Fluid Power and Control System Overview
Fluid power systems deal with generation and transmission of a pressurized
fluid at an appropriate variation of the rate of energy for using it for following a system
demand with minimum error. Applications of fluid power systems are several. Typical
requirements met by such systems are
 control of flow rates in chemical process plants,
 control of fin or wing movement of submarine or aeroplane for manoeuvring
vehicle motion,
 motion of a metal cutting or forming tool.
Notable advantages of a fluid power system over other power transmission
systems, say a gear transmission system, are as follows. The former system provides
precision control through elimination of mechanical backlash and an option of
conveying power between a distant pump and actuator without much loss in power.
A fluid-power system is referred to as pneumatic if the fluid is air, which
possesses compressibility. On the other hand, if the fluid is incompressible, the system
is referred to as hydraulic system. For their good heat-dissipation and lubrication
characteristics and seal-material compatibility over a wide temperature range,
petroleum-based oil is the most common variety constituting about 80% of about 1000
million litre of hydraulic fluid sold every year [1]. Other hydraulic fluids like synthetic
fluid, water glycol fluid and water-in-oil emulsions are normally used where fire can be
a hazard. High water content fluids (HWCF), containing more than 90% water and
multiple additives for property improvement, are emerging recently as an alternative to
the reducing oil reserve.
Pneumatic systems are often preferred in small systems, say in a mechanical
hammer in a forging shop, where air is taken from and finally released to the
atmosphere. However, in a larger system, hydraulic systems are preferred for their
superior controllability on account of incompressibility.
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Major components of a fluid-power system are
pump or compressor, where power is imparted to a fluid by pressurizing it by the
mechanical movement of a component , say a piston
actuator, where the fluid power is used to execute the intended operation like
moving a control surface or a press through an appropriate mechanism
control valves, by which fluid power supply between the pump or the compressor to
the actuator is monitored
accessories like reservoir, filter, heat exchanger, accumulator, intensifier and
connector or hose through which the fluid power is transmitted
Symbols of some fluid-power components have been shown in Figure 1.1 – (a)
and (b) respectively denoting a pump and a compressor, (c) a motor or alternatively an
IC engine that is meant to provide torque to the shaft of the fluid-power generating unit,
(d) and (e) the actuators, (f) to (i) the control valves and (j) to (p) the accessories. In
Figures 1(a), (b), (e), (f) and (g) filled and unfilled triangles have been used to represent
hydraulic and pneumatic equipment respectively. Many of these symbols have been
used in Figure 1.2 representing simple fluid-power control systems. In the system
drawing, if there is any fluid-identifying triangle in a valve symbol, it could be omitted
conventionally.
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(a) Compressor (b) Pump
(c) Motor
(e) Rotary
(d) Linear Actuator Hydraulic Actuator
(f)
(g)
4-Port 3-Way Direction Control Valves
(f) Manual, 3-Position
(g) Pneumatic, Automatic, Infinite Position
(j) Sump or (k) Air
Reservoir Tank
(h) Non
Return Valve
(i) Pressure
Relief Valve
(n) Air
(p) Non(l)
(m)
Preparation (o) Branched intersecting
Filter Cooler
Pipes
Unit
Pipes
Figure 1.1: Symbols for Basic Hydraulic and Pneumatic Components
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(a)
(b)
Figure 1.2: Fluid-power Systems – (a) Pneumatic, Manual,
Open-Circuit (b) Hydraulic, Automatic, Closed-Circuit
In Figure 1.2, the pump and the compressor have been shown as coupled with
motor. While the compressor in Figure 1.2 (a) draws air from the ambient to charge an
air tank, the pump in Figure 1.2 (b) raises the oil from a reservoir. The pneumatic
circuit shown in the figure is an open one. At the compressor inlet, air enters from the
ambient and the unfilled triangles at the exit ports of the direction-control and pressurerelief valves indicate the air discharge to the ambient. However, the hydraulic circuit is
always a closed type. As shown in Figure 1 (b), the pump receives oil from the
reservoir and the valves allow the oil to flow back to the same reservoir, though drawn
at different locations for convenience.
