Characteristic analysis of high-efficiency hydraulic system

Technology
Explanation

Characteristic analysis of high-efficiency
hydraulic system
HIRONAKA Tsuyoshi・TSUKUI Ken

1level approaching its maximum.
In this study, as a means of confirming the effectiveness and feasibility of this hydraulic system, we modeled
the entire system and built simulation technology that enables performance prediction. In addition, by utilizing
our simulator we clarified the design parameters that affect the performance and set the prospects for improving
the performance of the hydraulic system by coordinating
the control of each component.

2

Introduction

Recently, many countries have promoted carbonneutral activities from the viewpoint of the environment
and energy saving. Exhaust gas regulations for passenger
cars have also become stricter year by year. In responseto this trend, the automobile industry has accelerated its
efforts to improve the overall efficiency of vehicles. Thismeans that automobile transmissions are required todeliver higher efficiency.
The transmission uses a pump as its hydraulic powersource. Reducing the output from the pump section(specifically, reducing any excessive flow discharged
from the pump) will help improve the fuel efficiency.
That is why KYB has conventionally promoted efforts toimprove pump efficiency. However, there is limited scopefor further improvements in efficiency of the pumps
alone. Then, to achieve further improvement of efficiency
in the pump section, this study focused on a hydraulicsystem that combines two or more pumps and selector
valves and tried to confirm the feasibility.
As a means of confirming the effectiveness andfeasibility of this hydraulic system, we established an 1D
simulation Note 1) technology that enables performance
prediction. This paper explains the effort and the resultsof its evaluation.

Overview of KYB Hydraulic System

As shown in Fig. 1, the hydraulic system that wedeveloped in this study (hereinafter "the KYB hydraulicsystem") consists of a hydraulic pump driven by anengine (hereinafter "the mechanical pump"), anotherhydraulic pump driven by a separate electric motor(hereinafter "the electric pump"), and a change-overmechanism consisting of a solenoid and a selector valve.
The mechanical pump has two ports: main and sub.
These ports can be switched over by the change-overmechanism to deliver two-stage variable discharge.
The following describes the principle of operation of
the KYB hydraulic system. Normally, the discharge rateof the mechanical pump changes in proportion to the
speed of the engine. In other words, the pump may

Note 1) A method of simulation that expresses phenomena of
products and components mathematically and calculates
them spanning two or more physical phenomena.

Discharge totransmission

Main
EngineSub
Sub

Mechanicalpump

Abstract
Exhaust gas regulations for passenger cars have recently become stricter year by year as many countries
promote carbon-neutral activities from the viewpoint of
the environment and energy saving.
In response, manufacturers have sought the development of transmissions with higher fuel efficiency as one
solution as they accelerate their efforts to improve theoverall efficiency of their vehicles.
KYB has recently improved the efficiency of the pumpused as the power source for the transmission, reaching a

Motor

Solenoid

Change-over mechanism makes themechanical pump two-stage variable

Fig. 1― 10 ―

Selector valve

KYB hydraulic system

Electricpump

KYB TECHNICAL REVIEW No. 67 OCT. 2023

3

Feasibility Study

We checked in advance the effectiveness of the KYB
hydraulic system by conducting a system performance
evaluation using a simplified 1D simulation model. This
chapter describes the details of this effort. The 1D
simulation model we used was commercially available
SimcenterTM AmesimTM software (SIEMENS, Germany).
3. 1 Overview of Simulation Model
The simulation model basically consists of thefollowing three major sections as shown in Fig. 2:[1] Flow control section (hydraulic system): Uses theengine speed as input and controls the pump flowrate through electronic control of the solenoid andmotor.
[2] P ressure control section: Uses two or more voltage/electromagnetic valves to control the dischargepressure from the hydraulic system (hereinafter "thecontrol pressure").
[3] T ransmission control section: Controls thetransmission gear ratio according to the control
pressure.
To determine the effect of pump output reduction inthis simulation, driving data (engine speed and control
pressure) measured in the automobile’s fuel efficiency
evaluation mode was taken as input conditions. Then, thepump discharge with these input conditions wascalculated. In this study, we modeled two systems: theKYB hydraulic system and the conventional single
mechanical pump system. By comparing the pump output

