What is an Actuator?
It is a component of a machine that is responsible for moving or controlling a mechanism or system, e.g by actuating (opening or closing a valve). It requires a control signal and a source of energy. The control signal is relatively low energy and maybe electric current, hydraulic fluid pressure or pneumatic pressure. When the control signal is received, the actuator responds by converting the energy to mechanical motion.
Electro-mechanical actuators are similar to mechanical actuators except that the control knob or handle is replaced with an electric motor. Rotary motion of the motor is converted to linear displacement. There are many designs of modern linear actuators and every company that manufactures them tends to have a proprietary method. The following is a generalized description of a very simple electro-mechanical linear actuator.
Hydraulic actuators or hydraulic cylinders typically involve a hollow cylinder having a piston inserted in it. An unbalanced pressure applied to the piston generates force that can move an external object. Since liquids are nearly in-compressible, a hydraulic cylinder can provide controlled precise linear displacement of the piston. The displacement is only along the axis of the piston. A familiar example of a manually operated hydraulic actuator is a hydraulic car jack. Typically though, the term “hydraulic actuator” refers to a device controlled by a hydraulic pump.
Pneumatic actuators, or pneumatic cylinders, are similar to hydraulic actuators except they use compressed gas to generate force instead of a liquid. They work similarly to a piston in which air is pumped inside a chamber and pushed out of the other side of the chamber. Air actuators are not necessarily used for heavy duty machinery and instances where large amounts of weight are present.
So generally an actuator is a mechanism by which control system acts upon an environment. The control system can be simple(a fixed mechanical or electronic system), software based(printer driver, robot control system), a human or any other input.
There is a general move in the aerospace industry to increase the amount of electrically powered equipment on future aircraft. This is generally referred to as the “More Electric Aircraft”. The MEA concept can be subdivided into fly-by-wire, where the hydraulically powered flight actuators are electrically signaled, and power-by-wire, where the actuators are directly powered from the aircraft’s electrical system. Electrically powered flight actuators can take one of two principal configurations, the electromechanical actuator (EMA) with mechanical gearing, and the electro hydro-static actuator (EHA) with fluidic gearing between the motor and the actuated surface.
Types of Actuators.
Linear Actuators and Rotary Actuators. Linear actuators are manly used to operate trailing edge flaps, trim tabs, elevators, rudder, aileron etc and rotary actuators are used in air conditioning, landing gears and fuel systems.
Linear actuators as the one seen above are used to control doors, trims, flaps, air inlets and numerous other movable components in a modern aircraft. Depending on their setting they come in different sizes and are located on various parts of the aircraft control systems. Let us take a look at the landing gear system which employs actuators too.
Diagrammatic representation of the landing gear and its actuator systems
An aircraft has a lot of these actuators in various parts as seen in the pic below.Actuators on an Airbus A380-841 aircraft.
- Hydraulic actuators are rugged and suited for high-force applications. They can produce forces 25 times greater than pneumatic cylinders of equal size. They also operate in pressures of up to 4,000 psi.
- Hydraulic motors have high horsepower-to-weight ratio by 1 to 2 hp/lb greater than a pneumatic motor.
- A hydraulic actuator can hold force and torque constant without the pump supplying more fluid or pressure due to the incompressibility of fluids
- Hydraulic actuators can have their pumps and motors located a considerable distance away with minimal loss of power.
- Hydraulics will leak fluid. Like pneumatic actuators, loss of fluid leads to less efficiency. However, hydraulic fluid leaks lead to cleanliness problems and potential damage to surrounding components and areas.
- Hydraulic actuators require many companion parts, including a fluid reservoir, motors, pumps, release valves, and heat exchangers, along with noise-reduction equipment. This makes for linear motions systems that are large and difficult to accommodate.
- Electrical actuators offer the highest precision-control positioning. An example of the range of accuracy is +/- 0.000315 in. and a repeatability of less than 0.0000394 in. Their setups are scalable for any purpose or force requirement, and are quiet, smooth, and repeatable.
- Electric actuators can be networked and reprogrammed quickly. They offer immediate feedback for diagnostics and maintenance.
- They provide complete control of motion profiles and can include encoders to control velocity, position, torque, and applied force.
- In terms of noise, they are quieter than pneumatic and hydraulic actuators
- Because there are no fluids leaks, environmental hazards are eliminated.
- The initial unit cost of an electrical actuator is higher than that of pneumatic and hydraulic actuators. According to the example from Bimba Manufacturing, an electrical actuator can range from $150 to greater than $2,000 depending on its design and electronics.
- Electrical actuators are not suited for all environments, unlike pneumatic actuators, which are safe in hazardous and flammable areas
- A continuously running motor will overheat, increasing wear and tear on the reduction gear. The motor can also be large and create installation problems.
- The motor chosen locks in the actuator’s force, thrust, and speed limits to a fixed setting. If a different set of values for force, thrust, and speed are desired, the motor must be changed.
- The benefits of pneumatic actuators come from their simplicity. Most pneumatic aluminium actuators have a maximum pressure rating of 150 psi with bore sizes ranging from ½ to 8 in., which translate into approximately 30 to 7,500 lb. of force. Steel actuators have a maximum pressure rating of 250 psi with bore sizes ranging from ½ to 14 in., and they generate forces ranging from 50 to 38,465 lb. of force.
- Pneumatic actuators generate precise linear motion by providing accuracy, for example, within 0.1 inches and repeatability within .001 inches.
- Pneumatic actuators typical applications involve areas of extreme temperatures. A typical temperature range is -40°F to 250°F. In terms of safety and inspection, by using air, pneumatic actuators avoid using hazardous materials. They meet explosion protection and machine safety requirements because they create no magnetic interference due to their lack of motors.
- In recent years, pneumatics has seen many advances in miniaturization, materials, and integration with electronics and condition monitoring. The cost of pneumatic actuators is low compared to other actuators. According to Bimba Manufacturing, for example, the average pneumatic actuator costs $50 to $150. Pneumatic actuators are also lightweight, require minimal maintenance, and have durable components that make pneumatics a cost-effective method of linear motion.
- Pressure losses and air’s compressibility make pneumatics less efficient than other linear-motion methods. Compressor and air delivery limitations mean that operations at lower pressures will have lower forces and slower speeds. A compressor must run continually operating pressure even if nothing is moving.
- To be truly efficient, pneumatic actuators must be sized for a specific job. Hence, they cannot be used for other applications. Accurate control and efficiency requires proportional regulators and valves, but this raises the costs and complexity.
- Even though the air is easily available, it can be contaminated by oil or lubrication, leading to downtime and maintenance. Companies still have to pay for compressed air, making it a consumable, and the compressor and lines are another maintenance issue.
AviaConnect ( M B Dube)