WO2010128942A1

WO2010128942A1 - Antenna for electronic pen

Info

Publication number: WO2010128942A1
Application number: PCT/SE2010/050496
Authority: WO
Inventors: Bengt Falkenberg; Lars Rosengren
Original assignee: Anoto Ab
Priority date: 2009-05-05
Filing date: 2010-05-05
Publication date: 2010-11-11

Abstract

An electronic pen comprises a casing, a tip protruding from said casing, which tip is intended to be applied to a substrate, wherein the position of the tip on the substrate is determinable, and a communication unit, which is arranged to transmit position data to an external unit. The communication unit comprises a metal piece having a slot extending in the metal piece, whereby said metal piece forms a slot antenna, said metal piece being arranged inside or forming at least part of said casing.

Description

ANTENNA FOR ELECTRONIC PEN

Field of the Invention

The present invention relates to an antenna arrangement in an electronic pen.

Background Art Electronic pens for transforming handwritten information into digital information have been introduced in the market during recent years. For instance, the present applicant Anoto AB has developed such electronic pens.

The Anoto technology is based on an electronic pen comprising a small built-in camera, a built-in processor and a memory, combined with a paper with a dot pattern. When writing with the pen, the camera continuously captures images of the dot pattern on the paper. Concurrently, the built-in processor determines, from the dot pattern in the images, the momentary position of the pen tip. In this way, the writing may be captured as a sequence of determined positions of the pen tip.

The electronic pen may communicate with an external unit, for instance transmitting captured writing to the external unit. Thus, the captured writing may be further processed in the external unit or it may be forwarded to a processing unit, where the data is processed. The electronic pen may communicate with a computer, a mobile phone, a personal digital assistant or any other external unit that is able to receive and process or forward data. The communication between the electronic pen and the external unit may be achieved e.g. through a docking station, in which the pen is placed, through any kind of wired connection, such as a USB connector, or through a wireless connection. In order to enable a wireless connection, the electronic pen needs to be provided with an antenna, as briefly disclosed in e.g. WO2008/002239. The antenna may be connected e.g. to a Bluetooth™ module for operating according to the Bluetooth™ standard. The antenna is preferably placed inside a casing of the electronic pen. It would not be aesthetically appealing to have an antenna protrude from the casing. Further, such an antenna may obstruct a user's handling of the pen and the antenna may act constraining during writing with the pen.

Electronic pens have previously been designed in a plastic material. Thus, the casing of the pen would not affect the signal emitted from the antenna inside the pen. However, due to the antenna being arranged inside the pen casing, the use of a metallic or electrically conducting material in the pen casing would seriously deteriorate the antenna signal. Therefore, the robust and impact-resistant characteristics of a metallic material would not be available to a casing of an electronic pen without providing a new antenna arrangement.

US 2006/0232570 discloses an electronic pen, which comprises an antenna for receiving or transmitting a signal. The antenna is electrically connected to a PCB board. The electronic pen comprises a first and a second casing, which cooperate to positively secure the PCB board in a fixed position within the casing. Each of the casings is a plastic moulded component that is electrically plated with a conductive metal. The metal plated casings act as a shield electrically protecting the PCB board from extraneous signals. Only the portion of the antenna beyond the casing is outside the shield whereby improved antenna performance is achieved.

Summary of the Invention

It is an object of the invention to provide an electronic pen which may be more robust and impact-resistant. This and other objects, which will appear from the description below, are at least partly achieved by means of an electronic pen according to the independent claim, preferred embodiments being defined by the dependent claims.

According to the invention there is provided an electronic pen, comprising: a casing, a tip protruding from said casing, which tip is intended to be applied to a substrate, wherein the position of the tip on the substrate is determinable, and a communication unit, which is arranged to transmit position data to an external unit, wherein said communication unit comprises a metal piece having a slot extending in the metal piece, whereby said metal piece forms an antenna, said metal piece being arranged inside or forming at least part of said casing.

Thanks to the use of a metal piece as the transmitting element, it is now possible to provide a casing of the electronic pen in a metallic material without the casing obstructing a signal from the antenna. Thus, it is possible to use a metal casing that makes the electronic pen resistive to shocks and careless handling of the pen. Further, the metal casing may provide a shield protecting the printed circuit board and other components of the electronic pen from extraneous signals.

Brief Description of the Drawings

The embodiments of the invention will now be described in more detail with reference to the accompanying schematic drawings. Fig. 1 is a schematic view of an electronic pen system.

Fig. 2 is a top view of the inner parts of an electronic pen according to one embodiment illustrating a first embodiment of a slot antenna.

