There are numerous tools in use to help withmechanical and electrical engineeringapplications. A rotary actuator is one of them and is used to create both rotary movement and apply torque. In its most basic form, a rotary actuator is a largely mechanical device that produces rotary motion from linear motion in a specified direction. However, mechanical systems are not as popular as their electrically-powered equivalent, or in some cases, energy storage powered (e.g. with springs), hydraulic and pneumatic systems. They work in four major ways depending on the motor type in use:
- Electric motor: this type produces continuous motion
- Servomotor: this type produces motion to a fixed angular position
- Stepper motor: this type also produces motion to a fixed angular position
- Torque motor: this type generates torque, which in turn induces rotation, or is balanced by opposing torque, and thus it does not necessarily produce any motion.
Power sources for actuators
There are both electrical actuators and fluid actuators based on the power source. Electrical actuators include:
Stepper motors, as the name suggests, move in precise steps or stages of fixed measurement with controlled speed or angular amount. When coupled with a position encoder or single datum sensor, stepper motors can act as rotary actuators by moving to any angular position with high precision.
Servo motors are made up of several components including a motor, gear train, position encoder, and a control system. Motors are typical of electric type by sometimes hydraulic types could be used. Gear trains reduce the multiple rotations of motors to high torque revolution. The position encoder serves to determine the output shaft’s position to enable the control system to instruct the desired output position accordingly. When the position encoder determines the output shaft’s position, any variation with the desired position generated by the control system raises an error signal, which causes a rotation of the motor and gear train to reflect the desired position. Such servo motors are widely used across various applications.
A memory wire is a recent development in the ultra-lightweight actuator category. A current is applied to a wire causing it to heat up and change shape once the transition temperature is exceeded, causing a torque on the output shaft. The wire changes back to its original shape after the power is removed and it cools down.
Fluid power actuators are of pneumatic, hydraulic and vacuum types.
Hydraulic and pneumatic actuators
Hydraulic and pneumatic actuators are very similar in their construction. They are effective for driving larger actuators that require the resilience offered by fluid systems. There are two working principles for fluid actuators including:
- Piston and cylinder mechanism: in this system, a gear method induces motion from linear piston to add cylinder mechanism.
- Rotating asymmetrical vane mechanism: in this system, vanes swings through a cylinder of two radii of different size causing a differential pressure that results in unbalanced force and torque on the output shaft. This system has sealing and leakage problems, unlike the piston and cylinder type.
Vacuum power could be used to drive rotary actuators where it is available. Not to be confused with pneumatic power, this system works in the absence of air or at very low pressures. However, it is not very common these days having been used in automatic windscreen wipers until the early 1960’s. the system relied on the manifold vacuum of petrol engines to drive a quarter-turn oscillating vane actuator. However, it was not reliable since it only worked at low engine power consumptions.
Applications of actuators
Rotary actuators have numerous applications and thus, they are of diverse sizes, operating speed and types. For instance, zero-power actuators are applied as display devices such as the air core gauge. Valve actuators, on the other hand, are popular in the petroleum industry for the control and pipeline and process valves. In other cases, actuators are used in both military and civil engineering applications such as dam gates, sluice gates, machinery actuators, and extensible military machinery semi-permanent structures. Car wipers also depend on actuators for their function.
In conclusion, electrical actuators are the most popular actuator types in the market. There are also other actuator types as well including hydraulic and pneumatic. Some even depend on stored energy such as in springs for their power.