Basic Hyd Elect.

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Title: Electro-Hydraulics 24 For Private Circulation only

Electro Hydraulics

1. Introduction:

Electro-hydraulic systems are made up of hydraulic and electrical components:

The movements and forces are generated by hydraulic means (e.g. by cylinders)

Signal input and signal processing are effected by electrical and electronic components (e.g.electromechanical switching elements or stored-program controls i. e. PLC)

Electrical signals can be transmitted via cables quickly and easily and over great distances. Mechanicalsignal transmission (linkages, cable-pulls) or hydraulic signal transmission (tubes, pipes) are far morecomplex and sluggish.

1.1 Advantages of electro-hydraulicsIn the field of automation, signal processing is generally effected by electrical means. This enhances theoptions for the use of electro-hydraulic systems in automatic production operations (e.g. in a fully automaticpressing line for the manufacture of car wings)

Many machines require complex control procedures (e.g. plastics processing). In such cases, an electricalcontrol is often less complex and more economical than a mechanical or hydraulic control system.

1.2 Fields of application of electro-hydraulicsElectro-hydraulics are used in a wide range of sectors, such as:

the machine construction ( feed systems for machine tools, force generators for presses and in thefield of plastics processing)

automobile construction (drive systems for production machines),

aero plane construction (landing flap operation, rudder operation),

in shipbuilding (rudder operation).

1.3 Design of an electro-hydraulic systemThe following schematic diagram shows the two principalsub-assemblies in an electro-hydraulic system:

signal control section with signal input, signalprocessing and control energy supply

hydraulic power section with power supply section,power control section and drive section

An electrical signal is generated in the signal controlsection, where it is processed and then transmitted to thepower section via the interface. In the power section, thiselectrical energy is converted first into hydraulic and theninto mechanical energy.

1.3.1 Power SectionThe power section of an electro-hydraulic system comprises of all the assemblies, which ensure the powersupply, the power control and the working movements of a system. In most cases, the power section

hardly differs from the power section of a "purely" hydraulic system, with the exception of the actuationmode of the valves.

1.3.2 Power Supply SectionThe power supply section is divided into energy conversion and preparation of pressure medium. It is inthis part of the system that the hydraulic energy is generated and the pressurised fluid correctly prepared.In the energy conversion process the electrical energy is converted first in to mechanical energy and thenin to hydraulic energy . The following components are normally used:

electric motor or combustion engine

pump

pressure gauge

protective devices

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The pressure medium is prepared using the following components:

tank with fluid level indicator

filter

cooler

heater

temperature indicatorIn electro-hydraulic systems, the task of power control is performed by valves. According to the tasks they

perform within the system, these valves can be divided into four groups: directional control valves

non-return valves

pressure valves

flow valves

1.3.3 Drive sectionThe working movements are performed in the drive section of the system. In the drive section, hydraulicenergy in the pressure medium is converted into mechanical energy with the help of cylinders or motors.The power consumption of the drive components in the drive section determines the requirements withregard to the design of the components in the power supply section and the power control section. Allcomponents must be designed for the pressures and flow rates occurring in the operating section.

1.3.4 Signal control sectionThe signal control section of an electro-hydraulic system differs considerably from the signal control sectionof a purely hydraulic system. In hydraulic system, the corresponding functions are chiefly performed byoperating staff. In electro-hydraulic systems, the signal control section is divided into two function areas:

Signal input (sensor technology) and signal processing (processor technology).

1.3.4.1 Signal inputWith signal input, a general distinction should be made between signals given by the operator (via push-buttons, switches etc.) and signals transmitted within the system (limit switches, proximity switches,temperature sensors, special indicators, pressures switches etc.)

1.3.4.2 Signal processingIn electro-hydraulic systems, signal processing is effected either via electrical circuits or PLCs. There arealso purely pneumatic and, the less popular, hydraulic circuit options for the processing of signals. Here,signal processing is effected by electrical circuits.

1.3.4.3 InterfaceThe valve solenoids form the interface between the signal control section and the power section of anelectro-hydraulic system. DC magnets with an operating voltage of 24 V are generally used to actuate thesolenoid valves. AC solenoids are also used in the voltage range from 110V-220V.

