The Fourteenth Scandinavian International Conference on Fluid Power, May 20-22, 2015, Tampere, Finland
HYDRAULIC HYBRID ACTUATOR: THEORETICAL ASPECTS AND
SOLUTION ALTERNATIVES
Matti Linjama*, Mikko Huova*, Matti Pietola**, Jyri Juhala**, Kalevi Huhtala*
*Department of Intelligent Hydraulics and Automation, Tampere University of Technology
P.O. BOX 589, 33101 Tampere, Finland
[email protected]
** Department of Engineering Design and Production, Aalto University
ABSTRACT
This paper presents and analyzes a hybrid solution, in which the hydraulic energy storage element is
integrated to the hydraulic actuator. The approach results in a new system layout–a distributed hybrid
system–in which only mean power is transmitted between the actuators and the high power peaks are
handled locally. Three different implementations are discussed. A multi-actuator excavator load cycle is
analyzed and dimensioning of the components is discussed. Limitations of the approach are also discussed.
KEYWORDS:
hydraulic hybrids, hybrid actuator
1. INTRODUCTION
A traditional hydraulic system is based on a centralized pump unit that produces hydraulic power for all the
actuators of the system. The main advantages of the solution are its simple layout and its low price. The
main disadvantage is poor energy efficiency because one pump can produce only one pressure level for the
system. Several approaches have been suggested to improve efficiency: hydraulic transformers [1, 2], pump
controlled actuators [3], hydraulic hybrids [4, 5, 6], distributed valve systems [7], and the use of several
pressure sources [8–11]. Electrohydraulic actuators are another strong trend; these are used routinely in
modern airplanes and also in some industrial applications [12, 13]. The concept typically includes a variable
speed electric motor, a fixed displacement pump, and a low-pressure accumulator as an oil reservoir and a
cylinder. The drawback of this solution is the one-to-one connection between the electric power source and
the hydraulic power. This means that the electric motor must be able to produce the peak power of the
actuator, which results in either low maximum power or a bulky solution. The problem can be significantly
reduced by integrating the electric motor and the hydraulic pump into a compact unit [14], but the electric
motor and the hydraulic pump must still be able to produce the maximum power of the actuator.
Hydraulic hybrid solutions are already used in commercial machines. Komatsu’s hybrid excavator has an
electric swing actuator, while others are traditional hydraulic actuators [4]. A supercapacitor is used as the
energy storage and the estimated reduction in the fuel consumption is 25%. Caterpillar has presented a
hydraulic hybrid excavator, which uses distributed control valves and a hydraulic pressure accumulator as
the energy storage [5]. Only a swing drive has an energy recovery function and the estimated reduction in
the fuel consumption is also 25%. In both cases, the reduction of the fuel consumption is moderate because
only the swing actuator has an energy recovery function and because boom actuators use throttling control.
Significantly larger fuel savings are possible if all the main actuators have an energy recovery function and if
they are controlled without throttling. Kobelco has introduced a solution in which both the swing drive and the
boom lift actuator have energy recovery [6]. The throttle-free pump control is used and the impressive 40–
60% reduction in the fuel consumption has been achieved. The pumps are driven by electric
motor/generators and the system layout is an electric series hybrid. The drawback of the solution is that it is
complex having three pumps, four speed variable electric motors, a supercapacitor and a Ni-MH battery. A
solution based on hydraulic transformers has been theoretically analyzed in [2] and the simulation results
showed a fuel consumption reduction of up to 50% for the wheel loader. However, the hydraulic transformers
are not commercially available. Hybrid solutions based on the utilization of three pressure levels have been
presented by Lumkes and Andruch [8], Erkkilä et al. [9], Vukovic et al. [10] and Ketonen et al. [11].
The common feature of the previous hybrid solutions is that they have centralized energy storage. The
benefits of the approach are a simple layout and the possibility to transfer energy from one actuator to
another. The drawback is that big power must be transferred between the energy storage and the actuators.
An alternative approach is studied in this paper. The hydraulic accumulator is integrated into the actuator
and it is used to cover the peak power of the actuator. Only the mean power is transmitted into the actuator
while the power peaks are handled locally at the actuator. Although the idea is simple, the authors have not
been able to find any related publications.
2. THEORETICAL ANALYSIS
2.1.
System Layout
The two possible system layouts are shown in Figure 1. Both alternatives are based on the idea that big
power is kept at the actuators while mean power is transmitted to the actuators via a low power hydraulic line
(version (a)) or an electric line (version (b)). The latter version requires an integrated pump and a low-power
electric motor at the actuator. In both cases, following power flows are needed:
1) From the mean power source to the accumulator (accumulator charging)
2) From the accumulator to the actuator (satisfaction of power peak demands)
3) From the actuator to the accumulator (energy recovery)
4) Optionally, from the mean power source to the actuator
It is assumed that there is no need for the power from the actuator or the accumulator to be fed back to the
mean power line, which simplifies the mean power supply system.
Figure 1. Two possible ways to implement hybrid actuators.
2.2.
