2261 Introduction

control mode, this There are many available input and output devices,

Figure 22.6 Proportional-Integral-Derivative Control Mode Diagram
Figure 22.7 Sequencing with Dead Band

serving many basic and specific needs, along with ranges of quality and other features as required for the job, and the Controls Application Engineer quickly becomes familiar with many of these in great detail. The Energy Professional may choose to delve into the sea of products, but can also be very effective by providing only performance-based generic requirements and managing the project from a more macro view.

For input and output devices, there are basic distinctions between Transducers, Switches, Sensors, and Transmitters which are useful to understand.

• Transducers: These are the core of any instrument, and are used to convert the basic physical phenomena of interest into a form more useful to the instrument.


— Temperature: bimetal coil, two-phase gas bellows, thermistor, RTD, etc.

— Pressure: Bourdon tube, diaphragm, strain gage, etc.

The term 'transducer' is also commonly applied to output signal form changing equipment, such as converting an analog electronic signal to a corresponding pneumatic signal for use by a pneumatic actuator.

• Switches: Devices that can have two states (on-off, open-close) etc., used for regulating on-off electrical circuits as inputs or to actuate other equipment or devices in a two-position manner. Solenoid valves, relays, etc. are in this class of instruments.

• Sensors: These are "passive" input devices that can be read directly by the controller with no signal conditioning. They are usually limited to short cable lengths and further limited to transducers with inherent linear outputs.


— RTD (resistance temperature detector) that can be wired and read directly to the controller.

• Transmitters: These are the typical analog input instrument. A transducer is coupled with some form of pneumatic or electronic signal conditioning. The transmitter output is a linear, standard value easily decoded as an input. Often, the signal conditioning allows for stable signal over a large distance to allow remote location of the device, hence the term 'transmitter.'


0-100 degF 0-100 degF 0-100 degF 4-20mA output

^ 3-15 psig ^ 4-20 mA (milliamps) ^ 0-10 VDC (Volts DC) ^ 3-15 psig

0 0

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