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PROPORTIONAL VALVES - BASIC PRINCIPLES
Steve Skinner, Eaton Hydraulics, Havant, UK
Copyright

Eaton Hydraulics 2000
BASIC SYSTEM
1)
Consider a simple hydraulic
system consisting of a reservoir
(A), electric drive motor (B), pump
(C), relief valve (D), filter (E), flow
control valve (F), directional control
valve (G) and cylinder (H).
H
2) Movement of the cylinder is
controlled by the flow control
valve (which determines the
speed of movement) and the
directional control valve (which
determines which way the
cylinder moves).
G
F
B
C
E
D
A
BASIC SYSTEM
When the solenoid valve is
energised, the cylinder piston will
extend or retract at a speed
determined by the flow control
valve. The solenoid valve itself
therefore has no control over the
cylinder speed.
BASIC SYSTEM
A three position solenoid valve can:
- extend the cylinder
BASIC SYSTEM
A three position solenoid valve can:
- extend the cylinder
- retract the cylinder
BASIC SYSTEM
A three position solenoid valve can:
- extend the cylinder
- retract the cylinder
- stop the cylinder
The solenoid valve is therefore acting
much like a switch in an electrical
circuit.
In one position the light is switched
off...
... and in the other position it is
switched on but there are no
intermediate states.
However, another type of switch can
be used for controlling a light bulb
known as a dimmer switch.
In this case, the switch can be turned
to any position between fully off and
fully on to vary the brightness of the
bulb.
In this case, the switch can be turned
to any position between fully off and
fully on to vary the brightness of the
bulb.
In this case, the switch can be turned
to any position between fully off and
fully on to vary the brightness of the
bulb.
In this case, the switch can be turned
to any position between fully off and
fully on to vary the brightness of the
bulb.
In this case, the switch can be turned
to any position between fully off and
fully on to vary the brightness of the
bulb.
In this case, the switch can be turned
to any position between fully off and
fully on to vary the brightness of the
bulb.
In this case, the switch can be turned
to any position between fully off and
fully on to vary the brightness of the
bulb.
In this case, the switch can be turned
to any position between fully off and
fully on to vary the brightness of the
bulb.
BASIC SYSTEM
1)
A proportional directional
valve can be thought of as the
dimmer switch equivalent of an
electrical switch.
2) The valve spool can now be
moved not just to one of
three discrete positions but
anywhere in between.
The direction of the spool
movement away from the
central
position
still
determines which way the
cylinder moves but the
amount of spool movement
also controls the speed of
the piston.
BASIC SYSTEM
So in effect the proportional
directional valve is acting as both a
directional valve and a flow control
valve.
SWITCHING SOLENOID VALVE
A conventional solenoid valve can be
thought of as a simple switching valve.
It is controlled by some form of
electrical device which simply switches
the electrical current on or off.
SWITCHING SOLENOID VALVE
A conventional solenoid valve can be
thought of as a simple switching valve.
It is controlled by some form of
electrical device which simply switches
the electrical current on or off.
SWITCHING SOLENOID VALVE
A conventional solenoid valve can be
thought of as a simple switching valve.
It is controlled by some form of
electrical device which simply switches
the electrical current on or off.
PROPORTIONAL VALVE
A proportional directional valve
however will be controlled by an
electrical device more like a dimmer
switch.
PROPORTIONAL VALVE
By varying the current to either
solenoid, the amount of spool
movement can be varied and hence the
amount of flow through the valve can
be controlled.
PROPORTIONAL VALVE
By varying the current to either
solenoid, the amount of spool
movement can be varied and hence the
amount of flow through the valve can
be controlled.
PROPORTIONAL VALVE
By varying the current to either
solenoid, the amount of spool
movement can be varied and hence the
amount of flow through the valve can
be controlled.
PROPORTIONAL VALVE
By varying the current to either
solenoid, the amount of spool
movement can be varied and hence the
amount of flow through the valve can
be controlled.
PROPORTIONAL VALVE
By varying the current to either
solenoid, the amount of spool
movement can be varied and hence the
amount of flow through the valve can
be controlled.
PROPORTIONAL VALVE
By varying the current to either
solenoid, the amount of spool
movement can be varied and hence the
amount of flow through the valve can
be controlled.
