In this issue, I’ll pick up my column on signal conditioners. As I noted previously (June 2008), some of the 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.
More on Signal ConversionThere are several methods of signal conversion. I covered thermocouple, millivolt and multiple mixed inputs and outputs previously. Others include:
Three-Terminal Potentiometer to High-Level DC Signal. There are two applications here: Retransmission to the control room of the gate position on a hydro-electric station, or adjustment from the control room of a remote electropneumatic damper positioner.
Potentiometer to Pulse-Width Modulation. One application example is manual adjustment of percentage input of a heat process. The heat source could be gas, electricity or a heat transfer medium.
Strain Gauge Output to High Level DC. Examples are weight measurement, batch loading applications and pressure measurement.
Alternating Voltage and Current Inputs. AC inputs can be converted to signals representing voltage, current, power, reactive power, kVA, power factor and -- with a variable frequency output -- pulses representing watt hours per count. Monitoring and alarm annunciation functions can minimize the demand component of energy costs.
Signal Input/Output Configuration. An increasing number of models now are field configurable in respect of magnitude and type of input and output signal. This can be done by potentiometer and DIP switch adjustments; alternatively, by a PC. An LCD display is available on some models to show the process signal or as an aid to field configuration.
Rate-of-Change-of-Signal Limiter. Suppose your process cannot tolerate more than a certain rate of change of heat input, pressure, flow, etc. Your converters could offer an adjustable output ramp rate limit in the face of a wildly varying signal input.
Alarm Choices. Multiple alarm relays or open collector outputs can be specified and configured as high, low, deviation or rate-of-change alarms.
Communications. Models are available with RS232, RS485 or Ethernet communication capability, providing Internet access to process signals and alarm conditions. Remote ranging and some configuration items are also offered.
Intrinsic Safety. In this context, intrinsic safety refers to instruments and low voltage circuits designed to prevent the release of sufficient energy to ignite volatile gases. Signal conditioners are available that comply with the rules of intrinsic safety in specified hazardous atmospheres.