p 031984
DESCRIPTION
P03194TRANSCRIPT
The Third Workshop on Digital Fluid Power, October 13 - 14, 2010, Tampere, Finland
DIGITAL HYDRAULIC POWER MANAGEMENT SYSTEM – TOWARDS LOSSLESS HYDRAULICS
Matti Linjama, Kalevi Huhtala Department of Intelligent Hydraulics and Automation Tampere University of Technology, Tampere, Finland
ABSTRACT
This paper discusses the general characteristics of digital hydraulic power management system. The principle is new and studied only in few research publications. Functionality, controllability and losses are discussed, and the conclusion is that the technology makes almost optimal power management possible. The technology also improves the energy storing capacity of the accumulator by factor of 2-3 when compared to traditional constant pressure systems.
KEYWORDS: Digital hydraulics, pump, motor, transformer, power management
1. INTRODUCTION
Two main application areas of hydraulics are hydrostatic transmission and control of hydraulic actuators. The focus of this paper is in the latter one. The efficiency of hydraulic actuation systems is usually very poor. Many tasks require small or even negative average mechanical power some examples being unloading of a truck or turning of an excavator, but they take big and continuous power from the prime mover in traditional hydraulic systems. The reason is that the design of hydraulic systems is poor from the energy efficiency point of view. All key components have already relatively good efficiency but system efficiencies remain below 10 percent. The result is excess fuel consumption, emissions, cooling systems and economical losses [1].
1.1. How to Measure Energy Efficiency?
The poor energy efficiency of hydraulic actuation systems is not fully recognized. Efficiency is poor indicator because of its limitations. Good efficiency is not needed if the actuator moves seldom or its power level is small. Also, efficiency is not defined for negative actuator power, which is very important to consider in the calculations. The correct indicator is energy loss, i.e. time integral of the power loss over the complete work cycle, which must be minimized. As the energy loss of hydraulic systems is under consideration, the input power into is the product of the rotational speed and torque of
the prime mover, and input energy Win is its time integral. The change of energy stored in hydraulic accumulator(s) must also be considered. Thus, energy loss is:
, ,1 1
acc actN N
loss in acc i act ji j
W W W W (1)
where Wacc,i is the change of energy in the i:th accumulator and Wact,j is work done by j:th actuator. It is important to consider complete work cycle when calculating energy losses. For example, analysis of the digging motion only gives all too small losses because return movement is neglected.
1.2. General Features of Energy Efficient Systems
The theoretical principle of the energy efficient hydraulic system is simple: losses must be small in all actuators. This means instantaneous power matching in all situations including negative actuator power. As hydraulic power is the product of flow and pressure, the possibilities for power matching are constant pressure plus variable displacement actuator, variable pressure plus fixed displacement actuators, and variable pressure plus variable displacement actuators. Important features of power matching are fast and accurate control of pressure and/or actuator displacement, and ability to handle negative flow rates.
Matching of negative actuator power implies that the system must have energy sink. This is preferably hydraulic accumulator because the transformation of energy into another form is avoided. Another option is to move power to other actuators having positive power requirement. Third option is to move power into the prime mover.
Hydraulic actuators can have very high peak power while the average power is much smaller. In order to avoid over-sizing of components, a good design slogan is “mean power from prime mover, peak power from energy storage”. Again, hydraulic accumulator is preferred energy storage component because energy transformations can be avoided and power density is good.
Further features of energy efficient hydraulic systems are that good components are used and throttling is avoided as far as possible. Valve control may be necessary in many applications because sufficient stiffness and controllability is difficult to achieve without any throttling. However, surprisingly small pressure differential is enough to introduce stiffness and good controllability [2]. If system pressure is 35 MPa and valve losses are 0.5 MPa per notch, the valve induced power losses remain below three percent.
The general features of energy efficient hydraulic system are summarized in Figure 1.
Figure 1. Power flow in energy efficient hydraulic system. Essential features are possibility for two directional power flows, hydraulic energy storage, exact
power matching according to consumer demands, and small losses in all power paths (denoted by red bend arrows).
