Fundamentals of Electrohydraulics (Collection of ... · © Festo Didactic GmbH & Co., D-73770 Denkendorf, ... Production and interpretation of standard hydraulic and ... „Electrohydraulics

095051 GB

Fundamentals of

Electrohydraulics

Collection of

Transparencies

T A P B

T A P B

A

a b

B

P T

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Order no.: 095051

Designation: E.-HYDR-FOLIEN.

Description: D:OT-TP601-GB

Edition: 07/2001

Author: Dieter Scholz

Graphics: Doris Schwarzenberger

Layout: 18.07.2001, Beatrice Huber

© Festo Didactic GmbH & Co., D-73770 Denkendorf, 2001

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

© Festo Didactic GmbH & Co. • Electrohydraulics

This set of overhead transparencies has been designed for use with the basic level

of the technology package TP600 “Electrohydraulics”. The set of overhead

transparencies and the technology package form part of the Learning System for

Automation from Festo Didactic GmbH & Co.

The 30 transparencies in this set have been optimised from the didactic and

methodological points of view. Each transparency is accompanied by a short text

which provides the trainer with a quick overview of the training contents of the

transparency concerned.

� Basic principles of electrohydraulics

� Function and use of electrohydraulic components

� Basic principles of electrical engineering

� Production and interpretation of standard hydraulic and electrical circuit

diagrams

The text sheet includes a complete illustration of the transparency, in certain cases

with additional explanations and legends which the trainer can enter on the

transparency in the course of a training session.

The advantages of this concept are:

� The trainer can add to the transparencies step by step in the course of a training

session.

� The trainer is thus able to create a lively and interesting atmosphere.

� The provision of accompanying text means less preparation work for the trainer.

The enclosed CD-ROM contains the entire overhead transparencies and

accompanying text of this edition in an electronically presentable form in the files

„Electrohydraulics _transparencies.pdf“ and „Electrohydraulics _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 version for Windows 95/98/NT for installation in the directory

„Acrobat_Reader“. Please start the file „rs405eng.exe“ and follow the subsequent

dialogue.

Training contents

New!

Electronic presentation

Contents


Electrohydraulic press _______________________________________ Transparency 1

Schematic layout of an electrohydraulic installation ______________ Transparency 2

Electrical power supply unit __________________________________ Transparency 3

Electrical circuit ____________________________________________ Transparency 4

Measuring circuits __________________________________________ Transparency 5

Ohm’s law _________________________________________________ Transparency 6

Rectifiers __________________________________________________ Transparency 7

Analogue, digital and binary __________________________________ Transparency 8

Momentary and sustained contact switches _____________________ Transparency 9

N/O, N/C and changeover contacts ___________________________ Transparency 10

Limit switches _____________________________________________ Transparency 11

Proximity switches _________________________________________ Transparency 12

Solenoid (electromagnet) ___________________________________ Transparency 13

Electrohydraulic converters (solenoid valves)___________________ Transparency 14

Dry solenoids – wet solenoids________________________________ Transparency 15

Solenoid valve connections__________________________________ Transparency 16

Relays and contactors ______________________________________ Transparency 17

Direct activation – indirect activation __________________________ Transparency 18

Signal inversion ___________________________________________ Transparency 19

Latching__________________________________________________ Transparency 20

Circuit diagram ____________________________________________ Transparency 21

Hydraulic power pack_______________________________________ Transparency 22

Hydraulic circuit ___________________________________________ Transparency 23

Electrical power – hydraulic power ____________________________ Transparency 24

Power losses______________________________________________ Transparency 25

Pressure switches__________________________________________ Transparency 26

4/3-way solenoid valve _____________________________________ Transparency 27

4/3-way solenoid valve, piloted ______________________________ Transparency 28

Electrical safety____________________________________________ Transparency 29

Hydraulic safety ___________________________________________ Transparency 30

1


Electrohydraulic press

� Motor with hydraulic pump

� Control panel

� Valve manifold

� Cylinder

The hydraulic press is controlled via the electrical control panel.

