Safe tree felling using expansion agent
© The Author(s) 2017
Received: 28 October 2016
Accepted: 22 February 2017
Published: 3 March 2017
We discuss about safe tree felling methods. In previous work, tree felling robots that were equipped with cutting instruments have been investigated in order to keep workers’ safety. However, these robots may take damage when trees fall down against them. We propose a tree felling framework that maintains the workers and robots both in safe. In the proposed framework, a robot inserts a slow-acting expansion agent at the trunk of a tree. While the agent is expanding, the workers and robots escape from the tree. Also, we propose two drilling methods that are able to realize the proposed framework. In the experiments, conventional drilling methods that have been used for rocks did not divide logs. On the other hand, the proposed methods realized the divisions without any crack in the product side of them. The proposed methods were effective against standing trees too.
25 million (ha) (66%) of Japanese ground is covered by forest. The total volume of tree trunks (about 5 billion [m3]) is increasing year by year. The background of the increasing is the low self-sufficiency of wood in Japan. Compared with 1966, the total volume has been increased to 5.5 times in 2012. Self-sufficiency of wood has been decreased from 71.4 to 27.9%.
From these facts, Forestry Agency of Japan is targeting increasing the self-sufficiency to 50% until 2020 . However, the number of forestry workers has been decreased from 0.26 million (1965) to 0.10 million (1990) . One of the reasons of the decreasing is the high accident rate of forestry industry. In forestry, 1700 industrial accidents occur in a year. Half of them are occurring while tree felling tasks. According to 1000 rate,1 forestry is 28.7, which is quite higher than manufacturing (2.8) and construction (5.0) [8, 9].
One of the reasons why the number of accidents is not decreasing is that the falling direction of a tree is unpredictable. Even when typical Japanese workers use a sophisticated tree cutting method, ukekuchi-oikuchi cut, the direction is not sufficiently predictable. Also, we have no way to measure the center of mass of a tree before falling down in current technology. For example, we can not select a good wave frequency for non-destructive inspection that uses ultrasonic sound wave or some other waves against trees. Tree does not let high frequency wave pass it. If we use a low frequency wave, non-destructive inspection does not assure its precision.
From the above, the establishment of safe tree felling methods is an urgent need. In order to fill it, forestry machines have been developed. Feller buncher and harvester are popular forestry machines. However, these machines are not in use around steep or soft ground, because of their large weights. Also, dense forest gets rid of the use of them, because of their sizes. Currently, a large number of workers are using chainsaw in order to divide a tree, although it has much possibility of accidents.
Smaller sized robots may fit the conditions like steep ground, soft ground, or dense forest. Sugano et al. developed a pruning robot that climes a tree using two robot arms . Ishigure et al. developed a pruning robot that cuts the branches of a tree using a chainsaw. This robot is able to climb a tree using 4 wheels . Meaclem et al. investigated a biped mechanism of a felling machine that climbs between trees using two legs like a monkey . Kolb et al. proposed a tree trunk detection system for a semi-autonomous tree felling robot . Gui et al. developed an anti falling mechanism for tree pruning robots . Especially, Sugano et al. developed a remote control robot that is equipped with chainsaw at the end effector of its robot arm . However, a falling tree may crash these robots, because they must be close to the tree in order to cut it even when the tree falls down. Inserting remotely reacting materials in a tree is a new idea to keep the safety of robots.
In this paper, we propose a tree felling framework in that a robot is able to escape safely before a tree falls. In the proposed framework, a robot inserts a slow-acting expansion agent at the trunk of a tree. While the agent is expanding, the workers and robots are able to escape from the tree safely. We propose two drilling methods that realize the proposed framework.
In "Materials, methods and proposition" section, the propositions are described. In "Experiment" section, the effectiveness of the proposed drilling methods and conventional methods were examined through fracture experiments. In "Discussion" section, discussion is presented based on the results. In "Conclusion" section, this work is summarized.
Materials, methods and proposition
Type of expansion agent.
