Mitsubishi Demos '3D Touch Panel'
Mitsubishi Electric Corp prototyped a capacitive touch panel that can detect the distance between a finger and the panel and demonstrated it at Interaction 2009, which took place from March 5 to 6, 2009, in Tokyo.
Mitsubishi Electric calls the touch panel " 3D touch panel" because it can determine not only the x- and y- (plane) coordinates of a finger but also its z- (normal direction) coordinate. The prototype has a 5.7-inch screen with a resolution of 640 x 480 pixels (VGA).
The prototype is intended for use in mobile devices with a small touch panel. For example, the company envisions mobile devices equipped with a "mouseover function," which changes the image of an icon when the mouse pointer is placed on it.
This time, Mitsubishi Electric had a demonstration of moving a finger toward thumbnail icons and icons on a map and changing their shapes.
"We think the mouseover function is more useful in smaller screens," the company's spokesperson said.
By calculating the time variation of capacity in the z-axis direction, "the acceleration of the finger approaching the panel can be detected," the spokesperson said. With this method, the panel can determine the speed of the approaching finger.
"If the backlight is designed to be red when the finger moves fast and blue when it moves slowly, emotional changes of the user can be reflected on the panel," the spokesperson said.
Switching between two detection methods
The prototype can detect the distance of an approaching finger while using ITO transparent electrodes.
When the finger is not in contact with the panel, the capacitance change is smaller compared with when the finger is in contact with the panel, requiring high detection sensitivity. Copper electrodes would provide a sensitivity high enough to detect an approaching finger, but they are not transparent. But, if transparent electrodes are used, the sensitivity becomes lower than that with copper electrodes because the resistance value of the panel increases.
Therefore, Mitsubishi Electric decided to switch detection methods depending on the position of the finger. When the finger is approaching the panel, it is defined as "proximity state." When the finger is in contact with or very close to the panel, it is "contact state." In the proximity state, priority is given to the sensitivity. And, in the contact state, the resolution is prioritized.
In the proximity state, the panel can detect a capacitance change of about 0.3pF, and about 8 to 19pF in the contact state, according to the company.
The resolution in the x-axis and y-axis directions in the proximity state is equivalent to 10mm. In respect to the z-axis direction, "it is possible to determine the distance of the finger as long as the it is less than 20mm," the spokesperson said. The resolution in the z-axis direction is 256 stages.
In the contact state, the resolution in the x-axis and y-axis directions is equivalent to 0.2mm, the company said.
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In addition, the response speed varies depending on the position of a finger. In the proximity state, it is about 50ms and about 10ms in the contact state. In respect to power consumption, "there is no difference between the two states," the spokesperson said.
To improve the sensitivity in the proximity state, multiple sensors are connected to increase the sensor area. In general, sensitivity improves in proportion to sensor area.
Furthermore, a function called "sensor shield control" was added to reduce the parasitic capacity of the circuit. The electrodes in the x-axis direction and those in the y-axis direction are located ones on top of the others. When sensors are operating, the sensitivity lowers due to the increase of parasitic capacity caused by the electric potential difference between the upper and lower parts of the electrodes.
Therefore, Mitsubishi Electric reduced the parasitic capacity by applying signals of the same phase and potential. Also, the sensor shield control function eliminated the need for a shield layer that reduces electromagnetic noise generated from an LCD panel.
Only few changes made
The prototype is based on a capacitive touch panel that is available on the market, and "only a few changes were made to it," according to the spokesperson. Mitsubishi Electric just added switch elements to connect multiple sensors and improved the detection circuit to reduce its parasitic capacity, he said.
"Our research and development division does not consider that those improvements will significantly increase costs," he said.
The commercialization schedule of the new touch panel has not been determined yet. But when it is commercialized, "it will first be used for our products," the spokesperson said. The company plans to verify its resistance to environment as well as operability in actual devices for the commercialization.
The capacitive method employed for the prototype is a projection type, which is suitable for mobile devices and difficult to be applied to large screens. But Mitsubishi Electric is now working on measures to meet the requirements of screens that are 10 inches or larger. The new detection method cannot be realized with surface capacitive touch panels, according to the company.