Fig. 3 is an exploded perspective view of inner parts of the electronic pen. Fig. 4 is a perspective view of a slot antenna according to a second embodiment.

Fig. 5 is a perspective view of a slot antenna according to a third embodiment. Fig. 6 is a perspective view of a slot antenna according to the first embodiment.

Detailed Description of a Preferred Embodiment Referring to Fig. 1, a general description of a system using an electronic pen 100 will first be described. The electronic pen 100 is used for writing on a substrate 102. During writing, an electronic representation of the writing is captured. The electronic representation of the writing may be processed in the electronic pen 100 or may be transferred from the electronic pen 100 to an external unit 104 for further processing. The substrate 102 may be any kind of surface on which writing may be performed, such as a sheet of paper, a whiteboard surface, a plastic film, or a display surface. The electronic pen 100 may be provided with a writing implement 106 being arranged to leave a trace on the substrate 102 during writing. Thus, both a physical and an electronic copy of the writing may be created. Alternatively, the electronic pen 100 may be provided with a stylus, leaving no trace on the substrate 102. This may be particularly suitable if the electronic pen 100 is used for writing on a display surface. The electronic pen 100 may also be arranged to allow switching between using a writing implement and a stylus, such that a user may select whether a trace is to be left on the substrate 102.

The position of the electronic pen 100 in relation to the substrate is continuously determined during writing. A sensing mechanism for determining the position of the electronic pen 100 may be arranged in the electronic pen 100, in the substrate 102 or in an apparatus arranged in a predetermined relation to the substrate 102.

The electronic pen 100 may, according to a first embodiment of the sensing mechanism, comprise one or more accelerometers (not shown) for determining its position. The accelerometers may sense the movement of the electronic pen 100 and may therefore determine writing in relation to a starting point. The accelerometers may be supplemented with a mechanism for determining the starting point, such as a specific position on the surface. The starting point may be indicated to the electronic pen 100 in various ways, such as by pressing a button or by applying the electronic pen 100 onto the substrate 102. An example of an electronic pen using accelerometers and gyroscopes to determine its position is provided in US 5,902,968, which is incorporated herein by reference.

The substrate 102 may be provided with a position-coding pattern 108, which is illustrated as a hatching of the substrate 102 in Fig. 1 and also schematically shown in the magnified area A in Fig. 1. In such case, the electronic pen 100 may be able to determine its position by detecting the position-coding pattern 108 in the vicinity of the writing tip of the electronic pen 100. The electronic pen 100 may thus, according to a second embodiment of the sensing mechanism, comprise an image sensor for detecting the position-coding pattern 108. The pattern may be optically readable and the image sensor is thus optical. The pattern can, however, be based on a parameter other than an optical parameter. In such a case the sensor must of course be of a type which can read the parameter concerned. Examples of such parameters are chemical, acoustic or electromagnetic marks. Capacitive or inductive marks can also be used. However, it is preferable for the pattern to be optically readable, as it is then relatively simple to apply it onto different products and in particular onto paper.

The electronic pen 100 may further comprise an optical module for forming an image of the position-coding pattern 108 on the image sensor. The position of the electronic pen 100 may then be determined by decoding the imaged portion of the position-coding pattern 108. Examples of position-coding patterns are provided in US 5,221,833; US 5,477,012; US 5,652,412; WO 00/73983; WO 01/26032; WO 01/71643; US 6,330,976; and US 7,263,224.

According to a third embodiment of the sensing mechanism, the substrate 102 provides a surface that is able to detect the position of the electronic pen 100 on the surface when the electronic pen 100 is applied to the surface. The substrate 102 may thus be able to detect the touching of the surface with the electronic pen 100. There are a lot of different ways to detect the electronic pen 100 being applied to the surface, e.g. by resistive pressure detection, or by detection of interference with an electromagnetic or ultrasonic wave that is transmitted close to the surface. The substrate 102 may alternatively comprise tracks providing a multitude of overlapping antenna coils. The sensor emits electromagnetic signals that can be detected by a special electronic pen, which is provided with either active or passive circuitry in the tip of the pen. The position of the electronic pen 100 within the magnetic field created by the antenna coils of the substrate 102 can then be detected.

