When you are planning or modifying your process control and monitoring system, the odds are you will be reaching for your inventor's hat. This is because there are always some functions that cannot be performed by the normal control system, or there may be unforeseen requirements that need an extra device of some kind.
Look round your process and you might see small hockey-puck shaped components on the heads or inside the covers of temperature sensors. You might find others in a different package around the plant and inside control enclosures, mounted on a bulkhead or on a DIN rail. There is a good chance that these are signal conditioners, doing jobs that complement that of your main control system. These are essential items in the inventor's tool kit.
They are stand-alone components whose internal circuitry usually is powered by the normal 115 V AC line or by a 24 V DC supply. They can alternatively be loop powered, which means that the power can come from an incoming or an outgoing 4 to 20 mA circuit driven by its own DC power supply.
Signal conditioners have at least one pair of input and one pair of output terminals. The power supply pair can be omitted if you take loop power from a 4 to 20 mA input or output signal. Outside that, you can have a rich variety of features that can only be briefly covered in this column. Call or surf your instrumentation supplier for specific details and application examples.
Input/Output Options and How They Are AppliedSignal Isolators. Here, the conditioner may come as a millivolt in and voltage out model, with isolation, adjustable gain and offset, high input impedance and a robust low impedance output signal.
A current in/current out isolator also would 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 V DC and 4 to 20 mA DC. Five-way isolation applies between input, output, power supply, relay contacts and ground. The isolation is commonly designed to withstand 700 Vrms AC and 1,000 V peak.
Another option is inversion, whereby the output increases 0 to 100% as the input decreases 100 to 0% of working range. This could, for example, make a reverse-acting control loop into direct-acting. Gain and offset adjustments usually are provided. This would allow you, for example, to adjust the working range of a control valve. Major benefits of isolation are reliable operation in electrically noisy plants and the elimination of common-mode and ground-loop problems.
Signal Conversion. There are several device configurations to consider.
Thermocouple to DC Milliamp with Isolation. This gives a robust, interference-free signal and enables use of copper extension cable in place of the more expensive and higher resistance thermocouple extension cable. Two things to bear in mind: First, most low-cost models come with only one choice out of a selection of the most popular thermocouples. This would be factory configured and not field configurable. Second, the output signal would conform to the same nonlinear law as the thermocouple. This means that you must take account of the inaccuracy if you use a linear receiving indicator or recorder.
Millivolt, Volt and Milliamp Signal Scaling. The gain function may be selected to be linear, follow a mathematical function or follow a custom curve. A square root function requirement is common when handling flow signals. A custom curve would be one way to linearize a grossly nonlinear final control element that otherwise would harm control stability.
Some models have multiple inputs that can be manipulated mathematically to form the output.
V/f Converters. The output of voltage-to-frequency (V/f) and frequency-to-voltage converters can be monitored over great distances over a telephone line then easily reconverted to represent a process measurement at the receiving end using an f/V converter. Models are available with multiple and mixed inputs and outputs.
In one case, a BTU/hr measurement was required, calculated from mass flow rate times the temperature differential. 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 totallized as BTUs on an electric counter.
Later versions of conditioner can derive the same two results using only one conditioner with multiple inputs and outputs.
Three-Terminal Potentiometer to High Level DC. I'll mention two applications here: Retransmission to control room of gate position on a hydroelectric station. Manual adjustment of a remote electropneumatic damper positioner.
Potentiometer to Pulse-Width Modulation. Manual adjustment of percentage power on a contactor-controlled electric heater.
Strain Gage to High Level DC. Signal conditioners have been used for weight measurement and batch loading.
Alternating Voltage and Current Inputs. Signal conditioners have been used to convert signals representing power, reactive power, kVA and power factor, or for monitoring and alarm annunciation to minimize the demand component of energy costs.
Signal Input/Output Configuration. An increasing number of models are field configurable in respect of magnitude and type of input and output signal. This can be done by manual adjustments and DIP switches or by a PC. The upmarket models linearize thermocouple and RTD signals. An LCD display is available on some models to show the process signal or as an aid to field configuration.
Alarm Choices. Multiple alarm relays or open collector outputs can be specified and configured as high, low, deviation or rate of change of signal.
Communications. Models are available with RS232 or RS485 communication capability, providing Internet access through a browser to process signals and alarm conditions. Remote ranging and some configuration items are also possible through your browser.