What's an linear actuator?
A "linear actuator" is an actuator that can either push with a thrust force or carry a load linearly to specific positions. While the structures are similar, the differences in actuators start with their source of power, which can be hydraulic (fluid), pneumatic (air), or electric (AC/DC).
How do actuator work?
Pneumatic actuators are composed of a simple piston inside a hollow cylinder. The position of a pneumatic actuator is controlled by regulating the air into the valves, which moves the piston within the cylinder housing against the spring force. When there's no air pressure, the spring force moves the piston back to its original home position.
Electric actuators convert the rotational force of an electric motor into a linear force with a specific mechanism, such as a ball screw. By rotating the actuator's screw with the motor, the ball screw nut will move along the screw forward or backward. The position of an electric actuator is controlled by regulating voltage and current.
The right type of linear actuator to use is determined by how well it can meet the application requirements, such as load, speed, accuracy...etc. For example, pneumatic actuators can provide a higher speed, but electric linear actuators provide the most precise control since air and fluid pressure are more difficult to control than electricity. For the average load that does not require extremely high forces from hydraulic actuators, the solution is either a pneumatic actuator or an electric actuator.
Here are 15 factors to consider when deciding between pneumatic actuators or electric actuators.
|Pneumatic Actuators||Electric Actuators|
|Force||Depends on air pressure||Depends on screw pitch/lead|
|Motion Control Capability||Low||High|
|Cost||Lower initial cost||Lower total cost of ownership|
|Data Collection||Still developing||Highly developed|
|Environment||Harsh, hazardous||Refer to IP rating|
|Temperature||Higher ambient temperature||Lower ambient temperature|
|Ideal Application||End-to-end postioning||Multi-point positioning|
Pneumatic actuators are simpler in their design while electric actuators use more complex components, such as the ball screw and the electric motor. The simple design of a pneumatic actuator is also more compact than an electric actuator, but when you consider all the other components that are necessary to maintain that air pressure, it can actually take up more space.
Force & Speed
Traditionally, pneumatic actuators provide higher speeds and lower forces when compared to electric actuators. However, a few factors, such as screw pitch/lead from an electric actuator, or the number of pistons in the pneumatic actuator, can affect the comparison. For a pneumatic actuator, the force is calculated by multiplying the piston area (force factor) by the air pressure in the cylinder. For an electric actuator, the linear force is converted from the motor's torque.
It's difficult to maintain a set speed or force consistently when you're dealing with compressed air. Since voltage and current are easier to control, electric actuators can maintain force and speed much better even without closed-loop feedback. Ball screws or rack and pinion mechanisms on electric actuators also act as a gear reduction ratio, so forces can be increased with sacrifice to speed.
Pneumatic actuators typically operate from 80 to 100 PSI, and electric actuators convert the motor torque and RPM to a linear force and linear speed. To increase the force or speed of a pneumatic actuator, more pistons and/or air PSI is needed. To increase force or speed or acceleration for an electric actuator, more torque from a bigger or longer motor is needed.
Accuracy & Repeatability
Electric actuators dominate accuracy and repeatability, which makes them ideal for multi-point positioning applications. Since voltage and current are easier to control than air pressure, electric actuators can accurately control their position and repeat that position with the same motion profile. Pneumatic actuators are typically chosen for simple end-to-end positioning applications since they simply cannot achieve the same accuracy and repeatability as electric actuators.
Electric actuators use either servo motors or stepper motors, which already offer high stop accuracy and torque control capability. The holding torque from the motors also prevents position drifting.
Motion Control Capability
With more precise control of torque, speed, and acceleration/deceleration patterns, electric actuators can do more with motion profiles than pneumatic actuators.
For example, the following motion profile depicts what an electric actuator or motor can do.
It will be difficult to repeat this exact motion profile with a pneumatic actuator.
Electric actuators are the most capable of repeating specific motion profiles due to their precision and accuracy. Pneumatic actuators are limited in their motion profile generation, and motion profiles are more difficult to change once implemented. For this reason, pneumatic actuators are chosen for single-axis, end-to-end positioning applications. Due to high repeatability, electric actuators are often chosen for multi-point positioning applications and where multiple axes need to be synchronized.For electric actuators, hundreds of target positions can be saved and stored for multi-point operation. Vibration and shock loads can be minimized with modified motion profiles, such as S-curves, whereas pneumatic actuators need a hard stop and spring.
Absolute position control has also been advancing for electric actuators for quite some time. For example, in addition to servo motors with absolute encoders, closed-loop stepper motors with built-in multi-turn mechanical absolute encoders can also help minimize footprint by eliminating the external home and limit sensors. The difference is that the mechanical absolute encoder does not need a backup battery like an absolute encoder. Pneumatic actuators have started to offer absolute feedback, but they're not as common.
Efficiency & Cost
Another major benefit of electric actuators is efficiency. Pneumatic actuators operate at about 10~25% efficiency, which is even lower than hydraulic linear actuators at ~40%. Electric actuators operate at around 80% efficiency.
Efficiency affects power costs in the long run.
“Compressed air is one of the most expensive sources of energy in a plant. The overall efficiency of a typical compressed air system can be as low as 10 to 15%.”— U.S. Department of Energy: Energy Tips-Compressed Air, August 2004.