Electrical signals are used to activate the valves in the hydraulic installation.

The electrohydraulic press is used to form rectangular troughs.

Control pane

Motor withhydraulic pump

Valve manifold

Actuator(cylinder)

Function units

© Festo Didactic GmbH & Co. TP 601, 01Transparency

Electrohydraulic press

2


Schematic layout of an electrohydraulic installation

Division of installation into signal control section and hydraulic power section.

Input and processing of electrical signals.

Solenoid valves form the interface between the electrical signal control section and

the hydraulic power section.

Controls the flow of hydraulic fluid by means of solenoid valves. Actuators convert

hydraulic energy into motions.

��

Signal control section

Signalinput

Hydrauliccylinder

Solenoidvalve

Hydraulicpowersupply(conversion)

Powersupply

Signalprocessing

Power section

� �

��


Hydraulic power section

��


Schematic layout of an electrohydraulic installation


Power section

� �

��

3


Electrical power supply unit

Connection to mains supply.

Supplies signal control section with specified or maximum voltage and current

values.

� Voltage transformation

� Rectification

� Smoothing

� Stabilisation

� Fuse protection

In mobile hydraulic systems, rechargeable battery systems or generators are used

as a power supply for the signal control section.

230VAC

24VDC

TR

50 °C

G C T T

Transformer Rectifier Smoothing Stabilisation Fuse

Functions in

power supply unit


Electrical power supply unit

230VAC

24VDC

TR G C T T

4


Electrical circuit

A circuit consists of at least a power source, a load device and connecting lines.

Electrical current will flow only in a closed circuit.

Electrical current flows from the positive terminal of the power supply to the

negative terminal.

Closed circuit without power supply.

Introduction of a power supply to complete the circuit.

A B

+

Technical direction

of current flow

Figure A

Figure B


Electrical circuit

A B

5


Measuring circuits

The voltage drop across a load device is measured in parallel with the device.

Electrical current is measured in series.

++

Voltage measurement Current measurement

Voltage measurement

Current measurement


Measuring circuits

++

--

6


Ohm’s law

Describes the relationship between current (I), voltage (V) and resistance (R).

With ohmic resistances, the current rises in linear proportion to the voltage.

The value pairs for a constant resistance lie on a straight line. The current I is

proportional to the voltage V.

Mathematical formula for Ohm’s law: V = R • I

I = V : R

+

R

A

2.0

1.5

1.0

0.5

0100 20

V

I

30 40V

10 V

0.5 A

0.33 A

Voltage V

20 V

1.0 A

0.66 A

30 V

1.5 A

1.0 A

40 V

2.0 A

1.33 A

A

V

V = R • I

I =RV

Resistance R

Relay

Solenoid coil

Ohm’s law

Graph


Ohm’s law

+R

-

A

2.0

1.5

1.0

0.5

0100 20

V

I30 40V

Resistance R 10 V

Voltage V

20 V 30 V 40 V

A

V

Relay

Solenoid coil

7


Rectifiers

Rectifiers convert AC to DC.

Diodes are conductive in only one direction.

The polarity of the negative half-wave of the alternating voltage is reversed.

The pulsating voltage is smoothed using a capacitor and filters (reactance coil,

resistor).

~

=

VV

t

V

tt

C R

L

Diodes

Voltageretification

Voltagesmoothing


Rectifiers

~

=

VV

tV

tt

8


Analogue, digital and binary

Analogue values describe continuous functions, such as the continuous filling of a

bucket of water.

Digital values describe integer parts of a maximum value. A dice, for example, shows

one of six possible numbers.

Binary values are a special form of digital values. They can assume only two values,

such as in the case of a switch, which can only be on or off.

