yaw, pitch and roll. Dedicated commercial setups measure transverse motion and straightness in addition to longitudinal translation, but typically require a physical change of the optics assembly (for example the use of a Wollaston prism for tilt measurement).
The presented system offers a compact solution to measure simultaneously x-y-z displacements and roll-pitch-yaw movement from only one direction [3].
The key component of the system, the interferometer unit, resembles a bare-minimum Twyman-Green interferometer consisting of a collimator for light coupling, a 50/50 beam splitter, the reference mirror, the measurement mirror and an image sensor (Fig. °1). The restriction to a minimum number of optical components reduces errors induced by multipath reflections and wave front aberrations, saves cost, enables miniaturisation and easy alignment.
The reference mirror and the measurement mirror are aligned in a way that a fringe pattern is projected onto the sensor. The interferogram consists of a spatial sinusoidal signal whose frequency per unit length depends on the tilt between the reference mirror and the measurement mirror as well as the wavelength of the light source.


Fig.°1 Conceptional system setup (left). The captured and transformed signal (right).
The developed system (see Fig.°2) is a combination of a length, angle and straightness interferometer based on the interferometer principle described above. The device is able to measure lateral motion in the range of centimeters and angles of up to multiple mrad simultaneously.

  
Fig.°2 System design (left) and the optical setup (right)

Die Arbeiten werden durch das Forschungsprojekt “Metrology for movement and positioning in six degrees of freedom” (JRPIND58 [6DoF]) des European Metrology Research Programme (EMRP) gefördert. Dieses wird von den im Rahmen von EURAMET am EMRP teilnehmenden Ländern und der Europäischen Union gemeinschaftlich finanziert.

References:

[1] K. Roufas, Y. Zhang, D. Duff, M. Yim, Six Degree of Freedom Sensing For Docking Using IR LED Emitters and Receivers, Lecture Notes in Control and Information Sciences Volume 271, 2001, pp 91-100

[2] Dai G., Pohlenz F.; Danzebrink H.-U.; Xu M.; Hasche K.; Wilkening G. 2004 Metrological large range scanning probe microscope Rev. Sci. Instrum. 75 4, 962-969

[3] S. Strube, G. Molnar, H.-U. Danzebrink, Compact field programmable gate array (FPGA)-based multi-axial interferometer for simultaneous tilt and distance measurement in the sub-nanometre range 2011 Meas. Sci. Technol. 22 094026 doi:10.1088/0957-0233/22/9/094026.