WO2003052678A1

WO2003052678A1 - Touch screen display shielding

Info

Publication number: WO2003052678A1
Application number: PCT/EP2002/013194
Authority: WO
Inventors: Dan Westberg
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2001-12-17
Filing date: 2002-11-25
Publication date: 2003-06-26
Also published as: AU2002366346A1

Abstract

The present invention concerns a touch screen display and a portable electronic device including a touch screen display, which display has a display layer (16) and a touch screen panel with at least one first electrically conducting touch screen layer (20), said portable electronic device having an electromagnetic shielding barrier (30), wherein at least one capacitor (24) is connected between the touch screen layer (20) and the electromagnetic shielding barrier (30) for providing a high frequency shielding barrier.

Description

TITLE: TOUCH SCREEN DISPLAY SHIELDING

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a touch screen display and a portable electronic device including a touch screen display having arrangements for shielding the display from high frequency radiated electromagnetic fields. DESCRIPTION OF RELATED ART

A well-known problem with electronic devices of any kind is that they can be disturbed by radiated electromagnetic fields. Transmitting antennas generate electromagnetic fields, where the field strength depends on the distance from the antenna. Occurrence of electrostatic discharge (ESD) can also generate radiated electromagnetic fields that could cause malfunction to devices and components. There are in most countries governmental quality restrictions on manufacturers of electronic devices to have a proper protection against radiated electromagnetic fields on their products. Since the market today offers an abundance of handheld electronic devices which get more and more complicated and expensive, the problem with shielding against radiated electromagnetic fields becomes important to solve in a simple and cheap manner for manufacturers.

The development of mobile phones and Personal Digital Assistants (PDA:s) has come to a phase where developers try to maximize the size of the display at the same time as they try to minimize the total device volume. This results in a compact design where components are packed tight together very closely to each other and to the display. There is also a trend towards using higher and higher frequencies in for instance the field of cellular phones. Most electronic equipment has some kind of protection against radiated electromagnetic fields, such as a shielding barrier of some kind. Shielding can for example be implemented using a metal frame in connection with a metal chassis .

The perfect protection would obviously be to create a homogenous Faraday's cage around the electronic equipment and in that way avoid the risk of disturbances from radiated electromagnetic fields to the sensitive circuits. Having a display obviously creates an opening in the imaginary Faradays cage where electromagnetic fields find a coupling path into the electronic circuits. The larger the display the bigger the hole in the shielding barrier. Another problem with devices including a radio transmitter, such as e.g. mobile phones is that the antenna is situated close to the display causing further radiated electromagnetic disturbances into the opening that is formed by the display. This can, if it is not rectified, damage the display. JP-10163671 describes a touch panel connected to the shielding of a plasma display. However the touch panel is connected directly to the shielding via metal fittings. If the touch panel were to be used for shielding purposes it should be provided with a shielding layer. The document is however silent about this, why it is doubtful if the touch panel is used for shielding at all.

The problem with the opening caused by the display is addressed in US 4,786,767 where a touch screen is having an improved ESD shielding. An additional low reflectance transparent electrically conducting surface is applied on the display. The surface is configured to be electrically connectable. This allows the surface to provide shielding against the passage of electromagnetic and radio frequency interference through the touch screen. The layer is electrically connectable to a suitable ground such as through silver ink busbars or the like.

Another solution is to add a vaporized thin coat of metal . Unluckily that degrades the contrast of the display and hence a lower readability is experienced. It is also hard to connect the metal coat to a zero volt reference like to the chassis or to a metal frame of any kind.

US 5,398,046 shows a faceplate for an interactive display terminal . The touch screen section has an integrated shielding barrier in the form of a faceplate assembly having an electrically conductive grid connected to a ground potential. The grid has a plurality of optical pipes with metal walls, which provide protection. This special technical solution of the touch screen function and the shielding barrier consequently needs a so called "honeycomb grid" .

The prior art shielding barriers for electrically conducting touch panels are either too expensive, too bulky or contrast degrading because of the use of an extra shielding layer on the touch panel when shielding against radiated electromagnetic fields. There is thus a need for simpler and cheaper shielding against high frequency electromagnetic radiation.

SUMMARY

It is an object of the present invention to overcome the above mentioned problems and find an improved shielding against high frequency magnetic fields for electronic devices with touch screen displays. It is also an object of the present invention to overcome problems with visual ergonomics due to extra conductive and ESD-protective layers on the touch screen.

Using capacitors to create a high frequency connection between the touch screen layers and the rest of the shielding barrier meets this object. By using one or both of the conductive layers of a touch screen as high frequency shielding, an improved shielding against radiated electromagnetic fields is obtained.

The object is further met by a touch screen display and a portable electronic device having such a display comprising a touch screen panel with at least one touch screen layer. The layer is connected to a shielding barrier with capacitors as mentioned above .

