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99 000 RPM Contact-Less Digital Tachometer
Featuring LCD display and automatic DATA hold function
By Ibrahim Kamal
Last update: 4/4/08

Overview
This article describes how to build a contact -less tachometer (device used to count the revolutions per minute of a rotating shaft) using a 8051 microcontroller and a proximity sensor.

As the name implies, 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 an AT89S52 (or AT89C52) microcontroller, an alpha-numeric LCD module and and a proximity sensor to detect the rotation of the shaft whose speed is being measured.

A 600 mA.h Ni-Cd battery provides months of regular use of this device before it needs to be recharged.


Key Features:
Measures up to 99 000 RPM
Instantaneous measurement
Automatic DATA Hold Function

LCD display
Ni-Cad Rechargeable battery

Important: this tachometer uses a proximity sensor. In case you don't know how to make a proximity sensors, and/or how to operate them, please refer to this article first.

Contact less tachometer principle of operation
The idea behind most digital counting device, frequency meters and tachometers, is a micro-controller, used to count the pulses coming from a sensor or any other electronic device.

In the case of this tachometer, the counted pluses will come from proximity sensor, which will detect any reflective element passing infront of it, and thus, will give an output pulse for each and every rotation of the shaft, as show in the picture. Those pulses will be fed to the microcontroller and counted.

To understand how a micro
controller counts pulses, and deduce the frequency of those pulse, please refer to this tutorial about building a frequency meter, that elaborates the process of frequency counting.

The main difference between this tutorial about tachometer and frequency meters, is that we need the reading in pulses per minutes (to count revolutions per minutes), but in the same time, we don't want to wait a whole minute before getting a correct reading. Thus we need some additional processing to predict the number of revolutions per minute in less than a second.

Instantaneous measurement algorithm
To be able to deduce an RPM reading in less than second, while constantly refining the reading's accuracy, a simple algorithm have been developed, where a counter and a timer are used. Counter and timers are part of the internal features of a micro-controller, (like the AT89C52 used in this project) and they can be easily configured through programming.

The schematic below, shows how the timer and the counter are used for this task; The counter is connected i such a way to count pulses coming from the proximity sensor, while the timer is used to precisely feed the counted value to the microcontroller every filth of a second, and
reset the counter to 0. The microcontroller can now take an average of the last 3 readings (saved in C1, C2 and C3) and calculate the average numbers of pulses per fifth second, then multiply this value by 5, to get the number of pulses per second, then multiply this value by 60 to get the number of pulses per minute, which represents the measured RPM. The only purpose of calculating an average reading is that it will allow to get more

C1, C2 and C3 are used to store the last 3 reading
stable reading and prevent display flickering.


The electronic Circuits
This device is composed of 2 electronic circuits: the Sensor, which is a slightly modified proximity sensor, and the microcontroller board, which analyses pulses coming from the sensor, process them and display the result on the LCD display.

The microcontroller board:

Circuit explanation:

The LCD connections in the green shading is a standard for most of alpha numeric LCDs, the only feature I added is to be able to control the back light via the 80c52 microcontroller. The LCD protocol can seem complicated to some of you, and an article should be released soon to explain it.

The part in the blue shading is also standard in any 8051 microcontroller circuit, which includes the reset circuitry along with the crystal resonator that generates the clock pulses required.

The power supply, shaded in light red, regulates a 9V rechargeable Ni-CD battery and also provides a very simple battery monitor, with a green and a red LED, showing whether the battery need to be recharged or not.

The switch SW1, shown in the upper yellow circle, is used to enable/disable the measurement or the counting process. When the switch is pressed, the device measures the RPM of the shaft under test, and constantly updates the reading on the LCD, when the switch is released, the last reading is held unchanged on the display, as long as the device stays on. When the switch is pressed again the old reading is replaced by the new one.

The wire connection P1, which is connected to the output of the sensor, is connected to the pin 3.4 of the microcontroller, this pin has a dual function which is to count incoming pulses and increment a 8, 13, or 16 bit register according to the configuration of the timer T0.

As you may have noticed, this schematics misses tow important items to be called a tachometer: The C code loaded into the microcontroller, which will be discussed later, and the proximity sensor, which will feed the pulses to be counted.

The modified IR proximity sensor:
This schematic show the slight modification over the one proposed in this tutorial, which is the fact that the emitter LED uses a current limiting resistor of a higher value, to allow it to be turned on for a long period of time, because in this specific application, we need to turn the IR emissions on or off, but we don't need to inject high currents to reach high ranges... I recommend the reading of this article that fully covers all the aspects of this sensor.

The CTRL line, is an input coming from the microcontroller (at the wire connection: P4), turning the IR emissions ON and OFF, and the OUT line, is the output of the sensor, which is fed to the microcontroller (at the wire connection: P1).

After analyzing both the main board holding the microcontroller and the sensor, here is a simple
diagram showing how they are connected together. You will have to refer to the above schematics to see where P1, P2, P3 and P4 goes in the main board, as well as the other lines concerning the sensor.

