LVDT Position Sensor Application Notes

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 Number TDK Lambda Mounting Phoenix Contact Mounting ASG Product

Output Volts DC Number Devices TDK Lambda Part Number Mounting Phoenix Contact Part Number Mounting AFG 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
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

GE Position Sensors on Power Gen Turbines

How they work and how to use them with a third-party LVDT signal conditioner designed specifically for Power Gen Applications

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 to be the most advanced and easy to use single channel LVDT signal conditioner on the market today. It can operate practically any inductive or LVDT-type 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 application note should dispel much of the confusion around the operation of both GE LVDTs and inductive half-bridge sensors.

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

This application note 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 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 usually 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. While the most obvious difference is the tap on the primary, it does not otherwise seem to be much different from a conventional LVDT, but its electrical operation is extremely different. Although it is tempting to view it as a center tap, it is not a center tap at all. Most often it is a 30% tap, but, depending on the model, it can sometimes be a 25% tap. The voltages shown are typical of those used by GE, but, as noted above, the device is a transformer and will function with whatever AC voltage is used to operate it. For the sake of clarity, typical GE 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 appearing at the tap is 30% of the excitation voltage being applied to the LVDTs primary, and is in phase with the primary voltage. The way the windings are connected, this 30% voltage, e tap, 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 proper 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, the common connection of the primary and the series-connected secondaries is connected to J1-3, and secondaries' high output is connected to J1-4. A 3-wire configuration of an S2A only applies 1/2 of the usual input voltage to the primary, so it is good practice to shift jumper J7 over to increase the excitation output to the LVDT's primary by 50%. After this 3-wire connection for a "Buck-Boost" LVDT has been completed, the S2A can be operated and the LVDT calibrated in the normal manner shown in the module's manual.


Operating GE Inductive Half-bridge Position Sensors with an S2A Signal Conditioner

First, it is necessary to understand how GE half-bridge sensors function. 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 that have a 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 has the same impedance, resulting in an Eout between pins B and C that is 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 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 no 180 degree phase shift at "Null" as with an LVDT. These features specifically facilitate interfacing 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 on the functioning of these sensors. The choice of 7.07 Vrms (20 V p-p) by GE is merely dependent on the requirements of their control system. Second, the excitation frequency was 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 application note has focused on the GE half-bridges used in power plants, these same considerations apply to using an S2A or any of its derivative signal conditioner products with any half-bridge sensor.


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 D and E are not connected to the S2A at all. 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.

Using 60 Hz GE Half-bridge Sensors with an ASG S2A signal conditioner in Steam Power Plants

This treatise 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 60 Hz and some even used 115 Volts AC, 60 Hz. Systems integrators and utilities should both be aware of the risks of continuing to use a 50-year old design of 60 Hz operated GE half-bridges instead of the newer 3 kHz operated sensors. These 60 Hz units do not utilize much ferromagnetic material beyond their core so their windings have fairly low inductance. As a result, their impedance is largely based on the resistive component of their windings. Much of the AC input power gets dissipated in the resistance of these windings, so these sensors tend to get hot during operation.

Because of the effects of thermal expansion and contraction on the windings, these 60 Hz units have a history of developing intra-winding shorts, but which do not immediately show up in significant output changes because there are so many turns of wire on each winding. Furthermore, their 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 number of internal winding shorts can and often does increase, causing a net calibration shift that could result in a significant output error.

Despite these issues, some integrators have had success operating old 60 Hz GE half-bridge sensors with an ASG S2A LVDT signal conditioner by operating them with the S2A's 1 kHz excitation frequency. 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 60 Hz sensors utilize an obsolete 1960s design, and that contactless inductive position sensing technology and reliability have improved substantially in the intervening half century.

Click to View S2A Signal Conditioner
Heavy Duty LVDTs Avert Emergency Shutdown of Steam Turbine

Alliance Sensors Group’s PG Series LVDTs Prove Their Durability Once Again

The PG series of heavy duty LVDT position sensors from Alliance Sensors Group have become the defacto standard of the power gen industry for measuring steam valve position on power plant turbines, having demonstrated outstanding performance, reliability, and durability over many years of operation.

