Chapter 3 - Sensors

Abstract

The linear variable differential transformer (LVDT) is an accurate and reliable method for measuring linear distance. LVDTs find uses in modern machine-tool, robotics, avionics, and computerized manufacturing. The LVDT is a position-to-electrical sensor whose output is proportional to the position of a movable magnetic core. The core moves linearly inside a transformer consisting of a center primary coil and two outer secondary coils wound on a cylindrical form. Measurement of temperature is critical in modern electronic devices, especially expensive laptop computers and other portable devices with densely packed circuits which dissipate considerable power in the form of heat. Knowledge of system temperature can also be used to control battery charging as well as prevent damage to expensive microprocessors. Resistance Temperature Detectors (RTDs) are accurate, but require excitation current and are generally used in bridge circuits. Thermistors have the most sensitivity but are the most nonlinear. Modern semiconductor temperature sensors offer high accuracy and high linearity over an operating range. Charge coupled devices (CCDs) contain a large number of small photocells called photosites or pixels which are arranged either in a single row (linear arrays) or in a matrix (area arrays). CCD area arrays are commonly used in video applications, while linear arrays are used in facsimile machines, graphics scanners, and pattern recognition equipment.