As we continue to modernize our transmission grid, the control room, whether in a utility, RTO or ISO, grows more and more crucial. We've asked Charles Davis, Director of Engineering at Mitsubishi Electric's US Visual and Imaging Systems Division, to share his views on the display technologies impacting the control room today and in the future.

EET&D : What type of display technologies are in use today in control rooms?

Davis : There are many, among them older front projection systems, edge-blended direct throw rear-projection systems, legacy lamp-based rear projection cubes, dynamic tile boards, thin bezel LCD, and LED-based rear projection cubes.

For new purchases, however, most utilities and ISOs consider only thin bezel LCD panels with LED backlights and LED-based DLP rear projection cubes. Some applications can manage with thin bezel LCDs, as long as they can tolerate the larger image-to-image gap, but LCD is far from ideal in a control room operating 24x7x365. Inherent aging artifacts can lead to color shifts, image retention and image staining, and the panel lifetimes are, at best, only in the 50,000-hour range.

For mission-critical control rooms, LED-based rear projection cubes are a far better choice. LED cubes have much smaller image-to-image gaps than thin bezel LCD. They have extremely long lifetimes (approximately 80,000 to 100,000 hours), they don't suffer from aging artifacts, and they require little to no maintenance over the display lifetime.

EET&D : What types of display technologies are trending as future options?

Davis : The three that come to mind are LED upgrade engines, OLED panels and narrow pixel pitch direct LED technology.

One of the major downsides to the lamp-based technology still in use in many control rooms is that lamps must be replaced frequently, at a cost and downtime that are undesirable in a control room environment. A new alternative is to replace lamp-based engines with an LED-based upgrade engine. Since all the structural components and screens can be reused, the downtime and overall cost to make the change is greatly reduced. There is considerable interest in this option.

OLED technology enables very thin displays with very high contrast ratios. As this technology expands in the consumer market, I expect it will make its way into the control room. Today the panels are very expensive to produce and there are questions regarding lifetime, but as the technology matures it may become a viable option.

There is a lot of buzz about direct LED technology, familiar for its use in stadium video scoreboards but now becoming practical for many indoor, high-resolution applications. Advantages include the fact that direct LED components are seamless, very bright and relatively thin. Currently, however, this technology is extremely expensive, the pixel size is too large for most control rooms and many questions remain unanswered about long-term reliability and performance. Still, I believe this technology will become more viable over the next few years.

EET&D : What is the difference in a hardware-based controller versus a software-based controller in a video wall?

Davis : All video wall controllers are software-based to some extent. A hardware-based controller has a centralized processing unit with display management software that allows users to position any computer or video source anywhere on the display wall and scale it to the desired size. Graphics cards drive the displays and capture cards can accept a wide range of analog or digital inputs. In some cases, the controller may accept IP sources.

A software-based controller is more of a network solution. The software runs on off-the-shelf PCs and servers that manage and decode content accessed from the network to drive the displays. Network-based solutions are very attractive for control rooms that are running SCADA applications, since they can run on a server and the content can be displayed from the application pixel-for-pixel on the video wall. Software-based controllers also open the door for collaboration between users across multiple sites.

EET&D : What is the difference between a liquid-cooled versus an air-cooled video wall? What are the pros and cons of each?

Davis : Proper cooling is the key to maintaining the lifetime of the LEDs in a rear-projection system. Even a small rise in the operating temperature of the LED can greatly diminish its lifetime. Both the liquid-cooled and the air-cooled systems can do a good job of controlling the temperature and insuring long life if designed properly.

The liquid-cooled system consists of a pump, hoses, reservoir and radiator. The pump is used to circulate liquid coolant through a cooling block to control the temperature of each LED. The heated liquid is then pumped to a radiator which is cooled by a fan and then returned to the cooling block.

The air-cooled system is similar in concept, but it uses a heat sink,' a one-piece design that combines a passive radiator with a heat pipe mounted directly to each LED module. A fan circulates air across the fins of the radiator to remove the heat.

Both of these methods provide excellent temperature control. The major downsides to liquid-cooling are that it requires a more complex setup more prone to failure, and that the pump must be replaced every 30,000 to 40,000 hours. The coolant must also be removed and replaced, requiring special care because the corrosive substance ethylene glycol is typically used.

To avoid the need for scheduled maintenance and resulting downtime, air-cooled systems are preferable in LED light engines.

EET&D : What factors should be taken into an account in designing the video wall?

Davis : We need to look very carefully at what content will be displayed, how it will be managed and how it will be arranged on the wall. We also need to consider how many operators will work in the control room and where they can best be placed in relation to the wall. The process is similar to creating a blueprint for a building, and it takes as much attention and discussion. Many users initially ask for the highest resolution displays available, but the viewing distances and content dictates what the required resolution must be. You may pay for a higher resolution that the operators will never see.

EET&D : How do you decide on the proper size and resolution for the video wall displays?

Davis : Pixel pitch, the distance between the center points of two adjoining pixels, is what's crucial, given how far the operators will be positioned from the video wall. That's because a video wall will be made up of a large number of individual displays that form a combined image. If our total pixel count is the same, it really doesn't matter if those individual displays are XGA, SXGA, UXGA or WUXGA. On the other hand, if the pixel pitch is too large or too small, readability will be compromised.

To determine the maximum pixel pitch we can use, we consider the fact that an image, in order to be sharp, must be free of any pixel structure. Since the limit of the human eye's ability to discern individual objects is about one arc minute, or 1/60th of one degree in our field of vision, we need pixels that are one arc minute or smaller. We use that fact, together with the distance from the closest operator to the video wall, to calculate the maximum pixel pitch of our displays.

To determine the minimum pixel pitch, we need to know the distance of the farthest control room operator and the font size of the text to be displayed. Our goal is to use a display with pixels large enough that text and graphics are readable in the back of the room.

Once we know our content requirements, minimum and maximum pixel pitch, we can realistically look at display options. At this point initial cost, reliability and lifetime maintenance costs come into the equation.

EET&D : Of those, are certain factors more important than others?

Davis : Once we have a video wall design that will do the job we need to do, utilities consistently put reliability at the top of the list when choosing the individual displays. Ease of maintenance and total cost of ownership usually run a close second, and energy efficiency is becoming a factor as well. In all these categories, LED rear projection cubes score high marks and are currently the ideal choice for 24x7x365 control room applications.

EET&D : Thank you, Chuck, for your invaluable insights. Our readers will no doubt appreciate your thoughtful analysis of current and future display wall technology and its impact on the control room.
 

About the Author

Charles Davis, Director of Engineering, Mitsubishi Electric US Visual and Imaging Systems Division, manages the design and development of display technology.