Proportional hydraulics Collection of Transparencies
095061 GB
Order no.: Designation: Description: Edition: Author: Graphics: Layout:
095061 PROP.H.FOLIEN D:OT-TP701-GB 10/2002 Dr. Frank Ebel Thomas Ocker, Bärbel Böhland 09.10.2002, Petra Schwarz, Wolfgang Schreiner
© Festo Didactic GmbH & Co., 73770 Denkendorf/Germany, 2002 Internet: www.festo.com/didactic e-mail:
[email protected] The copying, distribution and utilization of this document as well as the communication of its contents to others without expressed authorization is prohibited. Offenders will be held liable for the payment of damages. All rights reserved, in particular the right to carry out patent, utility model or ornamental design registration.
Foreword
This set of overhead transparencies has been designed for the basic level technology package TP700. The set of overhead transparencies and the technology package form part of the Learning System for Automation and Technology from Festo Didactic GmbH & Co. The 28 overhead transparencies which comprise this set have been designed in accordance with dictactic and methodological principles. Each transparency is accompanied by a short explanatory text, which gives the instructor a brief overview of of the training content covered by the transparency. Training contents
• • • •
Fundamentals of proportional hydraulics Function and use of proportional hydraulic components Electronic components for the actuation of proportional valves Comparison with electro-hydraulic control systems
The text sheet includes a full illustration of the transparency, partly supplemented by additional explanations and designations, which can be added to the transparency by the instructor in the course of training. The advantages of this concept are: • The instructor can expand the transparencies step by step in the course of training • Lessons become more interesting • The available explanatory texts reduce the amount of preparation required by the instructor New! Electronic presentation
The enclosed CD-ROM contains the entire overhead transparencies and accompanying text of this edition in an electronically presentable form in the files „Prophydraulics_transparencies.pdf“ and „Prophydraulics_text.pdf“. In addition to the screen presentation, which can be made in any order, the contents can be printed out and text and graphics can be used for your own training preparations, insofar as the functionality of the required Adobe® Acrobat® Reader permits this. This freely distributable software is available on the CD-ROM in the currently valid English Windows version for installation in the directory „Acrobat_Reader“. Please start the file „*.exe“ and follow the subsequent dialogue.
© Festo Didactic GmbH & Co. • Proportional hydraulics
Contents
Proportional hydraulic system ________________________________Transparency 1 Mobile hydraulics __________________________________________Transparency 2 Proportional valve activation _________________________________Transparency 3 Honing machine ___________________________________________Transparency 4 Electro-hydraulics __________________________________________Transparency 5 Proportional hydraulics______________________________________Transparency 6 Electro-mechanical transducers _______________________________Transparency 7 D.C. switching solenoid______________________________________Transparency 8 Proportional solenoid with intermediate ring ____________________Transparency 9 Proportional solenoid, stroke-controlled_______________________Transparency 10 Proportional solenoid, force-controlled ________________________Transparency 11 Proportional solenoid, components___________________________Transparency 12 Electronic activation _______________________________________Transparency 13 Proportional amplifier ______________________________________Transparency 14 Ramp shaper _____________________________________________Transparency 15 Minimum current – Maximum current _________________________Transparency 16 Output stage (V/I converter) ________________________________Transparency 17 Pulse duration modulation __________________________________Transparency 18 Hysteresis _______________________________________________Transparency 19 Flow characteristic curves___________________________________Transparency 20 Control edge patterns ______________________________________Transparency 21 Piston opening ratio _______________________________________Transparency 22 Piston overlap ____________________________________________Transparency 23 Pressure relief valve, piloted ________________________________Transparency 24 Pressure regulating valve, piloted ____________________________Transparency 25 Flow control valve _________________________________________Transparency 26 4/3-way valve ____________________________________________Transparency 27 4/3-way valve, piloted _____________________________________Transparency 28
© Festo Didactic GmbH & Co. • Proportional hydraulics
1
Proportional hydraulics
Title
Proportional hydraulic system
Guide
• • • •
Signal input
Festo Didactic
Classification into signal control section and power section. Signal processing has been further enhanced by the provision of setpoint value and current signal conversion. The multiple function of proportional valves reduces the number of hydraulic components to a minimum. Proportional solenoids form the interface between the signal control section and the power section.
