In the two previous entries we have seen the criteria to take into account when choosing an actuator or motor to use in our Arduino projects, and the main types of rotary motors that we have available, with their advantages and disadvantages.
Finishing with the selection guide seeing the main types of linear actuators available, with their operation, characteristics, advantages, and disadvantages.
The explanation of the operation of each type of actuator will be quantitative and without equations, since entering into details of each of them would give rise to a chapter of an electronics book. The objective is to expose in a simple way how they work and their advantages and disadvantages.
Electromagnets
An electromagnet is a device that allows attracting ferromagnetic objects, such as Fe and some of its alloys.
An electromagnet is formed by a coil wound around a ferromagnetic core. When a current is applied to the coil, a magnetic field is generated inside it, which generates an attractive or repulsive force on other materials.
The ferromagnetic core of the electromagnet increases the power of the magnetic field and reduces losses due to dispersion.
We can use an electromagnet in our projects to create a small crane, or installed in a robotic arm to lift objects, or even to fix a robot to a metal plate.
Magnetic actuators
Magnetic actuators are similar to an electromagnet, but in which the magnetic core is movable. At the end, there is often a piece of plastic, for example, with a small clamp.
When the current circulates, the core is attracted into the interior of the electromagnet. There is a spring that returns the internal pin to its original position when the current ceases.
This type of magnetic actuators is widely used. They are very fast, but have the disadvantage of having a small travel.
Linear actuators
Linear actuators are formed by a direct current motor and a rod driven by a worm gear, inside a compact integrated unit.
This type of actuators is available in a wide variety of lengths, some of them very long. They are capable of exerting great forces. The disadvantage is that they are quite slow devices.
These types of linear actuators are capable of moving large loads and are used, for example, to mechanize awnings, lift loads.
Motors with worm gear and slider
Similar to the previous ones, these actuators are also formed by a direct current motor that drives a worm gear. But in this case, there is an intermediate element that moves along the path.
The slider is registered so that it does not rotate with the screw. This register can be by means of one or several guides, or a lateral register on one or both sides.
In this case, the characteristics of the actuator will depend on the motor used, being able to find a DC motor or a stepper motor.
Linear motors
Linear motors work in a similar way to a stepper motor, but instead of rotating a rotor, a slide moves linearly.
They are made up of one or several rails, along which alternating polarity magnets are placed. Depending on the typology of the motor, on, between, or around the rail there is a slide with coils controlled electronically.
By varying the polarity of the magnetic field generated in the coils installed on the slide.
Linear motors have high speed, high torque, and high precision in movements. They are used, for example, as substitutes for endless slides in industrial CNC machines. You can also find them in magnetic levitation trains, and even in advanced weaponry such as railguns.
However, linear motors have a prohibitive price for home projects. (If you ever have access to one of these, you can lend it to me, I want to play with one
Hydraulic and pneumatic
Of course, we could not talk about actuators without talking about hydraulic or pneumatic devices.
In these systems, a fluid (oil or water in hydraulic systems, air in pneumatic systems) is driven along a series of pipes and actuators. Control is achieved by acting on electronic valves located along the ducts.
Hydraulic systems are capable of exerting great forces with great precision. They are widely used, from excavators, presses, and all kinds of automations.
Pneumatic systems have less precision and force, as they work with gas and this is a compressible fluid. However, they are faster than hydraulic systems. They are also widely used in industry, to operate tools, expel parts, or make movements.
However, hydraulic and pneumatic systems generally require the use of expensive and bulky equipment, so they are beyond the reach of most home projects. Seriously, do not try to use these systems if you are not very clear about what you are doing.
Summary table
The above information is summarized in the following table. Of course, it will depend on the particular model of each chosen motor, but in broad strokes and by way of summary,
| Characteristics | Control (*) | |
|---|---|---|
| Speed | Force/Torque | |
| Electromagnet | – | ▲High |
| Magnetic actuator | ▲High | – Medium |
| Linear actuator (1) | ▼Low | ▲High |
| Linear actuator (2) | ▼Low | ▲High |
| Motor with worm gear and slider (1) | ▼Low | ▲High |
| Motor with worm gear and slider (2) | ▼Low | ▲High |
(*) With encoder, all of them pass to absolute control of position and speed (1) Driven by DC motor (2) Driven by stepper motor
Conclusions
Most of the devices invented by humanity to generate movement are rotary machines. To generate linear movement, it is most common to use a rotary motor accompanied by some mechanism that converts the movement into linear.
The electromagnet is a simple machine that we can use to attract ferromagnetic objects and lift loads. They can exert great forces, but act at small distances. In addition, we have no speed or position control.
Electromagnetic actuators use an electromagnet to attract a small actuator. They are very fast, but the action distance is very small, and the force they can exert is usually small.
Linear actuators allow moving exerting large loads and can have quite long runs. In return, they are slow mechanisms. They can be combined with other types of mechanisms (levers, scissors, joints, etc.) to expand their range of action and speed, but decreasing the exerted force. They can be used, for example, to retract an awning, lift platforms, open a door, for example.
Another common device is a motor with worm gear and slider. In general, they have low speed, high precision, and high torque, although it depends largely on whether it uses a DC motor or a stepper motor. They are very common in linear applications that require high precision, such as CNC machines or 3D printers.
Finally, linear motors are rare and extremely expensive. On the other hand, hydraulic and pneumatic solutions are only recommended for professionals and installers with experience, as they require a large number of elements and safety measures.
Download the code
All the code from this post is available for download on Github. 