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Actuators 101: What is an Actuator, what do they do

What is an Actuator Pneumatic Actuators Hydraulic Actautors Rotary Actuators Electric Linear Actuators

An Actuator is a device that requires an energy source input usually electrical energy, an external signal input in some form to tell the actuator what to do, and then the device actuates. The output in the form of a motion, can be either rotary or linear, and is used to achieve a desired outcome in a system.

The term Actuator comes from the act of Actuating something, in other words to Actuate is to operate something.  So to simplify the expression of what it does, an actuator reads a signal and then it actuates, or it operates. Actuators are typically part of an overall system or machine or device. It is a component within that machine that does something by making it move. 

For an actuator to work it requires an energy source input, usually electrical energy. It also requires an external signal input in some form to tell the actuator what to do, and then the device Actuates.  The output is usually in the form of a motion that can be either Rotary or Linear that's used to achieve the desired outcome in a system. 

History of Actuators

Actuators have been around for over a 100 years and their name came from what they do, they Actuate something. That is they move something by opening or closing, pushing or pulling, lifting or dropping etc. The most common type of actuator that you use every day is the solenoid actuator to lock and unlock your car door, or an electric linear actuator used to open and close the trunk in a car. Those are very common type of Electro-mechanical actuators that are used extensively in our daily lives. Before electricity was created they were still made but would be human controlled, such as a latch on a door.

Where are Actuators used?

There are over 50 Actuators used in a modern car, auto's probably have the most actuators we would use as part of our daily lives. A car uses them in the fuel injectors, valves for the fuel supply and management, heating and cooling systems, even the entertainment systems can use them to open and close speakers, GPS screens and so on.

Actuators 101 - What is an actuator and how does it work

what is an actuator

Let's look at a typical example of an actuator system used in our every day lives. The heating in a car has both a hot and cold temperature settings, as well as a fan with different force levels. The temperature setting is actually controlled by an actuator that regulates how much air flows over a heat exchanger. That actuator controls the air flow position, the more it flows over the heat exchanger the hotter the air is, conversely, the further away it is from the heat exchanger the cooler it is. 

actuators in cars

Is a Relay an Actuator?

A Relay is also sometimes considered to be a form of Electrical Actuator, meaning the relay actuates and electrical signal or connection.  Even though this may sound like an electrical component with no moving parts, it actually does have a moving component to it. A relay is a magnetically charged coil that opens and closes a connector via an electro-magnetic field. So, technically, it is a form of actuator, just on a small scale.

For the purpose of this article, we will focus more on Linear Actuators. This example is intended to illustrate how the term "Actuators" is actually very broad and can also cover Rotary Actuators, Solenoids, and other types too. 


Solenoid Actuators

Sticking with the Automotive, let's explore another very common actuator type, and that is the Solenoid Actuator. Solenoids work like a relay, they take in an electrical current and create an electro-magnetic field, its that magnetic force that then makes a rod move in and out. Typically, the higher the magnetic field that's supplied to the solenoid actuator, the more force it creates, and visa-versa. These are very simple on/off type actuators, which means very few control options. For example, solenoid actuators have no real control over speed or force, and also a very limited stroke length. It is rare to find a solenoid Actuator with more than 2" (inches) of stroke.

solenoid actuators

The central locking on car doors are the most common types of solenoid Actuator used. they simply connect and disconnect the latch from the door handle. The control mechanism is also very simple; a single pulse of 12v DC electricity is sent to the solenoid to actuate it, and a spring is what makes it return.

Below is a typical solenoid actuator, as used in most cars. If they look unfamiliar, it's because most people don't see inside the door panels of a car. 

type of actuator

Piezo Actuators

These actuators movement comes from being energized by voltage and they require very large voltages to make them expand and contract, typically over 200V. The Piezo material is a type of ceramic, it is very brittle and will have many layers with metal plates between each layer so each piezo stack gets energized.

Large amounts of voltage is required for very small change in length, typically a Piezo will only expand by about 1% of it size, but their force is very high, this means that you can amplify the expansion of the Piezo stacks to get more movement, but trading force for distance. The amplification could be done mechanically, such as with a lever, but Piezos are typically used in applications where you need very high precision and control. They are most commonly used as fuel injectors for cars, where the Piezo actuator controls the fuel volume entering the cylinder; where the control level needs to be down to the microns (one millionth of a meter). 

