**2. Actuators technology for damping and active isolation: An overview**

Undesired noise and vibrations are since ever a major problem in many human activities and domains. Airplanes, space trusses and satellites, cars, machine tools and large bridges,

all can be disturbed in their normal functions by vibrations and noise. Actuators play a critical role in the active control of vibration and different technologies must be considered in order to obtain compact and efficient smart structures.

Selection and use of these technologies is greatly influenced by the user's technical knowledge, the project's budget, available energy sources, and performance tradeoffs. For example, pneumatic actuators don't deliver high force output, but are well suited when a cost-effective, easy start-up solution is required. Hydraulic actuators generate a lot of noise and can leak nasty fluid, but are ideal for high force applications that require precise control. Electromechanical actuators have high energy requirements and are more difficult to install and maintain, but are preferred for complex, multi-axis, motion control applications.

*Pneumatics*: pneumatic actuation is the conversion of compressed air into, typically, linear force. Typical applications involve extreme temperature and magnetic systems because pneumatic actuators don't have the magnetic field issues of electric motors. Position feedback with proximity sensors is used in modern control-loop systems, bringing pneumatics beyond simple bang-bang applications.

Pressure losses and the compressibility of air make pneumatics less efficient than other actuator technologies. In addition compressor and delivery system limitations dictate that pneumatic systems operate at lower pressures, providing lower forces and lower bandwidths than other systems. Pneumatic cylinders typically operate with compressed air at 100 psi or less, in contrast with hydraulic cylinders, which operate on pressurized hydraulic fluids at over 500 psi. Speed, force and bandwidth are directly connected with these characteristics.

*Hydraulics*: hydraulic actuators are suitable for rugged applications that require high force output. However, hydraulic systems generate noise and, without proper maintenance, they can leak. More equipment is needed as well: hydraulic systems require a fluid reservoir, motors and pumps, release valves, and equipment to reduce noise and heat levels. Moreover external sensors are needed to determine piston velocity, acceleration and position in a closed-loop system. Hydraulic systems can deliver much tighter control than pneumatic systems and higher force density than any other actuator technologies. Bandwidth is better than pneumatic actuators but still under hundreds of Hertz.

*Electromechanical*: electromechanical actuators can be based on rotatory motors (using ball screw, roller screw or belt drive), linear motors or moving coils. This type of actuator have high dynamic performance, with accelerations greater than 20 g and velocities of 10 m/sec and eventually higher. Sub-micron resolution and repeatability are commonplace. Because the actuator is directly coupled to the load, there are fewer components with the chance of failure, which adds long term value.

*Piezoelectric*: piezomotors and piezoactuators rely on the electromechanical response of crystals. Electrical excitation causes the crystals to slightly change shape and distort, therefore generating large forces and small displacements. Exciting the crystals at a high frequency generates smooth, precise motion, making piezoelectric actuators suitable for applications with very fine positioning and high bandwidth requirements.


**Table 1.** Actuators technology comparison
