Sensor transmitters are measurement or signal conditioning packages that provide standard, calibrated outputs from sensors or transducers. Outputs types include current loops, variable voltage levels, frequency or pulse signals, timers or counters, relays, and variable resistance outputs. Sensor transmitters can also provide radio frequency (RF) signals and transistor-transistor logic (TTL) outputs. Current loops such as 4 – 20 mA are suitable for sending signals over long distances. Typically, a current is imposed on the output circuit proportional to the measurement. Feedback is used to provide the appropriate current regardless of line noise and impedance. Variable voltage outputs are simple, usually linear functions of the measurement. Frequency or modulated frequency outputs for sensor transmitters include amplitude modulation (AM), frequency modulation (FM), sine waves, and pulse trains.
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Types of Inputs
Sensor transmitters can receive three types of inputs: signal inputs, sensor inputs, and inputs from special devices. Signal inputs include DC voltage, DC current, AC voltage, and AC current. They also include frequency and charge, signals that often require filtering and amplification. Sensor transmitters receive sensor inputs from accelerometers, thermocouples, thermistors, and resistance temperature detectors (RTD). They also receive inputs from strain gauges, many of which use a Wheatstone bridge, and from both linear and rotary variable differential transformers (LVDT and RVDT). Inputs from special devices include signals from encoders and signal per cycle counts from counters and tachometers. Timers, clocks, relays, and switches can also provided special inputs to sensor transmitters.
Sensor transmitters differ in terms of form factor, mounting style, and user interface. Some products are integrated circuit (IC) chips that mount on printed circuit boards (PCB). Others are PCBs that attach to enclosures or plug directly into computer backplanes. Sensor transmitters that stack in modular bays or slots; and that mount on panels, chassis, racks, or DIN rails are also available. DIN is an acronym for Deutsches Institut für Normung, a German national organization for standardization. Stand alone sensor transmitters are benchtop or floor-standing units with a full casing or cabinet. For user interface options, sensor transmitters may include a touch screen, an integral front panel and display, or a hand held device that offers remote programming features. Computer programmable sensor transmitters interface to a separate supervisory or host computer.
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