A Digital RPM Meter is a measuring instrument which can measure the rotational speed of a rotary machine digitally. In industries we can use this type of digital tachometers which will reduce human effect. The industrial name of this instrument is "T
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Contactless Digital Tachometer Using 8051 Microcontroller
m § What is tachometer?? § Uses. § Types. § Working Principle. § Comparison Between Analog and Digital Tachometers. § Calibration.
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§ A tachometer is an instrument designed to measure
the speed of an object or substance. § A M M also called a M M § The word is formed from Greek roots: M 3 meaning speed3 and M3 meaning measure.
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§ The first mechanical tachometers were based on
measuring the centrifugal force3 similar to the operation of a centrifugal governor. § The inventor is assumed to be the German engineer Dietrich Uhlhorn; he used it for measuring the speed of machines in 1817. Since 18403 it has been used to measure the speed of locomotives.
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§ Tachometer is used for measuring rotational speed. § Can be used to measure speed of a rotating shaft. § Can also be used to measure flow of liquid by
attaching a wheel with inclined vanes.
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§ ven medical science uses tachometers. § If a turbine like small device is placed in the vein or on
an artery3 the doctor can make use of the tachometer to measure the blood flow rate from the spinning speed of the turbine in the device.
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§ Tachometers on automobiles3 aircraft3 and other vehicles show the rate of rotation of the engine's crankshaft3 and typically have markings indicating a safe range of rotation speeds. § In vehicles such as tractors and trucks3 the tachometer often has other markings3 usually a green arc showing the speed range in which the engine produces maximum torque3 which is of prime interest to operators of such vehicles.
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§ very engine spins within the limits of its design.The
pistons inside the engine pump the crankshaft to spin.This spinning crankshaft sends horsepower to the street. § A tachometer counts the number of rotations the crankshaft is making a minute PM).
§ If the PM of an engine is allowed to go outside of
specific limitations3moving parts begin to generate enough heat and friction to become damaged. § Use a tachometer to know when to shift and when to stop pushing a motor for more.
§ Tachometers can be classified on the basis of data
acquisition Ȃ contact or non contact types.. § They can also be classified on the basis of the measurement technique Ȃ time based or frequency based technique of measurement. § They can also be classified as analog or digital types.
m Analog Tachometer § ÷as a needle and dial type of interface. § No provision for storage of readings. § Cannot compute average3 deviation3 etc.
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Digital Tachometer § ÷as a LCD or LD readout. § Memory is provided for storage. § Can perform statistical functions like averaging3 etc.
m Time Based § The tachometer calculates speed by measuring the time interval between pulses. § More accurate for low speed measurement. § Time to take a reading is dependant on the speed and increases with decrease in speed. § The resolution of the tachometer is independent of the speed of the measurement.
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Frequency based § The tachometer calculates speed by measuring the frequency of pulses. § More accurate for high speed measurement. § Time to take a reading is independent of speed of rotation. § The resolution of the
tachometer depends on the speed of the rotating shaft.
m m m Contact Type § The tachometer has to be in physical contact with the rotating shaft. § Preferred where the
tachometer is generally fixed to the machine. § Generally3 optical encoder / magnetic sensor is attached to shaft of tachometer.
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Non Contact Type § The tachometer does not need to be in physical contact with the rotating shaft. § Preferred where the tachometer needs to be mobile. § Generally3 laser is used or an optical disk is attached to rotating shaft and read by a I beam or laser.
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m § Contact type Ȃ The wheel of the tachometer needs to
be brought into contact with the rotating object. § Non Contact type Ȃ The measurement can be made without having to attach the tachometer to the rotating object.
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m § Time Measurement Ȃ The tachometer calculates speed
by measuring the time interval between the incoming pulses. § Frequency Measurement Ȃ The tachometer calculates speed by measuring the frequency of the incoming pulses.
Display 5 digits large LCD ange: 2.5 - 993999 PM Distance: 50 to 13000 mm; 12 to 40 inches. Measurement angle: at less than 120 degrees. ange selection: Auto Laser Output Power: <1mW class II Memory: Last value3 Max Value3 Min. Value Time base: Quartz crystal Circuit: xclusive one-chip LSI circuit Battery: 4 X 1.5V AA Weight: 300g/0.65lb Size: 190 X 72 X 37 mm
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§ It is used to generate pulses proportional to the
speed of the rotating shaft. § Can be achieved by the following ways: § Attaching a disk3 which has an alternate black and white
pattern3 to the shaft and reading the pulses by a I module pointed towards it. § Using a slotted disk and a U shaped I emitter detector pair to generate waveforms.
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§ ÷all effect sensors Ȃ These make use of the ÷all effect
to generate pulses proportional to the speed of the shaft. § Passive magnetic sensors Ȃ These make use of variable reluctance to generate pulses.
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m § The output of the sensors may be noisy. § The output may have to be amplified. § It has to be digitized.
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§ Not essential3 but is generally the norm to have a
microcontroller. § Compute the speed. § Can store the readings. § Can output values to a display unit. § Give out warning signal when speed reduces / increases. § Transfer data to external controller.
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§ Used to output the values to the operator. § Can be used to view the stored values.
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§ These are generally the ones that display the speed of
your car. § The interface is needle and dial arrangement.
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§ Generally speed is converted to voltage through the
use of an external frequency to voltage converter. § The tachometer can also act as a generator and produce a voltage that is proportional to the speed of the shaft. § This voltage is then displayed by an analog voltmeter.
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§ Accuracy § Precision § ange § Acquisition Time § Contact type / Non Contact type § Portable / Fixed § Digital / Analog § Cost ÷
m § Why calibrate? § Wrong calibration = Wrong readings § Calibration compensates for wear3 tear and other
degrading effects.
§ ÷ow to calibrate? § Calibration is done by comparing the reading from
tachometer to a standard speed. § Necessary changes are made so that the actual reading matches the desired reading.