1.4 D.C CircuitThe following illustration shows a simple DC circuit consisting of a voltage source, electrical lines, a switchand a consumer (in this case a lamp)

1.4.1 Technical direction of currentIf the switch contact in the above circuit is closed, a current I flows via the consumer. The electrons

travel from the negative terminal to the positive terminal of the voltage source. Before scientists becameaware of the existence of electrons, the current direction was described as from "plus" to "minus". Thisdefinition is still valid today - it is termed the technical direction of current.

1.4.2 Electrical conductor

Electric current is the directional movement of charges. In the field of control technology, copper is themost frequently used conductor material. The electrical resistance of copper is particularly low.



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1.4.3 DC Circuit1.4.3.1 Ohm's lawThe relationship between voltage, current strength andresistance is described by ohm's law. Ohm's law statesthat in a circuit with constant resistance the currentstrength changes in proportion to the change in voltage:

if the voltage increases, the current strength also

increases- if the voltage falls, the current strength alsodecreases

V = R* I V = voltage (V)

R = Resistance ()I = Current strength (A)

1.4.3.2 Electrical powerIn the field of mechanical engineering, power can be defined in terms of the work performed. The faster atask is performed, the greater the required power. Power therefore means work per unit of time. In thecase of a consuming device in a circuit, electrical energy is converted into kinetic energy (e.g. electricalmotor), light radiation (e.g. electrical lamp) or thermal energy (e.g. electrical heater, electrical lamp). The

faster the energy is converted, the greater the electrical power. In this case, therefore, power meansconverted energy per unit of time. It increases with increasing current and increasing voltage. Theelectrical power of a consuming device is also called electrical power consumption

P = V* I V = voltage (V)I = Current strength (A)P = Power (W)

1.4.3.3 ElectromagnetismThe solenoid coils, relays and contactors used in electro-hydraulics work on the principle ofelectromagnetism:

every conductor through which current flows builds up a magnetic field around itself

the direction of current in the conductor determines the direction of the field lines.

The current strength in the conductor influences the strength of the magnetic field.

To increase the magnetic field, the conductor through which the current flows is wound in the form of a coil.If the field lines are then superimposed on the coil windings, the main direction of the magnetic field can beestablished. If the coil possesses an iron core, the iron is also magnetised. This makes it possible togenerate considerably greater magnetic fields than can be achieved using an air-core coil with the sameamount of current.

1.4.3.4 ElectromagnetAn electromagnet must meet two conflicting requirements:

minimum current input (low energy consumption) and

maximum power through a strong magnetic field.

To simultaneously meet both criteria, electromagnets aremade up of a coil with iron core.

Illustration of an electrical coilInductive resistance with alternating voltage

If an alternating voltage is applied to a coil, the current - and thus also the magnetic field - is constantlyincreased and decreased. The change in the magnetic field induces a current in the coil. The inducedcurrent counteracts the current generated by the magnetic field. The coil therefore puts up a resistanceagainst the alternating current. The resistance is called inductive resistance.



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1.4.3.5 Inductive resistance with direct voltageIn the case of direct voltage, the voltage, the current and the magnetic field only change upon switch-on.In this case, therefore, the inductive resistance is only active at the time of switch-onThe unit of inductance is the "Henry" (H):The signal control section in electro-hydraulic system is made up of electrical or electronic components.Depending on the task to be performed, the signal control section can vary in design:

relatively simple controls use either electro-mechanical components with contacts (e.g. relays) or acombination of components with contacts and electronic components with contacts.

for complex tasks, on the other hand, stored-program controls (PLC's) are mostly used.

The circuit examples in this programme are primarily based on electro-mechanical components.

1.5 Power supply unitElectro-hydraulic control systems are generally supplied with electricity not from their own voltage sources(e.g. batteries) but from the mains supply via a power supply unit.

1.5.1 Safety noteThe components of the power supply unit form the power current system in an electrical circuit. Thesafety regulations for power current systems must therefore, be observed

1.5.2 CapacitanceA capacitor consists of two metallic plates with an intermediate insulation layer (dielectric). The greater thecapacitance of a capacitor, the more electrical charge carriers it stores at the same voltage. If a capacitor isconnected to a direct voltage source, a charging current flows for a short time. The two plates areelectrically charged in opposing mode. If the connection to the voltage source is then interrupted, thecharge remains stored in the capacitor until the charge is dissipated via a consuming device (e.g. aresistor)

The unit of capacitance is the "Farad" (F) 1F = 1 As/V

Modules of a power supply unit

1.5.3 Electrical input elements

Switches are installed in a circuit to open or close the flow ofcurrent to the consuming device. These switches are dividedinto the two main groups "push button switches" (push-buttons)and "control switches". Both switch types are available foroperation with normally closed contacts, normally opencontacts or changeover contacts.

