What control engineers need to look for in a display

Dec. 8, 2016
Workstation HMIs require operator considerations

As controls engineers, we seldom make component selection decisions based on how a human operator interacts with the products we choose. If we do our job well, humans rarely need to see or touch the control devices in any particular system. We may think about maintenance requirements and use connector-style sensors versus hardwired versions. We may choose a computer-programmable VFD rather than one that requires manually entering all the parameters. One aspect of our job that does need to take human interaction into account is the human-machine interface (HMI). Other than designing operation panels with logical button and switch layout and HMI screens that are informative and easy to understand, the physical location of the control panels also requires some forethought. Graphical HMIs or operator interfaces and control panels consisting of buttons, switches and indicators share many of the same concepts.

If the enclosure material selection is made early in the design cycle, it can affect other decisions made later. A nonmetallic enclosure reduces the total weight supported by the mounting fixture; a metallic enclosure can require a less complex mounting bracket and may not need additional mounting point reinforcement. Nonmetallic enclosures are resistant to a wider range of chemicals than metallic versions. However, the HMI display and other actuators must be rated at the same level of resistance as the enclosure. Some HMI manufacturers offer optional environmental covers to increase resistance to some environments.

Standard machines and equipment typically have well-defined operator interface designs, refined over the machine’s development cycle. Special machines and custom applications often have an evolutionary aspect to design. Additional controls and I/O are often added as the system is tuned and finalized. Designing an HMI enclosure with ample room for expansion and additions will result in a more finished look and feel at the end of commissioning.

The positioning of the display, height and viewing angle are detailed in ANSI/HFES 100-2007 Human Factors Engineering of Computer Workstations and ISO 9241. Although these documents are written for computer workstation design, many of the concepts and recommendations are directly applicable to industrial installations.

When using a touch-sensitive display, the viewing angle of the display and the size of touch areas are particularly important. In the United States, the anthropometric eye height for the average worker is about 62 inches (1,575 mm). When a worker is shorter or taller than average, an increased viewing angle requires a larger defined touch area for useful input. Constantly looking up or down at an HMI also contributes to increased worker fatigue and eye strain.

When HMIs are used to display considerable amounts of data that the operator must access throughout the manufacturing cycle, consider mounting the display on an adjustable-arm device that allows for vertical and angular adjustment. HMIs that display small amounts of information or require little operator input at any one time may not benefit from a flexible mounting arrangement and may not justify the added expense. When the interface requires a large amount of data input, an external keyboard can sometimes be added for better operator efficiency.

If the HMI application includes manual machine control, mounting the display on a swing arm allows the operator to be in multiple locations during operation. Visual feedback from the actuators is often helpful. Many swing-arm systems allow vertical adjustment to increase operator comfort while using the HMI, and attaching handles to the enclosure allows the operator to more positively position and steer the assembly. A swing-arm-mounted HMI may also offer the ability to be moved completely out of the way when not being used. If a swing-arm assembly is selected, an under-engineered system will amplify any machine vibration and allow the arm to start to sag over time. Using high-quality equipment to build the swing arm is not wasted money.

Some manufacturers offer handheld devices that attach to the control system with an umbilical-cord-like cable. The devices may include operation buttons, switches and hardwired emergency-stop buttons. These portable HMIs usually have a rugged housing and are cushioned against shock. Similar to a robot teach pendant, these devices allow greater freedom of movement around the machine while visually monitoring manual operation. The display area of these devices is limited; simple screen design with a larger number of pages is preferable to complex screens with small text and icons.

HMI software is becoming available for tablets and smartphones. While they cannot offer a wired emergency stop, they do offer complete freedom to move around equipment while monitoring and controlling machine operation. Security issues that arise when using wireless technology in a control system are addressed in ANSI/ISA-100.11a-2011 and parts of IEC 62443.

A console-style workstation allows ample room to mount an HMI and control devices without crowding too many mechanisms in a small space. A desk-like console naturally appears to be a main area of control and can be an excellent place to coordinate overall machine operation. A console workstation often provides additional enclosure space. If using the extra space to mount additional equipment—PLCs, drives, power supplies—consider the maintenance person who must work on the equipment. Most electricians would likely prefer not to lie on the plant floor to do voltage checks.

As controls engineers, one of our customers are the operators who operate the machine or system. We must produce our designs with their comfort and efficiency in mind. The goal of our design should address ease of use as much as efficient production.

About the author
Thomas Stevic is a controls engineer at Star Manufacturing, an engineering and production company in Cincinnati. Contact him at [email protected].

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