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The linear variable differential transformer (LVDT) is a position sensor that uses a primary transformer winding and two identical secondary transformer windings. These windings are wound around a hollow tube which provides a cavity for a movable core. The movable core is attached to the part of the system whose position or motion is being measured. The movable core provides a magnetic linkage between the primary and secondary windings. Recall that when ac voltage is applied to the primary side of the transformer, the amount of secondary voltage for a transformer is determined by the number of windings and the type of core material. When the movable core is near the secondary winding, it will produce the maximum amount of voltage, and when the core is not near the secondary winding, the secondary winding will produce the least amount of voltage because the transformer essentially has an air core at that time.
Fig. 1 shows an example of an LVDT and its electronic circuitry. From the electronic circuit you notice that the two secondary windings of the transformer are connected as series opposing transformers. The output of each transformer is connected to an op amp and the remainder of the circuit changes the ac output to dc. When ac voltage is applied to the primary winding of the transformer, the output of each secondary will be 180° out of phase with the primary. If the movable core is positioned near the right end, the voltage from that secondary will be greater than the other. When the core is positioned near the left end, the voltage from that secondary will be greater. If the core is an equal distance between the two ends, the voltage will be zero because an equal amount of positive and negative voltage will be produced. The amount of voltage will be zero for two reasons when the core is an equal distance between the two secondaries. First, the amount of voltage produced by the positive secondary will be equal to the amount produced by the negative secondary, which means the op amp will see their sum as zero. The second point to remember is that since the core is an equal distance between the two secondaries, it will have a minimum amount of core in each winding, which means the amount of voltage produced by each secondary will be minimal. When the LVDT's output is zero, it's called the null point for the sensor.
Above: Fig. 1 (a) Electrical diagram of the primary and two secondary
windings of an LVDT sensor. (b) Examples of operating LVDT positioning
sensors.
Since the remainder of the circuit is used to convert the output signal to a dc differential voltage, the output voltage will be positive with respect to ground when the movable core is near one end. When it's near the other end, the output voltage will be negative with respect to ground.
The LVDT is used to sense position because it's very accurate and it has very little friction when movement occurs. The LVDT is simple to troubleshoot since it's essentially a transformer. The first measurement should be the amount of primary voltage. If primary voltage is present, the core can be moved from one end to the other and the output voltage should change from positive dc to negative dc.
