Signal Conditioners Revisited
June 5, 2008
Some little boxes, wired or attached to parts of your process, could be signal conditioners. They all have inputs and outputs and commonly perform some functions not being done by the main control system. They may have been added as new control requirements show up.
Some look like hockey-pucks, freestanding or attached to the head of a temperature or some other kind of sensor. Others, in a different package, can be found around the plant -- inside control enclosures, mounted on a bulkhead. Others called DIN-rail mounted are snapped onto a DIN rail and lined up like books on a shelf.
An abbreviation for Deutsches Institut für Normung (German Institute for Standardization), DIN has pioneered many industrial standards. Those for compact plug-in mounting and rectangular panel instruments have gained worldwide adoption, aiding interchangeability and reducing panel fabrication costs.
The internal circuitry of signal conditioners usually is powered by the normal 115 VAC line or by a 24 VDC supply. Alternatively, they can be loop powered, which means that the power for the device has to come from an incoming or outgoing 4 to 20 mA process signal driven by its own DC power supply. Outside that, you can have a rich variety of features and signal types. Call or surf your instrumentation supplier for specific details and application examples.
The simplest converters have two input and two output terminals.
Typical input signals include
- AC or AC voltage or current, derived from temperature or other sensors.
- Variable frequency AC or pulsed DC voltages.
- Resistance from a platinum resistance thermometer, strain gauge bridge or three-terminal potentiometer.
Typical output signals include:
- DC or AC voltage or current.
- Relay contact or logic for alarm.
- Other switching functions.
Signal Isolators. Any of these conditioners can come with isolation up to 2kV. A current-in/current-out isolator also could offer adjustable gain and offset. It would have near zero input impedance (current sink) and very high output impedance (current source).
Voltage/current and current/voltage isolators also are available. De facto standard process signals are 0 to 10 VDC and 4 to 20 mA DC. Five-way isolation applies between input, output, power supply, relay contacts and ground. The isolation is designed to withstand 700 Vrms AC and 1,000 V peak. Some models can withstand up to 4,000 V peak. Major benefits of isolation are reliable operation in electrically noisy plants and the elimination of common-mode and ground-loop problems.
Another option is inversion, whereby the output increases from 0 to 100 percent as the input decreases from 100 to 0 percent of working range. This could, for example, convert a reverse-acting control loop into direct-acting. (Gain and offset adjustments usually are provided.) For example, this would allow you to adjust the working range of a control valve.
Signal ConversionThermocouple Input to DC mA Output with Isolation and Linearization of the Thermocouple Curve. This provides a robust, interference-free output signal and enables the use of copper cable in place of the more expensive thermocouple extension cable. (Remember that a thermocouple cable can not be re-used for any other purpose.)
Volt, mV and mA Signal Scaling. The gain function may be selected to be linear; to follow a mathematical function; or to follow a custom curve. A square root function requirement is common when handling flow signals. A custom curve could be used in two ways:
- To linearize a grossly non-linear final control element that otherwise would degrade control stability.
- To linearize the signal from an uncommon non-linear sensor.
Math Functions. Some models have multiple inputs that can be manipulated mathematically to form the output.
Voltage-to-Frequency (V/f) and Frequency-to-Voltage Converters. The output of V/f converters can be monitored over great distances using a telephone line, and then easily reconverted to represent a process measurement at the receiving end using an f/V converter. V/f converters have a place in variable-frequency motor controller applications.
Multiple and Mixed Inputs and Outputs. In one case, a BTU/hr measurement was required and calculated using the following method. Start with the equation: mass flow rate x (T2 – T1). Inlet and outlet temperatures T1 and T2 were taken from two RTDs into a three-input conditioner. The third input was a frequency signal representing flow from a mass flowmeter. The temperature difference, T2 – T1, was obtained, then multiplied by the frequency signal. The resulting DC output was scaled to show BTU/hr.
A second conditioner took in the DC BTU/hr signal and converted it to a frequency, where each cycle represented a fixed number of BTUs. The frequency was suitably scaled and totalized as BTUs on an electric counter.
Later versions of a signal conditioner can derive the same two results using only one conditioner with multiple inputs and outputs.
I’ll have more on signal conditioners in August.