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Fundamentals• PURPOSE OF THE SUSPENSION SYSTEM
– Maintain correct vehicle ride height – Reduce the effect of shock forces – Maintain correct wheel alignment – Support vehicle weight – Keep the tires in contact with the road – Control the vehicle’s direction of travel
Main Components of a Suspension
Springs
Shock Absorbers
Strut
Tires
SpringsSupport the weight of the vehicle, maintain ride height, and absorb road shock
bounce : vertical movement of suspension
jounce : compression, upward movement
rebounce : extension, downward movement
The spring bounces at its natural frequency.
Sprung Mass : Mass carried by the spring, like the vehicle body, transmission, frame
Unsprung Mass : Mass that is not carried by the springs, like the tyre, wheels.
TYPES OF SPRINGS
COIL SPRINGS
LEAF SPRINGS
TORSION BAR
AIR SPRINGS
Coil springs
Diameter and length determine whether the spring is strong or flexible
No inter-leaf friction therefore smoother ride
Variable rate achieved by varyingMaterials of different thickness
Progressive winding of the spring
Variable rate providesLower spring rate under non-load conditions
Higher spring rate under load conditions
Leaf springs
Multi leafStacked steel plates of different lengthsSpring compress to absorb road shocksLeaf springs bend and slide for suspension movement
Mono leafTapered leaf spring- thick in middle and tapers toward sidesComposite material
Mounting configuration Longitudinally in pairsSingle traverse mounted leaf spring
Torsion Bar
Torsion bar straight or L-shaped bar of spring steel
Torsion bar twists to provide suspension
Mounting configuration Longitudinally – mounted solidly to the frame at one end and other end to the moving part of the suspension
Traverse mounting possible.
Air springs
Air spring is a rubber cylinder filled with compressed air
Movement of the piston attached to lower control arm affects the air compression to provide the spring action
On board compressor provides air through a valve at the top of air bag
Valve opens to add or release air according to the load requirements
Most popular on passenger cars, buses, and heavy trucks
Shock absorbers/ DampersControl movement
SpringSuspension
Turns K.E thermal/ heat and dissipates though hydraulic fluidLocated in pressure tube and at the end of suspension which forces the fluid in to the piston of shock absorberHas a piston with orifices to control flow of hydraulic fluidResistance to the flow depends on:
Number and size of orificesSpeed of movement of the suspension
Velocity sensitive faster the movement of suspension more resistance
Types of damping
Viscous
Coulomb
Structural
Fd
x
• Damping force linearly dependent on velocity
• Pure viscous damping force is an ellipse w.r.t displacement
• With the spring force added, the Damping force is inclined forming a hysteresis.
• Fd = πcwx
Viscous damping
Fd
x
• Damping force dependent on direction of velocity
• Pure coulomb damping force is a rectangle w.r.t displacement
Coulomb Damping
StrutsA common type of damper unitFirst main job:
Damping functionVelocity sensitiveSimilar structure of shock absorberMajor structural component of suspension – replace the upper control arm and ball joint
Second job:Structural support support the spring
hold tire in aligned positionBear side load placed on suspension Affect riding comfort, steering, braking, wheel alignment and wear on suspension
Suspension typesAbility of opposite wheels to move independent of
each other – Dependent – Independent– Semi dependent
Dependent: A live axle holding both wheels parallel to each other and perpendicular to the axle
Change in Camber of one will affect the same in otherIndependent: Not connected at all or connected through
universal joints with a swing axleAllows wheels to rise and fall without affecting opposite
wheelsAnti roll bars are classified as independentSemi-dependent: Swing axle is used, but the wheels are also
connected with a solid tube deDion axle
Ride Control
Objectives of a suspension steering stability good handling Maximize passenger comfort Limits of vibration Exposure Limit Fatigue Reduced comfort boundaries Design aspects Vibration isolation Suspension travel Road Holding
Ride Control
Vibration isolationResponse of sprung mass to the excitation from the ground
Judicious selection of damping required for control over the natural frequency of sprung mass and ride comfort
Suspension travel The deflection of the suspension spring or the relative motion between the sprung and the unsprung mass
Road Holding Vibration effects normal load acting b/w the tire & road
In turn effects the cornering force, tractive effort and the braking force developed by the tire
Suspension for AFVRequirement:
