Shape Sensing for Continuum Robotics Using Optoelectronic Sensors with Convex Reflectors

Abstract

Three-dimensional shape sensing in soft and continuum robotics is a crucial aspect for stable actuation and control in fields such as minimally invasive surgery, engine repairs and search and rescue operations, as the estimation of complex curvatures while using continuum robotic tools is required to manipulate through fragile paths. This challenge has been addressed using a range of different sensing techniques, for example, Fibre Bragg grating (FBG) technology, inertial measurement unit (IMU) sensor networks, or stretch sensors. Previously, an optics-based method using optoelectronic sensors was explored, offering a simple and cost-effective solution for shape sensing in a flexible tendon-actuated manipulator in two orientations. This was based on proximity-modulated angle estimation and has been the basis for the shape sensing method addressed in this paper. The improved and miniaturised technique demonstrated in this paper is based on the use of a convex shaped reflector with optoelectronic sensors integrated into a tendon-actuated robotic manipulator. Upgraded sensing capability is achieved using optimisation of the convex reflector shape in terms of sensor range and resolution, and improved calibration is achieved through the integration of spherical bearings for friction-free motion. Shape estimation is achieved in two orientations upon calibration of sensors, with a maximum Root-Mean-Square Error (RMS) of 3.37°.

Optoelectronic Shape Sensing for Flexible Robotic Applications

“…As the optoelectronic sensor is small, it is taken to be a point source of light, that propagates conically. Depending on the orientation of the reflector against the optoelectronic sensor, the distance d will vary, and the calculation for this is derived in [21]. Once d is known and using the assumption of a convex reflector with a constant radius with centre C, as well as known s area of the receiving phototransistor, Equation 1 can be used to calculate the portion of light intensity that is received.…”

“…Based on the previous work by the authors [21], some points were mentioned to aid in improving the performance of the shape sensing. These have been implemented in this prototype.…”

“…In order to maintain within the calibration conditions, the deformations are limited to a small range and within the elastic limit of the material. Optoelectronic sensors offer another optical-based method for shape sensing, that will be presented in the following section, building on the work by the authors in a preliminary paper demonstrating this novel shape sensing technique [20] [21]. Optoelectronic sensors comprise of a housed light emitting diode (LED) and phototransistor (PT).…”

“…In the subsequent sections, the sensing principle, system design, calibration process, as well as shape sensing evaluation experiments will be described. Some improvements are made to the system, compared to the earlier initial prototype demonstrated in [21]. This includes design of a power switching circuit, that alternates power between pairs of optoelectronic sensors during operation, in order to eliminate instances of interference between sensors, which was a previous potential source of error of shape estimation.…”

“…Lastly, a new calibration procedure is designed, using a linear frame that can slide vertically along the robotic structure and fix the motion of a number of units while the pairs of sensors are calibrated consecutively. This is a simplified process compared to the calibration process in [21], which utilised a number of rigid fixtures around the structure of the robotic manipulator. These improvements will be further explored in the following sections.…”