Piezo Actuators

Pneumatic Actuators

These types of actuators use pressurized gas or air in a cylinder created my a high-pressure pump to move a piston to create linear motion. Like hydraulic actuators, the design of pneumatic linear actuator has been around for a long time. An air compressor is used to pressurize the air or inert gas in a tank, and that high pressure air is used to make the actuators piston slide in and out. Once the piston in the actuator has reached the end of the travel, a valve switch is then moved to open the valve to the other end of the actuator where again high-pressure air then pushes the piston in the actuator the other direction. 

Pneumatic actuators

Benefits of using pneumatics are:

  1. High speed is possible and is controlled by the pressure valve and volumetric capacity of the system.
  2. Fairly high forces can be achieved.
  3. Little sound is emitted apart from the pump pressurizing the tank.
  4. Very long strokes are possible.
  5. Extremely high cycle reliability and durability.
  6. The Actuators can actually be very small and compact in size since they are quite simple in construction. 

Drawbacks of pneumatic actuators:

  1. Additional equipment is required such as a tank and high-pressure pump.
  2. The entire system can not be allowed to leak of the system fails.
  3. Air is a compressible gas, meaning when a pneumatic actuator is moving a high force, there is always a lag because the gas/air will naturally compress first before it moves the piston inside the actuator. This means there will be a lag in the system. Hydraulic Actuators do not have this problem.
  4. Very low positional control is achievable. Watch the video below where we use Lego to demonstrate the lack of control compared to a mechanical Actuator, and using a DTI (Dial Test Indicator) to show the difference

Where are pneumatic actuators used?

They are used where high-speed motion is required, upwards of 30 inches per second. Once installed they are hard to move from one place to another as they require a lot of installation time. These Actuators are found on the assembly lines of manufacturing factories as they are ideal for performing millions of cycles with no maintenance, and they can move very quickly. 

Hydraulic Actuators

Hydraulic Actuators operate in exactly the same way as Pneumatic actuators, except instead of using high-pressure air or gas they use a non-compressible liquid called hydraulic fluid. Because the fluid is non-compressible it has a huge advantage over pneumatics, these systems are capable of immense forces. This is why you see them used exclusively on heavy duty construction equipment like diggers, dump trucks, forklifts trucks, tractors etc.

hydraulic Actuators

How do Hydraulic Actuator work?

Hydraulic Actuator use high-pressure fluid to push a piston backwards and forwards where the switching is done through valve switches. These systems require high-pressure pumps, high-pressure valves and piping, and a tank to hold hydraulic fluid in. So, if you have a lot of space and money and require a very high amount of force, hydraulics could be the way to go.

Benefits of using hydraulic actuators are:

  1. Moderate speed is possible and is controlled by the pump speed.
  2. Extremely high forces can be achieved. 
  3. Very long strokes are possible.
  4. Extremely high cycle reliability and durability.
  5. The Actuators can be very small and compact in size since they are quite simple in construction. 

Drawbacks of hydraulic actuators:

  1. Control. Hydraulic Actuators have very little precision control.
  2. Hydraulic fluid is required for the system to work, and the fluid is very toxic. If the system fails, it could leak.
  3. When the hydraulic pump is operating it can be very noisy, and the higher the required force, the louder the noise.
  4. Hydraulic fluid relies on predictable viscosity, so it does not flow smoothly through pipes and valves etc., this requires additional energy to push fluid at high pressure through pipes and fittings. As a result, hydraulic systems are very inefficient to operate and use, especially in a varying climate.
  5. Price. These systems are expensive to buy and install. 

Rotary Actuators

A rotary actuator is an actuator that produces a rotary (spinning) motion, which makes them ideally suited to opening and closing valves. There are many different ways to create rotary motion, and thus many ways to create a rotary Actuator. The differences come in from the type of application. For example; In the picture below you can see that rotary motion is being created by a rack-and-pinion style movement where the "rack" is being controlled as a piston. The piston can be either hydraulically controlled, or pneumatically controlled. So, what would the difference be? If the rotary actuator below is hydraulically controlled then the the forces exerted could be huge, which could suit industrial applications requiring large forces to open and close a valve. If this rotary actuator is pneumatically controlled, then the actuator may require less force to rotate the main shaft, which will, in turn, be used to perform the required tasks. 

rotary actuators

Rotary Actuator principle

The motion produced by a rotary actuator may be either continuous rotation, as seen in an electric motor, or movement could be a fixed angular rotation. With a rotary actuator that's pneumatically or hydraulically controlled they are more likely to be a fixed angular rotation type, this is because the rack or piston that rotates the main shaft can only move so far and so the rotational movement is restricted by the linear stroke available. If more rotation is required, the piston would need to slide further, and a different gear ratio is used to translate the motion. 

rotary actuator gear ratio

Servo Rotary Actuator

Another type of rotary actuator is a servo motor and stepper motor. These are electrically controlled actuators that have a constant rotational movement, and also offer very precise rotational control. 

