Semi-Active Suspension, Extending the Limits

製品紹介

Semi-Active Suspension, Extending the Limits
Alexander Alonso Torres１Introduction

The avalanche of SUVs and the arrival ofautonomous driving are pushing vehicle manufacturers
to improve ride-comfort compromise during last years.
Semi-active suspension is a very powerful tool that
increases considerably vehicle behavior comparing
with a standard one. Although there are some
limits, studying each application carefully thesecan be extended.
In the last years KYB has developed a semiactive suspension system, and it’s now enteringinto the market as TIER1, as a complete system
shock absorber & software supplier. During thedevelopment KYB has manage to solve somelimitations related to lack of robustness and noise.
This will be the case study for this article.

２

Semi-Active System Architecture

Semi-Active System architecture is built by fourlevel sensors, three G-sensors, an ECU（“ElectronicControl Unit”）and one actuator on each shock
absorber.

control strategy analyses vehicle status by reading
input signals. If needed the ECU transmits anoutput current command to each shock absorberactuator in order to increase each damping force
and correct/adapt vehicle dynamics deviation. SeeFig. 1.

Fig. 1

System Architecture

From shock absorber point of view, thestandard technology has been combined with afast response electro valve. The aim is to adapt
the damping force – piston speed system to adamping force – piston speed – command current
dependent system. See Fig. 2.

Working principle of the system is defined inthree steps following the general rule Sense >Define > Act. The level sensors measure wheel –body relative displacement for each wheel and theG-sensors located on the car body, two of them on
the front axle and the third one on the rear are in
charge of measuring the vertical accelerations.
These measured signals from sensors added tomany other information available directly from theCAN Bus（such as vehicle speed, steer wheel
movement, longitudinal and lateral acceleration,
braking status, others…）are treated as input forFig. 2

the ECU control strategy. Each 2ms software― 68 ―

 amping force, piston speed and commandD
current dependent system

KYB技報３第56号

2018―４Semi-Active Control Strategy

During driving life, Semi-Active system willneed to manage many different situations. By
mixing vehicle status with driver and road inputs,
all these life situations are collected and SemiActive control strategy must be able to cover allof them. See Fig. 3.

Fig. 6

Fig. 3Sw strategy functions for road inputs

during a period of time. This function detects
wheel hoping case and adds a step of current toVehicle driving live situations

fix it. See Fig. 7.

For the purpose of this study pure driver input
live situations have been discarded. So from now
on, we will focus just on road input cases. See Fig.
4.

Fig. 4Fig. 7

Vehicle driving live situations simplified

Vehicle behavior must accomplish three main
targets（Fig.5）and each of them is ensured bysome functions or tools as described in Fig. 6.

Anti Wheel Hop Function description

Once minimum safety conditions have beenstablished, control strategy is set to manage ride
control. “Sky Hook” theoretical function has beenchosen for this purpose. See Fig. 8.

Fig. 5Vehicle behavior targets

Fig. 8

SkyHook functionality

For safety reasons highest priority target is to

As described in system architecture description,

secure road contact. For this, a base minimum
damping has to be set. This is handled by “BaseCurrent” function and basically it applies a current

Semi-Active system has three G-sensors
distributed in vehicle body in charge of measuring
bounce, pitch and roll body rates. See Fig. 9.

command depending on vehicle speed.
To provide robustness to base damping in caseof abrupt bumps and wheel hopping, “Anti Wheel

After calculations using bounce, pitch and rollrates, vehicle speed signal and vehicle tuning
parameters, Sky Hook function provides a continuousHop” function increases the command current

damping command to the system（Fig.10）.

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Semi-Active Suspension, Extending the Limits

Up
Variable Damping

Variable Damping

min

Rebound
Variable Damping

Rebound

DF［N］

Bump

min

Bump

min
Piston Speed［m/s］
min

Variable Damping

Dowm

Fig. 9Fig. 12

Modal Speeds: Bounce, Pitch and Roll

Karnopp Switching Theory Description

the reference position as soon as possible, whileduring cases 2 and 3 it is allowed to adjust thedamping according to the damping command
defined until now.
Taking a look to following road input example（speed breaker）, a clear effect of “Karnopp”
function can be seen. See Fig. 13.
Fig. 10

SkyHook function DF calculation

In order to increase ride control efficiency atend stroke, the “Stroke Sensitive Control” function
gain multiplies Sky Hook damping force command.
See Fig. 11.