In any control system, mechanical, fluid-power or electrical, the control is either
passive or active. A device is called active or passive depending on whether it receives
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any external command or not. The external command could be manual or electrical. In
an automatic control system, the electrical excitation comes from a data acquisition and
control system, called DAQCS in short. Figures 1.1(f) and (g) represent two active
direction-control valves and Figures 1.1 (h) and (i) represent two passive valves,
namely a non-return valve and a pressure relief valve. The manual control to the valve
in Figure 1.1 (f) is provided by a hand lever, whereas the control input to the valve in
Figure 1.1 (g) is set by a bi-directional solenoid in response to the electrical excitation
from DAQCS.
The internal features of the non-return valve, a poppet type pressure relied valve
and a spool-type solenoid-operated directional control valves are shown in Figures 1.3
(a), (b) and (c) respectively. As apparent in Figures 1.1 (f) and 1.3 (a), the non-return
valve has a ball, whose position depends on the balance of the pressure force and the
spring force acting on it from opposite sides. It could either sit on its seat blocking the
flow or could float off the seat, thereby allowing the flow through the valve. While the
non-return valve is meant for allowing the flow only in one direction, the pressurerelief valve does not allow the system pressure to rise beyond a limit.
Within a pressure-relief valve, there is a spring with a setscrew at one end and a
ball, poppet or spool on the other end that could be lifted off its seat in case the fluid
pressure at the outlet of the pump or compressor reaches a set limit. Figure 1.3 (b)
shows a ball-type relief valve. By turning the knob of the setscrew, the pre-compression
of the spring and hence the pressure at which the flow cracks open could be varied.
Figure 1.3(c) depicts an infinite-position, 4-port, 3-way, direction-control valve
excited by electrical command signal. In the figure, the markings s, c1, c2 and r stand
respectively for the supply port receiving high-pressure fluid from the pump, two
communication ports between the valve and an actuator and a return port for passing
the low-pressure fluid to a reservoir. Three possible ways of setting the ports are open
in two alternate directions and blocked, or neutral. These could be achieved by moving
a spool (1) within a bush (2) that has all the port connections laid within its body. While
the spool together with the bush are referred as the spool valve, all the other parts of the
valve that provide the spool driving force is called the force motor. Of course, the
excitation signal set by the controller to the coil (3) of the force motor causes both the
direction and the magnitude of the driving force to vary.
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s
r
x
(b)
Q
5
3 8
1
c2
(a)
r
4
c1
9
6
7
(c)
Figure 1.3: Schematics of (a) Non-Return Valve (NRV), (b) PressureRelief Valve (PRV) and (c) Direction-Control Valve (DCV)
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The spool in Fig. 1.3(c) has a rod with three discs, called the lands. At the valve
neutral, the lands completely block Ports s and r, thereby stopping the main flow
through the valve. Displacement of the spool in either direction from the neutral creates
orifice-like openings at one side of the land at s and one side of one of the lands for r.
Hence, flow between the valve and the actuator takes place either through Ports c1 and
c2 either along the path s  c1  c2r or along the path s  c2  c1r. Since
a port is called as metered or unmetered depending on whether its opening depends or
does not depend on the excitation signal, in Fig. 1(c) Ports s and r are metered and
Ports c1 and c2 are unmetered. Depending on whether the widths of the metered ports
are smaller, equal or greater than the widths of the corresponding lands, a valve is
termed as overlapped, critically lapped or underlapped respectively.
The force motor in Fig. 1(c) has a coil (4), an armature (5), two permanent
magnets (6 and 7), a central pole piece (8), two end pole pieces (9 and 10) and two cone
drives (11 and 12), each coupled with the armature by a spring (13 or 14). A number of
axial spokes on either end of an annular cylinder constitutes the armature. The other
ends of the spokes are extended to the respective end spring of conical diaphragm type.
