1.1

Background and objectives

With the assembly of the International Space Station (ISS), astronauts shall perform more space walks in upcoming years than they have been conducted since spaceflight began. More than 160 space walks totalling 960 clock hours, or 1900 man-hours, will be performed to assemble and maintain the station. In addition to these planned EVAs (Extra Vehicular Activity) the ESA Exploration Programme intends to have several manned missions to the Moon and to Mars in the coming decades. To accomplish the increasing number of requirements and objectives for these EVAs new technologies need to be pursued to support and optimize the tasks of the astronauts. In particular a new information management and display system needs to be developed.

In the frame of the ESA GSP (General Study Programme) Lusospace LdA has performed a study aiming at proposing a baseline design for a future direct visualisation display tool to be used during EVAs. This activity included the review of novel technologies for different sub-systems, the identification of potential applications and the design of a preliminary configuration for a future direct visualization display tool for space applications.

1.2

Visualization of information during an EVA

In the current NASA space suit, also known as Extravehicular Mobility Unit (EMU), the astronaut has to rely exclusively on the information that he carries in his EVA cuff checklist [1]. The EVA cuff checklist, shown in Fig. 1, is a booklet of 25 procedural pages bound by a coil spring to an aluminium alloy bracket and attached to an armband.

Fig.1.

EMU cuff- Checklist

The checklist contains text and simple graphics of procedures used for EVA tasks and in case of EMU malfunctions. It is cumbersome to use, time-consuming to assemble, information and data formats are limited, and it is difficult to maintain proper management control. Additionally, mounted in front of the upper torso of the EMU, there is a DCM (Display and Control Module) that contains a small number of visual displays and electric and mechanical controls required only for the operation of the EMU.

The development of a tool to efficiently display information to the astronaut during an EVA has been a topic of concern for both ESA and NASA in the last decades [2],[3],[4]. However, the technology available at the time would lead to the development of a very complex system with large dimensions, high mass and power consumption. All these drawbacks dramatically reduced the advantages of such tool against the current existing cuff checklist. Nowadays novel technologies can be used in the design of a very compact, reliable and efficient information management and display tool. The system can be designed as a HMD (Head Mounted Display), if the display area follows the user head movement, or as a HUD (Head Up Display), if the display area is fixed in front of the user head. In both cases the goal is to have the information being displayed as close as possible of the astronaut FOV. Therefore the astronaut does not have to divert his attention from the tasks he is performing. In addition the display can be either “non see-through” or “see-through” depending if the display area blocks or not the background. Fig.2 illustrates the information data and format that could be displayed to the astronaut using a HMD/HUD during an EVA.

Fig.2.

Illustration of the information that can be delivered to Astronaut during an EVA.

2.1

Commercial HMD

At present, a wide offer of HMD products is commercially available. Some examples are the “Nomad HMD” from Microvision and the “EG-8 HMD” from MicroOptical, which are lightweight and compact in size. These two models are presented in Fig. 3.

Fig.3.

Example of commercial HMD. (a) Nomad from Microvision. (b) EG-8 from MicroOptical.

The specifications of the available commercial HMD were reviewed. These systems are not sufficiently robust and have limited performance for space applications. Their poor image quality and low image source luminance prevents the use of the HMD against a bright background scenario. The exception is the Nomad system. However it occludes an important part of the user visual field and is monochromatic.

2.1

NASA research activities

NASA has conducted several researches for the implementation of HMDs in its EMU. This effort resulted in the production of four HMD prototypes [2], [5]. The Wright-Patterson AFB HMD (1987), the Hamilton Standard HMD (1988), the APA Optics HMD (1991) and the Technology Innovative Group HMD (1991). Fig. 4 illustrates two of these HMDs.

Fig.4.

Two NASA HMD prototypes. (a) Wright-Patterson AFB HMD layout [2](b). Technology Innovative Group HMD layout [5].

All the NASA HMDs are mounted on the bubble helmet. This prevents the use of the ExtraVehicular Visor Assembly. Moreover these systems have high power consumptions 5 to 20 W and use high voltages to control the image source (mini CRT). The Hamilton Standard HMD model is the only one that uses a LCD backlighted by a halogen lamp as image source.

2.3

Structure of DVDT

A DVDT for space use should have the following subsystems:

The Central processing unit is the computer that manages the DVDT. It manages the information displayed to the astronaut and the DVDT interfaces. The image source is a microdisplay that receives the video signal from the central processing unit. The microdisplay image is transmitted through the relay/image optics and then is reflected to the eye by a combiner for see-through HMD, or transmitted (directly or reflected by a mirror or prism) to the eye in the case of a non see-through HMD.

2.4

Image sources

One of the first image sources to be used was the micro CRT (1-inch diameter Catodic Ray Tube). The main advantages of the CRT were low cost, easy availability, bright images and good image quality. However, CRTs, even miniature ones, have inherent drawbacks which include weight, size (primarily depth), power requirements, high anode voltage, and heat generation. Due to these drawbacks the current preferred display technologies are based on Flat Panel Displays (FPD). FPD can be less than one inch in diagonal, creating a high-resolution image, typically smaller than a postage stamp. Typically FPDs have low power consumption (<1W) and voltage requirements, low heat output, and low weight. Therefore they are good candidates to be used as image source on space HMD/HUD.

