Author: admin
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12.9 Human-Machine Interfaces in PLC
Programmable logic controllers are built to input various signal types (discrete, analog), execute control algorithms on those signals, and then output signals in response to control processes. By itself, a PLC generally lacks the capability of displaying those signal values and algorithm variables to human operators. A technician or engineer with access to a personal…
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12.4 Ladder Diagram (LD) programming
In the United States, the most common language used to program PLCs is Ladder Diagram (LD), also known as Relay Ladder Logic (RLL). This is a graphical language showing the logical relationships between inputs and outputs as though they were contacts and coils in a hard-wired electromechanical relay circuit. This language was invented for the express purpose of making…
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12.3 Logic programming for PLC’s
Although it seems each model of PLC has its own idiosyncratic standard for programming, there does exist an international standard for controller programming that most PLC manufacturers at least attempt to conform to. This is the IEC 61131-3 standard, which will be the standard presented in this chapter. One should take solace in the fact…
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12.2 Input/Output (I/O) capabilities of PLC’s
Every programmable logic controller must have some means of receiving and interpreting signals from real-world sensors such as switches, and encoders, and also be able to effect control over real-world control elements such as solenoids, valves, and motors. This is generally known as input/output, or I/O, capability. Monolithic (“brick”) PLCs have a fixed amount of I/O capability…
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Chapter 12 Programmable Logic Controllers
Every control system may be divided into three general sections: input devices (sensors), controllers, and output devices (actuators). The input devices sense what is happening in the process, the controller decides what to do about it, and the output devices manipulate the process to achieve the desired result. A programmable logic controller or PLC is a general-purpose controller, applicable to many different types of…
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11.3 Interposing relays
In addition to directly performing logic functions, electromechanical relays may also be used as interposing devices between mismatched sensors, controllers, and/or control devices. A very simple example of a relay used to interpose between mismatched devices is shown in the following circuit diagram, where a delicate toggle switch is used to control a bank of high-power lights…
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11.2 Relay circuits
Electromechanical relays may be connected together to perform logic and control functions, acting as logic elements much like digital gates (AND, OR, etc.). A very common form of schematic diagram showing the interconnection of relays to perform these functions is called a ladder diagram. In a “ladder” diagram, the two poles of the power source are…
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Chapter 11 Relay control systems
The word “discrete” means individual or distinct. In engineering, a “discrete” variable or measurement refers to a true-or-false condition. Thus, a discrete control system is one designed to operate on Boolean (“on” or “off”) signals supplied by discrete sensors such as process switches. A form of discrete control taught in every introductory course on digital electronics involves the…
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10.4 On/off electric motor control circuits
An electric motor is often used as a discrete control element in a control system if driving a pump, conveyor belt, or other machine for the transportation of a process substance. As such, it is important to understand the functioning of motor control circuits. Of all the available electric motor types, the most common found…
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10.3 Solenoid valves
A very common form of on/off valve used for pneumatic and hydraulic systems alike is the solenoid valve. A “solenoid” is nothing more than a coil of wire designed to produce a magnetic field when energized. Solenoid actuators work by attracting a movable ferrous armature into the center of the solenoid coil when energized, the force of this…
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Chapter 10 Discrete control elements
The word “discrete” means individual or distinct. In engineering, a “discrete” variable or measurement refers to a true-or-false condition. Thus, a discrete control element is one that has but a limited number of states (usually two: on and off). In the case of valves, this means a valve designed to operate either in “open” mode or “closed” mode,…
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9.8 Flow switches
A flow switch is one detecting the flow of some fluid through a pipe. Flow switches often use “paddles” as the flow-sensing element, the motion of which actuates one or more switch contacts. Recall from section 9.1 that the “normal” status of a switch is the resting condition of no stimulation. A flow switch will be in its…
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9.7 Temperature switches
A temperature switch is one detecting the temperature of some substance. Temperature switches often use bimetallic strips as the temperature-sensing element, the motion of which actuates one or more switch contacts. An alternative design uses a metal bulb filled with a fluid that expands with temperature, causing the switch mechanism to actuate based on the pressure this…
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9.6 Level switches
A level switch is one detecting the level of liquid or solid (granules or powder) in a vessel. Level switches often use floats as the level-sensing element, the motion of which actuates one or more switch contacts. Recall from section 9.1 that the “normal” status of a switch is the resting condition of no stimulation. A level switch will be…
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9.5 Pressure switches
A pressure switch is one detecting the presence of fluid pressure. Pressure switches often use diaphragms or bellows as the pressure-sensing element, the motion of which actuates one or more switch contacts. Recall from section 9.1 that the “normal” status of a switch is the resting condition of no stimulation. A pressure switch will be in its “normal” status…