Electropneumatics

ELECTROPNEUMATICS

By: Irfan Ariyanto ST. MS.

Part 1Introduction

Pneumatics review

Pneumatics deals with the use of compressed air to do mechanical works (produce motion and to generate forces).Pneumatic drives have the task of converting the energy stored in compressed air into motion.

Pneumatic linear cylinder: Pneumatic swivel cylinde r:

Pneumatics review

Some of the many applications of pneumatics are:- Handling of workpieces (such as clamping, positioning,

separating, stacking, rotating)- Packaging- Filling- Opening and closing of doors (such as buses and trains)- Metal-forming (embossing and pressing)- Stamping

Pneumatics review

Pneumatic application example:

Control engineering

� Pneumatic drives can only do work usefully if their motions are precise and carried out at the right time and in the right sequence.

� Coordinating the sequence of motion of the pneumatic drives is the task of the controller.

� Control engineering deals with the design and structure of controllers.

� Controlling – open loop control – is that process taking place in a system whereby one or more variables in the form of input variables exert influence on other variables in the form of output variables by reason of the laws which characterize the system. (DIN 9226, Part 1)

Control engineering

� Controls must evaluate and process information (for example, pushbutton pressed or not pressed). The information is represented by signals.

� A signal is a physical variable, for example:- The pressure at a particular point in a pneumatic system.- The voltage at a particular point in an electrical circuit.

� Signal form: Analog, Digital, Binary.� Controller classification based on the signal form: Analog

controller, Digital controller, and Binary controller.

Control engineering

Analog

Digital

Binary

Control engineering

Signal flow in an open loop control system.

Command execution takes place at high power level to produce high force output.

Amplifies signals from low power level to high power level.

Takes place at low power level. Signal inputs are processed (logically and / or sequentially) to produce signal output.

Electropneumatic control system Pneumatic control system Electropneumatic control sys tem

→ In an electropneumatic control, the signal control sect ion is made up of electrical components.

Electropneumatic control system

� Electropneumatic controllers are shown in two separate circuit diagrams - one for the electrical part and one for the pneumatic part.

Electrical circuit diagram Pneumatic circuit diagram


An example of a modern electropneumatic controller:


Advantages of electropneumatic controllers:� Higher reliability (fewer moving parts subject to wear).� Lower planning and commissioning effort, particularly for

complex controls.� Lower installation effort, particularly when modern

components such as valve terminals are used.� Simpler exchange of information between several

controllers.

Part 2Basics of Electrical Theory

Basic of Electric

� Electric current: electron flow from negative potential/ voltage to positive potential.

� It is technically equivalent (and widely used) to say that electric current is flow of positive charge from positive potential to negative potential.

� Current only happens in a closed circuit!

Basic of Electric

� Direct Current (DC) is constantly flow while Alternating Current (AC) changes over time.

� Relationship between voltage/potential (V, in Volt), current (I, in Ampere), and resistance (R, in Ohm).V = I . R (Ohm’s Law)

� Electric Power (P, in Watt):P = V . I

Solenoid / Coil

� When DC current is applied, a solenoid/coil produces magnetic field which its strength is proportional to the current.

� A ferrous materials is added as the coil’s core to strengthen the magnetic field.

� Magnetic field produced by coils can be used to pull ferrous metal parts such as contacts or pistons.

Solenoid / Coil

� When AC current is applied, a solenoid/coil produces fluctuating magnetic field which, in turn, produce ‘resistance’/’reactance’ to the current (remember Lenz’s law).

� The reactance magnitude is proportional to coil’s inductance (H, in Henry).

1 H = 1 Volt second / Ampere = 1 Ohm second

Capacitor

� When connected to a DC voltage, capacitor charges its plates with electric charges, stores them, and discharges the electric charge when connects to a load.

� Capacitance (in F, Farad): measure of how much electric charges can be stored in a capacitor.

1 F = 1 Ampere second / Volt = 1 Coloumb / Volt

Diodes

� Diodes are made up by semiconductor materials.� Diodes allow current in a direction only and block current

in the reverse direction.� In allowed current direction, diodes have very low

resistance. While in the reverse direction, diodes have very high resistance.

