Friday, April 18, 2008


My ultimate favourite when it comes to electrical actuator, the (not-so) humble solenoid.

So, the solenoid is an electrical actuator, like the DC motor or the valves that are used in modern day industries. What it basically does is convert electrical energy into mechanical energy.

The solenoids that I will be talking about in this post is basically the ones referred to in engineering, referring to a kind of transducer which convert energy to linear motion.

So what is a solenoid? Below are some pictures of solenoids:

So solenoids, be it electromechanical (AC/DC), hydraulic or pneumatic driven, when given energy will produce a linear force.

Let's take a closer look into the insides of a electromechanical solenoid:

Inside the solenoid is motor wire coiled around in a special way. This special coil of motor wire, when energized (current flowing through the wire), creates a magnetic field.

In the picture above, we see that the motor wire has been coiled around a metal rod. This is known as the plunger. The magnetic field attracts or repels this rod that is usually made of iron or steel, applying a force on it. When the field is turned off (no more current flowing through the motor wire and therefore the coil no longer is energized), the spring that is connected to the plunger will return it to its original state.

So we see that the plunger can either be pulled into the shaft where the motor wire is coiled, or pushed out of the shaft, limited only by the spring attached at the back.

This leads me on to the two kinds of solenoids that one can have: a push solenoid, or a pull solenoid.

In a push solenoid, the plunger is normally kept inside the shaft. When the magnetic field is turned on, it applies a force on the plunger, forcing it out of the shaft. This gives us a pushing force at the end of the plunger. When the magnetic force is turned off, the spring will bring the plunger back into the shaft.

Unlike the spring of the push solenoid, the spring of the pull solenoid keeps the plunger out of the shaft of the solenoid. When the magnetic force of the pull solenoid is turned on, it 'pulls' the plunger back into the shaft.

So after so much talk about how a solenoid works, how can we control one?

Well, we see that the solenoid is polarized, meaning that it can only work in one direction: push or pull. We cannot make a push solenoid pull, or a pull solenoid push.

So all we need is a MOSFET and/or a relay that will trigger the current flow to the solenoid.

Do take caution when powering the solenoid as powering the solenoid at high currents for extended periods of time could lead to overheating and melting. The power rating should not be less than the voltage times the current draw of your solenoid. However, it is fine to go over this rating for intermittent periods of time.

Video Time!!!
Now time for a video as to what a solenoid can do. These are videos of a soccer robot from a Australian robot soccer team (a very good one by the way), that are using the solenoid as the 'kicker' that is propelling the ball forward:

That's all for this post. I will add in more stuff on the other points later :)


~YM~ said...

good post there..very informative indeed. makes me remember what i had studies long time ago..X'D

Mattan said...

Thanks! If there is anything that you would like me to write about, please comment :)