Linear Displacement Transducer
A linear displacement transducer is a device that measures displacement. Unlike a strain sensor, it is not pressure-based. It calculates displacement by transforming it into an electrical signal. Also, the output of a linear displacement transducer is a direct function of the input.
Primary and Secondary Coils
A linear displacement transducer is basically made up of three coils. These loops of wire are classified as primary and secondary. The transducer has one primary coil which creates a magnetic field, and two secondary coils which are placed opposite to one another on either side of the primary coil. The two secondary coils are symmetrical, meaning they are wound in opposite directions.
When alternating current is driven to the primary coil, voltage is inevitably induced in each secondary coil. The manner in which the current is transmitted depends on the position of a mobile armature, which is actually a magnetic core. As long as the armature remains at the center, the voltages of the two secondary coils are the same. But since they are on opposite sides, the voltages cancel out and the voltage output is zero.
Displacement of the Core
But every time the armature is displaced from the balanced position, it necessarily strays away from the center. The balance of voltages shifts when this happens. The voltage increases on one secondary sensor while decreasing on the other. The primary coil records this imbalance and an output that represents this movement is thus obtained from the sensor. This is why the voltage output of a linear displacement transducer is a direct function of the input, which is actually the displacement of the moveable magnetic armature.
The level of voltage output is always proportional to the displacement—the distance moved by the core. This is why the displacement being measured by the transducer is considered linear. The direction of displacement is determined through the phase of the voltage. The core moves without friction making the displacement measurement reliable.
The linear displacement transducer is normally housed in a casing highly permeable to magnetic forces. This protects the internal components from external magnetic fields, thereby ensuring that the output attained is clean data.