These LVDTs’ robust construction is clearly illustrated by the following pictures from a power plant in which the failure of the anti-rotation mechanism on the shaft of the hydraulic actuator operating a modulating steam valve while the turbine was operating resulted in extreme misalignment of the LVDTs’ extension rods, as shown in Figure 1 below.



Figure 1 Extension rods of LVDTs severely misaligned after anti-rotation device on cylinder shaft failed.

Because the LVDT extension rods are relatively large in diameter and utilize a ball joint coupling to the anti-rotation arms, as shown in Figure 2 below, the rods did not fracture at their coupling threads, as might otherwise have been expected. Instead, the ball joints conformed to the extension rods’ angular misalignment, relieving bending stress on the extension rods’ coupling threads that can lead to fracture.


Figure 2 Close-up of the ball joint couplings on the LVDT core extension rods at the anti-rotation arms.

Plant maintenance personnel straightened out the LVDT extension rods by correcting the anti-rotation problem after rotating the mechanism back into position WITHOUT SHUTTING DOWN THE TURBINE, as shown in Figure 3 below. Keeping the turbine operating during this repair avoided a costly shutdown.


Figure 3 Extension rods re-aligned only by adjusting the anti-rotation mechanism back into position.

Features enhancing the reliability and durability of ASG’s PG LVDTs in power gen applications:

  • A large shaft which totally captivates the LVDT’s core so it never can break off and fall way
  • Double shaft seals to prevent the ingress of contaminants that could compromise reliability
  • Ball joint couplings to overcome shaft misalignment which can lead to coupler joint failures
  • Screw clamp terminals that totally eliminate potential connector contact and sealing issues
  • ½ / ¾ NPT hub for easy connection of cord grips or liquid tight and sealtite flexible conduit


Specific differences between ASG's S2A and SC-200 LVDT signal conditioner modules

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 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.



Click to View Most Commonly Used Signal Conditioners.




How to Use the Failure Warning Output at J2-2 of an ASG LVDT Signal Conditioner

The LVDT signal conditioner module offers an open-collector failure warning output signal which is available at J2-2 on the blue terminal block connector. This output is a connection to the collector of an internal transistor switch, whose factory default mode of operation is Normally Open (NO). In NO mode a device connected between the +24 V DC supply and J2-2 will have no current flow when the module is operating without error, producing a logic High (+24 V) at J2-2, but if a failure occurs, current will flow producing a logic Low (0) at J2-2.

For modules with version 2 firmware, the open-collector switch's mode of operation can be changed in the field from Normally Open (NO) to Normally Closed (NC) by using the SET FOP command over the RS-485 bus. In the NC mode, the device connected between the +24 V DC supply and J2-2 will have current flow if the module is operating without error, producing a logic Low (0) output at J2-2, but if a failure occurs, current flow will stop, producing a logic High (+24) output at J2-2. To determine if this change is possible, find the module's firmware version by using the RS-485 bus command VERSION.

It is important to note that there is a built-in time delay between when a failure is detected and when the transistor actually turns on, which is normally the factory default of 200 ms, but can be reprogrammed in the field from 0 to 900 ms in 100 ms increments using the SET FD command over the RS-485 bus. This time delay prevents "nuisance trips" from unusual transients such as nearby lightning strikes, etc., and applies only to the NO or NC Failure Warning Output at J2-2. It does not affect any other fault indicators built into the S2A like the front-panel LEDs flashing or the analog output being driven out of range.

The types of devices that are typically connected between the +24 V DC supply and the open collector failure warning output terminal J2-2 include:
1. A 10 kilohm "pull-up" resistor for generating a TTL digital logic signal between J2-2 and ground in the event of a failure that can be transmitted to the control room and the DCS or operations data recorder.
2. A magnetic DC relay that requires 50 mA or less to operate, which can operate additional devices such as an audible alarm or an annunciator light. The voltage developed across the relay coil can also be used as in the same way as that across a pull-up resistor to provide a TTL digital logic output signal.
3. A solid-state relay that functions in the same manner as (2) above.
4. An LED array (with appropriate series resistors) for local fault indication.
5. A low wattage 24 Volt incandescent lamp for local fault indication.

