LVDT Signal Conditioning FAQ

How do I work and operate GE Position Sensors for Power Generation Turbines with an ASG S2A LVDT signal conditioner?

By: Ed Herceg  Chief Technology Officer 


GE Position Sensors for Power Generation Turbines

How they work and how to operate them with an ASG S2A LVDT signal conditioner

LVDTs (Linear Variable Differential Transformers) are very commonly used as position sensors in power plants throughout the world.  Working from AC voltages and frequencies not available from power lines, LVDTs require a signal conditioner to provide the necessary operating power. Alliance Sensors Group's model S2A module, designed specifically to be used in power gen applications, is considered by many power gen users be the most advanced and easy-to-use single channel LVDT signal conditioner on the market today. It can operate practically any LVDT or half-bridge (LVRT) position sensor currently in use. 

However, users and systems integrators can be confused by unusual LVDT configurations, particularly in regard to the General Electric 185A1328 and 311A5178 series of LVDTs typically used with their gas turbines.  Furthermore, for many years, GE has been using a contactless position sensor known as an inductive half-bridge for measuring the position of the operating shafts of steam turbine control valves.  These sensors, often called VRTs or LVRTs, are used to provide position feedback from modulating throttle and governor valves, as well as to give open or closed condition feedback from bypass, stop, and interceptor valves.  They are also used to monitor valve position on some turbine feedwater pumps.  Some typical GE half-bridge part numbers include the 119C9638, 119C9639, and 196C8768 series.   Again, there is some confusion among many systems integrators about how to connect these GE half-bridge sensors to an LVDT signal conditioner and how to calibrate them. This paper should help dispel any confusion about operating an S2A with a GE gas turbine LVDT or steam turbine half-bridge sensor.


Operating GE Gas Turbine LVDTs with an ASG S2A LVDT Signal Conditioner

The first section of this paper shows how these GE LVDTs differ from conventional LVDTs and how to operate them with an S2A LVDT signal conditioner module.  To understand the differences between GE Gas Turbine LVDTs and conventional LVDTs, it is important to review the characteristics of an ordinary LVDT.  Regardless of the method of construction actually used to make a conventional LVDT, it has a primary winding and two identical secondary windings that are usually connected in series opposition, with a movable permeable core to couple the primary to the secondaries, as shown in Figure 1 below.

The electrical output of a typical LVDT as a function of its core's position is shown in Figure 2 below.  Note that the plot of its AC output amplitude versus position shows the classical "V" shape commonly associated with an LVDT, with a minimum value called null at the center of its range of motion, and an increased output amplitude for core positions on either side of null. More important is the fact that the phase relationship of the differential AC output to the primary input voltage shifts abruptly by 180° as the core moves through null. It is this 180° phase shift that permits a user to know on which side of null the core is positioned.  The voltages and frequency shown are typical of those found in the operation of a conventional LVDT. It is important to note that the actual excitation voltage utilized makes very little difference to an LVDT's performance. Only the excitation frequency is important for proper operation.

The configuration of the LVDTs used by GE for various position sensing functions on their gas turbines is shown in Figure 3 below.  The most obvious difference is the tap on the primary, but otherwise it does not seem to vary much from the view of a conventional LVDT. However, its electrical operation is really substantially different.  Although it is tempting to view the primary tap as a center tap, it is not a center tap at all. Most often it is a 30% tap, but on a few models it is a 25% tap. The voltages shown are typical of those used by GE, but, as noted above, the sensor is a differential transformer and will function with whatever AC voltage is applied to operate it. For the sake of clarity, typical GE I/O values are used for the explanation of how this variety of LVDT works.

