The correct answer is: D. Heart wall motion.
A linear variable differential transformer (LVDT) is a type of transformer used to measure linear displacement. It consists of a primary coil, a secondary coil, and a magnetic core that can move between the coils. When an alternating current is applied to the primary coil, a magnetic field is created. This magnetic field induces a voltage in the secondary coil. The voltage in the secondary coil is proportional to the displacement of the core.
LVDTs are used in a variety of applications, including:
- Heart rate monitors
- Position sensors
- Force sensors
- Displacement sensors
In the context of heart rate monitors, LVDTs are used to measure the movement of the heart wall. This information is used to calculate the heart rate.
In the context of position sensors, LVDTs are used to measure the position of a moving object. This information is used to control the movement of the object.
In the context of force sensors, LVDTs are used to measure the force applied to an object. This information is used to control the force applied to the object.
In the context of displacement sensors, LVDTs are used to measure the displacement of an object. This information is used to control the movement of the object.
The other options are incorrect because they do not describe a linear displacement.