According to a fourth embodiment of the sensing mechanism, the substrate 102 may be arranged in a predetermined relationship to a detector. The position of the electronic pen 100 may then be determined e.g. through triangulation. The detector may comprise two emitter/receiver pairs that emit and detect an electromagnetic wave or ultrasound wave. The emitted wave is reflected by the electronic pen 100 and the position of the electronic pen 100 may be determined by comparing the time difference in the detected signals. The sensing mechanism may be arranged to detect a sequence of positions as the electronic pen 100 is used. Thus, writing or drawing with the electronic pen 100 on the substrate 102 will result in a sequence of positions being obtained. This sequence of positions may form an electronic representation of the writing. However, a position may be associated with a certain function. Thus, the sensing mechanism may alternatively detect that a position is within an area associated with a certain function. In this case, the sensing mechanism may only provide an area identification. As a further alternative, the sensing mechanism may provide a combination of sequences of positions and area identifications. Also, the determination whether a position is associated with a certain function may be performed at a later stage of processing the positions. Thus, the sensing mechanism may still only provide a sequence of positions. The electronic pen 100 is further arranged to communicate with an external unit 104. The electronic pen 100 may transmit the detected sequence of positions to the external unit 104 for further processing therein. However, the electronic pen 100 and the external unit 104 may alternatively or additionally communicate other kinds of information. For example, the electronic pen 100 may transmit images of the position-coding pattern 102 to the external unit 104, and the corresponding positions may be determined in the external unit 104 instead. Further, the electronic pen 100 may transmit function calls for initiating an operation in the external unit 104, which may e.g. be triggered by detection in the pen 100 of a position within an area associated with a certain function. The electronic pen 100 may also be arranged to transmit any other kind of information generated or stored in the electronic pen 100 to the external unit 104. Moreover, the external unit 104 may transmit information to the electronic pen 100, such as reactions to the detected positions, updates of software of the electronic pen 100 or specifications of associations between positions and functions.

Since the electronic pen 100 is arranged to communicate with an external unit, the invention is specifically suited for an embodiment wherein the sensing mechanism is arranged in the electronic pen 100. However, the electronic pen communicating with an external unit may still be used for other sensing mechanisms wherein the detected positions are transferred to the electronic pen 100, e.g. via a wired connection between the detector and the electronic pen 100. Further, the electronic pen 100 may be arranged to communicate other information than position information to the external unit 104. Thus, it is not necessary that information of detected positions is provided in the electronic pen 100.

The electronic pen 100 may be arranged to communicate with an external unit 104, which processes the information received from the electronic pen 100. However, the external unit 104 may also be arranged to forward the information to a processing unit. The external unit 104 may be any kind of unit that is able to communicate and handle data information, such as a computer, a mobile phone, a Personal Digital Assistant, etc. Where the external unit 104 is a computer, the processing of the information from the electronic pen 100 may typically be performed in the computer. However, where the external unit is a mobile phone, the mobile phone may typically be arranged to forward the information to a processing unit, such as a server. The information may be forwarded via a telephone network and a computer network, such as the Internet or a local area network, for reaching the processing unit. Of course, the mobile phone may itself process the information, whereas the computer may be arranged to forward the information. Referring now to Figs 2 and 3, a general description of an electronic pen 100 will be provided. The electronic pen 100 comprises a shell or casing enclosing the pen components. The electronic pen 100 may further comprise a writing implement 106 having a writing tip 110. The writing implement 106 may be movable between a retracted position completely inside the casing of the electronic pen 100 and an extended position, wherein the writing tip 110 of the writing implement 106 projects from the casing of the electronic pen 100. Alternatively, the writing implement 106 may be arranged in a fixed relation to the electronic pen 100, and the electronic pen 100 may comprise a cap covering the writing tip 110, which may be removed for unveiling the tip.

The writing implement 106 may be arranged to leave a pigment trace on the substrate during writing. For instance, the writing implement 106 may be in the form of an ink cartridge for any kind of pen, such as a ballpoint pen, a rollerball pen, a fountain pen or a felt tip pen. The ink cartridge may be in the form of a pen refill, which may be substituted when the pen runs out of ink. Alternatively, the writing implement 106 is fixed and may be provided with additional writing material, if needed. Using this alternative, a graphite lead may be used for leaving the pigment trace. The writing implement 106 may thus realize a mechanical pencil. According to another alternative, the writing implement 106 is arranged not to leave any trace on the substrate. In this regard, the writing implement 106 may be realized as a stylus having a tip for pointing at desired positions. Also, the electronic pen may be provided with both an ink cartridge and a stylus, such that it is possible to switch between a mode where a pigment trace is made on the substrate 102 and a mode where no trace is made.

