In-process measurement and control of tube slippage in rotary draw bending by vertical-cavity surface-emitting laser sensors

Rotary draw bending (RDB) is widely used for the production of tubular components in sectors such as automotive, aerospace, and construction. The process is affected by several quality issues, including springback, wrinkling, and cross-section ovalization. Among these, tube slippage at the clamp–tube interface is particularly critical, as it alters the final geometry and damages the tube surface. Despite its industrial relevance, slippage remains largely unaddressed in terms of in-line monitoring and control. Its small displacement scale and occurrence within the clamping zone make detection particularly challenging. This work presents a novel in-process measurement and feedback approach for tube slippage, based on compact VCSEL (Vertical-Cavity Surface-Emitting Laser) optical sensors integrated directly into the clamping dies. The system enables real-time detection of relative axial displacement at the tool–workpiece interface with high resolution and supports closed-loop correction of clamping and bending parameters. A signal processing and estimation framework is developed to quantify slippage from the sensor data, which is then used to drive a correction strategy without interrupting the production cycle. Experimental validation confirms the proposed approach effectiveness in detecting and compensating slippage, demonstrating its potential for integration into modern intelligent forming machines. Thanks to the integrated sensing and control scheme, slippage events as small as 0.1 mm are detected and compensated without disrupting the bending sequence.