PROPORTIONAL SOLENOID
So unlike a conventional
solenoid valve, the electrical
current flowing through the
coil of a proportional valve
needs to be regulated not
just switched on or off.
B
The
construction
of
a
proportional
solenoid
is
however similar to that of an
on/off solenoid.
A
The solenoid consists of:
F
D
C
E
- a coil (A)
- a frame (B)
- an armature (C)
- a pole piece (D)
- a push-pin (E)
G
The armature is enclosed in a
core tube (F) and the whole
assembly is often fully
encapsulated in a plastic resin
material (G).
PROPORTIONAL SOLENOID
When a voltage is applied to the
coil connections, an electrical
current will flow through the coil.
PROPORTIONAL SOLENOID
1) In turn, the electrical current
creates a magnetic field which is
concentrated in the metal frame,
pole piece and armature.
2) There is however a gap in the
magnetic circuit between the
pole piece and armature so a
force is created which acts to
close this gap and complete the
magnetic circuit.
PROPORTIONAL SOLENOID
1) The push-pin connects the
solenoid to the valve spool and
normally moves the spool against
a spring.
2) The force created by the
solenoid is determined by the
strength of the magnetic field
which itself is proportional to
the current flowing through the
coil.
PROPORTIONAL SOLENOID
Increasing the coil current will
increase the solenoid force and
hence move the spool a greater
amount against the spring.
PROPORTIONAL SOLENOID
Increasing the coil current will
increase the solenoid force and
hence move the spool a greater
amount against the spring.
PROPORTIONAL SOLENOID
Increasing the coil current will
increase the solenoid force and
hence move the spool a greater
amount against the spring.
PROPORTIONAL SOLENOID
I
F
I
F
The solenoid is designed so that
the relationship between the
solenoid force (F) and the coil
current (I) is a linear one. This
means that the solenoid force
depends only on the coil current.
SWITCHING SOLENOID SPOOL
A further difference between a
switching solenoid valve and a
proportional valve is in the design
of the spool.
SWITCHING SOLENOID SPOOL
1) With a switching valve, the
spool is designed to achieve
minimum pressure drop when the
valve is energised.
Q
2) Which would mean that to
control low flow rates, the
amount of spool opening required
would be very small and difficult
to control.
S
S
Q
PROPORTIONAL SPOOL
Q
1) A proportional valve spool
therefore has wider lands with
notches cut into the edges.
S
S
PROPORTIONAL SPOOL
Q
2) So although the maximum flow through
the valve may be reduced (compared to a
switching valve) low flows in particular are
more easily controlled and the opening of
the valve is more gradual.
1) A proportional valve spool
therefore has wider lands with
notches cut into the edges.
S
S
PROPORTIONAL SPOOL
Q
Depending upon the maximum flow to be
controlled, different spools can be
fitted to a particular valve which have
different shape, size or number of
spool notches.
S
S
DIRECT ACTING PROPORTIONAL RELIEF VALVE
1) Proportional valves can also be used to control pressure. In this case
a proportional solenoid is used to push a poppet against a seat via a
spring. The greater the solenoid force the greater the pressure
required to push the poppet off its seat and open the valve.
DIRECT ACTING PROPORTIONAL RELIEF VALVE
1) Proportional valves can also be used to control pressure. In this case
a proportional solenoid is used to push a poppet against a seat via a
spring. The greater the solenoid force the greater the pressure
required to push the poppet off its seat and open the valve.
2) This provides a direct acting
relief valve function but like most
such valves, it is only possible to
pass small flow rates through the
valve.
TWO-STAGE PROPORTIONAL RELIEF VALVE
To control higher
flow
rates,
the
proportional
direct
acting relief valve
can be used as the
pilot stage of a twostage
relief
(or
reducing valve).
TWO-STAGE PROPORTIONAL RELIEF VALVE
To control higher
flow
rates,
the
proportional
direct
acting relief valve
can be used as the
pilot stage of a twostage
relief
(or
reducing valve).
BENEFITS OF PROPORTIONAL SYSTEMS
REMOTE CONTROL - CONVENTIONAL SYSTEM
In order to adjust the speed of
an actuator in a conventional
system, the flow control valve has
to be mounted in a convenient or
accessible position. This may
often mean running high pressure
hydraulic pipes to and from an
operator’s desk.
