Machine Design

Matching Position and Velocity on-the-fly

Labeling, packaging, and bag-folding machines synchronize cutoff shears and other actuators to the same velocity as the piece they cut or sense. Thus, the controllers must provide precision positioning and velocity matching between the two components. Although several different motion controllers might fit an application, the programming tools that come with it can make the difference between success and failure.

Motion controllers for these kinds of jobs require a segmented electronic cam, floating-point electronic gearing, and high-speed interrupt commands to handle on-the-fly position and velocity matching. For example, a typical flying cutoff machine controller uses the electronic cam to match the position and velocity of the rotating knife to the material on the web. The knife’s motion profile is divided into four position and velocity segments: acceleration, velocity matching, deceleration, and wait segments. The velocity of the knife is matched to the web speed in segment two because the change in position with respect to time is dx/dt or velocity. An encoder on the web is the cam source which ensures matched positions. Additional commands allow advance or retard (cam floating zero), amplitude (cam scale), and frequency (cam length) to be programmed dynamically. Thus, the speed of the knife is programmed on-the-fly to the required cutoff distance and frequency.

One controller also has other programmable commands for similar applications. For instance, in labeling machines, the labels hit the material on the web when the controller matches their velocity and position. The controller does this with floating-point electronic gearing to match velocity, and with the segmented electronic cam mode to match position. Packaging or bag making applications also can use the third feature, the highspeed interrupt. This interrupt command includes a programmable window to look for registration marks to protect against jamming machines.

Information for this article was provided by Gary J. Hagar, Acroloop Motion Control Systems Inc., Minneapolis. For more information on synchronizing systems

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