Variable speed control section

Pressure control section
Current
commandTransmission
model

Hydraulic/electromagnetic valve model

Controlpressure
Flow control section(hydraulic system)
Enginespeedcommand

KYB hydraulic system

Single mechanical pump(conventional)

Pump
model

Selector
valve

Mechanicalpump

Mechanicalpump

Electric pump

Driving data
Input conditions

Target conditions(required flow)

流量［L/min］
Flowrate [L/min.]Targetpressure [MPa]
目標圧力［MPa］(engine speed, target
pressure)

Time [sec.]Time[sec.]
時間［s］

Fig. 2

Overview of simulation model

calculated in the same operation pattern between the twosystems, we checked the effectiveness of the KYB
hydraulic system.
3. 2

 onfirmation of Effectiveness of KYB
CHydraulic System
Fig. 3 shows a simulation comparison between the
single mechanical pump system model and the KYB
hydraulic system model (flow rate and pump output).
According to Fig. 3(a), both models meet the flow raterequired for the transmission. The Figure also indicatesthat the KYB hydraulic system successfully curbs theflow rate over a wider range compared with the single
mechanical pump system. The KYB hydraulic systemachieves this by reducing the discharge from themechanical pump by directly connecting the sub pump to
the tank via the selector valve in the time zones where
the single mechanical pump system has a highly
excessive flow. Accordingly, the KYB hydraulic system
is superior to the single mechanical pump system in
reducing the pump output over the entire time zone asshown in Fig. 3(b), showing a pump output reduction
rate of 48.3%. Thus, the effectiveness of the KYB
hydraulic system was confirmed.
RequiredflowSinglemechanicalpump

Flow rate [L/min.]discharge more flow than required depending on theoperation mode of the vehicle. This increased pump
output leads to lower fuel efficiency. The KYB hydraulicsystem is designed to solve this problem. In an operation
mode where the transmission requires a low flow rate,
the system diverts the discharge from the mechanicalpump (via the sub port) (hereinafter "the sub discharge")
to a tank via the selector valve and lets the discharge
from the mechanical pump (via the main port)(hereinafter "the main discharge") alone flow to thetransmission section, thereby reducing the output of themechanical pump. In an operation mode where thetransmission requires a high flow rate, the solenoid isactivated to operate the selector valve to merge the sub
discharge with the main discharge, ensuring normal
operation of the transmission. Moreover, some operation
modes of the vehicle, such as kick down, may quickly
require a high flow rate. In this case, the electric pump is
activated in addition to the mechanical pump, tocompensate for the insufficient flow rate. In this way the
KYB hydraulic system can supply the exact amount offlow required to the transmission with fine control.
Compared with the system consisting of a single
mechanical pump (hereinafter "the single mechanicalpump system"), the KYB hydraulic system can be
expected to substantially reduce the pump output,
namely, improve the fuel efficiency of the vehicle.

― 11 ―

Time [sec.](a)Flow rate

KYB hydraulicsystemCharacteristic analysis of high-efficiency hydraulic system
Singlemechanicalpump
Pump output [W]

KYB hydraulicsystemTime [sec.](b)
Fig. 3

4

Pump output

Simulation comparison resultsC hallenges and Countermeasures of KYB
Hydraulic System

The KYB hydraulic system is susceptible to a surge
pressure occurring when the selector valve is changed
over, as shown in Fig. 4. Upon the change-over, the sub
discharge will flow into the main line to merge with themain discharge, causing an abrupt change in pressure(surge pressure) in the circuit. This surge pressure
appears as a shock-like kick back in the vehicle. It is
desirable to reduce the surge pressure as far as possible.
We then conducted, against a hydraulic circuit with aselector valve that may cause a surge pressure, acomparative evaluation of simulation and actual vehicles,a parameter study Note 2) using the simulation, andoptimization Note 3) of the valve form. This chapter
describes the details.

4. 1 Comparative Study of Surge Pressure
Photo 1 shows a valve change-over test machine andFig 5 shows its hydraulic circuit diagram. The hydrauliccircuit for this test has a check valve on the sub dischargeline in parallel with the mechanical pump and a selector
valve on the sub discharge line at a further position. The
sub discharge flows into the tank when the selector valve
is open (hereinafter "the selector off") or flows throughthe check valve to merge with the main discharge when
the selector valve is closed (hereinafter "the selectoron"). In this test, a surge pressure was measured when
the selector valve was operated with the pump running at
a certain speed to deliver a certain discharge pressure.
We also created a simulation model of the same
configuration as that of the actual test machine tocompare the simulation results with the test results. In
this comparative evaluation of the surge pressure, themagnitude of surge pressure and the response time after
the change-over (the time from the issuance of a current
command to the occurrence of a surge pressure) wereselected as evaluation items.

