Hydraulic control system

1. Hydraulic control systems
Subject: Control Engineering

2. COMPONENTS OF HYDRAULIC
SYSTEMS:
COMPONENTS OF HYDRAULIC SYSTEMS:
•The major components are:
•Prime mover
•Pump
•Control valves
•Actuators
•Piping system
•Fluid

3. Few supporting components are:
• Filters
• Strainers
• Storage tank
• Heat exchangers
• Pressure gauges
• Sensors
• Protective devices
• Control devices

4. Major components
• Prime mover :
It is the device which
develops the mechanical
power. It is a power
producing device. The type
of prime mover will depend
on the system. After passing
through fluid power system
this power is again available
as mechanical power.

5. PUMP
• Hydraulic pumps are used in hydraulic drive
systems and can be hydrostatic or hydrodynamic.
• A hydraulic pump is a mechanical source of power
that converts mechanical power into hydraulic
energy (hydrostatic energy i.e. flow, pressure).
• It generates flow with enough power to overcome
pressure induced by the load at the pump outlet.
When a hydraulic pump operates, it creates a
vacuum at the pump inlet, which forces liquid from
the reservoir into the inlet line to the pump and by
mechanical action delivers this liquid to the pump
outlet and forces it into the hydraulic system.

6. Control valves:
• The pressurised fluid supplied by the pump is
required to diverted to various parts of the
system.
• Also controls various parameter of flowing
fluid.
• Classified into three types:
1.Pressure control valves
2.Flow control valves
3.Direction control valves
• As name suggests , these valves control the
respective parameter of fluid.

7. Actuators
• Actuators convert the fluid power contained
in pressurized fluid to mechanical energy.
They are the muscles of the system.
• They provide the mechanical motion to the
desired part and the desired actuating force.
• The actuators can be divided into linear and
rotary actuators.
• Example of linear actuators is single acting
cylinders and rotary actuator is limited
rotation motor.

8. Piping system:
• They carry fluid containing the energy to
various parts of system.
• After transmitting energy, the return oil is
brought back to the reservoir.
• Due to the high pressure involved, design of
piping system require extreme care.
• Bursting of piping could prove to be a serious
matter leading to damage of equipment or
injury to personnel.

9. The supporting components
Filters:
• Hydraulic filters remove dirt and particles from fluid in a
hydraulic system.
• A hydraulic filter helps to remove these particles and clean
the oil on a continuous basis. The performance for every
hydraulic filter is measured by its contamination removal
efficiency, i.e. high dirt-holding capacities. Almost every
hydraulic system contains more than one hydraulic filter.
Accumulators:
• These are storage devices. They can cater for small time
fluctuations in the energy. This may be due to power
failures. They provide a small pool of pressurized oil which
can be used during emergencies.

10. • Electrical devices:
• They provide much flexibility in operation.
• They are used in control of fluid power.
Electrical control enables us the remote
operation, automation and sequencing. Some
of the electrical components include
solenoids, torque motors, and limit switchers.

11. Pressure-control valves
• Pressure-control valves are found in virtually every
hydraulic system, and they assist in a variety of
functions, from keeping system pressures safely below
a desired upper limit to maintaining a set pressure in
part of a circuit.
• Types include relief, reducing, sequence,
counterbalance, and unloading. All of these are
normally closed valves, except for reducing valves,
which are normally open. For most of these valves, a
restriction is necessary to produce the required
pressure control. One exception is the externally
piloted unloading valve, which depends on an external
signal for its actuation

12. Pressure Relief valves
• Most fluid power systems are designed to
operate within a present pressure range. This
range is a function of the forces the actuators in
the system must generate to do the required
work. Without controlling or limiting these
forces, the fluid power components (and
expensive equipment) could be damaged. Relief
valves avoid this hazard. They are the safeguards
which limit maximum pressure in a system by
diverting excess oil when pressures get too high.

13. Flow control valves
• Flow control valves manage the flow by decreasing or
increasing the opening at the throttling point. This helps to
determine speed of movement for the actuators. The
simplest design for a flow control valve is a needle or
longitudinal slot mounted in the pipeline and connected to
a screw that adjusts the opening at the throttling point.
• These are called throttle valves and they are regularly
used in combination with a check valve, i.e. the throttle
check valve for speed control in one direction of flow. A
disadvantage of throttle valves is that at varying loads a
change in pressure drop will change the flow; thus, the
speed of the moving actuator will also be affected.

14. Classification of flow control valves
1. Needle valves :
Needle controls the
area of orifice , which
causes change in flow
rate through the
valve.
2. Globe valves :
Controlling element is
disc or globe.
3. Gate valves : Flow
control achieved by
the movement of the
gate.

