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Technology
- -- What exactly is a touch sensor?
- Tanabe
Touch sensors don't have any mechanical operations like a regular switch, so it's possible to make the touch-sensitive part flat, round, or any shape you want. This gives them great design potential.
Because of this, touch sensors are mainly used in high-end home appliances in Europe, where design is a high priority. However, fueled by growing interest in Japan, Korea, and China, the touch sensor market is expected to see rapid growth in the next few years. - -- What are the features of OMRON's Touch Sensor?
- Tanabe
We use electrostatic capacity detection to detect human touch. This involves measuring the minute electrostatic capacity between a person's finger and an electrode to detect touching. The main feature here is the use of a microcontroller in the detector circuit and software for the detection. - This method allows for a wide range of sensitivity settings to meet diverse customer requests, and makes it possible to use many different materials for the touch panel, such as wood, stone, and semi-transparent mirrors.
- Another feature is that we provide development tools. These tools can be connected to a personal computer for easy parameter setting and operation monitoring, so our customers can develop their own Touch Sensors.
- -- I understand that the electrostatic capacity detection was a real challenge.
- Kinoshita
Though we started out developing the Touch Sensor based on electrostatic capacity detection technology that we had accumulated with Pressure Sensors, once I got going I realized that there's a fairly large difference in the ways that Pressure Sensors and Touch Sensors are used.
The Pressure Sensor requires absolute accuracy in electrostatic capacity detection, and this is achieved by making delicate corrections to temperature and parts fluctuations. Touch Sensors, though, have to be inexpensive, so the costs that are involved in these kinds of corrections are simply not feasible.
Also, electrostatic capacity is greatly affected by the surrounding environment, so if there are metal objects nearby, or if there are any temperature changes, the accuracy can fluctuate a great deal. I quickly realized that it wouldn't be possible to use this method of detecting absolute accuracy. - -- So how did you solve this problem?
- Kinoshita
I started looking into electrostatic capacity variation. We thought that by using a microcontroller to process the amount of variation in the detected electrostatic capacity we could possibly cancel out the impact of environmental changes and noise, and detect only human touch.
I decided early on to use a microcontroller because of the advantage it gave us in being able to write the customer's individual settings to the EEPROM memory for easy customization. I got stuck, though, on what to use for the electrostatic capacity detection method and went through about six months of trial-and-error experiments. - Tanabe
The fact is, the main reason for using a microcontroller was the increased freedom that it gave us. Because of this, we wanted to allow ourselves a wide range of selection in the microcontroller to be used. However, because the first detection method we considered required a microcontroller with an AD converter, the number of limitations actually increased and the benefits of using a microcontroller seemed to fade. - -- Were you able to find a detection method that the microcontroller could handle?
Fig.1
Electrostatic capacity detector circuit- Kinoshita
I lost sleep for six months thinking about this (laugh), and then I came up with the detection method shown in Fig. 1. The principle itself is not new; it's a very ordinary detection method.
To measure Vb in Fig. 1, it uses a fairly simple function that determines whether the input voltage to the microcontroller's general-purpose I/O port is High or Low. However, this is not the conventional use of this function.
People wouldn't normally think of using it like this, but the idea was extremely important here because it let me start thinking of ways to use the microcontroller.
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