In somewhat more detail, the object is met by a touch screen display and a portable electronic device including a touch screen display, where the display has a display layer and a touch screen panel with at least one first electrically conducting touch screen layer. The portable electronic device and/or the display layer has an electromagnetic shielding barrier and at least one capacitor is connected between the touch screen layer and the electromagnetic shielding barrier for providing a high frequency shielding barrier.

With the present invention there is no need for an extra shielding layer. The touch panel itself is used as shielding using a high-frequency connection between the touch screen and the main shielding barrier without affecting the functioning of the touch screen.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention believed to be novel are set forth with particularity in the appended claims. The invention itself however, may be best understood by reference to the following description with the accompanying drawings , where :

Fig. 1 shows a cellular phone in which the invention is implemented

Fig. 2 shows a side view of the shielding in the phone of fig. 1 and Fig. 3 shows a simplified view depicting an embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS Referring now in detail to Fig. 1, there is shown a portable electronic device, in the form of a cellular phone 10, which includes a touch screen 12 and an antenna 14. Electronic devices having a touch screen could e.g. also be a PDA (Personal Digital Assistant) , a smart phone (combined mobile phone and PDA) , a lap top computer or a handheld game machine. Even larger electronic devices such as PCs and television sets could possibly be equipped with touch screens now or in the future .

In the example of a cellular phone, the device is subject to strong high frequency electromagnetic fields through the antenna 14. The frequency is decided by the type of network used, where certain frequency bands are to be used for transmission and reception of speech and data. There is generally a trend towards higher frequencies as these types of networks are designed to transfer more and more data. Examples of such networks are GSM, DAMPS, CDMA, WCDMA and GPRS.

Fig. 2 shows a side view of parts of the interior of the cellular phone of fig. 1. The phone includes phone logic 28 and phone shielding 26 surrounding the phone logic 28.

The phone shielding could be part of the frame of the phone .

The touch screen of fig. 1, also called touch sensitive display comprises an LCD display (Liquid Crystal Display) 16 or display layer, on top of which a first 20 and a second 18 touch screen layer are provided. The two touch screen layers together form a touch screen panel . The touch screen layers are provided e.g. with a touch sensitive foil or a corresponding technique known as such identifying also the location of a touch point on the display. The touch screen layers 18 and 20 preferably comprises a plurality of activation areas for activating the function of the device provided with the display.

The display layer 16 also preferably comprises at least one display area for showing variable data, such as telephone numbers, short messages, calculations etc.

The user activates functions usually by using a finger and a light pressing on the touch screen. A pen-like object or the like can also be used, in case the identification technique used at the time allows. The size and positioning of these activation areas and the display area can vary. The activation is, .done by using the two touch screen layers, which are both electrically conducting. How this is done is something well known within the art and is not further explained here. The touch screen layers 18, 20 are however connected to the phone logic 28 in order to be able to take care of the touch inputs. The LCD 16 is likewise electrically connected to the phone logic 28 in order to be controlled. The LCD layer 16 is provided with a shielding foil 30 having zero voltage. The shielding foil 30 is surrounding the whole of the side of LCD layer 16 or the whole perimeter of the layer. The shielding 30 could equally as well have been provided in the form of a box. The LCD shielding 30 is connected to the phone shielding 26. In this way the LCD shielding 30 becomes part of the phone shielding 26. The display does not have to be a LCD, but other types of displays are also feasible as long as they are covered with a touch screen layer.

Both of the conductive touch screen layers 18 and 20 are in fig. 2 used as a part of the shielding barrier for high frequency electromagnetic radiation by connecting them to the LCD shielding 30 using capacitors 22 and 24, respectively. It is also feasible to use only one of the touch screen layers as a part of the shielding barrier. By using capacitors, a low impedance, high frequency connection between the touch screen layers and the rest of or the main shielding barrier is obtained. In a simple version of the invention the LCD shielding barrier 30 and one touch screen layer 20 are in themselves the plates of a first capacitor 24, while the LCD shielding barrier 30 and the other touch screen layer 18 are the plates of a second capacitor 22. The layers 18, 20 and shielding foil 30 are then separated by a suitable medium having good dielectrical properties or a dielectrical constant suitable for the capacitor. This medium can be some kind of glue or a plastic foil. As is well known within the art, the capacitance is decided by the area of the plates, the distance between them and the dielectrical constant of the medium. The medium is selected and the different layers are dimensioned, spaced apart and possibly displaced in relation to one another, in such a way that a capacitance suitable for short circuiting the touch screen layer and the LCD shielding for a high frequency is obtained.

As an alternative the capacitance could naturally be connected between a touch screen layer and the shielding of the phone instead of the LCD shielding. It is also possible to have only one of the layers being shielded by a capacitor, while the other layer is not.

