Cycloids take the edge off sharp motion profiles

June 1, 2000
In a feed-to-length, web-pulling application using nonrigid materials such as plastics, films, and paper, the web frequently breaks from shock loads induced by abrupt acceleration and deceleration changes. The problem is often the motion profile itself. One of the worst, but most often used profiles for such applications is a trapezoid. The profile's transition points produce the shock loads in the web material when feed rates change

Advanced motion controls make it easy to customize acceleration and deceleration profiles to smooth out the transitions. For this application, a cycloidal profile provides uniform loading over the entire acceleration and deceleration pattern and minimizes the shock at transition points. The smooth patterns also demand less torque so motors can be smaller. Eliminating these shock components allow feed rates to be increased as much as 2.5 times without breaking the web.

Although this cycloidal approach has proved successful for many applications, occasionally other factors creep in that require a more complex solution. For example, some web materials introduce a jerk component (change in acceleration) into the profile between transitions. Another variable is called stiction, a grabbing motion caused by a momentary change in the coefficient of friction between rollers and sticky web materials such as plastic. In these cases, a load spike develops at the transition between stationary and dynamic motion. However, modern diagnostic software lets machine builders graphically see what is happening during the acceleration. This can be viewed in the velocity, torque, or position mode. Once the problem has been identified, the designer can selectively eliminate the shock components. For this example, one of two procedures may minimize the stiction effect: Applying tension gradually in the area where the torque spike was identified (to smooth the release of the material), or take advantage of the electronic motion control's flexibility.

In the first method, the torque determines the cycloid acceleration profile because web tension is directly proportional to applied torque. Initially, the torque is applied as usual to the point where the spike was seen. This somewhat lengthens the web. Then the torque is quickly reduced and reapplied. The plastic's memory causes the web to remain elongated while the reduced torque drops the friction force. This produces a relaxing effect which lets the web release with minimal jerk, reducing the risk of breakage.

In the second approach, the electronic motion profile can be viewed as a map, with the actual profile depicted as a series of points connected on the curve. The more points, the smoother the curve. Some controls position moves from point to point on the curve using an array table to develop a path.

A more sophisticated interpolation process called continuous path generation includes incremental control over both position and velocity. The key is that the velocity control has feed-forward capability, and speed and position commands are controlled simultaneously. The result is a smooth uninterrupted move no matter how complex the profiles. Also, there is no need to create an array table of point plots for the curve. The controller automatically plots the path by entering the start point, end point, and desired profile.

Portions of this article were contributed by John Downie, Yaskawa Corp., 2121 Norman Dr. S, Waukegan, IL 60085, (847) 887-7219, Fax: (847) 887-7310, www.yaskawa.com

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