1.3. Alternatives for Energy Efficient Systems
Let’s start from the constant pressure systems where the well known example is secondary controlled motors. Losses are relatively small and controllability is nowadays good also near zero velocity. Up to 70 percent energy recuperation has been demonstrated in the active wave compensation [3]. The approach has recently been extended to hydraulic cylinders having discretely adjustable force [4]. The challenges of the secondary control are that it does not work properly with small or unknown inertia and that large accumulators are needed for energy storing due to constant pressure approach. A new variant of the constant pressure systems is the combination of the multi-chamber cylinder and distributed valve approach, in which about 50 percent reduction of losses has been demonstrated when compared to traditional load sensing system [5]. Throttling control is used but valve losses are minimized by adjusting effective piston area stepwise.
The best known variant of the variable pressure systems is Load Sensing (LS). It is not energy efficient approach, because it does power matching for one actuator only and because traditional valves and pumps cannot handle energy recuperation, i.e. negative flow rate. Better approach is electric LS system with bi-directional distributed valve system where valves can be traditional [6] or digital [7]. Typical reduction of power losses is 30–40 percent when compared to traditional LS [6, 7] and losses can still be reduced by using pressurized tank line [8]. The fundamental drawback of any LS approach is that energy cannot be easily stored into hydraulic accumulator because high-bandwidth pressure control is needed. Thus, energy recuperation requires special pump with Mooring function.
Pump controlled actuators is another class of variable pressure systems. Each actuator has its own pump, which can be driven by common prime mover or by individual electric motors. The common prime mover approach yields long hosing and reduced performance. Pump losses are also significant because they work at partial displacement most of the time [9]. If each pump has its own electric motor, the benefit high power
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The machine is rotated by the prime mover having sufficient inertia in order to suppress torque ripple caused by the machine. Rotational speed can be constant or variable. The machine has certain maximum time-averaged flow rate Qmax, which depends on rotational speed, geometrical displacement and volumetric losses as in normal pumps or motors. The average flow rates have following constraints (outflow positive):
1) Absolute value of flow at each outlet is smaller than or equal to Qmax 2) Sum of positive outlet flows is smaller than or equal to Qmax
3) Sum of negative outlet flows is bigger than or equal to –Qmax
The most important feature of DHPMS is that each outlet (excluding LP port) can be controlled independently. Pressures at outlets have practically no effect on losses and transformation of pressure happens automatically. This means, for example, that it is possible to take energy from the HP accumulator to load even if pressure in accumulator is smaller than load pressure. Also, the accumulator can be charged from any load pressure independently on accumulator pressure. This feature allows best possible utilization of the energy capacity of the accumulator. Figure 3 shows some possible power flows of DHPMS.
Figure 3. Some possible power flows of DHPMS.
2.2. Detailed Operation Principle of DHPMS
The DHPMS consists of several units each having two states: Pump oil to exactly one of the outlets or receive oil from exactly one of the outlets. So far, two different implementations have been presented, reciprocating piston [14] and fixed displacement unit (e.g. gear pump-motor) [15]. Figure 4 shows one unit of the piston type DHPMS. If the pre-compression and pressure release phases are neglected, exactly one valve is open at each time instant. When the piston moves in the extending direction, oil is pumped into LP, HP, A, B or C outlet depending on, which valve is open. When the piston moves in retracting direction, oil is sucked or “motored” from one outlet. The
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pal flow rator outlets iss assumed tnot have “h
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ossible flowpal flows of
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he outlet in f principal imum flow 0 0 0 etc.)
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o several or, the 3Qmax If N =15, % of Qmax,
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question anflow rates ito outlet A,to pump 40
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nd zero meais that they, then it is p0 % of maxto some oth
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4. Control
ssume that phydraulic caessure is prow values
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POWER M
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1. Control
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PMS Based
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whatever product here are
1) Increase the resolution of the flow rate such that power matching is accurate enough. This means bigger number of pistons or fixed displacement units.
2) Use hydraulic capacitance to decrease pressure gradient caused by inexact flow rate. Correct average flow rate and pressure are achieved by repetitive switching between two closest flow rates. This approach was successfully used in [14, 15].
