Bridge conditioners are instruments that provide excitation and support for strain gages, Wheatstone bridges, load cells and sensors. They also include circuitry for signal conditioning, amplification, and processing. Bridge conditioners provide four-way galvanic isolation between their input, output, excitation, and power supply circuits. Electrical isolation breaks the galvanic path between the input and output signals. Physical wiring between the input and output is not required. Normally, the input is transferred to the output by converting it to an optical or magnetic signal that is reconstructed on the output. In this way, bridge conditioners prevent unwanted signals from traveling along the input line.
Bridge conditioners 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. In terms of sensors, current loop converters receive inputs from accelerometers, thermocouples, thermistors, and resistance temperature detectors (RTDs). They also receive inputs from strain gauges, many of which use a Wheatstone bridge, and from both linear and rotary variable differential transformers (LVDTs and RVDTs). Inputs from special devices include not only signals from encoders, but also signal per cycle counts from counters or tachometers. Timers, clocks, relays, and switches can also provided special inputs to bridge conditioners.
There are several filter architectures for bridge conditioners. Analog filters are designed with resistors and capacitors and are used in applications with low noise requirements. Digital filters are designed with solid-state components and include both finite impulse response (FIR) and infinite impulse response (IIR) filters. Filter functions are often programmable and may include anti-alias features that correct the signal misrepresentation that occurs during slow sampling. Low pass and high pass filters block or attenuate signals at frequencies above or below a specified cutoff frequency. Band pass filters block or attenuate signals at frequencies outside of the low pass and high pass cutoff frequencies. Band stop frequencies block only a portion of the frequency spectrum while all-pass filters allow all frequencies to pass.
Bridge conditioners provide several types of outputs. Some devices provide variable levels of voltage or current, or output frequencies or pulse signals. Other devices provide timer, counter, relay, and resistance outputs. Bridge conditioners that include integral amplifiers vary in terms of programmable gain, gain range, maximum output, and bandwidth. Amplifier gain, the factor by which the input signal is multiplied, can be adjusted from a touch screen, a front panel display, or a computer monitor. Some advanced bridge conditioners include application software for real-time monitoring and mathematical and statistical functions. Bridge conditioners that interface to computers use a variety of communications protocols, including Ethernet, DeviceNet, and IEEE 1394.
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Current Loop Converters
Current loop converters convert an analog or digital signal to a current loop output such as 4-20 mA or 0-20 mA.
Current-to-voltage converters scale and convert current signal input to the desired output voltage range.
Data Input Modules
Data input modules accept sensor and other signal output for data acquisition systems. They may include signal conditioning prior to the analog-to-digital conversion stage.
Data Output Modules
Data output modules or cards transfer amplified, conditioned, or digitized signals.
LVDT and RVDT Signal Conditioners
Linear variable differential transformer (LVDT) and rotary variable differential transformer (RVDT) signal conditioners are devices that translate the linear or rotary movement of a ferromagnetic armature into an AC voltage that is proportional to the armature position.
Sensor transmitters are measurement or signal conditioning packages that provide a standard, calibrated output from a sensor or transducer in the form of a current loop output (e.g., 4-20 mA).
Voltage Converters and Voltage Inverters
Voltage converters and voltage inverters accept voltage input and provide a scaled voltage output. Conversion types include scaling up a low-level signal, voltage doubling, and inversion (converting a positive voltage to negative and vice versa).