How does a system measure the speed of a servo motor using brushes on the commutator?

Question

I have a servo motor with 4 brushes on its commutator. The motor is connected to a controller that enables the user to control the speed and at the same time it has an analog RPM meter which most probably measures the speed of the motor through the two additional brushes on its commutator.

After doing normal maintenance for the motor (checking the brushes, and greasing the bearings), the RPM meter became very inaccurate like reading about half of what the speed should be. I have tried and tried, but matter what I do, I can't calibrate it back to how it was.

I was wondering what is the mechanism for this speed measurement to see if I can find another way to measure the speed continuously through these brushes.

Here's a motor like the one I have:

Answer 1

After some searching i figured out that this motor is combined with tachogenerator and it's output is 14v/1000rpm. Most probable causes at my opinion may be: 

• motor and tach terminals were accidentally swapped
• tach brushes were swapped or flipped which causes bad contact to commutator and thus big voltage drop. Typically brushes should fit better after some minutes of spinning.
• lube or dirt marks on tach commutator or brushes, if exist, should be removed with solvent
• previous causes may be present in main commutator, resulting in low real speed.
• theoretically, main and tach brushes can be made of different materials and, if swapped, work not as good as before.