Simulation of a Mechatronic Machine Lumped fluid theory, Flow types.
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Transcript of Simulation of a Mechatronic Machine Lumped fluid theory, Flow types.
Simulation of a Mechatronic Machine
Lumped fluid theory, Flow types
Simulation of a Mechatronic Machine
Summary of previous lecture
The two properties that are essential for modelling hydraulics are viscosity and bulk modulus.
The bulk modulus effects to the stiffness of the hydraulic components and for this reason the flexibility of the whole system.
Properties of liquids
it
i
ctctoe BV
V
BV
V
BV
V
BB
2
2
1
111
Simulation of a Mechatronic Machine
Lumped fluid theory
Simulation of a Mechatronic Machine
The lumped fluid theory is commonly used modelling approach for hydraulic systems.
The theory can be applied to hydraulic systems in which the effect of acoustic pressure waves is insignificant.
In mobile machines, the pipelines are short and work cycles are relatively slow in which case the pressure waves are of no significance.
Hydraulipiiri
Simulation of a Mechatronic Machine
In the lumped fluid theory, the hydraulic circuit is divided into volumes in which the pressure is assumed to be constant.
- Differential equations are formed for the volumes. Equations are solved in order to obtain the pressures of the system.
- The different volumes are assumed to be separated by throttle valves through which the fluid can flow. The flow rates through the throttle valves can be calculated using the pressures.
- The direction, pressure and flow valves as well as long pipelines used in real systems, are replaced by throttles which control the flow rate between the different volumes.
Simulation of a Mechatronic Machine
xx ,
Example
111 ,, VBp
222 ,, VBp
Q
Simulation of a Mechatronic Machine
Hydraulipiiri
222 ,, AeAA VBp
Idealisointi
111 ,, AeAA VBp pp
111 ,, BeBB VBp Tp
Simulation of a Mechatronic Machine
Pressures in the different locations of the hydraulic circuit can be calculated using the flow continuity equation.
iii Qm
ooo Qm is the densityQ is the flow rate
iiQ ooQQ
Simulation of a Mechatronic Machine
The mass of the volume can be written as follows:
where V is the volume under consideration.
- The mass of the volume changes if the in and out going flows are not equal:
VM
oi mmMdt
d iiQ ooQQ
Simulation of a Mechatronic Machine
iiQ ooQQ
Employing the equation of mass flow one can get:
Which gives us:
Assuming that the in going fluid density is the same as the out coming density above equation can be written in the form:
ooii QQVdt
d
ooii QQdt
dVV
dt
d
oi QQdt
dVV
dt
d
iii Qm
Simulation of a Mechatronic Machine
Using the relation in the equation of
bulk modulus leads to
By substituting this result to equation of flow continuity gives us
d
V
dV
dV
dpVBe
eB
dpd
oie
QQdt
dV
B
V
dt
dp
Simulation of a Mechatronic Machine
Reorganize of the equation leads to
Where the change of the volume dV/dt means, in practice, the motion of the piston
- As a result, first order differential equation for the unknown pressure is formed. The equation is called the flow continuum equation.
dt
dVQQ
V
B
dt
dpoi
e
oie
QQdt
dV
B
V
dt
dp
Simulation of a Mechatronic Machine
Flow types