As the Fukushima Daiichi Nuclear Power Station (hereafter refers to as 1F) became the high dose environment by the Great East Japan Earthquake, remotely operated robots were required in order to reduce workers’ radiation exposure. We developed a quadruped robot to walk the stairs and narrow passages with carrying burdens such as investigation tools. This robot investigated water leakage from vent pipes at underground of 1F unit 2[1].

There are various works towards the decommissioning such as measuring doses of radiation, cutting pipes, connecting wires and more. It is desirable to carry various work tools and to unload them at destinations with remotely operated robots. To this end, we have developed carrying and unloading functions of the robot[2]. In addition, we have developed cooperative carrying functions that two quadruped robots carry and unload a burden which is too long or heavy for individual robot. As a result, it was realized that two robots carried the pipe of 48kg while getting over a step of 100mm and unloading it at a destination.

Phase 1: Research and Development Phase

1. (1) Components:
   N/A

1. (2) Location:
   reactor building, turbine building, etc.

1. (3) Materials:
   N/A

1. (4) Condition:
   disaster site, high radiation environment

· Quadruped robots

We have developed two types of quadruped robots, Type I and Type II. Figure 1 (a) and (b) show type I and type II respectively and Table 1 shows specifications of each robot. These robots can walk at maximum velocity of 1 km/h with maximum payload. These robots are able to walk up stairs of 220mm in height.

Type I carried the small camera vehicle with the handling arm for loading/unloading the vehicle on the top of this robot at the time of the investigation of vent pipes[3]. The weight of this arm, which is 8kg, reduces capable payload of burdens. We developed type II which was improved to extend walking area of each leg by adding one joint on each leg, and this enabled new loading/unloading task by using its two fore legs instead of handing arms (Figure 2). Though there is a need to be on its belly with the new task before loading/unloading, it allows that the robot is able to load/unload a heavy burden up to 24 kg/unit. As there is a risk of toppling over forward by moving the heavy burden forward, hind legs are stretched out forward and support the overturning moment.







· Cooperative carrying system

Many  tasks  are  supposed  such  as  carrying,  loading  and  unloading  various  burdens  including long-heavy materials like pipes in the reactor building which has many narrow passages. In such an environment, transport work of a long heavy product using a plurality of smaller robots is required. Figure 3 shows the concept of cooperative carry of the long burden mounted on the top of two robots, the leader and the follower. Two robots have to keep constant interval between them to prevent from dropping the burden. We built the cooperative system which communicated the control information between two robots to enable the follower robot to follow the leader robot automatically.



A crawl gait, is adopted at the time of cooperative walking. As shown in Figure 4, this gait consists of two steps that the first step is body moving (Figure 4(a)) and the second step is leg swinging (Figure 4(b)). The quadruped robot repeats these steps. At the first step, the robot moves its center of gravity (CoG) to its target inside its supporting triangle. This triangle is defined by three leg tips except for the next swing leg. At the second step, the swing leg is moved to its target. It is able to keep that easily by equalizing two amounts of body moving in the first step. Therefore the crawl gait is suitable for the cooperative walking.



· Synchronize control

To keep the interval constant, the two robots’ bodies must be stationary even though either the follower or the leader executes the leg swinging step. Figure 5 (a) shows the interval in the case that the two robots are operated asynchronously. This case is that the swinging leg of the follower lands later than the leader. In this case, the interval increases because the leader starts the body moving step while the follower still executes the leg swinging step. Figure 5 (b) shows the case of synchronous operating. In this case, the leader halts after its leg swinging step until the follower lands, and the interval of the two robots body keeps constant. Figure 5 shows the case that the leader finishes the swinging leg step earlier. When the follower lands earlier, so that the follower will wait.



· Determination of body movement amount
To move the two robots synchronously, it is necessary to equalize deciding the amount of body of the two robots. A method of deciding the amount of body movements for each horizontal and vertical direction is explained as below.

Horizontal direction
When the amount of body movement is simply decided based on one robot, there is an anxiety that the projected CoG of another robot goes to outside of the supporting triangle and another robot is falling down. The reason of this is that the shapes of supporting triangle in each robot are different and the triangles are used to decide the amount of body moving. Figure 6 shows the procedure of deciding the body movement in horizontal direction(X and Y directions). The shapes of two supporting triangles, which are blue and red, are different. The process is explained as below.

Procedure 1: Overlay two supporting triangles of both two robots to match each the current position of projected CoG.
Procedure 2: Find the position whose distance is the maximum from the overlaid figure boundary.
Procedure 3: Decide the found position in procedure 2 as the target position of projected CoG.



Vertical direction
To keep the burden horizontally on the top of two robots, it is necessary to equalize the body height of them. These robots are able to perceive the heights of the leg tips by calculating the distances which are from their leg tips to their base of leg. In Figure 7, the leader starts to climb the step and the follower walks on the land. Figure 7 (a) shows the case that two robots move the body to the individual height of target position. The height of target position, which is shown the blue dot line, is defined by the distance which is from the robot’s leg tips to its top position of current body. In this case, the burden is likely to inclined fall. Figure 7 (b) shows the case that two robots move their bodies to equalized height. The equalized height is defined as the average of two robots’ height of target. In this case, two robots are able to keep the burden on their top.



· Cooperative unloading
After two robots carry the burden to the destination cooperatively, they unload the burden. The robots folded their legs and landed the body. After that, the each robots start unloading by using two legs at the same time with synchronize control (Figure 8).

These cooperative carrying and unloading have not applied to the actual plant yet. Chapter 5 shows experiment results.

· Cooperative carrying

We verified the developed control method that two robots were able to carry the burden cooperatively by using type I and type II. Carrying the pipe (weight: 48kg, length: 2m, diameter: 114mm) while getting over 100mm step was experimented by two robots. As a result, these robots were able to carry it without dropping (Figure 9) and the balance of interval between these robots was suppressed to 24mm or less during walking.

Figure 9　Experiment of cooperative carrying (Movie)

· Cooperative unloading
Cooperative unloading was also verified by using type I and type II with the aforementioned pipe. As a result, these robots were able to unload it to the target position (Figure 10) and we confirmed that two quadruped robots were able to carry and unload the long-heavy burden with the developed cooperative control system.

Figure 10 Experiment of cooperative unloading (Movie)