I Know, It Hertz
I am pretty sure I am going to get some flack from what I am going to say in this article, but before you conclude that I don’t know what I am talking about, hear me out. It is not my intent to turn people against using one of the most excellent components for accurate control of fluid power systems. In this, as in all the other articles, I want to stimulate your thinking. We all need to be sure that we represent the best in fluid power professionalism. This includes looking at the energy consumption of the systems we use.
One of the favorite valve categories in fluid power is the servo. Since its inception, the servo valve has allowed remarkable control of speed and position in processes controlled with fluid power.
I recently had an opportunity to take part, along with a number of Certified Fluid Power Accredited Instructors, in a review of the new Certified Fluid Power Electronic Specialist study guide. As we went through the material, we were reminded that the standard flow rating for a servo valve is determined using a 1000-psi pressure drop through the valve; 500 psi P to A or B, and 500 psi A or B to T. I was half expecting and certainly hoping that there would be a collective gasp of incredulity as this was presented, but then we were all experienced professionals and had come to accept this as normal. The 1000-psi pressure drop is accepted as the characteristic necessary for control.
I think that a lot of us do not automatically think of pressure drop in terms of energy consumption. Oh, I know we immediately recognize that there will be some more kilowatts pumped into the system and that will mean more Btu’s to remove. We use our rules of thumb and various charts to select the heat exchanger and the fine filtration. It is simply accepted as the cost of control.
Let’s think about that for a moment with energy in mind. In a system where I need 2000 psi at an actuator controlled by a servo valve, I would need to have 3000 psi available. That means that right from the get-go, I am sacrificing 33% of the energy needed for the system. So the question is this: why do we need the high pressure drop?
In the study guide, the servo valves are defined as either “flapper-nozzle” or “jet-pipe” in their control mechanism. High-pressure fluid is used to shift the valve spool and also to develop a high-velocity flow through the jet-pipe or nozzle. The result is that the servo is a very dynamic valve. Like a racehorse waiting at the gate, the spool is excited, just waiting for the command to go. Because of the pressure and consequent high velocity, the spool can move very fast and will respond quickly to subtle changes in the system. This speed is very important in developing the control expected of the servo valve.
However, we do not always need the speed. There are times when accuracy and repeatability are the issues and speed is not the biggest factor. There is a class of servo valves that are not often discussed but that could provide an excellent alternative to the customary high delta P valves. These valves are driven by a stepper motor. This is a rotary motor with digital positioning. They require no pressure drop to change the spool position, and because they are digital, some operate at zero hysteresis. I have seen an application where there was remarkable positioning control of a cylinder using this type of servo with only a 75-psi pressure drop through the valve. The downside is that the response time is relatively slow, but we need to ask what response time is necessary for our application.
The response times that are listed for valves are based on their ability to jump from one extreme to another in a certain time frame. The distance is typically 90% of the full potential movement of the spool, and it is rated by how many times it can do it within a span of one second. Some of the flapper-nozzle and jet-pipe servo valves have spools that can travel almost all the way from one side to the other 250 times in one second and are said to have a response time of 250 hertz. The stepper-motor driven valves have a much slower response time–about 75 hertz. At 250 hertz, the spool in a typical servo valve can leap from one extreme to the other in 4 milliseconds. The stepper-motor driven valve takes 13.3 milliseconds to cover the same distance. To give you an idea of how fast that is, the lights in the room where you are reading this article are probably flickering on and off at 60 hertz, but you perceive it as a continuous light source. These spools can bounce around inside the valve bodies up to four times faster than the light bulb flickers. That is pretty impressive and makes for a good sales presentation.
However, for many of our applications, the major activity of the valve is within a very narrow band of spool motion. When we are holding a cylinder at some location and want to keep it within one kazilienth of an inch, the spool usually has to travel a very short distance, modulating to hold the cylinder position. Moving within this narrow band may dramatically reduce the necessary speed of the spool, and the fact that a valve may provide zero hysteresis may be more valuable to the control system.
I just thought of something while I was writing and I need to take a ten-minute break to do some calculations. If some of you want to stick around and help out, that would be great. The rest of you can take a break but be back in ten minutes…
For those of you who are staying, I need to figure out the acceleration required to achieve a 250-hertz and a 75-hertz spool movement before I go too far out on a limb. I am going to assume a total spool travel of one inch. I am then going to assume a travel distance of 0.0625 inches for the valve modulation. The result will show the performance differences between the valves in the critical modulating position. I appreciate your help. The results will be the basis for the next statements when everybody returns.
Ok, we are ready to resume. This is what we found out: A system capable of 250 hertz and with a spool travel of one inch would have to see an acceleration of about 500 g’s. That is the reason for the high pressure drop. The pressure provides the necessary force to develop that rate of acceleration. A 75-hertz valve with the same travel needs less than 50 g’s of acceleration. Now, let’s take a look at the critical time for both valves to jump 0.0625 inches responding to a command signal. The faster valve will cover the distance in 6.2 x 10-7 seconds. The slower valve will cover the distance in 6.8 x 10-6 seconds. It is true that the 250-hertz valve is 11 times faster, but the actual time difference is 0.0062 milliseconds. This would be considered a tie in a photo finish.
The point, as always, is this: Choose the components that will do the job without consuming more than the necessary amount of energy. Don’t be awe-struck by a hertz rating. Figure out what is actually needed. Forget the fudge factor, shun the shortcut, and reject the rule of thumb. Do the math. Choose the best.
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