Linear Actuator

Another type of electromagnetic actuator that converts an electrical signal into a magnetic field that produces linear motion is called a Linear Actuator. They are also called a linear solenoid.

The linear actuator works on the same basis as the previous tutorial, the electromechanical relay, which can also be controlled by open and bipolar transistors or MOSFETs.The "Linear Actuator" is an electromagnetic device that converts electrical energy into a mechanical thrust or tensile force or movement.

Linear Actuator
Linear Actuator

Linear actuators consist mainly of an electric coil wrapped around a cylindrical pipe with a ferro-magnetic actuator or "piston", which is free to move or slide in the form of "IN" and "OUT" of the coil body.Solenoids can be used to electrically open doors and latches, open or close valves, move and operate robotic limbs and mechanisms, and even operate electrical switches simply by energizing the coil.

Both solenoid types are available as linear and rotational, holding (continuously energetic) or latching type (ON-OFF pulse), and latch types are used in energetic or closed applications.Linear solenoids can also be designed for proportional motion control when the piston position is proportional to the power input.

When the electric current passes through a conductor, it creates a magnetic field around itself.The direction of this magnetic field relative to the north and south poles is determined by the direction of the current within the wire.Then, with the electric current passing through the wire coil, it becomes an "Electromagnet", forming its own north and south poles.

The power of this magnetic field can be increased or reduced by controlling the amount of current passing through the coil or by changing the number of rotations or cycles that the coil has.Below is an example of an electromagnetism.

Magnetic Field Produced by Solenoid Coil

Linear Actuator

When an electric current passes through the coil windings, it acts like an electromagnet, and the piston inside the coil is pulled towards the center of the coil by the magnetic flux pattern inside the body of the coil, which compresses the coil, the small spring attached to one end of the piston.The strength and speed of the piston movement is determined by the strength of the magnetic battery produced in the coil.

When the feed current is made "OFF" (without energy), the electromagnetic field previously produced by the coil collapses, forcing the piston back to its original immobile position.This back-and-forth movement of the piston, solenoids are known as "Stroke", that is, the maximum distance that the piston can travel in the direction of "IN" or "OUT", for example 0 – 30 mm.

Linear Actuator Structure

This type of solenoid is often called Linear Solenoid due to the linear directional movement of the piston.Linear solenoids are available in two basic configurations, "Tensile type" because they pull the connected load towards them when energized, and "Push type", which moves in the opposite direction by pushing it away from itself when given energy.Both push and pull types are usually done in the same way with the difference in the position of the return broadcast and the design of the planer.

Tensile Type Linear Solenoid Structure

Linear Actuator

Linear solenoids are useful in many applications such as electronically activated door locks, pneumatic or hydraulic control valves, robotics, automotive engine management, irrigation valves for watering the garden.

Rotary Actuator

Most electromagnetic solenoids are linear devices that produce a linear back and forth force or movement.However, there are also rotating solenoids that produce angular or rotary motion clockwise, counterclockwise, or from a neutral position in both directions (duplex).

Linear Actuator
Rotary Actuator

Rotary solenoids can be used to replace small DC motors or stepper motors, where the angular motion is very small, and the rotation angle is the angle that is moved from the beginning to the end position.

Commonly found rotary solenoids have movements of 25, 35, 45, 60 and 90 degrees, as well as multiple movements in this respect, such as 2-position self-restore or zero rotation return.

Rotary solenoids produce a rotational motion when energized, deerable, or when there is a change in the polarity of an electromagnetic field, when a permanent magnet changes the position of the rotor.Their structure consists of a magnetic disc attached to an output shaft placed on the coil and an electric coil wrapped around a steel frame.

When the coil is energized, the electromagnetic field produces multiple north and south poles, which push the adjacent permanent magnetic poles of the disc and cause the rotating solenoid to rotate at an angle determined by its mechanical structure.

Rotary solenoids are used in vending machines or gaming machines, valve control, with special high-speed, low-power or variable positioning solenoids, and variable positioning solenoids with high force or torque are available, such as those used in dot matrix printers, typewriters, automated machines or automotive applications. .

