Hydraulics Explained

This page is primarilly for my own use, but others might find it usefull as well. I'm an electrical engineer (EE) by degree and working with a group that is into testing and controling hydraulic components and systems. A hydraulic system uses oil to do something like stop your car or control the flaps on an aircraft. Some of it is really cool like shaker tables where you're trying to break something by shaking it about. I havn't meet an engineer yet who doesn't like to try to break something.

At the moment my job is to program the software and deal with the electronics to control all of this. I'm not totally up on hydraulics so I'm putting what I learn here.

Hydraulic Pressure Control

Pressure is created by flow. You push more fliud into an enclosed area and the pressure rises -- like a baloon. You blow air into a baloon and the pressure rises. Since the baloon is made of rubber it expands until the rubber streaching creates a force that equals the pressure of the air inside the baloon. Steel tubing does the same thing, but expands such a small amount that you'll never see it.

Well, I'm EE so I don't think that way.

I think of things in terms of electrons, capacitors, resistors, and coils. I finally understood pressure during a 5 hour drive (ack!) home and pictured this:

Just think about this without hydrailics for a moment. Start out with no charge on the capacitor and both switches open. For a moment, let's say the battery has a voltage of 3000 V. The battery also has a limit as to how much current you can draw from it. We're going to close SW1 for just a moment. Electrons will flow from the + plate of C to the battery creating a charge across C. SW1 was opened before C could charge to 3000 V. Let's say it got to 1000 V. If we close SW1 again, the capacitor keeps charging above 1000 V.

With me so far?

OK, let's keep SW1 open. At this point there is a voltage potential across C. If we close SW2 for a moment, that voltage potential is now across the resistor, R, which causes current to flow. This pulls charges off of the capacitor at a rate limited by the resistor. Pulling charges off of the capacitor decreases the voltage across the capacitor.

Still with me? This is where it gets more interesting.

The goal is to keep the voltage across the capacitor about the same while keeping SW2 closed. The way to do this is to close SW1 to bring the voltage across C up to the voltage we want, say 1000 V. Soon as it hits a little above 1000, we'll open the switch until it drops a little below 1000. Then we close the switch again and the cycle repeats. This creates a ripple in the voltage that we have to live with. There is one thing that needs to happen for the switch flipping to work -- the battery must be able to supply enough current to keep the capacitor charged while the resistor draws off current from the capacitor.

Now we get to how this relates to hydraulics.

In hydraulics, the battery is a hydraulic power source (basicly a pump), the switches are valves, the resistor is whatever is in the system, and the capacitor is the nature of the pipe between the pressure control switch (SW1) and the system. Opening the pressure control valve allows fluid to flow into the system. If the fluid can't get through the system fast enough, it accumulates before the system and the pressure rises there. Since the valve can be partway open, the constant opening/closing doesn't occur. There is still ripple, however, because the valve is constantly moving. This constant movement inproves system performance and isn't noticed because the lines act like the capacitor and filter it out.