This LVDT hookup is generally known as a "Buck-Boost" configuration.  As can be seen in Figure 3, the voltage at the tap is 30% of the excitation voltage that is being applied to the LVDTs primary, and is in phase with the primary voltage. The way the windings are connected, this 30% voltage, etap, is being added to the differential secondary voltage, which consists of e1, which is in phase with the primary voltage, to which has been added e2, which is 180° out of phase with the primary voltage. The turns ratio of these GE LVDTs is usually about 5:1, so, with a 7.07 V ACrms input, the full scale output of the secondaries, without the effect of the 30% tap, would be about 1.4 V ACrms, either in phase with the primary excitation or 180° out of phase with it. However, the 30% tap inserts an additional 2.1 V ACrms that is in phase with the primary excitation to be added to the voltages already in the secondaries. The result is plotted in Figure 4, which shows that the total AC output from the series connection of the two secondaries and the 30% tap is a voltage between 0.7 to 3.5 V ACrms, with no 180° phase shift as the LVDT's core moves from one end of its range of motion through null to the other end of its range.

The electronics used by GE in their controllers requires this unusual AC output, but almost all these "Buck-Boost" LVDTs can be successfully operated with an ASG S2A signal conditioner module by using the appropriate connection configuration. The first thing to do is to ignore any connection to the primary tap. Instead, connect the LVDT in a 3-wire configuration, as shown in the S2A instruction manual. In this configuration, the high end of the primary is connected to J1-1 (or J1-2, if the output directional sense is reversed), the common connection of the primary and the series-connected secondaries is connected to J1-3, and the secondaries' high output is connected to J1-4. It is also necessary to move jumper J10 to its alternate position and to shift jumper J7 over to its high output position to increase the excitation to the LVDT's primary. After this 3-wire connection for a "Buck-Boost" LVDT has been completed, the S2A can be operated and the LVDT be calibrated in the normal manner shown in the S2A module's manual.

Operating GE Steam Turbine Half-bridge (LVRT) Sensors with an S2A Signal Conditioner

First it is necessary to understand how GE half-bridge sensors function to see how to use them with an S2A signal conditioner.  Figure 5 shows the schematic of a typical GE half-bridge sensor. Note that pins D and E play no role in the operation of the sensor, but are merely part of an "interrupt jumper" system which is used to notify the turbine control system that the connector has been removed.  As shown in the schematic, the sensor consists of two identically-wound inductor coils connected in series encircling a movable permeable core long enough to overlap a portion of each coil. An AC voltage ein is applied to pins A and C.  If the core is located symmetrically between the coils, each coil will have the same impedance and, assuming the output has no load, voltage eout between pins B and C will be 1/2 of ein.

If the core is moved to include more of the coil connected between pins A and B, the inductance, and therefore the impedance, of that winding will increase, while the inductance, and hence the impedance, of the coil connected between pins B and C will decrease.  The result will be a drop in the voltage eout.  If the core were moved in the other direction, the reverse action would happen and eout would increase. 

Thus, the half-bridge acts as an AC voltage divider. Over a limited range of motion, and excited at an appropriate input frequency, this operation can be reasonably linear if the change in impedance of each winding is largely due to its inductance rather than to its DC resistance, as shown in Figure 6 below.

The shaded symmetrical areas represent the different AC output levels developed by different sensors. Several things stand out in Figure 6. First is that the output at mid-range is not zero as with an LVDT, but is half of the AC excitation. Second, there is not a 180° phase shift at "null" as with an LVDT. Both features specifically facilitate interfacing these sensors with GE's Mark 2 - 6 turbine control systems.

There are two other very important points of note in the operation of half-bridge sensors, both of which concern the excitation of the sensors. First, the magnitude of the AC excitation voltage has no bearing at all on the functioning of these sensors. The choice of 7.07 Vrms (20 Vp-p) by GE is merely dependent on the requirements of their control system. Second, the excitation frequency is chosen to make sure the impedance of the two windings is dominated by their inductive reactance at the chosen frequency, so that the DC resistance of the windings has a fairly small overall effect on the winding impedances.

When looking at Figure 6, it is easy to understand why it can be unduly difficult to set up and calibrate these GE half-bridge position sensors in the field.  The primary reason is that a half-bridge sensor does not have a uniquely identifiable point in its range of motion like an LVDT's null point. Fortunately, ASG's S2A LVDT signal conditioner was designed not merely to work with inductive half-bridges, but to make their operation emulate that of an LVDT. Thus, anyone familiar with the techniques for calibrating an LVDT position sensor installed in a valve position feedback system using an S2A module will be able to utilize those very same techniques to calibrate a GE half-bridge sensor connected to an S2A module. 

