Now, look at how this is analogous to the flow of hydraulic fluid in a fluid conductor. The hydraulic conductor has a hole down the middle, and that is its secret to success. But if we look at it closely, imagine that there is a conductor filled with an incompressible fluid. Never mind that the fluid would just dribble out the ends; allow that we can take a new, external molecule of fluid at one end and push it into the filled conductor. That molecule would push against an internal molecule, displacing it. This would displace its neighbor, and so on down the length of the tube. At the opposite end, one molecule of fluid would be ejected and leave the conductor. Thus, like a column of pool balls, Figure 3, the filled tube would transmit the motion of an incoming ball and eventually displace a ball on the other end. If the circuit is completed, then the motion of the fluid molecule, or molecules, constitute a fluid flow.

Figure 3. Like a column of pool balls, a tube filled with hydraulic fluid molecules transmits the motion of an incoming molecule and eventually displaces a molecule on the other end. If the circuit is completed, then the motion of the fluid molecules constitute fluid flow.

The mechanisms that allow electrical flow compared to those that allow fluid flow are fundamentally different. However, it is not erroneous or misleading to think of the electrical current using the pool ball analogy. In fact, such visualization may help to demystify electrical current!

Speed of light and speed of sound
This is an opportunity to discuss the time for the above described actions to take place. There are really two questions:

  1. What is the speed of the fluid molecule or electron as it travels through the circuit?
  2. After the molecule or electron is jammed into one end of the conductor, how long does it take for the corresponding molecule or electron to jump out the other end of the conductor?

Whether the medium is electronic or hydraulic, the respective answers are quite similar. In the case of the fluid system, it is common practice to calculate the fluid velocity. This is the flow rate divided by the area of the conductor, and it is normally in the order of a few ft/sec. For a given molecule to travel from the reservoir through the entire circuit and back to the reservoir, can take a few seconds to a few minutes in a typical hydraulic circuit.

Similarly, the rate at which electrons drift around a circuit is relatively slow. In a wire that carries 1 A of current and has a diameter of 2 mm, the average electron velocity is only about 0.024 mm/sec! Some observers call this the drift velocity of an electron, because it is so low. It can take the better part of an hour for an electron to make one complete circuit in a hand-held flashlight. Clearly, these electrons are not moving at the speed of light. The fluid molecules in a typical hydraulic circuit are travelling much faster than the electrons in a typical electrical circuit. To get an appreciation for electronics and the speed of light requires examining the second question.

For the second question, imagine the fluid situation and look at the pool ball metaphor. If you jam a ball into one end of the conductor, how quickly does the emerging ball pop out the other end? We could look at the physics in detail, and observe that the compressibility of the balls comes into play. However, in the end, the time for emergence of the exiting ball is governed by the speed of sound through them. The action of shoving a molecule of fluid into one end of a conductor is felt at the other end of the conductor at a time delay determined by the speed of sound in the fluid. Similarly, in the electronic case, the cause (jamming an electron into one end of our conductor) results in a different electron jumping out the other end. The time between cause and effect is governed, approximately, by the speed of light and the length of the conductor.

Making waves
What we have here are two quite different phenomena, both in the fluid case and the electrical case. The first is the drift or transport of particles through their respective conductors, while the second is the transport of a cause through the conductor. The latter is often called wave propagation. When you jam fluid into one end of a conductor, a force is required. In fluid systems, that is best visualized with pressure. The input action causes a pressure wave to travel along the conductor, and it is the wave reaching the other end that forces the emerging molecule out of the tube. Analogously, it takes force to jam an electron into a wire, and that force is best visualized with the concept of voltage. A voltage wave is sent down along the wire at the speed of light, and it forces the emerging electron out on the other end.

So, current, measured in amperes, is the flow of electrical charges, and its hydraulic analog is the flow of fluid molecules, measured in volume per unit of time, for example, in.3/sec. The actual velocity of the particles, be they electrons or molecules of fluid, is relatively low, taking minutes or even hours for a single particle to make a complete loop around a circuit. On the other hand, the motivating force — pressure in the fluid case and voltage in the electrical case — can travel about the circuit as a wave at very high velocity. For a pressure wave, the speed is determined by the speed of sound in that fluid system. In electrical circuits, the voltage wave travels at speeds approaching the speed of light. Next month, we'll explore the pressure-voltage analogy further.

Pressure compensation from a fixed-displacement pump?


Say goodbye to variable-displacement, pressure-compensated pumps. One of Jack Johnson's books introduces and explains in detail a pressure-compensated system using a fixed-displacement pump. The system does not rely on relief valves and is as efficient as a conventional system using a variable-displacement pump. An Engineering Analysis of the Pulse width Modulation Method of controlling Output Pressure of a Hydraulic Power Unit contains 128 pages of information covering complete comparative test results,. More importantly, though, you can learn how to design and build your own lowcost, constant-pressure hydraulic power units for both servo and conventional systems.

Electrohydraulic Pressure Control is the title of another publication from Johnson. This is the first book directed to the subject of electrohydraulic pressure and force control. You can learn the reasons why electrohydraulic pressure control can sometimes be a real challenge and why it is sometimes accomplished with ease. One of the key issues is when to use integral control or not. You will also learn how the nonlinearities in a hydraulic system can affect system performance and what you can do to deal with them. This book is a must-have for anyone who wants to control pressure or force, with or without electronic programmability.

Click here for ordering instructions and more information on these and other books covering fluid power and control technology.