Digital Tachometer

ABSTRACT

Basically a Tachometer is one which shows the speed of a rotating part in rpm which

may be either the input/output of a motor or moving wheel or shaft, etc. Here we

design a Tachometer which performs the same operation as above.

The rotating shaft is used as an input which is fed to a digital circuit consisting of XNOR

gate, resistance and capacitance. The part of input is directly passed as one of the input

to the XNOR gate and another input to XNOR gate is passed through RC circuit. The

XNOR logic doubles the frequency of input signal fed to UP counter thus increasing

precision in measurement.

On commercial scale Tachometer finds application in automobile industries where in it is

used to measure the speed of vehicles like car, buses, bikes, etc. It is also used in small

scale in laboratories to measure speed of rotating machines like motor, generator, etc.

Digital Tachometer

Chapter 1

Introduction to Digital Tachometer

1.1 What is Digital Tachometer?

Basically a Tachometer is one which shows the speed of a rotating part in rpm which

may be either the output of a motor or moving wheel or shaft, etc. It can also be used to

measure flow of liquid by attaching a wheel with inclined vanes.

1.2 What Are the Different Types of Tachometers?

Tachometers can be classified on the basis of data acquisition – contact or non

contact types

They can also be classified on the basis of the measurement technique – time

based or frequency based technique of measurement

They can also be classified as analog or digital type

1.2.1 Classification Based on Data Acquisition Technique

o Contact type – The wheel of the tachometer needs to be brought into contact

with the rotating object

o Non Contact type – The measurement can be made without having to attach the

tachometer to the rotating object

1.2.2 Classification Based on Measurement Technique

o Time Measurement – The tachometer calculates speed by measuring the time

interval between the incoming pulses

o Frequency Measurement – The tachometer calculates speed by measuring the

frequency of the incoming pulses

Digital Tachometer

1.3 Comparisons

1.3 .1 Comparison between Analog and Digital Tachometers

Sl. No. Analog Tachometer Digital Tachometer

1. Has a needle and dial type of interface Has a LCD or LED readout

2. No provision for storage of readings Memory is provided for storage

3. Cannot compute average, deviation, etc Can perform statistical functions like averaging, determination of shaft velocity, etc.

1.3.2. Comparison between Contact and Non Contact Tachometers

Sl. No. Contactless type tachometer Non-contact type tachometer

1. The tachometer has to be in physical contact with the rotating shaft

The tachometer does not need to be in physical contact with the rotating shaft

2. Preferred where the tachometer is generally fixed to the machine

Preferred where the tachometer needs to be mobile

3. Generally, optical encoder / magnetic sensor is attached to shaft of tachometer

Generally, laser is used or an optical disk id attached to rotating shaft and read by a IR beam or laser

1.4 Typical Specifications of a Non Contact Type Tachometer

Display 5 digits large LCD /LED

Range: 2.5 - 99,999 RPM

Distance: 50 to 1000 mm

Resolution: 0.1 RPM<1000 RPM (2.5 to 9,999 RPM) 1.0RPM >1000 RPM

Measurement angle: at less than 120 degrees.

Range selection: Auto

Laser Output Power: <1mW class II

Sampling Time: 1.0 seconds (over 60 RPM)

Memory: Last value, Max Value, Min. Value

Time base: Quartz crystal

Digital Tachometer

Circuit: Exclusive one-chip LSI circuit

Battery: 4 X 1.5V AA

Weight: 300g/0.65lb

Size: 190 X 72 X 37 mm

1.5 Typical Specifications of a Contact Type Tachometer

Display 5-digit LCD/LED Display

Range selection Automatic range selection

Time Base 4MHz Quartz Crystal

Sampling Time 1 second (>60 rpm); >1 second (10 to 60 rpm)

Accuracy ± (0.1% of reading + 2 digits)

Photo Tachometer Distance 2 to 12” (5 to 30cm)

Operating Temperature 32 to 122oF (0 to 50oC)

Operating Humidity 80% RH Max.

Power supply 9V Battery

Battery Life 40 hours (approx.)

Dimensions 461700: 4.9 x 2.0 x 1.3” (124 x 51 x 33mm)

Weight 461700: 4.0 oz. (114g)

1.6.1 Disadvantage of Analog Tachometer

1. Less accuracy

2. Difficult to take reading as pointer takes long time to get stable.

3. No provision of storage of values

4. Cannot compute average deviations

Digital Tachometer

1.6 .2 Advantage of Digital Tachometer

1. Accuracy is high as display is used

2. Easy to take reading as little time delay is provided.

3. Memory is provided for storage of values

4. Can compute average deviations

1.6.3 Disadvantage of Digital Tachometer

1. Since the digital tachometer is held in hand, slight variation in its position

changes its speed.

2. If the speed is varying at every instant then the digital display fluctuates

and makes it difficult to read the actual speed.

Digital Tachometer

Chapter 2

Working Principle

2.1 General Concept of Digital Tachometer

Tachometer consists of an optical sensor which generates pulses proportional to the

speed of rotation. It counts the number of pulses sensed by the sensor and number of

pulses per second gives the rps and when multiplied by 60 gives rpm.

2.2 Block Diagram of a Digital Tachometer

Fig 2.1

2.2.1 Sensing the input signal

It is done by two ways:

1. Optical Sensing

1.1. Uses electromagnetic wave (Laser, Infrared). The variation in signal reception

produces pulse.

1.2. Can be achieved by the following ways:

1.2.1. Attaching a disk, which has an alternate black and white pattern, to the

shaft and reading the pulses by an IR module pointed towards it.

Digital Tachometer

1.2.2. Using a slotted disk and a U shaped IR emitter detector pair to generate

waveforms.

