Human-machine interface (HMI) can offer a digital version of almost any button, knob or light, but analog still has its place on machines, whether it’s for its ruggedness, for no-look interaction or for situations in which touchscreen operation isn’t an option.

We posed several questions regarding these time-tested components to a panel of industry veterans to see what light their experience and expertise might shed on the subject.

How are end users implementing stack lights on multi-station assembly lines?

Will Healy III, Balluff: End users are expecting more from their stack lights. Ultra-reliable LEDs with good visibility are becoming the de facto standard on the shop floor. As manufacturing is moving toward more flexibility, manufacturers are looking for the stack light to follow suit. As factories receive new projects for the line or if site regulations change, stack lights that can be reconfigured easily are needed. RGB LEDs allow for electrical or software configuration of the color and help to ease the integration of those new requirements. Multiple vendors are now offering control-configurable RGB LED stack lights via the universal standard IO-Link.

Will Healy III is marketing management director at Balluff.

Lee Clore, Onyx Industries: Stack lights can serve a number of purposes in multi-station manufacturing as a first line of defense to keep a line running optimally.

Just like on a small machine, operators on long assembly lines can see the equipment status in their areas of responsibility in real time via an indicator light. This may include visual feedback for normal running operation, warning, or fault conditions—green, yellow, red, respectively. They can sometimes actuate blue or white stack light color segments to get the attention of supervisors, maintenance or raw-material fulfillment personnel.

Stack lights along a longer assembly line allow the operators at each machine segment to follow work flow upstream and downstream. This can let them know if they are holding rate—keeping pace—if a work cell upstream may be down for a bit or if a station downstream is backed up. This visual feedback can initiate resource management by a supervisor to add an operator or lead-hand to deal with a condition or bottleneck. It may lead to a maintenance call. Supervisors, lead-hands and other personnel responsible to keeping operators on task can be anywhere up and down the assembly line. Well-placed stack lights, in line of sight to normal work areas, can capture support personnel’s attention quickly and from a distance.

The specific details of a down situation may or may not be communicated by a stack light. Typically, that data is conveyed with pilot lights or at the HMI level. However, those devices require that the operator be at the control panel. On a long assembly line, the stack light may provide less data than other devices; however, its impact is immediate with a wide coverage area. The addition of a buzzer adds an audible element to capturing the attention of plant personnel.

Lee Clore is owner/controls designer at Onyx Industries.

Robb Weidemann, Banner Engineering: Some multi-station lines use a stack light at every station, with or without a common stack light indicating overall line status. Other multi-station lines are using only limited stack lights for overall line status, but supplementing with smaller local indicators. Wireless stack lights are a trend on larger lines because they simplify cabling and allow for future expansion and flexibility.

Robb Weidemann is senior business development manager at Banner Engineering.

Alvaro Sanchez,Rockwell Automation: When it comes to signaling solutions and their functions, you can look at them on three levels: plant, machine and operator. At the plant level, stack lights and alarms are used for overall work-flow optimization and to broadcast status to many operators, potentially the entire plant floor. For plant-wide indication, larger horns, beacons and virtual signaling through mobile devices are the norm. At the machine level, where status is being broadcast to a smaller group of operators, the signaling solutions are smaller and focused more on improving line productivity. Similarly, at the operator level, the focus is on improving productivity using panel-mount alarms, pilot lights and illuminated push buttons to help an individual operator. The different uses for signaling and their impacts to productivity are best shown in Figure 1.

Alvaro Sanchez is product manager at Rockwell Automation.

Sopan Khurana, Patlite: Typically, floor managers need to keep track of multiple assembly lines and be able to quickly understand the status of each station or each line. The most common way that end users achieve this is by implementing one three-color stack light on each station to indicate a critical error or line stoppage—red lit; call for more materials or maintenance—amber lit; and normal operation—green lit. Today, there are several modern improvements, such as field-programmable stack lights that can be made entirely a single color at a time with 21 colors possible, greatly improving visibility across the plant floor, as well as MP3 voice-enabled devices that are able to communicate more complicated status information.

Sopan Khurana is applications engineer at Patlite.

Michael Day, Siemens: Modern discrete manufacturing lines consist of multiple machines arranged in cells, and multiple cells are arranged into manufacturing lines. More manufacturers are using andon systems, which provide overall line status and performance summaries in a textual format on a large board or display, while using stack lights at the machine level for a 360° visual and sometimes audible status indication. A best practice is to mount the stack lights at the highest point on the machine, or even elevated high above the machine on a pole to provide the best line of sight for workers and supervisors. Another trend we are seeing is mounting the stack light inverted from the ceiling above the machine. The ceiling mount presents other challenges in connecting to the control system of the machine but is a viable solution with benefits. Modern multi-station lines are finding network solutions like AS-Interface useful for connecting the machine to a stack light instead of hardwiring, especially for ceiling-mount applications. AS-Interface is unique in that it’s not only a single cable with two-conductor fieldbus for communications, but it also provides the needed 24 Vdc needed to power the stack light.

Stack lights are simple to implement and provide quick status indication for a machine. Red indication might identify a machine stoppage of some sort requiring user intervention. Green typically indicates normal operation. Stack lights are also used to identify other information—for example, the raw-material hopper’s getting low and needing to be replenished could be indicated by a flashing blue.

All in all, discrete manufacturers with multi-station assembly lines like the simple indication a stack light provides, and there are no signs of this technology being replaced anytime soon, only innovated more.

Michael Day is industrial control products market development manager at Siemens.

Michael O’Neill, Werma-USA: Multi-station assembly lines and processing lines tend to cover a lot of real estate. Sharing critical machine status becomes very important. With the advent of wireless transmitters and receivers, it is very easy to share the status changes of equipment throughout long assembly lines. This improved communication via signal devices such as stack lights maximizes the signal/information exposure to improve response times, increase productivity and reduce costs.

Michael O’Neill is president at Werma-USA.

Zach Tinkler, Schneider Electric: Whether an assembly line has an operator at each station or not, typical stack lights for assembly lines will have a red-yellow-green light configuration and will place them just off a remote control station or at the top of the control box. Most of these stack lights will be pole-mounted to give additional height for maximum exposure. In many cases, these stack lights will also have sound annunciation. The red light indicates warning of a maintenance issue or a jam; a yellow light would be caution of potential issues but not a line down situation; and finally the green light is to indicate the line is moving smoothly without fault. On multi-station assembly lines, there will be a stack light at every station.

Zach Tinkler is U.S. product manager, control and signaling at Schneider Electric.

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