Actuators 101. What is an Actuator, what do they do, how do they work.

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. 

Actuators 101 what is an actuator

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

Lets 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 also with different force levels. The temperature setting is actually controlled via 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, that is that the relay actuates and electrical signal or connection of some type.  Even though this may sound like an electrical components 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 as you can see it is a form of actuator just on a small scale.

For the purpose of this article, we will focus more on Linear Actuators. The term Actuators is actually very broad and can cover Rotary Actuators, Solenoids, and other types too. 

actuators

Solenoid Actuators

Sticking with the Automotive these let's explain 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 is created and visa versa. These are very simple on/off type actuators with little control options other than to turn them on or off. For example you have no real control over speed or force, in fact to make it worse solenoid actuator also has very limited stroke. Its 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 to a solenoid actuator and that's 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. They probably look unfamiliar but that is because most people don't get to see inside the door panels of a car. 

type of actuator

Piezo Actuators

These actuators movement comes from been 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, its very brittle and will have many layers with metal plates between each layer so that 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 move movement and trade-off force for stroke. The amplification could be done mechanically almost like a lever idea but they 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 and the control level needs to be down to the micron level. 

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 force can be achieved 
  3. Little sound is emitted apart from when the pump needs to replenish the high-pressure 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 cannot be allowed to leak of the system fails
  3. Air is a compressible gas and as such it means as a pneumatic actuator is moving a high force, there is always a lag because the gas will naturally compress first before it moves the piston inside the actuator. This means there will be a lag in the system. Hydraulic Actuators solves 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 or when a large amount of linear distance is needed to be covered quickly, such as inches per second or 30 inches per second. Once installed they are hard to move from one place to another as they do 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 major advantage, these systems are capable of immense forces and is why you see them used exclusively on heavy duty construction equipment, 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 all that hydraulic fluid in. So, if you have a lot of space and money and require a very high amount of force then 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 force can be achieved 
  3. Very long strokes are possible
  4. Extremely high cycle reliability and durability.
  5. The Actuators can actually be very small and compact in size since they are quite simple in construction. 

Drawbacks of hydraulic actuators:

  1. There are drawbacks to using hydraulics from an operating standpoint. The main one being control. You have very little precision control when it comes to these systems.
  2. Hydraulic fluid is required for the system to work and if this leaks the fluid can be very toxic 
  3. When the hydraulic pump is operating it can be noisy, and the higher the force required the higher the noise
  4. Hydraulic fluid has a very low viscosity, so it does not flow smoothly through pipes and valves etc, this all requires energy to push all that fluid at high pressure through all these pipes and fittings etc. As a result hydraulic systems are very inefficient to operate and use. 
  5. These systems are expensive to buy and install. 

Rotary Actuators

A rotary actuator is an actuator that produces a rotary motion, which makes them ideally suited to opening and closing valves. There are many different ways to create rotary motion and thus a rotary Actuator. The differences come in the form of the application. For example in the picture above you can see that rotary motion is been created via a rack an pinion style movement where the "Rack" is been controlled as a piston. The piston can be either hydraulically controlled or using high pressure air and gas to be pneumatically controlled. So what would the difference be?. If the rotary actuator above is hydraulically controlled then the the forces exerted can be huge and so this would suite industrial applications when large forces are required 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 is what will 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 then the piston needs to slide further, alternatively and different gear ratio is used to create 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, but 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 that via some gears the speed is reduced and torque increased 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 control system that translates into 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 you can see 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 due to the motor rotating the leadscrew directly or via some gears, the nut (as shown in yellow below) slides up and down the leadscrew in a linear motion and creates that 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 engineering 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 from the motor gives different speed linear strokes. And therefore the more force from the motor is applied to the leadscrew, the more force is given to the Nut that slides up and down the Leadscrew. The nut is attached to what we call the Rod, and its this Rod that is what you attach things to 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 to be used by the sliding Rod. 

There are different ways to create the torque of 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, but there is a trade-off, the higher the force, the lower the speed, conversely, the higher the speed the lower the force. To gain extra speed for a given force means having to use a larger input motor, and that requires more current and a larger motor and thus more money.

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 try to move away from its magnetic field. Whilst both are very different, they share the same name and they both result in what their name suggests...they actuate.  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 stroke of the actuator required is small. Perhaps you need to move something small or not very far then 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 less due to the smaller motors built into them.

Lets summarize what is an Actuator?

Actuators comes in many different forms, from rotary to linear, the type required is dependent on the application they are to be used for. 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. 

electro mechanical linear actuators

 

 

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