� When applied to AC source, diodes rectifies the AC voltage and current.

Diodes

Electrical circuit measurement

� Multimeter is the most popular device to measure electrical components and circuits.

� Multimeters can be set to be:- Voltmeter to measure voltage (DC, AC)- Ammeter to measure current (DC, AC)- Ohmmeter to measure resistance.

� Modern multimeters can measure AC frequency, and capacitance of capacitors.



Safe measurement procedure:� Know the limit/maximum voltage and current of your

multimeter !� Switch off voltage source of circuit.� Set multimeter to desired mode. (voltmeter or ammeter,

AC or DC, resistance)� Check zeroing for pointer instruments. Adjust if

necessary.� When measuring DC voltage or current, check for

correct polarity. ("+" probe of device to positive pole of voltage source and “-” or “GND” (ground) / “COM” to negative pole of voltage source).


� Select largest range. (*)� Switch on voltage source.� Observe pointer or display and step down to smaller

range. (*)� Record measurement for greatest pointer deflection

(smallest measuring range). (*)� For pointer instruments, always view from vertically

above display in order to avoid parallax error. (*)

(*) : not necessary for digital multimeter which is equipped with autorange feature and free from parallax error.


� In measuring voltage, voltmeter (multimeter) is connected in parallel to the load.

� Internal resistance of voltmeter is designed so high in order to have good accuracy.


� In measuring current, ammeter (multimeter) is connected in series to the load.

� Internal resistance of ammeter is designed so small in order to have good accuracy.

Q: What would happen if you accidentally use ammeter to measure voltage source ??


� When measuring an electric component in an electric circuit, make sure that the component is separated /isolated/disconnected to the rest of the electric circuit.Why ?? (see an example of measuring R2 below)

R (measured) = R1 R2 / (R1 + R2)(wrong!)

R (measured) = R2(right)


Daily practices of electrical component measurement: � Example 1 : Checking conductor’s/cable’s continuity.

Whenever you see very small resistance when measuring two conductor ends, it means the ends are electrically connected.

R small ( ≈ 0) → good cableR high / infinite → broken cable

R small ( ≈ 0) → A1 and B5 are pair ends of a cable.


� Example 2 : Checking coil or transformer condition.

R moderate (at specific value) → good inductorR high / infinite → broken inductor


� In analog / pointer multimeter, set the range correctly (smallest possible range) to minimize error in reading the value and get more accurate results.

10V rangeReading: ±9 (8.5 – 9.5) V

100V rangeReading: ±10 (5 – 15) V

Part 3Components and assemblies of electrical signal control section

Switches

� Switches:- Control switches which are mechanically held in the selected position. Ex: Light switch at your home.- Push button switches maintain in the selected position only when they are actuated/pressed.

Normally open contact- connects the contact when the button is pressed;- disconnects it when the button is released.

Switches

Normally closed contact- disconnects the contact when the button is pressed;- connects it when the button is released.

Changeover contactclose one circuit (connect the contact) and open another (disconnect the other contact) in one switching operation.

Displacement and pressure sensors

� Sensors have the task of measuring information and passing this on to the signal processing part in a form that can easily be processed.

� In electropneumatic controllers, sensors are primarily used for:- To detect the advanced and retracted end position of the piston rod in cylinder drives- To detect the presence and position of workpieces- To measure and monitor pressure


Limit switchLimit switch is mechanically actuated when a machine part or workpiece is in a certain position. Normally, actuation is effected by a cam.

Possible contact usage

Displacement sensors


Reed switchIt consists of two contact reeds in a glass tube filled with inert gas. The magnetic field causes the two reeds to close, allowing current to flow. A permanent magnet is attached to a moving machine part / workpiece to create the magnetic field.


Inductive proximity sensorThe oscillator generates a high-frequency alternating magnetic field that is emitted from the front of the sensor. If an electrical circuit is introduced into this field, the oscillator is attenuated. The downstream circuitry, consisting of a flip-flop and an amplifier, evaluates the behavior of the oscillator and produces the output (voltage supply).


