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
[email protected]
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:
N acc
Wloss
Win
N act
Wacc ,i
i 1
Wact , j
(1)
j 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.
Consumer
A
Consumer
B
PA=pA×QA
PB=pB×QB
PHP=pHP×QHP
Hydraulic energy
storage HP
Prime
mover
Pmech= ×
Hydraulic Power Management System
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
density of hyydraulics is lost. The general
g
challlenge is thhat each pum
mp and its electric
mootor must bee dimensionned accordin
ng to the peeak power of
o the actuattor.
Hy
ydraulic trannsformers mix
m the con
nstant presssure and variable presssure approaach. The
system has a cconstant preessure rail and
a each acttuator has its own transformer, whhich fits
thee pressure according to
t the load
d. Again, bboth analoggue [10] an
nd digital [[11, 12]
sollutions exxists. Hydrraulic enerrgy recupeeration is straightforrward, butt large
acccumulators are neededd because off the constaant pressure rail. Transfformer lossees seem
alsso to reducee the degreee of energy recuperation
r
n quite mucch [9, 13].
Thhe newcomeer is digital hydraulic power
p
mannagement sy
ystem, which has been studied
in [14–16]. Inn the basic form, the solution coonsists of onne “pump-m
motor-transfformer”
haaving a num
mber of inddependent outlets.
o
Thiss eliminatess the need for severall pumpmootors or traansformers and simpliffies the meechanical deesign. Presssure and flo
low rate
(inncluding diirection of flow) of each outlett can be controlled
c
independen
i
ntly and
preessure transsformation happens
h
auttomatically.. There is practically no limitationn for the
preessure amplification, which
w
allow
ws the full uutilization of
o accumulaator energyy storing
cappacity. It has been reccognized thaat the functtionality off the machin
ne is very vversatile
wh
hen comparred to the earlier
e
solutions. It cann satisfy all the conditiions for thee highly
effficient hydrraulics incluuding optim
mal utilizatioon of accum
mulators. Th
hus, the new
w name
“D
Digital Hyddraulic Pow
wer Manageement Systeem” (DHPMS) is inttroduced annd used
hereafter. A drawback
d
off the machine is its cenntralized naature, which
h means lonng hoses
in many appliications. Thhis may requ
uire valve coontrol, whicch increasess losses.
Thhis paper annalyses DH
HPMS appro
oach in genneral level. The operaation princiiple and
funnctionality of the maachine are first discuussed follow
wed by th
he analysis of the
controllabilityy and lossess. Several ap
pplication aalternatives are also preesented.
2. OPERATI ON PRINC
CIPLE OF DIGITAL
D
H
HYDRAULIIC POWER
R MANAGE
EMENT
SYSTEM
2.11. Generall Functionallity
Thhe Digital Hydraulic Power Management
M
System (DHPMS) has
h a num
mber of
ind
dependent outlets. Onne of them
m is low-ppressure (L
LP), which
h is normaally the
preessurized taank line. Seccondly there is an optioonal outlet for high-preessure accuumulator
(H
HP), which is used as the energy
y storage. F
Finally, theere is pre-d
defined num
mber of
acttuator outleets (A, B, C, D, etc.) deepending onn the designn of the macchine. The ddrawing
sym
mbol is shoown in Figurre 2.
D
Figure 2. Drawing ssymbol of DHPMS.
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.
From prime
mover to outlet
From outlet to
prime mover
From outlet to
another
Any combination
Etc.
Etc.
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
priinciple is exactly
e
the same as in digital ppump-motorrs [17], butt the DHPM
MS has
additional vallves for extrra outlets.
Figgure 4. On
ne unit of thhe piston typpe DHPMS.
Thhe state of valves
v
is chhanged at bo
ottom dead centre and top dead ceentre of thee piston.
