Read the speed of universal motor

I have an universal motor of washing machine(it’s in picture),

and I want to control it’s speed, i made a standerd dimming circuit based on the zero-crosse detector, but I want to ensure the stability of speed when increasing the load on the motor thus i want to read it’s speed, I know the coil marked in red is responsible for the feedback but I have no idea how to read the motor speed through it(like if i have to read the voltage or the frequancy … etc), so how can i read it’s speed and what circuit i need for that?

If you don't know what is in the package at the end I would probably start by testing for a voltage first whilst turning the main shaft by hand with the meter on a higher range and then stepping down until you get something showing.

If the voltage is very low there is more chance it is simply a sensor (possibly a hall effect)
You could probably count the pulses then for a complete revolution and do the math from there.

It could also be some form of brake for which a voltage may need to be applied.

Quite hard to tell but if there are any numbers on it or you know the make and model of the washing machine you could also trace back to what it is from the many manuals that are online.

The gauge of the wires to it can also often be a clue so if it is the same as the main voltage feed chances are it needs a devent voltage which would indicate a brake.

Much lighter gauge would indicate a sensor or lower voltage device.

In fact, it is a volt generator (a magnet inside the coil), but what struck me is that the output voltage when it's speed 80% is greater than the output voltage at speed 100%, sothat i confused about how to read it.

Are you sure it is not being used as a braking system or some form of soft start ?
From your description it sounds plausible.

Are you sure it is not being used as a braking system or some form of soft start ?

Its likely to be an AC tachogenerator. Standard for washing machine motors. The output
frequency measures the speed to detect overload conditions. The speed is controlled by
selecting a couple of different field coils usually.

Interestingly there are several types of motor used in washing machines, universal ones
like this, 3-phase induction motors, and direct-drive BLDCs.

3-phase appeared once the control circuitry for them became cheap - they require vector / field-oriented control methods to provide position / force / velocity control. Basically software became
sophisticated enough that the hardware can be simplified - induction motors are cheaper to
make and the only parts to wear out are the bearings.