A most important device that is used to provide
velocity feedback is the tachometer.
It is also known as rpm gauge, and revolution counter.
A tachometer is employed in a motor to calculate the
rotational speed of a shaft.
The output is displayed as RPM (revolution per
minute) in an analog device.
The two common types of a tachometer
1) AC tachometer
2) DC tachometer
1) AC tachometer:
It possesses primary and secondary stators with fixed
windings, and a rotor with permanent magnet.
If the rotor is stationary, a constant output voltage will be
If the rotor is moving, proportional to the speed of a rotor
This type of tachometer cannot provide information of
direction with only one output winding.
2) DC tachometer:
DC tachometer is the most commonly used instrument in
It is a DC generator implemented to provide an output
voltage that is proportional to the angular velocity of the
In this mechanism, the rotor and rotational part will be
attached directly. It has a stationary device called
as commutator, which is connected with the split slip
It is used for picking the induced output signal from the
Laser surface velocimeter:
A laser surface velocimeter (LSV) is a non-
contact optical speed sensor measuring
velocity and length on moving surfaces.
Laser surface velocimeters use the laser
Doppler principle to evaluate the laser
light scattered back from a moving
LSV is based on Doppler effect (or Doppler shift) that
is the change in frequency of a wave (or other periodic
event) for an observer moving relative to its source.
They are widely used for process and
quality control in industrial production
A piezoelectric sensor is a device that uses
the piezoelectric effect, to measure changes in
velocity, pressure, acceleration, temperature, strain,
or force by converting them to an electrical charge.
The prefix piezo- is Greek for 'press'
Principle of operation:
The way a piezoelectric material is cut
produces three main operational
Two main groups of materials are used
for piezoelectric sensors: piezoelectric
ceramics and single crystal materials.
Schematic of piezoelectric
Piezoelectric sensors are versatile tools
for the measurement of various
They are used for quality
assurance, process control, and for
research and development in many
An accelerometer is a device that measures proper
acceleration(Time rate of change of velocity).
They are typically used in one of three modes:
1) As an inertial measurement of velocity and position;
2) As a sensor of inclination, tilt, or orientation in 2 or 3
dimensions, as referenced from the acceleration of
gravity (1 g = 9.8m/s2);
3) As a vibration or impact (shock) sensor