One of the biggest challenges, and also one of the best opportunities, for a controls engineer is to add value to a project with the operator interface or HMI. There are many trade-offs to be considered between what control program parameters should be exposed to operators, supervisors, or maintenance personnel under different levels of password protection, and what should be hidden.
Similar trade-offs apply to the alarm system. If you add too many trivial alarms, the operators soon begin to ignore all alarms. If there are too few or insufficiently detailed alarms, you'll get call-backs to the machine manufacturer's service department about issues that could be easily handled by the owner's crew -- this is if they had enough information.
We once built a packaging machine that used a servo axis in electronic gearing mode to precisely synchronize the speed of two sections of the machine. The gear ratio needed to be fine-tuned on each machine to correct for slight mechanical tolerance issues, but once set, the numbers would never change for that particular machine unless some major components were replaced.
Foolishly, I put this parameter (suitably password-protected to allow only the highest level maintenance access) on the HMI for the convenience of our maintenance people during commissioning. All was well until soon after the third machine was commissioned in the customer's factory.
We started getting calls about bizarre erratic behavior in the machine. After considerable troubleshooting and a site visit, we discovered that someone in the plant had been monkeying around with this setting. We set the number back to its correct value, and then I removed that control from the screen.
A week later, we received a call from the plant manager insisting that they must have access to that control restored since they needed to adjust it periodically to keep the machine running properly. No amount of logic or reasoning by our sales, engineering, or service departments could convince this fellow that any change to the setting would just make things worse. So I proposed a "fix."
I put the control back on the HMI, but I wrote it to a dummy location that did nothing. I also put some additional logic in the program that popped up a message any time the new dummy control was touched: "Warning: stop the machine before attempting any adjustment of this value." I also added logic to the program so that after any change in the dummy value the first time the machine was restarted it would run slowly for the first 10 seconds while displaying a prominent blinking message that said "Recalibrating."
The plant manager was happy with his modified HMI and the comment was relayed back to me that all of the earlier problems had been the result of our failure to notify them that the gear ratio was not to be adjusted unless the machine was stopped.
This entry was submitted by Kim L. Ground and edited by Rob Spiegel.
Kim L. Ground holds a BA degree in distributed studies chemistry/math/physics from the University of Colorado at Denver. For the past 25 years he has been employed as a controls engineer for companies building custom high-speed packaging and electroplating machinery. His particular areas of interest include PC-based controls, mechatronics, and integration of mechanical and wet chemical processes.
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