In the near future, the time may come when woodworking products and electronic circuits can be integrated. A research group from Ochanomizu University, Tokyo University of Technology, Yahoo, and the University of Tokyo has developed a technology that partially carbonizes the surface of wood with a laser processing machine and uses it as part of electrical wiring and sensors.
Since the wiring is so-called charcoal, the electrical resistance value is higher than that of copper wiring used for a general printed circuit board (PCB). Therefore, it is not suitable for large currents, but it can handle signals with a current of several milliamperes. So far, the research team has prototyped a capacitive touch sensor (pictured at the beginning) and a switch circuit that can detect the opening and closing of a door (pictured below) by combining charcoal wiring and a microcomputer board.
Expanding the applications of laser processing machines
Speaking of laser processing machines, cutting, drilling, and engraving are common uses. Its performance and price vary depending on the type of laser oscillator, but in recent years, inexpensive products of several hundred thousand yen have appeared. It is known as a familiar tool not only for industrial use but also for so-called DIY.
If wiring and sensors can be formed directly on the surface of wood using such a laser processing machine, it will be easier to combine electrical functions with woodwork products. This is because there is no need to wrap vinyl-coated wires inside the product or attach individual touch sensors.
Examples of methods for forming electrical wiring and sensors on the product housing itself without using PCBs are as follows. It is a method of printing conductive ink with an inkjet printer and a method of cutting out a conductive sheet with a cutting plotter.
In such a method, it is necessary to prepare a conductive material for electrical wiring separately from the parts of the housing. In that respect, if the method is to change the surface of the wood itself into conductive charcoal with a laser processing machine, it is not necessary to prepare a conductive material separately. When disposing of the product, it is not necessary to separate the conductive material.
According to the research team, Ayaka Ishii, there is also the merit that "because the wood itself is used, the natural texture is not easily impaired."
Dare to blur the laser and irradiate
In order to carbonize wood with a laser processing machine and use it as an electric circuit, it was necessary to devise something different from cutting and engraving. Generally, the laser machine adjusts the height of the stage to match the focal point of the laser. However, in such normal usage, the wood is burned, so it does not remain as conductive charcoal.
Therefore, Ishii et al. Searched for the operating conditions of a laser processing machine suitable for forming charcoal. First, the height of the stage of the laser machine was lowered by about 5 to 6 mm from the usual level, and the focus was intentionally blurred. On top of that, the laser output was suppressed and the irradiation speed was set faster than when cutting.
Then, by repeatedly irradiating the same place with a laser and gradually heating the wood, highly conductive and uninterrupted charcoal was obtained. Ichiro Shiio, an emeritus professor at Ochanomizu University, explains, "By irradiating the laser little by little, the water in the wood was removed and only carbon was finally left."
For the pattern creation of charcoal wiring and sensors, we have developed and used a dedicated pattern design tool using CAD (computer-aided design).
This time, the research team dared to challenge laser processing in the atmosphere in order to emphasize the ease of DIY. "If processed in an atmosphere of an inert gas such as nitrogen, it may be possible to obtain even higher quality charcoal wiring," (Mr. Shiio).
We also know that the type of wood is important for obtaining high quality charcoal wiring. Medium-hardness wood, such as lauan and cypress, gained relatively high conductivity when carbonized. Hard wood such as beech and oak, and soft wood such as sugi tended to not provide the desired conductivity.
Changes in characteristics settle down after half a year
What is worrisome is the change over time in the wiring and sensors. According to Mr. Ishii, the surface resistivity of charcoal wiring was the lowest immediately after production, and tended to gradually increase over time. However, data have also been obtained that the surface resistivity rises up to about 1.5 times and then settles at a constant value six months later. In addition, the capacitive touch sensor has a track record of operating even after more than 8 months have passed since it was manufactured.
So far, the research team is verifying laser machining on flat wood. Technically, it is possible to form charcoal wiring on the surface of wood with curved surfaces such as spheres.
The load sensor is interesting in predicting the possibility of charcoal wiring. Ishii et al. Are also testing whether the charcoal wiring formed on the back side of the seat of a wooden chair can be used as a load sensor. When a person sits down, the wiring is distorted and the electrical resistance value increases, so it was possible to detect the change in voltage with the analog-to-digital (AD) converter of the microcomputer and monitor the presence or absence of a person.
The research team will continue to improve the functionality of wiring pattern design support software, and will continue to study ways to make the design and processing of charcoal wiring easier. Specifically, "We aim to create software that calculates and presents the surface resistivity in advance by specifying the wiring pattern and laser processing conditions" (Mr. Ishii).
As mentioned above, this technology is intended for DIY only, and there are still some hurdles to apply to commercial woodwork products. For example, protection of charcoal wiring. "Since the charcoal wiring formed by the laser processing machine is exposed, it is necessary to paint with varnish to protect the surface" (Mr. Shiio)
Until now, little has been known about the method of integrally forming electrical wiring on the surface of wood. In the future, unexpected products that combine woodworking products and electronic circuits may appear.