Prooper sequeencing of valve
v
openings allowss pumping to or moto
oring from any of
ou
utlets. Idle mode is allso possiblee by keepiing LP valve open co
ontinuouslyy. Some
examples of control
c
sequuences are:
Suctioon phase froom LP, pum
mping phasse to A: Pum
mp to A po
ort, power iis taken
from pprime moveer
Suctioon phase frrom A, pu
umping phaase to LP: Motor fro
om A port,, power
recupeeration to prrime mover
Suctioon phase frrom HP, pu
umping phaase to A: Hydraulic power
p
flow
ws from
accum
mulator to poort A. Addiitional pow
wer is needed from prim
me mover, iif pHP <
pA. Power recuperration to priime mover exists if pHPP > pA.
Suctioon phase from A, pum
mping phas e to B: Hy
ydraulic pow
wer from A to B.
Additiional powerr is needed from primee mover if pA < pB. Pow
wer recuperration to
prime mover exissts if pA > pB.
It is importannt to note thhat suction and pumpinng phases happen
h
at different
d
tim
me. This
meeans that above
a
discuussion is valid
v
for aaverage pow
wers only. Energy iss stored
tem
mporarily innto the inerrtia of the prime
p
moveer and big inertia is neeeded if thee system
haas one unit oonly.
Thhe piston tyype DHPMS
S is achieveed by conneecting severral units in parallel. A simple
example is shhown in Fiigure 5. On
nly one acttuator outleet is shown
n because oof space
lim
mitations, buut additionaal outlets can be added by simply adding
a
moree valves.
Figure 5.. A piston type DHPM
MS with fou
ur units andd one actuato
or outlet.
Anotheer type of DHPMS
D
is based on fixed displacement unnits, such as
a gear pum
mpmotor. One unit iss shown in Figure 6 annd parallel connection
c
can be mad
de similarlyy as
in the piston
p
type machine
m
as shown in Figure
F
7. Th
his system hhas the samee functionallity
than piiston type unit
u with thhe exceptioon that flow
w is smooth
th and that pumping and
a
motorinng of each unit
u happenns at the sam
me time. Im
mportant bennefits of thiss approach are
smoothh flow, relax
xed valve rrequirementts, faster reesponse andd easier con
ntrol [15]. The
T
challennge may be efficiency oof the machine.
In bothh types of DHPMS
D
the hydraulic power
p
at ou
utlets is trannsformed in
nto mechaniical
power at common
n axis. Thuus, the pow
wer flows of Fig. 2 gooes through
h the comm
mon
mechannical axis.
Figure
F
6. F
Fixed displaacement uniit as a unit oof DHPMS..
Figure 7. DHPMS based on thhree fixed displacemen
d
nt units.
2.33. Controllability of Flow
F
Rate
Thhe sum of all
a outlet flows of DH
HPMS is zeero if externnal leakage is neglecteed. This
meeans that onne outlet is uncontrollab
u
ble and it siimply proviides or receiives oil usedd by the
oth
her outlets. This speciaal outlet is LP
L in the noormal case.
2.33.1.
Pistoon Type DH
HPMS
It is
i claimed tthat any floow rate is po
ossible withh digital pum
mp-motor [17], but thiss is true
forr the averagge flow only. If only one
o piston ppumps oncee per second
d, for exam
mple, the
ressulting flow
w rate is verry irregularr and unsuittable for mo
ost applicattions. Accuumulator
cann be used too smooth thhe pressure ripple, but it results inn slow presssure dynamiics. The
hig
gh bandwiddth pressurre control is essentiaal in variab
ble pressure energy eefficient
systems and therefore the
t dampin
ng element must be small
s
and flow
f
rate m
must be
sm
mooth. Smoooth flow ratte reduces also
a torque ppulsation at the cranksh
haft.
