A Review of Active Mechanical Driving Principles of Spherical Robots <s> Universal Wheel <s> HIT spherical robot has been designed on the principle of carrying out the turning and driving motions independently in order to reduce dynamic complexity and realize real-time detection. After introducing the decoupling principle implemented in the working processing, we derive the dynamic equations about the two fundamental motions, i.e., rolling and turning. Hardware and software of the open-loop control system are introduced briefly and, in the following, experiments show that the robot can fulfill common exploration applying the proposed decoupling method. <s> BIB001 </s> A Review of Active Mechanical Driving Principles of Spherical Robots <s> Universal Wheel <s> This paper presents a novel design of an omni-directional spherical robot that is mainly composed of a lucent ball-shaped shell and an internal driving unit. Two motors installed on the internal driving unit are used to realize the omni-directional motion of the robot, one motor is used to make the robot move straight and another is used to make it steer. Its motion analysis, kinematics modeling and controllability analysis are presented. Two typical motion simulations show that the unevenness of ground has big influence on the open-loop trajectory tracking of the robot. At last, motion performance of this spherical robot in several typical environments is presented with prototype experiments. <s> BIB002
Another design that incorporates the principles of barycenter offset is BHQ-3 BIB002 . The BHQ series of robots derive their name from the fact that their dynamic model was established from the Boltzmann-Hamel equation. This design is a combination of the hamster wheel and the previous IDU design. It can be conceptualized as a universal wheel type system: the interior drive mechanism can rotate freely on the inside of the robot due to the combination of wheels attached to it. The IDU in the BHQ-3 is designed such that the internal mechanics will not shift when encountering bumpy terrains ( Figure 4 ). Two DC drive motors control the robot: one motor controls the orientation of the IDU, and one motor controls the speed of the drive wheel. This allows the ball to move with a zero turning radius, creating a higher degree of holonomy than the previously described robots. In addition to having the ability to maneuver itself by the use of barycenter offset, the robot's velocity is also controlled by the angular velocity of the driving wheel. As the wheel spins faster, the translational velocity of the robot increases. This means that the internal weight of the IDU is not the only factor controlling the momentum of the robot. This particular robot is able to travel in water, sand, as well as up a small slope. However, it may have high energy loss due to friction from the sponge wheels as well as the inability to roll unpowered down a slope, depending on the motors and control method used. The HIT Spherical Robot BIB001 was designed to behave in a manner where the steering and driving mechanisms are independent of one another ( Figure 5 ). In a pendulum-based designed, the steering and turning mechanisms are dependent on each other, creating a non-holonomic robot. For a robot to be able to move in any direction regardless of orientation (holonomy), the steering and driving mechanisms must be mechanically independent. HIT is only controlled by two motors: a turning and a driving motor. The turning motor (Item 1, Figure 4 ) rotates the entire inner assembly of the robot along a rim at the equator, and the driving motor (Item 2, Figure 4 ) shifts the center of gravity of the robot, causing it to move. 