Underactuation

Underactuation is a technical term used in robotics

Robotics

Robotics is the branch of technology that deals with the design, construction, operation, structural disposition, manufacture and application of robots...

 and control theory

Control theory

Control theory is an interdisciplinary branch of engineering and mathematics that deals with the behavior of dynamical systems. The desired output of a system is called the reference...

 to describe mechanical devices that have a lower number of actuator

Actuator

An actuator is a type of motor for moving or controlling a mechanism or system. It is operated by a source of energy, usually in the form of an electric current, hydraulic fluid pressure or pneumatic pressure, and converts that energy into some kind of motion. An actuator is the mechanism by which...

s than degrees of freedom

Degrees of freedom (engineering)

In mechanics, degrees of freedom are the set of independent displacements and/or rotations that specify completely the displaced or deformed position and orientation of the body or system...

.

The classic inverted pendulum

Inverted pendulum

An inverted pendulum is a pendulum which has its mass above its pivot point. It is often implemented with the pivot point mounted on a cart that can move horizontally and may be called a cart and pole...

 is an underactuated system: it has two degrees of freedom (one for its support's motion in the horizontal plane, and one for the angular motion of the pendulum), but only one of them (the cart position) is actuated, and the other is only indirectly controlled. Although naturally extremely unstable, this underactuated system is controllable.

In a device designed for gripping, for example, a greater number of actuators means that the device increases in versatility, but this comes at the cost of size, complexity, cost and weight of the device. Underactuated devices can be more efficient, simpler and more reliable than their fully actuated alternatives. Although the control of underactuated devices is theoretically more complex than that of fully actuated systems, the combination of underactuated control with naturally stable or nearly stable mechanical systems can be very effective, as has been demonstrated by research on bipedal walking robots.

Further reading

• Saliba, M. & de Silva, C.W. (1991). An Innovative Robotic Gripper for Grasping and Handling Research IEEE Journal of Robotics and Automation, p975-979.

External links

• Canudas-de-Wit, C. On the concept of virtual constraints as a tool for walking robot control and balancing Annual Reviews in Control, 28 (2004), pp. 157-166. (Elsevier)
• Non-Linear Control of Underactuated Mechanical Systems