# Analysis, Design, and Experimental Research of a Novel Wheelchair-Stretcher Assistive Robot
## 1 Introduction  
It is generally known that population aging is a global phenomenon because of the increasing levels of health care. In 2019, the number of people aged 65 years or over has become 703 million in the world, and it is estimated to double in 2050 [1; 2]. With the aging population, there are more and more disabled and semi-disabled elderly people. Despite that bedridden elderly needs to go to the toilet in a wheelchair or go out for some fresh air, it is impossible to do on their own [3].  
Researchers have done a lot of work on enabling the elderly to smoothly move between bed and wheelchair. For instance, bed chair assist robot \"Resyone\" is developed by Panasonic Company in Japan [4]; a bed-chair robot system \"AgileLife\", that can be docked with wheelchair, is made by NextHealth Company [5]; a multi-functional wheelchair of assisting the elderly to get up and downthe bed is proposed by Harbin Engineering University [6]; an integrated multi-functional nursing bed for the bedridden elderly with weak body but normal intelligence is developed by Institute of Robotics of Beijing University of Aeronautics and Astronautics and Xinsong Robot Automation Co., Ltd. [7,8]. By changing the structure of the bed, the above studies enable patients to move between the bed and the wheelchair. There is also some research on the transport device from bed to stretcher, such as a patient transport device developed by SA.VIR Company in Italy [9], taxiing air cushion designed by Getinge Company in Sweden [10], \"PowerNurse\" transfer equipment invented by Astir Technologies Company in American [11], SE series medical electric transfer vehicle developed by Ningbo Kai Medical Technology Co., Ltd. [12], a transfer robot researched by the Institute of Health Equipment of the Academy of Military Medical Sciences of the Chinese people's Liberation Army [13], and intelligent transport robot for patients designed by Yanshan University [14].  
In addition, some researchers use humanoid robots to study the transfer and transport of patients, such as lifting transfer robot \"RIBA\", invented by the Institute of Physical and Chemical Research in Japan [15], the wounded transfer robot BEAR developed by Vecna Company in the United States [16], and the humanoid back-hugging transfer robot \"Baize\" designed by Hebei University of Technology [17].  
In summary, we can see that:  
1. The technology of transport devices in moving patients from bed to stretcher, or from stretcher to bed, is more mature; but it is difficult to transform the stretcher to a wheelchair.
2. The research on the multi-function nursing bed mainly focuses on the change of bed structure. There are two main forms; one is that the middle part of the bed structure is changed directly into a wheelchair; the other is that one side of the bed structure is transformed into a wheelchair. However, for these two methods, not only the sheets need to be customized, but also the mattress under the body needs to be moved with the patient which resulted in a deterioration of the patient's experience.
3. The research technology of humanoid assistive robot is advanced, but it is expensive and is far away from industrial application.
4. It has always been a difficult problem for patients to go to the toilet. Although the research institutions have put forward some ideas, they have not been able to successfully develop the right products.  
Based on these, a low-cost and practical wheelchair-stretcher assistive robot is proposed. The robot could transform between wheelchair and stretcher. In wheelchair form, it could move autonomously, so that the elderly can easily complete the basic daily activities such as going to the bathroom and outdoors. In stretcher form, it could automatically adjust the height and dock with the bed. The robot can also work with transport device to move patient between bed and stretcher which promote comfort and prevent harm to patients.