Approaching the "Zenith": Bistable LCDs in a Retail Environment

Previous electronic shelf-edge labels based on standard twisted-nematic segmented LCDs have allowed updates of price only. Limiting the information shown meant ultra-low-power requirements for the battery-operated displays in constant use, which in turn supported the need for low cost on the part of retailers. Bistable passive-matrix displays allow retailers to supply more information while also enabling battery options that last for years. This article discusses a bistable LCD and RF protocol designed to provide both functionality and ease of use for retailers.

by Cliff Jones

LABELING is one of the few remaining aspects of the retail arena yet to be automated. Paper labeling in a retail environment has worked for centuries, but gives the retailer limited flexibility and poor reactivity, and represents an unwanted operational cost. Many retailers would embrace a solution that can automatically update product information, pricing, and promotional content; one that responds within seconds to market opportunity and is linked directly to the stock and point-of-sale computer systems. Existing electronic shelf-edge labels (ESLs) provide only a part of the solution and are frequently limited to showing price only. Despite being electronic, these labels still require conventional paper labeling to be added for each ESL to provide the necessary product information. A matrix display would be ideal – but how is all of the required information shown on a display that both meets the right price points for such a cost-conscious sector and copes with the need for the information to be on display 24 hours a day and 7 days a week? The answer is to use a bistable display.

The enormous potential of bistable ESLs for the retail sector has been recognized by most players by use of a bistable-display technology. In particular, reflective bistable LCD technologies are suited to this application, not only due to their ultra-low power, but also because they can be addressed as a passive matrix. This helps the display meet the cost requirement because it avoids the need for a TFT backplane. This is true of bistable cholesteric displays (developed by Kent Displays Inc.1), weakly anchored bistable nematics (the Binem mode from Nemoptic2), and the grating-aligned display from ZBD3 (shown in Fig. 1).

 

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Fig. 1: A grating-aligned display used in a retail application.

 

ZBD is now developing a new display technology and combining it with an ultra-low-power RF product and appropriate software to produce an integrated system. It is this bistable LCD and RF combination that is the main focus of this review.

Grating-Aligned Bistable LCD

ZBD's first approach toward using grating alignment to induce bistability in a nematic device was to employ the bi-grating to define two orthogonal directions for the liquid-crystal alignment, both within the plane of the liquid-crystal cell. Although bistability was demonstrated, there was a fundamental difficulty in that the out-of-plane electric field applied to the internal surfaces of the LCD in the usual fashion, resulting in a switching torque between the in-plane bistable states that is too low. This meant that the voltage required to operate the latch between the states was impractically high (greater than 60 V). A breakthrough was made in 1995 by Bryan-Brown, Brown, and Jones,4 who realized that a zenithal bistable display, one in which the bistable alignment states are between a low tilt (planar to the cell) and high tilt (parallel to the applied electric field), could operate at voltages suitably low to be readily accessed using STN drivers. This zenithal bistable surface was demonstrated in a device that used a deep monograting formed from a material that aligns the contacting liquid crystal perpendicular (i.e., homeo-tropic) to the local grating surface.

A simple way of visualizing how zenithal bistability arises is to consider the local alignment of the liquid-crystal director close to a deep grating surface: the grating at the groove tops and bottoms tends to favor the vertical-alignment state, whereas the side-walls of the surface will favor a horizontal state. In practice, the two states are differentiated by the presence of nematic defects; in one state, the nematic director deforms elastically around the grating continuously (the C-state), whereas in the other state (the D-state) this elastic deformation is reduced by forming defects (i.e., disclinations) at the top and the bottom of the grooves. Latching between the two states occurs for voltage pulses of the appropriate polarity. Negative voltages on the grating always latch to the C-state while positive voltages latch to the D-state. The latching torque arises due to the flexo-electric polarization that occurs in any nematic liquid-crystal material undergoing a splay or bend deformation. Usually, this polarization is very weak and plays only an insignificant role in conventional LCD behavior. However, in ZBDs, the deformation close to the grating is strong and localized to the top and bottom of the grooves, precisely the areas where it is needed to nucleate and annihilate the nematic defects. This means that latching can be induced with fields as low as 1.6 V/μm (i.e., 8 V in a 5-μm-spaced display).