1.5.4 Control switch

In control switches, the two switching positions aremechanically inter-locked. A switching position ismaintained until the switch is activated once again.

1.5.5 Push-button

A push-button only opens or closes a current circuit fora short time. The selected switching position is activewhile the push-button is pressed.

1.5.6 Normally open contactIn the normally open version, the circuit is open when the push-button is in the normal position; i. e. not pressed. The circuit isclosed when the control stem is actuated; current then flows to theconsuming device. When the control stem is released, the push-button is returned to its original position by spring pressure, and thecircuit is then interrupted.

Normally open contact : sectional view and circuit symbol



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1.5.7 Normally closed contactIn the normally closed version, the circuit is closed when the push-button is in normal position. The springaction ensures that the contacts remain closed until the push-button is pressed. When the push-button ispressed, the switching contact is opened against the spring pressure. The flow of current to the consumingdevice is interrupted.

Normally closed contact: sectional view and circuit symbol

1.5.8 Changeover contactThese contacts combine the functions of normally closed andnormally open contacts in one unit. Change over contact are used toclose one circuit and simultaneously open another. It should benoted, however, that both circuits are momentarily interrupted duringchangeover.

Changeover contact: sectional view and circuit symbol

1.6 SensorsSensors are used to record information about the status of a system

and to pass this information on to the control. In electro-hydraulicsystems, sensors are mainly used for the following tasks:

measurement and monitoring of pressure and temperature ofthe pressure fluid,

recording the proximity i.e. the position or the end position ofdrive components.

1.6.1 Limit switchA mechanical limit switch is an electrical switch which is activated when a machine

Part or a workpiece is in a certain position. Activation is generally effected by a cam activating a movableoperating lever. Limit switches are normally equipped with changeover contacts capable of performing

closing, opening or changeover of circuits.

1.6.2 Pressure switchPressure switches are used as control or monitoring devices.They can be used to open, close or change between circuitswhen a pre-set pressure is reached. The supply pressure acts ona piston surface. The resulting force acts against an adjustablespring pressure. If the pressure is greater than the force of thespring, the piston is moved and actuates the contact assembly.

In pressure switches with mechanically actuated contactassemblies, a diaphragm, a bellows or a Bourdon spring can be used in place of the helical spring.

Piston pressure switch: sectional view and circuit symbol

Recently increasing use has been made of diaphragm pressure switches, where the contact is no longermechanically actuated but electronically switched . This also requires the use of pressure or force-sensitive sensors which exploit one of the following physical effects:

the resistance effect (diaphragm with strain gauge, change in electrical resistance with shape,change),

the peizo-resistive effect (change in electrical resistance with change in mechanical tension),

the piezo-electric effect (generation of an electrical charge through mechanical stress),

the capacitive effect (change in capacitance with change in mechanical stress).



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The pressure-sensitive element in this process is created through diffusion, vapour-depositing or etchingon the diaphragm. A suitable protective electronic circuit supplies an amplified analogue signal. Thissignal can be used for pressure indication or for further switching operations.

1.6.3 Relay and contactorThe representation of relays and contactors in the electrical circuitdiagram is identical as is their operating principle.

Relays are used to switch relatively small output and currents;

Contactors to switch relatively large outputs and currents.

1.6.4 RelayRelays are electro-magnetically actuated switches. They consist of ahousing with electromagnet and movable contacts. Anelectromagnetic field is created when a voltage is applied to the coil ofthe electromagnet. This results in attraction of the movable armatureto the coil core. The armature actuates the contact assembly. This contact assembly can open or close aspecific number of contacts by mechanical means. If the flow of current through the coil is interrupted, aspring returns the armature to its original position.

Relay: sectional view and circuit symbol

Application examplesThere are various types of relay; e.g. time-delay relays and counter relays. Relays can be used for variousregulating, control and monitoring functions.

as interfaces between control circuits and load circuits,

for signal multiplication,

for separation of direct current and alternating current circuits,

for delaying, generating and converting signals and

for linking information.

1.7 Terminal designations and circuit symbolsDepending on design, relays possess varying numbers of normally closed contacts, normally opencontacts, changeover contacts, delayed normally closed contacts, delayed normally open contacts anddelayed changeover contacts.

