A tachometer encoder can be used for measurement of speed:

Shaft Encoders are digital transducers that are used for measuring angular displacements and velocities.

Shaft Encoders can be classified into two categories depending on the nature and method of interpretation of the output:

Incremental encoders:

• The output is a pulse signal that is generated when the transducer disk rotates because of the motion that is being measured.
• These are often called a tachometer, is normally used in systems that rotate in one direction only and require simple position and velocity information.

Quadrature encoders:

• It has many pulse tracks on its transducer disk. When the disk rotates, several pulses trains equal in number to the tracks on the disk are generated simultaneously.
• They have dual channels (A and B), phased 90 electrical degrees apart. These two output signals determine the direction or rotation by detecting the leading or lagging signal in their phase relationship.
• Quadrature encoders provide very high-speed bi-directional information for very complex motion control applications.