Documentation

LVDT/RVDT basics

The linear variable differential transformer (LVDT) and rotary variable differential transformer (RVDT) are electromagnetic displacement transducers designed to provide output voltages proportional to linear and rotary displacement respectively.

The LVDT consists of an AC-excited primary coil and two identical secondary coils on a non-contacting magnetic core, as shown in the cutaway view in the figure below. The RVDT is, in principle, like the LVDT.

When the core is moved away from the center position, the mutual inductance of the primary coil with one secondary coil increases and with the other secondary coil decreases due to the change in the reluctance paths. The induced voltages are no longer equal and an output voltage appears.

LVDT/RVDT wiring

Concerning how the connections between both secondary coils are implemented, there are three possibilities.

For each of the output voltage graphics, the primary is excited with a sine-wave voltage that the frequency of which, depending on the design, may be from 47 [Hz] to over 10 [kHz]. As in any transformer, the output voltage is a function of the turns ratio and the coupling efficiency between the primary and secondary windings.

How to connect your device

This section describes connection methods for your measured or simulated LVDT/RVDT/Synchro/Resolver.

LVDT/RVDT measurement

When measuring an LVDT/RDVT with the IO421-1 submodule, the following connection methods can be used.

2-wire (primary as reference)

This connection method converts the widest range of LVDT/RVDT sensors and is the most sensitive to excitation voltage variations, temperature and phase shift effects.

3-wire (derived reference)

In this connection method, excitation is on the primary side (as with the 2-wire method) but the converter reference is the sum of A + B, which has a constant amplitude for changing core displacement. This system is not sensitive to temperature effects, phase shifts, or oscillator instability. To use this method, the driver is configured in 4-wire mode and the A LO and B LO inputs are connected together.

4-wire (derived reference)

LVDT/RVDT simulation

When simulating an LVDT/RVDT with the IO421-3 submodule, the following connection methods can be used. In all connections, the primary is excited with a sine-wave voltage.

4-wire LVDT

The default configuration is the 4-wire LVDT/RVDT:

3-wire LVDT

To simulate a 3-wire LVDT/RVDT, connect the A LO and B LO pins as shown below:

2-wire LVDT

To simulate a 2-wire LVDT/RVDT, use A HI and A LO pins only:

	Note
	The phase lock between the excitation and the signal output can take up to 1 second after connecting a valid excitation signal or starting the model. Consequently, no signals are immediately present at the output.

Synchro/Resolver

Synchros and resolvers are essentially transformers. They have a primary winding and multiple secondary windings, and like a transformer, their primary winding is driven by an AC signal. Synchros and resolvers are very similar; however, there are some differences. As shown in Figure 1, a synchro has:

In contrast, as shown in Figure 2, a resolver has:

Electrical representation of a Synchro:

Figure 1

Electrical representation of a Resolver:

Figure 2

While a synchro and resolver are electrically very similar to a transformer, they are mechanically more like a motor. The primary winding in a synchro or resolver can be physically rotated in relation to the secondary windings. For this reason, the primary winding is called the "rotor"; the secondary windings (which are fixed) are called "stators."

Synchros and resolvers are often used to track the rotary output angle of a closed-loop system, which uses feedback to achieve accuracy and repeatability. A synchro/resolver can be turned continuously and, since its secondary winding outputs are analog signals, it provides "infinite" resolution output. As the shaft of a synchro transmitter is turned, the angular position of its rotor winding changes in relation to its secondary (stator) windings. The relative amplitude of the resulting AC output signals from the secondary (stator) windings indicates the rotary position of the transmitter’s shaft.

Two-speed configuration: For better resolution, you can use one channel as "coarse" and the other channel as "fine" dial.

	Note
	The phase lock between the excitation and the signal output can take up to 1 second after connecting a valid excitation signal or starting the model; consequently, no signals are immediately present at the output.