1. General Concepts
The linear variable differential transformer (LVDT) is a sensor used to measure displacement in the industrial environment. An LVDT consists of three coils of wire wound on a hollow form. A core of permeable material can slide freely through the center of the form. The inner coil is the primary coil, which is excited by an AC source as shown. Flux formed by the primary is linked to the two secondary coils, inducing an AC voltage in each coil. When the core is centrally located in the assembly, the voltage induced in each primary is equal. If the core moves to one side or the other, a larger AC voltage is induced in one coil and a smaller AC voltage in the other because of changes in the flux linkage associated with the core.
The rotational variable differential transformer (RVDT) is used to measure rotational angles and operates under the same principles as the LVDT sensor. The LVDT uses a cylindrical iron core, while the RVDT uses a rotary ferromagnetic core.
A resolver is, in principle, a rotating transformer. If we consider two windings, A and B, and if we feed winding B with a sinusoidal voltage, a voltage is induced into winding A. If we rotate winding B, the induced voltage is at maximum when the planes of A and B are parallel and at minimum when they are at right angles. Also, the voltage induced into A varies sinusoidally at the frequency of rotation of B so that EOA = Ei sin(theta). If we introduce a third winding (C), positioned at right angles to winding A, as we rotate B, a voltage is induced into this winding and this voltage varies as the cosine of the angle theta, so that EOC = Ei cos(theta).
If you can measure the relative amplitudes of the two winding (A and C) outputs at a particular point in the cycle, these two outputs are unique to that position. The rotation of winding B causes the magnitude of the induced signals to vary as a function of the angular position. The voltage induced in A is in quadrature (at a 90o offset) to the voltage induced in C.
Using the output of the two windings gives an absolute position, since each position has a different combination of A and B. The frequency also changes with the velocity and you can also determine the velocity.
The data output from the two phases is usually converted from analog to digital by means of a resolver-to-digital converter. Resolvers typically achieve a resolution up to 65,536 counts per revolution.
Developer Zone: LVDT Fundamentals
Developer Zone: How to Choose Among LVDT/ RVDT/ Potentiometer/ Optical Encoder/ Ultrasonic/ Magnetostrictive/ Other Technologies
Developer Zone: Measuring Position and Displacement with LVDTs
Developer Zone: Resolvers
Developer Zone: Basics of Feedback with Resolvers
Developer Zone: LVDT Definition
Developer Zone: RVDT Definition
Developer Zone: Variable Reactance Transducer
Developer Zone: Types of Accelerometers
Developer Zone: Displacement/ Location/ Position Sensors
Developer Zone: Transformer Transducer
2. Signal Conditioning Required
The output of an LVDT is an AC waveform and therefore does not actually have a polarity as such. The magnitude of the output of an LVDT raises regardless of the direction of movement from the electrical zero position.
In order to know in which half of the coil the center of the armature is located, you must consider the phase of the output as well as the magnitude. The output phase is compared with the excitation phase and it is either in or out of phase with the excitation, depending upon which half of the coil the center of the armature is located. The electronics therefore, must combine information on the phase of the output with information on the magnitude of the output. This allows you to know exactly where the armature is rather than how far from the electrical zero position it is located.
LVDT signal conditioners generate a sine wave for the primary coil and synchronously demodulate the secondary output signal, so that the DC voltage that results is proportional to core displacement. The sign of the DC voltage indicates whether the displacement is to the left or right. The signal conditioning for LVDTs consists primarily of circuits that perform a phase-sensitive detection of the differential secondary voltage. The output is a DC voltage whose amplitude relates the extent of the displacement, and the polarity indicates the direction of the displacement.
A broad range of LVDTs is available with linear ranges of at least ±25 cm down to ±1 mm. The time response is dependent on the equipment to which the core connects. The units of an LVDT measurement are typically in mV/V/mm or mV/V/in, which indicates that for every volt of stimulation applied to the LVDT there is a definite feedback in mV per unit distance. A carefully manufactured LVDT can provide an output linear within ±0.25% over a range of core motion and with a very fine resolution, limited primarily by the ability of hardware to measure voltage changes.
A resolver requires that it receive an excitation AC signal. This signal must have a frequency that is adjustable to the specifications required by the individual resolver. The excitation voltage must also have different settings for the varied resolvers. To obtain the signal at least two channels are necessary to take measurements from both of the sensing coils. From these two measurements you can take an arctangent to obtain the angle of the resolver.
KnowledgeBase 270AHCLL: Using the SCXI-1540 with a Device that Requires External Excitation
KnowledgeBase 2QDA4IRA: How do I Connect My Resolver to My DAQ Equipment?
KnowledgeBase 27J8ETCR: SCXI-1540 Troubleshooting
KnowledgeBase 2ZOAHTIP: How Do I Exactly Produce my Sensor Excitation with the SCXI-1540?
3. NI Supported Devices
Terminal Block: SCXI-1315