[2]

[1]

Surge pressure

Controlpressure

Check
valve

Selector valve

Voltage

Main

Time [sec.][1] Surge pressure [2] Response time

Fig. 4

Valve change-over test machine

Relief valve

Voltage [V]Control pressure [MPa]Photo 1

Sub

Torquemeter

Surge pressure caused by valve change-overNote 2) To set analytical models and conditions as parameters
and repeatedly conduct analysis while changing theparameter values.
Note 3) A n approach to find a best parameter value by
minimizing or maximizing a design target (an objectivefunction) of a system.

Fig. 5

Hydraulic circuit of valve change-over test machine

Fig. 6 shows a comparison of pressure at the time ofchange-over between the simulation and the actualmachine test. According to Fig. 6, the simulation results
closely matched those of the actual machine test witherrors within just ±3% for all the evaluation items except
response time (the time from the selector on to theselector off) whose error was 8.8%. It was confirmed thatthe simulation can reproduce the behavior of the actualmachine with an accuracy within ±10% for all the― 12 ―

KYB TECHNICAL REVIEW No. 67 OCT. 2023

evaluation items.
Thus, the simulation model we built in this study
proved its high validity, making it possible to predict thesurge pressure upon change-over of the selector valve.

Pressure [MPa], Voltage [V]Selector off -> Selector on
Control pressure(actual machine)
Control pressure(analytical model)
Voltage

Selector on -> Selector off

Error 3.0%

Error -2.3%

Error -0.8%
Error 8.8%

Time [sec.]

Comparison of selector valve between analyticalmodel and actual machine

Fig. 6Note 4) A property of a system or structure that it is unlikely tobe affected by various disturbances.

4. 2 Implementation of Parameter Study
To identify design parameters that may affect the surge
pressure and/or system response time, we carried out aparameter study using the simulation model created in
section 4.1.
Parameter values were adjusted by halving or doubling
the design median value. Otherwise, parameter values
were changed to an extent that would not hamper thechange-over function. We eventually conductedsimulation in several tens of patterns. Table 1 shows anexample of sensitivity trends of design parameters that
affect the surge pressure and/or system response time.
The positive (+) and negative (-) signs in Table 1 indicate
an increase and a decrease in the relevant performance/
characteristics respectively.
Table 1

Parameter sensitivity for various evaluation items(example)
Evaluation item

Targetmodel

PipingSelector
valve

Check
valve

Parameter change

substantially affected by the selector valve and piping.
In the piping model, the surge pressure performance ishigher with lower rigidity, a larger diameter, or a longer
length. This is probably because the piping has anincreased accumulator effect to be more capable ofabsorbing surge pressures. In the check valve model, thesurge pressure performance is higher with a smaller spool
diameter or a shorter lap length. On the other hand, aback flow is more likely to occur when the check valve isclosed, which may degrade the robustness Note 4) as asystem. This issue should be taken into account during
design. Description of the selector valve model is omitted
here and will be included in the next section.
From the results of the parameter study, we also
confirmed that there is basically a trade-off relationshipbetween the surge pressure performance and the
responsivity.
4. 3 Study on Valve Design Optimization
From the results of the parameter study in section 4.2,
we determined that the opening characteristics of theselector valve and the spring specifications (including
spring constant and initial load) substantially affect thesurge pressure performance and responsivity (see Table1). In light of this, we optimized the form of the selector
valve to enhance the valve change-over performance.
The following describes the details of the optimization.
Please note that, prior to optimization, we reviewed thevalve configuration by simulation and then modified theconfiguration so as to hopefully deliver even higher
change-over performance.
Table 2 shows predictor variables, objective functions,and limiting conditions for optimization. Since there isbasically a trade-off relationship between surge pressure
performance and responsivity as indicated by the results
in section 4.2, the objective function was set so as tooptimize the sum of the weighted surge pressure andresponse time, thereby allowing the valve to have both
good surge pressure performance and good responsivity.
In terms of limiting conditions, some parameters thatcannot help having errors for design reasons, including
stroke and spring constant, were given variations. Then,it was judged whether the sub discharge was diverted
properly when the valve was changed over. This madethe selector valve, which had already been optimized,
more robust.

