[PV Robot Front Line] Cleaner Robot by Sinfonia Technology

Washes panel surfaces using water, brush, wiper

2014/06/16 09:30
Kenji Kaneko, Nikkei BP CleanTech Institute

Sinfonia Technology Co Ltd unveiled the "solar panel cleaner robot Resola" prototype 1 in November 2013 (Fig. 1). The company is planning to release an upgraded mass-produced model as a new product in the fall of 2014 (Fig. 2). Robot cleaners make us visualize iRobot Corp's "Roomba," and the "Resola" is like a solar panel version of the Roomba. It recognizes the surrounding environment with its sensors and cleans solar panels, automatically tracking its route according to its own decision. While the Roomba vacuums dusts, the Resola wets and cleans panels with a brush and a wiper.

To be priced at about 1.2 million yen

Provided with a built-in Li-ion battery and a water tank (2.7L), the Resola can clean about 75m2 (45 panels) in 30 to 45 minutes with a full tank; the tank will run out of water in about the same length of time. The rechargeable battery runs the robot for about five hours per charge and can then be replaced with another one (Fig. 3). The body weighs 22 to 23kg when the tank is full, which can be carried by one person (Fig. 4).

It can be used on panels tilted at 5 to 20° and run across panels spaced up to 3cm lengthwise, 5cm widthwise and 1cm up or down from each other. The Resola is compatible with both single- and poly-crystalline silicon panels but incompatible with thin-film types such as CIS and amorphous silicon panels. Sinfonia Technology's suggested retail price is about ¥1.2 million (approx US$11,685).

I witnessed the prototype 1's demonstration on the test panels at Sinfonia Technology's Ise Plant. The Resola was placed facing upward on the lower left corner of an array (unit of connected panels) tilted at 20°, and switched on. In a few seconds, it started climbing up the 20° slope. It detected that it had reached the upper end of the panel, stopped and rotated right by 90°.

The cleaning process started from here. The Resola sprayed water forward, horizontally running on the upper edge of the panels from left to right (Fig. 5). It washed the wet panel surface with the nylon rotational brush attached under the body and pushed away the dirty water with the wiper. The water ran down the 20° tilted panel toward the bottom.

President suggests idea himself, commercializes cleaner

The Resola detected that it had reached the right end of the array, turned downward by 90° and stopped after moving down a little (Fig. 6). Rotating left by 90°, it started running and cleaning horizontally from right to left this time. By repeating these actions, the robot washed the panels in series from the upper side (Fig. 7). As the panels were sloping, the water containing the dirt and dust gradually ran down the panels and was eventually removed when the Resola, running along the lower edge of the panels, pushed the water off under the mounting systems (Fig. 8).

"We assumed a usage that a robot automatically cleans solar panels while a mega (large-scale) solar power plant is being inspected," said Mitsuo Tsume, manager of the New Business Planning Div, Sinfonia Technology. As far as I saw, in the prototype 1's demonstration, this concept was already achieved. Product Development Manager Nakamura said, "It was exceptional that we could shape the idea into a product in just six months."

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Non-slip shoe sole material applied for work in the rain

The company sticked to the idea of cleaning panels with water. Some existing solar panel cleaner robots blow off the dust with air, but this method does not always sufficiently remove the dust. When water is used, on the other hand, most robots are externally supplied with water through a hose, etc.

Embedded with a built-in water tank, the "Resola" requires no incidental work using hoses and pipes. While the prototype 1 discharges water forward from the front of the robot, the mass-produced model delivers water by drops from under the body.

"We decided to make an improvement, finding out that water sometimes does not fall on the panel if it is discharged forward from the front and is then blown away by wind," Sinfonia Technology said.

Nakamura said, "What we paid the most attention to was workers' safety." The Resola can easily slip as it moves along wet sloping panels. Should the 20kg body slide down the panels toward a worker, the person could get injured. Accordingly, the quality of the material used in the crawler (the rubber belt on the drive wheels) that touches the panels becomes important.

Sinfonia Technology secured safe operation on panels sloping up to 20° by looking for a rubber material that hardly slips, adopting the rubber and the pattern used for the soles of shoes that prevent slipping even in rain (Fig. 9).

The "slip" issue in operation on a sloping surface could slightly affect the robot's straight drive and rotation. The Resola can calculate the amount of travel based on the number of crawler rotations and information gained from the built-in gyroscope. Even if it appears to be running stably, however, the crawlers are, in fact, slipping little by little, and, in many cases, the route tends to slightly curve downward. The robot is programmed to rotate 90°. However, in practice, it deviates slightly.

Image recognition drives cleaner run along panel 'lines'

Based on these circumstances, the route should be corrected by coordinating the information acquired through multiple sensors mounted on the robot. The Resola features an infrared CCD camera (on its front side), which allows the robot to detect the "lines" of panels through image processing, run straight by following the guidelines and correct the route when it starts to deviate (Fig. 10).

The "lines" are the two to four electrodes (bus bar electrodes) between and in cells (power generating elements). The Resola is not compatible with thin-film-type solar panels because they have no such "lines." With an inclination sensor built in, the robot detects and corrects the body's inclination with this sensor when rotating 90° at the end of the panel. In addition, it features an ultrasonic sensor on each corner of the body as the last measure to stop without falling from the edge of an array. This sensor is also used to detect the width of space between panels.

"Unexpectedly, it took a long time to program the CCD camera to accurately detect the support lines in various circumstances," Nakamura said.

While repeating test operations at different sites and times, it became difficult to detect the "lines" with the panel surface easily reflecting sunshine due to the sun's direction or the robot detecting the reflection of a building near the panels as "lines." Sinfonia Technology said it overcame these challenges by improving the image recognition software technology.