The electronic pen 100 may comprise a pen down detector 112 for determining whether the tip 110 of the electronic pen 100 has been applied onto the substrate 102. Thus, the pen down detector 112 detects when a user puts down the electronic pen 100 on the substrate 102 for commencing input of data with the electronic pen 102. The pen down detector 112 may be a force sensor. The force sensor may be arranged at the opposite end of the writing implement 106 to the tip 110. The writing implement 106 may be movable such that a force is applied to the force sensor when the tip 110 is pressed down on the substrate 102. Alternatively, detection that the pen 100 is applied to a substrate may be accomplished by the image sensor being able to acquire an image of a substrate in focus. As a further alternative, the electronic pen 100 may comprise a light source and a light detector at a front end of the pen. The light source may be arranged to transmit light pulses and upon detection by the light detector of reflected light from a substrate within a predetermined time frame, the pen 100 may be determined to be applied to the substrate 102. Any combinations of these techniques may also be used.

The electronic pen 100 may further comprise a sensing mechanism 120 for recording information in order to determine the position of the tip of the electronic pen on the substrate 102. The sensing mechanism 120 may start detecting the position of the electronic pen in response to a signal from the pen down detector 112 that the electronic pen 100 has been applied to a substrate 102. However, as explained above, the sensing mechanism 120 may be arranged in the substrate 102 instead.

Where the sensing mechanism 120 is arranged in the electronic pen 100, the sensing mechanism 120 may be arranged to acquire an image of the substrate 102 depicting a portion of a position-coding pattern 108 on the substrate 102. The position-coding pattern 108 may have characteristics that enable the pattern to be distinguished from the substrate 102 on which it is applied. Thus, the electronic pen 100 comprises a corresponding sensor that is able to image the substrate 102 to separate the pattern 108 from the substrate 102.

In a preferred embodiment, the position-coding pattern 108 is optically detectable. The sensing mechanism 120 may then comprise an optical system 122 for forming an image of the substrate 102 close to the writing tip 110 onto an optical image sensor 124, such as a charge coupled device (CCD) or complementary metal-oxide-semiconductor (CMOS) sensor, which is arranged to acquire a two-dimensional image. The optical system 122 may comprise a lens 126 for focussing the image onto the sensor surface. The optical system 122 may also comprise a light guide 128, which may include one or more mirrors for guiding the light to the sensor 124. Also, the optical system 122 may comprise a wavelength filter for filtering out unwanted wavelengths. The pattern may have especially strong reflective or absorptive characteristics for certain wavelengths. By only letting these wavelengths reach the sensor surface, the image quality is improved.

The electronic pen 100 may further comprise a light source (not shown). The light source may emit light towards the portion of the substrate 102 that is being imaged. Thus, the imaging conditions may be improved. The light source may further be arranged to emit light in a wavelength range, which the position-coding pattern 108 will interact with in a strong manner. For example, the light source may emit infrared light and the position-coding pattern 108 may be printed on the substrate 102 with a pigment that strongly absorbs infrared light. The optical system 122 may further have a filter that blocks wavelengths shorter than infrared light to further improve the quality of the image acquired by the sensor 124.

The sensing mechanism 120 in the electronic pen 100 may alternatively comprise a sensor, which is arranged to read another parameter than an optical parameter. For example, the sensing mechanism may comprise a spectrometer for detecting a pattern using a chemical parameter, e.g. the chemical composition of the substrate being varied in order to produce the position-coding pattern. As another alternative, the sensing mechanism may comprise a detection head in which a driving coil and a receiving coil are disposed. These coils interact with a scale in which a magnetic modulating section such as closed loop coils is disposed, e.g. in the form of a metal pattern on a resin substrate. The detection head is relatively moved with respect to the magnetic modulating section to modulate a magnetic flux generated by the driving coil. A primary varying magnetic flux which is generated when the driving coil is AC-driven is modulated by the scale, so that a magnetic pattern of a predetermined period is formed. The magnetic pattern is coupled with the receiving coil of the detection head. As a result, an induced voltage which is varied in accordance with movement of the scale appears in the receiving coil. As a yet further alternative, the sensing mechanism may comprise accelerometers in order to detect the movements of the electronic pen relative a starting position. The sensing mechanism may then further comprise a detector for determining the starting position, such as a barcode reader for reading a barcode in a predetermined position of the substrate.

The electronic pen 100 may further comprise an analyzing unit 130, which is arranged to determine the position of the electronic pen 100 based on the data acquired by the sensing mechanism 120. The analyzing unit 130 may be integrated with the sensing mechanism 120 in order to directly determine the position of the electronic pen upon the position data being acquired. For example, the analyzing unit 130 may be realized as a customer- specific integrated circuit adapted for the purpose (for example ASIC, application-specific integrated circuit), which may also hold the image sensor 124. As an alternative, the analyzing unit 130 may be arranged to receive the acquired position data, such as images, from the sensing mechanism 120. The analyzing unit 130 may then still be realized as a customer- specific integrated circuit, or of some form of programmable integrated circuit (for example PROM, FPGA) or an ordinary processor running a special software program for determining the position based on the acquired position data. Also, the electronic pen 100 may be arranged to merely acquire the position data and transmit the position data to an external unit 104 for analysis. In such case, the analyzing unit would be arranged in an external unit 104.