Figure 7 below shows that a GE inductive half-bridge connected to an S2A signal conditioner displays exactly the same type of AC output as would be developed if the S2A were connected to an LVDT. When using an S2A with a half-bridge sensor, the front panel LEDs function in the same way during calibration as if it were an LVDT, as does the Null Output voltage available at J4-1 and J4-2.  While this app note has focused on the GE half-bridges used in power plants, these same considerations apply to operating any inductive half-bridge sensor with an S2A or any of its derivative LVDT signal conditioners.




The connection diagram for hooking up GE half-bridges to an S2A signal conditioner module is shown above. Hook up the GE half-bridge connector's pins to the numbers on the black plug, J1, as follows: Pin A goes to J1-1, pin B goes to J1-4, and pin C goes to J1-2.  Pins E and F are not connected to the S2A at all. Also, move jumpers J7 and J10 over to their alternate positions. If the directional sense of the S2A's analog output is reversed from the desired output, either interchange the connections to J1-1 and J1-2, or flip the INVERT switch, DS2-3, inside of the S2A.


50/60 Hz GE Half-bridge Sensors used with ASG S2A Signal Conditioners in Steam Power Plants

This paper was based on the most common GE half-bridge sensors likely to be found in power plants today, all of which operate at 3 kHz.  However, early on, GE used some half-bridge sensors that were operated with 24 Volts AC at 50/60 Hz and some even used 115 Volts AC, 50/60 Hz. Both systems integrators and power gen utilities should be aware of the risks of continuing to use a more than half-century old design of 50/60 Hz-operated GE half-bridges instead of the newer 3 kHz-operated sensors.

These 50/60 Hz units do not contain much ferromagnetic material beyond their core so their windings utilize many turns of fine wire to achieve the needed inductance.  As a result, their winding impedance has a large resistive component, so that much of the AC input power gets dissipated in the resistance of the windings, leading these sensors to get quite hot internally during normal operation.

Because of the effects of thermal expansion and contraction on the winding insulation, these 50/60 Hz units have a history of developing intra-winding shorts, but which do not immediately produce significant output changes because there are so many turns of wire on each winding.  Furthermore, sensor output linearity is also poor to begin with, typically ± 2% at FSO, and ± 5% at 110% of FSO, which often initially masks the effects of any such internal shorts. But over a period of time, the internal winding shorts can and often do increase, causing a net calibration shift that could result in a significant sensor output error.

Despite these issues, several integrators have had success using these old 50/60 Hz GE half-bridge sensors with an ASG S2A LVDT signal conditioner by operating them with the S2A's 1 kHz excitation frequency. In fact, some old sensors had such a low winding impedance at 1 kHz that it was necessary to operate them at 3 kHz. Typically, those integrators have utilized old stock of unused spares or ordered new units built to the old design, mostly because their utility customers are reluctant to change sensors over to newer, more reliable products, particularly in nuclear power plants, because of issues with certifications, testing, and related documentation.  Even so, it is important to bear in mind that these 50/60 Hz sensors utilize a genuinely obsolete design, and that contactless inductive position sensing technology and sensor reliability have improved substantially in the intervening half-century or more.

To download a PDF of this FAQ, please click here.

Where can I find Datasheets for your LVDT Signal Conditioners?


LVDT Signal Conditioning


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How do I use the RS-485 Port of your LVDT Signal Conditioners?

Using the RS-485 port of ASG S2A/SC-200 LVDT signal conditioners   Aug 1, 2019


ASG's LVDT signal conditioners incorporate an RS-485 two-wire multi-drop serial communications interface for up to 16 devices. This port enables half-duplex serial communication by which a module can be set up and calibrated remotely and system data can be read or stored on a PC running an ASCII terminal program like Hyper Terminal with a 2-wire RS-485 converter for the computer's com port or USB port connected to terminals J3-1 and J3-2 of the signal conditioner. The PC's com port parameters are: 9600 bps, no parity, 8 data bits, and 1 stop bit (9600, NP, 8, 1), with echo on and no flow control. Be sure that the RS-485 connection for Data A (D-) is connected to J3-2 and Data B (D+) is connected to J3-1. Always follow the data polarity (D) indicated above, regardless of the letters for data lines used by the RS-485 converter.


