By Peter Achten, INNAS BV
In a presentation at the 16th Scandinavian International Conference on Fluid Power held in Tampere, Finland, Peter Achten discussed fluid power technology, the state of the industry and commented on where it’s all headed.
Images have always been strong instruments in conveying messages. Ideas that get deep-rooted in your head are called memes. Often without knowing or realizing it, they inﬂuence your decisions and your behavior. They are strongly contagious and can go “viral.” Memes are often used by internet trolls, but also in marketing and journalism.
The term “meme” was coined in 1976 by Richard Dawkins, an Oxford professor and evolutionary biologist. Whereas genes determine your physical characteristics, he said, memes determine your behavior. Memes can be tunes, catch-phrases, fashions, slogans. They propagate by means of imitation. And, of course we are manipulated constantly on social media. If you think that the fluid power community doesn’t have our own memes, then you are wrong.
Let’s take a small mental tour and deﬁne our own ﬂuid power genes. What is really inherent to ﬂuid power, both good and bad? But also, what are our memes? What is tradition, and superstition? And ﬁnally, what does it all mean for the future of ﬂuid power?
First, our genes. The quintessence of hydraulics is the ﬂuid. Fluid transmits and controls power and can store energy, transport heat to coolers and carry debris to a ﬁlter. These characteristics of ﬂuid enable robust and flexible machines with unparalleled power, force and torque density. They also deﬁne the genes of hydraulic systems.
Our ﬁrst “gene” is that ﬂuids are compressible. Oil, for instance, is 2 to 3 orders of a magnitude more elastic than steel. And especially below 50 bar, bulk modulus strongly depends on the air content. Another key characteristic is that ﬂuids are viscous, and that viscosity depends on temperature and pressure.
The third inherent characteristic of ﬂuid power is that there is always some internal leakage — something you don’t have in a gear transmission. In a simple transmission with equal gears, output speed matches input speed. This is different in a hydrostatic transmission. Although pump displacement equals motor displacement, output speed is always lower than the input speed. This is especially important for low rotational speeds when internal leakage can result in control issues, such as dead band. Finally, we can’t have negative pressures (at least not in the real world.) A practical consequence is that cylinders can’t pull unless they are double acting.
But how about the idea that high power losses, noise and pulsations are unavoidable. And our systems must be complicated and expensive. And ﬁnally, the industry is extremely conservative and there are no innovations. These are all memes! There are no fundamental physical reasons why any of this is true.
Let’s choose three memes. First is the idea that hydraulic systems, by deﬁnition, have high power and energy losses. Second is that hydraulic systems and components are expensive and that costs can’t be reduced. And third, our market doesn’t want to innovate. Do we have to accept this?
Take a closer look at power losses. Energy efﬁciency, considering worldwide concerns about climate change is, of course, an important topic. But efﬁciency not only relates to global warming, it affects fuel costs, and the need for coolers, and high frictional losses that go hand-in-hand with accelerated wear.
Efﬁciency is the key. Yes, we are inefﬁcient — often extremely inefﬁcient. It is almost unbelievable that we still exist on the market. Most pumps and motors don’t make the 90% efﬁciency point and often run below 70%. And valves simply control by means of energy dissipation, by deﬁnition.
But does this mean that inefﬁciency belongs to hydraulic systems and there is nothing we can do about it? There is no physical law, such as Carnot’s law for heat engines, that tells us we have to accept high losses. We don’t need to be inefﬁcient! But there is no easy solution, not just one single innovation which will suddenly make hydraulics systems efﬁcient. We need to come up with a whole range of innovations.
Let me give you one example of such an innovation. The most dominant loss in swash plate pumps and motors is in the contact between the piston and its cylinder. Strong lateral forces are the main cause for high friction losses. Floating-cup pumps and motors can solve this issue. Tests show that compared to the hydrostatic piston force, even at high pressures (350 bar) friction is negligible, less than 0.01%. And that is good news, not only for the efﬁciency of the entire drive train, but also for lifetime and maintenance costs. At bauma, Bucher Hydraulics presented their new AX pump and motor line based on the INNAS ﬂoating-cup principle. They call it a “revolution.”
But losses in pumps and motors are peanuts when compared to losses in valve blocks, load sensing valves and pressure compensators. To minimize these losses the answer is simple: displacement control must replace valve control. There are three ways of doing this, by three different means of power distribution. The ﬁrst option is by electric power distribution. Hydraulic systems are reduced to the minimum. Sometimes this system is referred to as zonal hydraulics.
The second option is based on mechanical power distribution. Instead of one pump supplying oil to several loads, each load is controlled by its own variable displacement pump. Pumps are driven through mechanical transmissions or by stacking pumps in line. In the third option, energy is distributed via a hydraulic power line, a common pressure rail (CPR). This system needs secondary control and hydraulic transformers.
Let’s forget the mechanical option. I have no conﬁdence it can compete with the others. That leaves us with two choices.
First, the system with electric power distribution needs electrohydraulic actuators which should fulﬁll these demands: The pump should run in a wide range of operating speeds; friction losses should be low, also at close to zero speeds; leakage losses need to be low, to reduce dead band and nonlinear behavior; the pump must have low noise and pulsation and, if possible, work in all four quadrants, that is as a pump and a motor in both rotational directions.
These machines are not yet available: they need to be developed. I like this system because of its modular approach. This development will surely beneﬁt from the electrification trend. But electrohydraulic, speed-controlled actuators are by no means simple. Each actuator has its own miniature hydraulic circuit, isolated from the others, with its own reservoir, ﬁlter and cooler. The reservoir is often an accumulator, not for energy recuperation but to avoid cavitation for the four-quadrant operation And due to friction and leakage, the system is often non-linear and suffers from dead band.
But the market demands accuracy and high dynamic performance. Thus you will need multiple inner- and outer loop controllers, backstepping sliding-mode controllers, and other advanced control technologies. My main concern is that the system essentially is velocity controlled, which makes it jerky.
In the case of a hydraulic power distribution, the common pressure rail is the backbone of this system, much like the electric power grid in your home. All loads receive the same common pressure. The main power supply can be an internal combustion engine or electric motor. Essential for this system are hydraulic accumulators. And because pressure level in the accumulator varies, the CPR-system is not a constant pressure system. CPR systems also require hydraulic transformers, which are not readily available on the market. We have a job to do.
However, CPR systems basically need only one reservoir, cooler and ﬁlter. They offer excellent opportunities for energy recuperation and power management. Control is not based on velocity or ﬂow control, but on pressure or load control. You are controlling acceleration, not speed, which greatly enhances accuracy. Finally, CPR systems are extremely efﬁcient. Compared to conventional valve control, losses can be reduced by nearly 90%.
I don’t know which approach will win out. Both offer tremendous advantages in terms of energy efﬁciency and power losses — the OEM industry will make the choice. But, one conclusion is certain: poor energy efﬁciency is not a necessary evil. It is not a law which is carved in stone, we can change this. Inefﬁcient hydraulic systems are a meme, just like the meme that says that climate change is a hoax.
Read more in Part 2, .”Reducing costs and fostering innovation.”
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