Surge pressure
performance(Off -> On)

Responsivity(Off -> On)

Surge pressure
performance(On -> Off)Responsivity(On -> Off)

Diameter up(+) sensitivity:High(-) sensitivity:
Middle(+) sensitivity:High(-) sensitivity:
Low

Length up(+) sensitivity:
Middle(-) sensitivity:
Middle(+) sensitivity:
Middle(-) sensitivity:
Low

Rigidity up(-) sensitivity:High(+) sensitivity:
Middle(-) sensitivity:High(+) sensitivity:
Low

Openingcharacteristics

Sensitivity:
High

Sensitivity:
Low

Sensitivity:
MiddleSensitivity:
Low

Spool friction up(+) sensitivity:
Middle(-) sensitivity:
Middle(+) sensitivity:
Low(-) sensitivity:
Low

Spool diameter up(+) sensitivity:High(-) sensitivity:High(+) sensitivity:
Middle(-) sensitivity:
MiddleTable 2Spring force up(-) sensitivity:High(+) sensitivity:High(+) sensitivity:
Low(-) sensitivity:
Low

Predictor variable

Spool diameter
down

No effect

No effect(+) sensitivity:High

No effect

No effect(+) sensitivity:
MiddleLap length down

No effect

No effect

From the results shown in Table 1, we obtained
insights about the overall sensitivity trend. The surge
pressure performance is substantially affected by thepiping and check valve while the response time isParameters used to optimize the valve design
Objective function

Limiting condition- Underlap- Evaluation function- Judgement of the- Stroke- Spool diameter- Orifice- Spring constant- Initial load
- Opening notch

expressed in surge
pressure andresponse time

valve's change-overoperation withvariations in design
values― 13 ―

Characteristic analysis of high-efficiency hydraulic system

Beforeoptimization
After
optimization
Voltage

Voltage [V]Pressure [MPa]Fig. 7 shows a comparison of surge pressure/responsetime (the selector off -> the selector on) before and after
optimization. It can be confirmed in Fig. 7 that the
optimized model shows substantially lower surge
pressures and shorter response time than the valvechange-over model created in section 4.1. This analysis
method was found to be effective for finding optimal
design parameters that can improve both surge pressure
and response time at the same time.

67% lower

84% lower

78% shorter

47% shorter

Time [sec.]
Fig. 7

Evaluation of surge pressure characteristics of
optimized model

89% lower 40% shorter

Note 5) Acronym for Permanent Magnet Synchronous Motor.
PMSM is driven through synchronization between the
motor rotation and the frequency of the AC current
flowing into the coil.
Note 6) A motor control method that selects from the same
effective current to maximize the output torque.
Note 7) A motor control method that sends a current in a
direction that cancels the magnetic flux of thepermanent magnet, enabling high-speed operation while
reducing the torque.
Note 8) MATLAB® and Simulink® are registered trademarks of
The MathWorks, Inc.

Beforeimprovement
Optimized
Voltage

49% shorter

The KYB hydraulic system model includes an electric
pump in addition to the mechanical pump and theselector valve. The electric pump model used in thefeasibility study in Chapter 3 was specified to operate
without delay according to speed commands. Efficiency
and responsivity were not factored into the model design.
This study in turn requires a model for the KYB
hydraulic system that can determine feasibility andefficiency of the system operation. Therefore, the electric
pump, which is one of the system components, also
needs to reproduce its efficiency and response time.
Then, we modeled the electric pump, which is one of
the components to be installed in the KYB hydraulicsystem, based on data collected on the actual machine.
This chapter describes the details of this effort. The target
model accuracy for efficiency and response time was set
to within ±10%.
5. 1 Evaluation with Initial Electric Pump Model
Fig. 9 shows the configuration of the actual electric
pump. The pump section includes a PMSM Note 5) and avane pump and uses torque maximizing control Note 6) andflux weakening control Note 7) strategies to control the
motor current. The electric motor was modeled using
simulation software products MATLAB® and Simulink® Note 8),
which are often used to build control models, instead of
the Simcenter Amesim used to model the hydraulicequipment. By coupling the motor and pump models we
had created, we conducted an analysis of the electric
pump. In terms of pump operation, the actual speed ofthe pump motor was fed back and controlled to minimize
deviations from the speed commands.

