Electrohydraulics Basic Level Textbook TP 601

Transcription

1 Electrohydraulics Basic Level Textbook TP 601 Festo Didactic en

2 Order No.: Edition: 03/2006 Authors: C. Löffler, D. Merkle, G. Prede, K. Rupp, D. Scholz Graphics: Doris Schwarzenberger Layout: , Beatrice Huber Festo Didactic GmbH & Co. KG, Denkendorf, Germany, 2006 Internet: 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.

3 Contents Preface Areas of application of electrohydraulics Basic control engineering terms Hydraulic and electrohydraulic control systems Advantages of electrohydraulic control systems Fundamentals of electrical technology Direct current and alternating current Ohm's Law Mode of operation of a solenoid Mode of operation of a capacitor Mode of operation of a diode Measurement in electrical circuits Components and assemblies in the electrical signal control section Power supply unit Push button and control switches Sensors for measuring displacement and pressure Relays and contactors Programmable logic controllers Overall structure of the signal control section Solenoid actuated directional control valves Exercises Design and mode of operation Designs and hydraulic performance data Performance data of solenoid coils Electrical connection of solenoid coils Design of an electrohydraulic control system Procedure for the development of a control system Procedure for the planning of a control system Application example: Design of a sawing machine Procedure for the realisation of the control system 122 Festo Didactic GmbH & Co. KG TP 601 3

4 Contents 6. Documentation of an electrohydraulic control system Function diagram Sequence table GRAFCET Hydraulic circuit diagram Electrical circuit diagram Terminal connection diagram Safety measures for electrohydraulic control systems Dangers and protective measures Effect of electric current on the human body Measures to protect against accidents with electric current Control panel and indicating elements Protecting electrical equipment against environmental impact Safety recommendations for electro-hydraulic systems Relay control systems Use of relay control systems in electrohydraulics Direct and indirect actuation Logic operations Signal storage Delay Sequence control with signal storage via double solenoid valves Circuit for evaluating control elements Sequence control system for a sawing machine 238 Index 265 Standards Festo Didactic GmbH & Co. KG TP 601

5 Preface Electrohydraulics are in use in numerous areas of industrial automation. To give a few examples, woodworking machines, machine tools, process engineering plants, presses, plastics processing machines and conveyor systems worldwide are operated using electrohydraulic control systems. However, electrohydraulic control systems are also in use in a wide variety of ways such as in mobile hydraulics which include agricultural vehicles, road construction or street cleansing. Changing requirements and technical development have distinctly changed the appearance of control systems. The relay in the signal control section increasingly has been replaced by programmable logic controllers in many areas of application in order to meet the increased need for flexibility. In the power section too, advanced electrohydraulic control systems feature new concepts adapted to the needs of industrial practice. To mention just a few keywords such as control block, bus networks and proportional hydraulics. The close interaction of fluid power engineering with microelectronics, sensors and information technology leads to numerous innovations in the area of oil-hydraulic drives and control systems. These in turn open up new fields of application to electrohydraulics with more challenging tasks in process control. As an introduction to the subject, the textbook first of all explains the design and mode of operation of the components required for the construction of an electrohydraulic control system. The following chapters describe the procedure for the planning, design and realisation of electrohydraulic control systems, using fully detailed examples. All readers of this book are invited to contribute with tips, suggestions and constructive criticism in order to improve the book. March 2006 The authors Festo Didactic GmbH & Co. KG TP 601 5

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7 1.1 Areas of application of electrohydraulics Hydraulics involves the use of hydraulic fluids. Hydraulic fluids are most commonly used to perform mechanical work. Mechanical work is necessary in order to carry out movements and generate forces. The function of hydraulic drives is to convert the energy stored in hydraulic fluid into kinetic energy. The following are used as hydraulic drives: Cylinders (linear drives) for the generation of straight-linear movements, Motors and semi-rotary actuators drives (rotary drives) to generate rotary movements. Hydraulic linear cylinder (Storz Hydrauliksysteme GmbH) and hydraulic motor (Sauer-Danfoss GmbH & Co) Hydraulic systems are used if high performance levels, excellent heat dissipation, regular movement, high switching dynamics or extremely high forces are required. Applications of electrohydraulics can be found both in stationary and mobile hydraulics. Characteristic of electrohydraulics is the receiving, processing and output of signals. The valves forming the interface to the power section are also solenoid actuated. Important branches of electrohydraulics are: Machine building, where electrohydraulic system for example are used to feed machine tools, for the generation of force for presses or in injection moulding machine for plastics processing Automotive engineering, where electrohydraulic systems are the preferred option for the actuation of construction machinery or also for the control of steering mechanisms on agricultural machinery Steel and power station construction. Here, electrohydraulic systems are used for theatre technics, lifting platforms, for the control of locks, weir gates and movable bridges Festo Didactic GmbH & Co. KG TP 601 7

8 Aircraft construction, where for example the actuation of the landing flaps and rudders is realised electrohydraulically Ship building, where for example the rudder or the cargo cranes are controlled electrohydraulically Application example In the case of modern CNC controlled machine tools, tools and workpieces are clamped hydraulically. Feed functions and spindle drives can also be hydraulically realised. Rotary indexing machine 8 Festo Didactic GmbH & Co. KG TP 601

