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A tachometer is just a little DC motor. Usually permanent magent, no field necessary. Ouput is relatively linear based on speed.
Cons: Can be electrically noisy, expensive.
In the old days they were used quite a bit. Modern systems use encoders. They are cheap and easy. In this application you don't need 1024 Pulses Per Rev (PPR) or 2500 is also common. Those are also Quadrature encoders. Essentially giving 4x the PPR. You're right, you don't need that kind of precision. We use those for high accuracy feed to position lines. Also for motor speed to gearbox speed comparisons, where on a hoist we want to be sure the gear train is intact.
I remember in one point of this thread someone talking about a Black and White tape and an optical sensor. Or a disc with notches that breaks a beam in an opto sensor. At low RPM however 1 PPR may not be enough. There is a tradeoff. You want a good pulse train at low speeds, but you don't want the Frequency of the pulses at high speed to exceed the working frequency range of whatever device you are counting pulses with. For example, a 5000 PPR Encoder traveling 10,000 RPM generates 833KHz. ((5000 x 10,000)/60) This may exceed the working range of the input device. Now I doubt your motors are traveling 10,000PRM, and that anyone will get a 5000PPR encoder. Even as low as 10PPR would be good. That way it's not too long between pulses at low speeds for a controller to be able to calculate RPM.
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