Capacitive proximity sensorAn electrostatic field is generated between the anode and the cathode of the capacitor. A stray field forms at the front ofthe sensor. If an object is introduced into this stray field, the capacitanceof the capacitor changes. The oscillator is attenuated and then the circuitry switches the output (voltage supply).


Optical proximity sensorOptical proximity sensors use optical and electroni c means for objectdetection. Red or infrared light is used. Semiconductor light- emitting diodes (LEDs) are particularly reliable sources of red or infrare d light.

One-way light barrier

The one-way light barrier has spatially separate transmitter and receiver units.The output is switched if the light beam is interrupted by a moving workpiece.


Reflective light barrier

The transmitter and receiver are mounted together in one housing.The output is switched if the light beam is interrupted by a moving workpiece.

Diffuse reflective optical sensor

The transmitter and receiver are mounted together in one unit. If the light hits a reflective object, it is redirected to the receiver and causes the output of the sensor to switch.


Piston-actuated pressure switchLimit switch is mechanically actuated when a machine part or workpiece is in a certain position. Normally, actuation is effected by a cam.

Pressure sensors

Sensitivity knob


Analogue pressure sensorsThe piezoresistive measuring cell of a pressure sens or. Variable resistor 1 changes its value when pressure is applied to the diaphragm. Via the contacts 2, the resistor is connected to the electr onic evaluating device, which generates the output signal.

Relays and contactors

RelayA relay is an electromagnetically actuated switch. When a voltage isapplied to the coil, an electromagnet field results. It causesthe armature to be attracted to the coil core. The armature actuates therelay contacts, either closing (in normally open relay ) or opening them (in normally closed relay).A return spring returns the armature to its initial position when the current to the coil is interrupted.


Time relay- Pull-in relay: energized after a set delay. (delay ed activation)- Drop-out relay: de-energized after a set of delay. (delayed release)

Pull-in relay (delayed activation)

internal circuit of pull-in relay

Relays and contactorsPull-in relay

circuit diagram using a pull-in relay


Drop-out relay

internal circuit of drop-out relay

Relays and contactorsDrop-out relay (delayed release)

circuit diagram using a drop-out relay


ContactorContactors operate in the same way as a relay. Typi cal features of acontactor are:- Double switching (dual contacts)- Closed chambers (arc quenching chambers)These design features allow contactors to switch mu ch higher currentsthan relays .


Contactors are used for the following applications:- High currents (high power electricity) of 4 to 30 kW are switched via the main contacts of power contactors.- Control functions and logical associations are swi tched by auxiliarycontacts.

In electropneumatic controllers, electrical currents and power are low.For this reason, they can be implemented with auxil iary contactors. Mainor power contactors are not required.

Programmable Logic Controller (PLC)

� Programmable logic controllers (PLCs) are used for processing of signals in binary control systems.

� For complex signal control processing, PLCs has replaced relay usage because of their easy use, simplicity, and compactness.



� The main element (CCU) is a microprocessor system.� Programming of the microprocessors determines:

- Which control inputs (I1, I24 etc.) are read in which order- How these signals are associated- Which outputs (O1, O2 etc.) receive the results of signal processing.

In this way, the behavior of the controller is not determined by the wiring (hardware), but by the program (software).


Comparison of relay signal processing and PLC signal processing.

Programmable Logic Controller (PLC)Code/program

Part 4Electrically actuated directional

pneumatic valve

Electropneumatic directional valves

� They belong to signal output (final control elements) stage in signal flow of electropneumatic control system.

� Interface between electric signal and pneumatic/ pressure signal.

� Work in two forms of energy: electrical and compressed air.

� The electropneumatic valve can be used for:- switching on or off air supply.- actuate (extend or retract) cylinder drives.


Application example

(coil de-energized) (coil energized)


� Construction type of electropneumatic valves:- Return spring → return to normal position when coil is de-energized. - Double coil/solenoid without return spring → retain the actuated position (not return back to previous position) when the coil is de-energized. Energizing the opposite coil is done to return the valve back to the previous position.

