Stepper motors, DC motors, servos and solenoids - there are so many things to choose from when you want to add some motion to your project, but which one is the most appropriate for a given situation? They all have their own unique quirks, pros and cons, and weird units like what is torque and what on earth is a kilogram centimetre? Well, all these questions and more are what we're going to be tackling today.

Hey everyone, Jared here. In this guide, we're going to be talking about some of the most common types of electrical actuators, but we won't be going into precise and detailed instructions on how to wire up and code a microcontroller for these actuators. More so, we'll be looking at the actuators themselves and some applications to hopefully equip you with the ability to know exactly what actuator you need for what job.

But that brings up a good question. What is an actuator? Simply put, an actuator is a device that produces motion from energy. DC motors, servos, steppers, solenoids, these are all examples of actuators as they convert electrical energy into movement. Actuators can also be pneumatic, meaning air powered, hydraulic, fluid powered, but we will be sticking with just electrical actuators.

There is such a wide range of these to choose from, each with their own abilities and limitations, but in general, you'll find that actuators produce either rotational or linear motion. Linear motion is motion in a straight line. So let's start by looking at linear actuators.

Now, the name is a little bit confusing, but when we say linear actuator here, we mean one of these. It has a little DC motor in here and a gearbox that converts that rotational motion into nice straight linear motion. Now, a linear actuator produces a force. Ah, what a good opportunity to talk a little bit about force. If we go to the data sheet of this linear actuator, we can find that it has a maximum output of 128 newtons.

Now, unless you're into physics, a newton isn't really a very intuitive unit of measurement. That's why there exists another much easier unit of force, the kilogram force. One kilogram force is equal to 9.8 newtons. So if we converted this from newtons, we get about 13 kilograms of force. Now, that is something that we're used to, and we know what it feels like. If you hold a one kilogram weight in your hand, like this one litre bottle of water, it applies one kilogram of force on your hand thanks to gravity. So you can easily imagine what 13 kilograms of force would feel like.

So that means if we hold this linear actuator above this set of scales here and extend it, it will read about 13 kilograms, and this is the maximum force that it can output. And of course, this also means you can lift up 13 kilograms with this linear actuator.

I'm going to give you an infinitely helpful little trick here. This printer weighs 8 kilos, so we should be able to lift it perfectly fine with our linear actuator. But what if we wanted to push it along the table? How do we know how much force we need? There is quite a bit of math involved in trying to figure that out, but these luggage scales are an incredibly helpful tool in finding this out. So we're going to wrap our scales around what we want to measure, give it a pull, and as you can see, we need about five kilos of force to move this thing. So 5.5 kilos to move it, our linear actuator here is going to have no trouble pushing that backwards and forwards.

There is a huge selection of linear actuators to choose from. This is a tiny one, it extends in and out about one centimetre and can apply 13 kilograms of force. But there are ones that can extend 10 centimetres, 30 centimetres, even a meter, and be able to output hundreds of kilos of force. But those ones might start to get a bit pricey. LinearActuators tend to have a mechanism called a worm gear, which turns the rotation of the internal DC motor into linear motion and prevents it from going the other way. Once a linear actuator has extended out to a distance, it tends to stay there, even when it's not powered on, due to the holding force or static loading.

Linear actuators are simple to operate, requiring just two wires to extend and retract by applying voltage in different directions. To control them with a microcontroller, a motor driver is needed due to the power draw being more than the microcontroller can supply.

A common application of a linear actuator is in sit-stand adjustable desks, where it moves the desk up and down in a linear motion. Linear actuators are not always the fastest, but they are valued for their holding force, allowing them to maintain a set position without power.

For more in-depth information and specifications on linear actuators, a guide is available for reference.

Moving on to servos, they are actuators that produce rotational motion and output a rotational force called torque. Torque is a rotational force measured in kilogram centimetres, and it can be understood as a force being applied at a distance through rotation.

An example with three identical servos, each outputting 10 kilogram centimetres of force, illustrates how the force applied at different distances results in different torques. The concept of load, which refers to a weight, force, or torque, is also introduced for further discussion.An actuator must overcome or move a load. An example of a load is a door in an automatic garage. The weight of the door itself is a load that the motor in the garage door opener must overcome. Another example is a DC motor. If powered on without anything connected, it would spin freely, known as no load conditions. However, if a load is applied by grabbing onto the motor shaft, the speed decreases. As the load increases, the speed decreases, and the current linearly increases. Under stall conditions, the actuator draws the most current and generates the most heat, which can quickly damage the actuator.

