Backlights evolution for human machine interface (HMI) panels
Technology Update: White LEDs show high-temperature advantages for touchscreen HMIs or operator interface backlighting compared to cold cathode fluorescent lamp (CCFL) backlit liquid crystal displays (LCDs).
Mark T. Hoske
From cathode ray tube (CRT) displays to cold cathode fluorescent lamp (CCFL) backlit liquid crystal displays (LCDs), and now white light-emitting diode (LED) backlit LCDs, touchscreen HMIs or operator interfaces have come a long way since introduction in the early 1990s. Flat-panel displays have become very popular in the automation industry with applications such as industrial computers and human machine interfaces (HMIs) connecting to PLCs and embedded controller boards.
Understanding the history and development of a technology can be a tremendous aid in properly using it for a given application. HMIs, and in particular the backlights behind the HMIs, have evolved significantly for industries such as automotive, medical, packaging, pharmaceuticals, food and beverage, transportation, and many others.
Understanding the technology behind an HMI is very important when deciding which to use. Parameters such as display life, picture quality, and display brightness all come into play when deciding which HMI will best suit an application and its environment. Modern HMIs with LCDs are made up of several components. One of the most significant components of the LCD is its backlight, responsible for the light emanating from the screen. When flat-panel LCDs replaced the older CRT displays like the ones used in older Panelview 1200s and 1400s from Rockwell Automation, there was a significant advancement in each of the above noted parameters. LCDs with a CCFL backlight tremendously enhanced the quality, brightness, and life span of the HMIs as compared to the old CRT displays. CCFL backlight for LCDs began with a life span of only 5,000 hr. The CCFL backlit LCDs improved over time, increasing from 10,000 to 15,000 to 20,000 hr, and all the way up to 50,000 hr of backlight life. Life span for CCFL backlit LCDs depends on the HMI ambient temperature and humidity. CCFL backlit LCDs for more expensive thin-film transistor (TFT) LCDs operating at 25 C (77 F) normally last about 50,000 hr, but the backlight life span dramatically changes with temperature. For example, if the temperature of the LCD rises to about 40 C (104 F), CCFL backlight life deteriorates to roughly 20,000 hr (Figure 1).
As for relative humidity’s effect on life span, the “wet bulb” temperature of a CCFL backlit HMI is 39 C (102.2 F). Once a CCFL backlit LCD reaches its maximum humidity rating at a given temperature, the backlight will give up (Figure 2).
CCFL challenges relate to how they work. The CCFL light source is classified as an electronic component, and is, in simplest terms, a gas-discharge light source, which produces output from a stimulated phosphor coating inside the glass lamp envelope. The typical CCFL is a hollow glass cylinder, coated inside with a phosphor material composed of rare earth elements and sealed with a gettered (impurity-free) electrode at both ends (Figure 3).
Ultraviolet energy at 253.7 nm is produced by ionization of the mercury and penning gas mixture by the application of high voltage to the electrodes. UV energy from the mercury discharge stimulates the phosphor coating, producing light. A CCFL can be described as a transducer converting electrical energy into light energy. Surprisingly, a CCFL backlit LCD requires a supply voltage of more than 1000 V, typically 1,200-1,500 V. A power-hungry inverter board drives the CCFL backlight (Figure 4).
Despite the name, cold cathodes don’t remain cold as they operate; they can get painfully hot, which is problematic for CCFL backlit LCDs. The increase in the backlight temperatures significantly reduces life span and can cause erratic operation. HMIs are shipped globally, including India, Mexico, China, and the Middle East, where ambient temperatures reach 45 C (113 F) with 95% humidity.
For longer life and higher reliability, high-end manufacturers are turning to high-efficiency white LEDs, solid-state devices that consist of a chip of semiconducting materials doped with impurities to create a p-n junction. As in other diodes, current flows easily from the p-side (anode) to the n-side (cathode). Figure 5 shows LED construction.
Unlike CCFLs, white LEDs do not have gases and phosphors requiring high voltages but can operate in the range of 5 V to 24 V without an inverter board, generating less heat than CCFLs. White LED backlit LCD displays generally last twice as long as CCFL backlights—75,000 h to 100,000 hr without temperature effects (Figure 1). CCFL backlit LCD displays have 25 C operating temperatures; white have LED-based backlights capable of operating up to 60 C (140 F) in humidity up to 95% (Figure 6).
LED-based lighting has high efficiency as measured by light output per unit power input. Standard touchscreen HMI brightness is often measured in NITS (lumens / sq meter). An HMI with white LED backlights normally exhibits roughly 500 nits, compared to the CCFL backlit LCD displays at roughly 200-300 nits. With a white LED backlit LCD, the backlight life span increases twofold, and brightness increases.
Redundancy is another advantage. When a CCFL bulb fails, the entire LCD display is impossible to read. With the white LED backlit HMI, if one of the many LEDs burns out, the display dims in proportion, but it still can be used for important machine operation. With HMIs in use in many applications and environments, indoors and outdoors, these are key advantages for white LED use in operator interfaces.
White LED backlit LCD displays, with a longer life span, more brightness, and higher temperature ratings, have made significant headway in standard LCD TV displays and notebook computers. Sony has used LED backlights in higher-end slim Vaio notebooks since 2005. In 2007, Asus, Dell, and Apple introduced white LED backlights into some notebook models. In October 2008, Apple announced use of LED backlights for its notebooks, and for a new 24-inch Apple cinema display.
As for the touchscreen HMIs, the Uticor division of AVG Automation introduced the first white LED backlit HMI, with 100,000 hr of backlight at 60 C and covering 95% humidity. Displays are available in 4-, 6-, 8-, 10-, and 15-in. sizes. In areas such as the Indian subcontinent, it is normal to have 45 C ambient with 55-60 C inside the control panel.
- Edited by Mark T. Hoske, content manager, CFE Media, Control Engineering, with information from Vikram Aditya Kumar, vice president, AVG/Uticor.
Inception: Retrofit engineering a white LED for backlighting
Inspiration for white LEDs throughout an HMI product line had a hot beginning.
Shalabh Kumar, AVG Automation CEO, said, “I got the idea for [LED backlighting] when landing in Ahmedabad, India, and the pilot said it was 42 C [108 F] at 7:30 p.m. I asked what the temperature was during the day, and it was 48 C [118 F]. In a manufacturing plant, a panel can easily be subjected to 60 C [140 F], which rapidly decreases operating life. Then we talked to manufacturers whose machines use operator interfaces using CCFL displays. A year and a half life for a screen is not unusual.”
“While some screen manufacturers try to make the bulbs easy to replace, no one we talked to actually replaces them in the field. They replace the entire operator interface [OI], which adds considerable expense,” Kumar said.
Operator panels using white LEDs last up to 6 years, compared to 1.5 to 3 years maximum for CCFLs, with a much higher operating temperature range; in rugged environments, especially in high-temperature areas, the higher initial cost is easily made up by avoiding multiple OI replacements during the same operating period, he suggested.
Kumar said, in the March 14 interview, that AVG is the only company in the world that offers white LED backlight as standard, throughout its Uticor Tough Panel line of HMIs.
- Mark T. Hoske is content manager, CFE Media, Control Engineering.
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