Servo rotary Actuator

These types of actuator are commonly used in Robotics, and consumer electronics where rotational movement and torque is produced by a rotary motor. The speed is reduced and torque increased by a gear system to create the rotary motion. To get precise control, the actuator will have a sensor that measures position. This is usually in the form of a hall sensor or encoder that sends a signal back to the 'brain' to translate into a position. A great feature of servo motors is that they can be made very small and used in very tight places. 

Converting rotary motion from a servo actuator motor to linear motion

 converting rotary motion from a servo motor into linear motion

Because rotary servo actuators are so commonly used and relatively inexpensive to buy its has become a popular way for people to create linear motion. Through simple linkages and some form of linear guiding system it is possible to create linear motion. The stroke that results will be directly proportional to the lever arm length as seen in the picture above. The longer the arm from the servo actuator, the longer the stroke will be; however the downside is that the force will be reduced because the torque is proportional to the arm length. 

Below torque equation for rotary actuators

torque equation for rotary actuators

Electro-Mechanical Linear Actuators.

With Electric Linear Actuators, rotary motion from an AC or DC motor is converted to linear motion via a Leadscrew. A Leadscrew is basically a helical gear machined onto a Rod. As the Leadscrew rotates from to the motor, the nut (as shown in yellow below) slides up and down the leadscrew in a smooth linear motion, translating rotational movement into linear movement - Hence the name "Linear Actuator". This is very different to a solenoid actuator, which is still a form of Linear Actuator, but in the industry, engineers typically differentiate the two by calling them "Solenoid Actuators" and "Linear Actuators" even though both output linear motion.

linear actuator a type of actuator

With Electric Linear Actuators, having different length Leadscrews gives you different stroke lengths. Turning the leadscrew faster or slower with the motor gives different linear speeds. The more force from the motor that can be applied to the leadscrew means more force is given to the nut that slides up and down the Leadscrew. The nut is attached to the Rod, and the Rod that is what you see and attach to the mounting bracket in order to create that linear motion.  The more torque that is able to be applied to the leadscrew, the more linear force will be available for the sliding Rod. 

There are different ways to create the torque in an actuator. Adding gear between the motor and the leadscrew is the most common method; the higher the gear ratio, the more force is created. There is a trade-off: higher forces mean lower speed, conversely, higher speed means lower force. Additional speed for a given force would require a larger input motor, which is physically bigger and draws more current to function; both the size and the power make it more expensive.

Electric Linear Actuators

An electric actuator is a device that converts the rotational motion of a motor into linear motion, or takes electrical current to create an electro-magnetic field and uses magnetism to force a metal object away from its magnetic field. While these types are very different, they share the same name and both provide actuation.  This means they all provide both push and pull movements in either a linear or rotational movement. 

For a more detailed overview on how an Electric Linear Actuator works, we created this article "Inside a Linear Actuator - How an Actuator works"

If you are looking to purchase an electric linear actuator we have created an article called “Don’t by a Linear Actuator until you read these 5-steps” this can help you avoid some common problem before spending any money.

Micro Linear Actuators

micro actuators

 Micro Actuators or Mini Linear Actuators are used in applications where space is limited or the required stroke of the actuator is small. Perhaps you need to move something tiny a very short distance, a Micro Linear Actuator would be ideal for such an application.  Typically Micro Actuators strokes are 10mm to 100mm and are very compact in size.  One of the downsides of a Micro Actuator is that forces tend to be a lot lower due to the small motors built into them.

Lets summarize: What is an Actuator?

Actuators comes in many different types, from rotary to linear, hydraulic and pneumatic, solenoid and electro-mechanical. Each type has an ideal application. Large industrial rotary actuators that are hydraulically driven are great for opening huge oil-pipe valves, and micro actuators can be powered by small 12v power sources with great accuracy and precision for robotics and small applications. For more details on Acuators we have written a white paper that goes into a little more depth in the world of Actuator. Please read that article here

FIRGELLI®  actuators are specially designed and produced with high-quality materials to give you the perfect balance of power, control, and price to build your automation systems.

electro mechanical linear actuators

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