1 Introduction TO RESOLVER and LVDT sensors

Figure 1-1 shows LVDT sensors for industrial applications. Figure 1-2 helps to understand the construction and the theory of operation. The sensor is essentially a transformer with one primary winding and two secondary windings connected in series. Excitation signal V_EXC drives the primary winding. The movable magnetic core couples the signal to secondary windings V₁, V₂. The amplitude is proportional to the position (displacement) of the core.

Figure 1-1 LVDT Sensor

Figure 1-2 LVDT Sensor Principle

Figure 1-3 shows a variable reluctance resolver used typically in motor control applications. Figure 1-4 reveals that the construction and the operating principle is very similar to LVDT sensors. Resolvers have one primary and two independent secondary windings SIN and COS. The secondary windings are electrically in right angle from each other. The excitation signal V_EXC drives the primary coil. The magnetic core distributes the signal between secondary windings with respect to the magnetic core position (angle). The host system decodes the angle from secondary windings voltages V_SIN and V_COS.

Figure 1-3 Variable Reluctance Resolver

Figure 1-4 Resolver Sensor Principle

Table 1-1 compares common excitation signal requirements.

The analog front-end, that senses voltages on secondary windings, has high input impedance. This means that secondary windings are not loaded and the sensor does not transfer any real power. For this reason, the primary winding appears mainly as an inductive load to the excitation amplifier. Also, the amplifier delivers only reactive power.