� In term of controlling mode:- directly controlled valves → coil + armature assembly directly controls / switches the valve’s ports.- pilot (indirectly) controlled valves → coil + armature assembly controls pilot air supply which, in turn, actuates the valve’s piston and switches the ports.

Electropneumatic directional valvesNormally closed 3/2-way directional valveWhen energized, compressed air is passed to consumi ng devices.



Normally open 3/2-way directional valveWhen energized, compressed air supply to consuming devices is stopped.



Manual override operationDone by screwing in the small piece A, so that the cam mechanismpushes the armature up.

Screw in

Electropneumatic directional valvesPilot controlled 3/2-way valve


Electropneumatic directional valvesPilot controlled 5/2-way solenoid valve

(coil de-energized)Pilot air port


(coil energized)

Electropneumatic directional valvesPilot controlled 5/2-way double solenoid valve

(all coils de-energized)

Pilot air port


(left-coil energized, right coil de-energized)


(left-coil de-energized, right coil energized)


For pilot controlled 5/2-way double solenoid valve:� If both coils / solenoids are energized or de-energized,

then the pressure force on the left and right side of the piston assembly are equal. Which means the piston stops at current position.

� That means this type of valve retains its actuated position.

� To return the piston to previous position, corresponding coil must be energized while the other coil must be de-energized. (i.e: energize left coil and de-energize right coil to push the piston to right; de-energize left coil and energize right coil to return the piston to left)

Electropneumatic directional valvesPilot controlled 5/3-way valve with exhausted norma l / mid position.

(initial / mid position, all coils de-energized)Return spring to mid position


(left-coil energized, right coil de-energized)

The spring is pressed right


(left-coil de-energized, right coil energized)

The spring is pressed left


There are 3 variations of mid-position of pilot controlled 5/3-way double solenoid valves :

� Pilot controlled 5/3-way valve with exhausted mid position (described above).

� Pilot controlled 5/3-way valve with closed mid position.� Pilot controlled 5/3-way valve with pressurized mid

position.

Electropneumatic directional valvesPilot controlled 5/3-way valve with exhausted mid p osition

Since no pressure (all cylinder port are vented out), force on the piston is zero, and it is freely movable.

Pilot controlled 5/3-way valve with closed mid posi tion

Force on the piston is zero, but it is not freely movable. (there would be pressure increase opposing the movement)


Pilot controlled 5/3-way valve with pressurized mid position

Force on the piston is not zero but at very reduced magnitude.


Comparison of directly controlled and pilot controlled solenoid valves:

Directly controlled Pilot / indirectly controlled

� Coil + armature directly opens or closes air flow supply to consuming devices.

� It needs bigger coil and armature to allow higher air flow rate.

� Bigger coil means higher thermal dissipation when it is energized.

� Simple construction.

� Coil + armature opens or closes pilot air flow. Pilot air flow controls opening or closing of the main valve.

� Pilot air mechanism can exert higher force on the valve. Higher flow rate is available.

� Small coil is enough to control the pilot air.

� More complicated construction.

Selecting Electropneumatic Valves

1. Establish the selection based on:- The task of the valve must perform. (see ref. page 75)- The valve’s behavior in case of the power supply failure:

A spring-return 3/2-way or 5/2-way valve switches to its initial position and the piston rod of the cylinder returns to its initial position.

A pilot controlled 5/3-way valve with mid position also switches to its initial / mid position. If the working ports are exhausted in the initial position, the cylinder is not subject to force. If the ports are pressurized, the piston rod extends at reduced force. If the ports are closed, the motion of the piston rod is interrupted.

A double-solenoid valve retains its current position. The piston rod completes the current motion.


2. Check manufacturer’s catalogue and examine the data sheet/ performance data:

Pneumatic (include pilot control and main stage) data : nominal size, nominal flow rate, pressure range, response time (see ref.page 83)

Electrical (include coil and plugs) data : operating voltage, electric power, duty cycle, protective circuit, switching indication, signal conversion, class protection


Pneumatic data:

Nominal size of valve → narrowest area in which the air flows through, then it is converted to an equivalent circle area. Diameter of the equivalent circle is the valve nominal size.

