Actuators

Types of Actuators by Power Transmission

There are three main types of linear actuators which are distinguished by their respective methods of power transmission. These include the pneumatic, hydraulic, and electric linear actuators, each with specific pros and cons which determine the situations in which they are best used. 

Both pneumatic and hydraulic linear actuators are made of a piston residing within a hollow cylinder. An external pump or compressor is used to pressurize fluid and move the piston. The piston can then be returned to its original position by force applied by either a spring in the case of single-acting systems or by pressurized fluid in the case of double-acting systems.  Electric linear actuators work by rotating a lead screw or worm gear within the actuator housing which moves a lead or ball nut [1]. The lead ball or nut can be moved either forwards or backwards depending on the direction that the lead screw rotates, and limit switches may be used to stop the rotation once the lead ball or nut reaches the end of the stroke. A gear system may be used to attain the correct torque ratio [2]. 

Types of Actuators

Actuators are components that allow for controlled positioning or movement with mechanical or electro-mechanical power transmitted in any one of a variety of methods. Actuators are categorized by both their type of motion and their means of power transmission. For clarity a chart of some of the many types of actuators is provided below [1][12].

Characteristics

Pneumatic Actuators

Energy Efficiency

Double-acting systems have return strokes that are faster and stronger in addition to often having a longer lifespan than their single-action counterparts, however, they are more expensive. Conversely, single-action systems offer a simpler more compact design, and only require half the compressed gas to operate[3]. Pneumatic actuators are the least energy efficient system with efficiency ranging between 23 to 30%. The efficiency is also dependent on a variety of factors including air and seal quality, wear, and leaks, requiring maintenance to maintain efficiency. As a result maintenance and operating costs of pneumatic actuators may be much more significant than the cost of the components themselves. Data from the U.S Department of Energy on air supply efficiency and compressor cost is shown below[4].

Other Characteristics

Most pneumatic actuators have max pressure ratings of 150 or 250 psi with bore sizes ranging from 1/2-8" and 1/2-14" in the case of aluminum and steel actuators respectively. This results in a generated force ranging from 30 to 7500 lb and 50-38,465 lb. Pneumatic actuators may have an accuracy of linear motion within 0.1 inches and a repeatability within 0.001 inches. Pneumatic actuators may be used in a variety of environments including those with temperature ranges of approximately -40°C - 120°C, or which require machine safety or explosion protection requirements due to their lack of magnetic interference and lack of hazardous materials. Limitations of the compressor and air delivery systems may cays lower pressures which in turn result in both lower forces and lower speeds. Compressors must maintain their pressure continuously regardless of if they are in use or not, increasing energy usage. Furthermore, air quality may be diminished by contaminants such as oil or lubrication which can increase maintenance requirements. Pneumatic actuators are able to provide greater force than electric actuators, but less force than hydraulic ones [1].

Hydraulic Actuators

Hydraulic actuators can produce the greatest amount of force of the three types, with great force to volume and force to weight characteristics.  Unlike pneumatic actuators, hydraulic ones can maintain their force and torque without supplying more fluid. Hydraulic actuators undergo little power loss when pumps and motors are located far away. Hydraulic actuators can also leak over time, which can lead to a loss in efficiency in addition to cleanliness issue and damage to surrounding components. Similarly to pneumatic actuators, hydraulic systems require maintenance whose price will add up [5]. Another drawback of hydraulic actuators is their requirement of many components such as fluid reservoirs, motors, pumps, release valves, heat exchanges, and noise-reduction equipment. Hydraulic actuators may have a force to volume ratio 25 times greater and a horsepower to weight ratio 1 to 2 hp/lb greater than pneumatic actuators. Hydraulic actuators are moderately energy efficient at about 40% [4]. 

Piezoelectric Actuators

The application of a voltage across certain materials causes the materials to expand via the piezo effect. Such a process requires high voltages in return for miniscule expansions, resulting in very fine position control. Piezoelectric actuators have tiny ranges of motion and undergo hysteresis which makes them more difficult to control with high repeatability. Additionally they are very expensive, slow, and require a force to return them to their initial position [12]. On the other hand piezoelectric actuators have fast response, high acceleration rates, high power generation, high mechanical power density, and a compact design. Furthermore, piezoelectric actuators only consume power when in use, can operate at cryogenic temperatures, can operate in a vacuum, and are unaffected b magnetic fields. There are several kinds of piezoelectric actuators, such as the strip and stack actuators which includes the discrete stack and co-fired stack types. [10]The expansion of the piezoelectric actuator is equal to the applied voltage times the piezoelectric coefficient (d33), a material specific coefficient. The piezoelectric coefficient is a measure of the materials efficiency at transferring between electrical and mechanical energy. The scale of some types of piezoelectric motion is a hundred μm supporting 7 kN/ cm² [11]. There are many different types of piezo electric actuators including the square, square with through hole, round, ring, tube, shear, and stripe types.

