A touch panel, working as a location recognition device, can ingeniously combine input and display interfaces, and therefore has the advantages of being space saving and user-friendly operated. Nowadays it has been generally applied to a wide variety of consumer or industrial electronics, for example, PDAs (Personal Digital Assistant), palm-sized PCs (Personal Computers), tablet computers, mobile phones, handwriting input devices for a smart phone, IAs (Information Appliances), ATMs (Automated Teller Machines) and POS (Points of Sale) etc., which can generally be seen in various kinds of application occasions in business and industry.
Among various types of location recognition devices, those using projected-capacitive touch technology are most popular. This technology utilizes conductive materials and capacitors to form a projection of an electric field, and then senses a change caused by other conductive materials such as fingers approaching or touching the electric field. Devices based on this technology can react more quickly due to high transmissivity and sensitivity. Furthermore, they are also good at durability since their touch screens can be made entirely of plain glass, allowing them to be immune to fire, smudges, and scratches. In addition to this, these devices have the capability of sensing as many fingers as can fit on their screens, and therefore provide a more human-oriented way to operate them. Compared to their counterparts, location recognition devices with projected-capacitive touch technology have been most favorable as a consequence of the aforementioned merits.
Projected-capacitive touch panels use a capacitive sensor to sense multiple locations where pointing objects such as a user's fingers land on the capacitive sensor. For instance, where pointing objects locate on the touch panels can be determined by measuring a capacitance change between a sensing electrode and a constant potential reference, such as ground, in this system. Projected-capacitive touch panels are made with a first conductive layer formed on a substrate in the first place, and then multiple driving electrodes, electrically insulated one from another, are formed by etching. A second conductive layer is sequentially formed on another substrate and multiple electrically insulated sensing electrodes are also formed by etching. A plurality of intersections are then formed on the surface of the projected-capacitive touch panels, wherein a driving electrode is paired with a sensing electrode at each intersection because the electrodes on one of the two layers are arranged in rows and those on the other layer are in columns; the intersections of each row and column represent unique touch coordinate pairs. A conventional location recognition device usually comprises a said projected-capacitive touch panel, a controller, a driving circuit coupled to each of driving electrodes and a sensing circuit coupled to each of sensing electrodes. Said capacitive sensor is included in the sensing circuit.
As shown in FIG. 1A, a controller (not shown) charges multiple driving electrodes 10 in sequence in the first place during which an inductive electric-field channel E is formed between a driving electrode 10 and a sensing electrode 20. Each inductive electric-field channels E has a capacitance value. When an inductive electric-field cannel E is formed between the driving electrode 10 and the sensing electrode 20 without any other medium to intervene, the capacitance between the electrodes is a constant value, and the voltage is also kept to a constant basic voltage value. When a touch object such as a finger intervenes in the inductive electric-field channel E, the variation of the capacitance happens and, in consequence, the voltage drops there. By this characteristic, a contact on the touch panel can be located, as shown in FIG. 1B.
The principle of the above projected-capacitive touch panels will be explained hereafter using conductors and physical properties thereof, such as electric charges and capacitances. Refer to FIG. 2A, the capacitance for a certain conductor is a constant value, which relates to the size and shape of the conductor but does not relate to the material the conductor is made of, the number of electric charges or whether there is an electric charge on the conductor. A conductor in electrostatic equilibrium is an equipotential body. For an independent conductor, its electromotive fore is related with the electric charges it carries. When the number of electric charges increases, the strength of the electric field provoked by them at each of the points also proportionally increases. As a consequence, the work done by electric-field force to move an electric charge q also increases by the same multiple. The electric charge q is proportional to the electromotive force U, and the capacitance C is given by C=q/U. Refer to FIG. 2B, the charges on a plate A is +q in a vacuum, and the electromotive force is given by U. A plate B is put near plate A as shown in FIG. 2C, and the same charges −q and +q are induced on two sides of plate B by plate A due to electrostatic induction. The electromotive force U on plate A is equal to the electric field induced by the charge +q on plate A and the charges −q and +q on plate B, which is summed by the law of algebraic addition. Refer to FIG. 2D, the charge +q on plate B is neutralized when plate B is connected to the ground, so the electromotive force on plate A decreases. The capacitance increases as the charge q on plate A remains the same. A dielectric object D is put between plates A and B as shown in FIG. 2E. The negative charge −q is induced by the dielectric object D near one side of plate A, and the positive charge +q is induced by the dielectric object D near one side of plate B. Thus, the electromotive force on plate A decreases, and the capacitance increases.
Back to FIGS. 1A and 1B, the conventional identifying method of the projected-capacitive touch panel is disclosed as below. The controller charges multiple driving electrodes with a driving circuit, coupling electric charges to each of the driving electrodes simultaneously, sequentially, or group-by-group, and then coupling the electric charges to each of the induced channels through the electric field. Then, each capacitive sensor in the sensor circuits is respectively coupled to a corresponding induced channel in order to transfer the induced charges in the channel to its corresponding capacitive sensor for measuring. Usually, the controller performs multiple measuring cycles before measures the charges on the capacitive sensor. When a touch object, for example, finger or stylus approaches the sensing electrodes 20, the touch object is taken as a virtual ground. It changes the capacitance between the sensing electrodes and the ground. Thus, the capacitive sensor measures the variation of the charges for identifying the location of contact.
However, the change of the capacitance causes the variation of the voltages, which is measured by the projected-capacitive touch panel with the principle of the electric field, in order to identifying the contact of the touch object. When there are medium other than the touch object contacting the touch panel, it also changes the capacitance value between the driving electrode 10 and the sensing electrode 20 and the voltage is dropped which intervenes the identification and generates noises. Dielectric properties of the other medium cause different effects. For example, when there is polar medium (e.g., liquid) on the touch panel, it may be induced by the positive charges on the driving electrode and generates polarization phenomena. As the induced channel between the sensing electrodes and the driving electrodes is intervened, the variation of the charges between them becomes very weak. The polar medium centralizes the negative charges near the driving electrodes and the positive charges far from the driving electrodes respectively, and there are two problems because of the centralizing phenomenon. One is that the sensing electrodes drops due to the increased capacitance value near the driving electrodes. The other is that the induced electric field is erroneous due to the sensing electrodes far from the polar medium generating erroneous location of contact.
Moreover, different shapes of liquid covering on the touch panel cause different effects. When the shape of liquid is a stripe as FIG. 3A and the extending direction of the stripe is the same as the extending direction of the driving electrodes, it makes the capacitance value increase at the liquid covering area but the locating of the contact is not intervened. While the extending direction of liquid intersects the extending direction of the driving electrodes or the sensing electrodes as FIG. 3B, it induces erroneous locating of the contact. Therefore, a method for accurately identify the location of contact is needed.