If the initial cost is important to you, pneumatic actuators are the way to go. In comparison, electric actuators have higher initial costs but lower operating and maintenance costs. However, when you factor in the long-term total cost of ownership, electric actuators actually come out on top. This is because both air and fluid power take more work to maintain and are less efficient than electric power.
Short-term costs consist of the system cost, but the total cost of ownership includes replacement costs, air line installation, and maintenance. Remember that air compressors also need electricity to operate.
According to the Department of Energy, "24 percent of the annual cost of compressed air is due to maintenance, equipment and installation while 76 percent is due directly to the cost of electricity for the compressor."
Total cost of ownership comparisons between pneumatic and electric linear actuators include many variables and assumptions. Sometimes, it comes down to the design of tube fittings, how well the systems can be maintained, and how you use it. For applications that do not require precision or continuous service life, pneumatic actuators can save some money.
Data collection can lead to better efficiency or predictive maintenance. Electric actuators are the winners in this category, too.The control side of electric motors and actuators has been advancing for a long time, so they use more sophisticated controls. Data collection is easier to implement since many of these functions are already included. More industrial network communication protocols, such as EtherNetIP, Profinet, and EtherCAT, are readily available to connect to a variety of PLCs, HMIs, and IPCs. Although pneumatic actuators are also advancing, it may be challenging to catch up to a point where the data can be used to control a process in real-time.
Environment, Temperature, and Noise
Since electric actuator systems can include more sensitive components, such as motors, encoders and sensors, pneumatic actuators are more well-suited for hazardous environments. However, pay attention to the ingress protection (IP) rating and/or specifications in order to understand what environment it can handle exactly.
Pneumatic actuators can sometimes handle a wider ambient temperature (about -20~350°F) than electric actuators (40~150°F), but when pneumatic actuators work in high ambient temperatures, the air seals can fail, and operation could be sluggish. High heat can also affect the bearing grease life of an electric motor and affect metal expansion properties, which can increase friction and wear in an electric actuator.
Pneumatic actuators are also noisier than electric actuators due to compressed air. However, this has also improved over the years.
If you don't like maintenance, electric actuators are the way to go.
The maintenance needs of a pneumatic actuator are very high compared to an electric actuator. There needs to be a constant supply of compressed air from a reservoir tank, which is not easy to maintain. In addition to the actuator, there are more components to maintain, such as the compressor, valves, fittings, muffler, lubricator, filter regulator lubricator, solenoid, and air tubing.
An electric actuator requires minimal maintenance since there are fewer components that could wear out due to the minimized friction from the bearings and linear guides. An occasional greasing may be necessary. If you take out the actuator from the equation, an electric motor can be considered a non-maintenance item as the cost of repairing a motor often exceeds the cost of buying a new motor.
Preventing air leaks is important when using pneumatic actuators. As seals wear, the force produced by the pneumatic actuator will vary, thus making both accuracy and repeatability even worse. Pneumatic actuators depend on tight rod and piston seals to prevent air leakage due to wear and tear. Sometimes, it could take a long time to adjust or regulate the airflow.
Both pneumatic actuators and electric actuators offer medium L10 service life based on bearing life. However, the life of an electric motor can be calculated, while the life of a pneumatic actuator can only be estimated. Predicting when the air seals would fail is very difficult, so periodic maintenance is a must for pneumatic actuators.
The key to extending life for pneumatic actuators is to keep the rod and piston seals secure. Wear and tear of the seals is unavoidable. If air leakage increases, efficiency, force, speed, and responsiveness will suffer. The key to extending life for electric actuators is to keep operating temperature low. Always operate within specifications for both.
The design differences between pneumatic actuators and electric actuators lead to differences in their characteristics. As a result, the coarse characteristics of pneumatic actuators make them ideal for basic, end-to-end positioning applications while the precision of electric actuators makes them ideal for multi-point positioning applications with advanced motion profiles or multi-axis synchronization.
Pneumatic actuators can work in more hazardous environments than electric actuators. However, high temperatures can decrease service life for both types of actuators. If advanced operation, such as closed-loop feedback or data collection, is necessary, electric actuators present an integrated option that should be easier to use for years to come.
In the simplest terms, the choice between pneumatic actuators and electric actuators comes down to simplicity, precision, efficiency, and maintenance.
Pneumatic actuators are smaller, are easy to set up, and can easily meet simple, short-stroke, end-to-end positioning requirements. Electric actuators, on the other hand, can meet stricter requirements with their superior precision and repeatability, and they're well-suited for long stroke, multi-point positioning applications with advanced motion profiles.
Although initial costs favor pneumatic actuators, it's important to consider the total cost of ownership, which includes the initial purchase cost, operating costs, and maintenance costs. Remember that the air compressors used for pneumatic actuators also use electricity. Operating and maintenance costs for electric actuators can be lower in the long run.
The case for switching pneumatic actuators to electric actuators makes sense for applications requiring position, speed, acceleration, and force with better accuracy and repeatability. They are also better at data collection and synchronized multi-axis applications.
Here is the product link of electric linear actuators,
Mini Linear Actuator A
Linear Actuator A2 With Potentiometer
Industrial Linear Actuator B
Heavy Duty Linear Actuator C
Heavy Duty Linear Actuator F