On 1

Va

lue

+

1

6

5

4

3

2

0

Vo

lum

e V

Filling time t Experiments with dice

Lig

ht

Time tSwitchingtime point

Off 0

full

half-full

empty

Analogue values

Digital values

Binary values


Analogue, digital and binary

+

-

Experiments with dice Time t

Vo

lum

eV

Va

lue

Lig

ht

Filling time t

9


Momentary and sustained contact switches

Actuation of contacts.

A momentary contact switch has a “rest position” and an “actuated position”.

A sustained contact switch has two detents. It thus has two rest positions.

Momentary and sustained switches can close or open current paths or switch from

one current path to another.

3 3

4 4

Momentary contact switch

Sustained contact switch

© Festo Didactic GmbH & Co. TP 601, Transparency 09

Momentary and sustained contact switches

10


N/O, N/C and changeover contacts

Contact normally open in rest position.

Example: Momentary-contact switch

N/O and N/C contacts in a single housing; one contact is open while the other is

closed.

The designations of the switch contacts are governed by the European standard

EN 50012.

Connection(N/C contact)

42

1

3 1

4 2

Connection(N/O contact)

Switch elements

N/O

N/C

Changeover contact


N/O, N/C and changeover contacts

11


Limit switches

Determining the position of a positioning slide.

Electrical contacts are actuated when a defined intermediate or end position is

reached.

Limit switches can be connected up as N/O, N/C or changeover contacts.

Switch leverStem (insulated)

Contact

Snap-actionmechanism


Limit switches

12


Proximity switches

Signal is generated by the presence of any material with good conductivity in an

oscillating magnetic field.

Signal is also generated by all insulators with high dielectric constants in an

electrical field.

Signal is generated when light barriers are interrupted or when light is reflected

back to an optical sensor.

Signal is generated by magnets whose fields close the built-in contacts in the switch.

Magnet

Metal

MetalorInsulator

reflectivecompo-nent

inductive

capacitive

optical magnetic

Reed switch

Inductive

Capacitive

Optical

Reed switches


Proximity switches

13


Solenoid (electromagnet)

A magnetic field with concentric field lines is created around a conductor carrying an

electrical current.

A coil carrying an electrical current develops a uniform magnetic field in its axial

direction due to the overlapping of field lines.

By appropriate alignment of elementary magnets, an iron core can be made into a

switchable electromagnet.

+

Field around

a conductor

Coil

Coil

Iron core


Solenoid (electromagnet)

+

-

14


Electrohydraulic converters (solenoid valves)

Convert electrical signals into hydraulic switching operations.

The switching armature is drawn into the coil, which acts as a magnet.

This produces a displacement of the actuating piston in the hydraulic 4/2-way valve.

A B

P T

Switching armature

4/2-way solenoid valve

BT A P L


Electrohydraulic converters (solenoid valves)

A B

P T

15


Dry solenoids – wet solenoids

The solenoid is separated from the hydraulic fluid; a seal is required on the stem.

� Simple construction

The armature chamber of the solenoid is filled with hydraulic fluid, the solenoid

operates in oil.

� Low friction

� Little abrasion

� Good heat dissipation

� Long service life

seal

Dry solenoid Wet solenoid

Dry solenoid

Advantage

Wet solenoid

Advantages


Dry solenoid Wet solenoid

16


Solenoid valve connections

� Solenoid with plug base

� Cable with connector socket

Electrical signals are transmitted via standard connectors.

� Reliable alignment of connections

� Connections can be attached and detached quickly

� Protected against dust and water

2

1

3

Protective earth connection

Plug base

Electricalconnections

Connector components

Advantages


Solenoid valve connections

2

1

3

17


Relays and contactors

Switches which are actuated and held in their actuated positions

electromagnetically.

Used to separate control and power circuits.

