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The dc motor and its role in suture inspection

10 October 2015

Medical suture specialist, PharmSouth, has expanded its range with a new inspection machine that will save labour and material costs and achieve a much higher quality product. maxon dc motors and controllers played no small part in this achievement.

PharmSouth’s new machine inspects the sutures for defects; these are common as the sutures are made up from lengths of braided or twisted polymer, which can break, stick out or bunch up, leading to suture breakages and discarded product. maxon technical sales engineer, Mark Gibbons was in at the beginning of the machine building project and suggested PharmSouth used a maxon EPOS2 50/5 controller and utilise the dual encoder mode of operation to solve the issue with the elasticity of the suture.

The machine can inspect sutures ranging in diameter from 0.2mm to 1.5mm at speeds of up to 150m/min. The suture is reeled on an off-wind spool, then threaded via a three-axis inspection head, over a measuring encoder wheel. From here it passes through a three-axis diameter gauge, beneath a camera, to a knotting station (used to mark out any flaw positions with a knot) and it is finally wound on a take-up spool. The inspection head provides two digital signals: ‘major flaw’ or ‘minor flaw’. 

The off-wind and on-wind spools use maxon RE 40 brushed dc motors in conjunction with maxon motor postioning controllers EPOS2 24/5 (off-wind) and EPOS2 50/5 (on-wind). The off-wind runs in tension (current) mode, utilising a dancer arm whose input to the EPOS2 motor controller is via an optical potentiometer.

The suture is reeled on the take-up spool using a traverse screw, driven by a maxon EC45 brushless dc motor via an EPOS2-P 24/5 controller. This runs in ‘master encoder mode’, taking its positional ratio to the on-wind from the suture diameter, to ensure the spool collects the suture evenly. The master encoder mode electrically gears the guiding traverse motor to the speed of the suture, and this ratio is changed for each formula depending on the various thicknesses of suture. 

All three maxon controllers are linked via CANbus with the traverse EPOS2-P 24/5 motion controller, which powers the axis, instructing the other nodes and containing the control program. The EPOS2-P 24/5 controller runs an IEC 61131-3 program controlling the speed/tension and sensing of flaws, utilising the EPOS ‘position marker’ function. This permits the use of digital input in order to capture the instantaneous position at the time of the input being triggered.

This stored position is then used to reference the jog of the suture to different inspection/work points throughout the machine. The EPOS2-P initiates the camera to take the defect picture while simultaneously instructing the knotter to tie a knot to mark the defect. The EPOS2-P 24/5 program also processes flaw positional data, to determine if multiple flaws are only one large flaw, by implementing a monitoring window.

This inspection machine is a good example of the way Southampton based PharmSouth has revolutionised the suture processing industry over recent years. The company has developed a range of manufacturing machines, from pack winding to the attaching of the needle and stiffening the ends.

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