Category: Education
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16.3 H1 FOUNDATION Fieldbus Data Link layer
Like so many other industrial data networks, FOUNDATION Fieldbus is an “unswitched” or “broadcast” type of network. This means all data transmissions by all devices on a network are sensed by all the other devices. In other words, there are no private messages between two devices on a shared network: every device “hears” every transmission…
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16.2 H1 FOUNDATION Fieldbus Physical layer
Layer 1 of the OSI Reference Model is where we define the “physical” elements of a digital data network. The H1 FF network exhibits the following properties: Two-wire (ungrounded) network cable 100 ohm (typical) characteristic impedance DC power is conveyed over the same two wires as digital data 31.25 kbps data rate Differential voltage signaling…
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Chapter 16 FOUNDATION Fieldbus instrumentation
FOUNDATION Fieldbus is a standard for digital field instrumentation enabling field instruments to not only communicate with each other digitally, but also to execute all continuous control algorithms (such as PID, ratio control, cascade control, feedforward control, etc.) traditionally implemented in dedicated control devices. In essence, FOUNDATION Fieldbus extends the general concept of a distributed control…
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Chapter 15 Digital Data Acquisition and Networks in Control Systems
The advent of digital electronic circuitry has brought a steady stream of technological progress to industrial instrumentation. From early applications of digital computing in the 1960’s to the first distributed control systems (DCS) in the 1970’s to the “smart” transmitter revolution of the 1980’s, digital technology has expanded on the capabilities and information-sharing ability of…
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14.6 Proper care and feeding of pneumatic instruments
Perhaps the most important rule to obey when using pneumatic instruments is to maintain clean and dry instrument air. Compressed air containing dirt, rust, oil, water, or other contaminants will cause operational problems for pneumatic instruments. First and foremost is the concern that tiny orifices and nozzles inside the pneumatic mechanisms will clog over time. Clogged orifices tend…
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14.5 Analysis of practical pneumatic instruments
To better understand the design and operation of self-balancing pneumatic mechanisms, it is helpful to examine the workings of some actual instruments. In this section, we will explore three different pneumatic instruments: the Foxboro model 13A differential pressure transmitter, the Foxboro model E69 I/P (electro-pneumatic) transducer, the Fisher model 546 I/P (electro-pneumatic) transducer, and the…
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14.4 Comparison of Op-Amp Circuits With Analogous Pneumatic Mechanisms
Self-balancing pneumatic instrument mechanisms are very similar to negative-feedback operational amplifier circuits, in that negative feedback is used to generate an output signal in precise proportion to an input signal. This section compares simple operational amplifier (“opamp”) circuits with analogous pneumatic mechanisms for the purpose of illustrating how negative feedback works, and learning how to…
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14.3 Pilot valves and pneumatic amplifying relays
Self-balancing mechanisms consisting solely of a baffle/nozzle detector coupled to a feedback bellows, while functional, are not always practical as field instruments. Nozzles and orifices must be made rather small in diameter in order to minimize compressed air usage4 , but this means the mechanism will require significant time to alter its output pressure (i.e. to…
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14.2 Self-balancing pneumatic instrument principles
A great many precision instruments use the principle of balance to measure some quantity. Perhaps the simplest example of a balance-based instrument is the common balance-beam scale used to measure mass in a laboratory: A specimen of unknown mass is placed in one pan of the scale, and precise weights are placed in the other pan until…
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14.1 Pneumatic sensing elements
Most pneumatic instruments use a simple but highly sensitive mechanism for converting mechanical motion into variable air pressure: the baffle-and-nozzle assembly (sometimes referred to as a flapper-and-nozzle assembly). A baffle is nothing more than a flat object obstructing the flow of air out of a small nozzle by close proximity: The physical distance between the baffle and the nozzle…
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Chapter 14 Pneumatic instrumentation
While electricity is commonly used as a medium for transferring energy across long distances, it is also used in instrumentation to transfer information. A simple 4-20 mA current “loop” uses direct current to represent a process measurement in percentage of span, such as in this example: The transmitter senses an applied fluid pressure from the process…
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13.7 Troubleshooting current loops
A fundamental principle in instrumentation system troubleshooting is that every instrument has at least one input and at least one output, and that the output(s) should accurately correspond to the input(s). If an instrument’s output is not properly corresponding to its input according to the instrument’s design function, there must be something wrong with that…
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13.6 4-wire “passive” versus “active” output transmitters
Some self-powered (4-wire) analog electronic transmitters are designed to behave as electrical loads rather than as electrical sources. Such transmitters are commonly referred to as having passive or sinking 4-20 mA outputs, as opposed to the active or sourcing 4-wire transmitters previously described: At first this seems needlessly confusing. Why build a self-powered transmitter requiring a second power supply in the circuit to…
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13.5 2-wire (“loop-powered”) transmitter current loops
It is possible to convey electrical power and communicate analog information over the same two wires using 4 to 20 milliamps DC, if we design the transmitter to be loop-powered. A loop-powered transmitter connects to a process controller with only two wires, which is why loop-powered transmitters are synonymously known as 2-wire transmitters: Here, the transmitter is not really…
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13.4 4-wire (“self-powered”) transmitter current loops
DC electric current signals may also be used to communicate process measurement information from transmitters to controllers, indicators, recorders, alarms, and other input devices. Recall that the purpose of a transmitter is to sense some physical variable (e.g. pressure, temperature, flow) and then report that quantity in the form of a signal, in this case a 4…