Lossless Power Analysis
Consider a hydraulic cylinder with piston areas AA and AB, stroke xmax and maximum pressure pmax. The
maximum power of the actuator is:
Pmax
Fmax vmax
pmax AAvmax
(1)
The maximum power available from the MP line is:
PMP, max
QMP, max pMP
Pmax
(2)
where the design parameter tells how big a part of the actuator power the MP line can produce. The power
needed from the accumulator is:
PAcc , max
1
Pmax
(3)
The average power needed by the actuator is:
tC
Fv dt
Pavg
0
tC
(4)
where the integration interval tC covers the complete load cycle of the actuator. The MP line must cover the
average power of the actuator, and the lower limit for is thus:
Pavg
Pmax
(5)
2.3.
Accumulator Sizing
If ideal pressure transformation between the accumulator and the actuator is assumed, the accumulator
sizing can be made according to its energy balance. Consider the work cycle shown in Figure 2. The
maximum power is needed to extend the piston and the same power is recuperated during the retracting
movement. The average power required is zero; thus, it is impossible to charge the accumulator from the MP
line. If MP line were used, the energy would cumulate to the accumulator. All the energy is first taken from
the accumulator and then recuperated; this cycle represents the worst case. The energy needed in the first
half of the work cycle is:
Emax
Fmax xmax AA pmax xmax
(6)
and this is the worst case scenario for the accumulator capacity. In reality, the situation is much better
because actuators do not work against the maximum force for the whole stroke and because the MP line can
be used to produce part of the energy needed. If the load cycle is such that full capacity of the MP line can
be used, the amount of energy needed from the accumulator is reduced to:
Emax
1
Fmax xmax
(7)
Figure 2. The worst case load cycle.
The typical actuator of a medium size excavator has a diameter of 130 mm and a stroke of 1 m. This results
in the worst case accumulator energy of 465 kJ, if the maximum pressure is assumed to be 35 MPa. This
kind of energy storage capacity is achieved with about 45 l accumulator (p0 = 11 MPa, pmax = 35 MPa). The
oil volume of the A-chamber of the cylinder is 13 l and a rule of thumb is that the accumulator volume is 3.5times the volume of the cylinder chamber. This means that the physical size of the accumulator is
significantly bigger than the size of the actuator in the worst case scenario.
3. IMPLEMENTATION OPTIONS
3.1.
Implementation with a Speed Variable Pump-motor
One way to implement the hybrid actuator is to use the speed variable pump-motor. A possible
implementation is shown in Figure 3. The system has a 2:1 differential cylinder and it is controlled by the
speed variable pump-motor. The inlet of the pump-motor is connected to either the tank line or to the
accumulator line, which allows the pressure boost without increasing the size of the electric motor.
Figure 3. Hybrid actuator with a speed variable pump-motor.
Figure 4 shows the different control modes of the system together with the pressure ranges. It is assumed
that accumulator pressure is 10 MPa and the maximum pressure is 20 MPa. The logic valves are not shown
in order to simplify the pictures. It is seen that the only difficult situation is the large overrunning load with the
extending direction of the movement. This kind of situation occurs rarely and it could be handled with the
proportional valve between the B-chamber and the accumulator.
Figure 4. Driving modes of the system with the speed variable pump-motor.
3.2.
Implementation with a Multi-chamber Cylinder
The drawback of the solution presented in Section 3.1 is that it requires an electric servomotor with a peak
power that is about half of the maximum power of the actuator. This makes the solution expensive and
unnecessarily bulky, especially if the mean power is much smaller than the peak power. A secondary
controlled multi-chamber cylinder is one approach that can be used to solve the problem [15]. The idea is to
have a cylinder with four piston areas in ratios 8:4:2:1 and to connect these chambers to either the highpressure line or the low-pressure line via low resistance logic valves, as depicted in Figure 5. The result is an
actuator with 16 different equally spaced output forces, which can be utilized in the secondary control. The
experimental results in [15] have shown that it is possible to implement energy efficient motion with a high
ratio between the peak and mean powers. The challenge of the approach is controllability with small inertia
loads, which can be solved by using slight resistance control [16]. The difference between the original
publication [15] and Figure 5 is subtle: the high-pressure accumulators are now integrated on the actuator
and, thus, the high power is handled locally.
Figure 5. Hybrid actuator implemented using a four-chamber cylinder [15].
3.3.
Compact Implementation of a Constant Pressure Accumulator
The solutions presented in Sections 3.1 and 3.2 require a constant pressure accumulator. As the energy
storage capacity of the accumulator depends on the ratio between its maximum and minimum pressure, very
big accumulator is needed, which negates the compact hybrid actuator idea. One solution to this problem
has been presented in [17]. That solution is a piston accumulator in which the piston has four areas as
shown in Figure 6. This allows almost full utilization of the energy storing capacity of the gas volume.
Figure 6. Multi-area piston accumulator [17].
3.4.