Actuators
Signal processing, control
Setpoint generator
Signal amplification
A
B Proportional valves
p
PLC
P
P
Power supply
701-01-GB
~
Power section
T
M
Power supply Power section
Proportional hydraulics
p
Festo Didactic
PLC
P Energieversorgung
~
Steuerteil
Proportional hydraulic system 701-01-GB
A
B
P
T
M
2
Proportional hydraulics
Title
Mobile hydraulics
Guide
• • • •
A
Festo Didactic
Origins of mobile hydraulics. Long hose connections between control valves and hydraulic drive components. Ratio of hand lever movement to piston deflection is approximately proportional. Control pattern: High-precision controllability at start of hand lever displacement, large flow rate change during hand lever movements at end deflection. In this instance, flow is not released until end deflection due to the positive piston overlap.
B
High-precision controllability
Piston B deflection
Flow rate [Q]
A
B Hand lever movement
Piston overlap A T
701-02-GB
B
P
A
Proportional hydraulics
Festo Didactic
High-precision controllability Piston B deflection
Flow rate [Q]
A
B Hand lever movement Piston overlap A
Mobile hydraulics 701-02-GB
3
Proportional hydraulics
Title
Proportional valve activation
Guide
• • •
Festo Didactic
Setpoint values are converted into magnetic flux and amplified. By means of the proportional solenoid, magnetic flux is converted into piston deflection within the hydraulic valve. Actuation of proportional directional, pressure and flow control valves.
Directional valves
Pressure valves
Proportional amplifier
Proportional solenoid
Flow control
701-03-GB
Proportional hydraulics
Festo Didactic
Proportional valve activation 701-03-GB
4
Proportional hydraulics
Title
Honing machine
Guide
• • •
Festo Didactic
Typical machine tool employing a proportional hydraulic control system. Continuous path control on machine tools: Controlling of path, speed and acceleration. Smooth transition of upward and downward stroke of honing machine in accordance with time function.
Cyl. stroke advanced 1
retracted 0
701-04-GB
Time
Proportional hydraulics
Festo Didactic
Cyl. stroke advanced 1
retracted 0
Time
Honing machine 701-04-GB
5
Proportional hydraulics
Title
Electro-hydraulics
Guide
• • • •
Festo Didactic
Example for conventional electro-hydraulics: Control of velocity stages. Use of parallel switching flow control valve for control of velocity stages. Digital switching characteristics: only preset flow rates can be invoked (example: flow control valve). Extensive hydraulic circuit requirement.
01
23
Cylinder stroke
3 2 Y4
Y3
1 0 Time [t] Y1
P
701-05-GB
Y2
M
Y1
1
1
1
0
0
0
0
Y2
0
0
0
0
1
1
0
Y3
0
1
0
0
1
0
0
Y4
0
0
1
0
0
0
0
Proportional hydraulics
Festo Didactic 01
23
3 Cylinder stroke
2 Y4
Y3
1 0 Time [t] Y1
P
Y2
Y1
1
1
1
0
0
0
0
Y2
0
0
0
0
1
1
0
Y3
0
1
0
0
1
0
0
Y4
0
0
1
0
0
0
0
M
Electro-hydraulics 701-05-GB
6
Proportional hydraulics
Title
Proportional hydraulics
Guide
• • • • •
Festo Didactic
Example for proportional hydraulics: Control of velocity stages. Hydraulic circuit requirement considerably reduced, electrical signal control section extended. Additional requirements for commissioning: Setting of setpoint values and ramp functions. Analogue valve cross section openings according to setpoint values and thus different flow range, i.e. actuator speeds. Flexibility thanks to electrical control, e. g: an additional velocity stage requires only one additional setpoint value.
01 2
3 4
IY1 Current Time [t]
IY2 Cylinder stroke 4
IY1
IY2 Y1
3
Y2 2
P
1 0
M
Time [t]
Y1
701-06-GB
Y2
Proportional hydraulics
Festo Didactic 01 2
3 4
IY1 Current Time [t] IY2 Cylinder stroke 4
IY1
IY2 Y1
3
Y2 2
P
1 0 M
Time [t] Y1
Proportional hydraulics 701-06-GB
Y2
7
Proportional hydraulics
Festo Didactic
Title
Electro-mechanical transducers
Guide
•
• • • •
Proportional solenoid: Further development of switching solenoid. The magnetic flux causes a force which remains constant throughout the stroke. The force acts only in one direction. The extent of the magnetic force is proportional to the magnetic flux. Due to its force, the servo drive of the proportional solenoid is particularly suitable for the direct actuation of standard proportional valves. Plunger coil: Also an electro-dynamic principle, whereby a force acts on a current carrying conductor in the magnetic field. Depending on the direction of current the plunger coil is deflected up or down. Torque motor: The effects of the permanent and electro magnets are superimposed. The magnetic force causes a torque to the right or the left. The servo drive, plunger coil and torque motor have a high natural frequency, which makes them suitable for the indirect actuation of highly dynamic proportional and servo valves.