� Hinged-armature design

� Single-break contacts

� Used for logic gating of signals

� (generally) changeover contacts

Each main contact is of double-break type (contacts used as main contacts),

suitable for

� all power ratings from small to very large

� only N/O and N/C contacts

+

+

Contacts

Return spring

Return spring

Contacts Coil

Coil

Iron core

Armature

Hinged-armature solenoid Plunger-armature solenoid

Definition

Special feature

of relays

Special feature

of contactors


Relays and contactors

+

+-

-

18


Direct activation – indirect activation

Connection of a solenoid valve via a switch (Fig. A).

Connection of a solenoid valve via a relay (Fig. B).

� Separation of control and power circuits

� The current passing through switch S1 (Fig. B) is lower, and the service life of the

switch is significantly greater.

A B

24 V

0 V

1 224 V

0 V

S1 S1 K1

Y1 K1 Y1

1

3 3 3

4 4 4

Direct activation

Indirect activation

Advantage of

indirect activation

24 V

0 V

1 2


Indirect activationDirect activation

24 V

0 V

1

19


Signal inversion

The output signal is inversely proportional to the input signal.

Signal inversion at relay K1.

Signal inversion at switch S1.

The switching elements are shown under zero-voltage conditions.

24 V

0 V

1 2

S1 K1

0

10

1

Y1

0

1

B

S1 K1

K1 Y1

31

42

24 V

0 V

1 2

S1 K1

1

00

1

Y1

0

1

A

S1 K1

K1 Y1

3 1

4 2

Figure A

Figure B

24 V

0 V

1 2

S1 K1

0

1

Y1

B

S1 K1

K1 Y1

31

42

24 V

0 V

1 2


Signal inversion

S1 K1

0

1

Y1

A

S1 K1

K1 Y1

3 1

4 2

20


Latching

Latching only with indirect activation.

Actuation of switch S1: Relay K1 reverses holding the current path of K1 closed.

The actuation of switch S2 interrupts the latched current path.

K1 K13 3

4 4

24 V

0 V

1 2 3

S1

K1 Y1

3

4

S21

2

S1 S2

0 0

K1 & Y1

0

1

0

1

0 0

1 1

1

dominant on

1


Latching

K1 K13 3

4 4

24 V

0 V

1 2 3

S1

K1 Y1

3

4

S21

2

S1 S2

0

K1 & Y1

0

1

0

1

0

1

1

21


Circuit diagram

Lines of potential with constant voltage are drawn horizontally.

Current paths are drawn vertically and numbered consecutively.

Circuit symbols are shown in the direction of current flow.

Switching elements are generally shown under zero-voltage conditions.

Latching to guard against sticking of relay contacts in current paths 6 and 7.

K23

4

S2

K2

3

4

A2

A1

K13

4

S1

S0

K1 Y1 Y2

3

3

4

4

K2 K11 1

2 2

A2

A1

K1 K23 3

4 4

24 V

0 V

1

2 3 4 5 6 7


Circuit diagram

K23

4

S2

K2

3

4

A2

A1

K13

4

S1

S0

K1 Y1 Y2

3

3

4

4

K2 K11 1

2 2

A2

A1

K1 K23 3

4 4

22


Hydraulic power pack

Hydraulic power pack for use in stationary hydraulic systems.

Power is delivered via the pressure medium using a pump and drive motor.

The pack incorporates devices to remove air, water and solid particles from the

hydraulic fluid.

Baffle plate

Pressure relief valve(safety valve)

Hydraulic pump

Suction pipe

Suction filter

Return line

Air filter

Filler filter

Fluid level indicator

Cleaning aperture

Drain screw

Pressure line

Electric motor n = 1500 rpm


Hydraulic power pack

23


Hydraulic circuit

Hydraulic systems consist of open or closed circuits.

Hydraulic fluid is used as an energy carrier.

This energy is output via cylinders and motors.

Relieved hydraulic fluid is returned to the tank or hydraulic pump.

