Linear Solenoid Actuator

INTRODUCTION

To properly understand the concept of linear solenoid actuators, we must understand the basics first.

A linear solenoid is an electromagnetic actuator. An electromagnetic actuator is a device that converts electrical energy into a magnetic field, creating movement or motion in the process.

In the simplest terms, a linear solenoid also known as a Linear Electromechanical Actuator (LEMA) is an electromagnetic device that is capable of producing mechanical energy or motion from electrical energy. Its working principle is very similar to that of the electromechanical relay device.

Basically, a linear solenoid is made up of a tube with a solenoid coil wrapped around it. Inside the tube is an iron-core or plunger with enough space for it to slide in and out of the solenoid coil.

Solenoids are very useful in the creation of mechanical energy or motion. They are used in the operation of robots, opening/closing of valves, doors, etc.

Apart from linear solenoids, there are several other types of solenoids. The rotary solenoid is a very common solenoid form. 

Magnetic fields are formed when electrical current is passed through a conductive material, and the flow of current within the coil determines the direction of the magnetic field.

A linear solenoid is made up of a coil of wire (solenoid core) around an iron core (electrical conductor), thus when current is passed through it, a magnetic field is formed and the coil becomes an electromagnet having permanent North and South poles.

The strength of the magnetic field formed is dependent on two things:

  • The magnitude of the current flowing through the coil and
  • The number of loops on the coil

As current passes through the coil, the coil begins to behave more like an electromagnet and a magnetic flux is created which attracts the plunger to the middle of the body of the coil. This movement of the plunger causes the small spring attached to it to get squeezed and compacted. The magnitude of the magnetic flux created influences the force that acts on the plunger, making it move and the speed with which it moves towards the coil.

Once the flow of current to the coil is cut off, the electromagnetic field produced completely breaks down and the coil loses its electromagnetic properties. The plunger which was formerly drawn to the coil in its electromagnetic state is pulled back to its initial position by the energy stored in the body of the compacted spring. As the plunger moves to and fro the coil, solenoid strokes are formed. This is basically the utmost distance the plunger can move, it is calculated in millimetres (mm).

Structure of the Linear Solenoid

The linear solenoid is so called because it causes the plunger attracted to it to move and act in a straight line. Based on the direction in which the plunger moves, there are two types or forms of linear solenoids.

  • Pull Type – Here, when there is a flow of current, the electromagnetic force of the coil works to bring the object closer to itself.
  • Push Type – When there is a flow of current, the electromagnetic force of the coil acts on the object, taking it further away from itself in the opposite direction.

What makes the difference between the two types of linear solenoids is the position of the spring (return spring) attached to it and the makeup of the plunger.

Pull-type Linear Solenoid

When a “push” or “pull”, “in” and “out” or “open” and “close” movement is required, linear solenoids come in very handy. They form the principal mechanism of robotic motion, door locks, irrigation valves, self-propelled engines, etc.

They come in different forms sealed tubular type, and open and closed frames.

Rotary Solenoid

Another common type of electromagnetic actuators is the Rotary Solenoid. While the linear solid is concerned with movement in a straight line, rotary solenoids effect rotational movement around a fixed point. This movement could be clock-wise, anti-clock-wise, or move in both directions.

Where the rotational movement needed is small, DC motors and stepper motors can be substituted with rotary solenoids. In determining the angular orientation, you consider where the movement started from, and the point at which it ends.

A rotary solenoid can move 25, 35, 45, 60 and 90 degrees. It can also move through a particular angle in several ways:

  • 2-position Self-restoring movement also known as Return to Zero rotation, e.g. 0˚ to 45˚ to 0˚.
  • 3-position Self-restoring Rotation e.g. 0˚ to 60˚ to 0˚ to -60˚
  • 2-position latching

Just like in the case of the linear solenoid, a rotary solenoid produces movement, rotational movement, when current is passed through it. It also produces movement when there is a change in the position of the electromagnetic coil. This is due to changes in the polarity of the electromagnetic field.

The structure of the rotary solenoid is made up of a coil of wire wrapped around a steel object. Just above the coil, there’s a magnetic disc linked to the output shaft. When current flows through the coil, the electromagnetic field created produces several North and South poles which repulse the standard north and south poles of the magnetic disc. This causes a rotational movement of the disc. The direction it turns is determined by the mechanical structure of the solenoid.

Application

  1. The rotary solenoid is an integral part of valve controls, cameras, vending machines, etc.
  2. Some rotary solenoids have low power but a sufficient torque magnitude. They are used in automotive machines and applications, dot matrix printers, etc.

Using AC Current in Solenoids

As already mentioned earlier, linear solenoid energize their system by allowing the flow of DC voltage through the coil. However, AC current can also be used. This can be achieved by changing the supply with the use of full-wave bridge rectifiers. Smaller solenoids that make use of DC current can be operated using MOSFET switches or transistors.

As an inductive device, the solenoid coil generates large amounts of electromotive force (emf). To forestall any damage caused by high emf voltages, the Fly-wheel Diode is usually used.

How to Reduce Energy Leakage in Solenoids

As mentioned above, most solenoid coils, including the linear solenoid, are inductive devices. This means that they generate heat from some of the electrical energy in the coil. This is because of the wire’s resistance.

This means that when connected to a source of electrical energy for some time, its temperature increases and keeps on increasing as long as it is connected to the source. Also, the increase in temperature has an effect on the electrical resistance of the wire which in turn increases the flow of electrical current, thus increasing its temperature further.

If the coil receives a constant supply of current, the temperature will rise steadily, especially if the power supply never stops. As this steady rise in temperature would eventually damage the solenoid, it is important to check the amount of current flowing through the coil. This can be achieved by controlling the supply of voltage to the solenoid. The illustration below shows to do this simply by linking the solenoid in series with an appropriate resistor:

In the diagram above, the NC contacts are closed. This allows the current to flow freely into the coil by stopping the free flow of resistance in the system. Once a sufficient amount of current has entered into the system, the contacts automatically open up and a series connection is formed between the solenoid coil and the holding resistor.

With the help of the technique above, the coil can receive a constant flow of supply voltage. This is because the consumption of energy and the generation of heat have been cut down drastically.

However, it must be noted that the holding resistor would also produce some amount of heat. This is due to the effect of Ohm’s Law.

An even better way to cut down the loss of energy and production of heat is the Solenoid’s Duty Cycle also known as the Intermittent Duty Cycle. An intermittent duty cycle involves continuously switching the coil ON and OFF at appropriately fixed times. This would help to ensure that just enough current gets into the system enough to keep the plunger working even when it’s been switched off without letting in current steadily.

The ratio of the time the solenoid switch is ON to the absolute total of the ON and OFF time for one operation cycle is what is called the Duty Cycle of the Solenoid. Simply put, the total duty cycle time is a cumulation of the OFF time and ON time.

 The illustration below further explains the concept of the duty cycle. Thus, if the total ON time is 25 seconds and the total HOT time is 75 seconds, it means the total cycle time is 100 seconds. The duty cycle of the solenoid coil is 25%. As per:

(25/100)*100 = 25%

Using this formula, the total ON time of the coil can be calculated, as far as the values of the OFF time and the total cycle time.

As an example, consider a coil with a switch off time of 35 seconds, and total cycle time of 60 seconds. Thus, the switch on time of the coil is 25 seconds.

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