4 Actuator Control
6.3 Dynamic Modeling and Simulation
Also, numerical problems will be encountered if for some reason the correct solution includes zero flow through an orifice. In that case the orifice equation must be replaced with a restriction flow equation that reflects the laminar regime, i.e., Q∝Δp.
6.3 Dynamic Modeling and Simulation
Physically, dynamic simulation of a hydraulic system is mainly characterized by:
• acceleration of all mechanical parts
• compressible fluid
Dynamic simulation allows for a much more detailed investigation of the performance of the hydraulic system, however, it does also require that further estimations of especially system damping and oil stiffness in order to predict behavior correctly.
Steady state simulation corresponds to solving a set of algebraic equations whereas dynamic simulation corresponds to solving a mixed set of differentials and algebraic equations. The differential equations are time dependant, i.e., the problem is an initial value problem, where the pressure in the pressure nodes, the volumes of the accumulators and the position and velocity of all mechanical degrees of freedom must be known at the start of simulation.
Most of the basic equations shown in Table 6.2 remain the same. Most notable is the difference when considering flow continuity of a pressure node. If the fluid is compressible then conservation of mass yields the following differential equation for a volume of fluid = pressure node:
V This equation gives the pressure gradient in a given volume of fluid, V. The net-flow into the volume is Q (positive if flow enters the volume) and the time derivative of the expansion (displacement flow) is V (positive if the volume is expanding). If this value is positie at a certain time the pressure is going up and vice versa. The effective stiffness of the fluid, which greatly depends on temperature, dissolved air, hosings and tubings, is β. In this case we have a 1st-order differential equation and only one initial condition is required, namely the initial pressure in the volume: p.
As an example consider the volume shown in Figure 6.2 which is bounded by 2 cylinders, 2 motors, 2 pumps and 2 orifices.
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p
Figure 6.2. A volume bounded by cylinders, motors, pumps and orifices.
The differential equations for the pressure gradient of this volume may be written using Equation 6.3 with the following values for Q and V :
Just like a mechanical system typically contains more than one body a hydraulic system will typically need to be modelled with several volumes. In a hydraulic system the different volumes are typically separated by either: Pumps, motors, cylinders or orifices.
Displacement flows in volumes are typically caused by hydraulic cylinders or accumulators. If we consider the steady state equations for an accumulator then the time derivative yields a linear equation containing both the pressure and volume gradient:
g
In the above equation the polytropic exponent n depends on whether the compression/decompression of the accumulator is predominantly adiabatic. In general Equation (6.5) is a simplification because n may vary significantly, hence, care should be taken in the modeling if the performance of the hydraulic system is very sensitive to the value of n.
Equation (6.5) introduces another state variable, namely the fluid volume of the accumulator. The initial fluid volume is readily derived from the total accumulator volume (which is constant), the preload conditions and the current pressure:
0
Hence, if a volume is connected to an accumulator it has two states: the pressure and the fluid volume of the accumulator and their gradients must be solved simultaneously using Equations (6.3) and (6.5).
In general, a purely hydraulic system may be solved numerically in the following steps:
1. Identify all volumes in the system and set up the pressure build up equation for each and identify all accumulators and set up the (Circuit diagrams useful here).
2. Identify all orifices and their dependancy on the motion of mechanical parts in valves.
3. Determine initial pressure for each volume and initial fluid volume for each accumulator.
4. Calculate pressure gradients for each volume and volume gradient for each accumulator.
5. Calculate acceleration of all movable mechanical parts in valves.
6. Update pressure in each volume and volume of each accumulator.
7. Update position and velocity of all movable mechanical parts in valves.
8. If the analysis is not yet concluded then go back to 4.
Typically som mechanical bodies, cylinders or valves will be part of the system. This means that their position and velocity must be updated simultaneously in order to update volumes, orifice flows and net-flows into volumes for the next step.
As may readily be observed from Equation (6.7) the position and velocity of the mechanical system is needed in order to do a dynamic simulation of the hydraulic system. In fact, any type of dynamic simulation of a hydraulic system requires a simultaneous dynamic simulation of the actuated mechanical system and, depending on the level of detail, also of the movable mechanical parts within the hydraulic valves.
Hence, dynamic simulation of hydraulics is closely connected to dynamic simulation of mechanics.
The mechanical system may, in general, be divided into a number of bodies. In the planar case, the governing dynamic equations for a body are:
M
In Equation (6.7) m is the mass and J is the mass moment of inertia with respect to the mass center of the body. Furthermore, r is the coordinates of the mass center of the body and θ is the rotation of the body, both measured relative to some reference coordinate system. ∑ F is the sum of all the forces acting on the body, normally divided into applied forces (gravity, springs, dampers, actuators, friction, wind resistance, rolling resistance etc.) and reactive forces (connections with other bodies and ground).
is the sum of the force moment of
∑ M ∑ F with respect to the mass center and all the
moments (force couples) acting on the body.
The dynamic equilibrium equations contain 3 scalar equations corresponding to the 3 degrees of freedom of a body. All of them are differential equations, and in general needs to be solved numerically. Being 2nd order differential equations 2 initial conditions is required for each coordinate, namely the initial position and initial velocity:
θ θ
r r
Knowing the positions and velocities of the bodies will typically also be necessary in order to determine the resulting forces and moments, i.e., the right hand side of Equation (6.7).
