November 2013 Newsletter

The month’s newsletter will delve into the art (and technology) of operator interfaces.

Most machinery has an operator interface; whether it is an ATM, a vending machine, a dishwasher, a CNC lathe, an industrial furnace, etc… We all know of good and bad interfaces on the machines that we use often. Let’s start with what is likely the evolution of the operator interface for electrical machinery.

The first operator interfaces were a combination of a few push buttons and pilot lights. They were very basic and filled the need for 19th and mid-20th century equipment.

The next generation was more complex. They had multi-position selector switches, lighted push buttons, thumb-wheel switches and 7-segment displays. Some had strip-chart-recorders for process variables. And they had lots of wires, used lots of I/O points, required lots of programming, and were lots of problems at times. But sometime in this generation of operator interfaces, graphical over lays were incorporated. Basically a picture or drawing of the equipment was laid over the operator controls. For large or complex equipment, the visual -aid was a huge step in the right direction for the operator.

Next came the OIT (operator interface terminal). The OIT had a serial communication or discrete connection with the PLC. These were a step in the right direction, but had limited graphics and used function keys for navigation and control. The serial versions eliminated a lot of the required wires and I/O points. To its credit, the OIT gave way to the present day HMI (human machine interface).

Today’s HMIs are varying sizes and configurations of industrial touch screens. They are feature-loaded including; flash drive ports for data collection, multiple PLC connection schemes (Ethernet, Serial, Profibus, etc.), high resolution graphics, and a wealth of canned configuration functions. They can easily display graphs, alarms, bitmap images, and process animations. Some are even integrated with a PLC such that interface, I/O, and controller are in a single package.

Even with advanced technology there can be bad interfaces.

  1. Don’t make the interface more complex than it has to be. Keep it as simple as possible. If it only needs a push button and pilot light, don’t install an HMI.
  2. Keep frequently used controls separate from least used controls. Keep operator controls separate from maintenance controls. Use password protected screens for changes to items like recipes, PID parameters and alarm set points.
  3. Incorporate visual aids when the process or operation is more complex. A picture or drawing can be worth a thousand words.
  4. Place the interface in a location that keeps operator foot steps to a minimum.Often times interfaces are put where they are easiest to install, not in a location that best assists the operator. Also mount the interface at heights and angles that are ergonomic.
  5. Incorporate machine diagnostic into the interface such that machine problems are easily identified. Utilize help screens for operating, maintenance, and safety procedures. See our June newsletter section on Andon Indicators for more information on this topic.
  6. Take time with the operating team to specify the requirements for a new operator interface. It is more productive for the designer to have this information at the beginning of the design stage of an equipment project.
  7. Finally, providing a manual on using the operator interface can increase the learning curve for new operators and can provide a reference for features that are not used very often.

A Motor Tip

If you are experiencing a lot of 3-phase motor failures, check your system for voltage imbalance. According to the US Department of Energy, ‘voltage unbalance is probably the leading power quality problem that results in motor overheating and premature motor failure’. Google ‘motor system tip sheet #7’ for more details.