The analyzing unit 130 may be arranged to analyze an image of a position-coding pattern 108 and decode the position coded by the portion of the position-coding pattern that is depicted in the image. The analyzing unit 130 would naturally have instructions that are dependent on the encoding scheme of the position-coding pattern 108 such that the decoding performed in the analyzing unit 130 is adapted to the specific position-coding pattern to be decoded. Examples of such decoding algorithms are provided in WO 01/26032 and WO 03/001440.

The electronic pen 100 may further comprise means for providing feedback to a user. Feedback may be provided in many different ways. For example, the electronic pen 100 may comprise a vibrator, which may cause the electronic pen to vibrate in order to e.g. signal to the user that a certain position associated with a function has been successfully detected or that the electronic pen 100 is not able to decode or read positions properly. As an alternative, the electronic pen 100 may comprise one or more light sources, such as LEDs, for providing information to the user. The LEDs may e.g. indicate when the electronic pen is switched on or when data is being transmitted to an external unit. As a further alternative, the electronic pen 100 may comprise a display 140. Thus, text or picture messages may be provided to the user regarding the handling of the electronic pen 100. As a yet further alternative, the electronic pen 100 may comprise a speaker (not shown) for providing audio messages to the user. Of course, the electronic pen 100 may comprise one or more of these feedback means in order to provide feedback in more than one way to the user. The electronic pen 100 may then be arranged to provide different forms of feedback through different means.

The electronic pen 100 further comprises a communication unit 150 for communicating with an external unit 104. The electronic pen 100 may be arranged to communicate with the external unit 104 both through a wired connection and through a wireless connection, which will be further described below. The wired connection may e.g. be established by the electronic pen 100 comprising a connector for receiving a plug or for being introduced into a receptacle. The connector may be any kind of electrical connector, such as a USB connector. The wired connection may alternatively be established by the electronic pen 100 being placed in a specially designed cradle, which in turn may comprise a wire for connecting to the external unit 104. The electronic pen 100 may then comprise an interface for forming a connection with the cradle. The interface may be freely designed, since the cradle is specially designed for the electronic pen 100, and thus many alternative techniques may be feasible. For example, any kind of plug/socket connection may be used.

The power supply for the electronic pen 100 may be obtained from a battery, which may be mounted inside the electronic pen. As an alternative, the power supply may be provided via a cable from an external power source.

The electronic pen 100 may further comprise a printed circuit board 160. The printed circuit board 160 may comprise a processor 162 which may be arranged to control the different parts of the electronic pen 100, such as the pen down detector 112, the sensing mechanism 120, such as the image sensor 124 and light source as exemplified above, the feedback means, and a communication unit 150 for communicating with an external unit 104. According to one embodiment, the processor 162 may also be arranged to analyze the position data acquired by the sensing mechanism. The processor 162 may also be connected to a storage memory for storing, for example, position data. The different parts of the electronic pen 100 may be connected and/or mounted on the printed circuit board 160. The printed circuit board 160 may be arranged to extend in a longitudinal direction of the electronic pen 100, which enables the electronic pen 100 to be manufactured with a small diameter. The electronic pen 100 may also comprise a carrier, which may form a mounting unit for controlling the mounting of the parts of the electronic pen inside the casing of the pen. An example of such a carrier is provided in WO 05/0574471.

In order to provide communication through a wireless connection, the communication unit 150 of the electronic pen 100 comprises an antenna 152. The antenna 152 is formed by a metal piece 154, in which a slot 156 is arranged. The slot 156 forms a through-hole in the metal piece 154 and has a lengthwise extension that is chosen with regard to the frequency of the signal that is desired to be transmitted by the antenna 152.

The antenna 152 is connected on one side of the slot 156 to a ground plane and on the opposite side of the slot 156 to a generator for generating a driving frequency that may drive the transmission of the antenna. The antenna 152 may comprise projections 158 for forming these connections. According to an alternative, a plurality of slots 156 is provided in the metal piece. Then, connections may be formed to connect the sides of each of the slots to the generator. The sides of the slots may be connected in parallel to the generator. The placement of the connections, as described in further detail below, may need to be adjusted in order to ensure that the impedance of the antenna arrangement does not introduce an impedance mismatch to the generator. Typically, the impedance of the antenna arrangement should be 50Ω.

