Eliminate Unsightly “Bars” Across your Substrate with Direct Drive Technology
Reprint of Kollmorgen Technical Article posted January 08, 2014, by Tom England
Coating and lamination applications demand precise speed regulation in order to avoid velocity ripple that causes uneven coating and undesirable horizontal bars across the substrate. The key to achieving the most uniform coating is minimizing the variations in velocity as well as in metering of the coating material. Coating and laminating applications are characterized by compliance caused by the elasticity of the web, which in turn produces variations in torque requirements. Web handling machines face the challenge of handling these loads while avoiding velocity ripple that can cause uneven coating and unsightly horizontal bars across the substrate. As an example, consider film coating, where depositing a dark film onto the substrate material at varying velocity would result in a series of dark and light “bars” across the material. As web speed and quality requirements have increased, the inevitable inaccuracies in the mechanical transmission and servo system have become a limiting factor on coating and laminating uniformity.
Effect of Mechanical Transmissions Mechanical transmissions can have a negative impact on coating and laminating quality by causing tooth or belt chatter. Backlash is inevitable in any mechanical transmission system. Transmission components such as lead screws, gearboxes, and belts and pulleys all contribute error between the motor and the load. Even when a geared system is tuned very tightly, within a short period of time the gears will wear and backlash will begin to occur. Backlash causes the roller and cylinder to rapidly accelerate and decelerate as each gear teeth bounce back and forth against each other. The result is uneven coating of the substrate manifested by the appearance of alternating light and dark horizontal lines on the product.
Advent of Direct Drive Systems
In some cases these concerns can be adequately addressed by selecting high-quality mechanical transmission components with ultra-precision tolerances. However, the added expense can become prohibitive and the components will still wear eventually. The ultimate solution is a direct drive rotary system, which eliminates the transmission altogether. In direct drive systems, the motor directly drives the load. The accuracy of a direct drive solution can be up to 60 times better than that of traditional systems and audible noise can fall by more than 20 dB. Other measures such as servo response (bandwidth), machine parts reductions, and reliability also can improve dramatically.
When the load is directly coupled, there is no limitation on the inertia mismatch between load and motor (Provided there is no compliance introduced in the coupling method – see blog post “Reflecting” on Inertia Ratios). The servo loop gains can now be increased significantly to provide the necessary servo stiffness to achieve excellent speed regulation to optimize product quality.
When the transmission components such as gearboxes, belts, rack and pinion, and pulleys are eliminated, the servo system becomes free of such negative factors as compliance, backlash, and component wear. The accuracy increases, inertia-matching requirements relax, acceleration and deceleration improve, maintenance becomes unnecessary, and the product life increases by a significant degree.
Information on Kollmorgen’s family of Direct Drive Motors can be viewed at the link below-