71% lower

Voltage [V]Pressure [MPa]4. 4 P erformance Evaluation of Optimized
Selector Valve on Actual Machine
To determine the effect of the optimized selector valve
obtained through simulation, we conducted a
confirmation test on the actual machine. Fig. 8 shows thesurge pressure and response time of the original selector
valve in section 4.1 and of the optimized selector valve.
According to Fig. 8, the optimized selector valve shows
about 80% lower surge pressure and about 40% shorter
response time than the original counterpart.
This clarified that the optimized selector valve can also
substantially improve both properties in the actualmachine in the same manner as for the simulation. We
thus confirmed the validity of the simulation and theeffectiveness of the optimization program.

5 Building of an Electric Pump Model

Time [sec.]
Fig. 8 Comparison of results of surge pressure tests

Fig. 9― 14 ―

Configuration of electric pump

KYB TECHNICAL REVIEW No. 67 OCT. 2023

Fig. 10 shows equivalent circuit diagrams of the PMSM
circuit converted through coordinate transformation to thed-axis (in the direction of the main flux) and the q-axis(shifted from the d-axis by 90˚). In the coordinatetransformation shown in Fig. 10, the motor current canbe divided into a component I q that generates a motor
torque and a component I d that causes the rotor to
produce a magnetic flux. These current components areindividually controlled. Modeling of the electric motor inthis study was based on the electric circuit diagrams
shown in Fig. 10. Parameter settings for the model usedmeasurements of the actual machine, which is thecomparison target, and calculated values from thespecifications.

analytical model was about twice (about +100% error)that of the actual machine. With such a big difference, it
was considered that the model must be improved.

Fig. 11

Fig. 10 Equivalent circuit diagrams of motor

Table 3 shows the results of comparative tests ofelectric pump efficiency between the model and theactual machine. Table 3 indicates that the simulation
gives higher efficiency than the actual machine under all
the test conditions. This is probably because the motor
model was created by considering electrical andmechanical losses, but not considering magnetic
hysteresis and eddy currents. The efficiency errors may
be attributable to the fact that the dependency of motor
inductance on current and temperature was not taken into
account. The magnitude of efficiency error is not lessthan 10% on the whole and over +100% under the highdischarge pressure and low speed conditions in particular.
Thus, the efficiency of the pump model was found todiffer largely from that of the actual machine.
Table 3

Evaluation of responsivity of initial electric pumpmodel

5. 2 Improvement of Motor Model
Based on the results in section 5.1, we reviewed the
motor model and parameters with consideration given toiron loss. Fig. 12 shows d, q-axis equivalent circuit
diagrams with iron loss taken into account 2) . The
equivalent circuit diagrams in Fig. 12 express the ironloss caused by rotor rotation in equivalent iron loss
resistance Rcp. Also, Ldq and Lqd are used to reflect the d,
q-axis mutual inductance. We then created another motor
model based on these equivalent circuit diagrams. Since
the equivalent iron loss resistance Rcp shown in Fig. 12 isa variable parameter relative to Id, Iq, or the motor speed
N, the iron loss obtained in the actual machine test wasused to formulate an approximate expression, which wasapplied to the model. The inductances were set in tabledata as Id/Iq-dependent parameters and then subjected totemperature correction before insertion in the equations.
The motor temperature was set to a value predicted fromthe actual machine data.

Efficiency evaluation of initial electric pumpmodel
Test conditions(speed and discharge pressure)

Model efficiencyerror

Low speed,
High pressure

Mid speed,
Mid pressure

High speed,
Low pressure

＋113％

＋12％

＋14％

Fig. 11 shows the results of comparative tests ofelectric pump responsivity between the model and theactual machine. In this study, the pump responsivity wasevaluated by the time from the step time of the speedcommand signal (start time) until the discharge flow
reaches 90% of the normal flow rate.
According to Fig. 11, the response time of theFig. 12Equivalent circuit diagrams of motor with iron loss
taken into accountFig. 13 shows the T-N (torque and number of
revolutions) characteristics of the actual motor, the motor
model in section 5.1 (hereinafter "the initial motor
model"), and the motor model in this section (hereinafter"the improved motor model"). In Fig. 13, the horizontal
axis indicates motor speed and the vertical axis represents
motor torque. The torque-speed (T-N) characteristics areplotted for each of the effective values Ie of the threephase current.