Type of expansion agent
In forest, we must use an expansion agent that does not damage the surrounding environment (1). Also, the expansion agent must make sufficient stress in the trunk in order to cut the fibers (2). Moreover, the expansion agent should be safe in handling (3).
If we use gunpowder , it is able to divide the trunk. However, the left residual material makes pollution around the tree. Also, handling it inside forest is dangerous. On the other hand, making the stress using some devices such as an air bag is not in use, these devices do not make sufficient stress. We need to select the expansion agent that is able to make more pressure than the strength of wood (Table 1) at least.
Non-explosive demolition agent (NDA) is a candidate of the expansion agent. NDA is frequently used for breaking up rocks inside forest. The chief component of NDA is calcium oxide that makes expansion pressure in hydration reaction with water. Commercially available NDA is able to make about 80 (N/mm2), which is more than the strength of wood. Calcium hydrate, which is left after the reaction of calcium oxide and water, does not make damage forest. Calcium hydrate is frequently used for the sterilization of forest. NDA is safe in handling. It does not any license in Japan, because of its safety.
In this paper, we use NDA for validating the proposed framework. For NDA, we used Power Blister of Taiheiyo Material Co. Ltd. in the experiments. This NDA makes solid cap quickly around a hole in that it is inserted.
Strength of wood (N/mm2) 
In order to fell a tree using NDA, the way of drilling is important.
Is able to fell a tree.
Easiness of automation.
Quality of wood.
On the basis of the requirements, we propose two drilling methods as in Figs. 5 and 6. In both proposed methods, holes are placed close in horizontal direction, in order to stop the extension of a crack in longitudinal direction.
In proposed drilling method A, two holes are drilled vertically with distance d 1. For the lower hole, a cut that reaches to the hole is made using a chainsaw. Above the upper hole, another cut is made with a distance d 2 from the upper hole where d 2 is larger than radius d/2 of the hole. This drilling method divides the trunk by chaining the two holes by the pressure of expansion agent. This method utilizes the specification of a tree where vertical crack is easily made by weak pressure. The fibers in the trunk of a tree are directing to the longitudinal direction of a tree. Cracks are easily made along the fibers. The distance between the upper hole and the cut above the hole is required in order to avoid the crack in production parts (above the cut). If this cut does not exist, the pressure around the upper hole affects all surrounding directions. This cut stops the pressure to the upward. The robot that performs this method must use two mechanical tools (drill and chainsaw).
Drilling the log.
Putting NDA in water for 4 min.
Filling the holes by NDA.
Confirming the result 14 h later.
In Experiment 3, we made holes as in Fig. 7. In this experiment, we confirmed the effectiveness of the NDA when the holes are opened holizontally with some cuts. If holizontal cuts were enlarged by the NDA, these methods were good solutions.
In Experiment 4, we made holes using the proposed drilling method A. In Experiment 5, we made holes using the proposed drilling method B. These two experiments were done in order to confirm the effectiveness of the proposed methods.
In Experiment 6, we examined the both proposed drilling methods in forest in order to confirm their applicability on the field. This experiment was done under the temperature of 9.0–13.7 (°C) in a forest of Kamaishi City in Iwate Prefecture of Japan. The altitude was around 10 (m). The slope gradient was 0–25 (°). For method A and B, we used cedar trees with diameters of 277 and 234 (mm). Before the experiment, the trees around the targeted tree were cut, in order to assure the safety of the experiment.
All of the holes were filled by NDA.
Drilling parameters (Experiment 1)
Number of holes
In Experiment 2, we made Log H that had 3 holes with 30 (mm) diameter using previous method B as in Fig. 8(4). The first hole was made at the center of the log. The lest of it was made at 60 (mm) left/right from the center.
Drilling parameter (Experiment 3)
Position of holes
About 2/3 of the log
20 (mm) from the cut
10 (mm) beyond the hole
10 (mm) beyond the hole
Center and 50 (mm) left from the first hole
10 (mm) beyond the hole
50 (mm) from the edges
10 (mm) beyond the hole
60 and 110 (mm) from the right edge
In Experiment 4, parameters \(d_1\) and \(d_2\) were set at 80 and 20 (mm) respectively for Log N.
