Less common are proportional pneumatic valves or even various electrical actuators. Hydraulic machinery uses solenoids to control the flow of oil to rams or actuators. The fin servo system consists of hydraulic actuators and valves and an electrohydraulic power supply. Most landing gear are operated by electric motors or hydraulic actuators. Control valves are normally fitted with actuators and positioners. It employed 10 sensors sending signals to microprocessor-controlled hydraulic actuators at each wheel. Hydraulic systems use high fluid pressure, such as the hydraulic actuators for bulldozers and backhoes. A HIL simulation must include electrical emulation of sensors and actuators. There may be independent airbag systems which contain sensors, logic and actuators. Most linear motors have a low load capacity compared to other types of linear actuators. Linear motors have been used for sliding doors and various similar actuators. However, piezoelectric actuators or magnetostrictive actuators are alternative options. This electricity in turn is used to drive motors forming the actuators of mechanical systems. Uses include magnetic levitation, linear propulsion, linear actuators, and liquid metal pumping. Mechanical arms are composed of multiple beams connected by hinges powered by actuators.