The terminal designations of the relays are standardised (DIN EN 50 005, 50011 - 13):

relays are designated K1, K2, K3 etc.

the coil terminals are designated A1 andA2.

the contacts switched by the relay arealso designated K1, K2 etc. in circuitdiagrams.

The terminals of auxiliary contacts (relaycontacts) are designated by two digitnumbers:

the first digit is the ordinal number, the second digit is the function number.

1.7.1 ContactorContactors work on the same basic principle as relays. The typical features of a contactor are:

double-break (2 break points per contact),

positive-action contacts and

closed arcing chambers (spark arresting chambers)

Contactors possess several contact elements, normally between 4 and 10. There are also different typesof contactors with various combinations of normally close contacts, normally open contacts, changeovercontacts etc. The contacts are divided into main contact elements and auxiliary contacts (control contacts).



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Outputs of 4-30 kW are switched via main contact elements.

The auxiliary contacts can be used to simultaneously switch further control functions orlogic operations.

Contactors which only switch auxiliary contacts (control contacts) are called contactorrelays (control contactors)

For the purpose of classification, contactors with main contact elements for powerswitching are called power contactors (main contactors).

In line with DIN 40 719, contactor combinations for switching on three phase motors designated by theletter K(for contactor) and M (for motor) as well as serial number. The serial identifies the function of thedevice; for example: K1M = mains contactor, three phase, variable pole, single speed motor.

1.8 SolenoidsIn electro-hydraulics, valves are actuated via solenoids. An iron core (armature) is installed in the coilwinding of the solenoid. A non-magnetic plunger is embedded in this armature. If the coil is then suppliedwith current, a magnetic field is formed which energises the armature. The plunger connected to thearmature then switches the valve gate (see illustration on next page).

Solenoids have two end positions.

The first end position is achieved during conductive continuity

While the second end position is achieved in de-energised state via a return spring

(electro-magnetic, decay position A)

In each switching operation, the plunger additionally pressesagainst the return spring of the valve, thus reducing its force inthe direction of attraction.

At the beginning of the travel movement themagnetic force is small. The motion of the armaturetherefore begins with a small idling stroke (positionA).

The control gate of the directional control valve isnot switched(position B) until a greater magneticforce has been reached.

Stroke/force characteristic of a DC solenoid

1.8.1 DC and AC solenoidsThere are solenoids for direct current or alternating current. AC solenoids for 230 V are used less and lessfrequently for reasons of safety (dangerous touch voltage).

ArcingWhen a solenoid is switched off, the flow of current is interrupted. The collapse of the magnetic fieldcreates a voltage peak in the opposite direction. A protective spark suppression circuit is essential toprevent damage to the solenoids or the contacts.

Types

DC solenoids are produced as wet or dry designs, whereas AC solenoids are always dry solenoids.

1.8.2 Wet solenoid

In wet solenoids, the armature chamber of the solenoid contains hydraulic oil, in which the solenoidswitches. The housing of these solenoids have to be seal tight (towards the outside). The armaturechamber is connected to the tank port to prevent high pressure on the solenoid. The advantages of thisnowadays common type of solenoid are:

absolute seal-tightness and low friction due to theabsence of a dynamically stressed seal at the plunger,

greatly reduced corrosion inside the housing and

cushioning of switching operations.



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1.8.3 Dry solenoidThe term "dry solenoid" means that the solenoid is isolated fromthe oil. The plunger is sealed off from the oil in the valve body by

a seal. Therefore, in addition to the spring force and the frictionof the control gate, the solenoid has to overcome the frictionbetween plunger and seal.

1.8.4 4/2-way solenoid valve

The 4/2- way valve shownis activatedelectromechanically and returned by spring action. The attachedD.C. solenoid is a "magnet which switches in oil" (wet magnet). Thearmature also operates in oil and ensures low wear, excellent heatdissipation and a cushioned armature stop. The armature chamberis connected to the tank port. The valve has two power ports A and

B, a pressure port P, and a tank port T. 4/2-way valve, electro-magnetically actuated

1.8.5 4/3 way solenoid valveIn the no-load condition, the 4/3 way solenoid valveswitches to the mid- position. In this mid-position all portsare blocked. This valve possesses no storage capability.

