TPT May 2017

T E CHNOLOG Y

Length and speed measurements for non-destructive testing with laser precision By Polytec, Germany

tracking of the defect position through the ma- chine, without contacting the surface. Laser surface velocim- eters (LSVs) use the laser Doppler principle to evalu- ate the backscattered laser light from a moving object and to determine the exact motion and position of a tube in the test stand. The combination of methods provides a fast and complete testing of the tube. A series of laser surface velocimeters are used at the ultrasonic test stand to measure length and

TO provide quality testing of steel tubes, a suite of non-destructive testing techniques are combined to facilitate a fast and complete assessment of the tubes. Where length and velocity measure- ments on rotating tubes are needed, laser surface velocimeters are used to provide critical position data during the testing procedure. Inspection facilities include processes suitable for non-destructive testing of seamless steel tubes such as electromagnetic inspection, magnetic powder inspection (MPI), metallurgical tests and ultrasonic testing. Depending on the application and specifications, ultrasonic testing is applied over the whole length and perimeter of the tube for a comprehensive failure analysis. Testing of both longitudinal and transverse defects as well as the inspection of wall thickness and lamination are critical for quality control. In order to track defects through the machine the defect detection method is combined with a position measurement technique to eventually mark the defect upon exiting the machine for further processing. Laser surface velocimeters are a solution for precise and reliable

LSV measurement principle

very different speeds. This measuring technique allows the user to control the forward and rotational movement of tubes during non-destructive testing with maximum precision and therefore helps to optimise the overall process performance. The LSV emits two laser beams, which overlap at a certain distance – the working distance. The volume in which both laser beams are superimposed is called the measurement volume. In the measurement volume the overlapping laser beams generate an interference pattern of bright and dark fringes. The distance between those fringes is called the fringe spacing and is a system constant for the LSV. It depends on the wavelength of the laser light and the angle between the laser beams. If a surface moves through the fringe pattern then the intensity of the light scattered back is modulated between bright and dark. As a result of this a photo receiver in the sensor generates an AC signal. The frequency of the signal is directly proportional to the velocity component of the surface in measurement direction. This makes laser velocimetry a useful technology for a highly reliable control process leading to a cost-efficient manufacturing of tubes and pipes.

velocity of the tubes. The tubes that are passing through the facility are rotating with a speed of 2m/s. Each inlet and outlet of the stand is equipped with two LSV sensor heads. Uniquely, with the LSV is its possibility to detect the speed component in the direction of its current measurement. By using two correctly aligned LSVs, the longitudinal and lateral (rotational) motion of the tubes can be distinguished and clearly derived. The paired sensors at the inlet and outlet of the ultrasonic test stand provide measurement data for determining the tube’s position while travelling through the stand. The translational speed is quite low in relation to the rotational speed. Thus, the respective sensor head must be aligned exactly parallel to the direction of motion in order to avoid any superposition with the lateral motion. Otherwise the translational speed would be influenced, causing errors in length and position measurements. The precise alignment of each sensor is achieved by using a high precision mounting plate and by following a special adjustment procedure. With the aid of precision screws the angular position can be set precisely before starting routine operation. Non- contact and precise laser surface velocimeters, combined with suitable accessories, provide precision length and speed measurements even in the case of superimposed motions with

Polytec – Germany Website: www.polytec.com

LSV 2000

52

www.read-tpt.com

MAY 2017