A coupler, coaxial with the spring, is meant for connecting the spool. The figure shows
typical magnetic flux lines emanating axially outward from a magnet (6 or 7) to go
through the respective end pole-piece (9 or 10) that guides the path axially inward to
cross an axial air gap and enter the armature (5) within which it becomes inward radial
to cross a fixed radial air gap to enter the central pole piece (8) and eventually turns
axially outward to enter the originating magnet.
The spool driving force is expected to be zero in absence of any current in the
coil (4) and the axial air gaps on its either side of the armature (5) equal. For non-zero
coil current, electro-magnetic flux paths are induced through the coil that turns in both
the end pole pieces from axially outward to axially inward direction, in between
crossing one axial air gap, the armature and the other axial air gap in succession. The
magnetic and electro-magnetic fluxes in the figure are additive in the left air gaps and
opposing in the right air gaps. As a result, a net force towards left acts on the armature
that eventually displaces the spool towards left. The steady state displacement of the
spool maintained by the balance of the electromagnetic, spring and flow forces depends
on the magnitude of the coil current. In case the direction of the coil current is revered,
the spool displacement also gets reversed. Thus, the openings of the metered ports
could be varied infinitesimally in a controlled manner.
The active control for reversing either the linear or rotary motion of the actuator
has been shown as manual in Figure 1.2 (a) and as automatic in Figure 1.2 (b). While
the rotary actuator in Figure 1.1 (e) is unidirectional and hydraulic, the actuator shown
in Figure 1.2 (a) is bi-directional and pneumatic. Figure 1.2 shows the flow path
through the direction-control valve in three blocks. At the neutral position of the valve,
four ports are explicitly shown, each in the form of a T. These remain disconnected
from each other at the neutral position. A port is meant to supply the fluid from the
pump or the compressor, a port takes the discharge to either the reservoir or the ambient
and the other two ports are meant to supply and receive the flow to and from the
actuator. Thus, at the neutral, the flow communication with the actuator remains cutoff.
The manual operation of the valves allows the spool to acquire any of three
possible positions shown by the three blocks – blocked at the neutral, and open at the
other two positions depicted by the right and left-side connection. A continuous
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variation of the electrical excitation to the solenoid in an automatic valve allows a
continuous variation of the port openings up to a maximum on either side. Of course,
the excitation signal is calculated in real time by a controller hardware following a
control law. Figure 1.4 depicts an electrohydraulic control system with automatic
feedback of the piston displacement acquired by an LVDT. The input module IM of a
real-time controller gathers this feedback signal. There are a real-time processor RTP
and an output module OM in the controller. The RTP also receives the digital input of
the demand that is appropriately converted and compared with the feedback signal. A
control algorithm needs to be developed and loaded on the RTP so as to calculate the
output that would be minimizing the difference between the demand and the feedback.
Of course, the output is communicated through the OM and amplified suitably in an
amplifier prior to directing the amplified signal to the coil of the force motor of the 4/3
infinite-position DCV, where a 4-port 3-way direction control valve is referred as 4/3
DCV in short.
Amplifier
Command
Signal
IM
Host PC
OM
RTP
Real-time
Controller
Electronic
Fast actuation response is a desirable feature of any automatic control system.
Therefore, the moving element and its total movement within the control valve are
made quite small, and so is the radial clearance between it and the corresponding
stationary part. This calls for using filters to avoid clogging of the clearance passages.
In fact, the maximum opening in the main flow path in the variable-area ports of the
valves are mostly in the sub-millimeter range. Thus, a large pressure loss occurs in such
a port, leading to substantial temperature rise of the fluid. Therefore, coolers are
necessary for containing the rise of fluid temperature. It is customary to use an air
preparation unit for a pneumatic system. Such a unit comprises an additional filter, a
pressure reducing valve and a pressure indicator. The purpose of integrating the valve
in the unit is to reduce the pressure loss in the system. This would be clear at a later
stage, when the working of a pressure-reducing valve will be discussed in detail.
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Figure 1.4: Schematics of an electrohydraulic feedback control system
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