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Production, assembly and packaging systems worldwide are driven by electropneumatic control systems. The change in requirements together with technical advances have had a considerable impact on the appearance of controls. In the signal control section, the relay has increasingly been replaced by the programmable logic controller in order to meet the growing demand for more flexibility.
Modern electropneumatic controls also implement new concepts in the power section to meet the needs of modern industrial practice. Examples of this are the valve terminal, bus networking and proportional pneumatics.
In introducing this topic, this textbook first looks at the structure and mode of operation of the components used for setting up an electropneumatic control. The following chapters then look at the approach to project planning and the implementation of electropneumatic controls using fully worked examples. Finally, the last chapter looks at trends and developments in Electropneumatics.
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November 1997 The Authors TP201 Festo Didactic 5 Chapter 1 Chapter 1 Introduction Festo Didactic TP201 6 Chapter 1 1. 1 Applications of pneumatics Pneumatics deals the use of compressed air. Most commonly, compressed air is used to do mechanical work – that is to produce motion and to generate forces. Pneumatic drives have the task of converting the energy stored in compressed air into motion. Cylinders are most commonly used for pneumatic drives.
The following section covers the basic terms used in control engineering. Control (DIN 9226, Part 1) 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 controlling 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 A device closes metal cans with a lid. The closing process is triggered by 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). TP201 Festo Didactic 9 Chapter 1 Fig. 1. : Assembly device for mounting lids on cans 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 ? The pressure at a particular point in a pneumatic system voltage at a particular point in an electrical circuit Festo Didactic TP201 10 Chapter 1 Fig. 1. 4: Signal and information TP201 Festo Didactic 11 Chapter 1 A signal is the representation of information The representation is by means of the value or value pattern of the physical variable.
An analog signal is a signal in which information is assigned point by point to a continuous value range of the signal parameter (DIN 19226, Part 5). 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. 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 (DIN 19226, Part 5). A pressure measuring system with a digital display shows the pressure in increments of 1 bar.
There are 8 possible display values (0 to 7 bar) for a pressure range of 7 bar. That is, there eight possible value ranges for the information parameter. If the signal rises or falls, the information changes in increments. A binary signal is a digital signal with only two value ranges for the information parameter. These are normally designated o and 1 (DIN 19226, Part 5). A control lamp indicates whether a pneumatic system is being correctly supplied with compressed air. If the supply pressure (= signal) is below 5 bar, the control lamp is off (0 status).
If the pressure is above 5 bar, the control lamp is on (1 status). Analog signal Application example Digital signal Application example Binary signal Application example Festo Didactic TP201 12 Chapter 1 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 (DIN 19226, Part 5). Fig. 1. 5: Classification of controllers by type of information representation Logic controller Application example
A logic controller generates output signals through logical association of input signals. The assembly device in Fig. 1. 3 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. A sequence controller is characterized by its step by step operation. The next step can only be carried out when certain criteria are met. 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. Sequence controller Application example TP201 Festo Didactic 13 Chapter 1 A controller can be divided into the functions signal input, signal processing, signal output and command execution. The mutual influence of these functions is shown by the signal flow diagram. ?Signals Signal flow in a control system from the signal input are logically associated (signal processing).
Signals for signal input and signal process are low power signals. Both functions are part of the signal control section. the signal output stage, signals are amplified from low power to high power. Signal output forms the link between the signal control section and the power section. execution takes place at a high power level – that is, in order to reach a high speed (such as for fast ejection of a workpiece from a machine) or to exert a high force (such as for a press). Command execution belongs to the power section of a control system. Fig. 1. 6: Signal flow in a control system ?At ?Command
The components in the circuit diagram of a purely pneumatic controller are arranged so that the signal flow is clear. Bottom up: input elements (such as manually operated valves), logical association elements (such as two-pressure valves), signal output elements (power valves, such as 5/2-way valves) and finally command execution (such as cylinders). Festo Didactic TP201 14 Chapter 1 1. 3 Pneumatic and electropneumatic control systems Both pneumatic and electropneumatic controllers have a pneumatic power section (See Fig. 1. 7 and 1. 8). The signal control section varies according to type. In a pneumatic control pneumatic components are used, that is, various types of valves, sequencers, air barriers, etc. an electro-pneumatic control the signal control section is made up of a electrical components, for example with electrical input buttons, proximity switches, relays, or a programmable logic controller. The directional control valves form the interface between the signal control section and the pneumatic power section in both types of controller. Fig. 1. 7: Signal flow and components of a pneumatic control system TP201 Festo Didactic 15 Chapter 1 Fig. 1. : Signal flow and components of an electropneumatic control system Festo Didactic TP201 16 Chapter 1 In contrast to a purely pneumatic control system, electropneumatic controllers are not shown in any single overall circuit diagram, but in two separate circuit diagrams – one for the electrical part and one for the pneumatic part.
For this reason, signal flow is not immediately clear from the arrangement of the components in the overall circuit diagram. Structure and mode of operation of an electropneumatic controller Fig 1. 9 shows at the structure and mode of operation of an electropneumatic controller. The ? The ? The electrical signal control section switches the electrically actuated directional control valves. directional control valves cause the piston rods to extend and retract. position of the piston rods is reported to the electrical signal control section by proximity switches.
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