1. Field of the Invention
The present invention relates to an electronic pen input device and coordinate detecting method therefor, and more particularly to an electronic pen input device which can generates a movement information thereof using a 3-axis accelerometer and an optical 3 dimensional measuring device and a coordinate detecting method therefor.
2. Description of the Related Art
These days, input devices having 2 dimensional sensor arrays such as LCD tablets or digitizer tablets in order to input a pen writing information to PDAs or computerized devices are widely used. These input devices require a relatively wide area 2D sensor array, that is a separately provided detect surface. Accordingly, the portage thereof is inconvenient and it occupies space, and moreover it is expensive. Lighting in view of the technical trend, the PDA becomes smaller and smaller, eventually to be a form of wrist watch or wallet. This trend leads the display of the PDA to be narrow and there will be no more operating space of the pen input device making the writing movement unnatural.
To overcome the above described disadvantage, it is very proper to utilize a pen input device operable within any space without any physical tablet so that there is unlimited input space of the pen input device. To input a document or a picture using a self-detective electronic pen, it must be accomplished that the tip position of the electronic pen relative to any reference coordinate must be continuously calculated. However, most of the writing operation is performed in such a manner of the writing while downing and contacting with the ground and moving while separating upward from the ground. To determine the continuous position of the pen in a coordinate, it is required a means for accurately detecting the position both while in touch and out of touch.
Conventional pen input devices are generally classified into two categories, those having an outside detection method and a pen-inside detection method.
As an outside detection method, there have been proposed a 3-axis detection method (U.S. Pat. No. 5,166,668), and a method using a difference in a time of flights of an electronic wave (U.S. Pat. No. 5,997,958) or a microwave (U.S. Pat. No. 4,478,674). However, in these methods, the signals are sent from the pen and are received outside of the pen. So, in case of mobile terminals, there must be a receiving portion at the main body of the terminals, resulting in a difficulty in delivering.
Also, there was provided a method of determining the tip position of a pen inside of the pen, including a method using a ball rotation (U.S. Pat. No. 5,027,115) and a method detecting the force against the pen (U.S. Pat. Nos. 5,111,004, 5,981,883). However, these methods have a difficulty in determining when the pen is detached from the writing surface. There was also proposed a method using a 2-axis or 3-axis acceleration sensor in which the position of a pen is determined by double integration (U.S. Pat. No. 5,247,137, WO94/09,447, U.S. Pat. No. 5,587,558). Also in this case, it is difficult to mount the acceleration sensor to the pen tip and there can exist a large error in position of the pen because there is no consideration of the inclination angle of the pen centerline. Moreover, the double integration can cause an accumulated position error.
To compensate the error due to the pen inclination, A.T. Cross company (U.S. Pat. No. 5,434,371) proposed a structure in which the acceleration sensor is attached to a tip of a pen and a signal processing portion is located at the upper portion of the pen. However, due to this distance therebetween, there is a much effect of electrical noise and also is no space to mount an ink pot. On the other hand, company Seiko (Japan patent application Hei 6-67,799) proposed a method for determining the position and the inclination angle of a pen in which the position is determined by 2-axis acceleration sensor and the inclination angle thereof is determined through integrating the angular velocity of the pen using 2-axis gyro sensor. Richo K. K. (U.S. Pat. Nos. 5,902,968, 5,981,884) proposed to use 3-axis acceleration sensor and 3-axis gyro sensor. However, in Richo, the writing surface must be perpendicular to the direction of gravity which restrains the pen handling and furthermore, due to the accumulation error and drift error, the acceleration sensor may have an error of order of square of time and the gyro sensor of order of time.
To overcome the inaccuracy in position detecting due to the error accumulation, Intersense Inc. recently adopted an ultrasonic wave sensor so as to decrease the accumulated position error, resulting in an inconvenience in delivery.