# Dynamic Balance of the Head in a Flexible Legged Robot for Efficient Biped Locomotion
## 2 Rollo Robot and FE Model  
### Rollo in Details  
Rollo, shown in Figure 1, was conceived as an avatar robot for people affected by Amyotrophic Lateral Sclerosis (ALS) disease, able of being a controller using advanced brain-computer interfaces. ALS is a chronic neurodegenerative disease that reduces human motion and interaction capabilities. A biped robot may be used to assist people affected by ASL [22; 39], and Rollo robot has been specifically designed for those patients, given its simple architecture, lower power supply, and high vertical stability.  
Rollo is a biped, flexible-leg wheeled-feet robot. The two legs are connected with a beam-like structure at the top, horizontal and parallel to the ground. Each leg is connected to the feet on the lower side. The robot head is located on the beam linking the two legs. Each leg is made of one or more flexible modules. The robot feet are constituted by two active driving wheels, two passive wheels to guarantee vertical stability, and one heavy motor, chosen to increase stability and lower the center of mass of the robot. The weight of the robot naked structure is almost 23.5 kg, and it is 30 kg considering also non-structuralelements and accessories; the overall dimensions in the configuration with two flexible modules for each leg are about 80 \(\times\) 40 \(\times\) 30 cm.  
Two locomotion types are embedded in Rollo: human-like walking alternating the motion of the feet, the focus of the paper, and simpler locomotion with the feet always parallel, refer to [22] for more details. In the human-like locomotion type, the flexible modules of each leg absorb energy and give it back, net of dissipations, in the next step, i.e., as it happens in human walking. The human-like walking mechanism minimizes the amount of external energy required for walking.  
Rollo is driven by an input, which may be externally given to the robot by a joystick, a mobile, brain-computer interface, or other communication systems. The input parameters required to control the robot are the speed, locomotion type, and the direction (forward, backward, turn left, turn right) [22]. Tests on Rollo locomotion were performed in [22], in the configurations with two (Figure 1 right) and three (Figure 1 left) flexible modules for each leg. The tests highlight better locomotion performance in the configuration with two flexible modules for each leg and better capability to turn left or right for the robot with three flexible modules in each leg; however, undesired oscillations of the robot head were detected in each configuration [22]. The oscillations were neglected in previous works focused on the design of the robot and locomotion control logic [22]. Head oscillations should be avoided because they can compromise the interaction with people for which it is conceived. In particular, Rollo is an avatar robot, and the head of the robot (constituted by an iPad) has the webcam onboard, reproducing in real-time the external environment on the PC of the patient. Furthermore, the iPad of the robot reproduces in real-time the video of the patient for the interaction with people. Oscillations of the Rollo head are undesired because they compromise Rollo communication capabilities.  
Rollo design requires modifications to attenuate the undesired head oscillations. The design changes must not change the robot nature, i.e., mass balance and human-like locomotion logic. Therefore, the necessary changes have to comply with some constraints: the robot mass cannot increase more than 1% of the total mass, the center of gravity vertical  
Figure 1: Flexible-wheeled biped robot Rollo.  
height cannot increase more than 3 cm, otherwise the robot vertical stability is compromised. Moreover, Rollo's overall dimensions and electrical range should not change.