Note that the open collector can be used with external devices as long as there is a ground connection from the module in addition to the connection to J2-2, but be careful not to produce a "ground loop".


Hot Swapping S2A LVDT Signal Conditioning Modules

Under the right conditions, an S2A module can be "hot swapped" with another module having the same internal DIP switch settings and loaded with values in certain EEPROM locations that match those in the original module. If S2As are connected in a multi-module array, this process will take the selected module off-line, but if done carefully, will allow the remaining modules in the array to operate normally.

To prepare the replacement module for a hot swap, the configuration data of the original module, which should have been obtained previously and saved for a possible hot swap, is needed. If this data is not available, connect to the RS-485 port of the original module with a PC and terminal program, and use the Config command to get it. Using that data, set the internal DIP switches and jumpers of the new module to match, then power it up and use its RS-485 port as noted above to load the values of ADC HI, ADC Lo, In Pot, Gain, and, if needed, FOP, FD, and LF using the Set command for each value.

To do the hot swap, first disconnect only the input power positive from the original module. Do this by pulling that module’s power input fuse, by tripping its circuit breaker, by turning off its power switch, or by loosening the screw terminal of J4-4, the red plug, and carefully removing the wire connected to it. Do not remove the red plug with any power present. Once the unit has been depowered, the plugs may be removed from the original module in this order: J4 (red), J3 (green), J2 (blue), then J1 (black). Insert the plugs into the replacement module in reverse order: J1 (black), J2 (blue), J3 (green), and last, J4, (red), but with no input power positive. Finally, restore the input power positive or reconnect the wire to J4-4.

Techniques for Measuring the Primary Impedance of an LVDT

Periodically the need arises to be able to measure the primary impedance of an LVDT to ensure that it will work with a particular version of signal conditioning electronics. Typical LVDT signal conditioner systems are built to work with a primary impedance of 200 Ohms or higher, but some systems cannot supply enough excitation power to operate an ordinary industrial LVDT. If a user is uncertain of the primary impedance of a particular LVDT, it may be necessary for that user to actually measure it.

However, before explaining how to do it, it is important to be certain that the user understands that he cannot measure impedance using the resistance ranges of a multimeter, even though they are both measured in Ohms. Those ranges of a multimeter indicate DC resistance (DCR).

Impedance is an AC phenomenon, which is a function of and determined by frequency. There are instruments called Impedance Meters that can directly measure the impedance of an R-L-C circuit at some particular frequency. These meters usually have a variable frequency source or some selection of common reference frequencies available to provide an impedance reading. There are also Impedance Bridges, which are older laboratory devices to measure impedance.

In the absence of either type of direct reading instrument to measure impedance, there is a very fundamental method of measuring the scalar (magnitude) component of the primary impedance of any LVDT on the lab bench using Ohm's Law for AC circuits, as described in the steps below:

1. Make sure the core of the LVDT is approximately at its Null position inside the LVDT's bore.

2. Hook up an audio frequency sine wave oscillator to the LVDT's primary leads or connector, being sure both of the LVDT's secondaries are not connected to anything.

3. Set the oscillator at or near the LVDT's nominal excitation frequency, typically 2.5 or 3 kHz.

4. Use a precision AC voltmeter with enough bandwidth to measure at the above frequency the voltage being applied to the LVDT's primary, which should be its nominal AC input, typically 3 V.

5. Use a precision AC milliameter, again with enough bandwidth to measure at the frequency above, connected in series with the LVDT's primary to measure the AC current being drawn.

6. Use a calculator to divide the RMS Voltage from step 4 by the RMS current in step 5. If the AC current is in milliamperes and the AC excitation in Volts, using Ohm's Law for AC circuits, the quotient is the magnitude in kilohms of the primary impedance at the excitation frequency.

7. To know the LVDT's input power in milliwatts, multiply AC voltage (V) by AC current (mA).

8. For an LVDT that is already connected to an LVDT signal conditioner and being powered at or near the LVDT's nominal excitation frequency, it is only necessary to follow steps 1, 4, 5, and 6 above to gather the parameters needed to calculate the LVDT's primary impedance.