By Lori DeBoer
Cholesteric reflective displays can now be addressed in a fraction of the time that it used to take because of a new dynamic drive developed by ALCOM researchers at the Liquid Crystal Institute, Kent State University.
The new drive scheme addresses bistable cholesteric reflective displays at a rate of one line per millisecond. This is a substantial improvement over previous drive schemes, which took 20 milliseconds per line to address the cholesteric display on a passive matrix. It is now possible to have sub-second, page-turn rates for page-size, high-resolution displays.
The new drive scheme takes advantage of the nonlinear switching mechanism that occurs in a cholesteric liquid crystal. When a field is suddenly removed from a cholesteric liquid crystal in the homeotropic aligned state, the material rapidly transforms to a transient planar texture in the period of a millisecond. By manipulating the voltage during this narrow time interval, the final state of the liquid crystals can be controlled.
The development of this technology required science, perseverance and some serendipity, according to Xiao-Yang Huang, one of the co-inventors, along with ALCOM principal investigators Deng-ke Yang and Phil Bos.
Huang, a doctoral student in physics at Kent State University, began studying transition mechanisms in Spring 1993, under the guidance of Deng-ke Yang. Huang developed dynamic dielectric measurements, measuring the capacitance of liquid crystal cells during transitions. When he first began his measurements, he had equipment that could only measure transitions of 50 milliseconds. After he developed his own equipment that could measure transitions at .05 milliseconds, he was able to observe an extremely fast transition from the homeotropic state to the transient planar state.
The discovery, which coincided with the theoretical model developed by Yang and Peter Pfalffy-Muhoray, gave the research team some hope. However, many obstacles had to be overcome before the fast transition state could be harnessed for use. Since the transient planar state resulting from the fast transition is not a stable state, more voltage waveforms must be applied to achieve stability.
In May 1994, Huang was testing some voltage waveforms and discovered that if you changed the voltage level at a series of different levels there is a one-millisecond period that could be used to control the final result of the display. But the whole waveform was still about 120 ms long, which offered no advantage over the 20 ms per line rate. However, Huang noticed that in order to achieve a stable final state in a display pixel, he only needed to apply different voltages during the one ms period; all the other voltages were the same. He realized that the pipeline algorithm – which he learned about when he was helping his wife with her computer architecture class two years earlier — would be helpful. This is a widely used method to fully utilize the CPU capability by sharing the time among the peripheral processes in the computer. Based on this concept and on optical studies of the transitions of cholesteric textures by doctoral student Z.J. Lu, the team designed a three-stage drive sequence: preparation, selection and evolution. Because of this sequence, there can be about 100 lines in the preparation and evolution stage, while information is passed to one particular line in the selection stage.
To develop the actual drive mechanism, Huang worked with electronics group that included Merrill Groom, instrumentation engineer, and Nick Miller, display technician, supervised by Phil Bos. Because the waveform had never been used before, and other LCD displays used much lower voltages, Groom and Miller had to build in addressing prototype from the ground up. Groom, drawing on his background in analog electronics, developed a power supply unit. Miller, with his digital circuit experience, designed a logic controller to control the more than 10 different voltages used in the whole drive system. The most recent addition to the team is John Ruth, a postdoctoral fellow who is working on systems integration. The first successful run of the prototype occurred in late December.
Huang noted that the LCI was able to draw people with a variety of backgrounds to expedite the process. LCI’s awareness of industry needs also helped, he said. “If you’re from the pure science view, once you theoretically predict something and experimentally prove it, that’s all.”
Huang received his undergraduate degree in physics from the Peking University in Beijing, and his master’s degree in physics from the University of Massachusetts-Dartmouth. Huang will finish his dissertation on this topic and graduate from Kent State University sometime this year. He will present these findings at the Society for Information Display (SID) meeting in Orlando, Florida, for which he received a student travel grant from SID.
Article for the ALCOM Update, a liquid crystal industry newsletter.