It is now possible to use a metal casing that makes the electronic pen 100 resistive to shocks and careless handling of the pen. The antenna is formed in the metal casing by providing the at least one slot in the casing. Further, the metal casing may provide a shield protecting the printed circuit board 160 and other components of the electronic pen 100 from extraneous signals. Thanks to the use of an antenna 152 made in metal, the advantages of a metal casing may be obtained while the electronic pen 100 is able to transmit information wirelessly. Further, there is no need of providing an antenna 152 outside the metal shield. Further, the electronic pen 100 is intended to be held by a user. In testing performed by the inventors, it was found that antenna operation was significantly less affected by the pen being held by a user when a slot antenna was used compared to use of a dipole antenna. Thus, the design of the slot antenna within the electronic pen may not be very critical to ensure that the influence of the signals transmitted by the antenna is within acceptable limits. One possible explanation for the effects on antenna operation by a user holding the pen may be that the body of the user may influence the antenna to detune the antenna operation frequency and introduce impedance mismatching. A dipole or monopole antenna sets up primarily an electric field, leading to high wave impedance close to the antenna. This means that the antenna is affected by the user and when designing the electronic pen there is a need to consider how the antenna will be affected by a user holding the pen. The slot antenna on the other hand sets up a primarily magnetic field and therefore provides low wave impedance close to the antenna. For this reason, the slot antenna may be less affected by a human body close to the antenna than a dipole or monopole antenna.

The opening formed by the slot 156 in the metal piece 154 of the antenna 152 may be used to present an opening in the electronic pen 100 for a display (not shown). The slot antenna 152 may provide a support to which the display may be attached, wherein the support is close to the outer surface of the electronic pen 100. In fact, the slot antenna 152 may be formed in the casing of the electronic pen 100, whereby the display may be arranged at the outer surface of the pen 100. Alternatively, the metal piece 154 of the slot antenna 152 is arranged inside the electronic pen 100 and a cover for the display may be arranged in the casing of the pen 100.

The metal piece 154 may be formed by virtually any metal material. However, a material which has a good electric conductivity and which is easily connected to the ground plane and to a generator providing a driving frequency is advantageous. For example, the metal piece 154 may be constituted by brass, which is relatively light weight and may be easily shaped. Further, it is easy to attach the connections to the metal piece by means of soldering. The metal piece may alternatively be constituted by aluminium, stainless steel, copper, zinc, or any other conductive material. The metal piece 154 may alternatively be connected to the ground plane and to the generator by means of screws that are applied through the metal piece 154 and into the printed circuit board 160. As a further alternative, the printed circuit board 160 may comprise two latches that may be arranged to protrude into corresponding recesses in the metal piece 154. The latches may be spring-biased so as to snap into the recesses of the metal piece 154, when the metal piece 154 is properly mounted over the printed circuit board 160.

The slot 156 forms a through-hole in the metal piece 154. This through-hole may be filled by an electrically insulated material, while the slot antenna 152 still maintains its antenna characteristics. Thus, a cover glass of the display may be arranged in the slot. Alternatively, the slot 156 may be filled by a plastic material. This may be especially suitable, if the slot antenna 152 is formed in the casing of the electronic pen 100, whereby the plastic material in the slot 156 would prevent access into the interior of the electronic pen 100.

The metal piece 154 may be shaped in several different ways. According to one embodiment as illustrated in Fig. 4, the metal piece 254 is at least part of the casing of the electronic pen 100. Thus, the casing of the electronic pen 100 constitutes the antenna 252. In this embodiment, the metal piece 254 would be a substantially cylindrical tube encompassing the interior of the electronic pen 100. The slot 256 may be formed in a longitudinal direction of the tube. The metal piece 254 may further comprise two projections (not shown) projecting to the inside of the tube for facilitating attachment of the metal piece 254 to the ground plane and the generator. These projections may be formed at the sides of the slot 256. However, the angle between the projections in a radial cross-section of the tube may be up to 180° or even more. In another embodiment as illustrated in Fig. 5, the metal piece 354 is arranged at the interior of the electronic pen 100, i.e. inside the casing of the electronic pen 100. In this case, the casing may still be formed of a metallic or other electrically conducting material. However, the casing should comprise a portion of an electrically insulated material covering the slot 356 of the antenna 352 such that the effect of the antenna is not affected. Alternatively, the casing may be formed of any electrically insulated material. The metal piece 354 may be shaped as a cylindrical tube that extends in the longitudinal direction of the electronic pen 100. The tube needs at least be sufficiently long to accommodate the desired length of the slot, which is advantageously formed in the longitudinal direction of the tube. However, the tube may also extend further along the longitudinal direction of the electronic pen 100 in order to provide a protection against shocks and a shield against extraneous signals to components of the electronic pen 100. In this embodiment, the metal piece 354 may also be provided with projections 358 for facilitating attachment of the metal piece 354 to the ground plane and the generator.

