1. A.D. PATEL INSTITUTE OF TECHNOLOGY
NAME: HIRPARA CHIRAG
EN.NO.: 160013119006 (E2)
DIVISION: 02
SUBJECT: DYNAMICS OF MACHINERY(2161901)
TOPIC: FREQUENCY MEASURING INSTRUMENTS
BRANCH: MECHANICAL ENGINEERING
2. Frequency Measuring Instruments
Most frequency-measuring instruments are of the mechanical and
electrical type and are based on the principle of resonance.
There are two types of frequency measuring instruments
1. Tachometers
2. Oscillators
3. Fullarton Tachometer
• This instrument consists of a variable
length cantilever strip with a mass
attached at one of its ends.
• The other end of the strip is clamped,
and its free length can be changed by
means of a screw mechanism.
4. • Since each length of the strip corresponds to a
different natural frequency, the reed is marked
along its length in terms of its natural frequency.
• In practice, the clamped end of the strip is pressed
against the vibrating body, and the screw mechanism
is manipulated to alter its free length until the free
end shows the largest amplitude of vibration.
• At that instant, the excitation frequency is equal to
the natural frequency of the cantilever; it can be
read directly from the strip.
5. Frahm Tachometer
• Frahm’s reed tachometer is used to measure the
frequency of vibrations.
• The instrument consists of a number of reeds in the form
of cantilever carrying small masses at their free ends.
6. • The reed with natural frequency rear to that of excitation
frequency to be measured will vibrate at resonance to
produce large amplitude of the vibration.
• Each reed has a different natural frequency and is marked
accordingly. Using a number of reeds makes it possible to
cover a wide frequency range.
• when the instrument is mounted on a vibrating body, the
reed whose natural frequency is nearest the unknown freq
uency of the body vibrates with the largest amplitude.
• The frequency of the vibrating body can be found from the
known frequency of the vibrating reed.
Disadvantage
• The main disadvantage of these instruments is the smaller
angel of frequencies which can be measured
7. Quartz Oscillators
• Mechanical oscillators that resonate based on the
piezoelectric properties of synthetic quartz.
• Excellent short term stability, but poor long term accuracy
stability due to frequency drift and aging.
• Highly sensitive to environmental parameters such as
temperature and vibration.
• A simple quartz oscillator (like those is a stopwatch) is
known as an XO.
• Test equipment usually contains either a TCXO (temperature
controlled quartz oscillator), or an OCXO (oven controlled
crystal oscillators). An OCXO offers the best performance.
8. Rubidium Oscillators
• The lowest priced atomic oscillator.
• A good laboratory standard. Their
long-term accuracy and stability is
much better than an OCXO, and they
cost much less than a cesium
oscillator.
• Rubidium oscillators do not always
have a guaranteed accuracy
specification, but most are accurate
to about 5 10-10 after a short warm
up.
• However, their frequency often
changes due to aging by parts in 1011
per month, so they will require
periodic adjustment to get the best
possible accuracy.
9. Cesium Oscillators
• Cesium oscillators are the primary
standard for time and frequency
measurements and the basis for
atomic time, because the second is
defined with respect to energy
transitions of the cesium atom.
• Cesium oscillators are accurate to
better than 1 10-12 after a short
warm-up period, and have excellent
long-term stability.
• However, cesium oscillators are
expensive (usually $30,000 or more
USD), and have relatively high
maintenance cost. The cesium beam
tube is subject to depletion after a
period of 5 to 10 years, and
replacement costs are high.
