An actuator is a “mover” in a system that controls the moving mechanism. It is just the inverse of a sensor. They need a power source and an control signal. Actuators translates the control signal to mechanical movement .
Depending on the actuation type, they are classified as:
- Linear actuator - actuating movement in a straight line
- Rotary actuator - For continuous angular movement
- Semi rotary actuator - limited angle actuation <360 degree
Depending on the power source, actuators are classified as :
- Hydraulic
- Pneumatic
- Electric
- Thermal or magnetic
- Mechanical
Hydraulic
A hydraulic actuator consists of fluid motor that uses hydraulic power for mechanical operation. The fluid motion is linear, rotational or oscillatory which gives a corresponding mechanical motion. Compression of fluid is very difficult and any movement is gradual in nature therefore, a hydraulic actuator can exert a large force but with limited acceleration.
The hydraulic cylinder is a hollow tube with a piston. The piston moves only in one direction fitted with a spring attachment for the return stroke in a single acting system with fluid pressure only acting at one side.In double acting system pressure is applied on each side of the piston; and difference in pressure between the two sides of the piston moves the piston to one side or other.
Types of hydraulic cylinder
| Type | Description |
|---|---|
| Gravity return Single acting | Retraction is done by weights. There are two types – Push and Pull… |
| Type | Description |
|---|---|
| Spring return Single acting | Springs are used for retraction. Two types – Push and Pull |
| Type | Description |
|---|---|
| Piston rod on one side Double acting | Piston rod on only one end. Direction of the fluid determines the extension of the cylinder. Fluid through the blank end and out through the rod end into the reservoir is cylinder expansion. The reverse direction is retraction. |
| Type | Description |
|---|---|
| Piston rod on both sides Double acting | cylinder with a rod extending from both ends. Used in application where both end of the cylinder is needed for a productive actuation Because of their design they can bear heavy loads. |
| Type | Description |
|---|---|
| Trough rod Tandem cylinder | Same as the double acting cylinder. Used when same speed is required in both the directions, both ends of the rod can be utilized to do work and the non-working end is used to indicate or signal the position of the load. In some applications, the rod is fixed at both the ends and the cylinder body carrying the load moves on the rod. |
| Type | Description |
|---|---|
| Displacement Tandem cylinder | The rod is displaced from inside a tube by pumping hydraulic fluid into the tube. The volume of the rod leaving the tube is equal to the volume of fluid entering the tube, hence the name. Retraction is by weight , spring or external forces. |
| Type | Description |
|---|---|
| Telescopic | Used in application for long strong length and short retraction. They elongate in steps with sleeves holding the next length of the rod within. The load lifting capacity increases with each nested section. Stoppers are used to avoid the complete retraction of the sections |
Pneumatic
Pneumatic actuators convert the energy from air at high pressure into rotational or linear motion. The force produced is high from a very small pressure difference and is mostly used for quick response application at the start or stop eg: main engine controls. They are safer, cheaper, and often more reliable and powerful than other actuators. These forces are often used with valves to move diaphragms to affect the flow of air through the valve.
Types of pneumatic cylinders
Pneumatic cylinders also are divided into various similar categories as hydraulic system with respect to cylinder design. The similar types are not explained below.
Based on application Light , Medium and Heavy duty are the three classification based on pneumatic applications.The materials of the components for these applications differ. The difference between these three types are
| Components | Light Duty | Medium Duty | Heavy duty |
|---|---|---|---|
| Seamless Cylinder tubes | Aluminium or brass, plastics | Brass, Iron or steel | Bronze, iron or steel |
| Piston | Aluminium alloy | Aluminium alloy, brass, cast iron | Aluminium alloy, brass, cast iron |
| Piston rods | EN8 steel ground or polished or chrome plated | EN 8 steel, chrome plated, Ground and polished stainless steel | Ground and polished stainless steel |
| Mounting brackets | Aluminium alloy | Aluminium, Brass, Iron castings | High tensile castings |
Based on Cylinder action
| Type | Description |
|---|---|
| Diaphragm cylinder | Piston is replaced by diaphragm made of hard rubber. So only short strokes are possible which makes it good for calmping,embossing, riveting applications |
| Type | Description |
|---|---|
| Rolling diaphragm cylinder | Piston is replaced by diaphragm made of hard rubber. But the diaphragm rolls out along the inner walls and so can do slightly longer strokes of 50 to 800 mm. |
Based on cylinder design
| Type | Description |
|---|---|
| Cable Cylinder | Piston does not have rod instead is connected to an external system by nylon covered cable. The movement of the air controls the movement of the inner piston which moves the cable and subsequently the load attached. They have stroke length of 2000mm |
| Type | Description |
|---|---|
| Sealing band Cylinder with slotted cylinder barrel | Piston does not have rod instead is connected to an external system by means of a carriage which moves the load, the cylinder has seals on both ends which open and close alternatively to enable the carriage movement. The seals are designed to provide air tight sealing to the system |
| Type | Description |
|---|---|
| Cylinder with Magnetically Coupled Slide | Piston does not have rod instead is connected to an external system by magnetic coupling. Having strokelength of 50 to 4000 mm, they have no leakage, minimum wear as there is no direct contact with moving element. |
| Type | Description |
|---|---|
| Impact cylinders | Used in high energy applications like stamping, punching or embossing with limited stroke length so that velocity does not diminish. They are regular pistons attached to an air reservoir |
| Type | Description |
|---|---|
| Duplex cylinder | Similar to a tandem cylinder. But the pistons are capable of independent motion through separate air supply control. Application include lever actuation, sorting devices where the |
| Type | Description |
|---|---|
| Position sensor for cylinders | In automation where the end of one set of functions needs to initiate the next step these types of cylinders are used. The sensing mechanism is done through mechanical levers, pressure sensor or magnetic sensor . |
Electric
Being one of the cleanest form of actuating devices, electrical energy is used as a power source. Electric energy is used to control a motor that opens and closes valves.
Twisted and coiled polymer (TCP) or supercoiled polymer (SCP) These actuators ae a type of electrical actuators that are energized by power. Electric power is used to energize the TCP which is actually a helical spring of Nylon coated with Silver or Gold. On energizing the spring heats up due to joules heating ( resistance heats up the conductor) and the normally extended spring contracts. The springs are held extended by weights.
For details on electric actuators see also : Types of electric actuators
Thermal or magnetic
These are compact, lightweight, economical actuators with high power density that can be energized by thermal or magnetic energy. Shape memory alloys are used for the construction of these actuators.
Mechanical:
Mechanical actuators convert one form of mechanical movement into another. Working on a combination of elements like gears and train or pulley-chain. Both rotational and linear movement is possible with such arrangement. The powering system may be physical or sometimes electrical too.
Graphical representation of cylinders
Thing to consider for actuators.
While performance of actuators is based on the speed, acceleration, and force and energy efficiency. The points to consider for determining the performance of the actuators are as follows
Force
-
Static load - force capability of the actuator while not in motion.
-
Dynamic load - force capability while in motion.
Speed
Speed is taken at no load conditions. and the rate of speed depends on the initial speed and the force needed
Operating conditions
Actuators are commonly rated using the standard IP Code rating system. Dangerous environments will have a higher IP rating than those for personal or common industrial use.
Durability
This will be determined by each individual manufacturer, depending on usage and quality
Soft actuators:
Used in applications that need delicate handling like the biological handling, they are made of elastic soft materials which are batch manufactured by micromachining, moulding or free form fabrication. Alternatives are sought out in 3D fabrication technics where molecular changes are made before component assembly by printing techniques.