REMOTE CONTROL - PROPORTIONAL SYSTEM
With a proportional system however,
where control valve adjustment is
electronic, only low-power electrical
cables need to be connected
between the operator’s desk and the
valve.
PLC REMOTE CONTROL - PROPORTIONAL SYSTEM
More commonly these days, machine
control is carried out by a digital
electronic controller. Here again, the
ability to control proportional valves
electronically provides a simple
interface between the hydraulic
system and the electronic controller.
PROPORTIONAL PRESSURE CONTROL
The use of proportional
directional and pressure
control valves means
that
all
hydraulic
functions of a machine
(movement and force)
can
be
controlled
electronically.
SOLENOID VALVE RESPONSE TIME
A further benefit of proportional
valves is the ability to electronically
control the speed of operation of
the valve.
SOLENOID VALVE RESPONSE TIME
0.015
Depending upon its size and voltage
supply, a conventional switching solenoid
valve will have an energisation response
time of approximately 15 milli-seconds.
S
SOLENOID VALVE RESPONSE TIME
0.040
The de-energisation response time will
be only slightly slower (typically around
25 ms) since the return spring produces
less force than the solenoid .
S
PROPORTIONAL VALVE RESPONSE TIME
S
The speed of movement
of a proportional valve
spool however can be
determined
by
the
electronic signal fed to
the valve solenoid. By
gradually increasing or
decreasing
the
the
signal
(known
as
ramping), it is possible
to achieve energisation
and
de-energisation
response
times
of
several seconds.
PROPORTIONAL VALVE RESPONSE TIME
1.000
The speed of movement
of a proportional valve
spool however can be
determined
by
the
electronic signal fed to
the valve solenoid. By
gradually increasing or
decreasing
the
the
signal
(known
as
ramping), it is possible
to achieve energisation
and
de-energisation
response
times
of
several seconds.
S
PROPORTIONAL VALVE RESPONSE TIME
2.000
The speed of movement
of a proportional valve
spool however can be
determined
by
the
electronic signal fed to
the valve solenoid. By
gradually increasing or
decreasing
the
the
signal
(known
as
ramping), it is possible
to achieve energisation
and
de-energisation
response
times
of
several seconds.
S
PROPORTIONAL VALVE RESPONSE TIME
3.000
The speed of movement
of a proportional valve
spool however can be
determined
by
the
electronic signal fed to
the valve solenoid. By
gradually increasing or
decreasing
the
the
signal
(known
as
ramping), it is possible
to achieve energisation
and
de-energisation
response
times
of
several seconds.
S
LIFT EXAMPLE - CONVENTIONAL SYSTEM
The reason why it is useful to be able to control the speed of spool movement
of a valve is to reduce shock in a system. This is achieved by controlling the
acceleration and deceleration of the actuator. Suppose, for example, that the
simple hydraulic system described earlier is used to operate a passenger lift in
a hotel.
LIFT EXAMPLE - CONVENTIONAL SYSTEM
When the solenoid valve is energised to lower the lift, the valve spool will move
across very rapidly. This means that the cylinder will accelerate very quickly up
to its maximum speed (determined by the setting of flow control valve F). This
sudden starting of the lift provides a very uncomfortable ride for its
occupants.
F
LIFT EXAMPLE - CONVENTIONAL SYSTEM
Similarly, when the lift reaches its destination, the solenoid valve will shut off
very rapidly causing a sudden stopping of the lift and again a very
uncomfortable situation for the occupants. In real hydraulic systems, the
shocks generated by sudden starting and stopping of actuators create high
peak pressures which are one of the principle causes of fluid leakage.
LIFT EXAMPLE - PROPORTIONAL SYSTEM
If the solenoid valve and flow control valve are replaced with a proportional
valve then not only can the speed of the lift be adjusted electronically, but
also its stopping and starting can be controlled.
LIFT EXAMPLE - PROPORTIONAL SYSTEM
The proportional valve can be opened sufficiently slowly to provide a smooth
acceleration of the lift up to its maximum speed.