Servos are popular actuators that specialize in precise control of position. They can be set to an exact angle and have internal feedback mechanisms to maintain the set angle. They are easy to use and have a limited range of typically 180-degrees of motion. Servos have three wires for power supply and a PWM signal to set the angle. They are commonly used in remote control planes to rotate the control surface of the aircraft.

DC motors are the simplest of all actuators. They generate internal magnetic fields to produce rotational motion when a voltage is applied across the two wires. They come in various sizes and shapes and are among the cheaper actuator choices. Under no load conditions, DC motors can spin very fast, but these speeds may not be suitable for all projects.DC motors are commonly used for applications that require simple, constant rotational motion without the need for precise angles. They are ideal for driving the propeller of an RC plane, for example, as they provide infinite rotational motion and can be controlled by adjusting the voltage supplied to the motor. However, DC motors typically do not produce a significant amount of torque, which may limit their ability to perform certain tasks, such as moving an RC car forward without a gearbox to increase torque. Therefore, it is common for DC motors to come with inbuilt gearboxes to reduce speed and increase torque, with various gear ratios available to achieve the desired torque and speed balance.

When using DC motors with a microcontroller, it is important to use a motor driver to power the motor with an external power source and control its speed and direction. This ensures that the motor does not draw too much power from the microcontroller, making it a straightforward process to manage.

Stepper motors, on the other hand, offer a unique operation compared to DC motors. They generate magnetic fields and use a team of coils to step the motor forward one position at a time, hence the name "stepper motor." Unlike DC motors, when a stepper motor is powered, it can hold its position, making it suitable for applications that require precise movement control. Stepper motors can also perform micro-stepping, allowing for even more precise control over their movement.

While stepper motors offer advantages such as precise movement control and the ability to hold position when powered, they are more complex to control and may have varying numbers of wires depending on their design. To effectively control a stepper motor, a stepper motor driver is required, along with a library on the microcontroller to facilitate easy control of the motor's direction and speed.

A notable application of stepper motors is in 3D printers, where they are used to precisely move the printhead using belts and pulleys. This demonstrates the versatility and precision that stepper motors can offer in various applications.DC motors are ill-suited for this, as they don't have the required level of precision, and servos are ill-suited as they have a limited motion.

Another great application is in a vending machine. That little spiral that holds your food and drink, that is spun by a stepper motor, as it only needs to rotate an exact amount from its current position.

And the final actuator on our list is the solenoid. These are beautifully simple actuators that produce nice and linear motion. A solenoid uses an electromagnet to push away this internal rod here, called a plunger, and a spring on the back here pulls it back to its original position, when the electromagnet is powered off. They are so simple because they are either on and off, and have two positions to represent those states. Typically, they don't have much of a throw, which is how far that plunger can travel, and they usually have a throw of about a few millimetres to maybe a few centimetres. However, they do move between these positions incredibly fast, way, way faster than a linear actuator.

They aren't, of course, without their downsides, and that is that they don't like to be powered for very long. Usually, they can only be powered for like 5 to 15 seconds at a time, sometimes more, sometimes less, and your data sheet should have this number on it. They also aren't the strongest of actuators. This outputs 13 kilograms of force, and this one puts out half a kilogram of force. Obviously, there are more powerful ones out there, but they tend to be on the weaker side in comparison.

As always, these actuators are too powerful for your microcontroller, and so an easy way to control it would be with a MOSFET wired up as a switch. A fantastic example of a solenoid in use is in a pinball machine. That flipper, the little paddle that you whack the ball with, is driven by a solenoid. You only need two positions, extended and retracted. You need it to move quickly between these two, and it isn't a huge distance that it's moving, so a solenoid is perfect.

So, solenoids are great for when you need something to move between position A and position B very quickly, but with not too much force.

Well, there we have it. Those are five of the most common electrical actuators you will encounter. You should now be equipped with an understanding of each enough to be able to choose which one is appropriate for your next project. Again, we have a bit more of an in-depth guide below, and if you think you know what actuator you want to use, it is worth checking out as we go over some of the specifications you will encounter with those actuators.

If you have any questions about actuators or need any clarification, feel free to let us know on our forums. We're all makers here, and we're happy to help. Till next time.