Velocity sensor
Specifications for electromagnetic velocity sensor
Velocity sensors can utilize the same principles of displacement sensor, and produce a
response as a function of time.
A typical velocity sensor, in a fixed direction can be based on a design similar to the LVDT.
In this case, a permanent magnet can be attached to the moving part which will induce a
differential voltage at the output proportional to the speed of the permanent magnet. Note
that unlike LVDT, a constant voltage is produced only when there is a constant movement.
Slide # 1
Acceleration sensor (Accelerometer)
What can the accelerometers measure?
•
•
•
•
•
•
•
Acceleration
Tilt and tilt angle
Incline
Rotation
Vibration
Collision
Gravity
Accelerometers are used in a wide variety of machines, specialized
equipment and personal electronics such as:
•
•
•
•
•
•
•
Self balancing robots
Tilt-mode game controllers
Model airplane auto pilot
Car alarm systems
Crash detection/airbag deployment
Human motion monitoring
Leveling tool
MEMS accelerometers are one of the most important and widespread MEMS devices used in our
everyday life
Slide # 2
Acceleration sensor (Accelerometer)
BASIC THEORY
Slide # 3
Acceleration sensor (Accelerometer)
CHARACTERISTICS
1. Sensitivity is the ratio of an electrical output
to the mechanical input. It is usually expressed
in terms of volt per unit acceleration. For
example the unit may be 1 V/g. It is typically
measured at a single reference frequency
Slide # 4
Acceleration sensor (Accelerometer)
CAPACITIVE
• In accelerometer there is an inertial or dead mass which is displaced, with respect to the housing, when the
there is acceleration
• The sensor measures the mass displacement by various means, which is proportional to acceleration for
small displacements
• Most accelerometers these days are micromachined out of Si and integrated to signal conditioning circuits
Cmc: Top plate capacitor
Cmb: Bottom plate capacitor
Slide # 5
Analog Devices ADXL50 accelerometer chip
• The sensors fixed plates are
charged with a 1 MHz signal with
180º phase shift.
• Under no acceleration the
distance between the plates is
the same (Fig. 16), and the
signal output is zero.
• When there is acceleration, the
center plate moves toward one
of the fixed plates and a signal is
produced
• The loop bandwidth for the
signal is ~1 KHz
Slide # 6
Heated plate Accelerometer
• In heated plate accelerometer there is a
hanging plate (cantilever) with integrated
heating and temperature sensing
elements (thermopile)
• The temperature sensed by the
thermopile changes due to the change in
proximity of the plate to the thermal sinks.
• These accelerometers are less sensitive
than the capacitive or piezoelectric ones,
but are much more stable with respect to
ambient temperature fluctuations and
electromagnetic/electrostatic noise

K g M 1  M 2 
Lsi DM1M 2
x: distance of a point on the cantilever from
the support
Slide # 7
Heated Gas Accelerometer
• This accelerometer consists of a micromachined
plate with a heater at the center and 4 temperature
sensing elements (Al/poly Si thermopiles) at the 4
corners
• In equilibrium condition, the differential temperature
between diagonally opposite sensing elements is
zero, but when there is acceleration or tilt, there is
differential temperature between elements, and a
signal is detected
• These accelerometers capable of measuring both
dynamic and static accelerations
• They have a large dynamic range from below 1g to
100 g, with a typical noise floor of 1 mg/Hz.
• Their response time is the order of ms, with typical
frequency response ~100 Hz.
• The sensitivity gets affected quite significantly by
ambient temperature
MEMSIC MX 2125 chip
Slide # 8
Piezoelectric Effect
• Appearance of an electric potential across certain faces
of a crystal when it is subjected to mechanical pressure
• The word originates from the greek word “piezein”, which
means “to press”
• Discovered in 1880 by Pierre Curie in quartz crystals.
• Conversely, when an electric field is applied to one of
the faces of the crystal it undergoes mechanical
distortion.
• Examples --- Quartz, Barium titanate, tourmaline. Quartz
crystals is one of the most stable piezoelectric materials.
Slide #