Dynamic Analysis and Design of an Innovative Sit-to-Stand Gait Trainer

Abstract This paper illustrates three examples of how to implement dynamic lifting support, including the solution adopted in the Up n’ Free® (Easy-Walking Inc., Maple Glen, PA). The Up n' Free system can lift the user from a seated posture to a standing position through the employment of a four-bar mechanism and gas piston. When the user is standing, the mechanism can provide partial compensation from gravity to help the user regain mobility. The compensation can be provided while standing without taxing the arms and shoulders with the use of crutches. By using this system, users can sit and stand without assistance from another person and can also receive compensation for gravity loads while walking. This group is not involved with the design and commercialization of the Up n' Free product but uses that design as a reference of comparison for two additional actuation solutions, one of which involves a cam acting on a rubber band. The piston in the Up n' Free is positioned between the bottom part of the walker frame and the four-bar mechanism. We propose a more compact configuration where the piston is located within the four-bar mechanism. This work discusses the pros and cons of the two gas spring cylinder configurations. Common problems arising with both configurations are the difficulty of regulating the minimum and maximum force at the seat, as well as the inability to define a force profile for the transition from sitting to standing. Thus, a new actuation system to implement the dynamic lifting support is proposed, where the force profile is modulated using a cam acting on an elastic band. We were able to define a linear profile of the force during the transition from sit-to-stand, thereby avoiding sudden increases in force that are typical of actuation solutions encompassing gas pistons acting on a variable moment arm.