Tacho or encoder?

Encoders are more reliable and cheaper, as long as your drive supports them thats the way to go.

Probably the cheapest and smallest solution today (when building in quanitity from the ground up) is to use a motor withough a commutator, with a driver circuit that switches the phases at the desired speed, possibly with some feedback detection from the same coils. I think this is what you will find in modern disk drives, for example.

For a one-off, use whatever you can get your hands on surplus - tacho, surplus encoder which could be quite coarse, even one pulse per revolution if the speed is high, etc.

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Depends on the application.

You have to be careful about the number of pulses per rev - I think you will find if there are not enough pulses then your phase amrgin will suffer. Also what's teh point in an encoder if you are using analogue control? You would need to convert back to analogue.

Naebad

How a tacho is used??

Now in my application am giving it to an opamp(error amplifier).

the application is to drive a DC servo motor(90Vmax,3A )

Thanks

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Yes, but do you also need to stop and hold the shaft at a fixed position, do you need to know the exact position of the shaft, etc.

Encoders can be used to generate a tacho-like signal, but at very low RPM including stopped position, the signal isn't as smooth as a true DC-tacho, and you may get small oscillations in the shaft position and audible noise. But if you need to know the exact position you *need* to use an encoder. I had a system once that used both a DC-tacho and an encoder with sine output. Tacho for velocity feedback and encoder to keep track of position during run, and sine output was used to lock the shaft in stopped position.

So, again, it depends on the application. You can't just say that a tacho is better than an encoder or vice verca.

When in doubt, try both ;)

I vote for the encoder. If the control system is analog you only need to feed the encoder output (1 bit for unidirectional systems, 2 to include direction) to a little logic and a "F to V" converter. benefits: better linearity (over almost 4 decades of speed), and retains analog control system (if that is a requirement). for digital control systems it is obviously no contest in favor of the encoder; you could also get as many bits absolute position as you can pay for. speed would be taken from the top three bits to include measure of acceleration.