3) The next bigger flow rate is selected and the excess flow is drained to tank. This approach is possible when distributed valves are used together with DHPMS, but it slightly increases losses.
3.2. Control of Power Balance
The hydraulic power of actuator outlets is:
,H act A A B B C CP Q p Q p Q p (4)
where subscript H refers to hydraulic power. Now the total hydraulic power is:
,H H act HP HP LP LPP P Q p Q p (5)
As the LP flow is not controlled, the hydraulic power can be balanced by selecting suitable HP flow. The boundary conditions are:
Hydraulic power must not exceed the maximum or minimum power available from the prime mover. Minimum power can be negative.
Accumulator pressure must stay within predefined limits
Too big transients should be avoided in order to reduce torque ripple.
Prime mover should work at its optimal operation range when possible.
3.3. Control of Torque of Prime Mover
Torque control is closely related to the control of hydraulic power, because their relation is
HP (6)
The average torque must not exceed the minimum or maximum torque of the prime mover. Short over torque is allowed if the system has sufficient inertia. An example of this is simulations presented in [14, 15] where flywheel was used together with very small prime mover. This approach requires careful and active control of hydraulic power. It is important to use smooth flow rates only in order to keep torque ripple at acceptable level.
3.4. Control of HP Accumulator
The purpose of the HP accumulator is to satisfy peak power requirements of the system and to allow the prime mover to produce mean power only. This downsizing of the prime mover reduces weight and losses, especially if Diesel engine is used as the prime mover. The selection of the control strategy of the HP accumulator is not trivial, because it depends on the system and its work cycle. The future actions should be known for the optimal control and some simpler approaches must be used in practice. The control problem is analogous to hybrid cars. One option is to control the state of the accumulator such that it is charged to about half of its maximum energy. Then it is possible react on both big positive and big negative power demands without running out of pressure range allowed.
A big benefit of the DHPMS approach is that it can fully utilize the energy storing capacity of the accumulator. Much smaller accumulator is enough than in constant pressure systems. This difference is highlighted by an example. The ideal gas equation of the accumulator is:
0 0 0 oilp V p V V (7)
where V0 is size of the accumulator, p0 pre-charge pressure and Voil is the volume of oil inside the accumulator. The energy stored in the accumulator is:
1
0 0 0 0 0 0
0
d1
oilVoil oil oil
V
p V V V V V V V VW p V (8)
Assume now that maximum pressure is 35 MPa and accumulator volume is 10 l. We assume for the “constant” pressure system that minimum pressure is 29 MPa. Energy storing capacity is maximized by using as high pre-charge pressure as possible and it is selected to be 26.1 MPa according to 0.9 pmin rule. The pre-charge pressure can be selected freely in the DHPMS and the optimal value is about 9 MPa (pmin = 10 MPa). Assuming = 1.4 gives energy capacity of 37 kJ for the constant pressure system and 100 kJ for the DHPMS, i.e. 270 percent more.
4. LOSSES OF DHPMS
In order to be competitive with electromechanical systems, the losses of DHPMS should be very small. As the piston type DHPMS is similar to digital pump-motor, its losses are also similar. Total efficiencies over 95 percent have been demonstrated by Artemis Intelligent Power by their radial piston digital pump-motor [17]. The efficiency remains good in very wide operation range. Merrill et al. [18] compared losses of the traditional swash plate unit and digital pump by simulations and found that digital machine has much better efficiency at low displacements and rotational speeds. These results are consistent with results demonstrated by Artemis.
Heikkilä et al. [16] studied efficiency of a six piston boxer DHPMS. The system suffered from internal leakage and too small flow capacity of the control valves. The
efficiency was about 80 percent and an important result was that efficiency does not drop in the power transfer mode.
There are several reasons for very good efficiency of the piston type digital machines:
1) Pre-compression can be optimized according to load pressure while the traditional valve plate can be optimized for one pressure only.
2) Pressure release function allows recuperation of the energy stored in the compressibility of fluid.
3) Displacement is adjusted by setting pistons into idle mode. Idle losses are very small.
