Dr. Günther Brandenburg, speaking at the IWEB conference at Oklahoma State University in 2011, talked of making the controls as Simple as Possible, but as Accurate as Necessary. His keynote address was titled "Advanced Process Models and Control Strategies for Rotary Printing Presses." This sounds like obvious advice, but we need several metrics to make sense of it.
For drives, we know that Simple as Possible is a minimum time spent tuning the drive speed regulator and tension regulator. We are done.
We are not quite as sure about as Accurate as Necessary. We lack the metrics. In many cases the only metrics we have are speed accuracy to 0.001% and tension control to 1%. Not provided is the response performance (units of radians/second) and maximum overshoot.
These metrics may be subjective requiring input from operators as well as process and electrical engineers. Metrics defining the accuracy required will determine the specifications of the drive, tension measurement and even mechanical considerations such as roller balancing tolerances, maximum roller inertia, bearings and speed reducers. The tension regulator dynamic response requirements will define the effort required to tune the drive.
My observations are that we are:
- Unaware of the required accuracy
- Installing devices that are accurate enough for most web-handling applications
- Not taking enough care with the entire drive train from the drive to the motor to the couplings, speed reducers and roller balancing
- Not tuning the drives to meet the necessary accuracy requirements
- Doing a good job of keeping things simple