Solenoid Switching

Usually linear or rotary solenoids work with the application of a DC voltage, but they can also be used with AC sinusoidal voltages using full wave bridge rectifiers to correct the feed, which can then be used to replace the DC solenoid.Small DC type solenoids can be easily controlled using Transistor or MOSFET switches and are ideal for use in robotic applications.

However, as we have seen in electromechanical relays before, linear solenoids are"inductive"devices, so some kind of electrical protection is required along the solenoid coil to prevent high back-to-back emk voltages from damaging the semiconductor switching device.In this case, the standard "Flywheel Diode" is used, but in the same way you can also use a zener diode or small valuable varistur.

Linear Actuator
Replacing Solenoids Using a Transistor

Reducing Energy Consumption

One of the main drawbacks of solenoids, and especially linear solenoids, is that they are "inductive devices" made of wire coils.This means that solenoid coils have resistance and convert some of the electrical energy used to operate them into "ISI" due to the I 2 R heating effect of the wire.

In other words, when they are connected to an electrical source for a long time, the bandaged coils can heat up, and the longer the power applied to a solenoid coil, the warmer the coil.Also as the coil heats up, the electrical resistance changes, reducing both the current flowing from the coil and the magnetic field power, since this directly depends on the amperage rotations.

Since the input power is always on with a continuous voltage input applied to the coil, there is no cooling of the solenoid coil.To reduce this self-induced heating effect, it is necessary to reduce the amount of time energy is given to the coil or reduce the amount of current flowing through it.

One method of consuming less current is to apply a sufficiently high appropriate voltage to the solenoid coil to provide the necessary electromagnetic field to operate and place the piston, but then the coils are activated to reduce the feeding voltage to a sufficient level to protect the piston. One way to achieve this is to connect a serially suitable "holding" resistance with the solenoid coil, for example:

Reducing Solenoid Energy Consumption

Linear Actuator

Here, the key contacts are closed by short-circuiting the resistance and transmitting the full feed current directly to the solenoid coil windings.After energizing, contacts that can be mechanically connected to the solenoids are serialized with the R H solenoid coil, the piston movement that binds the holding resistance is turned on.This effectively binds the resistance to the coil in series.

Using this method, the solenoid can be connected to the voltage source indefinitely (continuous duty cycle) using an appropriate power resistance, as the power consumed by the coil and the heat produced are greatly reduced, it can be increased from 85% to 90%.But the power consumed by resistance will also produce a certain amount of heat, I 2 R(Ohm Act),and this must also be taken into account.

Solenoid Duty Cycle

A more practical way to reduce the heat generated by the solenoid coil is to use an "intermittent duty cycle", i.e. PWM.The intermittent duty cycle means that the coil is repeatedly "ON" and "OFF" at an appropriate frequency to activate the piston mechanism, but not to allow the waveform to be deered during the OFF period.Intermittent task cycle change is a very effective way to reduce the total power consumed by the coil.

The Task Cycle (%ED) of a solenoid is part of the "ON" time when a solenoid is energized, and the ratio of "ON" time for a full cycle to the total "ON" and "OFF" time.In other words, the cycle time is equal to the opening time plus the closing time.The task cycle is expressed as a percentage, for example:

Linear Actuator
Linear Actuator

Then, if a solenoid is turned on or energized for 30 seconds and then "OFF" for 90 seconds before re-energization, a full cycle will have a total "ON/OFF" cycle time of 120 seconds, (30 +90), the task cycle of solenoids will be calculated as 30/120 seconds or 25%.This means that if you know the task cycle and shutdown time values, you can determine the maximum opening time of the solenoids.

For example, the shutdown time is equal to 15 seconds, the task cycle is equal to 40%, so the opening time is equal to 10 seconds.A solenoid with a Nominal Duty Cycle of 100% means that it has a continuous voltage rating and therefore can be left "ON" or continuously energized without overheating or damage.

In this tutorial on solenoids, we examined both Linear Solenoid and Rotary Solenoidi as an electromechanical actuator that can be used as an output device to control a physical process.The type of output device that we will examine in the next tutorial is DC Motor.