F
Power consumption
701-07-GB
[W]
x
N
N
S
S
x
Proportional soleonid
Plunger coil
Torque motor
5 - 40
0.2 - 5
0.02 - 4
20 - 1000
8 - 80
2 - 40
Working stroke
[Nmm]
Linear deviation
(%)
0.5 - 6
1-7
1-2
Critical frequency
[Hz]
10 - 150
100 - 200
100 - 300
Proportional hydraulics
Festo Didactic
F Proportional solenoid Power consumption
x Plunger coil
[W]
Working stroke [Nmm] Linear deviation
(%)
Critical frequency
[Hz]
Electro-mechanical transducers 701-07-GB
N
N
S
S
x Torque motor
8
Proportional hydraulics
Title
D.C. switching solenoid
Guide
• • • •
Festo Didactic
Magnetic field lines of a coil. Existence of a homogenous magnetic field within a coil without magnet core or armature. Radial air gap and axial air gap form different “resistances” in the magnetic field. The magnetic force acts on the armature mainly via the radial air gap, since in this case the gap between the core and armature remains small.
Radial air gap
Homogenous magnetic field
Magnetic field lines
701-08-GB
Axial air gap
Proportional hydraulics
Festo Didactic
D.C. switching solenoid 701-08-GB
9
Proportional hydraulics
Festo Didactic
Title
Proportional solenoid with intermediate ring
Guide
• • • •
Different magnetic forces act on the armature via the axial air gap. Reducing the magnetic field lines via the radial air gap by means of installing a non-magnetisable intermediate ring. The new distribution of the magnetic field line leads to a virtually constant magnetic force pattern within the working stroke range. 1 = Magnetic force characteristic in the axial air gap 2 = Magnetic force characteristic in the radial air gap 3 = Summation characteristic curve
Radial air gap Axial air gap a
Magnetic force [N]
b
6
c
3
3 2
1 = Magnetic force characteristic in the axial 0 air gap 2 = Magnetic force characteristic in the radial air gap 3 = Summation characteristic curve
1
701-09-GB
a
Working stroke b ≈ 1mm
c
Piston position
Proportional hydraulics
Festo Didactic
Magnetic force [N] 60
a b c
30
0 Piston position
Proportional solenoid with intermediate ring 701-09-GB
10
Proportional hydraulics
Festo Didactic
Title
Proportional solenoid, stroke controlled
Guide
• •
• •
The solenoid force is proportional to the electrical input signal. Poppet principle: The valve cone is pressed against the valve seat via a spring; the valve opens as soon as the hydraulic force on the valve cone exceeds the pilot force, whereby the entire spring/ plunger system is moved back. Spool principle: the valve piston is pushed against the spring, the stroke movement ceases when the magnetic force and the spring force are in equilibrium. Force/stroke characteristic curves: Magnetic force with specified current values in relation to plunger position; characteristic curve of a pressure loaded spring.
Poppet principle Characteristic force/stroke curves Magnetic force [N]
Characteristic spring curve
6
Spool principle 3
0 Piston Working stroke Idle stroke position
701-10-GB
Proportional hydraulics
Festo Didactic
Characteristic force/stroke curves Magnetic force [N] 60
30
0 Piston position
Proportional solenoid, stroke controlled 701-10-GB
11
Proportional hydraulics
Festo Didactic
Title
Proportional solenoid, force controlled
Guide
• • •
The solenoid force is proportional to the electrical input signal. Only poppet type valves can be operated via force controlled solenoids. This valve is used as a pilot pressure valve for directional or pressure control valves because of the reduced flow rates.
Force/current characteristic curve
P
Magnetic force F [N]
T Poppet principle (only) F~I P
Magnetic flux I [A]
701-11-GB
T
Proportional hydraulics
Festo Didactic
P Magnetic force F [N]
T
Magnetic flux I [A]
Proportional solenoid, force controlled 701-11-GB
12
Proportional hydraulics
Festo Didactic
Title
Proportional solenoid, components
Guide
•
Construction of proportional solenoids.