LIFT EXAMPLE - PROPORTIONAL SYSTEM
And likewise the deceleration can be controlled by slowing down the speed of
spool movement back to the centre condition.
MOTION CONTROL
Distance
In general therefore, proportional valves are capable of providing
full motion control in terms of:
time
MOTION CONTROL
Distance
1. A smooth and controlled acceleration of an actuator up to its
maximum speed.
Acceleration
time
MOTION CONTROL
Distance
2. Control of the actuator velocity and if necessary maintaining it
constant with varying loads.
Velocity
Acceleration
time
MOTION CONTROL
3. A smooth deceleration with minimal pressure peaks.
Distance
Deceleration
Velocity
Acceleration
time
FORCE CONTROL
Proportional valves can
also be used to control
the force output from an
actuator (for example in
press or plastic injection
moulding applications) by
controlling the pressure
applied to the actuator.
Force
time
FORCE CONTROL
In such cases it is often
necessary to control not
only
the
maximum
actuator pressure but
also the rate at which
the pressure is applied
or removed.
Force
time
FORCE CONTROL
In fact the machine
cycle may consist of a
series of ramps and
holding periods all of
which can be achieved
with
just
the
one
proportional valve.
Force
time
FORCE CONTROL
At the end of the
machine cycle the rate
at which the pressure is
reduced is also critical in
many processes.
Force
time
FORCE CONTROL
Motion and force control
can thus be achieved
using proportional valves,
and in some cases the
same valve can be used
for both motion and
force control. This is
usually referred to as
‘PQ’ control ie. the
control of both pressure
(P) and flow (Q).
Furthermore,
all
of
these control functions
can be achieved using
electronic inputs to the
valve thus providing a
simple interface to the
machine controller.
ELECTRONIC CONTROL
VALVE INPUT SIGNAL
As described earlier, the electrical current to the solenoid of a
proportional valve needs to be regulated and not just simply
switched on or off as is the case with a conventional valve.
VALVE INPUT SIGNAL
In theory, this could be achieved by using a dimmer switch type
component (ie. a variable resistor). Practical problems such as
heat generation and drift however mean that such a device
would not normally be used except for the very simplest
applications.
PROPORTIONAL VALVE AMPLIFIER
Normally, the current flowing through the proportional solenoid
will be controlled by some form of electronic amplifier. The
amplifier itself will require a power supply (usually 12 or 24 VDC)
and a command input signal.
24 V DC
PROPORTIONAL VALVE AMPLIFIER
The output of the amplifier (electrical current) is controlled by
the input signal so with zero input the output current is also
zero.
24 V DC
PROPORTIONAL VALVE AMPLIFIER
Increasing the input signal to the amplifier results in a
corresponding increase in output current to the valve solenoid.
24 V DC
PROPORTIONAL VALVE AMPLIFIER
Increasing the input signal to the amplifier results in a
corresponding increase in output current to the valve solenoid.
24 V DC
PROPORTIONAL VALVE AMPLIFIER
Increasing the input signal to the amplifier results in a
corresponding increase in output current to the valve solenoid.
24 V DC
PROPORTIONAL VALVE AMPLIFIER
The relatively large current required to drive the valve solenoid
(typically 2 to 3 amps) is provided by the power supply so the
current required from the input signal device is very small
(normally just a few milli-amps). The input control device can
therefore be a simple potentiometer.
24 V DC
PROPORTIONAL VALVE AMPLIFIER
In mobile applications the input device could by a joystick type
potentiometer.
24 V DC
PROPORTIONAL VALVE AMPLIFIER
In an increasing number of applications however, the input signal
is generated by the machine controller itself (eg. a PLC Programmable Logic Controller). This can then be fed directly to
the valve amplifier to generate the appropriate output.
24 V DC
VALVE TYPES
NON-FEEDBACK VALVE
Different types of proportional valves with differing levels of
performance are available to meet the requirements of a wide
range of applications. The simplest type of proportional
directional valve balances the solenoid force against a spring
compression force in order to position the spool within the
valve body.
NON-FEEDBACK VALVE
An input signal to the amplifier produces a corresponding
output current to the valve solenoid. This current creates a
force on the valve spool thus moving it across until the
compression of the spring balances the solenoid force.
A small input signal thus creates a small opening of the valve.