High loadGreater suspension travelProtection form land-mines & anti-tank weaponsWheel pairs of six/eight for ride over rough groundWide range of ambient temperature: -10 ˚C to 50 ˚C
EvolutionFixed suspensions Leaf spring suspensionsChristie suspension: coil springs inside armor hull and bell crankHorstmann suspension –combination of bell crank and exterior coil springsTorsion bar suspensionHydro-pneumatic suspension
Hydro –pneumatic suspension• Working principle:
As the road wheel rises the axle arm is lifted and this rotates the crank which moves the piston via the con rod; the piston displaces oil through the damper valve and moves the separator piston; this causes the gas to compress which produces the
spring force
Hydro-pneumatic suspensionAdvantages:
Progressive spring ratesofter rate around the static position and a stiffer rate near full deflection
Totally independent System is external more space inside the vehicle Better cross-country performanceSensitive equipment subjected to lower acceleration levels Reduced shock loadsvary the gas pressures of each unit
Variants:Hydro strutHydrogasTandem hydro strut for
torsion suspensionVariable damper & spring
Static Test and Dynamic Test
Static Test Load at Rebound,
Static, Bump Spring Characteristic Curve
Dynamic Test• 300 ± 200 mm, 0.1
Hz
• 250 ± 100 mm, 0.8
Hz
0
2
4
6
8
10
12
14
16
18
20
0 100 200 300 400 500 600
Estimated Value
Before Dynamic
After Dynamic
Lo
ad, T
on
s
Displacement, mm
Spring Characteristics
Endurance Test Results
0
20
40
60
80
100
120
140
160
0 60 120 180 240 300
FPC
SPC
DAMPER
Time, min
Tem
per
atu
re,
°C
Variation of Temperature with Time
Status of suspension technology
France
Leclerc/ EPC MBT
50t 6x6 Twin-cylinder hydro pneumatic– 425 total travel
AMX-40 MBT
43t 6x6 Torsion bar
AMX-32MBT
39t 6x6 Torsion bar
AMX-30 and AMX-30-S
36 5x5 Torsion bar
GermanyLeopard 2 55.15 t 7x7 Torsion bar
suspension and advanced friction dampers
Leopard1A1
40t 7x7 Torsion bar& hydraulic shock absorbers
Leopard1A4
42.2t 7x7 Torsion bar & hydraulic shock absorbers
TAM 30.5 6x6 Torsion bar
Leopard2 MBT
Leopard 1A1 MBT
IndiaVijayanta:40,4tonnes and 6x 6 configuration with torsion bar suspensionIndian production of Vickers MBT, UK
CI Ajeya:43.5 tonnes and 6x 6 configuration with torsion bar suspensionIndian production and upgraded T-72M1
IndiaArjun: Combat weight of 58.5 tonnesHydro- mechanical Suspension . 7 x7 road wheel configuration
Ex-tank: Combat Weight 47tonnes has Suspension type Torsion bar with Hydrogas struts. Indian production of T-72M1 series tank chassis fitted with the complete turret of the Arjun MBT
Israel
Mk 1 Merkava
60 t 6x6 Horstmann suspension with helical spring
MK3 Merkava
63t 6x6 concentric coil springs and have a hydraulic rotary damper
MK4 Merkava
65t 6x6 Horstmann suspension with rotary shock-absorbers
United Kingdom
Challenger 1 MBT weighs around 62 t and has hydro pneumatic suspension
The In-arm suspension is for high-performance system with minimal space claim and weight
Vickers MBT MK1weighs around 38.6tonnes and MK3 weighs around 38.7 tonnes have six road wheels on each side with torsion bar suspension
Tandem Hydro strut suspension
United KingdomHydro strut are used in Centurion, M109, and Cheiftian
Khalid MBT weighs 58 tonnes has hydro pneumatic suspension with 6 road wheels on each side
Cheiftian has 12 MK types and have Horstmann suspension systems
United States
M1A2 Abrams with combat weight of 63,086 kg has advanced torsion bar suspension
Suspension System for M1 MBT
The suspension systems of M48, M60 and centurion and T-series are being retrofitted with in-arm suspensions developed by Textron Marine called 14k, 10k and 6k systems (In-arm suspension)
Russian federation
T-72 S MBT
46.5 t 6x6 torsion bar suspensions with hydraulic rotary shock absorbers
T-80 BT-80 U
42.5 t46 t
6x6 torsion bar suspension
T-90 MBT
46.5 t 6x6 torsion bar suspension &hydraulic shock absorbers
Semi active rotary damper
BAE Systems Land Systems OMC (previously Reumech Ermetek) of South Africa and Horstmann Defense Systems of the UK teamed to develop, as a private venture, the Adaptive Damping System (ADS).
In-arm suspension
A compact light weight design to meet the requirements of air portable tracked vehicles
Options for in-arm suspension include variable height , Adaptive damping system, and suspension lockout as well as the development of super light weight with extensive use of titanium.
In arm suspension is used on surrogate vehicle chassis
Concept of active and semi active suspension
Acceleration of the sprung mass is sensed
Correction signal, as per control strategy, is fed to the
variable damper/ actuator.
Variable damping achieved by variable orifice, MR, ER
fluids, proportional or servo control valves.
Active suspensions consume very high power and are
more complex.