9 1.2 Basic control engineering terms Hydraulic 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 is the task of the control system. Control engineering deals with the design and structure of control systems. The following section covers the basic terms used in control engineering. Control DIN 19226, Part 1 defines the term control as follows: 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. The distinguishing feature of open loop control is the open sequence of action via the individual transfer elements or the control chain. The term open loop control is widely used not only for the process of controlling but also for the plant as a whole. Application example In an assembly device, locating holes on sub-bases for valves are sealed with blanking plugs. The closing process is triggered via the operation of a pushbutton at the workplace. When the pushbutton is released, the piston retracts to the retracted end position. In this control, the position of the pushbutton (pushed, not pushed) is the input variable. The position of the pressing cylinder is the output variable. The loop is open because the output variable (position of the cylinder) has no influence on the input variable (position of the pushbutton). Festo Didactic GmbH & Co. KG TP 601 9

10 Assembly device for fitting caps in locating holes 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 hydraulic system The voltage at a particular point in an electrical circuit 10 Festo Didactic GmbH & Co. KG TP 601

11 Pressure 7 MPa Time Signal and information: Signal/physical variable Pointer position Time Signal and information: Information, analogue Display Pressure MPa Time Signal and information: Information, digital Festo Didactic GmbH & Co. KG TP

12 Pressure Supply pressure Yes 1 No 0 Time Signal and information: Information, binary A signal is the representation of information The representation is by means of the value or value pattern of the physical variable. The different signal types are described in DIN 19226, Part 5. Analogue signal An analogue signal is a signal in which information is assigned point by point to a continuous value pattern of the signal parameter. Application example In the case of a pressure gauge, each pressure value (information parameter) is assigned a particular display value (= information). If the signal rises or falls, the information changes continuously. Digital signal A digital signal is a signal with a finite number of value ranges of the information parameter. Each value range is assigned a specific item of information. Application example A pressure measuring system with a digital display shows the pressure in increments of 1 MPa. There are 8 possible display values (0 to 7 MPa) for a pressure range of 7 MPa. That is, there eight possible value ranges for the information parameter. If the signal rises or falls, the information changes in increments. Binary signal A binary signal is a digital signal with only two value ranges for the information parameter. These are normally designated 0 and 1. Application example A control lamp indicates whether a hydraulic system is being correctly supplied with hydraulic fluid. If the supply pressure (= signal) is below 5 MPa, the control lamp is off (0 status). If the pressure is above 5 MPa, the control lamp is on (1 status). 12 Festo Didactic GmbH & Co. KG TP 601

13 Classification of controllers by type of information representation Controllers can be divided into different categories according to the type of information representation, into analogue, digital and binary controllers. Controllers Binary controllers Analogue controllers Digital controllers Classification of controllers by type of information representation Logic controller A logic controller generates output signals through logical association of input signals. Application example The assembly device for sub-bases is extended so that it can be operated from two positions. The two output signals are linked. The piston rod advances if either pushbutton 1 or 2 is pressed or if both are pressed. Sequence controller A sequence controller is characterized by its step by step operation. The next step can only be carried out when certain criteria are met. Application example Drilling station. The first step is clamping of the workpiece. As soon as the piston rod of the clamping cylinder has reached the forward end position, this step has been completed. The second step is to advance the drill. When this motion has been completed (piston rod of drill feed cylinder in forward end position), the third step is carried out, etc. Festo Didactic GmbH & Co. KG TP

14 Signal flow in a control system A control system can be divided into the functions signal input, signal control, signal output and command execution. The mutual influence of these functions is shown by the signal flow diagram. Signals from the signal input are logically associated (signal control). Signals for signal input and signal process are low power signals. Both functions are part of the signal control section. At the signal output stage, signals are amplified from low power to higher power. Signal output forms the link between the signal control section and the power section. Command execution takes place at a high power level that is, in order to move heavy loads (e.g. a lock gate) or to exert a high force (such as for a press). Command execution belongs to the power section of a control system. Command execution Power section Signal output Signal processing Signal control section Signal input Signal flow in a control system The components in the circuit diagram of a purely hydraulic control system are arranged so that the signal flow is clear. Bottom up: input elements (such as manually operated valves), logical association elements (such as shuttle valves), signal output elements (power valves, such as 4/2-way valves) and finally command execution (such as cylinders). 14 Festo Didactic GmbH & Co. KG TP 601

15 1.3 Hydraulic and electrohydraulic control systems Hydraulic and electrohydraulic control systems both exhibit a hydraulic power section. However, the signal control section is constructed differently. In the case of hydraulic control systems, this section is mainly carried out manually. It is rare for the signal control to be effected by means of a hydraulic circuit, which then comprises for example shuttle valves. In the case of an electrohydraulic control system, the signal control section is constructed using electrical components, which include for example electrical input keys, proximity sensors, pressure switches, relays or a programmable logic controller. In the case of both types of control, the directional control valves form the inteface between the signal control section and the hydraulic power section. Command execution Power component Cylinder Swivel cylinder Hydraulic motors Optical displays Hydraulic power section Signal output Final control elements Directional control valves Signal processing Signal input Processing elements Directional control valves Shuttle valves Pressure control valves Input elements Directional control valves (mechanically operated) Directional control valves (manually operated) Hydraulic signal control section Signal flow Hydraulic components Signal flow and components of a hydraulic control system Festo Didactic GmbH & Co. KG TP

16 Command execution Power components Cylinder Swivel cylinder Hydraulic motors Optical displays Hydraulic power section Signal output Final contol elements Electropneumatically operated directional control valves Signal processing Signal input Processing elements Relays Contactors Programmable logic controllers (PLCs) Input elements Pushbuttons Control switches Limit switches Reed switches Ind. proximity sensors Cap. proximity switches Light barriers Pressure-actuated switches Electrical signal control section Signal flow Electrohydraulic components Signal flow and components of an electrohydraulic control system 16 Festo Didactic GmbH & Co. KG TP 601