Linear Velocity Measurement
Author: John M. Cimbala, Penn State University
Latest revision: 19 February 2008
is a vector that consists of a magnitude (
) and a direction.
is defined as
the rate of change of the position vector with time
at an instant in time.
Technically, velocity is always a
, while speed is always a
. 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
Two words are used interchangeably to describe the measurement of velocity:
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)
linear velocity transducer
) 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.
if a magnetic field moves near an electrical wire,
current flows through the wire
Whereas an LVDT measures
, an LVT measures
An LVT consists of a rod called the
(a permanent magnet), and t
, as sketched to the right.
The core slides inside a hollow cylindrical tube called a
sketched. A DC voltage is generated when the core moves.
Since the two coils are wrapped with
, 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
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.
works by the same principle, but with
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
A radar-Doppler velocimeter is sketched to the right.
Here is how it works:
Radio waves of wavelength
(incident waves) towards a moving object.