4) Zero leakage seat valves can be used. Load holding is possible without any extra components.
It is important to remember that electrical losses can be big and they must be considered because the piston type machine requires continuous switching of valves.
DHPMS based on fixed displacement units utilizes traditional fixed displacement pump-motors and efficiency is similar, but control valves cause some extra losses. Losses also increase, if differences of flows are used to improve controllability. However, it is important to remember that total losses of the complete system can still be much smaller because of optimal power management.
5. APPLICATIONS OF DHPMS
5.1. DHPMS and Distributed Valves
Figure 10 shows some possible ways to connect DHPMS and a cylinder actuator via a distributed valve system. The small accumulator symbol means the damping element. The idea in each version is that DHPMS dynamically produces optimal supply pressure for each actuator and valves are used to achieve good controllability. Pressure losses of valves are minimized at each control edge in each case. Version (a) uses common LP-line for all actuators. Good properties are that differential connection is possible and that only one actuator outlet is needed per actuator. Version (b) has two adjustable pressures for one actuator. This may have more versatile controllability and improved stiffness in certain load conditions, but the cost is that two outlets are needed. Version (c) uses also two outlets for one actuator, but valve system is simplified. Differential connection is not possible with this version.
5.2
FigtheveswThco
5.3
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The valve requirements of the piston type DHPMS are very demanding as discussed in [14]. The requirements for the 15-piston machine with maximum flow of 100 l/min @ 1500 rmp are: durability of 109 cycles, response time below 2 ms, repeatability of 0.1 ms, flow capacity of 30 l/min @ 0.5 MPa, and energy consumption below 1 J per cycle. This kind of performance is very difficult to achieve and therefore it might be better to use several smaller valves in parallel. As discussed in [19], the replacement of one big valve with several smaller ones should yield faster response, smaller total size and smaller energy consumption. Additional benefits are that the valve system becomes fault tolerant and it is possible to control the opening profile. Recent research results show that one big and very fast valve is not the optimal way to control DHPMS and proper selection of the opening profile reduces pressure ripple [20].
6.2. DHPMS Based on Fixed Displacement Units
The easiest way to implement this type of DHPMS is to use machines with through axis. This rules out bent axis machines, for example. Valve requirements are much less demanding as shown in [15]. It might be good idea to use parallel connected valves in this solution also. As each machine has different displacement, the sufficient flow capacity can be achieved by increasing the number of parallel connected valves in bigger units, which allows the use of one valve type only.
7. CONCLUSIONS
Digital Hydraulic Power Management System is a newcomer for highly efficient hydraulic systems. Two different solutions have been presented so far: piston type DHPMS and DHPMS based on fixed displacement units. The prototype of the piston type DHPMS has already been implemented and the fixed displacement version works according to simulations.
It is expected that losses of the piston type DHPMS will be significantly smaller than in traditional transformer solutions. Even more important feature is its versatile functionality, which allows optimal power management. This means big potential in reducing losses in hydraulic systems. This is true for DHPMS based on fixed displacement units also even if losses of the machine itself are slightly bigger than in traditional machines. Yet one benefit of the DHPMS is that it can fully utilize energy storing capacity of accumulators, which means 2-3 times bigger energy storing capacity than in constant pressure systems.
The technology is at its infancy and lot of research is needed. The implementation of DHPMS based on fixed displacement units should be straightforward because commercial pump-motors can be used. The optimization of the switching between states needs further research. Also, losses should be measured and compared to other solutions. The difficulty in the piston type DHPMS is that it is difficult to find suitable “base machine”. The optimal machine is obtained by designing completely new one, but this is very demanding for universities.
Implementing the machine is the first step only. Control methods play very important role in DHPMS technology as in all digital hydraulic systems. These topics were only scratched in Chapters 2 and 3. The easiest version is the combination of DHPMS and
distributed valves (Figure 10). The direct connection (Fig. 11) is probably much more demanding. The transformer idea (Section 5.4) is new and its properties are no fully understood yet. The proper control of power and torque balance, and energy stored in the HP accumulator are challenging control problems as well.
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