frequency measuring instruments
- 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.
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