Achieving the Low-Cost Target

Superficially, it may appear as if incorporating grating structures to the internal surface of an LCD is a complex, unconventional, and inherently expensive process. The grating may have a pitch of between 0.6 and 1.5 μm, and have a depth-to-pitch ratio of between 0.8 and 1.2. Any offset of grating giving unnecessary photoresist between the bottom of the grooves and the ITO electrode will cause a voltage drop in the cell and potentially increased cell-to-cell variations in the operating voltage.

In practice, grating fabrication is a simple step from an LCD-manufacturing point of view, similar to the rubbing process it replaces. The manufacturing simplicity is one of the advantages of this technology.

All production devices to date have been manufactured by Varitronix in China. ZBD and its partners produce a sheet of PET carrier film with the inverse of the required grating shape on one surface. This is then used to emboss the grating into a photopolymer (Fig. 2).

The composition of the photopolymer is designed to produce the homeotropic alignment of the liquid crystal. The embossing machinery is simply two rubber-coated rollers through which the glass and film pass. The plate is then exposed to 365-nm UV light to cure the photopolymer and the film removed, leaving a perfect replica of the grating on the LCD plate. The fidelity of the grating is maintained for a wide range of process conditions; zero-offset is achieved by ensuring that the embossing pressure and Shore hardness of the roller are kept high.

The complex part of the process is the original production of the film. This is initiated by careful mastering of a single grating structure, which is then replicated hundreds of thousands of times. Any of a number of methods can be used to create the original master grating, including contact photo-lithography, projection photolithography, laser scanning, or diamond cutting. Each master needs to be perfect and involves optical characterization of its uniformity and inspection for defects. However, this has negligible impact on the final cost of the grating, due to the high level of replication involved. Once completed, the master shape is copied into nickel using the standard electro-forming process used in the CD/DVD industry; although the master is destroyed in this process, the first nickel copy can be used to produce further copies scores of times. The final step is to use the nickel to emboss the grating shape onto the film using a roll-to-roll embossing method common to the optical films industry. This film is then used by the LCD manufacturer to emboss the grating alignment layer onto one of the glass substrates of the LCD.

 

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Fig. 2: The embossing process used for ZBD grating fabrication.

 

Once completed, the plate is used to construct a standard twisted-nematic LCD (TN-LCD), spacing the grating plate 5 μm away from a rubbed polyimide plate to give a 90° twisted structure when the grating is in the low-tilt D-state and an non-twisted hybrid-aligned state when the grating is latched into the vertical C-state (thereby forming a hybrid aligned or HAN state), as shown in Fig. 3.

The display is completed using standard polarizers, liquid-crystal material, and supertwisted-nematic (STN) drivers. It can be operated in either the normally white (NW) TN or normally black (NB) TN mode by appropriate orientation of the polarizers. Indeed, another advantage of being an LCD is that it can benefit from the use of other established components common to LCDs, such as color-filter plates, transflective polarizers, internal reflectors, and other optical elements.

A number of factors favor the ZBD device for producing low-cost displays. Of course, the fact that there is a well-defined threshold to the bistability is ideal for line-by-line passive-matrix addressing. The simple embossing step is done with little capital expenditure and is run alongside standard TN or STN production. There are also cost savings over STN production, including the use of unpolished TN quality glass and cheaper LC materials. The additional cost of the grating film is counter-balanced by the lack of an optical compensator that would be required for an STN panel, each being of similar cost.

Achieving Optimal Device Performance

Next to cost, the ZBD device's appearance is designed to appeal to the retailer. High reflectivity, contrast ratio, and viewing angle are inherent to the design. Unlike a conventional TN, the optics are optimized for the display to be viewed without an applied field. The NW TN state has a low tilt throughout the cell, featuring high reflectivity and viewing angle, whereas the HAN state is devoid of twist and offers an exceptional black state.

Also important is the ability to display the same image content as used on a standard paper label. For this reason, the displays are typically made at 110 dpi, ensuring that the smallest text is still legible, as well as giving good response to barcode readers.

As with any of the competitive options, adding full color to the display is too expensive for large-scale deployment throughout a supermarket chain. However, retailers need only a limited amount of color, for example, to highlight promotional events. This can be done very simply for negligible cost by printing a dyed area onto the glass underneath one of the polarizers (see Fig. 4).

Because the ZBD device has a high-contrast ratio (typically over 20:1), the colored area of the display can be made completely dark – essential for distinguishing between promotional offers and standard product. Figure 5 shows an alternative display option that is closer to paper-white.