ASG offers a 1.8 m long 2-wire USB-to-RS-485 converter cable, USB-RS485-WE, p/n 5810-0001, having an orange wire with a red tip plug that is Data B (D+), and a yellow wire and tip plug that is Data A (D-). Normally no driver is needed for a USB-RS485-WE used with MS Windows 7 or later. For Windows XP or earlier, or MAC or Linux operating systems, driver software can be found on: In most cases, the optimum driver for any particular OS is the VCP version.


RS-485 user commands for S2A/SC-200 ASG LVDT signal conditioner modules


Note that all commands must be formatted to begin with UXX followed by a space, where XX is the numerical value between 00 and 15 of the module's decimal digital address as set up on DS2, switches 5, 6, 7, and 8 according to Table 1b in the S2A instruction manual, or by following the DIP switch settings diagram shown on the S2A module's left side label.


Note: Some Set command descriptions show in bold face the range of values that follow the command and a (space).


Analog In RUN mode, returns the nominal analog output value scaled in electrical units that depend on the setting of DS1, or the analog output range selected with the Set Aout command.


Cal Enters CALIBRATION mode; command is the same as pressing FULL SCALE and ZERO pushbuttons together.


Clrall In RUN mode, clears EEPROM of all RS-485 command settings used to override module's DIP switch settings. 


Config Lists the module's setup data and displays DIP switch settings and current EEPROM values. Specifically, it shows the module's firmware version, operating mode, digital address (00 - 15), date stamp, serial number, analog output setting (1 - 8), excitation frequency setting (0 - 3), output invert switch off or on, low frequency filter status (LF) off or on and filter corner frequency, excitation drive jumper (J7) in or out, failure output delay time (FD) and polarity (FOP) NC or NO, Lock status, and stored EEPROM values for ADC Lo, ADC Hi, Input pot, and Gain pot.  (Log and store all Config data and values by digital address to be able to reconfigure a hot swapped module at a later time).


Error In RUN mode, displays any setup or operations error code(s); for multiple errors, the error code sum is displayed.


Errsec In RUN mode, ON, OFF (default) toggles error indications and failure outputs from low DCR LVDT secondaries.


Errsig In RUN mode, ON (default), OFF toggles error indications and failure outputs for all errors found during setup.


Exit Required to exit CAL mode, or to exit any Set command writing a value to the module's EEPROM in RUN mode.


FS In CAL mode, sets the module's full scale output point at the LVDT core’s maximum position and is the same as pressing FULL SCALE pushbutton. Occasionally it may require setting a second time after using the Z command.


Help Shows all ASCII user commands available for execution over the RS-485 bus, including a few not shown in this list.


LEDs In RUN and CAL modes, outputs the status of the 3 green LEDs, displayed in S-E-P order, e.g.:  - * 0  means S LED is off, E LED is flashing slow, and P LED is on. + is a fast flash and ! indicates alternating solid and flashing.


Lock In RUN mode, locks the module against any changes and displays attempted tampering over the RS-485 bus.


Null In RUN mode, displays the Null Output voltage at any core position and is typically used to verify that the core of an LVDT is at null; may also be used to establish the symmetry of an LVDT’s endpoint outputs versus core position.


Read LF In RUN mode, when DS2-4 is ON, or the LF filter is invoked, shows the status and frequency setting of the supplemental low frequency low pass filter.


Recal FS In RUN mode, after a calibration has been completed, if the actual full scale output value is within ±4% of the nominal full scale output value selected by DS-1 or the Set Aout command, this command trims the actual full scale output value to match the selected full scale output value. The command may be repeated once to get the most precise FS output value. Recal can be set at module by pressing and holding the FULL SCALE button until the POWER LED blinks.


