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Modern Automotive Display Touch-Lens Interface

Modern Automotive Display Touch-Lens Interface

Technology News |
By eeNews Europe



The display interface is an area of particular focus in the automotive industry as poke-through resistive touch displays yield a more modern touch-lens in the integrated center control panel (Figure 1).

 

Figure 1: Touch-lens in integrated center control panel

 

Display interface requirement considerations for the touch-lens include functional, aesthetics, and performance & conformance categories. Figure 2 shows a representation of these interface requirements (green), enablers (yellow), and the key component (pink) of the system.

 

Figure 2: Touch-lens System Interface Diagram. For better resolution click here.

 

Addressing all of these considerations within the touch-lens component for automotive use significantly shrinks the solution space due to competing requirements that can be grouped in the following categories:

  • Functional – multi-touch sensing, touch with press-for-intent, force feedback haptics, and spatial gesture recognition
  • Aesthetics – dead-front look, minimize fingerprints, seamless front surface, and curved surface
  • Performance & Conformance – display visibility, acceptable birefringence, head impact regulatory compliance, and environmental performance.

Functional Requirements

Mechanical:

A high craftsmanship feel is generally provided by a hard surface. Therefore a curved touch-lens over an 8-inch display requires a birefringence-free polymer lens base material thickness of at least 2-2.5mm that impacts the selection of the touch technology.

Integrating touch sensing:

Projected capacitance (pro-cap) screens can be adapted to provide a high craftsmanship continuous curved surface touch-lens desired by automotive styling studios. Integrating the touch screen into a curved surface eliminates resistive touch screen, most infrared LED, and acoustic wave technologies as solutions since these technologies require a flat surface to operate. Technology companies are working on infrared and acoustic wave technologies for continuous curved surface touch solutions that may become suitable for automotive applications in the future.

Augmenting touch inputs:

Audible and tactile feedback augments the verification that the touch has been registered. Providing tactile feedback can be a challenge with the constraint of a relatively large lens. The haptic actuation and lens mounting features need to be carefully chosen to accomplish the desired wave propagation on a rigid curved surface. Adding multi-stage touch sensing or the press-for-intent feature increases the complexity of the mechanical mounting by requiring detection of a slight pressure while maintaining a mounting system suitable for haptic feedback.

Spatial Recognition:

Spatial recognition is a new feature gaining popularity and offers some unique challenges to the touch-lens. If the requirements are simple driver vs. passenger hand detection, projected capacitance solution may suffice. However, for more complex gestures and larger detection areas above the panel, IR LED reflection, or possibly IR camera technologies are warranted. This solution requires the use of an IR transmissive surface. Other technology used to recognize no-touch movements (such as ultrasound), may result in a decreased craftsmanship level (visible microphone openings).

Aesthetics

Dead-front Appearance:

Hiding of the display behind the continuous curved surface that provides partial or complete display hiding provides a high craftsmanship appearance that is a trend in the automotive space (example in Figure 4).

 

Figure 4: Dead-front example

 

This exciting styling opportunity gives the Tier 1 supplier or system integrator one of the biggest challenges in the center stack of the vehicle. Addressing this challenge requires the following:

  1. The ability to quantitatively assess visibility of the display for a given optical system and vehicle packaging
  2. Optimizing the design of the optical system
  3. Providing the best combination of display and efficient backlight

Providing a dead-front or black panel appearance requires an understanding of how the human eye works to optimize the light transmission while providing the desired hiding effect. Hiding the display opening when the display is off involves human factor studies. The contrast sensitivity function (CSF) per Figure 5 shows that a contrast of about 0.01 is necessary to hide the display opening.

 

Figure 5: Contrast Sensitivity Function

 

Using the CSF as a guide, different optical constructions may be considered to control the reflection level of the display opening versus the non-display area to minimize the contrast (also known as Michelson contrast, Cm) while maximizing display visibility. There are many interrelated factors to consider when selecting the lens optical configuration, such as transmission, polarization, retardation, display luminance and reflected background luminance. Since reflections can be used beneficially to hide the display opening, the lowest reflection lens configuration may not always be the best solution.

Taking the visibility mathematical function into account, various optical configurations may be examined to ascertain the required display luminances. Figure 6 shows that lower display luminances (higher optical system efficiencies) can be achieved with different optical other than the traditional neutral density filter (ND). However, the background reflection level must be considered to pick the best optical configuration based on actual in-vehicle jury evaluations.