Un-sprung mass
Sprung mass
Control & power supply
Variable Damper
sensor
sensor
Semi-Active suspension system concept
Un-sprung mass
Sprung mass
Control & power supply
Force actuator
sensor
sensor
Active suspension system concept
Requirements of spring and damper
Frequency Requirement Spring Requirement Damper
For ride For handling
For ride For handling
Near Natural frequency of unsprung mass
soft stiff low
Near Natural frequency of sprung mass
Little influence;
soft
high
High (> natural freq) Not influential
low high
Low (< natural freq) stiff Not influential
High
Variable damping
Electro-rheological fluid mixture of dielectric base oil and fine semi
conducting particles Its resistance to flow is related to the electric
voltage across it
Magneto-rheological fluidsmart material --mixing fine particles in a fluid of
low viscosity The particles form into fibrous chain like
structures in the presence of a magnetic field
MR Damper
The fluid must pass through the MR valve for flow
MR valve is a fixed size orifice with the ability to apply a magnetic field
results in an apparent change in viscosity of the MR fluid
Imparts pressure differential for the flow of fluid proportional to the flow required to move the damper rod.
The accumulator is used to avoid cavitation in the low pressure side of the MR valve
Accumulator
MR valveMR Fluid
Control strategies
The Conventional PID Optimum Control And Ricatti EquationPredictive ControlRobust Control / L1-Optimal ControlSkyhook Damping Control
The Conventional PID
Damping set to two values:• Maximum • Minimum
Setting parameter: Relative velocity (Rv) b/w sprung mass and unsprung mass
Rv is in the direction of absolute velocity of sprung mass damping is set to maximum
Robust and simple
Optimum Control And Ricatti Equation
Penalty function J = ∫(XT Q X + UT R U) dt Objective function is linear quadratic Ricatti solution X = AX +B An optimum control strategy is one that
minimizes the following:• rms value of the sprung mass acceleration• the rms value of the suspension travel• the rms value of the dynamic tyre deflection
Predictive Control
Input obtained by solving an open-loop optimal control problem over a fixed prediction horizon into the future
Applied input is determined on-line at the recalculation instant
Implemented until new measurements become available
As applied input is based on an optimal control problem, it is possible to take specifications into account
Robust Control / L1-Optimal Control
for plants with parametric or dynamic uncertainties
time-invariant and time-varying controllers Norm-based optimization is used to address
robustness and performance issues of the control system
Criteria: H∞, H2 and in particular the L1
norms, as well as multi-objective synthesis problems
Skyhook Damping Control
An optimum control strategy, followed for suspension control problems
The damper is considered to be positioned such as to connect its one end to the sprung mass and the other end to an inertial reference in the sky,
instead of the unsprung mass.
Csky
M
Base wheel
Testing of AFV suspensions
Control Room
Controllers
Oil Reservoir
Heat Exchanger
AccumulatorFilter
Pump
Suspension Test RigPower Pack
Actuator
Pit Mounted Hydraulic System
Specimen
Instron test rig• Specification
– Hydraulics• Powerpack:4 nos• Max pressure: 250 bar• Flow: 80 lpm/ power pack
– Vertical actuator• ±300mm stroke• Load: 25T dynamic, 50 t static• Frequency: 0.1 Hz/ ±250mm,
0.8 Hz/ ±100mm– Measurements:
• Load cell• Pressure: 0-1000 bar• Surface temperatures: 0-200˚C
One-axial dynamic test rig
Two-axial Quasi-static test rig
Hydraulic Power packHydraulic Power Unity of 550 KW able to power 1000 l/min @ 280 barIt consists of pumps for both motion and fluid cooling, drive motors, controls, fluid reservoir, and associated equipment and valves.
Acceptance Test rig Minimum low end specifications for production testing of
the proven design of MBT Arjun suspensions
It has a hydraulic actuator controlled by a electro-proportional valve and powered by a hydraulic power pack of 400 lpm at 250 bar
The actuator is configured in an inclined mounting, which does not need a special structural strong floor
The motion of the actuator is transmitted to the test specimen via bell crank mechanism
If proven, it can be used by the production agencies for qualification of the production line of HSUs.
Test specifications
Test Frequency (Hz) Amplitude (mm)
Cycle 1 0.1 200
Cycle 2 0.8 100
Sample Stroke Pattern
Acceptance Test rig
ActuatorLH & RH Mounting for test units Electric motor -Pump
Conclusion
State of the art is the passive type, hydro-pneumatic suspension, for the AFVCurrent research actively pursued on semi-active and active suspensionKorea, USA. etc., are ready to field AFV with semi-active Hybrid (passive suspension for some stations and other with active suspension/semi-active) is the next optionCVRDE fully prepared to take up development of semi-active suspensions for AFV’s