Linear displacement sensor LVDT

LINEAR DISPLACEMENT Motion control systems typically require a sensor in a feedback loop to ensure that the moving member or component reaches its commanded position. Position sensors are typically used on machine-tool controls, elevators, liquid-level assemblies, forklift trucks, automobile throttle controls, and numerous other applications. Position sensors measure either absolute or incremental displacement, and are made with a variety of different materials. Also, when the sensor’s power fails, some retain the measured position data, and other types lose the information.

One common device is a resistive position sensor or potentiometer. It may be a single-turn or a multi-turn rotary sensor with an element made of carbon or any one of a number of more durable, high-temperature, conductive-plastic film materials. Some types of potentiometers connect to the moving member through a rigid or flexible coupling while others are actuated with a string wrapped around its output shaft. Yet other position sensors include LVDT’s (linear variable differential transformers), sonically operated devices, and digital encoders.

String-Potentiometer Transducers String-type potentiometers are basically rotational devices that typically measure linear motion as shown in Figure 9.01. Because a cable connects between the moving member and the potentiometer, the cable must be preloaded with a spring, have a low coefficient of expansion under wide ranges of temperature (-65 to +125˚C), and have little tendency to stretch. Because of these constraints, its accuracy is somewhat less than that of a more rigid, shaft-coupled sensor.

Most modern potentiometers are extremely linear and are made with special alloys or precious metal wipers and conductive plastic resistive elements to withstand as many as 100 million revolutions. They are usually part of a voltage divider and provide an output voltage proportional to displacement, but they may also drive analog-to-digital converters to feed microprocessor-based instruments directly. However, when digital outputs are needed, a more common practice uses digital encoders that inherently supply higher accuracy.

LVDT:

 Linear Variable Differential Transformers LVDTs are electronic devices comprised of a special transformer with a movable metal actuator in its hollow core. The transformer contains two differential wound secondary winding's on either side of a primary winding. As the actuator in the core moves in and out,

LVDT measure linear displacements over a limited range. It consists of outer coils with an inner moving magnetic core. High frequency alternating currents (AC) is applied to the center coil. This generates a magnetic field that induces a current in the two outside coils. The core will pull the magnetic field towards it, so in the figure more current will be induced in the left hand coil. The outside coils are wound in opposite direction so that when the core is in center, the induced current cancel, and the signal out is zero volt. The magnitude of signal out voltage on either line indicates the position of the core. Near the center of motion the change in voltage is proportional to the displacement. But,Further from the center the relationship becomes non linear.