Minimizing Fingerprint and Reflections:

Another consideration for front lens aesthetics is the use of antiglare (AG) films to lessen the effect of fingerprints and reduce the clarity of specular reflections. AG films must be used with caution due to unwanted speckle and decreasing the sharpness of the TFT image (Figure 7) as the film is moves further from the TFT.

 

Figure 7: AG Film Blurring the TFT Image

 

The blurring performance of AG films may be quantitatively determined by obtaining the line-spread-function and associated modulation transfer function via FFT techniques [1].

Performance & Conformance

Display Visibility:

One of the challenges is to ensure vehicle center display visibility under all lighting conditions. Visibility problems occur when engineering principles coupled with human factor studies are not properly applied. The fundamental geometric requirement is that the display must be positioned and tilted so no window can be seen in a mirror placed on the display surface. If a window can be seen, a potentially unsafe specular sunlight condition could result in increased driver recovery time and possible retinal damage from seeing the sun’s reflection. No amount of antireflective films or antiglare treatments can solve the sunlight specular angle problem since the luminance of the sun is approximately 1.6×109 cd/m2.

To determine required display luminance, the reflection components must first be assessed from various sources per Figure 8.

 

Figure 8: Superposition of Reflection Components [1]

 

Since superposition applies, the process involves considering each source separately to obtain the total background reflection luminance. These three major lighting conditions must be considered:

1. Hemispherical illumination – from the cockpit interior where the diffuse reflectance of the display may be estimated by using an integrating sphere specular component excluded (SCE) measurement.

2. Specular illumination – from sources like white shirt, seats, etc. Once the specular reflectance, ζ, for the system is measured, the specular object luminance seen by the driver can be calculated per Equation 1.

3. Direct sunlight illumination of the display – The haze reflection component (Figure 9) is perhaps the least understood component of reflection and is the cause of many poor visibility implementations.

 

Figure 9: Reflection Components [2, page 188]

 

Using a classical Bi-Directional Reflection Distribution Function (BRDF) method to measure the reflectance as a function of angle from the specular position can cause the some of the reflection components (shown in red per Figure 10) to miss the detector due to multiple reflection surfaces.

 

Figure 10: BRDF Apparatus [1, page 230 adapted]

 

A more suitable method is to use a small signal reflection measurement method, shown in Figure 11, where the collimated light source beam size is large enough to encompass all of the reflection surfaces in the optical stack.

 

Figure 11: Small Signal Reflection Measurement Method [2]

 

 

Figure 12: Goniometric Small Signal Measurement System

 

Figure 12 shows a goniometric apparatus to measure the small signal reflection with example results per Figure 13.

 

Figure 13: Small Signal Measurement Example

The log scale shows the importance of positioning the display to maximize the viewing angle deviation from the sunlight specular angle.

It is not uncommon to calculate very high required display luminances on the order of 3000cd/m2 out of the TFT for dead-front touch-lens systems. This value is much larger than typical automotive TFT luminances of 500-850cd/m2 that do not have a “dead front” neutral density lens in front of the TFT. The luminance reduction of the display at center stack viewing angles also needs to be considered for different backlight brightness enhancement film configurations. By considering thermal maintenance from a system level, high luminance 3000 cd/m2 displays can be achieved, which dissipate approximately 20 Watts of backlight power for an 8” TFT.

Predictive Display Visibility Analysis:

Once the total reflected background luminance is determined, the required display luminance can be estimated using historical human factor studies. The required display luminance is a straight visibility line on a log-log plot. Figure 14 shows how various film configurations may be studied to determine the best solution to stay above the visibility line. This solution can then be used for a cost-driven optimized system.

Summary

The conflict between the aesthetics of a dead-front curved touch lens, display visibility need, and the functional aspects of the display interface create a dynamic new area of development for the automotive supplier. To be productive in this space will require the ability to assess optical performance and develop the best system solution.

References:

[1] Weindorf, P., Hayden, B., 3.4 Anti-glare Film Sharpness Measurement Investigations, SID 2012 Vehicle Displays and Interfaces Symposium Digest of Technical Papers.

[2] Information Display Measurements Standard, Version 1.03

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