― 15 ―

Characteristic analysis of high-efficiency hydraulic system

Torque maximizing
control zone

Flux weakening
control zone

Torque [Nm]Line graph: Actual machine●: Initial motor model
×: Improved motor model

Ie10
Ie20
Ie30
Ie40
Ie50
Ie60
Ie70

Speed [rpm]Flow rate
Speed command

Delay

Actual speed

Time [sec.]
Fig. 14

Speed [rpm]Fig. 13determined an equation to obtain the extension timing of
the vanes based on the actual machine's data, and thenincorporated a mathematical model for calculating thestart-up and response times of the pump model. Of
course, we could reproduce the response of the pumpmodel by incorporating components including vanes andcam rings. In this study, however, we did not select thisapproach because:[1] the calculation load is very high, particularly ittakes enormous time to analyze the fuel efficiency;[2] i t is necessary to identify unknown parameterssuch as the coefficient of friction of the vanes, and;[3] the contribution of the pump section to the delay ofthe entire electric pump is low.

Flow rate [L/min.]According to Fig. 13, there are two distinctive
sections: a constant torque section and a section where
the torque decreases as the speed increases. In thischapter, these sections are called by their current control
method: the constant torque section is called the torquemaximizing control zone and the slanted graph section iscalled the flux weakening control zone. When the resultsof the actual machine are compared with the results of
the initial motor model for the various common current
values in Fig. 13, the analytical model closely
reproduces the graph gradient, namely, changes in torque,
but the analysis values are higher than the measurements
by about +5% to +10% over the entire torque range. On
the other hand, the improved motor model shows motor
characteristics with an accuracy as high as within ±3%
error for all the current values in the torque maximizing
control zone. Also in the flux weakening control zone,
the error is limited to within about ±10%. Thus, we
successfully built, through improvements including theaddition of iron loss, an improved motor model that can
reproduce the characteristics of the actual motor with ahigher accuracy than the initial motor model.

Evaluation of T-N characteristics of motor models

5. 3 Improvement of Pump Model
Fig. 14 shows an example of measurements of motor
speed and discharge flow rate of the actual electric pump.
When the actual motor speed is compared with thedischarge flow rate in Fig. 14, there is a delay between
the start-up of the motor and the start of oil discharge.
From this finding, we assumed the following pump
behaviors:[1] After the electric pump is started, the pump vanes
remain retracted delivering no flow or no dischargepressure until the pump speed reaches a certain
level.
[2] When a certain speed level is reached, the pumpvanes extend to produce a flow and build adischarge pressure accordingly.
[3] When the running pump (motor) is stopped, thevanes retract to depressurize the pump.
We designed a pump model that can reproduce
behaviors [1] to [3] above for modeling with an operation
delay taken into account. On the assumption that thetiming for the vanes to extend depends on the angularvelocity or angular acceleration of the pump, weActual measurements of pump operation

Fig. 15 shows the pump model we created. By
incorporating models for determining the vane state(retracted or extended), we built a pump model that cansimulate a delay from the motor start-up until the shift tothe hydraulic circuit with the extended vanes, and thenincorporated the model into the electric pump model.
5. 4 E valuation of Improved Electric Pump
Model
The combined electric pump model created byincorporating the improved motor model in section 5.2
and the pump model in section 5.3 was evaluated for
efficiency and response time.

Fig. 15― 16 ―

Pump model with vane behavior taken into accountTable 4

Evaluation of efficiency of improved electric pumpmodel
Operating conditions

Modelefficiency error

Highpressure,
Low speedMidpressure,
Mid speedLowpressure,High speed

Between the power and theinverter

－3.0％

－1.2％

－0.1％

Inside the inverter

－9.4％

－2.5％

0.5％

Between the inverter andthe motor

0.2％

1.0％

1.2％

Inside the motor

－4.1％

－1.7％

－2.1％

Between the motor and the
pump

12.8％

2.0％

2.6％

Inside the pump

－0.8％

－0.2％

0.0％

Entire assembly (electric
pump)

