Should I specify an AC- or DC-operated LVDT linear position sensor for my application?

Mike Marciante – Applications Engineer (TE Sensor Solutions)

An AC-operated LVDT Linear Position Sensor does not contain any internal electronics and requires an external oscillator, carrier amplifier, or demodulators and filters to operate. A DC-operated LVDT Linear Position Sensor is comprised of an AC-operated LVDT and a carrier generator/signal conditioning module.  It maintains all the desirable characteristics of the AC-LVDT, but has the simplicity of DC operation. Applications often dictate the choice of an AC- or DC-operated LVDT. Here are some examples:

AC-OPERATED LVDT APPLICATIONS

Extreme temperatures: below -20°C or above 85°C: Constructed with appropriate materials, AC-operated LVDTs can operate in temperatures from -200°C to 500°C.  A DC-operated LVDT, on the other hand, is limited by the properties of the materials in the electronic signal conditioning module. 

Hard-to-Reach installations: While LVDTs with internal electronics may have a 20 year expected life, free-core, non-contact AC LVDTs have an even longer life expectancy. Their high reliability makes them ideal for installations in locations without easy access.  Without the need for internal electronic components, AC-LVDTs also can be offered in smaller package sizes to fit in compact locations. Remote electronics can be installed in an accessible location away from the LVDT.  

High Shock/Vibration Environments: Sensitive electronic components can be affected by shock and vibration. Installing an AC- LVDT allows the user to segregate the sensing element from the electronic circuitry. Connected by long cables up to 31 meters (100 feet), AC-operated LVDTs can work in hostile environments with remotely-located electronics that operate in benign areas.

DC-OPERATED LVDT APPLICATIONS

Easy and Fast Installation: DC input/DC output and factory-calibrated output allow for a simple and quick set-up. Using ASIC and microprocessors, internal electronics can provide for more complex processing functions as well as signal conditioning within the sensor housing. As there is need for calibration or reliance on amplification equipment, setup time is reduced as well as overall system cost.

Eliminates Signal Conditioning Requirements: The DC-operated LVDT can eliminate the volume, weight and cost of conventional external AC excitation, demodulation and amplification equipment (ideal for outdoor applications where a control panel may not exist). They also can produce digital outputs directly compatible with computer–based systems and standardized digital buses, which is desirable in metrology and subsea applications.

Loop-Powered Designs: Unlike voltage signals, current signals will not diminish over a long run of cable. This makes loop-powered 4-20 mA LVDTs ideal for applications where long cable runs in excess of 1,000 feet are required. This is a very useful feature in subsea and outdoor applications where control panels can be located far from the sensor.

Benefits of AC- vs. DC-Operated LVDTs at a Glance:

SCENARIOS FOR CHOOSING AN AC VS DC LVDT

1. A position sensor was required on the door to a large furnace where temperatures reach 175°C. Because of its 200°C temperature rating, TE Connectivity’s Macro Sensors HSTAR AC-operated LVDT and LVC-4000 signal conditioner were specified. While the LVDT was exposed to the high temperatures, the LVC-4000 was mounted remotely where the temperature was controlled to produce a 4-20 mA output.      

2. The Macro Sensors GHSIR Spring-Loaded 4-20 mA Loop-Powered LVDT  was specified to monitor structural components on a bridge. Because control panels on bridges can be far apart, the cable runs between an LVDT and its associated electronics could be as high as 1,000 ft. Using TE Connectivity’s Macro Sensors GHSIR allowed the customer to make the measurements with little concern over the LVDT’s proximity to the closest control panel.