15. Directional Control Valves
2 POSITION, FOUR WAY DCV

16. Hydraulic Integral Controller:
• This is shown in fig. the components are
similar to that of proportional control. But the
pilot valve in this case can divert the oil to two
ports. Each going to each side of a double
acting cylinder.

17. • The input and feedback link are the same. If an error input e is
given to input link, it moves the spool of the pilot valve. Oil
would be send to the corresponding port of the double acting
cylinder when pressurized oil goes to the double acting
cylinder.
• This movement of the piston is feedback to the feedback link.
Thus an output motion of the cylinder is produced
corresponding to the error input e.
• To show that the control action is integral. For an input
displacement e, a proportional discharge Q is produced by the
pivot valve
• Above Eel is the equation for an integral control. Thus the
above system gives a hydraulic integral control.

18. Derivative Controllers:
Basic definition:
• Derivative control is the type of control action in which, the
controller output is proportional to the rate of change of
the deviation.
• This mean that the controller output is related to the rate
of change of deviation.
• If the deviation is changing fast, then the controller output
will be high. If the deviation is changing at a slow rate, the
value of controller output would be low.
• The deviation control action start even before the error has
actually changed by that much. The slope of the change or
the trend is sufficient to initiate control action.

19. Mathematical representation:
• As per the definition, in derivative control, the
controller output m is proportional to rate of
change of error.
• Where is called the derivative time.
• If express above eel in the Laplace domain we
get,

20. • The block diagram will look as shown

21. Graphical representation
• Derivative control action is shown
graphically in fig. the error is given by
a curve as shown in the error
characteristics.
• The error increases at uniform rate,
remains constant and thereafter drops
at a constant rate.
• When error is increasing at a uniform
rate, the controller output has a
constant value as shown by the
controller characteristics. This is
because, for derivative control, the
controller output is proportional to the
rate of change of error.
• When the error curve becomes
parallel to the time axis we could see
that the controller output drops to
zero.
• So though is an error. But there is no
corrective action.

22. Hydraulic proportional integral
controller:
• This combines, proportional as well as integral
actions combined fig. shows the
constructional details of a hydraulic
proportional controller.

23. • In this, instead of a pilot valve we have a distributor
block and a swinging nozzle. The swinging nozzle is
connected to the input link. When the input link is
moved due to an input, the nozzle swings. This
changes oil flow rate through port A and B
• The swinging nozzle produces the proportional action
and the needle valve and the feedback cylinder
produces the integral action. Thus the combined
output is a proportional – integral action.

24. Mathematical representation:
• The three modes of control which we have seen now that is,
proportional, integral and derivative mode of control are not
always used in single mode.
• Proportional mode can be additively combined with integral
mode to get the benefits of both the modes. This is called
proportional – integral control.
• Proportional action
• Integral action

25. • When two are combined
• It can be represented as:

26. Hydraulic proportional derivative
controller:
• Given fig. shows the schematic diagram of a proportional derivative controller.
It consists of an input arm to which the input displacement x is given.
• Pilot cylinder has five ports. Pressurized oil comes to the pilot cylinder
through the center port. Return oil goes through the two end ports. Oil goes to
the two sides of the power cylinder depending up on the position of the spools.
The spool has two pistons connected to the common rod. Depending on the
input displacement and the error signal, the spools move to the right or left.
• The power cylinder has one piston connected to the piston rod.
Depending up on the oil flowing in to the power cylinder the piston will move in
appropriate direction. This will give the output displacement y to the piston rod.

27. • Working:
• When an input displacement is given to the input lever say to
the right the spool move to the right. This will cause the oil to
flow to the left side of the power cylinder. This will push the
piston to right by an amount y. the amount of piton
movement will be proportional to the input displacement x.
This is the proportional part of the controller.
• Simultaneously the rod of the power cylinder will move the
piston in the feedback cylinder. The movement of the piston
in the feedback cylinder will push the fluid to the right
pressurizing the fluid in the right side of the cylinder. This in
turn will cause a flow rate of oil from the right side to the left
side. This is nothing but a rate feedback.

28. • Graphical representation:
• A proportional –derivative
action cannot be adequately
described by a step change.
Because as we saw earlier, in
pure derivative the controller
output is present only during
the rising part of the step.
• There after the output drops to
zero at the flat portion of the
step. So we will use a ramp
signal as the error input and see
how the output would be
shown in fig.
• For a ramp signal

29. • Block diagram I s shown in fig.

30. Proportional – Integral – Derivative
Control (PID):
• This is also called the three mode control. It
combines the merits of all the three modes and is
widely used in processes. It is commonly known as
PID Control.
• It is nothing but the additive combination of
proportional, integral, and derivative control actions.

31. • mathematically a PID control action is
represented as
• In Laplace form,
•

32. Comparison of Electricl and Hydraulik
Systems

34. THANKYOU!