Co
onsider macchine with following
fo
feeatures:
Machiine has N piistons each having equaal stroke an
nd diameter
N is innteger of thrree
Pistonns follow sinnusoidal trajjectory
The phhase shift b etween pistons is equall
Thhen each outtlet has at leeast the folllowing relattively smooth flows [14
4]:
Q
3M
Qmax , M
N
N
,M
3
(2)
We caall these “p
principal fllows”. There are also
o several oother smoo
oth flow raates
especiaally, if the number
n
of ppistons is big. Howeverr, the 3Qmaxx/N is the sm
mallest smooth
flow raate and (N-3
3)Qmax/N thhe biggest below
b
Qmax. If N =15, then princiipal flow raates
are 0, 20 %, 40 %, 60 %
%, 80 % and 100 % of Qmax, for examp
ple. These are
obtaineed by follow
wing controll sequences:
u0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
u1
1
0
0
0
0
1
0
0
0
0
1
0
0
0
0
u2
1
1
0
0
0
1
1
0
0
0
1
1
0
0
0
u3
1
1
1
0
0
1
1
1
0
0
1
1
1
0
0
u4
1
1
1
1
0
1
1
1
1
0
1
1
1
1
0
u5
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
(3)
where one
o means that
t piston ppumps to th
he outlet in question annd zero meaans pumpingg to
some other
o
outlet.. The good property off principal flow rates iis that they
y can be freeely
mixed. If we use u2 to get 40 % of maxiimum flow to outlet A,, then it is possible
p
to use
u
shifted version of u2 (0 0 1 1 0 0 0 1 1 0 0 0 etc.) to pump 400 % of max
ximum flow
w to
outlet B,
B for exam
mple, and theere is still 20
2 % of flow
w available to some oth
her outlet. The
T
only lim
mitation is that
t each pisston pumpss to exactly one outlet.
Fiigure 8. Poossible flow
w combinations of two outlets wheen
princippal flows off the 15-pistton machinee are used.
As the pump and
d motor moodes of eacch piston un
nit are com
mpletely ind
dependent, the
above discussion
d
is
i valid for motoring also.
a
It is po
ossible to frreely mix an
ny of negattive
princippal flow rattes as well . The “con
ntrollability map” of a 15 piston
n unit for two
t
actuatoor outlets is shown in Figure 8. Each
E
dot sh
hows one poossible flow
w combination
and it is assumed that
t HP flow
w is zero. The
T map is different
d
forr each HP flow
f
value, but
it does not have “h
holes”.
2.33.2.
DHP
PMS Based on Fixed Units
U
An
n additionall feature of DHPMS based on fixxed units is that each unit can easiily have
diffferent displlacement, which
w
allow
ws us to improve contro
ollability. When
W
displaccements
aree different, it is also poossible to use
u differencce of flow rates
r
as welll. These addditional
degrees of freeedom leadd to question that whatt the optim
mal displacem
ments of unnits are.
Thhis optimal “coding” depends
d
on the numbeer of actuator ports as discussed in [15].
Fibbonacci codding (1:1:2:3:5:8 …) iss good for ssystems hav
ving HP porrt and two aactuator
po
orts. The coontrollabilitty maps fo
or some nuumber of units
u
are sh
hown in Fiigure 9.
Co
ontrollabilityy improves rapidly wh
hen the numb
mber of unitss increases
Figure 9. Controllaability map s for DHPM
MS based on
n fixed
dissplacement units whenn Fibonacci coding is used.
2.44. Controllability of Pressure
P
Asssume that pipeline
p
dynnamics can be neglecteed and the actuator
a
outtlet is conneected to
a hydraulic
h
caapacitance. It follows from the prressure builld up equatiion that thee rate of
preessure is prroportional to the diffeerence betw
ween inflow and outflow. As onlyy certain
floow values are availabble, it can be concluuded that exact contrrol of presssure is
im
mpossible. There
T
are ceertain discreete rates of ppressure av
vailable and
d zero rate ddoes not
generally exisst, if outfloow is nonzeero. Thus, itt is possiblle to contro
ol pressure ttowards
tarrget value (with
(
differrent rates) but
b it is noot possible to
t keep preessure at thhe target
vaalue. This reesults in seesaw type prressure behaaviour arounnd the targeet value.
