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Motion System Design
Field Report: Matters of the heart

Field Report: Matters of the heart

Stent-manufacturing machines use encoders for precise segmental compression at high speeds

Background: Interventional stent therapy opens clogged heart and peripheral vessels to restore normal blood flow, minimize damage to the heart muscle, and reduce heart attacks. The process involves placing and expanding an angioplasty balloon and metal stent at the blockage point in a blood vessel. The balloon must be folded to less than 0.1-in. diameter to fit inside the metal stent, which evenly crimps down around the balloon. Mounted on the end of a catheter, the balloon/ stent is inserted into a blood vessel and remotely maneuvered into position. As the balloon is inflated, the outward pressure re-expands the stent to force open the blockage. The balloon is then deflated and withdrawn, leaving the metal stent in place.

Problem: Manufacturing stent delivery systems was a slow, manual process — often taking 45 min. for one system. That changed when Machine Solutions Inc. (MSI), Flagstaff, Ariz., introduced automated equipment that delivers precision and repeatability, while cutting processing time to about 30 sec. The equipment pleats and folds balloon catheters and crimps metal stents, compressing them to a diameter small enough for insertion.

“Besides the enormous labor savings for stent manufacturers, automation also ensures a more precise process,” says Scott Mickelson, MSI project manager. Such precision is critical; the balloon must be uniformly folded to eliminate interference when the stent is crimped on and to ensure no irregularities when the balloon and stent are expanded.

Solution: The machines use segmental compression technology with RGH24/25 optical encoders from Renishaw Inc., Hoffman Estates, Ill., to deliver exact, predictable expandability. The encoders, mounted in a partial arc, control wave pleat and folder motion to 5-mm resolution.

MSI also uses Renishaw encoders for its SC family of stent crimping machines. Configured as diameter encoders, they continuously monitor crimp diameter as the stent is compressed around the balloon.

A combination of factors led to using these optical encoders: high resolution and repeatability, small size, no-wear optical design, LED-guided alignment simplicity, and the versatility of the flexible metal tape scale, which allows use in linear or radial applications.

The RGH24 encoder readhead measures 1.42 x 0.55 x 0.6 in. and weighs 0.4 oz. The RGH25 readhead, with separate interpolation electronics, has the same length and width, but is 0.51 in. high and weighs 0.3 oz. The encoders combine the compact readheads with flexible, adhesive-backed steel tape scale. Gold-plated and protectively coated, the reflective 20-mm pitch scale provides a linearity and slope-corrected accuracy of ±3mm/m.

Red and green LEDs on the readhead or interface speed setup and provide confirmation of optimum scale alignment. The LEDs are tied into a signal monitoring circuit, providing visual alerts of low signal amplitude to prevent unrecognized loss of encoder count.

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