# Step-Climbing Tactics Using a Mobile Robot Pushing a Hand Cart
## 1 Introduction  
Hand carts and shopping carts are used all over the world to help people convey baggage. In recent years, several robotic carts have been investigated, including an autonomous cart that follows the user [1; 2], a robotic cart that helps the operator when transporting heavy baggage [3], and transportation using a cart connected to an omnidirectional mobile robot [4; 5]. [PERSON] et al. studied two shopping assistant robots that can be used in supermarkets [6]. They proposed not only a cart robot, which has a shopping basket attached to the robot body for transportation of an item, but also a humanoid with dual manipulators, which can bring the shopping basket. If the partner robot can operate a cart freely, the possibilities of transportation using a cart for improvement are endless. Research on such carts that can be manipulated by a partner robot has been carried out. [PERSON] proposed a method of transportation of a hand cart using ASIMO (HONDA Co., Ltd., Tokyo, Japan), a legged locomotion robot with dual manipulators [7]. Similarly, [PERSON] et al. studied the locomotion of a wheelchair, rather than a cart, using a legged locomotion robot [8]. However, they showed how to push and transport a wheelchair using a personal robot with dual manipulators. A hand cart has a wheel mechanism that it uses to move. Excellent energy efficiency is one of the advantages of a wheel mechanism. It is easy for people to move a hand cart on a flat road. However, it is difficult for a vehicle that has a wheel mechanism to move up and down steps. Numerous research programs aimed at improving the mobility of wheel mechanisms on steps have been conducted, based on the following strategies: additional legs [9; 10], multiple wheels and variable wheelases [11], and a combination of an adjustable center of gravity (COG) and multiplewheels [12]. Reports of multiple vehicles cooperating for crossing irregular terrain include those of [PERSON] et al. [13], who considered a forklift system. Transporting a cart using a mobile robot that has numerous manipulators, similar to the human arm, is not an easy task. When the cart is heavier than the robot, controlling a robot to push or pull the cart is very difficult because the reaction force from the cart strongly influences the robot. In particular, one weakness for a wheeled vehicle is step climbing [14]. In transporting heavy baggage using a push-cart, it is very difficult to control the cart at a step because the incline of the vehicle is always changing, and the reaction force from the cart is also changing. As such, there have been no studies of a cart climbing a step using a partner robot. The research group of the present study has achieved cooperative step climbing using a wheelchair and a partner robot with manipulators [15] and have proposed a cooperative step-climbing system using a hand cart and a robot [16]. In the present paper, we intend to describe in detail the concept of step climbing of a hand cart and present the results of both theoretical analysis and experiments. The proposed step-climbing method uses the difference in velocity between connected vehicles, which considers not only the robot driving force, but also the wheelchair driving force. However, since a typical hand cart does not generate a driving force, the step-climbing method that we have proposed for a wheelchair cannot be applied to a hand cart. The present paper proposes a step-climbing method for a hand cart and a partner robot that uses an action that is similar to that performed by a human. Based on preliminary measurements of friction coefficients using the vehicles in wet and dry conditions on asphalt, concrete, wood, and interior flooring, the ground surface considered in the present study was assumed to have a friction coefficient in the range 0.6 to 0.9, which satisfies all of the above conditions.