Nominal flow rate of valve → measured in a condition to maintain pressure 6 bar for upstream flow and 5 bar for downstream flow.

Pressure range → the range of supply pressure at which the valvecan be operated.


Electrical data:

Operating voltage and power supply tolerance → range of allowed electric supply for the coils and the load capacity of power supply unit.

Coil’s duty cycle → maximum time for a coil in energized state. Measured in percentage of standard coil’s operating time (10 minutes). So, duty cycle 75% means the coil shouldn’t be energized more than 7.5 minutes.

The class of protection → how well a solenoid coil is protected against the ingress of dust and water


Electrical data:

Average pickup time → time difference between the event when coil is energized and the armature picks up.

Protective circuit of a solenoid coil → to protect from high voltage induction caused by sudden decrease of electrical current flowing in a coil, a protective circuit is commonly used.

Part 5Developing an electropneumatic control

system

Developing an electropneumatic control system

1. PROJECT DESIGN

1.1 Formulation of task definition� Positional sketch

� Determination of requirements (see list table 5.1 and table 5.2)

Pneumatic components catalogue may be used as additional information for drawing the sketch and determining the requirements.

Developing an electropneumaticcontrol system

1.2 Stipulation on how to implement the control sys tem� Conceptual design:

- relay processing or using PLC ?- separate installation of valve or installation of valves by mounting them on valve terminals ?- standard valves or valves with auxiliary functions ?- using modular (templates) control system design ?- using state-of-the-art bus system and valve terminals ?

� Components selection. See the catalogue/datasheet of:pneumatic drives, electropneumatic valves (ex. table 4.1), sensors, electrical control elements (relays, switches, pushbuttons, PLCs)


1.3 Graphical representation of control systems� Allocation table (ex. table 5.3)� Function diagram (displacement-step diagram,

displacement-time diagram) and/or function chart.


1.4 Design of control systemMake:� Pneumatic circuit diagram� Electrical circuit diagram and PLC program (if PLC is

used)� Terminals diagram� Parts list


2. IMPLEMENTATION

2.1 Procurements of components

Can be by purchasing new components or using available spare parts.


2.2 Installation� Attachments and mounting of components � Wiring the signal control section� Tubing power section part (pneumatic drives)


2.4 Commissioning� Loading PLC program (if using PLC)� Functional testing� Implementation of necessary changes� Testing and adjustment in under all conditions occurring

in real practice (SAT : Site Acceptance Testing)� Updating documentation of the system based on the

installed / tested configuration for maintenance purpose� Preparation of acceptance test certificate


Case study: Lifting device example

Positional sketch


Pneumatic drives / cylinders


Pneumatic drives / cylinders- Compressed air network 6 bar (6 kPa)- Cylinder 1A: stroke 500mm, min force 600N (which

means piston diameter need to be min 40mm), soft braking (end cushioning), braked immediately then remains still when electrical power fails.

- Cylinder 2A: stroke 250mm, min force 400N (which means piston diameter need to be min 32mm), soft braking (end cushioning), braked immediately then remains still when electrical power fails.

- Cylinder 3A (stopper): stroke 20mm, min force 40N, initial position is extended.


Valves- Since cylinder 1A and 2A must be braked immediately

when there is electrical power failure, we need 5/3-way pilot controlled double solenoid valve with closed mid position for each cylinder.

- Since cylinder 3A must extend in case there is electrical and/or pneumatic power failure, 3/2-way pilot controlled solenoid valve with return spring should be enough.

- One-way flow control valve to regulate cylinder 1A and 2A by means of controlling exhausting air.

- Additional valve to cut off air supply to the system in case there is electrical power failure or emergency stop.


Electrical control components � Electrical power supply 24V DC.� Sensors: 1B1, 1B2, 2B1, 2B2, 3B1 are reed switches, B5

is optical proximity sensor, B6 optical sensor is not considered for now.

� Relay signal processing / relay control (not using PLC).