NON-FEEDBACK VALVE
Increasing the input signal gradually opens up the valve and
allows more flow to pass through...
NON-FEEDBACK VALVE
... until the valve is wide open and passing maximum flow.
NON-FEEDBACK VALVE
1) For any input signal therefore, the spool is positioned by
balancing solenoid force against spring force. In practice
however, other forces also act on the spool. Flow forces in
particular are generated when flow passes through the valve
and these will act with the spring to oppose the solenoid force
and thus reduce the amount of spool opening.
NON-FEEDBACK VALVE
1) For any input signal therefore, the spool is positioned by
balancing solenoid force against spring force. In practice
however, other forces also act on the spool. Flow forces in
particular are generated when flow passes through the valve
and these will act with the spring to oppose the solenoid force
and thus reduce the amount of spool opening.
2) This simple type of
valve will therefore have
limitations both on the
maximum flow rate it can
pass and also with its
performance in terms of
accurate positioning of the
valve spool.
FEEDBACK VALVE
Valve performance can be increased by adding a spool position
sensor to the valve. This sensor provides an electronic
feedback signal to the amplifier and thus allows the spool to be
positioned much more accurately.
FEEDBACK VALVE
Increasing the input signal to the amplifier gradually opens the
valve flow path.
FEEDBACK VALVE
Increasing the input signal to the amplifier gradually opens the
valve flow path.
FEEDBACK VALVE
As before, flow forces will build up to oppose the solenoid
force and attempt to reduce the spool opening. Any reduction
in opening is now detected by the spool sensor however and
results in an increased output current from the amplifier and
an increased force from the solenoid to counteract the flow
forces.
FEEDBACK VALVE
1) As the pressure drop and flow rate through the valve
increase further, ultimately the flow forces will overcome the
solenoid force and act to reduce the valve opening as with the
simple valve but this will now occur at a significantly greater
flow rate than before. For a given size therefore, a feedback
type valve will pass a greater flow than the equivalent nonfeedback valve and the spool positioning will be more accurate.
2) The penalty for the improved performance however is a
higher cost valve and the fact that the amplifier needs to be
dedicated to the type of valve it is controlling (as opposed to a
multi-purpose amplifier used on the non-feedback valve).
TWO-STAGE SOLENOID VALVE
When higher flow rates
need to be controlled, a twostage valve becomes the
most
practical
solution
(rather than fitting larger
and larger solenoids).
As with direct acting valves,
a two-stage proportional
valve has many similarities
with its equivalent switching
valve, but there are also
significant differences.
TWO-STAGE PROPORTIONAL VALVE
Firstly, the main spool is
modified to incorporate the
spool metering notches as on
the direct acting valves.
This
provides
a
more
controlled
opening
and
closing of the valve flow
path.
TWO-STAGE PROPORTIONAL VALVE
Secondly, the pilot stage is
modified
so
that
the
solenoid current varies the
pressure created in the
ports leading to either end
of
the
main
spool.
Effectively, the pilot stage
operates as two proportional
pressure reducing valves.
TWO-STAGE PROPORTIONAL VALVE
Thirdly, the main spool
springs are replaced with
just one spring. This means
that the same spring is
compressed whichever side
of center the main spool is
moved thus avoiding the
need for two accurately
matched springs.
TWO-STAGE VALVE (NON-FEEDBACK)
Finally, a pressure reducing
module is sometimes fitted
between the main stage and
pilot stage to reduce the
pilot
pressure
when
operating at high system
pressures (typically greater
than 3000 psi).
TWO-STAGE VALVE (NON-FEEDBACK)
Energizing one of the pilot
stage solenoids creates a
pressure in one main spool
end-chamber proportional
to the solenoid current.
This pressure pushes the
main spool across until the
main spring compression
force balances the pilot
pressure force.
TWO-STAGE VALVE (NON-FEEDBACK)
Energizing the opposite
solenoid moves the main
spool in the other direction
but still compresses the
same main centering spring.
TWO-STAGE VALVE (SINGLE FEEDBACK)
When an increased level of
performance is required, a
spool position sensor can be
fitted to the main spool and
a single solenoid pilot stage
used. As before however,
this will increase the cost
of the valve and require a
dedicated amplifier.