This picture also shows what is meant by the connection of the sensor to the main board. The reason for separating the sensor from the main board, is to allow better performance sensors, or even other types of sensors to be connected to the device. In general, modular designs cost more, but is more useful in the prototyping phase...

The software
Here are only small relevant parts of the full C program, that was loaded into the microcontroller after being compiled to a HEX file. Those part of the code were selected as the ones that emphasize the main purpose of a microcontroller in such an application. For examples, function dealing with the LCD operation are not included in this description. Comments in green explains the program. The full code is available in the Project folder, downloadable at the bottom of this article
.
#include <REGX51.h>
#include <math.h>

unsigned int clk_tmp,clk_tmp2,clk_sec,clk_sec2;
unsigned intex_pulses,rps,rps_tmp,temp,rps_avg,rps_max;
unsigned int rps_his[5];
char a,b,c,d,e;
unsigned char count1,count2;
unsigned char scale = 4;

delay(y){ // A function to make software delays
unsigned int i;
for(i=0;i<y;i++){;}
}

setup_interrupts(){ // This function initialises the TIMER and the COUNTER to
EA = 1;             // be used in in the trachometre
ET0 = 1;      //set the Timer/counter 0
TR0 = 1;      //Enable Timer/counter 0 to count
TMOD = 0X25;  //counter 0 in mode 1 (16 bit counter),
              //timer 1 in mode 2 (auto reload from TH1)
TH1 = 0;      //start counter from 0
ET1 = 1;      //enable timer 1
TR1 = 1;      //Enable Timer/counter 1 to count
PT0 = 1;      //Setup the priorities of timer 1 and timer 0, a 0 gives a
PT1 = 0;      //higher priority.
}

void int_to_digits(unsigned int number){ //store the 5 digits of an integer
float itd_a,itd_b;                       //number in the variable a,b,c,d,e
itd_a = number / 10.0;
e = floor((modf(itd_a,&itd_b)* 10)+0.5);
itd_a = itd_b / 10.0;
d = floor((modf(itd_a,&itd_b)* 10)+0.5);
itd_a = itd_b / 10.0;
c = floor((modf(itd_a,&itd_b)* 10)+0.5);
itd_a = itd_b / 10.0;
b = floor((modf(itd_a,&itd_b)* 10)+0.5);
itd_a = itd_b / 10.0;
a = floor((modf(itd_a,&itd_b)* 10)+0.5);
}


clk() interrupt 3        //timer 1 interrupt
{
clk_tmp++;          //Software counter for the timing of the tachometer readings
clk_tmp2++;         //Software counter for the display refresh rate
if (clk_tmp2 > (1236)){  // update display
clk_tmp2 = 0;
rps_avg = floor(((rps_his[0] + rps_his[1] + rps_his[2] + rps_his[3] + ___
          ___rps_his[4])/5)*60);
}

if (clk_tmp > (6584/scale)){ // update the measured RPM
clk_tmp = 0;
if (P2_0 == 0){
rps = TL0;
temp = TH0;
temp = temp * 256;
rps = (rps + temp)* scale;
rps_his[4] = rps_his[3];
rps_his[3] = rps_his[2];
rps_his[2] = rps_his[1];
rps_his[1] = rps_his[0];
rps_his[0] = rps;
}
TL0 = 0;
TH0 = 0;
}
}

count_pulses() interrupt 1 //counter 0 interrupt
{
if (scale < 10)      // If the pulses are so fast that the internal counter
scale++;             // overflows, increase the variable 'scale' so that
}                    // so that readings are recorded at a higher rate

void main(){
   scale = 10 ;
   P3_3 = 0; // ini proximity sensor, OFF
   P3_4 = 1; // ini sensor input
   P1_1 = 0; //turn LCD backlight ON
   P2_0 = 1; //ini count/hold button
   ini_lcd(); // ini the LCD
   setup_interrupts();

   while(1){
      P3_3 = ~P2_0;
         if (P2_0 == 1){
         scale= 4;
         }
   }
}


To understand the functioning of this source code, you must have some basic microcontroller and C language skills.
The variable scale is used to control the rate at which the tachometer reads and resets the counter.

The housing of the tachometer
For the housing, an old floppy disk drive case is used, where the tachometer and the battery fits perfectly.
Here, those few pictures are worth a thousands words.
(click on a picture to enlarge)



Download the zip file for the project.
containing the PCB, Schematic and Example 8051 C51 code.
[note: i use ExpressPCB(FREEWARE) to design the schematics and the PCB]

Discussion (Last 15 posts preview...)
Preview of the last 15 messages discussing this page. Messages are sorted from the newest to the oldest.
Posted by:
rakesh5454
on: 03 Jul 2009
Re: 99 000 RPM Contact-Less Digital Tachometer
['Quote ]
Hi,