A novel display technology that has recently emerged is the OLED (Organic Light-Emitting Diode) [5]. OLEDs are electroluminescent microdisplays. OLED light emitting materials can be small organic molecules or long polymeric molecules. OLEDs are brighter, consume less energy and are easier to manufacture than other displays based on liquid crystals. Being an emissive technology OLEDs do not require backlight. Also they have low power consumptions (ex: <200 mW “eMagin” OLED), operate at low voltage (3-10V) and have Lambertian emission.

4.1

Optical design and image source

The optical system was designed for a diagonal FOV of 25° (HFOV 20,6° and VFOV 14,5°), an exit pupil of 7mm and eye relief of 30mm to permit the use of eyeglasses. The optical system main part is the 10mm thick polycarbonate (ZEONEX) prism. The optical prism has two off axis aspheric surfaces without rotational symmetry. These surfaces, Free Form Surfaces (FFS), can correct the first order aberrations and also the off-axis aberration [6]. Light emitted by the OLED display is collected by an achromatic lens and enters the prism at one of the FFS. Inside the prism, the light undergoes two total internal reflections, reaches the other FFS that is half-mirrored, and reflects in the direction of the user’s eye. Anti-reflection coatings are considered on the prim surfaces, except on the half mirrored FFS. The image is focused at infinity. Fig. 5 shows a schematic layout of the polycarbonate prism.

Fig.5.

Schematic layout of the optical system

In order to enable the vision of the outside background, a compensator prism is added to the back of the half mirror FFS (beamsplitter). Light coming from the outside background will cross the medium with minimal disturbances. The complete optical system was designed and optimized using ZEMAX software. The selected image source is the colour SVGA OLED from eMagin. Some of its specifications are: assembly envelope of 20x16x4mm; 852(x3)x600 pixels; a power consumption of 200 to 400mW; maximum luminance of 400 Cd.m^-2; and a weight of 1,8g.

Fig. 6 shows a photograph of the OLED displaying the image presented on Fig.2.

Fig.6.

Photography of the SVGA+ OLED microdisplay

4.2

Processing and control unit

The use of a PDA as control unit of an EVA HMD has been considered [7]. The selected processing unit is the DELL Axim X50v PDA. This selection was based on the following reasons: fast processor, manually selectable clock rate, adequate pixel resolution, connection to an external display and include accessories such as sound card and wireless card.

The VGA cable accessory will be used to connect the OLED display. In addition this PDA can accept a compact flash acquisition card. This card has the capability to acquire inputs from external sensors. This can be used to display for example the Bio-sensors data (such as: astronaut heart rate). The PDA microphone audio input will be connected to the EMU voice communications system. The voice signal will then be processed by voice recognition software (such as ScanSoft VoCom3200) in the PDA. This will enable the astronaut to control the DVDT. In addition a simplified keyboard control shall be also considered. The connections for the PDA 5-way navigation button will be re-routed to four buttons on the EMU left wrist, implementing functions such as: scroll up, scroll down, enter/select, and escape/cancel. A power off switch will exist as well.

Two high capacity lithium-ion batteries (3,7V; 2200mA. h) will be used to get not less than 8 hours autonomy.

Fig. 7 shows the DVDT block diagram.

Fig.7.

DVDT block diagram

5.1

Optical performance

For the proposed optical system the off-axis aberrations, astigmatism and coma were corrected. The image spot size is less than 66μm. In case of daylight operation were the astronaut eye pupil diameter is approximately 3mm the spot size is less than 55μm. In the 0^o field of view the transverse ray aberration is very small, less than 30μm; the maximum value obtained in the marginal field of view is 215μm. Vignetting (<1%) and distortion (<4%) are negligible.

Fig. 8 shows the bitmap simulation of an image transmitted through the optical system. This image completely fills the DVDT FOV and it was simulated using 10⁸ rays.

Fig.8.

Simulation of an image: left- at the OLED; right- as seen through the optical system

Fig. 9 shows the Modulation Transfer Function (MTF) of the optical system.

Fig.9.

Modulation Transfer Function

The maximum spatial frequency where the MTF is greater than 0.3, is 40cycles/mm (equivalent to 80pixels/mm). An MTF greater than 0.3 is a typical value for the discrimination of images with high spatial frequencies (such as 50cycles/mm). The SVGA eMagin OLED has a resolution of 67pixels/mm. Therefore the resolution achieved by the optical system (80pixels/mm) is superior to the SVGA+ OLED resolution (67pixels/mm). Preliminary thermomechanical analysis shows that the prism suffers negligible deformations when exposed to thermal gradients of 20^o. Table 1 summarizes the major DVDT specifications.

Table 1.

DVDT specifications