Category: Education
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13.3 Controller output current loops
The simplest form of 4-20 mA current loop is the type used to represent the output of a process controller, sending a command signal to a final control element. Here, the controller supplies both the electrical power and signal information to the final control element, which acts as an electrical load. To illustrate, consider the…
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13.2 Relating 4 to 20 mA signals to instrument variables
A 4 to 20 mA current signal represents some signal along a 0 to 100 percent scale. Usually, this scale is linear as shown by this graph: Being a linear function, we may use the standard slope-intercept linear equation to relate signal percentage to current values: Where, y = Output from instrument x = Input to instrument…
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Chapter 13 Analog electronic instrumentation
An “analog” electronic signal is a voltage or current proportionate to the value of some physical measurement or control quantity. An instrument is often classified as being “analog” simply by virtue of using an analog signal standard to communicate information, even if the internal construction and design of the instrument may be mostly digital in…
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12.10 How to teach yourself PLC programming
First and foremost, you need to get your very own PLC to work with. Computer programming of any kind is not a spectator sport, and can only be learned by significant investment of time and effort at the keyboard. In many ways, learning to program is like learning a new spoken or written language: there…
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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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12.3 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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11.2 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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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…