This website is fantastic.
I have a wee doubt. Can someone please tell me how to transmit these measured values from the micro-controller to a base station about 100 meters wirelessly. Please guide me. How do i integrate a transmitter. Any circuit diagrams and part models in particular i should be using?
Thanks.
Posted by:
tarungoel11
on: 29 Jun 2009
Re: 99 000 RPM Contact-Less Digital Tachometer
['Quote ]
hi,can you tell me what options to use to compile the code in keil eval version.Also i would be thankful if i can get any help in using the tach for closed loop speed control.
Posted by:
sudheer
on: 16 Jun 2009
99 000 RPM Contact-Less Digital Tachometer
['Quote ]
sir,can i purchase cotactless 99krpm tachometer? i am a b-tech student from india,kerala,if so ,pleeeeeeeeeees inform me about the cost,and send me immediately,thank you......
my contact number is+9847425581(name:shabil)
email id:shabilabbas@yahoo.com
Posted by:
sudheer
on: 16 Jun 2009
99 000 RPM Contact-Less Digital Tachometer
['Quote ]
sir, i have done ur contactless tachometer,but i did nnt get the output, no modification is made,and LCD is not displaying anything,have u provided full program 4 the circuit from the downloadable file? if not pleeeeeeeeeees send me the full program including the hex file,pleees sir pleees ,its very urgent,we are in trouble.reply fast
thank you
Posted by:
tahirmaqsood
on: 17 May 2009
speedometer
['Quote ]
Dear sir
i whant rpm mter cunvert to speedometer + destensmeter
plz help me for chanch the code

Thanks
Tahir
Posted by:
dbomber
on: 28 Apr 2009
99 000 RPM Contact-Less Digital Tachometer
['Quote ]
is there a problem if I use the AT89S52 instead of the AT89C52 ?? thatīs beacuse I want to use the ISP programmer. Also I want to know if all the 16x2 LCD have standard connections, for I can be sure to connect it the same way as shown in the tachometer project. Can somebody help ?? Thanks a lot !!
Posted by:
bjdowndown
on: 25 Apr 2009
Re: 99 000 RPM Contact-Less Digital Tachometer
['Quote ]

Quoting vipindaskb: why cant the AT89c51 be used instead of 89c52

C51 (4k Byte Flash) , C52(8k Byte Flash) but they're the same price .
Posted by:
phenom_x8
on: 22 Apr 2009
Data Logger with AT89s52 for my infra red tachometer.
['Quote ]
I am constructing an infra red tachometer using at89s52 microcontroller just like one in this website project ("contact less 99k RPM tachometer"). But instead of displaying the result on the LCD, I planned to use my PC as the display device (only as a display, not to count the RPM data(c1,c2,c3), because its still use the micro to count it ) . The result data stored in microcontroller EEPROM will be sent to computer via serial port or any port that possible, I m using VB6 to write a program to display the RPM measured. Is there anyone know how to connect my microcontroller to my PC so that my PC can read the data stored on my microcontroller (after googling, I recognized that I need data logger for this purpose, but all of them already embedded with the temperature measuring project. No single tutorial explain about this data logger specifically and separately) If There is a link about this, please let me know it . Thanks!!
Posted by:
erhemal
on: 22 Apr 2009
99 000 RPM Contact-Less Digital Tachometer
['Quote ]
sir.......
i am in problem to use register in this project........
there R7 & R8....
what is value of this register......
........
can i made,R8 is 10Kohm is place two 5Kohm in center of three terminals.......


plz give me quick anser................
plz help me..............
thank angin........
Posted by:
sukrut
on: 12 Apr 2009
99 000 RPM Contact-Less Digital Tachometer
['Quote ]
hi.i have made the tachometer using 89s52.so do i have to do any changes in the code for it.reply immediately
Posted by:
erhemal
on: 08 Apr 2009
99 000 RPM Contact-Less Digital Tachometer
['Quote ]
sir.
can u given me the specification about LCD in used in this project.....
I can no find any those type of LCD...
calculator LCD is used here....Yes...then which is program for that its same....
Posted by:
gaurav21gupta
on: 06 Apr 2009
99 000 RPM Contact-Less Digital Tachometer
['Quote ]
hi sir i m in final year elect. want to make the contact less tachogen can u plz provide me the details for various parts so as to make it easier to purchase(reflector strip, lcd module, sensor, controller etc).
also the steps to follow after assembling i.e. for programing the u-controller.
plzzzz... help
Posted by:
neil1221
on: 30 Mar 2009
99 000 RPM Contact-Less Digital Tachometer
['Quote ]
sir I have created this project but there is a problem..the reading is not accurate..for example it reads for about 5000rpm then jumps to 6500rpm then it continously change readings..it doesnt stay on 1 reading and the value it jumps so far from the previous reading that it displays,even if my rotating element is on stable speed, please help me sir to fix this..will I make some modification on the sensor or the program..please guide me to stabilized the readings...thank you sir...
Posted by:
erhemal
on: 28 Mar 2009
99 000 RPM Contact-Less Digital Tachometer
['Quote ]
hi
Posted by:
neil1221
on: 09 Mar 2009
99 000 RPM Contact-Less Digital Tachometer
['Quote ]
sir what specific LCD did you use here....can you provide us the data sheet so I can make some comparison to my LCD...
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IMPORTANT COPYRIGHT NOTE: Electronics and Robotics Articles by Ibrahim KAMAL are licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 United States License.