Introduction
Linear Variable Differential Transformers (LVDTs) is one of the conventional primary sensing tools for positioning. LVDTs are still used as position sensors in harsh conditions such as industrial plants, particle accelerators, nuclear power plants, etc. because of their low sensitivity towards heat, and their contactless sensing. Also, depending on the applications, LVDTs are conveniently customizable, these are known as custom LVDTs.
What is LVDT?
An LVDT is a passive inductive sensor that shares the same working principle as a transformer, that is mutual inductance. As in a transformer, an LVDT consists of a primary winding and a secondary winding wound around a ferromagnetic former, and a movable core is placed inside the former. The primary winding is positioned between two identical secondary windings as shown in the figure below:
The primary winding is energized with high-frequency AC voltages. The motion to be indicated is connected with the core actuator. As the core inside the former moves, the magnetic paths between the primary and secondaries change, thus giving secondary outputs proportional to the movement.
There will be radial and longitudinal magnetic forces on the core all of the time. It is possible to view these magnetic forces as magnetic springs that attempt to shift the center to its core location. In certain applications, this could be important.
What is meant by custom LVDTs?
LVDTs are used for various ranges of applications in manufacturing industries, ATMs, moisture testing, as well as in automation applications. According to the application requirements, the LVDTs may vary in design. The installation technique used, size, and measurement range are some of them. LVDT manufacturers customize the LVDT as per the client’s requirements; these LVDTs are generally called custom LVDTs.
The allowable custom modifications for LVDTs vary from manufacturer to manufacturer. Although some general modifications in custom LVDTs are listed below:
- Various mounting techniques. The mounting customization depends on the vibration of the application, where the device is to be mounted and the fitting required, and the size of the LVDT.
- The size of the LVDT is customizable to an extent.
- Electrical output requirement.
- Material choice. Corrosion-resistant materials are required for many industrial applications.
- Settings required for radiation protection in nuclear power plant applications.
- Customer specified types of connector, insulation material, wiring length, and wire colors.
Different types of custom LVDTs
LVDTs are customized to make them suitable for different applications. So, depending on the electrical property and application specifications, custom LVDTs are available in various types.
Captive Armatures:
These custom LVDTs are controlled by low friction assemblies, which avoid misalignments. Hence captive armature LVDTs are used for applications that require a long lifetime.
Unguided Armatures:
Unguided armature custom LVDTs offer a no-wear design that provides infinite resolution. In this type of custom LVDT, special protection is required for the linear transducer, since it is connected loosely to the measurands.
Force Extended Armatures:
These types of custom LVDTs are used where measurements need be taken from sluggish medium and slow-moving applications. To drive the armature constantly to its maximum possible extension, force extended armatures use internal spring mechanisms, pneumatic power, or electric motors.
Custom LVDT products/services
There are a lot of unique applications for LVDTs in industrial and other applications. Custom LVDTs are the best choice for such applications. Custom LVDT manufacturers provide a variety of LVDTs with some customizable features such as electrical output, fittings, radiation resistance, size, etc. Some of the custom LVDTs products are listed below:
AC custom LVDT
Usually, these are miniature LVDTs that cannot accommodate built-in signal conditioners. They are mostly used in applications such as robotics, medical uses, and optical components.
DC custom LVDTs
DC LVDTs come with embedded signal conditioners, so they are comparatively larger and used in industrial applications.
Spring-loaded LVDTs
These are used for high-resolution precision applications in tight places. Spring-loaded LVDTs are used for civil engineering, robotics, and optical components.
Custom connections for LVDTs
One of the best benefits of custom LVDTs is customizable connection methods. In our market, you can find different connection methods suitable for connecting LVDTs with steam turbines, threaded connections as per client requirements, mounting feet for preferred applications, and extra holdings to resist vibration.
Radiation resistant LVDTs
Such LVDTs are constructed with special radiation-resistant materials that operate continuously without decay. They are used to measure the position of the control rod in the nuclear plant, submarines, missile fins, etc.
LVDT signal conditioner
The LVDT output will be an AC sine wave where the amplitude is proportional to the off-center position of the LVDT core. The output AC has to convert into measurable DC voltage for digital systems. It is carried out using signal conditioners attached to the LVDT setup. The position of the core can be determined from the amplitude of the dc output, and the direction of the movement of the core can be determined from the polarity of the DC voltage.
Phase-sensitive demodulator chips such as the AD598 are available in the market. To suit unique application requirements, these chips are extremely scalable and flexible to use. Such circuits offer a wide range of input and output voltage, and frequencies, which enable them to be used in a variety of applications.
Applications for custom LVDTs
LVDTs have a wide range of applications, from simple displacement measurements to jet engine control near exhaust gases, and material thickness calculations of roll positions in hot-slab steel mills. LVDTs can also be used to measure force and pressure after some mechanical modifications. Other applications of LVDTs are listed below:
- LVDTs are used in gravitational wave interferometers: These are smaller LVDTs with high gain and position resolution below that of nanometers.
- Soil testing: An external rod that is connected to the core of LVDT will move as the load moves to compress the testing soil inside a chamber. The load-displacement will be measured by the LVDT.
- Robotic control: These are used for joystick control based heavy equipment robotics.
- Flat-panel display thickness checking: A variant of small size LVDTs is used to determine the thickness of the flat panel display during the inspection.
- Pressure gauges: LVDTs are used as the second sensing element in pressure gauges that use the bourdon tube as the primary sensing element.
- Aerospace applications: For monitoring flight controls and the steering mechanism.
Advantages of LVDTs
A major benefit of LVDT is that there is no direct interaction between the core and the coil form, and thus there will be no friction or wear. Other advantages are listed below:
- Sensitivity: The LVDT sensitivity is virtually independent of changes in atmospheric temperature during high-frequency excitation.
- Hysteresis: LVDTs have low hysteresis thus they provide excellent repeatability.
- Enables a wide range of displacement measurements with a range of 1.25mm to 250 mm.
- There is complete isolation between input and output, which makes an LVDT an effective analog computing element without buffer amplifiers.
- To an extent, LVDTs are rugged and shock-resistant, which makes them maintenance-free.
LVDTs will continue as significant transducers in future automation and robotics for measuring displacement and as a part of the control circuits. Since LVDTs have a long lifetime and shock resistance, overall they are very reliable transducers.