Electrical controls are the dominant method for controllingfluid power systems. The solenoid-operated directional control valve is the basic building block forelectrical control circuits. Just as the designer had to learn ISO graphic symbols to communicate about

fluid power circuits, he must also learn standard electrical symbology. This paragraph deals primarily withelectrical control circuits, not power circuits.

1.9 Switches and other electrical controls.Switches are the basic devices for generating control signals, enclosed list illustrate and briefly describethe symbols of most commonly used types of electric switches, contacts, relays, and motors.

2. RATINGS AND SPECIFICATIONSElectrical power circuits.Electrical power is primarily used to drive fluid power systems. Alternating current (AC) power circuits arerated at 110-120 V (voltages, 220-240 V, 440 V and 2.2 KV. They may be either single or three-phasesystems.

2.1 Electrical control circuitsDirect-current (DC) electrical control circuits are rated at 6, 12, 24, or 36 volts; the first three are the mostpopular. Alternating-current rating for control circuits are 6, 12, 24, and 110 volts. Control circuits canoperate at any voltage level suitable for the system. However, operating control systems at high voltagesis inefficient and introduces unnecessary hazards for personnel.

Switches are rated on the basis of system voltage and the maximum current they can handle safely. Atypical rating for a push-button switch designed for 1 hp duty is 20 amperes(A) and 120 V, or 10A and250 V. Some switch rating includes data on DC as well as AC current and voltage. Others specify furtherthat the rating given applies to a non-inductive load. Inductive loads can cause high current surges in thecircuit which the switch may not be able to handle.

When relays are used, it is necessary to specify coil and contact ratings. For example, a designer might

use a relay with a 6-V DC operating coil and contacts capable of handling a 10-A, 240-V load. Or he may



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want a relay to operate on the same voltage as the contact circuit; in this case the designer could specify110 V for this coil, and 20 A and 110 V for the contacts. Specifications for a relay can become as complexas those for a directional control valve. The designer must include the voltage ratings, the contact pattern,and any special control functions, such as time delay, latching, or stepping.

2.2 Designation of components in the hydraulic circuit diagram using numbers

The components in hydraulic circuit diagrams are given numbers. The designation is made up of a group

number and an equipment number.

The various control loop systems are consecutively numbered using the ordinal numbers 1,2,3 etc. Thepower supply section is not assignable to any or control loop system as it is responsible for several controlloop systems. For this reason, it is always designated by the ordinal number zero.

2.3 Group assignment

Group 0 all power supply elements

Group 1,2,3 . Designation of the individual control loop systems (normally one group number percylinder)

Each component in a control loop system is to be identified by an equipment number made up of theordinal number of the control loop system and a distinctive number.

2.4 Equipment numbering

.0 e.g. 1.2, 2.4 drive component, e.g. 1.0, 2.0

.1 final control elements, e.g 1.1, 2.1

.2,.4 even numbers: all elements influencing the forward ( clock wise) motion

.3, .5 flow, e.g. 1.3, 2.3 uneven numbers: all elements influencing the return (anti clock wise)

.01, .02 valve, 1.01, 1.02. elements between final control elements and drive component, e.g.throttle

In day-to-day operations, this designation system using group and equipment numbers has the advantagethat maintenance personnel are able to recognise the effect of a signal by the number of the element inquestion. If, for example a fault is ascertained in cylinder 2.0, it can be assumed that the cause is to besought in the 2nd group and, therefore, in elements whose first number is 2.

3. General safety recommendations

High pressures, temperatures and forces occur in electro-hydraulic system. Energy is also stored,sometimes in large quantities. A whole series of safety measures is necessary to rule out the possibility ofdanger to personnel and equipment during the operation of electro-hydraulic systems. In particular, thevalid safety regulations for electro-hydraulic systems are to be observed.

Regulations and standards

The following safety regulations apply for the field of hydraulics:

accident prevention regulations, directives, safety rules and the testing guidelines of the employer'sliability insurance associations,

regulations on pressure vessels, pressurised gas vessels and filling systems (pressure vesselregulations),

DIN standards, VDI directives, VDMA standard sheets and technical rules for pressure, vessels,containing in particular, notes and regulations on dimensions, design, calculations, materials andpermissible loads as well as stipulations on functions and requirements.

Electro-hydraulic systems must comply not only with the regulations on hydraulic systems but also with the

regulations on electrical systems and components (e.g. DIN VDE 0100).



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