Operational control� Enable continuous operation (“continuous cycle on”).� Enable single cycle operation (the sequence is

processed precisely once).� Enable emergency stop to cut off both electrical and

pneumatic power supply to the system.� Enable “reset” to return all pneumatic drives to their

initial position (1A and 2A retract, 3A extends).� Enable to stop continuous operation (“continuous cycle

off”). In case there is already a workpiece in the device, it must be transferred to the upper roller conveyor then drives 1A and 2A retract and 3A extends (initial condition).


Operational control


Product selection� See the manufacturer catalogue. For ex.:

- Festo DNGUL-40-500-PPV-A for cylinder 1A- Festo DNGUL-32-250-PPV-A for cylinder 2A- Festo STA-32-20-P-A for cylinder 3A- Festo MEH-5/3G-1/8 for valve actuating cylinder 1A and 2A- Festo GRLA-1/4 (cylinder 1A) or GFLA-1/8 (cylinder 2A) for speed regulating in cylinder 1A and 2A.- Festo MEH-3/2-1/8 for valve actuating cylinder 3A- Festo CPE14-M1H-3GL-1/8 for valve cut off air supply in case electrical power failure or emergency stop.


Allocation table


Displacement-step diagram


Pneumatic circuit diagram


Electrical circuit diagram:Controlelements


Electrical circuit diagram:Sensorevaluations


Electrical circuit diagram:Switching of sequence steps


Electrical circuit diagram:Circuitry ofsolenoidcoils


Electrical control section with PLC:1. Install the software of the PLC in your PC. The software

enables you write, compile, test / simulate, download (to PLC) a PLC program.

2. Write the PLC program / code in your PC. 3. Compile it, check out for errors (syntax errors, logic

errors).4. Fix any error of the program and recompile it.5. Test / simulate the program without the PLC (in software

environment only. Check whether the program (you has written) runs as expected.


6. Download the program to the PLC but do not connect the PLC to the coils/solenoid of valves yet. Test / simulate it until you are sure that your program runs as expected when applied to the PLC.

7. Connect the PLC to the coils / solenoids of your valves of your pneumatic system and see if the program runs perfectly as expected or needs to be adjusted / modified.

Part 6Documentation for an electropneumatic

system

Documentation for an electropneumaticsystem

� Good and systematic documentation helps reducing cost of developing a control system. It minimizes error in installation and testing.

� Accurate and complete documentation is important for maintenance of the system. It helps maintenance people to find out the problem of the system, replace a component, and fix the problem.

� To be clear, unambiguous, and easily understandable, documentation should follow relevant engineering guidelines or standards.

Documentation for an electropneumatic system

� Positional sketch (usually with a step table)� Function diagram (such as displacement-step diagram)

or function chart� Pneumatic circuit diagram� Electric circuit diagram� Terminal allocation list� Parts / components list

Function Chart

Function chart

� It is meant to replace displacement-step diagram.� It is easier for more complicated control system.� Complies with DIN standard.

Function Chart

Function chartExample: sheet metal bending device (fig 6.1)

Retract the clamping cylinder (2A)2B1retractingnone4

Retract the clamping cylinder (1A)1B1noneretracting3

Bend the metal2B2extendingnone2

Clamp the metal1B2noneextending1

CommentEnd of step detection

Movement of Cyl. 2A

Movement of Cyl. 1A

Step

Function Chart

Function chart

Displacement step diagram

Function chart

Electrical Circuit Diagram

� In an electrical circuit diagram, the components are represented by graphical symbols that are standardized according to DIN 40900. (see Fig. 6.22 to 6.27)

� Designation / label of every components per DIN 40719 Part 2:


Relay contact table� It lists where normally open and normally closed contact of a relay

are used in a electrical circuit diagram.� In electric circuit diagram, it is usually placed below the coil of a

relay.



Terminal designation of relay


Wiring using terminal strips� In developing a control system, components are usually

grouped and placed in different cabinets.

� Terminal strips are used to connect wires going in and out of a cabinet.


� By using terminal strips, it is easy to troubleshoot wiring problems and repair them.

� Terminal allocation list table should be arranged before installing terminal strips.


Electropneumatics

Documents

test simulate

pilot controlled

electropneumatic

electropneumatic

programmable

electrical

electrical

high power