2. Magnetic Sensing

2.1. Hall Effect sensors – These make use of the Hall Effect to generate pulses

proportional to the speed of the shaft.

2.2. Passive magnetic sensors – These make use of variable reluctance to generate

pulses.

2.2.2 Logic circuit

Logic circuit, combination of XNOR gate and RC circuit, doubles the frequency of

input signal and helps in getting more precise value, thus it reduces counting period to

0.5 second instead of 1 second thus increasing the precision.

2.2.3 Counter circuit

To count the number of pulses sensed in a given period of time. A 4029[1] decade

counter IC is used. The counter is further cascaded with another 3 4029 ICs to get 4 digit

display, as a single counter facilitate only one digit display.

2.2.4 Timer circuit

To give the time delay during which the counter circuit counts the number of

pulses. An astable multivibrator is used to perform this delay operation.

2.2.5 Display

In order to display the speed of object under observation in revolutions per

minute (rpm), a common cathode 7-segment display is used with a driver IC 4511[2] to

display the speed in rps as counted by the counter.

Digital Tachometer

Chapter 3

Project Stage-I

3.1 Modelling and Simulation using Multisim

3.1.1 Main Circuit

Fig 3.1: Simulated Circuit showing different blocks of the circuit

Digital Tachometer

3.1.2 Logic Circuit

Fig 3.2: Logic Circuit

The voltage source in the Fig 3.2 represents the sensor whose output resembles

that of sensor. The input wave is given as input to XNOR gates out of which one

input is fed to a RC circuit as shown in the circuit diagram. The doubling of

frequency can be showed from the following wave forms.

Fig 3.3: Input and output waveforms of logic circuit.

Digital Tachometer

𝒇 =𝟏

𝑻 (3.1)

Or 𝒇′ =𝟏

𝑻′ (3.2)

Or 𝒇′ =𝟏𝑻

𝟐

Or 𝒇′ =𝟐

𝑻

Or 𝒇′ = 𝟐𝒇 (3.3)

3.1.3. Astable Multivibrator

Fig 3.4: Output waveform of Astable Multivibrator

The astable multivibrator circuit is realised using IC NE555 [3]. Its output is inverted using

the not gate IC 74LS04 [4] . The values of passive elements in the circuit are designed

using the following formulae for a frequency of 0.66Hz i.e. 1.5 sec time period.

F =1.44

R1 + R2 ∗ C (3.4)

For a required frequency standard capacitance value is assumed.

Ton=0.69*R1*C (3.5)

TOFF=0.69*R2*C (3.6)

The application demands to have Ton =.5 sec and TOFF = 1 sec.

Digital Tachometer

3.1.4. Counter cascaded with 7 segment display driver

Fig 3.5: Pin to Pin diagram of IC 4029 cascaded with IC 4511

The BCD (binary coded decimal) counter (IC 4029) start counting upwards at each rising

edge of clock pulse. The output of BCD counter as the name says is in BCD i.e. it cannot

be displayed on a 7 segment display. To have a display on 7 segment display the output

of counter is given as inputs of a BCD to 7 segment driver (IC 4511), the output of whose

drives the 7segment display. The 1st pins of all IC 4029 (reset pin) are shorted, similarly

the 5th pins of IC 4511 (latch enable pin) are shorted and the output of astable

multivibrator is given to them; this arrangement counts up to 0.5 sec and holds the

count for 1 sec(as per multivibrator design).

Digital Tachometer

Chapter 4

Project Stage-II

4.1 Hardware Implementation (Bread Board Implementation)

Fig 4.1: Implemented Circuit with respective blocks labelled

Digital Tachometer

4.1.1 Working of sensor circuit

Fig 4.2: Circuit diagram of infrared sensor

The infrared(IR) sensor consists of IR emiter and IR reciever. An IR emitting diode is

suplied with a pulse of frequency 36kHz; since the IR reciever TSOP1736[5] is responsive

only when it recieves the IR signals with the carrier wave of frequency 36kHz. The IR

reciever will have its output low when it is not receiving any signal. The reflected signal

falls on the IR reciever causing its output to go low. This cointinous high and low output

results in pulse. This pulse is fed to the logic circuit. In order to reflect the IR rays a small

patch of white paint is painted on the shaft. The fig 4.3 shows the pulse generated by IR

receiver.

Digital Tachometer

Fig 4.3: Sensor Circuit and its output on CRO

4.1.2 Working of logic circuit

Fig 4.3: Logic Circuit and its practical output on CRO

4.1.3 Working of astable multivibrator (timer circuit)

The timer circuit consists of NOT gate and 555-TIMER, which sets the time duration

during which the speed is to be measured and displayed.

Digital Tachometer

Fig 4.4: Astable Multivibrator

4.1.4 Working of counter circuit and display

Fig 4.5: Counter circuit cascaded with 7-segment driver

Digital Tachometer

Chapter 5

Conclusion

5.1 Applications

1. Used in laboratories to measure speed of motor/generator/engine.

2.

5.2 Future Scope

1. Can be Implemented using microcontroller so that size gets reduced .i.e. it

becomes compact.

2. Circuit can be modified to get speed in rpm using DAC[11], ADC[12] & op-

amp[13].

5.3 Merits

1. Can work efficiently in any light conditions.

2. Being contactless, doesn’t put load on rotating shaft.

5.4 Short comings

1. Speed measured is displayed in rps not in rpm.

2. Range of IR emitter used is limited, which causes hindrance.

5.5 Conclusions

1. The Digital Tachometer designed is economical for lab purposes.

Digital Tachometer

References

1. IC 4029

2. IC 4511

3. IC NE555

4. IC 74LS04

5. IC TSOP1736