－5.5％

－2.7％

2.0％

Flow rate(analytical model)
Flow rate(actual machine)
Speed commandTime[sec.]
時間［s］
Fig. 16 Electric pump discharge flow rate over time
Note 9) A control gain for adjusting the control input inproportion to the difference between the target andcurrent values of the controlled object

Table 5 shows the response time of the analyticalmodel under various test conditions compared with theactual machine. According to Table 5, the response timeerror is within ±10% with the P control gain set to high.
On the other hand, it is between about 10% and 30%
with the P control gain set to low. This is probably
because a lower P control gain leads to a smallerdifference between the motor torque and the load torquein the transient state, resulting in a relatively stronger
impact of the motor torque error of the model on therotary action.
Table 5 Evaluation of responsivity of improved
electric pump model
Operatingconditions
Low pressure,High speed

Next, we evaluated the response time of the electric
pump model. The response time of the pump is affected
not only by the discharge pressure and pump speed but
also by the P control gain Note 9) setting for motor speed,which was added to the test conditions. In the testing, theP control gain setting for speed control was adjusted atthree levels: low, middle, and high.
Fig. 16 shows a comparison between the actualmachine and simulation regarding the response time ofthe electric pump under high-speed, low-pressure
operation with the high P control gain setting. Fig. 16
reveals that the simulation almost reflects the responsetime as well as the flow rate waveform including thetiming of the rising edge of the flow rate and the process
to reach the steady state.

回転数［rpm］
Speed
[rpm]

Table 4 shows a comparison of efficiencies during
operation of the electric pump. It should be noted inTable 4 that the efficiencies of the overall assembly(electric pump) are actual measurements calculated fromthe actual pump output, while the efficiencies of various
components are estimates calculated from theoreticalequations.
The efficiency error of the entire assembly is within±10% under any of the conditions. In particular, the error
for the mid-pressure and mid-speed operation or the lowpressure and high-speed operation is within ±3% not
only for the entire assembly but also for all components,
proving a very high model accuracy. For the highpressure and low-speed operation, the entire assembly
has -5.5% error, but some components show around 10%
error. These quite big errors are probably because themodel accuracy is low for rotary motion of components"between the motor and the pump" downstream of theelectric pump system.
Thus, the model still needs to be improved, but was
confirmed to reproduce the pump with high accuracy
particularly for the entire assembly with an efficiency
error of about ±5%.

Flowrate [L/min.]
流量［L/min］

KYB TECHNICAL REVIEW No. 67 OCT. 2023

High pressure,Low speed
Mid pressure,Mid speed

P control
gain

Response time
error

Low

12.8％

Mid

-12.0％

High

2.0％

Low

21.6％

Low

7.1％

Mid

4.3％

High

-7.9％

In this way, we successfully built an electric pumpmodel that can reproduce efficiency and response timewith the target accuracy by setting the P control gain forpump speed to a high value. Note that the KYB hydraulicsystem described in Chapter 6 uses a high P control gain
setting for performance evaluation.
P erformance Evaluation of Hydraulic
SystemWe combined the models built in the previous chapter(hereinafter "the detailed models") into a whole hydraulicsystem and evaluated the performance through simulationin order to determine whether the system is effective.

６― 17 ―

Characteristic analysis of high-efficiency hydraulic system

Singlemechanical pump

Required flow

Simplified model

Selector off

Time [sec.](a)

Detailed models

Flow rate

Pressure command

Flow rate [L/min.]Control pressure [MPa]Requiredflow

to the entire vehicle, leading to lower ride comfort. Thisissue might be resolved by cooperative control of theselector valve and the electric pump. Then, we took asimplified measure for reducing the surge pressure,which is described in this section.

Flow rate [L/min.]

6.1 P
 erformance Evaluation of Hydraulic
SystemThis section describes the results of system
performance evaluation of the detailed models which was
conducted in the same manner as for Chapter 3. Fig. 17
shows the results of the simulation. Fig. 17(a) shows thepump discharge flow rate, indicating that the pump meetsthe required flow rate. This graph also reveals that thedetailed models discharge at a lower flow rate as a wholethan the single mechanical pump system, implying thatany excessive flow has been reduced.
The pump output of the detailed models is lower thanthat of the single mechanical pump system model overthe entire range as shown in Fig. 17(b). The output
reduction rate is 48.3%, proving the effectiveness of thissystem.