In fig. 3 is shown an elevational view of a preferred embodiment of the invention, where the touch screen layers 18 and 20 are connected to the LCD shielding 30. Also the

LCD layer 16 is shown. Here there are several capacitors 22, 24 connected between layers 18, 20 and the LCD shielding 30. The capacitors 22 are here provided between the layer 18 and the LCD shielding 30 on two opposite sides of the touch screen layer 18, while two opposite sides of the layer 18 do not have any capacitors. On the sides of the layer 18 lacking capacitors, the other touch panel layer 20 has capacitors 24 connected to the LCD shielding 30. The number of capacitors is not a crucial matter for the functioning of the touch screen panel but if they are equally distanced along the perimeter of the LCD layer, they ensure a good protection against electromagnetic fields.

Since the layers are conductive at high frequencies the "opening" formed by the display is no longer an opening anymore in terms of radiated electromagnetic fields at high frequencies, but are instead an integral part of the radiated electromagnetic field protection.

By the well known formula X_c = 1/(2 fC) it is easily realized that the capacitive reactance X_c will be close to zero in connection with radiated electromagnetic fields which generally are of high frequencies.

The capacitors that are connected between the touch screen layers and the rest of the shielding barrier need to be safely attached to these surfaces . The touch screen layers could have screen printed connectors that the capacitor could attach to by using electrically conductive glue. The contacting of the capacitors to the main shielding barrier could be realised with some kind of contact spring means. Glueing could be an alternative also here. However, the attachment of the capacitors to the layers and to the rest of the shielding barrier is an open question which has to be adapted to mechanical design and should not be a limitation to the invention. The capacitors can as an alternative be provided in the form of thin film circuits which are bonded to the touch screen layers and LCD shielding.

The LCD shielding barrier 30 could be of many types and shapes. In the preferred embodiment the LCD shielding barrier 30 is formed as a plate of a conductive material, lying around the sides or perimeter of the LCD layer 16. Another possible embodiment is to have the LCD shielding barrier surrounding the touch screen like a frame. Other shapes and locations of the LCD shielding barrier are possible and the invention is not intended to be limited by any such constraints of the shielding of either the display or the portable electronic device. It suffices that there is a shielding.

It is important to realise that the touch screen panel could not be connected directly to the zero volt reference since that would lead to short circuiting of the panel .

As stated earlier the invention can be used with only one touch screen layer being capacitively coupled to the LCD or phone shielding. Having two layers coupled is however better, since then the shielding effectiveness will be doubled, while the functioning of each layer will not be influenced negatively. In both cases there can be more than one capacitor.

Claims

IDCLAIMS

1. Touch screen display (12) having a display layer (16) and a touch screen panel with at least one first electrically conducting touch screen layer (20) , said display layer having an electromagnetic shielding barrier (30) , c h a r a c t e r i z e d in that at least one capacitor (24) is connected between the touch screen layer (20) and the electromagnetic shielding barrier (30) for providing a high frequency shielding barrier.

2. Touch screen display according to claim 1, c h a r a c t e r i z e d in that the at least one first conducting touch screen layer (20) and the electromagnetic shielding barrier together make out the plates of the capacitor (24) , between which plates a medium having good dielectrical properties is placed.

3. Touch screen display as claimed in claim 1, c h a r a c t e r i z e d in that a plurality of capacitors (24) are connected between the touch screen layer (20) and the shielding barrier (30) .

4. Touch screen display as claimed in claim 3, c h a r a c t e r i z e d in that the touch screen panel includes a second electrically conducting touch screen layer (18) and that a plurality of capacitors (22) are connected between this layer and the shielding barrier (30) .

5. Touch screen display as claimed in claim 4, c h a r a c t e r i z e d in that the capacitors (24) between the first touch screen layer (20) and the shielding barrier (30) are provided on two opposing sides of first touch screen layer and that the capacitors (22) between the second touch screen layer (18) and the shielding barrier (30) are provided on two opposing sides of the second touch screen layer.

6. Touch screen display as claimed in any of claims 3 - 5, c h a r a c t e r i z e d in that the capacitors are provided along the whole perimeter of the display layer (16) and at equal distances.

7. Touch screen display as claimed in any of claims 3

- 6, c h a r a c t e r i z e d in that the capacitors are attached to the shielding barrier and to the at least one first touch screen layer (20) with electrically conductive glue .

8. Touch screen display as claimed in any of claims 3

- 6, c h a r a c t e r i z e d in that the capacitors are attached to the shielding barrier with a contact spring means .

9. Portable electronic device (10) including a touch screen display (12) having a display layer (16) and a touch screen panel with at least one first electrically conducting touch screen layer (20) , said portable electronic device having an electromagnetic shielding barrier (26, 30) , c h a r a c t e r i z e d in that at least one capacitor (24) is connected between the touch screen layer (20) and the electromagnetic shielding barrier (26, 30) for providing a high frequency shielding barrier.