Signal conditioning units bring low cost flexibility

Paul Boughton

Paul Wade reports reports on a new range of DIN rail signal conditioning modules.

Despite the rapid growth in the implementation of sophisticated oftware-based control systems, today's process industries still rely heavily on primary measuring sensors and associated conditioning electronics to provide accurate and reliable data from the processes. In many cases, straightforward control and measurement requirements in today's process industries are typically solved using over-engineered and expensive PLC solutions.
In contrast, Status Instruments' MEDACS range of DIN rail signal conditioning modules offer a high degree of functionality and configurability, allowing them to be used to solve a growing number of process control applications in a very cost effective and expedient manner.
MEDACS signal conditioners offer a new concept in front end conditioning which can be used to interface to a wide range of industrial sensors. Conventional signal conditioners connect to a sensor and measure the sensor parameter, converting this to an industry standard output such as 4-20mA. However, MEDACS can take the sensor inputs and then modify them to provide outputs with special functionality. The concept is aimed at specifiers of electronic equipment in large processing plants, system integrators and specialist panel builders.
Units can handle most common sensor inputs including mV, mA, temperature and frequency and, in addition, have two isolated discrete digital inputs. Outputs include an isolated 4--20mA re-transmission signal, a single change-over trip relay and dual normally-open relays. Each unit has a RS485 serial communications link, enabling data to be sent to and from a host using Modbus protocol. A abus jumper' system allows units to be easily bussed together, removing the need for complicated and expensive back plane wiring. For high density systems, cost effective dual channel versions are available.
Status has found that the versatility of MEDACS is best explained through real plant applications.
Level control in sewage lift stations (often known as wet wells) is typically provided by two or more pumps with a level controller to tell the pumps when to start and stop. A single MEDACS unit can field power a level device mounted in the lift station and provide the switching of the contactors for the pumps via two relays. Under normal operating conditions the duty pump will lower the level, but in conditions of high demand, the back up pump may be called in to assist. After each pump down cycle, MEDACS can be set up to rotate the duty assignment to ensure both pumps remain active and with even running hours. This control sequence has a number of parallel applications where motors and pumps are operated in pairs.
In another example, a Dual Channel MEDACS can be used to measure the readings from two pressure sensors fitted either side of a filter. The MEDACS can be set up to calculating the difference in readings and provide a 4--20mA signal proportional to the pressure differential. The built- in relays can be set for instance to switch on an air purge to clean the filter and activate an alarm should the differential become too great.
MEDACS can solve problems encountered when trying to measure flow over a wide range. Accurate measurement can be problematical due to the turndown of the flow meter providing inaccurate measurement at extremes of span. A Dual Channel MEDACS can take inputs from two different sized flow meters and then select the most appropriate instrument for the conditions, signaling a changeover valve to divert flow to the selected instrument. A continuous
4-20mA output proportional to flow is maintained by the MEDACS.

Paul Wade is with Status Instruments Ltd, Tewkesbury, Gloucestershire, UK. " target="_blank">www.status.co.uk

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