The embodiment of the antenna 152 shown in Fig. 2 is illustrated in more detail in Fig. 6. Here, the metal piece 154 is shaped as a plate. The plate 154 may be arranged on top of the printed circuit board 160. In fact, the plate 154 may merely form a frame around the slot 156. Thus, a minimum amount of material is used to form the slot antenna 152. The plate 154 may also be provided with projections 158 which project from the plane defined by the metal plate 154. These projections 158 may lift the plate from the printed circuit board 160 such that the plate 154 is not arranged immediately on top of the printed circuit board 160. The projections 158 may also facilitate attachment of the metal piece 154 to the ground plane and the generator. Also, the plate 154 may be provided with further projections 159 which merely serve to maintain a distance between the printed circuit board 160 and the plate 154 and do not provide any electrical connection.

The length of the slot 156 may essentially correspond to a wavelength of the frequency to be transmitted or the wavelength divided by a whole number. Thus, a resonance of the driving frequency may be obtained in the slot 156 and a strong transmission signal may be obtained. For example, the length of the slot 156 may essentially correspond to a half wavelength or a quarter wavelength or even an eighth of the wavelength. The antenna 152 may be arranged to communicate over any frequency for which it is feasible to form a slot having these characteristics. Thus, the antenna 152 may emit a frequency that is larger than 300MHz.

A slot antenna 152 for use in an electronic pen 100 is well suited to transmit signals within the 2.4 GHz ISM (Industrial, Scientific and Medical) radio band, which spans from 2.4 GHz to 2.5 GHz. This radio band is e.g. used for Bluetooth communication and Wi-Fi communication, which are suitable for transmitting data between computing devices. The wavelength of the waves within this band is about 120 mm. It may be possible to arrange a slot 156 in the slot antenna 152 having a length corresponding to the wavelength. However, this would imply that the slot 156 would extend along almost the entire length of the electronic pen 100, since a pen is typically in the order of 150 mm long.

For Bluetooth™ communication, it would be more preferable to arrange a slot 156 extending approximately a half wavelength in the metal piece 154. Such a slot 156 would thus be about 60 mm long and allows more freedom in designing the electronic pen 100, since there is no need of having a slot 156 extending along the entire length of the pen 100. Further, a half wavelength slot would be a very efficient radiator, since nodes may be formed in the closed ends of the slot 156 causing a resonance at the desired wavelength. According to one embodiment, the slot 156 is arranged to extend approximately a quarter wavelength in the metal piece 154. This would somewhat deteriorate the performance of the slot antenna 152 compared to a half wavelength slot, since the closed ends of the slot may not both form nodes and, therefore, the resonance at the desired wavelength is not as strong. However, it has been found that the quarter wavelength slot provides a satisfactory signal for communicating over Bluetooth™.

The cross-section of the slot 156 may be very small, such as in the order of 0.1 mm. This implies that the design of the slot antenna 152 is quite flexible, allowing a large degree of freedom to a designer of the electronic pen 100, even if the metal piece 154 is to form the casing of the electronic pen 100. Further, the slot 156 may be dimensioned to hold a display of the electronic pen 100 and the cross-section of the slot 156 may then be controlled by the size of the display. Thus, the slot may for example be 5-10 mm thick to accommodate a display.

Furthermore, the slot 156 need not be rectangular. The slot 156 may have a varying cross- section, as illustrated in Figs 4 and 6. Moreover, the slot 156 may form any shape in the metal piece 154. For example, the slot 156 may extend in a radial direction of a tubular metal piece 154. As a further alternative, the slot 156 may have a helical shape or a meandering shape. The slot 156 may even have the shape of a logotype or a name of the manufacturer of the electronic pen 100. Moreover, the slot 156 needs not be entirely confined in the metal piece 154, but may instead have an open end at an edge of the metal piece 154.

The projections 158 of the slot antenna for forming connections to the generator and the ground plane are arranged on opposite sides of the slot 156. The projections 158 may be formed of the same material as the metal piece 154 such that the projections 158 are integrated with the metal piece 154. Thus, the projections 158 may be formed in the same manufacturing step as the forming of the metal piece 154. However, the projections 158 may be arranged in another material than the metal piece 154 providing an electrical connection to the metal piece 154. The projections 158 may be arranged to be soldered to the printed circuit board 160 for forming a connection to the ground plane and the generator, respectively.

