In this unit we're going to be talking about accumulators which are already important component in a fluid power system. And by the end of this unit you should be able to describe why accumulators are used, and explain the different types of accumulators, and then we'll be going over some of the equations that come out of the governing physics that describe the behavior of accumulators. But before we do that, let's go on a field trip to show you a practical application of an accumulator. >> Valley Fair is the largest amusement fair in the upper midwest with over 75 rides and attractions. The extreme swing is the best swing-set in town where riders reach heights of 125 feet. And swing speeds of 60 miles per hour. Extreme swing is powered by four 300 horse power air compressors. Compressed air energy at a 105 psi is stored in two tanks acting as a giant accumulator. The accumulators release their energy using timed valve openings into a pair of 27 inch diameter pistons that drive the ride. Let's go for a ride and see it in action. [SOUND]. So now you have an appreciation for what accumulators can do. The main purpose of a Accumulator is to store energy. They are the potential energy element for food power systems. And you can store that energy in a raised way. You can store it by compressing a steel string. Or most commonly, you can store that energy by compressing a gas. And there's a, a couple of purposes for Accumulators. One is to provide pressurized fluid during brief high-powered demand periods. So that's what going in that. Field trip shoot. And the second main reason for it is to smooth out the pulsations that you get in the fluid. And those pulsations come. Typically from the pump as it rotates, puts pulsation into the pressure surges into the fluid and you don't want those pressure surges to profligate through down to the cylinder. So in the top we've got a schematic of a fluid power system. So we have a, a motor and a relief valve and here's a three-position valve and then a cylinder acting against a load. So this motor can apply pulsations that would go through to the load. So down here is the schematic for an accumulator where you can store energy. It's equivalent to a capacitor in an electrical system and if you add that accumulator into the system, then it's going to smooth out the pulsations coming from the pump. Now one of things you want to do in a practical system is to locate this accumulator physically. As close to the input of the valve as you can so that the input pressure to the valve is as smooth as possible. Here's one example of a potential application for an accumulator. This is a power of your ankle while you walk. Where along the. Our horizontal axis is one walking step. And this shows the power that your ankle produces. What happens is, right when you push off on your toe, your power peaks to about 200 watts. While the average power in your ankle is only about 13 watts. So in some of our research, we're doing exoskeletal ankles for use by individuals who've lost the power in their ankles, and we're using tiny hydraulics to do that. So this would be a perfect application for an accumulator, because the average power that you're putting our is relatively low, but you need this brief burst of power. And one of the things that accumulators are great for is producing very high power densities. That is you can release the power very quickly as we saw in the field tripler. Another way of thinking about a accumulator is compressed gas. So you think about blowing up a balloon and letting go of the neck, that balloon will fly away quickly. So this would be an example of a application where you might want to use an accumulator. And here is that hydraulic ankle foot arthosis power ankle that we have developed in our research lab. And down here you can see some tiny hydraulic cylinders and then hoses that go up to a power supply. And right now we don't have an accumulator in the system but we will put one in shortly. Here's a couple of other examples of accumulators in action. So around the left is a diaphragm accumulator. And it's purpose is to smooth out the ripples in the fluid supply lines. For a multiple fluid power systems. So it's sitting bolted right down onto a manifold. And the output of that manifold goes off to several systems. And then on the right is another application of a. accumulator. So this big black tank is an accumulator. And this is a hydraulic hybrid bicycle that was built for the Parker Chainless Challenge by a team of undergraduates at the University of Minnesota. What this accumulator does, among other things, is when you break the bicycle. Rather than the breaking energy going off as heat, instead there's a hydraulic pump in the system. And the hydraulic pump pumps fluid into the accumulator, compressing gas and storing the energy as compressed gas. Then when you want to take off from the stop light you can release the compressed gas in the cylinder and drive the bicycle. So that would be another example of a accumulator in action. Let's take a look at the types of accumulators. And there's, there's four main types but two of them are much more common than the other two. So up on the top left is what's called a weight loaded accumulator. And we've got a cross-sectional view here. And the red is the fluid being pushed in. And it's a cylinder, this is really just, like the picture shows a cutaway of a cylinder. And the rod of the cylinder is attached to a heavy weight. So the way that the energy is stored is that it's lifting that weight. Now this was the original type of accumulator, and it's no longer used, and the main reason it's no longer used is that the weight has to be pretty heavy to get reasonable energy stored, so these things are big and bulky. Over on the right, top right, is a spring loaded accumulator. Same principle as the weight loaded one where there's a piston that is moving up and down, responds to the fluid, but now that piston is compressing a spring. So here the energy is stored in the compressed steel spring. This can store quite a bit of energy, but again it's of limited use because once ou have a big enough spring to store a lot of energy, that spring ends up being quite heavy, so this one isn't used very much either. Down on the bottom are the two that are used more often and here the energy is stored by compressing a gas, an inert gas, and typically its nitrogen. So again, on the left we've got a piston that moves up, and as it moves up it compresses the nitrogen gas shown in yellow at the top of this cylinder. You can pre charge the cylinder through the top and so typically you pre charge it to somewhere around half the working pressure. Now this gas charge piston is good because it can have a pretty big compression ratio, up to 10 to 1. And you can have a long stroke to it. The downside of it is that it's somewhat inefficient because you have to have very tight seals around the piston in order to keep your compressed gas from escaping. Over on the right, is a bladder type of accumulator, which is the most common type. And here rather than in a piston, there's an internal bladder that separates the nitrogen gas from the fluid. So, as the pressurized fluid comes in, it compresses the bladder and the gas within the bladder, and this is very efficient because now you have no sliding seals. You do have to be a little bit careful about the design of these. Accumulators because the bottom material has to one where the nitrogen gas doesn't over long periods leak through it. And it has to be a material that can withstand the the oils that are against the other side. And, and over time the nitrogen gas will diffuse. Across the Bladder, but that can be days or months or after years of service and, again, at the top, there's a gas valve that you can use to pre-charge the nitrogen gas. On the desk, I've got a variation of a Bladder accumulator. This one is rather than a bladder it's a diaphragm but essentially the same principle and this one is good for about 3000 psi of fluid. So the fluid goes into this end. The nitrogen gases charge from this end and the size of this accumulator it says on the back it's 30 cubic inches. Here's a cut away cartoon of that bladder accumulator in operation. So here you've got it fully the bladder fully extended and full of the gas, that is pre-charged pressure. And here, the system pressure. The fluid is below that pre-charged pressure, so the bladder fully fills the system. Now you raise the system pressure above the preload pressure and fluid can enter the system through the poppet valve and compress the gas, the nitrogen gas. Now, the pressure of that nitrogen gas in this. Situation is almost the same as the fluid pressure. And this'll become important when we start to look at the equations that cover the physics. And then as the system pressure drops then the gas can provide energy to the to the fluid. And releases stored energy to provide needed food flow to the system. And here's an example of real system cutaway, just so you can see what these things look like. Over on the left is a piston-based accumulator, so fluid is in the bottom, and the gas would be up here in the top. And then over on the right is a cutaway of a. Bladder accumulator where the black is the bladder. Fluid comes in the bottom, surrounds the bladder, and the gas in on the inside. [NOISE]