Terminology

Two common terms which describe a piezoelectric actuator is free deflection (X_f) and blocking force (F_b). Free deflection is the displacement of the actuator when the max voltage is applied, while blocking force is the force applied when this displacement is restricted. The actuator is considered optimized for an application when it is in the max work condition, when the required force is applied at 1/2 the free deflection.  A force-deflection graph with important points can be found below. [12]

Loading

It is important to ensure that piezoelectric actuators are mounted in such a way that the applied stress is axial and only compressive. Fortunately there are a variety of end pieces which can be selected between to ensure proper loading [13].

Stack Actuator

The dimensions of the piezo element does not have any impact on the movement produced, however, stacking elements can multiply the effect. When the same voltage is applied across multiple stacked piezo elements there is a multiplicative effect on the movement produced. For example, a stack of 5 piezo elements would have five times the movement of a single piezo element at the same applied voltage. Piezoelectric actuators can be low or high voltage with max voltages between 200V-1000V, with co-fired actuators being low voltage and discrete stack actuators being high voltage. Co-fired multilayer stack actuators, or monolithic stacks, are composed of a complete ceramic and electrode pile which is sintered together. They are often rectangular since this shape is easier to cut and manufacture. Both types of stack actuators are available with protective casings for mechanical, environmental protection or to supply prestress [11].

Stripe Actuator

Stripe Actuators, or bending actuators, are another type of piezoelectric actuator. Stripe actuators are composed of two thin piezoelectric ceramic strips which are attached one on top of the other and electrically connected in parallel. One ceramic strip expands while the other contracts as the actuator is powered, causing the actuator to bend along its top face. Stripe actuators have greater deflection than other piezoelectric actuators but can apply a much smaller force. A cantilever mounted standard sized strip actuator may have a deflection of about 2.5mm but only supply a force of only 0.60N at a max voltage of 150V [12].

Electric Actuators

Electric actuators contain an integrated motor unlike either the pneumatic or hydraulic systems. It is able to provide a high degree of precision and repeatability without taking up a large amount of space. Electric actuators are much easier to control, offering immediate diagnostic feedback, the use of encoders for velocity, position, torque, and force control, and are able to be reprogramed with ease. Electric actuators are quieter than either pneumatic or hydraulic actuators, and do not contain fluids which may leak. These actuators have a greater initial cost, for example Bimba Manufacturing electric actuators range in price from between $150USD to more than $2000USD (2015)[1]. This cost is reduced over the lifetime of the actuator to be comparable to pneumatic actuators once replacement and operating costs, process efficiency is considered, especially at moderate scales where motion system components can be replaced separately [11]. They are not safe for all environments including flammable areas since their motors may overheat if left to run continuously [1]. This overheating may also increase the wear on the reduction gear. Another issue with electric actuators is that they must be capable of satisfying the instantaneously highest torque requirement, leading to the necessitation of a large motor which will often be used below its max performance [11]. In general, electric actuators have the highest energy efficiency of about 80% and this efficiency does not change dramatically with use [4].

Pneumatic and Electric Actuator Selection Criteria

Other Considerations

Fire Risk

Proper protocols and design may be required to reduce the fire risk presented by some hydraulic systems. While some hydraulic fluids are specifically designed to be fire resistant, and any good hydraulic fluid should have a high ignition temperature, low vapor pressure at operating temperatures, and a high flash point, once ignited some hydraulic fluids can burn vigorously. Since hydraulic fluid is under tremendous pressure this can result in a high power stream of flaming oil. Most cases of flued ignition occurs after a leak, with broken bearings being among the most common causes of fluid ignition [9].

Hydraulic Actuator Repair Estimator

Position Control and Stick Slip

Both pneumatic and hydraulic systems are made more difficult to control, in part since they can undergo stick slip which causes jolts of movement when moving after having stopped. This jerking movement can make it difficult to accurately position the actuator due to overshooting. This stick slip is caused by the transition from the relatively high static friction to the lower dynamic friction and by the Stribeck Curve. The Stribeck Curve shows the relationship between the friction and velocity of a lubricated recirculating bearing. The initial motion of the bearing pulls lubricant into the contact area between the surfaces reducing friction as velocity increase. After a certain point the friction will increase due to viscous lag. The exact behavior is determined by the type of lubrication used, so proper lubrication selection is important [6][7].