Alternatively, the steady state equations of the actuators, see Table 6.2, may be generalized to take into account the dynamics of the mechanical system. For the motor a simplified dynamic equation can be set up:
M
In Equation (6.8) the sign conventions are as follows: rotation of the shaft,θ(and its time derivatives; and ), is defined as positive in the same direction as the hydraulically generated torque on the shaft produced by a positive pressure drop across the motor. The applied moment on the output shaft, M, is positive in the opposite direction. The introduction of the hydro-mechanical efficiency,
θ θ
hmM
η´ , should be done with care. Normally, the hydromechanical efficiency is measured in a situation where the motor is motoring, i.e., the direction of the applied moment is opposite to that of the angular speed. In that case:
2 M
However, if the motor is working as a pump, i.e., the direction of the applied moment is in the same direction as that of the angular speed (negative load M<0) we get:
2 M
The effective mass moment of inertia, , and the applied moment, M, must be related to the output shaft of the motor. In general, they are both functions of
Jeff
θ and and may θ
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be determined from energy considerations. The applied moment may be computed as
In Equation (6.11) is the work done by the external forces/moments (gravity, friction, rolling resistance etc.) on the mechanical system actuated by the motor.
Wext
The effective mass moment of inertia may be computed according to:
2
In Equation (6.12) Ekin is the kinetic energy of the mechanical system.
Consider the system shown in Figure 6.3.
θ
Figure 6.3. A hydraulic motor is driving a winchdrum via a gearbox. The drum is connected to a payload and is also subjected to a certain rotational friction.
For an infinitesimal rotation of the motor shaft the work done on the system by the external loading can be computed and, subsequently, the moment applied to the output shaft of the motor:
i
The effecitve mass moment of inertia may be computed as:
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2
Notice, that the mass moment of inertia of the motor rotor and the gearbox have been neglected. Because of the high torque pr. volume ratio of hydraulic motors neglecting the inertia of the motor rotor and the gearbox is normally a reasonable assumption.
Next, consider the system shown in Figure 6.4. It is a four-wheel drive vehicle subjected to a total rolling resistance of , and propelled by a hydraulic motor via a geared chain drive
FR
1 2
z
i= z . Due to symmetry the vehicle is modeled as a planar system.
θ
Figure 6.4. A four-wheel drive vehicle propelled up an incline by a hydraulic motor is shown.
Power is transmitted from the motor to the wheels via chain drives.
As in the previous example a infinitesimal rotation of the motor is used to determine the moment applied to the output shaft of the motor:
{ }
The effecitve mass moment of inertia may be computed as:
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2
Having determined the effective mass moment of inertia it is possible to compute the eigenfrequency of the hydraulic-mechanical system composed of the lines to and from the motor, the motor and the mechanical system represented by the effective mass moment of inertia:
(
L)
In Equation (6.17) is simply the total volume of the fluid lines leading up to the motor and away from it.
VL
Similar to that of the motor a simplified dynamic equation can be set up for the cylinder:
(
p p)
F F FA x
meff ⋅= ⋅ 1−ϕ⋅ 2 − = tC − (6.18) In Equation (6.18) the sign conventions are as follows: piston travel,x(and its time derivatives; and ), is defined as positive when the cylinder is extracting. The applied force on the piston, F, is positive in the opposite direction. As in the case of the motor the introduction of the hydro-mechanical efficiency,
x x
hmC
η´ , should be done with care.
Equation (2.47) is used in its general form to get an expression for the friction force as a function of the hydromechanical efficiency:
tC
If the cylinder is extracting the theoretical cylinder force is defined as
(
1 2If the cylinder is extracting and is pulled by the load we have:
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( )
If the cylinder is retracting the theoretical cylinder force is defined as
(
2 1 negative value.If the cylinder is retracting and is pushed by the load we have:
( )
The effective mass, , and the applied force, F, must be related to the piston of the cylinder. In general, they are both functions of
meff
x and and may, similar to the values associated with the motor, be determined from energy considerations. The applied force may be computed as follows:
x
dx F −dWext
= (6.24)
The effective mass may be computed according to:
2
As an example consider the system shown in Figure 6.5.
Synthesis of Hydraulic Systems Page 13 of 16
m
Figure 6.5. A hydraulic cylinder is rotating an arm with a payload at the end.
For a infinitesimal extraction of the cylinder the work done on the system by the external loading can be computed and, subsequently, the force applied to the cylinder piston:
In Equation (6.26) it is utilized that the arm and the piston must have the same absolute velocity in point C.
The effective mass may be computed as:
( )
Having determined the effective mass it is possible to compute the eigenfrequency of the hydraulic-mechanical system composed of the lines to and from the cylinder, the cylinder and the mechanical system represented by the effective mass:
( )
2 2 1
2 eff n
V A V
k A
m k
⋅ + ⋅
= ⋅
=
ϕ β β
ω
(6.28)
In Equation (6.28) and are simply the total volume of the fluid leading up to the cylinder piston side and to the cylinder rod side, respectively. This include the volume in the lines as well as the volume in the cylinder. Hence, in general the volumes are functions of the piston position.
V1 V2