Brightfield microscopy metrology system for the evaluation of high precision medical robotic system

Abstract

The development of future surgical therapies has driven the efforts to increase the precision of robotic-guided manipulators beyond submillimeter to micron accuracies for applications in reconstructive microsurgery, vitreoretinal eye surgery, cellular level neurosurgery etc.

The design of high precision robotic systems requires high precision measurements to assess and optimize the device dexterity. Interferometric systems are routinely employed however, these are complex, requiring expert knowledge on its operation and results interpretation. Optical microscopes provide high magnifications making these suitable to perform high precision measurements based on visual inspection and interpretation in a simplified way.

In this paper we present a non-contact metrology approach based on bright field microscopy (BFM) to characterize the precision and kinematic performance of a custom-made 3 degrees of freedom linear parallel robotic manipulator for precision tasks producing a theoretical resolution of < 1µm.Our BFM is based on a 5 elements microscope objective lens (Celestron LLC, Torrance, California, United States 44308) having an equivalent focal length of 15.8mm. Magnification is adjusted though a tube slider adjusting the lens distance between a 5 MP camera and the object. The device is mounted in two positions: for lateral motion an optical breadboard and a v-clamp mount are used to position the BFM system vertically in close proximity to the robot. The end effector is provisioned with a reticle used to track its position. For planar motion the BFM is mounted on a custom-made end effector facing towards the X, Y workspace. The robotic system is controlled though a custom-made GUI operated though an FPGA (NI MyRIO 1900 NI, Austin, Texas, USA). The BFM device is used to track the robot motion within its workspace obtaining images for set trajectories. The data is processed using imageJ.

With the design resolution of our high precision robotic system of < 1 µm we caried out our test protocol. When operating the robotic system set to single rotational steps of 0.225° producing an end effector displacement of 0.6 µm the built system is capable of achieving a precision of 1.3 ± 0.3 µm. This is expected given that whenever the device starts moving it involves acceleration and deacceleration producing a deviation from the theoretical value of 0.7µm. The next test involves a continuous motion moving in a defined trajectory and a set speed of ~105 µm/s. For a theoretical set trajectory of 7.5 µm in the robot z-axis, we were able to obtain a displacement of 7.7 ± 0.23 µm reducing the motion error to 0.2 µm with respect to the design parameter with no evident deviation from the z-axis. This results from the fact that the robotic actuators are accelerated and deaccelerated only once for the full trajectory. Finally, we assessed the influence in performance of our device at higher speeds (310 µm/s) and a straight trajectory of 7.5 µm in the robot z-axis. This resulted in a displacement of 8.4 ± 0.23 µm having an impact on the motion error of ~1 µm with respect to the design parameter and showing no evident deviation from the z-axis.

The use of our BFM system has allowed us to validate the design of a high precision linear parallel robotic manipulator considering motion resolution, repeatability, vibration effects and kinematic performance. The use of this approach can be extended to other areas in engineering where high protection measurements are required without having to develop complex interferometric approaches.