SCXI is a signal conditioning platform for PC-based data acquisition (DAQ) systems used in instrumentation applications. An SCXI system consists of a shielded chassis that houses a combination of signal conditioning input and output modules, which perform a variety of signal conditioning functions. You can connect many different types of transducers, including LVDTs, directly to SCXI modules. SCXI operates as a front-end signal conditioning system for PC plug-in DAQ devices or DAQ modules in PXI measurement and automation systems.
Note: The NI SCXI-1540 is mature and is not recommended for new designs.
See Obsolescence Planning for Several Legacy Product Families for more information about migration options.
Figure 8. SCXI Signal Conditioning System
The National Instruments SCXI-1540 8-channel LVDT module provides the necessary conditioning to measure signals from transformer-based ratiometric position sensors, including LVDTs, rotary variable differential transformers (RVDTs), and resolvers. The SCXI-1540 offers both 4 and 5-wire connections for LVDTs and RVDTs. In addition, this module offers autocalibration without external hardware using NI-DAQ driver software. Each of these modules can multiplex its signals into a single channel of the DAQ device, and modules can be added to increase channel count. With random scanning capabilities, you can select only the channels from which you want to acquire data.
How to Connect LVDTs
LVDTs typically come in 4-wire (open wire) and 5-wire (ratiometric wire) configurations. Wires from the sensor connect to a signal conditioning circuit that translates the output of the LVDT to a measurable voltage. The two circuits in the figures below depict the external connections to the conditioning circuit.
Figure 9. 4-Wire Connection of an LVDT to a Signal Conditioning Circuit
Figure 10. 5-Wire Connection of an LVDT to a Signal Conditioning Circuit
Note: The color of the wires may vary.
4-wire and 5-wire configurations are differentiated by the way the signals from the first and second secondaries are conditioned. In the 4-wire configuration, only the voltage difference between the two secondaries is measured. The following equation relates the measured voltage to the displacement, where G is the gain or sensitivity:
The benefit of using a 4-wire configuration is that you require a simpler signal conditioning system. “This is at the expense of temperature stability and phase coherence between the primary excitation voltage and the resulting secondary voltages. Temperature changes can alter the LVDT’s magnetic induction efficiency. This causes a change in the perceived voltage for a given displacement. Because the 4-wire scheme is also sensitive to phase changes between the primary and the resulting secondary voltage, long wires or a poor excitation source can also cause problems.”
The 5-wire configuration is less sensitive to both temperature changes and phase differences between the primary and the secondaries. “The reason for the temperature stability lies in the fact that the voltage changes due to the changes in magnetic induction efficiency affect voltages VCH+ and VCH- equally with respect to ground and thus null the effects of temperature.” Similarly, phase information is determined at the signal conditioning circuitry without needing to reference the phase of the primary excitation source. Therefore, longer wires can be used between the LVDT and the signal conditioning circuitry. The following equation relates the measured voltage to the displacement, where G is the gain or sensitivity:
Each of the eight analog inputs consists of an instrumentation amplifier, a variable gain stage, a demodulation circuit, and a 250 Hz lowpass filter. Excitation voltage can be set for 1 or 3 Vrms and a frequency of 2.5, 3.3, 5 or 10 kHz.