Actuators are really complicated. There’s a whole little ecosystem around them, all to provide exactly the right type of movement for every application. In principal, an actuator’s just moving something a few inches or a few degrees, but the way it moves really matters. This has lead to actuators with different types of movement speeds, different levels of precision, different return systems, and a lot of complicated terminology about what you’re actually buying into.
3 Types of Performance
One of the key metrics is performance and the price it takes to achieve it. Consider for example, the damper actuator in your air vent. It doesn’t matter too much if it takes a minute or two to fully open as long as it works quietly and within the temperature range of your home. Compare this to an industrial fast-acting actuator on the safety systems in an oil refinery. When the plant alarm goes off that something’s might explode, you want that valve to slam shut with all the force of the hulk hitting the snooze button in the morning.
In your home, a little electrical actuator would work just fine. In that refinery however? That needs something special. There, you need a pneumatic system that can throw thousands of pounds of force into a valve near instantly. There’s just no other good way to get the job done in a fast manner. Electrical motors on the rise but, in these scenarios, electromagnetics aren’t ready.
There’s also scenarios where you simply need a lot of force. Consider city-scale water supply systems and emergency water towers. These require thousands of pounds of force to close off. It’s a wet environment, so electronics are a harder solution to deploy. Pneumatics becomes challenging because gasses will compress, it’s possible the actuator can get overwhelmed by the load. What isn’t hard and doesn’t compress? Hydraulics. A good system of hydraulic actuators can pick up entire buildings. It won’t be fast, but they make light work of heavy things.
Whatever you’re actuating, it’s going to have to return to it’s starting position at some point. This is where another performance gap begins to arise. How do we make it move back? How fast does it need to move back? How hard will it be to move it back?
For electrical applications, the simple answer is, as fast and as easy as it was to open in the first place. You reverse the polarity going to the motor and roll it open one inch or degree at a time. It’s pretty straight forwards to change direction here because you have complete control of the mechanism.
Pneumatic systems aren’t so lucky. In these systems, you may need to push the actuator back to the starting position. Consider, a pneumatic system which closes off a pipe. The valve blocks it completely and the force is equal such that it can’t just be pushed open again by the contents of the pipe. For this, we have spring return and double acting configurations. In these setups, you have either a spring which will push the actuator back to it’s start position or you can apply pressure to both sides of the actuator and actuate it in reverse altogether with similar performance to the first actuation.
There are also cases where you don’t need a built in return mechanism. Gravity or vaccuum pressures or something will provide enough de-actuating force. This can often be the case for hydraulics, where sucking the material is about as effective as pushing it. Such systems can be called Non-Spring Return.
Lastly, there are scenarios where we need exact control over the actuator. This can be for mixing valves, modulating to mix air, or simply precise manufacturing. In these scenarios, electrical motors are going to be king.
The nice thing for electrical motors is how many ways there are to measure their movement. There’s stepper motors, which precisely shift in “clicks” with little teeth inside the motor. There’s feedback sensors that can be built-in, reading the offset of the motor. There’s light and magneticly driven systems which detect either markings on a rotor or its moving electromagnetic field to detect position and speed. The power flowing in and out alone, can be used to calculate speed and position. On top of this, variable frequency and amplitude to control motors allows for absurd precision in how fast and how torque is delivered.
In pneumatics, there’s a measure of uncertainty due to the compression of the air throughout the system. It can lurch and be a little springy as a result. In such scenarios, you can attempt to control pressure evenly, but a guaranteed 100% rate of accuracy is going to be very difficult to achieve. If one thousandth of an inch or one one thousandth of a degree is going to matter, pneumatics can’t deliver accuracy without some sort of gearing-down to reduce that uncertainty.
Hydraulics are by contrast much more reliable with their precision. As long as the correct amount of pressure is applied, things move and ideally hold their position. It’s possible to move hundreds of thousands of pounds with sub-millimeter accuracy using these types of devices. Just look at theme park roller coasters and skyscrapers, where hydraulicly driven cranes line up bolt holes with less than a millimeter of wiggle room. These systems are of course, expensive as it gets, but they’re powerful, slow, and precise.
A Big World Out There
This immense variety isn’t everything, but it’s a good starting place to understand why we have so many options in actuators. We have built solutions optimized to near every application available. We have systems that are fast and safe. Systems that can fail in the open or closed position. Systems that can outlift the hulk and systems with the precision to move microscopic distances.