•
D.C. solenoid group.
• •
Representation of the most important components of a proportional solenoid. Electro-mechanical transducer: the electrical input signal is converted into a mechanical output signal.
Electrical connection
Venting screw
Non-magnetisable inner ring Control cone
Compensating spring
Core magnet
Plain bearing Guide rod (stem) Housing Stop/Guide disc Barrel magnet Exciting coil
701-12-GB
Armature
Proportional hydraulics
Festo Didactic
Proportional solenoid, components 701-12-GB
Proportional hydraulics
13
Title
Electronic activation
Guide
• • • •
Festo Didactic
Control stages of a proportional amplifier for magnetic flux generation. The input signal is set by means of ramp time, minimum or maximum current and pulse duration modulation. Both the amplification of the signal and the conversion by means of a V/I converter take place during the output stage. Proportional solenoid valves are to be actuated via an adjustable D.C. current of 0 to 800 mA.
A
B
P
T
V I
Y1
Y2
V I
Setpoint specification
701-13-GB
Ramp setting
Setting of amplifier characteristic
Pulse-duration modulation
V/I-converter
Proportional valve
Proportional hydraulics
Festo Didactic
A
B
P
T
V I
V I
Electronic activation 701-13-GB
Y1
Y2
14
Proportional hydraulics
Festo Didactic
Title
Proportional amplifier
Guide
•
Control panel of a setpoint and amplifier card.
•
W1 - W8: Input of up to 8 setpoint values.
•
R1 - R4: Setting of ramps. TIME = Ramp time.
•
IA - IB: Setting of initial, jump and maximum current.
•
24V, 0V: Supply voltage.
•
Liquid crystal display for representation of all parameters.
+24V
+24V
+24V
+24V
W1
A
I1
I2 R1
I3
OUT
W1 W2
R2 R3
W2
W3 W4
R4
IB W1 W2
W5 TIME
INT W1
W6 W7
FUNCTION
INT W2
D/E OUT W8 CONTRAST
0V
701-14-GB
0V
0V
B
FUNCTION IA IA BASIC IB BASIC IA JUMP IB JUMP
INT W1 IA MAX INT W2 IB MAX D/E DITHERFREQ. CONTRAST
OE A OE B
0V
Proportional hydraulics
Festo Didactic
+24V
+24V
+24V
+24V
W1
A
I1
I2 R1 R2 R3
I3
OUT
W1 W2
W2
W3 W4
R4
W2
W5 TIME
INT W1
W6 W7
FUNCTION
INT W2
D/E OUT W8 CONTRAST
0V
0V
0V
Proportional amplifier 701-14-GB
B
FUNCTION IA IA BASIC IB IB BASIC W1 IA JUMP IB JUMP
INT W1 IA MAX INT W2 IB MAX D/E DITHERFREQ. CONTRAST
OE A OE B
0V
Proportional hydraulics
15
Title
Ramp shaper
Guide
• • • • •
Left: Signal transmission in switching hydraulics. Right: Signal transmission in proportional hydraulics with ramp shaper. Use of ramps for adjustable acceleration or delay of hydraulic drive components. Angular setting: The ramp angle determines the adjustable speed of the valve piston. Elimination of pressure peaks and switching impacts in the hydraulic system. Displacement positions: 0: Retracted end position 1: Acceleration completed 2: Delay initiated (signal generator) 3: Forward end position
Displacement
Displacement Signal generator
3 2
3 0
Festo Didactic
3
0 1
23
Cylinder
1 0
0 t0
t 3 Time [t]
t0
t1
t2 t3
Time [t]
Y1 100% Control signal
Y1
0% t0
t1
t2 t3
Time [t]
Y1 1 Y1
1 Input signal
Y2 0
0 t0
701-15-GB
Signal generator
Time [t]
t0
t2
Time [t]
Y2
Proportional hydraulics
Festo Didactic
Displacement
Displacement 3 2
3 0
3
0 1
23
1 0
0 t0
t 3 Time [t]
t0
t1
t2 t3
Time [t]
Y1 100% Y1 0% t0
t1
t2 t3
Time [t]
t2
Time [t]
Y1 1 Y1
1
Y2 0
0 t0
Time [t]
t0
Ramp shaper 701-15-GB
Y2
16
Proportional hydraulics
Festo Didactic
Title
Minimum current – Maximum current
Guide
• • • •
Setting of minimum and maximum current for adjustment of proportional directional control valve to flow rate of hydraulic pump. Minimum current for adjustment of jump current to control edge. Maximum current or adjustment of maximum cross section of the opening in relation to flow rate QPump. A linear characteristic curve runs between minimum and maximum current.