Recal Z In RUN mode, after a calibration has been completed, if the actual zero output value is within ±4% of the nominal zero output value selected by DS-1 or the Set Aout command, this command trims the actual zero output value to match the selected zero output value. This command may be repeated once to get the most precise zero output value. Recal can also be set at the module by pressing and holding the ZERO pushbutton until the POWER LED blinks.


Reset In RUN mode, produces a "soft" reset of the module's processor so the module restarts as if it is powering on. Command is the same as pressing the FULL SCALE pushbutton three times for at least one-half of a second each.


Reset All In RUN mode, using prefix U90 instead of Uxx, this command performs a simultaneous "soft" reset on all modules connected to the RS-485 bus.  Each module on the RS-485 bus then restarts itself as if it is powering on.


Restore In RUN mode, resets module to factory set condition by cancelling all user setup values stored in EEPROM. It can also be invoked by pressing the ZERO pushbutton three times in a row for one-half of a second each time.


Set ADC Hi In RUN mode, writes an A/D converter high value into module's EEPROM. Command is used during a hot swap module reconfiguration to enter the ADC Hi value logged from the original module's Config command.


Set ADC Lo In RUN mode, writes an A/D converter low value into module's EEPROM. Command is used during a hot swap module reconfiguration to enter the ADC Lo value logged from the original module's Config command. 


Set Aout In RUN mode, permits setting the analog output range: 1 - 8, independent of the setting of DIP switch DS1.


Set Exf In RUN mode, permits setting excitation frequency: 0 - 3, independent of settings of DIP switches DS2-1, -2.


Set FD In RUN mode, permits the user to set the delay time before the failure warning output switch is activated from 0 to 900 msec in 100 msec increments: 0 - 9. The factory default delay time is set at 200 msec.


Set FOP In RUN mode, sets failure warning switch polarity: NC, Normally Closed (default) or NO, Normally Open.


Set Gain In RUN mode, writes a Gain pot value into module's EEPROM. Command is used during a hot swap module reconfiguration to enter the Gain pot value logged from the original module's Config command. 


Set In Pot In RUN mode, writes an Input Pot value into module's EEPROM. Command is used during a hot swap module reconfiguration to enter the Input Pot value logged from the original module's Config command.


Set Inv In RUN mode, permits inverting analog output by overriding setting, ON, OFF, of invert DIP switch DS2-3.


Set LF In RUN mode, sets the corner frequency of the supplemental low pass filter between 0.1 Hz and 10 Hz. If DS2-4 is not ON, command permits LF filter status to be changed: ON, OFF, and its corner frequency to be set.


Ver In RUN mode, returns the version number of the module's firmware.


Z In CAL mode, sets the module's zero output point at the minimum position of the LVDT's core; function is the same as pressing the ZERO pushbutton. Occasionally it may require setting a second time after using the FS command.




What are the differences between the S2A and SC-200 LVDT Signal Conditioners?

The S2A and SC-200 LVDT signal conditioners are very similar; however, each unit was designed with specific features for the markets it serves in mind, so there are a few differences between them.

The S2A was specifically designed for the power generation industry and has the following features:

  • LVDT excitation frequencies of 1, 3, 5, and 10kHz. The 1kHz and 3kHz frequencies were selected so the conditioner could be used with GE and Westinghouse steam turbine LVDTs and LVRTs.

  • If the S2A senses a sensor-related fault, the module's fault-response outputs are activated and in addition, the analog output is driven out of range. This is done so that in a redundant sensor configuration, by using an algorithm coded into the turbine's DCS control system, the faulty reading is identifiable and is not be accepted by the DCS with the other correct sensor readings.

  • The I/O screw-terminal plugs are color-coded and are removable for easy installation.


The SC-200 was designed for standard industrial applications and has the following features:

  • LVDT excitation frequencies of 2.5, 5, 7.5, and 10kHz. The 2.5kHz frequency is commonly used by LVDT manufacturers and the 7.5kHz frequency was selected to be used with Marposs analog pencil gaging probes. Alliance Sensors Group is an official distributor for Marposs USA.

