Choosing Signal Conditioning and getting it right !
Lawrence Farrant, product engineer at Status Instruments looks at some of the challenges facing process engineers with regard to signal comparison and how these can be addressed with the correct choice of signal conditioner.
W ith so many types of sensor available for monitoring processes - including temperature, level, flow, pressure, position, light, and so on - the list of applications which use signal conditioning is endless.
Any industry, heavy or light, from farming to power generation, will make use of signal conditioning units. Each industry will, however, present its own set of requirements and problems.
Some applications will require the comparison of signals and an output based upon the result. For example, the difference between input and output of oil flow to a burner will be the amount consumed. While this is normally done with the use of complex controllers or PLCs, advances in the functionality of signal splitters and multi-channel conditioners now mean some of this work can be carried out by the conditioning block itself.
As an example, Status Instrument's rail mounted SEM1700 range of single and dual channel conditioning and splitter products can be used to compare two temperature signals
Although basic signal conditioning has been used for many years, it can still present a challenge to the process engineer. There are, in fact, several things to be considered in any system that needs to transmit analogue signals representing engineering values from a measurement point to a monitoring or control point.
Many issues will affect how a signal is conditioned - from the mismatch of inputs to outputs, to the environment in which the system is located:
The output signal type and level from process sensors can cause problems in being matched to the input requirements for controllers or PLCs. It may be that a sensor with a (o to lo) mA output will need to be connected to a controller with a(o to io) V input, or even a different mA range. Matching the signals is the primary requirement of any signal conditioner.
Isolation of equipment inputs/outputs on the same control loop may be required if ground loops that can cause signal errors are to be avoided.
Ranging and scaling signals where sensor outputs are not in the same engineering unit or engineering unit multiple, and potential re-ranging if process conditions change, are potential pitfalls.
If the working parameters for a process change - if, for example, a scaled level sensor needs to be re-located or changed - can the conditioning block be easily re-configured to the new input settings?
A flow sensor representing (o to loo) I/min may need to be connected to a data logger needing an input representing (o to 50) I/min. If both are using a (4 to 20) mA signal, how will the conditioning unit cope with the mismatch?
Signal integrity is also important as a loss of signal or, what can be worse, an incorrect reading, may cause a control system to fail or run outside of limits. Integrity can be affected by many factors from inside and outside the control system, including physical ambient conditions, length of cable run and electrical interference. A robust system is therefore essential in maintaining signal fidelity.
Isolation between sensors that may become live in a fault condition can be a safety concern in some applications, so a conditioner with a suitable rating can be a major factor in choosing the correct unit for some engineers.
CHOOSING THE RIGHT EQUIPMENT
With so many factors to assess, sometimes more than one conditioner may be required. But, while a single unit that can fulfil all of the requirements can be very useful, such a sensor can be complex and expensive.
Due to the ever-growing number of sensor types and manufacturers producing them, it is important that process engineers choose equipment that can be easily interfaced. Controllers, PLCs, building management systems and displays will all need to be integrated to work with each other in many different combinations.
This is why most sensors, or the transducer the sensor may connect to, will look to convert its particular engineering value into something that can be interfaced with the vast majority of control systems on the market. To be practical, these normally accept only a limited number of inputs. The industry `standards' for analogue signal transmission are within the ranges of (0 to 20) mADC or ±(o to io) VDC.
Flexibility in modern signal conditioning technology is helping the process engineer to reduce the number of units required by making them multi-purpose, and easy and quick to commission. Many of the problems presented to the process engineer can be solved by the correct choice of signal conditioning unit.
Status Instruments' rail mounted SEMi7oo range of single and dual channel conditioning and splitter products has been designed with a view to overcoming many of the common problems associated with signal conditioning. Its worldwide power supply operating from 24VDC all the way up to mains voltage, 3k75V isolation between inputs and outputs, and a wide range of input and output configuration, gives the user flexibility in the field.
The dual channel unit can be used in two modes. In simple mode only the basic features of a signal conditioner/splitter need to be considered. In advanced mode the functionality offers users non lin, signal damping and system diagnostic tools, and other options including recording are available. Set up is achieved using FOC software and a standard USB lead.
From an article by Lawrence Farrant of Status Instruments Ltd.
Tel: +44(0)1684 296818
Website : www.status.co.uk
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