Hydraulicmotor

Motor

Filter

Motor

Hydraulic cylinder

Electrohydraulicpower valve

Safetyvalve

Safetyvalve

Hydraulicpump

Returnline

Safety valve

Feed pump(top-up forleakage losses)

Hydraulic pump

open closed


Hydraulic circuit

open closed

24


Electrical power – hydraulic power

Driven by electric current on the basis of a potential difference.

Electrical power is the product of voltage and current

Pelectr. = V • I (unit: watts).

Driven via fluid flow on the basis of a pressure difference.

Hydraulic power is the product of pressure and flow rate

Phydr. = p • Q (unit: watts).

Electric motor

+

P = V • Ielectr. P = p • qhydr.

Current I

Vo

lta

ge

V

Pre

ssu

re p

Flowrate q

Gear motor

Electric motor

Hydraulic motor


Electrical power Hydraulic power

+

-

25


Power losses

� Frictional losses

� Magnetic losses


� Volumetric losses

� Pressure losses dp

� Power losses expressed by P = dp • Q


� Volumetric losses

P = p • qhydr. loss �

Frictional andvolumetric losses

Frictional andpressure losses

Frictional,volumetric andmagnetic losses

Po

we

r lo

sse

s

Efficiency of electric motors

Efficiency of hydraulic pump

Valves, piping, tubing

Hydraulic motors

and cylinders


Power losses

26


Pressure switches

Actuation of electrical contacts when a defined minimum pressure is reached.

The minimum pressure (switching point) can be varied by means of an adjustable

spring.

Example shows switching at approx. 4 bar with rising or falling pressure.

Generally connected up in practice as a changeover contact.

Hysteresis

2

3

1

1

2 3

4 bar

pp

a

a/c

b

c

b

Switching point with rising pressure

Switching point with falling pressure

Symbolic representationin electrical circuit diagram

Symbolic representationin hydraulic circuit diagram

Pressurep

Time t


Pressure switches

2

3

1

1

2 3

Pressurep

Time t

4 bar

pp

27



Valve spool is directly actuated by solenoids.

The piston spool is displaced to the right and creates the connections P – A

and B – T.

Rest position produced by spring centring of piston spool. All inputs of the 4/3-way

valve are closed in this position.

T A P B

T A P B

A

a b

B

P T

a

a

b

b

Actuation on left

Rest position



A

a b

B

P T

T A P B

T A P B

28


4/3-way solenoid valve, piloted

Direct activation of pilot spool via solenoids.

The pilot piston spool is displaced to the right, and hydraulic fluid is directed to the

right-hand side of the main control spool, creating the connections P – A and B – T.

Rest position produced by spring centring of pilot and main piston spools. All ports

of the 4/3-way valve are closed at the main valve.

Simplified symbol

P T

A

A

B

B

P T

PB AT

Detailed symbol

Actuation on left

Rest position

P T


4/3-way solenoid valve, piloted

A

A

B

B

P T

PB AT

29


Electrical safety

� EMERGENCY STOP button placed in an easily accessible position

� Protection against voltages

� Protection by means of insulation

� Protection by means of earthed conductive housings

� Safety isolation by means of transformers

L1

24 VPE

N

1) Switch off2) Check3) Carry out work

EMERGENCY

STOP

Basic insulation

Additionalinsulation

220 V AC

Safety insulation Protective earth Safety isolation


Electrical safety

Safety insulation Protective earth Safety isolation

30


Hydraulic safety

Safety regulations must be observed.

� Switch off motor/pump

� Depressurise accumulator

� Lower load

� Check pressure

Work can now be carried out by authorised trained personnel.

m

1) Switch off motor/pump

2) Depressurise accumulator

4) Check pressure

3) Lower load


Hydraulic safety

m

Documents

Fundamentals of Electrohydraulics (Collection of ... · © Festo Didactic GmbH & Co., D-73770 Denkendorf, ... Production and interpretation of standard hydraulic and ... „Electrohydraulics