Some aspects of the technology are not important in retail applications. Although operation from –20°C to +60°C has been demonstrated, the retail products work from –5°C to +45°C. Similarly, current displays take typically 0.5 sec to update a page. These limitations are largely set by the 30-V maximum operating voltage chosen. The wider temperature range and update speeds of 100 msec/page can be achieved using 40-V drivers.

Optimizing Battery Life

From the outset, the aim has been to develop a battery life for each unit of over 5 years. The ultra-low power that a bistable display offers is obvious. The display may only need to be updated two or three times each day. For a 4-in. QVGA ZBD display, each update requires less than 30 mA (including both the display and driver electronics). This corresponds to over 200,000 updates for a typical battery, which at 50 updates a day would result in a potential lifetime of over a decade (although this may be longer than the shelf-life of a typical battery). For comparison, a similarly sized STN panel would consume about 20 mA. This is similar to the ZBD current because both use similar liquid-crystal material, cell spacing, and applied voltages. However, in this case, the STN must be powered continuously, and therefore an STN panel operating from the same battery would be limited to 25 hours. In practice, of course, a high proportion of the battery energy will be consumed by the communications system.

 

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Fig. 3: Layers involved in a twisted-nematic display construction at ZBD are shown above.

 

Even if the display is not powered for the majority of the time, the communications system needs to awaken periodically to check whether an update is required. Also, to ensure dependability, the customer needs the system to be two-way: All update events must be acknowledged by the individual label. An RF-based communications system offers both a simple infrastructure and the bandwidth required for matrix information. Of the various options available, it is the unlicensed ISM bands (868–915 MHz) that have enabled ZBD to achieve a protocol offering ultra-low power consumption. Each wake-up also includes a synchronization signal to ensure timing is maintained between consecutive wakeups – this helps keep label costs low by minimizing the accuracy requirements on the real-time clock circuitry. The bandwidth is 25 kbps, but time-division multiplexing helps ensure the maximum efficiency of the available bandwidth. Each label has its own ID and is encoded to ensure security. Multiple channels (10 channels at 868 MHz and 60 at 915 MHz) cater to overlapping networks or multiple communicators. At each wake-up, the entire transaction (RF receive, write-image-to-memory, and transmit acknowledgement) expends approximately 40 mA. With a wake-up frequency of between 6 and 10 sec, the standby and synchronization current uses, at most, 0.1 mAh each day. Thus, each label readily achieves the target battery life of more than 5 years from two button cells when updated up to five times a day.

A major benefit of the entire system is simplicity of installation. This should be contrasted with conventional ESL systems that require many routers in the ceiling of the outlet. For a large supermarket, this is both expensive and disruptive; for a smaller retailer, such systems are prohibitive. The ZBD-type system operates using a single RF transmitter that can communicate reliably with all labels in the store environment within a range that is greater than 100 m. It is interfaced directly to the existing point-of-sale infrastructure, for example, using a USB connection, and installation can be done in minutes. This helps ensure that the total cost of ownership for the labels is very low.

Conclusions

The combination of a simple bistable display technology with an ultra-low-power RF product is already proving powerful for the retail market, meeting the customer's needs for both small and large outlets. However, the application of either element or their combination is not limited to retail applications and growth in other sectors is already anticipated over the next few years.

Acknowledgments

The author wishes to thank his colleagues at ZBD, particularly David Dix and his team, for the details on the RF.

References

1D.-K. Yang, J. L. West, L.-C. Chien, and J. W. Doane, J. Appl. Phys. 76, 20 (1994). Kent Displays, Inc., at http://www.kent displays.com/products/products.html.

2I. Dozov, M. Nobili, and G. Durand, Appl. Phys. Lett. 70, 1179 (1997). Nemoptic at http://www.nemoptic.com/index.php.

3J. C. Jones, J. Soc. Info. Display 16/1, 143–154 (2008).

4G. P. Bryan-Brown, C. V. Brown, and J. C. Jones, U.S. Patent 6,249,332 (1995). •

 

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Fig. 4: Some of the ZBD options shown at SID's Display Week 2008 included the use of area color (top left shows the display with black corners and top right with the same display and the corners latched to red).

 

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Fig. 5: ZBD's display together with LCD geometries that give a whiter light state.

 


Cliff Jones is Founder and Chief Technology Officer at ZBD Displays, Ltd., Malvern Hills Science Park, Geraldine Park, Geraldine Rd., Worcs, WR 14 3SZ, U.K.; telephone +44-1684-585-313, fax –390, e-mail: jcjones@zbd.co.uk.