The projections 158 may be arranged at different distances to a short side 156a of the slot 156. However, it is preferred that the projections are arranged facing each other at an equal distance to the short side 156a of the slot 156. Further, the projections 158 are preferably arranged close to a longitudinal side 156b of the slot 156, but may be arranged at a distance from the slot 156. For example, where the metal piece 154 is formed as a cylindrical tube, it may be advantageous to provide the projections 158 at opposite sides of a radius of the cylinder, i.e. separated by 180°, which would enable the projections 158 to be connected to opposite sides of the printed circuit board.

The projections 158 may be arranged at the center of the slot 156. However, the projections 158 may alternatively be arranged closer to one short side 156a of the slot 156. The impedance of the antenna is dependent on the distance in the metal material between the connection points. Thus, the impedance between points at the center of the slot would typically be quite high. Therefore, in order to match the impedance of the antenna to the generator, the projections need to be properly positioned in relation to the sides of the slot 156. The impedance of the antenna also depends on the material of the metal piece, the driving frequency, and the cross-section of the slot 156. The generator for generating a driving frequency comprises at least one oscillator for providing a frequency. The oscillator may be an electronic oscillator, which may be coupled to a crystal for stabilizing the frequency. The generator further comprises circuitry for controlling the output frequency from the oscillator. The generator may be arranged on a chip that is especially adapted to generating a Bluetooth™ signal. The generator may be connected to a wire for transferring the radio frequency to the antenna 152. The wire may typically provide a 50Ω load, which is normally expected from a generator. A first projection of the metal piece 154 is connected to the wire for coupling the driving frequency to the slot antenna 152.

The printed circuit board 160 may provide a ground plane in a separate layer of the board design. Thus, it may easily be ensured that all circuitry on the printed circuit board 160 may be connected to the same ground potential. A second projection of the metal piece 154 may be connected to the ground layer of the printed circuit board 160 for connecting this projection of the slot antenna 152 to the ground plane. The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope and spirit of the invention, which is defined and limited only by the appended patent claims.

Claims

1. An electronic pen, comprising: a casing, a tip protruding from said casing, which tip is intended to be applied to a substrate, wherein the position of the tip on the substrate is determinable, and a communication unit, which is arranged to transmit position data to an external unit, c h a r a c t e r i s e d i n t h a t said communication unit comprises a metal piece having a slot extending in the metal piece, whereby said metal piece forms an antenna, said metal piece being arranged inside or forming at least part of said casing.

2. The electronic pen according to claim 1, wherein the metal piece forms said casing.

3. The electronic pen according to claim 1 or 2, wherein the metal piece is a cylindrical tube.

4. The electronic pen according to claim 1, wherein the metal piece is a plate.

5. The electronic pen according to claim 4, wherein the pen further comprises a printed circuit board and the plate is arranged upon the printed circuit board.

6. The electronic pen according to any one of the preceding claims, wherein the metal piece comprises two projections, which are arranged to be connected to a ground plane and a generator for generating a driving frequency, respectively.

7. The electronic pen according to claim 6, wherein the pen further comprises a printed circuit board and the projections are connected to the printed circuit board.

8. The electronic pen according to claim 7, wherein the projections are soldered to the printed circuit board.

9. The electronic pen according to any one of claims 6-8, wherein the projections are arranged to extend from positions close to the slot.

10. The electronic pen according to any one of claims 6-9, wherein the projections are arranged to extend from positions at equal distances from a short side of the slot.

11. The electronic pen according to any one of claims 6-10, wherein the metal piece is a cylindrical tube and said projections are arranged to extend from radially opposite positions in the cylindrical tube.

12. The electronic pen according to any one of the preceding claims, wherein the antenna is arranged to transmit a radio frequency in the band of 2.4 GHz to 2.5 GHz.

13. The electronic pen according to any one of the preceding claims, wherein a length of the slot essentially corresponds to a wavelength of a frequency to be transmitted by the antenna.

14. The electronic pen according to any one of claims 1-12, wherein a length of the slot essentially corresponds to a half wavelength of a frequency to be transmitted by the antenna.

15. The electronic pen according to any one of claims 1-12, wherein a length of the slot essentially corresponds to a quarter wavelength of a frequency to be transmitted by the antenna.

16. The electronic pen according to any one of the preceding claims, wherein a display of the electronic pen is arranged in said slot.

17. The electronic pen according to any one of the preceding claims, wherein the metal piece is constituted by brass.