Common Applications

Pneumatic Actuators

Perhaps the most ubiquitous application of pneumatic actuators is in their use in the engines of gas powered vehicles. The pressure from the ignition of the air-gasoline mixture within the ignition chamber is used to extend the piston [4]. 

Hydraulic Actuators

Hydraulic actuators are used extensively in aerospace applications. Hydraulic actuators are used for flap positioning, brake systems, landing gear movement, and cargo ramp opening and closing. [8]

Electric Actuators

Some common applications of electric actuators is in sliding doors, windshield wipers, and adjustable seats. [2]

Piezoelectric Actuators

The precise but fine movement caused by piezoelectric actuators makes them well suited for adjusting equipment such as lenses, mirrors, and machining tools. Piezoelectric actuators can also be used for small-volume pumps and hydraulic valve control. [10]

References

1. C. Gonzalez, "What’s the Difference Between Pneumatic, Hydraulic, and Electrical Actuators?," 16 April 2015. [Online]. Available: https://www.machinedesign.com/mechanical-motion-systems/linear-motion/article/21832047/whats-the-difference-between-pneumatic-hydraulic-and-electrical-actuators.
2. Butterfly Valves & Controls , "What Is An Electrical Actuator And How Does It Work?," [Online]. Available: https://valuebutterflyvalves.com/what-is-an-electrical-actuator/.
3. SEMCOR, "How Pneumatic Actuators Work and Advantages of Pneumatic Actuators," 2016. [Online]. Available: https://www.semcor.net/blog/advantages-of-pneumatic-actuators/.
4. A. Dietrich and Tolomatic, "Electric actuators vs. pneumatic cylinders: Total cost of ownership," 10 June 2015. [Online]. Available: https://www.linearmotiontips.com/electric-actuators-vs-pneumatic-cylinders-total-cost-of-ownership/.
5. Duff-Norton, "Hydraulic vs. Electric Linear Actuator: Which is Best?," 1 January 0001. [Online]. Available: https://www.columbusmckinnon.com/en-us/resources/duff-norton-blogs/hydraulic-vs.-electric-which-is-best/.
6. D. Collins, "What is stick-slip?," 1 December 2016. [Online]. Available: https://www.linearmotiontips.com/faq-what-is-stick-slip/.
7. Y. Wakasawa, "Stick-Slip Happens, But How?," 7 June 2016. [Online]. Available: https://www.hydraulicspneumatics.com/technologies/cylinders-actuators/article/21885171/stickslip-happens-but-how.
8. AeroToolbox, "Aircraft Hydraulic Systems," 2017-2020. [Online]. Available: https://aerotoolbox.com/hydraulic-system/#Design_of_an_Aircraft_Hydraulic_System.
9. If P&C Insurance Ltd, [Online]. Available: https://www.if-insurance.com/large-enterprises/insight/if-news/property/how-to-prevent-hydraulic-oil-fires

10. American Piezo, "Piezo Actuators," APC International, 2016. [Online]. Available: https://www.americanpiezo.com/piezo-theory/actuators.html. [Accessed 16 December 2020].

11. American Piezo, "Stack Actuators," APC International, 2016. [Online]. Available: https://www.americanpiezo.com/standard-products/stack-actuators.html. [Accessed 16 December 2020].
12. Collins, "What are piezo actuators?," WTWH Media LLC, 23 December 2015. [Online]. Available: https://www.linearmotiontips.com/what-are-piezo-actuators/. [Accessed 16 December 2020].

13. American Piezo, "Stripe Actuators," APC International, 2016. [Online]. Available: https://www.americanpiezo.com/standard-products/stripe-actuators.html. [Accessed 16 December 2020].

14. S. Alfayad, F. B. Ouezdou, F. Namoun and G. Gheng, "High performance integrated electro-hydraulic actuator for robotics – Part I: Principle, prototype design and first experiments," Sensors and Actuators A: Physical, vol. 169, no. 1, pp. 124-132, 10 September 2011. 
15. Industrial Media LLC, "Costs And Capabilities Of Pneumatic, Electric Actuators," 17 November 2016. [Online]. Available: https://www.manufacturing.net/home/article/13194104/costs-and-capabilities-of-pneumatic-electric-actuators.
16. Thomas, "Complete Guide to Actuators (Types, Attributes, Applications and Suppliers)," [Online]. Available: https://www.thomasnet.com/articles/pumps-valves-accessories/types-of-actuators/#electric_rotary.