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.

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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

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Acceleration sensor (Accelerometer)

BASIC THEORY

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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

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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 C mc : Top plate capacitor C mb : Bottom plate capacitor

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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

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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 

L si DM

1

M

2 x: distance of a point on the cantilever from the support

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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

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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.

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