[mA]I 80 0
Imax Setting
Amplifier characteristic
Setting I min
0 Setpoint value 10 Volt
∆Q ≈ 0
∆s = min.
701-16-GB
∆Q ≈ max.
∆s = max.
Proportional hydraulics
Festo Didactic
I [mA] 800
0
Setpoint value 10 Volt
Minimum current – Maximum current 701-16-GB
17
Proportional hydraulics
Title
Output stage (V/I converter)
Guide
• • • • •
Festo Didactic
The setpoint voltage is converted into a magnetic flux in the output stage. Closed-loop control of magnetic flux through comparison of setpoint value and actual value on the measuring shunt RM. The closed control loop is created by means of connecting the actual value via the feedback resistor RR. The closed control loop eliminates the effects resulting from operating voltage fluctuations and/or the effects resulting from proportional solenoid heating. The solenoid coil has an ohmic resistor RS and an inductive resistor LS.
VI Input signal R I Input resistance
VV Power supply voltage RS Coil: Ohmic resistor LS Coil: Inductive resistor Solenoid coil
RR Feedback resistor
RS LS
RM Measuring shunt
701-17-GB
Proportional hydraulics
VI
Festo Didactic
VV
RI
RS
RR
LS
RM
Output stage (V/I converter) 701-17-GB
Proportional hydraulics
18
Title
Pulse duration modulation
Guide
• • • •
Solenoid voltage
Festo Didactic
Recording of pulsed voltage signals and the associated magnetic flux. The output voltage is maintained at a constant level. The switch-on/ switch-off ratio (pulse duration) within a time period T influences the size of the effective magnetic flux. Clean-switching operations prevent excessive heating of the output stage and power dissipation is kept to a minimum. An oscillating output current is obtained thanks to pulse duration modulation. Sliding friction prevails at the plunger thanks to this dither effect and the hysteresis is improved.
V 24 V
T = Duty cycle
I eff = Effective magnetising current 0 Time V 24 V
T
I eff 0 Time V 24 V
T I eff
0 Time
701-18-GB
Proportional hydraulics
Festo Didactic
V
T = Periodendauer
24 V I eff 0Time V
T
24 V I eff 0Time V
T
24 V I eff
0Time
Pulse duration modulation 701-18-GB
19
Proportional hydraulics
Title
Hysteresis
Guide
• • • •
Festo Didactic
The friction of the valve piston spool causes a resistance to directional movement. Hysteresis: Differential current during approach of a pressure or flow rate value from different directions. Pulse duration modulation: Reduction of hysteresis by means of continuous slight oscillating movement of the piston spool (sliding friction). Avoidance of pressure peaks in the hydraulic system.
p [bar]
∆I
50 40 30 20 Pulse duration modulation
10
0
701-19-GB
100 200 300 400 500 600 700 800
I [mA]
Proportional hydraulics
Festo Didactic
p [bar] 50 40 30 20 Pulse duration modulation
10
0
100 200 300 400 500 600 700 800 I [mA]
Hysteresis 701-19-GB
20
Proportional hydraulics
Title
Flow characteristic curves
Guide
• • •
Festo Didactic
Representation of control piston with different control notches; Representation of the corresponding flow characteristic curves. Delta control edge pattern: Precision controllability for small volumetric flow rates, clear changes within the range of large volumetric flow rates. The stroke travel of a piston with control notches corresponds to the maximum length of the notch opening.
Q [l/min] 8 6 4 2 0 100
400
700 I [mA]
100
400
700 I [mA]
Q [l/min] 8 6 4 2 0
701-20-GB
Proportional hydraulics
Festo Didactic
Q [l/min] 8 6 4 2 0 100
400
700 I [mA]
100
400
700 I [mA]
Q [l/min] 8 6 4 2 0
Flow characteristic curves 701-20-GB
21
Proportional hydraulics
Title
Control edge patterns
Guide
• • • •
701-21-GB
Festo Didactic
Control edge pattern for the purpose of determining the flow behaviour of the valve. The adjustment of flow behaviour is linear, progressive, degressive or generally according to the task. Number of precision control openings on the periphery of the control piston depending on valve function. The sleeve system is easier to produce in terms of production technology.