  • The analog output is NOT driven out of range should the SC-200 sense a fault, but the module's other fault-response outputs are activated.

  • The I/O screw-terminal blocks are not removable.




How do I use the RESTORE Function for S2A or SC-200 LVDT Signal Conditioners?

An S2A or SC-200 module set for 4-20 mA output displays an output error during setup

1. Before taking any specific remedial action:
    a. Make sure that the analog output is not connected into a loop powered system.
    b. Make sure that there is a loop load resistor connected to the module's output terminals.
2. If the E and S LEDs are blinking and the P LED is not illuminated, perform the RESTORE function by depressing the front panel ZERO pushbutton 3 time in a row for a least 1/2 second duration each time.
3. If the RESTORE function was successful, the E and S LEDs will go off and the P LED may turn on. 
4. If the RESTORE function was unsuccessful, repeat the process, carefully observing the duration.
5. When successful, proceed to calibrating the module with the connected LVDT sensor normally.

What are the recommendations for DC power supplies and USB-based data acquisition hardware for use with ASG's signal conditioners and DC-operated sensors?

DC Power Supplies

There are two distinct types of DC power supplies that convert the AC power line voltage into DC voltage to operate electronic devices. Linear supplies use a transformer, rectifier diodes, a regulator, and filtering to produce relatively clean DC, but they are limited to a narrow range of power input voltages and are usually physically large. Switching supplies offer a wider range of power input voltages and are usually smaller in size, but often have switching frequency noise on their DC output. Both types are available in a variety of packages, but most DIN-rail-mounting units are switching supplies.

Because ASG's devices draw relatively low current, the chart of recommended power supplies shown below is based on the number of similar units being powered at the same time. All the power supplies listed below are available from national electronic component distributors such as Digi-Key, Mouser, Newark Electronics, or Allied Electronics at unit prices from $40 to $120. 

Output Volts DC Number Devices TDK Lambda Part Number Mounting Phoenix Contact Part Number Mounting ASG Product Application
12 V 12 DSP10-12 DIN rail 2868538 DIN rail, tabs DCSE LVDT
15V 8 DSP10-15 DIN rail     MR, ME, LV LVIT
24 V 6 DSP10-24 DIN rail     LVIT, S2A, SC-200
24 V 10     2868535 DIN rail, tabs LVIT, S2A, SC-200
24 V 16 DSP30-24 DIN rail 2868648 DIN rail, tabs LVIT, S2A. SC-200
24 V 16     2866446 DIN rail only S2A, SC-200

Data Acquisition Systems

For analog data acquisition (DAQ) input to a PC, the simplest products to utilize are generally USB-based instruments. There are several USB-DAQ choices offered by National Instruments (NI), or their subsidiary, Measurement Computing Corp. (MCC), (which offers more economical DAQ products) that work with ASG products. Typically, USB-based DAQ systems can operate from the 5-Volt DC power of a USB port, so an additional power supply is normally not needed. These DAQ systems work with all the popular NI software like LabVIEW and Signal Express.

DAQ systems are usually used with an analog voltage input, and the majority of ASG products offer a single-ended, ground-referenced DC voltage output up to 10 Volts full scale. Most DAQ systems have at least 8 single-ended analog inputs, and some have differential inputs as well. A DAQ with a differential input capability is of particular benefit for use with sensors that have their DC output off-ground, like some types of DC-LVDTs that offer either a 3-wire connection with a 1-6 Volt grounded output, or a 4-wire hook up with a 0-5 Volt output in which the return side of the output is floating 1 Volt above the ground of the sensor’s power supply.

Recommended USB DAQs for single-ended inputs are the MCC USB200 series and the NI USB-6000 series. Recommended USB DAQs for both single-ended and differential inputs are the MCC USB-1208 series and the NI USB-6008 series. All have a resolution of 12 bits, with a variety of sampling rates available. Detailed product information is available on the respective websites of the suppliers: or