The wind-up and the pitch

Senior Editor

Catheters; microstents for repairing aneurysms. These medical devices consist of precisely wound, small-diameter coils of extremely fine metal filaments. How fine? "We've made coils from 0.0004-in.-diameter platinum wire that has a breaking strength of just 1.2 gm," says Dale Henson, owner of Engineering by Design, a maker of miniature coil winders. "It is difficult for human hands to get wire that small off a spool and around the filament-dispenser pulleys without breaking, but the winding machines handle it with no problem." The trick: precise tension control.

Tension control

EBD winding machines use two independent servoloops to control filament tension to 0.1 gm. A dancer tensions the wire by means of a servodriven spring that applies torque to the dancer. Force feedback for the spring comes from a standard load cell. The load cell attaches to a pivot arm that magnifies force of the wire tension. An optical encoder on the dancer signals the spool motor to maintain slack so the dancer remains in the middle of its travel.

The approach accurately maintains tension, independent of pay-out speed. This is important because minute tension fluctuations and mandrel diameter variations can cause relaxed coil ODs to vary by more than the allowed diameter tolerance in some cases. The ability to precisely control tension has other uses. Changing preload on closed-pitch coils produces a soft tip at the distal end of a catheter and a stiff shaft for the proximal end, for example.

Angle control

Precise control of wind angle is equally important for making tight-toleranced coils. Wind angle for fine wires ranges from about ±30° from perpendicular, with minus angles rotated CW. "Larger-diameter wires can handle more severe angles, but we tend to focus on wires 0.003 in. and smaller," says Henson. Angle in this case is related to coil preload and pitch. Minimal angle or tension variances produce a detectable pitch variance. The relationship is different for every combination of small wire sizes. Pitch tolerances of ±0.0001 in. are not uncommon so control resolution must be better than that.

A vision system with two independent cameras run by a single processor monitors coils as they are made. One camera looks at the shadow of the wire as it wraps around the mandrel, which is itself a wire with a diameter of at least 0.001 in., and reports X-axis position. An algorithm figures Y-axis position based on system geometry. Angle is calculated with a second-order polynomial that approximates the trig function. The system senses wire angle to 0.01° and controls it to 0.05°. Adjusting the traverse position in response to wire-angle error returns the wire to the proper angle.

The system software programs for a single angle but also ramps between two angles along the coil length to fine-tune coil stiffness. Wire angle can additionally be programmed for a specific angle with the mandrel chucks stopped to minimize transition regions. This permits the combining of angle-controlled, tight-pitch coil sections with pitch-controlled, open-pitch sections, all on the same coil. The software also supports repeated patterns which simplifies data entry and setup time. This way, multiple small coils can be made on a single mandrel with visually discernible separations between coil sections.

The second camera monitors the wind area and is used primarily as a process-development tool. It could as well measure actual pitch and provide pitch-control feedback to the machine, but that is of limited value because a coil must be complete before taking measurements.

The coil-making process can be controlled and monitored remotely over a network. Here, a combination Ethernet router/modem from Rockwell Automation replaces an Ethernet switch and external PC used in previous models, greatly simplifying setup. "The arrangement doesn't use a company network, which is typically not an option because it violates most Internet security policies," says Henson.

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