SIGNAL CONDITIONING FEATURE


Supporting sensors in harsh environments


Karmjit Sidhu, VP, business development, Macro Sensors, looks into how partitioning electronics from sensors supports long-term operation in harsh environments


S


ensor reliability is critical in industrial applications associated with high


temperature, radiation and sub-sea conditions. Used to measure pressure, temperature, position and flow, such sensors often have to operate unattended for thousands of hours, providing accurate measurements that are fed into distributed control systems (DCS), programmable logic controllers (PLC) or proprietary control systems. Sensor outputs are used to monitor operating parameters to enhance system performance and diagnostics, as well as safety. In a typical power generation


application, for example, where system performance and safety are essential, the values of temperature and radiation (in nuclear power generation plants) must be monitored but often exceed the limits of sensor internal electronics. While sensors (resistive, magnetic, inductive and capacitive) chosen to monitor these values can be designed to be passive with no internal electronics, their output signal must be processed remotely away from the hostile environment. In these scenarios, cables must connect sensors with remotely- located electronics that power the sensors, amplify and demodulate their output. Output is then displayed on a suitable readout and/or input into a computer-based data acquisition system for statistical process control. In environments where sensors are used to monitor turbine health or regulate steam valves, temperatures can reach 250˚C. In nuclear power plants, radiation levels can be 100 Mega Rads. In these extreme conditions, the distance required between the sensor and electronics can range in excess of 30m, limiting the use of magnetic technologies. As a result, linear variable differential transformers or LVDTs are more commonly used in power generation and other applications with extreme environments due to their robustness and high Mean Time Between Failure (MTBF).


30m+. The signal conditioner has the ability to provide one of four different frequencies, analogue and digital outputs, and the ease of programming the zero and span values via a front panel push button display with LEDs. Configurable with LVDTs, RVDTs and VR


half bridge position sensors, the LVC 4000 signal conditioner digitally communicates and can be connected together in master/slave configuration with up to 16 devices simultaneously for multiple channel applications. Offering both 4-20mA (analogue), 0-10VDC (analogue) and RS485 (digital) outputs, the signal conditioner offers a choice of 2500Hz, 5000Hz, 7500Hz or 10,000Hz drive frequency. CE approved against EMI and RFI, units can be DIN rail mounted. Furthermore, the signal conditioner


Figure 1


Construction of the LVDT linear position sensor includes a primary and two secondary windings, as shown in Fig. 1, above. A low frequency signal in the


range of 2000 to 10,000Hz excites the primary windings. The core, attached to the moving member, couples the magnetic signal from the primary windings to the secondary via means of mutual coupling. When the core is in the middle (see Fig. 2, right) the output is zero since the mutual coupling is equal and opposite in each secondary winding, resulting in the cancellation of voltages. As the core moves from the centre, the phase also changes, indicating the position of the core with respect to one of the secondary windings. To provide excitation and signal


conditioning for the LVDT linear position sensor, complex electronics in the form of a signal conditioner are needed to generate the drive signal and read the output signals over some length of cable. As the LVDT can operate from 2000 to


10,000Hz, the signal conditioner must also be able to provide these frequencies as user selectable within the device. The signal conditioner must also be able to provide analogue and digital outputs that the user can read with today’s process control systems such DCS, PLCs or others.


SIGNAL CONDITIONER DESIGN The EAZY-CAL LVC 4000 push button programmable LVDT signal conditioner is an example of electronics designed specifically for operation with LVDT linear position sensors remotely located up to


/ INSTRUMENTATION


The signal conditioner when used with a high temperature LVDT (250˚C) for measuring valve position displacement. The core, as shown, is attached to the actuation member of the valve


INSTRUMENTATION | JULY/AUGUST 2014 23


can be used with LVDTs to monitor the operations of coal-powered power plants, gas turbines and nuclear power plants. The signal conditioner can also


interface with a low pressure differential pressure (dP) sensor used in heavy water holding tanks found in nuclear power plants. In this application, the core is attached to the dP diaphragm and encased within the dP body of the sensor. The LVDT, contained within the dP housing, is rated to 100 Mega Rads as it contains no integral electronics. A radiation- resistant cable is used to connect the sensor


from the radiation area to the safe area where the EAZY-CAL LVC 4000 signal


conditioner is located. Figure 2


Macro Sensors www.macrosensors.com Enter 669


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52