Velocity_measurement

Velocity_measurement - Linear Velocity Measurement Author:...

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Linear Velocity Measurement Author: John M. Cimbala, Penn State University Latest revision: 19 February 2008 Introduction Velocity is a vector that consists of a magnitude ( speed ) and a direction. Linear velocity is defined as the rate of change of the position vector with time at an instant in time. Technically, velocity is always a vector , while speed is always a scalar . However, most people (erroneously) do not distinguish properly between velocity and speed. In this learning module, we discuss various ways to measure the velocity of solid objects and flowing fluids. We are concerned here only with linear velocity, not angular velocity. Two words are used interchangeably to describe the measurement of velocity: anemometry and velocimetry . Velocity of solid objects There are several instruments used to measure the linear velocity of a solid object, and we highlight several of them here. Linear velocity transducer (LVT) A linear velocity transducer ( LVT ) is an inductive device that is similar in principle to the linear variable displacement transducer (LVDT) discussed previously. It utilizes the link between electricity and magnetism as found by H. A. Lorentz, namely, if a magnetic field moves near an electrical wire, current flows through the wire . Whereas an LVDT measures displacement , an LVT measures speed . An LVT consists of a rod called the core (a permanent magnet), and t electrical coils , as sketched to the right. wo The core slides inside a hollow cylindrical tube called a bobbin , as sketched. A DC voltage is generated when the core moves. Since the two coils are wrapped with opposite polarity , and since the magnet also has two poles (north and south), the south pole induces a voltage primarily in coil 1, and the north pole primarily in core 2. It turns out that the voltage is proportional to the speed of the core, and is relatively independent of position within some limited range near the center (typically about ± 15% to either side of center). Although the range is limited, LVTs are used in some types of machinery. Doppler radar velocity measurement You are familiar with the Doppler shift for sound waves. Namely, a noise (such as a car horn) moving towards you (or away from you) has an apparently higher (or lower) pitch or frequency, since the wavelength of the sound that reaches your ears is compressed (or stretched) due to the relative motion. Doppler radar works by the same principle, but with radio waves instead of sound waves. When radio waves strike a moving object, the frequency of the reflected radio waves is altered in a similar manner as the sound waves. A radar-Doppler velocimeter is sketched to the right. Here is how it works: o Radio waves of wavelength λ are transmitted (incident waves) towards a moving object.
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Velocity_measurement - Linear Velocity Measurement Author:...

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