## 1 Introduction  
Powered exoskeletons are nowadays used in various fields, such as agriculture, and medical and welfare services [1; 2; 3; 4]. They have a wide variety of applications in numerous fields. A powered exoskeleton in the field of rehabilitation [5; 6; 7; 8] has low output power for safety assistance. On the other hand, the assisting power used to transport heavy baggage tends to be high [9; 10]. Powered exoskeletons have also been developed for workers in a nuclear power plant [11; 12]. We have also been developing a powered exoskeleton for workers who transport heavy baggage in a nuclear power plant. In case of a nuclear hazard, workers need to wear radiation-protective equipment that weighs approximately 40 kg. Moreover, the worker is supposed to carry a heavy exploration robot, such as the PackBot [13], around in a nuclear reactor for efficient exploration. The target of our study is to develop a powered exoskeleton that is used to support workers in a nuclear plant who need to wear heavy radiation-protective equipment and carry an exploration robot. Several approaches have been proposed to control powered exoskeletons. One is based on myoelectric signals measured by electromyography (EMG) sensors [8; 14; 15; 16]. It estimates human intentions by measuring the action potential of the muscles, and the powered exoskeleton assists human action according to the intention. Since action potential occurs approximately 50 ms before the relevant muscle contracts, the powered exoskeleton enables rapid power assistance. However, it can easily be affected by human sweat in a hot environment. It is thus unsuitable for our intended application because workers often sweat in a radiation-protective equipment [17]. The equipment is composed of highly airtight materials so that temperature and humidity inside the equipment are high. Another approach is based on a force sensor/switch. Berkeley Lower Extremity Exoskeleton (BLEEX) [18; 19; 20; 21] uses pressure sensors that measure the force between the shoe of the powered exoskeleton and the foot of the wearer to control the exoskeleton according to the configuration of the foot relative to the ground. [PERSON] et al. [22; 23] also proposed using force sensors attached to the bottom of the wearer's feet to detect the pressure between the shoes of the exoskeleton and the ground. These exoskeletons control joint angle and angular velocity based on the given state of the leg, such as \"stance phase\" or \"swing phase\", estimated by the force switches/sensors. We have examined this approach. We place a force sensor on the wearer's back to measure the weight of the load on it. The powered exoskeleton controls itself to generate assist torque based on the floor's reaction force. The powered exoskeleton assists the worker to carry the load according to the measured weight of the load. It has an advantage of not being affected by human sweat. However, we found that this approach cannot distinguish among similar motions, such as \"walking forward\" and \"walking backward\". This means that the approach restricts the motion that can be assisted, and the motion needs to be designed in advance. Actually, the potential user needs to do many motions, including walking forward and backward, squat, going up and coming down stairs, run, one-leg standing, and so on. In this research, we focus on only three motions, standing upright position, walking forward, and walking backward. The motion \"walking backward\" is necessary because we suppose that it is hard to turn around in case that the passageway in the nuclear power plant is narrow. Furthermore, the outputs of a force sensor tend to be noisy, especially at the time of impact. Reactive assist control based on force sensors tends to be jerky due to noise. Low-pass filters can be applied but slow down the assist control. Moreover, they are likely to cause hardware trouble because of repeated impact during the walk because they are likely attached to the bottom of the foot of the powered exoskeleton.