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29-12-2012, 02:39 PM
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CONTACTLESS TACHOMETER REPORT
CONTACTLESS TACHOMETER



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ABSTRACT

A tachometer is a device that measures the rotation speed of a shaft or disk, as in a motor of other machine. In automotive use, it is used as a gauge showing the speed (RPM) of the engine shaft that is driving the transmission, usually in thousands of rotations per minute. What makes this device special is that it can very accurately measure the rotational speed of a shaft without even touching it. This is very interesting when making direct contact with the rotating shaft is not an option or will reduce the velocity of the shaft, giving faulty readings. This device is built on a microcontroller, an alpha-numeric LCD module, a battery and a proximity sensor or an infrared to detect the rotation of the shaft whose speed is being measured. If we were using proximity sensor, the counted pulses will detect any reflective element passing in front of it, and thus, will give an output pulse for each and every rotation of the shaft. But if we were using infrared, we will put the infrared on both shaft and the tachometer. Those pulses which we get from every rotation of the shaft will be fed to the microcontroller and counted.

INTRODUCTION

BACKGROUND OF STUDY

A contact-less tachometer will let you know how quickly something spins and is frequently used for buses, trains, tractors, trucks, cars and planes. This non contact tachometer version uses a sensor that will sense revolutions through pulses. A contact-less tachometer consists of a shaft encoder and electronic circuits. The output of the shaft encoder provides electric pulses. The frequency of these pulses is proportional to the rotational speed. A speed signal is obtained by processing the pulses from the encoder using an additional electronic circuit. When the wheel or shaft rotates, it has a mirror or a tab that obstructs the path of the light every time it revolves. Then, there is simply a chip to count the number of obstructions per minute. This one is extremely accurate and can handle some of the highest speeds.
This paper proposes a new solution for the processing of the pulses from the encoder to derive the speed signal. The solution takes advantages of new single chip low cost programmable microcontrollers. It is shown how to use hardware and software combined with a suitable method for the speed evaluation to design a high performance tachometer. These can work like an optical sensory as you point it like a laser at what you want to measure.

PROBLEM STATEMENT

A tachometer typically use a rotating target attached to a wheel, gearbox or motor. This target may contain magnets, or it may be a toothed wheel. The teeth on the wheel vary the flux density of a magnet inside the sensor head. The probe is mounted with its head a precise distance from the target wheel and detects the teeth or magnets passing its face. One problem with this system is that the necessary air gap between the target wheel and the sensor allows ferrous dust from the vehicle's under frame to build up on the probe or target, inhibiting its function.
In the other words, the normal tachometer requires physical contact between the instrument and the device being measured. In applications where this is not feasible for technical or safety reasons, it may be possible to use a contactless tachometer to take measurements from a distance. This contactless tachometer is not only useful in terms of safety, but it is also very efficient. The efficiency depends on both the proximity sensor and the reflective element passing in front of it. Thus, it is clear that the method of contactless tachometer is a technique that worthy of being developed.

CONTACTLESS ANALOGUE TACHOMETER

The tachometer employs a standard cathode-ray tube as a voltage to light-spot position actuator, a black white contrast edge on a diameter of the shaft end, and a photomultiplier light detector. The light spot is projected onto the shaft end and switched rapidly along the contrast edge about the shaft centre. Any detector output (at the switching frequency) is amplified, and by using a sampling function circle generator, the system is able to position the light-spot so as to minimize the detector output. Thus, this closed-loop system operates to maintain the light-spot on the contrast edge and the input to the circle generator is the tachometer output [3].
The instrument must be focused on the shaft end; no special calibration is required and the light spot readily locks onto a rotating contrast edge. An experimental instrument has been used on shafts down to 3 mm in diameter, which has a range of at least 20000rev/min in either direction, a bandwidth of about 1 kHz with a resolution of about 400rev/min limited by noise, owing to the effect of background light on the photomultiplier. Greater resolution may be obtained at the expense of bandwidth .

SENSOR

A sensor; is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. A sensor is a device which receives and responds to a signal. A sensor's sensitivity indicates how much the sensor's output changes when the measured quantity changes. Sensors that measure very small changes must have very high sensitivities. Sensors also have an impact on what they measure. Sensors need to be designed to have a small effect on what is measured; making the sensor smaller often improves this and may introduce other advantages.

INFRARED SENSOR

An infrared (IR) sensor is an electronic device that emits and/or detects infrared radiation in order to sense some aspect of its surroundings. Infrared sensors can measure the heat of an object, as well as detect motion. Infrared sensor is electromagnetic radiation with a wavelength between 0.7 and 300 micrometres, which equates to a frequency range between approximately 1 and 430 THz. IR wavelengths are longer than that of visible light, but shorter than that of terahertz radiation microwaves. Bright sunlight provides an irradiance of just over 1 kilowatt per square meter at sea level. Of this energy, 527 watts is infrared radiation, 445 watts is visible light, and 32 watts is ultraviolet radiation. But infrared sensors are usually designed only to collect radiation within a specific bandwidth. As a result, the infrared band is often subdivided into smaller sections.

MICROCONTROLLER

Microcontrollers must contain at least two primary components – random access memory (RAM), and an instruction set. RAM is a type of internal logic unit that stores information temporarily. RAM contents disappear when the power is turned off. While RAM is used to hold any kind of data, some RAM is specialized, referred to as registers. The instruction set is a list of all commands and their corresponding functions. During operation, the microcontroller will step through a program (the firmware). Each valid instruction set and the matching internal hardware that differentiate one microcontroller from another [4].
Most microcontrollers also contain read-only memory (ROM), programmable read-only memory (PROM), or erasable programmable read-only memory (EPROM). Al1 of these memories are permanent: they retain what is programmed into them even during loss of power. They are used to store the firmware that tells the microcontroller how to operate. They are also used to store permanent lookup tables. Often these memories do not reside in the microcontroller; instead, they are contained in external ICs, and the instructions are fetched as the microcontroller runs. This enables quick and low-cost updates to the firmware by replacing the ROM.
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