Proportional hydraulics
Festo Didactic
Control edge patterns 701-21-GB
22
Proportional hydraulics
Title
Piston opening ratio
Guide
• •
Festo Didactic
Example: Actuation of a 2:1 cylinder. The ratio of the valve flow cross sections is adjusted to the cylinder inlet and outlet by means of control edge pattern.
Cylinder 2:1
A
T
701-22-GB
B
P
T
Proportional hydraulics
Festo Didactic
Piston opening ratio 701-22-GB
23
Proportional hydraulics
Festo Didactic
Title
Piston overlap
Guide
•
Representation of positive, negative and zero overlap.
•
Alternative designations: Overlap, underlap and zero intersection.
• •
The opening of the valve port is dependent on the notched opening in relation to the control edge. The flow rate is also adjusted by means of the throttle opening ratio of the control piston. The number of radially configured notches on the control piston.
QL
QA positive
X >0
X
QB QA
zero
QL
X =0
QB
X QL
QA negative
X <0
701-23-GB
QB
X
Proportional hydraulics
Festo Didactic
QL
QA
X
X
QB QA
QL
X QB
X QL
QA
X QB
Piston overlap 701-23-GB
X
24
Proportional hydraulics
Title
Pressure relief valve, piloted
Guide
• • • •
Festo Didactic
Sectional view of a pressure relief valve. The mode of operation corresponds to that of a conventional, piloted pressure relief valve. Piloted stage: the force set for the proportional solenoid is transferred to the pilot poppet via the spring. As soon as the pressure acting on the pilot valve exceeds the value set, the poppet valve opens. The venting control flow generates a pressure drop at the main stage, whereby this opens.
Y
p [bar] 5 4 3 T
2
P
1 TY
P 0
701-24-GB
20
40 I [mA]
Proportional hydraulics
Festo Didactic
Y
p [bar] 50 40 30 T
20
P
10 TY P 0
200
400 I [mA]
Pressure relief valve, piloted 701-24-GB
25
Proportional hydraulics
Festo Didactic
Title
Pressure regulating valve, piloted
Guide
•
Sectional view of a piloted pressure regulating valve.
•
Reduction of system pressure to a specific value.
•
Application for subranges within a hydraulic system.
Y
A
P A
701-25-GB
PY
Proportional hydraulics
Festo Didactic
A PY
Pressure regulating valve, piloted 701-25-GB
26
Proportional hydraulics
Festo Didactic
Title
Flow control valve
Guide
•
Sectional view of a flow control valve.
•
Normal position: all ports are closed.
•
Depending on the piston deflection, control cross sections (holes in the piston, control sleeve) and thus the flow are changed.
AB
PT
A
B
Q l [ ] min 8
Flow characteristic curve
6 4 2 P
701-26-GB
T
0
100 200 300 400 500 600 I [mA]
Proportional hydraulics
Festo Didactic
AB
PT Q [
l ] min 8 6 4 2 0
Flow control valve 701-26-GB
100 200 300 400 500 600 I [mA]
27
Proportional hydraulics
Title
4/3-way valve
Guide
• •
Festo Didactic
Essential functional components: Proportional solenoids, main spool, reset spring. Multiple functions: Directional control, speed control, hydraulic clamping of power components (throttle opening ratio).
AB
PT
T B P A
701-27-GB
Proportional hydraulics
Festo Didactic
AB
PT
4/3-way valve 701-27-GB
28
Proportional hydraulics
Title
4/3-way valve, piloted
Guide
• • • • •
Festo Didactic
Pressure relief valves for pilot control of a 4/3 proportional directional control valve. Electrical signal to one of the two proportional solenoids increases the pressure inside the control chamber of the main spool to a specified value (relief pressure). The control piston is moved up to the equilibrium position with the opposing spring action. High control force due to minimum control pressure of 25 bar eliminates sensitivity to disturbance factors. This is however associated with continuous control oil consumption. Without an electrical signal, the control piston is centred by means of the springs.
AB
PT
P
P
T
T
Actuation B
Actuation A
T
701-28-GB
A
P
B
Proportional hydraulics
Festo Didactic
AB
PT
P
P
T
T
T
A
P
B
4/3-way valve, piloted 701-28-GB