making tachometer using microcontroller

how to get speed of motor instantly from pulses given by rotation of motor using microcontroller ?
by instantly i mean within 0.5 sec from when the motor starts moving...
thanks

What motor you have in mind, car engine or electromotor ?


Best regards,
Peter

i have a dc geared motor

IKA-TACH
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Contactless tachometer project is based on an ATMEGA48 AVR micro controller, and is able to measure very high RPMs, as well as very low ones. It is based on an IR (Infra Red) opto-couple to detect shaft rotation.

A tachometer is a device used to measure the number of revolutions per minute of a rotating shaft (see wikipedia’s definition). It is mainly used to measure the speed of motors in planes, cars, motor bikes or even bicycles.

The heart of this tachometer is an IR sensor, also called opto couple, which is a diode and photo diode in one package. This opto-couple will send IR beam on the shaft to detect it’s rotation. For that purpose, a small reflective sticker is added on the shaft, so that each rotation of the shaft will cause a “pulse” of IR light to be reflected. Illustration shows a beam or IR light being sent to a rotating shaft having a reflective sticker on it.

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Best regards,
Peter

A variety of gearmotors are available with encoders built into them. Do you get to pick the motor or do you have to use one provided and graft onto it an optical sensor ?
Your microcontroller code just has to count pulses generated (assuming by interrupt), and your micro also has to have a reasonably stable timer interrupt of some suitable interval over which to divide the counted pulses by.

Another way is to measure the back emf. This is done by periodically (once every - say - 0.5 secs) disconnecting the DC power to the motor (are you using PWM drive ?), and measuring the voltage generated by the motor as it free-wheels. The measurement itself can be typically done within 1 - 2 mS.

The back-emf voltage is directly & precisely proportional to speed.

This is known as sensor-less speed measurement.

The optical method shown in #4 is used in most of the measurement places and tacho generator is used in control system applications...

As your need continuous measurement tacho generator method is best.......

The only trouble with optical method is that the optics must be kept well aligned and clean at all times.

The tacho generator requires the physical attachment of the tachogenerator (which is basically a small generator) directly to your motor, in addition to whatever else you have attached. This may or may not be feasible in your case.

The back-emf method is identical to the tachogenerator, except that it requires NO additional attachments of any kind whatsoever. All it requires is a small amount of circuitry - as stated before. Since you are already using a uC, it should be quite easy to do.

Here's a couple of links --

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https://ww1.microchip.com/downloads/en/devicedoc/41233b.pdf

IKA Tach Video Demonstration
https://www.youtube.com/watch?v=R1qmK6vhzPg


Best regards,
Peter