Detailed modelHigh surge
pressure

Time [sec.](b)

Time [sec.](a) Flow rate

Fig. 18

Follow-up performance during KD

Detailed models

Pump output [W]Single mechanicalpump

Pump output

Time [sec.](b) Pump output
Fig. 17  Results of simulation

6. 2 Stability Evaluation of Hydraulic System
This section describes a simulation of an operation
pattern that needs instantaneous high flow rate andcontrol pressure, such as kick down (hereinafter "KD"),
to determine the follow-up performance.
Fig. 18 shows the results of analysis of flow rate andcontrol pressure. Fig. 18(a) indicates that the required
flow rate is mostly satisfied over the entire operating
range. Fig. 18(b) indicates that the control pressure
follows the pressure command, except for high surge
pressures with the selector off.
As described above, the surge pressure causes a shock

Fig. 19 shows an overview of the cooperative control
logic. The KYB hydraulic system is conventionallycontrolled to merge the main discharge with the sub
discharge if the main discharge alone is not sufficient toprovide the required flow. If this is not enough to meetthe required flow, the electric pump is started up tocompensate for the flow. This is a control logic only
intended to satisfy the required flow. In this case, a surge
pressure is likely to occur when the sub discharge isturned on/off with the selector valve, as shown in theupper section of Fig. 19. Then, we decided to add surge
pressure reduction control to the cooperative control
logic as shown in the lower section of Fig. 19. This
control system is intended to prevent abrupt changes inflow in the whole hydraulic system by decreasing/increasing the discharge of the electric pump in responseto an increase/decrease in sub discharge upon changeover of the valve.
Fig. 20 shows the results of the analysis of control
pressure in the KD evaluation using the model including
the surge pressure reduction control. It was confirmedthat the addition of this control can reduce the surge
pressure that occurs upon change-over of the valve by
about 50%.
In addition to the stability improvement verified in thissection, a further study on cooperative control will enable
us to reduce the frequency of change-overs, implement a― 18 ―

KYB TECHNICAL REVIEW No. 67 OCT. 2023

robust operation design, and improve the effect of pump
output reduction.

Fig. 19

Control pressure [MPa]The results obtained make it possible to accelerate thedevelopment with less frequent prototyping of actualmachines and design a hydraulic system with priority
placed on the main machine.
From now on, we will study optimal equipment
specifications and/or cooperative control suited tooperation modes required by customer specifications,with the aim of maximizing the system performance/
quality. We will also apply the technology to theprediction of the lubrication/cooling system for eAxle (anintegrated structure of the motor, reducer, and inverter)that is growing in demand in the automobile industry.

Cooperative control logic

Surge pressure reduction
control: No

Surge pressure reduction
control: Yes

Small surge pressure

Large surge pressure

Time [sec.]

Fig. 20 Effect of surge pressure reduction by cooperative
control

7(2) We carried out a parameter study using the model
built to identify parameters affecting the stability,
clarifying the design indices with which surge
pressure and responsivity can be controlled.
(3) We successfully set the prospects for improving theperformance of the hydraulic system by coordinating
the control of each component according to operation
mode.

Concluding Remarks

We have established a simulation technology thatenables performance prediction as a means of confirming
the feasibility of a high-efficiency hydraulic system andobtained the following findings:(1) We modeled a KYB hydraulic system consisting of amechanical pump, an electric pump, and a selector
valve to enable prediction of the system performancewith high accuracy. We then confirmed theeffectiveness of this hydraulic system.

References
1) SUZUKI, NAGASHIMA: "Performance Improvement of Vane
Pump via CFD Analysis", KYB Technical Review, No.61(October 2020)2) K ANO Yoshiaki, KOSAKA Takashi, HOJO Yoshihisa,
OHMORI Yoichi: "Novel d-q equivalent circuit model of
IPMSM for accurate torque control", Joint Workshop on Motor
Drives/Home and Consumer Appliances on March 1, 2022,
pp.115-124 (March 2022)

Authors 
HIRONAKA Tsuyoshi

TSUKUI Ken

Joined the company in 2009.

Joined the company in 2018.

Mechanical Component Engineering

Mechanical Component Engineering

Sect. , Basic Technology R&D

Sect., Basic Technology R&D Center,

Center, Engineering Div.

Engineering Div.

Engaged in research and development

Engaged in system analysis ofof hydraulic systems and solenoid

automobile transmissions.

valves.

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