Multi-pressure Implementation
An alternative to the multi-chamber cylinder is the combination of the standard cylinder and multiple pressure
sources. Again, the straightforward implementation with multiple accumulators and a loading pump yields a
bulky solution so some kind of integrated solution is needed. One such sketch is shown in Figure 7 in which
only one gas volume is used together with several independent pistons. The pressures have fixed ratios and
the usage of the accumulator changes all the pressure levels. The loading of the accumulator is made to the
highest pressure chamber only. The positions of the other pistons are controlled by active selection of the
control mode of the system. The proportional valves shown in figure are optional and they may be needed in
small inertia systems. The solution resembles the three pressure hybrid systems [8–11], but the principle is
different. The local energy storages are used and intermediate pressures are also generated locally.
Figure 7. Hybrid actuator implemented using a multi pressure piston accumulator.
4. EXCAVATOR LOAD CYCLE ANALYSIS
4.1.
Introduction
The analysis presented in Section 2.3 shows that the limited accumulator energy storing capacity is a big
challenge. The analysis is based on the worst case load cycle scenario and the situation may be much better
in practice. This is why an excavator load cycle is analyzed. A 21 ton wheeled type excavator equipped with
mobile proportional valves was used. The output powers of the actuators are calculated as a product of
piston force and velocity, and the piston force is calculated from the measured chamber pressures.
4.2.
Load Cycle
The load cycle is medium speed digging of macadam from the outermost position of the bucket. The unloading position is about 2.5 metres above the ground level and close to the machine. Figure 8 shows the
measured actuator positions and the output powers of the load cycle. The mean powers of the boom, arm,
bucket, and swing actuators are 1.21 kW, 3.79 kW, 2.95 kW, and 0.78 kW, respectively. These are very
small values when compared to the peak powers, which are 64.4 kW, 27.7 kW, 48.0 kW, and 22.1 kW.
Figure 8. Measured actuator positions and output powers of the digging cycle.
4.3.
MP Power and Accumulator Sizes
The minimum value for the MP power is the sum of the mean powers of the actuators, which is only 7.9 kW.
However, the MP power is selected to be 20 kW, which takes into account the fact that the load cycle is not
too aggressive and that losses occur in the system. Assuming that 5 kW is lost, 15 kW from the MP line is
still available to the actuators. The power curves show that the boom actuator requires the most energy and
the energy used for lifting is 210 kJ. The lift movement takes 7 s and if half of the MP power is available for
the boom actuator, the energy taken from the MP line is 52.5 kJ. The boom accumulator energy storing
capacity should thus be about 160 kJ. However, the starting point of the digging is ground level and in
practice longer lifting movements are possible. This argument yields to an accumulator that is about doubled
in size, and it can be concluded that an energy storing capacity of 320 kJ should be enough. The
corresponding accumulator size is about 30 l. The worst case energy storing capacity of Eq. 6 is much
larger, 1040 kJ (two 130 mm cylinders, xmax = 1.12 m, pmax = 35 MPa).
5. LIMITATIONS OF THE APPROACH
5.1.
The Energy and Power Capacity of the Accumulator
The worst case analysis presented in Section 2.3 shows that the physical size of the accumulator is clear
limitation with some types of load cycles. On the other hand, the excavator load cycle analysis shows that
the situation is much better in practice. Wide pressure variation is required in order to utilize the full capacity
of the accumulator, which calls for some kind of pressure transformation between the accumulator and the
actuator. The multi-area accumulator [17] is an interesting alternative for implementing this. The power
density of hydraulic accumulators is good, but the challenge is how to guarantee good efficiency with rapid
loading and unloading of the accumulator. Some kind of insulation or heat regeneration solutions are
probably needed in order to guarantee proper and efficient operation of the accumulator [18, 19].
5.2.
Control of the Energy Balance: Under-power and Stagnation
The proper control of the energy balance of the accumulator is essential for the correct operation of the
hybrid actuator. If the energy level is too low, it is possible that the actuator might be temporarily unable to
produce enough power and under-power situation could occur. Conversely, if the energy level is too high
and the accumulator cannot receive additional power from the actuator, stagnation occurs. Predictive and
learning control are possible solutions for these challenges.
5.3.
Temperature
The amount of oil in the hybrid actuator is small and it is easy to overheat the system if its efficiency is not
good enough. If the mean power is 10 kW and efficiency is 80%, there is still 2 kW heat generation, which
might be difficult to dissipate without active cooling. Thus, the highly efficient solutions are thus needed.
5.4.
Controllability
All the solutions presented in this paper are based on the concept of different control modes. The switching
between control modes must happen smoothly, which is challenging. Fast components and proper control
methods are needed in order to guarantee good controllability. Fortunately, a new fast and high capacity
on/off valve has come into the market [20].
6. CONCLUSIONS
A hybrid solution has been presented in which the energy storage elements are integrated into actuators.
The feasibility of the approach has been analyzed and three different solution alternatives are given. The
energy storing capacity of the hydraulic accumulators is the biggest challenge in order to make the solution
compact enough. The size of the accumulator depends strongly on the load cycle of the actuator and it can
be concluded that the approach has potential in many applications.
ACKNOWLEDGEMENT
The research was funded by the Academy of Finland (Grant No. 278464).
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