I would venture a guess that most, if not nearly all, of those reading this blog will have been challenged by some speed constraint in their past projects. In my own history, I have had more than one occasion where I have had a collision with speed.
Communication interfaces can have an impact on batch execution or order downloads or usage uploads or data collection. Servos, vision systems, batching systems and machine control can all have their challenges.
Packaging lines warrant some caution because they can challenge the limits of classic PLC control. On one occasion, I had a need to manage accumulating bi-flow conveyors on a packaging line. It didn’t seem too challenging at first—until I discovered the window of time I had to work with in counting the packages in and out. In addition to managing the accumulating conveyors, I also had to collect the production data and move it to a server for historical and reporting purposes.
Packages were flying by at such a pace that it required a high-speed counter to reliably capture the counts. The high-speed counter was running asynchronously to the PLC scan, as was the server. Keeping track of the high-speed counter in the PLC wasn’t too much of a problem, but getting the proper count to the server required a hand-shaking routine to make sure we didn’t miss an exchange or double up on a transmission. Being able to monitor communications and respond timely when communications fail is critical as well.
On one occasion that involved a packaging line with a vertical accumulator, the line speed was such that, without a dedicated controller, we weren’t going to be able to actuate the accumulator stop between boxes. Don’t forget to allow for the solenoid reaction times and at what air pressure. The result was to apply a dedicated controller on the accumulator and simply manage it as a separate machine on the line.
On another occasion, I was pulsing a collection of hose connections to determine what was connected to what and failed to consider the slew rate of the output card and the input card—in other words, the time you tell an output to go high to the time it takes to reach full voltage. Then how long it takes for the input card to see that signal. You also have to consider the PLC scan rate. When considering all of this, you have to have some margin in order to be certain that you’re going to see that exchange every time it happens.
Recently, I have heard some refer to the Nyquist theorem, which sounds like a good rule of thumb. Used in digital signal processing, it says that the scan rate should be two times the sample rate. In my case, after getting phantom trips and consulting with a colleague, I finally resorted to some basic engineering and ran the numbers only to find that my problem was entirely self-inflicted.
Cycle times on batching systems can also be challenging. Rates and purge times are not the only constraints. If you need to have data exchanges with a server, you need to consider the responsive capabilities of the network the server resides on.
If you’re not an experienced motion control resource, it’s easy to assume that a servo can handle whatever you throw at it, but you should test your theory as soon as possible or even invest in a test scenario before committing to a solution. Inertias and accelerations and accuracies have an influence on one another. Proper sizing is also very important, and be sure to account for a safety factor. In the case of servo applications, especially those with high speed and tight tolerances, more is not always better. A servo that is oversized for the application isn’t as good as it might first sound.
Vision with speed has challenges of its own as well. There are a number of things to consider when looking at the effect of speed on a controls solution. Today’s PLCs and PACs are far more capable than they used to be, providing the capabilities to execute high-speed applications in separate tasks. Yet without proper consideration, speed can really slow you down.
Ray Bachelor is chairman of the board and Larry Asher is director of operations at Bachelor Controls Inc., a certified member of the Control System Integrators Association (CSIA). For more information about Bachelor Controls, visit their profile on the Industrial Automation Exchange.
