###### Abstract
Force control is used to ensure the UT probe maintains constant contact with the part through path corrections. Force data in the context of robotic UT-NDT have traditionally utilised roller probe technology, used to provide local corrections to the estimated position of a known part within the robot's frame of references [9; 10; 11]. These examples have, however, used known digital-twin models with limited positioning corrections required. The main draw-back of roller probes are the uni-directional speed allowed along the tool's axis, along with shear forces causing degradation of the roller probe's polymer components. An alternative UT contact measurement device is the Conformable Wedge (CW) probe. Filled with water, the wedge's material has a near-identical UT wave-speed as water, preventing reflection at the boundary. The main drawback of conformable wedges used within a force control schema is the potential for bursting. This method is not suitable for sharp surfaces or high forces, which is defined dependant on the probe material and thickness. While CW probes are more sensitive to sharp corners than roller probes, the nearly-flat aperture of the sensor presents the advantage of additional information in the form of surface friction. Corroded parts prevalent within sectors such as remanufacturing require surface pre-preparation to clean and level rusted patches in accordance with ISO standard 16809:2017 [12]. In certain regions, operators may have insufficiently prepared the surface, or the surface thickness may not allow levelling. In these cases, surface roughness data are advantageous to map overlooked regions of the surface in the preparation stage. While laser data provides high-accuracy surface position feedback, the couplant fills pits on the surface, presenting it as smooth and preventing it from providing such a metric. For conformable and flat rigid surfaces separated by a thin film of fluid, the frictional force is a sum of the hydrodynamic friction due to viscosity, and the asperity or contact force [13]. The hydrodynamic friction term \(\tau\), contact load \(P\), area of the CW probe \(A\)contact friction term \(f_{c}\), and normal force \(F\) define the dimensionless coefficient of friction \(\mu\) in a mixed-lubrication regime;  
\[\mu=\frac{\int_{A}\tau\mathrm{dA}+\int_{A}f_{c}P\mathrm{dA}}{F}. \tag{1}\]  
Contributions of hydrodynamic and contact friction are proportional to the surface area exposed to each regime. Along the Stribeck curve [14], the hydrodynamic coefficient of friction is related in a highly non-linear way with the average load \(P\), wedge/couplant relative speed \(v\), and fluid viscosity \(\upsilon\) by the Hersey number \(H=\upsilon v/P\). The Stribeck curve is shown in Figure 1.  
This paper has sought to introduce an autonomous scanning procedure that address the issues with moulded part manufacturing and remanufacturing industries where an accurate digital twin is not always available. The presented method autonomously plans paths and deploys a UT sensor without prior knowledge of the surface through:  
* location of and planning over a surface using a noisy and incomplete representation of the surface,
* simultaneous UT probe alignment, scanning, and path corrections using local force and laser-sensor feedback. The novel path planning method presented overcomes limitations of traditional visual systems such as variable-lighting artefacts [15] and RGB/D specific artefacts [16] without the need for complex and potentially inaccurate point cloud corrections. Further, the requirement for apriori knowledge of the surface, such as imposed primitives, are removed. This is completed while taking full advantage of the data available to minimise the risk of collisions in-process. The presented method enables single-shot part localisation strategies that could not be used under previous vision-enabled path planning regimes that require greater levels of accuracy.