Behaviour / Road
sectionSection 1Car Body movement

NoneUpUpDown

DownUpDownDamper Stroke
direction

None

Compression

Rebound

ReboundCompression

ReboundCompression

Dampingsoftsofthard

soft

hard

hard

hard

Section 2

Section 3

Section 4

Section 5

Section 6

Section 7

Car body
Relative stroke speed

Fig. 13

Karnopp Switching Example

Once having managed body control, it’s time totake a look to comfort and increase the filtering.
“Digressive Control” is responsible to decrease
variable damping in case of impacts throw
aggressive road inputs. See Fig. 14.
Fig. 11

Stroke Sensitive Function DF

Until this point, a continuous damping commandhas been provided to achieve ride control. In orderto warranty vehicle flat behavior, the wheel andbody relation movement sense has to be taken
into account. For this purpose, software designer
needs to consider the “Karnopp SwitchingTheory”. See Fig. 12.
Body and wheel vertical displacement
combination can be split in four different cases:
body up and wheel in compression or rebound,body down and wheel in compression or rebound.

This simple function is very efficient in case of
single input, but for continues road inputs, whatFor cases 1 and 4, Semi-Active control strategy

usually it’s call “rough road”, Semi-Active control

sets minimum damping force in order to recoverstrategy uses a last function called “RoadFig. 14― 70 ―

Digressive Control Function DF calculation

KYB技報第56号

2018―４Condition”. See Fig. 15.

Fig. 15Road Condition Function Description

Fig. 16

If continuous wheel hopping happens, thefunction adapts requested variable damping

 own Steps track, with rebound end ofDstroke impact noise example

rebound stroke（positive）.

progressively until having the desired filtering.

４５

Theoretical Strategy Limitation

Highest limitation of semi active suspension is
the non-possibility to predict road inputs.
Nowadays autonomous driving technology isproviding many different solutions of preview
systems, but even combining some of them, they
are not robust enough to work properly 100% ofdriving time. This means that Semi-Active system
must cover robustly all live situations by itself. To
be considered that no preview systems have beentaken into account during this case study.
As the spring properties can’t be adapted, andtime consumption for movement stabilization isneeded. an optimization by “Karnopp Switching”
function has to be done by applying a minimum
damping in some maneuver steps.

Studied Solutions

In order to solve the issue, two different
solutions have been tested. First one, based on apassive solution, a HRS （“Hydraulic reboundstopper”）system have been used. See Photo 1 andFig. 17.

Photo 1

Let’s imagine a worst case, comfort oriented
vehicle, with short stroke shock absorbers and theHRS

lack of damping in some steps according to semi
active suspension functionality, this implies a highrisk of impact noise and lack of body control inaggressive road inputs, for instance, speed
breakers or down steps.
Fig. 16, shows a real case study measurement,
where some rebound end of stroke impacts can beseen just in the last down steps.
During this test the last 4 steps of 70mm height
have been enough to trigger rebound end ofFig. 17

HRS Damping force characteristics

stroke impact noise. This fact is very clear indownstairs right graphic, force vs displacement,

Basically, using a deformed inner tube and aplastic segment located in piston rod, a newchamber is created in rebound side, and it iswhere some force peaks appear just at the end ofincreasing damping force according to piston― 71 ―

Semi-Active Suspension, Extending the Limits

speed and rebound stroke.
This is a fantastic solution to improve shock

aggressive road inputs as speed breakers or down
steps. See Fig. 19.

absorber efficiency at the end of rebound stroke
because it absorbs a great quantity of energy, fastand progressively.
On the other hand, the implementation of thissystem has an important cost impact on shock
absorber price.
The second solution tested, has followed same
HRS philosophy, but in this case, it has been
implemented modifying software control strategy.
So, a new function has been implemented, andcalled DHS（“Double Hydraulic Stopper”）.See Fig.
18.

Fig. 19

６Fig. 18

DHS function DF calculation

The function adapts the damping command（activation point and gradient）according tovehicle speed and load condition.
DHS function avoids end of stroke impact noiseand provides robustness to ride control inFig. 20

 own Steps track, without rebound end ofD
stroke impact noise

Summary

KYB provided two different solutions to solve
impact noise and lack of ride control in aggressive
road inputs as speed breakers. Finally thecustomer decided to go on with DHS software
solution. See Fig. 20.
By intelligent combination of theoreticalstrategy and experimental function, KYB has
managed to improve noise filtering and avoid
extra cost to our Customer. Studied solution has
been implemented in mass production software
strategy.
KYB is now extending the limits of the system inSummary― 72 ―KYB技報

第56号

2018―４order to enhance even more the body control andcomfort level as expected from such an intelligent

would like to take advantage of this experience tocontinue adding value to electronic controlled

technology, KYB’s Semi-Active Suspension.
We believe that providing solutions to specificrequirements as in this project we also led tosystem which seems to be the future.
Last but far from least, I'd like to express mysincere appreciation to my colleagues greatbuild up our know-how and expertise on E/E（Electrical/Electronics）system development. We

cooperation during this project development.

Author 

Alexander Alonso Torres
Joined company in 2012.
Currently at KYB Europe GmbHas Semi-Active suspension teamcalibration and vehicle testing
engineer.

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