In Experiment 5, parameters \(d_3\) was set at 50 (mm) for Log O.
In Experiment 6, we used the same parameters as Experiment 4 and 5 for two standing trees, and used \(d_3 = 35\) for a tree and connected half holes by drill while grabbing the production part of the tree using a harvester in order to conduct the experiment in safe.
The two proposed methods might be effective to fell trees, because they could divide even the logs that take no pressure from the above. In actual trees, the tearing of fibers easily occurs by their self-pressure.
Proposed method B is quite easy to be automated, because it requires only drills. Automating proposed method A might be more difficult than that of B, because proposed method A requires two types of mechanical tools. But, we can not get rid of the use of proposed method A completely, because proposed drilling method A requires less amount of NDA than proposed method B.
The both proposed methods effectively kept the quality of woods. In the experiments, any crack occurred on the surface of the product parts.
Diameter of holes
The effective diameter of holes is larger than 25 (mm), because the holes with 25 (mm) did not extend any crack in Experiment 1. This reason might be the shortage of the volume of the expansion agent. From the cross section surface of Log E and G, we can confirm the effectiveness directly.
We can make tearing force by making additional cuts after drilling like Experiment 3. In this experiment, the pressure around the holes works only up and down. However, controlling the tearing force so that it affects only horizontal direction was difficult, because of the fibers that are placed in longitudinal direction. As a result, logs were torn along with the fibers. Proposed method A is effectively using the characteristic of the fibers. The crack in longitudinal direction is included in the division surface of resulting two piece of woods.
In the proposed method, we can control the reaction duration of the expansion agent. The reaction time of calcium oxide is quite rapid. In NDA, it is supressed by additional materials. Thus, we can easily control the duration by changing the amount of the additional materials.
The workers of Kamaishi Forest Ownership Association want to use it at night. If they set the expansion agent and alarms, they can sleep in a lodge while the reaction occurs. After the reaction, they can take the trees on the ground at the tomorrow morning.
Although the proposed methods allow a worker to fell a tree safer than previous methods, we need to automate the drilling method and insertion method in order to make sale product, because manual drilling and insertion cost a large time (currently about 20 min). If we consider the automation of drilling method, proposed drilling method B might be better than proposed drilling method A, because it requires only a type of cutting tool and might be realized by cheaper price. But, if we consider the amount of required expansion agent, drilling method A might be better. For the automation of the insertion, we can use a lapped expansion agent. Currently, the reason why insertion process costs much time is that expansion agent is half liquid. If we lap it, we can handle it like a solid. After the development of these automation techniques, the time to set expansion agent might be going to be 1 min or so.
We summarize the drawbacks of the proposed methods. Firstry, the proposed methods require NDA addition to mechanical system for cutting a tree. Even if the finantial cost of NDA is small, it is not zero. Secondly, there is no evidence that the proposed methods are able to control falling direction of a tree. We need additional experiments and improvements in order to assure controllability. Thirdly, we need additional devices that alart a person who comes close to a tree that took NDA in order to assure the safety of people. Especially when using NDA while workers are sleeping, exclusion zone of the forest must be set. To sign the exclusion zone, we need additional devise.
We proposed tree felling framework that take into account the safety of workers and robots. In the framework, we proposed two drilling methods too. The proposed methods effectively divided logs through the reaction of expansion agent with time delay. Thus, all of the workers and robots are able to escape from the tree before it falls down. From the experiments, the radius of the holes should be more than 25 (mm). The directions of the holes should be straight. The heights of the holes should be differentiated. In the proposed drilling methods, all of these features are took into account.
The number of injured workers in a year among 1000 workers.
CHK proposed the consept, analyzed and interpreted data, and wrote the manuscript. HA, AN, and JK acquired data. All authors read and approved the final manuscript.
A part of this research is on the support of Iwate Next Future Industry Seeds Growing Research. We would like to thank the cooperation of Kamaishi Forest Owner’s Association and Ishimura industrial Co. Ltd.
The authors declare that they have no competing interests.
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