3. POWER MANAGEM
M
MENT
Thhis chapter discusses power man
nagement sstrategies of
o DHPMS in generaal level.
Loosses are negglected in order
o
to keep
p the analyssis simple.
3.11. Controllability of Hydraulic
H
Power at Outtlets
Diiscussion inn Chapter 2 shows that pressure att each outleet of DHPM
MS can be w
whatever
bu
ut the flow rate
r has onlyy certain diiscrete valuees. As the hydraulic
h
po
ower is the product
of flow and pressure,
p
thee exact pow
wer matchinng is imposssible with DHPMS.
D
Thhere are
at least follow
wing approaaches to tack
kle this probblem:
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:
PH , act
QA pA QB pB QC pC
(4)
where subscript H refers to hydraulic power. Now the total hydraulic power is:
PH
PH ,act QHP pHP QLP pLP
(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
PH
(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:
p0V0
p V0 Voil
(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:
Voil
W
p dV
V 0
p0 V0
V0 Voil
V0
1
1
V0 Voil
V0 Voil
(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 LPline 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.
Figuree 10. Some possible waays to connnect DHPMS
S and cylind
der actuatorr via
distributedd valve system. Connecction to hyddraulic moto
or is similarr.
5.22. Direct C
Connection of DHPMS
S and Actuattor
Figgure 11 presents the diirect connecction of DH
HPMS and actuator.
a
Sym
mmetric acttuator is
thee easier casee and smoooth velocitiees are achievved at least by using prrincipal flow
ws. The
velocity resollution is pooor in this ap
pproach, butt this might be improveed by some kind of
sw
witching conntrol. Case (c)
( is more difficult as different floow rates aree needed at outlets.
Thhis case maay also be solved by
y switching control. The
T big ben
nefit of thee direct
connection is that losses are minimizzed, but its functionalitty is uncertaain.
Figuree 11. Directt connectionn of DHPM
MS and actuaator.
5.33. DHPMS
S and Consttant Pressurre Systems
DH
HPMS can be used to
t maintain
n constant pressures needed in constant ppressure
systems. Verrsion (a) of Figure 12 uses
u
energyy storing accumulator and
a active ppressure
control at connstant pressuure lines CP
P1 and CP22. Benefits are
a that pressures can bbe truly
constant and tthat energy storing cap
pacity of acccumulator is
i much bigger as discuussed in
Seection 3.3. The drawbback is thatt power floows throughh DHPMS from the cconstant
pressurre lines to HP
H accumullator and viice versa, which
w
increaases losses. Version (b)) is
closer to normal CP system
ms and big accumulators are neeeded for en
nergy storiing.
Importaant benefit is that theree are hardly
y any requirrements for smoothness of flow raates
of DHP
PMS outletss.
Figgure 12. Tw
wo alternativve ways to implement constant prressure liness by DHPM
MS.
5.4. DHPMS
D
as Transforme
T
er
A new idea is to use DHPM
MS without prime mov
ver. Then th
the torque balance
b
of the
machinne determin
nes its rotattional speed
d. Inertia lo
oad may bee needed in
n order to get
sufficieent controlllability off the rotatiional speed
d. The diffference to
o the norm
mal
transforrmer is thatt DHPMS ccan have anyy number of
o outlets as shown in Figure
F
13. The
T
controll problem is to contro l rotationall speed acco
ording to fl
flow deman
nds and torqque
balancee such that target
t
speedd is achieved
d.
Figuree 13. DHPM
MS as hydraaulic transfoormer.
6. PRA
ACTICAL CONSIDER
C
RATIONS
6.1. Piston
P
Type DHPMS
The currrent valve technologyy causes thatt the easiestt machine tyypes are rad
dial piston and
a
inline machines.
m
Both
B
have ssufficient sppace for con
ntrol valvess and are eaasy to modiify.
They have
h
also go
ood efficienncy although
h inline maachines are seldom useed in hydrauulic
applicaations. A diffficulty in bboth types iss that the nu
umber of pisstons is usuaally too smaall.
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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