The main function of directional valves in hydraulic systems is to direct and distribute flow between consumers. As far as the valve modeling is concerned, the valves are classified by the following main characteristics:
Number of external paths (connecting ports) — One-way, two-way, three-way, four-way, multiple-way
Number of positions a control member of the valve can assume — Two-position, three-position, multiple-position, continuous (can assume any position within working range)
Control member type — Spool, poppet, sliding flat spool, and so on
As an example, the following illustration shows a portion of a hydraulic system with a 4-way, 3-position directional valve controlling a double-acting cylinder, next to its schematic diagram.
Throughout Simscape™ Fluids™ libraries, Hydraulic (isothermal liquid) ports are identified with the following symbols:
P — Pressure port
T — Return (tank) port
A, B — Actuator ports
X, Y — Pilot or control ports
4-way directional valves are available in multiple configurations, depending on the port connections in three distinctive valve positions: leftmost, neutral, and rightmost. Each configuration is characterized by the number of variable orifices, the way the orifices are connected, and initial openings of the orifices. Ten 4-way directional valve blocks in Simscape Fluids libraries represent twenty most typical valve configurations. Configurations that differ only by the values of initial openings are covered by the same model.
The basic 4-Way Directional Valve block lets you model eleven most popular configurations by changing the initial openings of the orifices, as shown in the following table.
Basic 4-Way Directional Valve Configurations
All four orifices are overlapped in neutral position:
All four orifices are open (underlapped) in neutral position:
The other nine configurations are covered by the remaining 4-way directional valve blocks (A through K), as shown in the next table.
Other 4-Way Directional Valve Blocks
4-Way Directional Valve A
Contains two additional normally-open, sequentially-located orifices. Valve displacement to the left or to the right closes the path to tank.
4-Way Directional Valve B
Ports P and A are permanently connected through fixed orifice.
4-Way Directional Valve C
Ports P and B are permanently connected through fixed orifice.
4-Way Directional Valve D
Two orifices are installed in the P-A link. Port A never connects to port T.
4-Way Directional Valve E
Two orifices are installed in the P-B link. Port B never connects to port T.
4-Way Directional Valve F
Two parallel orifices in the P-A arm and two series orifices in the A-T arm.
4-Way Directional Valve G
Two parallel orifices in the P-B arm and two series orifices in the B-T arm.
4-Way Directional Valve H
Two parallel orifices in the P-B arm and two series orifices in the P-T arm.
4-Way Directional Valve K
Two parallel orifices in the P-A arm and two series orifices in the P-T arm.
The Directional Valves library offers several prebuilt directional valve models. As indicated in their descriptions, all of them are symmetrical, continuous valves. In other words, the control member in 2-way, 3-way, 4-way, and 6-way valves can assume any position, controlled by the physical signal port S. The valves are symmetrical in that all the orifices the valve is built of are of the same type and size. The only possible difference between orifices is the orifice initial opening.
These configurations cover a substantial portion of real valves, but the directional valves family is so diverse as to make it practically impossible to have a library model for every member. Instead, Simscape Fluids libraries offer a set of building blocks that is comprehensive enough to build a model for any real configuration. This section describes the rules of building a custom model of a directional valve.
All directional valve models are built of variable orifices. In Simscape Fluids libraries, the following variable orifice models are available:
To simplify the way variable orifices are combined in a model, their instantaneous
opening is computed in the same way for all types of orifices. The orifice opening is
always computed in the direction the spool, or any other control member, opens the
orifice. In other words, positive value of the opening corresponds to open orifice,
while negative value denotes overlapped, or closed, orifice. The origin always
corresponds to zero-lap position, when the edge of the control member coincides with the
edge of the orifice. In the illustration below, origins are marked with
0 for the orifice with variable area round holes (schematic on the
left) and for the orifice with variable slot (schematic on the right). The
x arrow denotes the direction in which orifice opening is measured
in both cases.
The instantaneous value of the orifice opening is determined as
|Instantaneous orifice opening.|
|Initial opening. The initial opening value is positive for initially open (underlapped) orifices and negative for overlapped orifices.|
|Spool (or other control member) displacement from initial position, which controls the orifice.|
|Orifice orientation indicator. The variable assumes +1 value if a spool displacement in the globally assigned positive direction opens the orifice, and -1 if positive motion decreases the opening.|
The number of variable orifices and the way they are connected are determined by the valve design. Usually, the model of a valve mimics the physical layout of a real valve. The illustration below shows an example of a 4-way valve, its symbol, and an equivalent circuit of its Simscape Fluids model.
The 4-way valve in its simplest form is built of four variable orifices. In the
equivalent circuit, they are named
B_T. The Variable Orifice block, which
is the most generic model of a variable orifice in the Simscape
Fluids libraries, is used in this particular example. You can use any other
variable orifice blocks if the real valve design employs a configuration backed by a
stock model, such as an orifice with round holes or rectangular slots, poppet, ball, or
needle. In general, all orifices in the model can be simulated with different blocks or
with the same block, but with different way of parameterization. For instance, two
orifices can be represented by their pressure-flow characteristics, while two others can
be simulated with the table-specified area variation option (for details, see the Variable Orifice block reference page).
The next example shows another configuration of a 4-way directional valve. This valve unloads the pump in neutral position and requires six variable orifice blocks. The Orifice with Variable Area Round Holes blocks have been used as a variable orifice in this model. Port T1 corresponds to an intermediate point between ports P and T.
Finally, let us consider a more complex directional valve example. The figure below shows basic elements of a front loader hydraulic system. Both the lift and the tilt cylinders are controlled by custom 3-position, 5-way valves, developed for this particular application. The valves are designed in such a way that the pump delivery is diverted to tank (unloaded) if both cylinders are commanded to be in neutral position. The pump is disconnected from the tank if either of the two control valves is shifted from neutral position.
To develop a model, the physical version of the valve must be created first. The following illustration shows one of the possible configurations of the valve.
The Simscape Fluids model, shown below, is an exact copy of the physical valve configuration.
All the orifices in the model are closed (overlapped) in valve neutral position,
P_T1_2. These two
orifices should be set open to an extent that allows pump delivery to be discharged at