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FALL 2004 MKM 503E MECHATRONIC SYSTEM COMPONENTS

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Sensor: The term sensor is used for an element which produces a signal relating to the quantity being measured

SENSORS AND TRANSDUCERS

Transducer: The term transducer is often used in place of the term sensor. Transducers are defined as elements that when subject to some physical change experience a related change.

SENSOR=TRANSDUCER

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PERFORMANCE TERMINOLOGY

Range and Span: Range of a transducer defines the limits between which the input can vary. The span is the maximum value of the input minus the minimum value.

Example: a load cell is used to measure forces might have a range of 0 to 50 kN and ists span is 50 kN

Error: Difference between the result of the

measurement and the true value of the quantity being measured.

Error = measured value – true value

Example: If a measurement gives a temperature reading of 25 °C when the actual temperature is 24 °C, the error is +1 °C.

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Accuracy: Accuracy is extent to which the value

indicated by a measurement system might be wrong. It is thus summation of all possible errors that are likely to occur, as well as the transducer has been calibrated.

Example: A temperature-sensing instrument might be specified as

having an accuracy of ± 2 °C. This means that the measurements can be expected to lie between + or - 2 °C of the true value.

Accuracy is is often expressed as a percentage of the full range output or full scale deflection.

Example: accuracy of ± 5 % of full range output, full range is 0 to 200 °C

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Sensitivity: The relationship indicating how much output you get per unit input.

sensitivity = output/input

Example: A resistance termometer’s sensitivity is 0.5 Ω/ °C.

This term is frequently used to indicate the sensitivity to inputs other than that being measured. There can be sensitivy of a transducer to environmental

conditions, such as temperature, fluctuations of the supply voltage, etc.

Example: A pressure sensor might have a temperature sensitivity of ±1% of the reading per °C in temperature.

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Hysteresis Error: Maximum difference in output for increasinfg and decreasing values.

Non-Linearity Error: Maximum difference from the straight line (an linear characteristic).

Example: an nonlinearity error of a pressure sensor may be quoted as ±0.5% of the full range.

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Repeatability/Reproduceability: Ability to give the same output for repeated applications of the same input value. 100 range full n valuesgive . min . max ity repeatabil = − ×

Example: a transducer for the measurement of angular velocity typically might be quoted as having a repeatability of ±0.01% of the full range at particular angular velocity.

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Stability: Ability to give the same output when used to measure a constant input over a period of time. A term drift is often used to describe the change in

output that occurs over time. Drift may be expressed as a percentage of the full range output.

The term zero drift is used for the changes that occur in output when there is a zero input.

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Dead Band/time: Dead band or dead space of a

transducer is the range of inut values for which there is no output.

Example: in flow meters, due to bearing friction, there is no output until the flow reaches to a particular value.

The dead time is the lenght of time from the

application of an input until the output begins to respond and change.

Resolution: A smallest change in the input until an observable change in the output occurs. For a

sensor giving digital output the smallest change in the output is one bit.

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DYNAMIC CHARACTERISTICS

Resonse Time: The time which elapses after a constant input, a step input, ia applied to the

transducer up to a point at which the transducer gives an output corresponding to some specified percentage, e.g. 95%.

Time Constant: This is the 63.2% of the respnse time. Rise Time: This is the time taken for the output to

rise to some specified percentage of the steady state output.

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Displacement, Position and Proximity Sensors - Potentiometers

- Strain Gauged Element - Capacitive Element

- Differential Transformers

- Eddy Current Proximity Sensors - Inductive Proximity Switch

- Optical Encoders - Pneumatic Sensors - Proximity Switches - Hall Effect Sensor

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Linear Potentiometer

Moving direction

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Various Applications of Strain Gauged Elements

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Push-pull sensor _{Capacitive proximity sensor} Capacitive Element Applications

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Hall Effect Proximity Sensor b Bi K_H = Hall U + Output (V) - 0

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Mechanical Proximity Switches

Lever Operated Roller Operated Cam Operated

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FALL 2004 MKM 503E MECHATRONIC SYSTEM COMPONENTS 31

Pyroelectric Sensors

t ∆ = ∆q k_p Pyroelectric Effect

Lithium Tantalate Curie Temp. 610˚C

Equivalent Circuit

Dual Pyroelectric Sensor Application

Unpolarized

Crystal Polarized Crystal

External Electric Field Applied during the heating Process

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Fluid Level Measurement

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Pressure Sensors

Bellows

LVDT Bellows

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Diaphragm Pressure Pressure Gauge

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Pressure Measurement

Orifice Plates

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Temperature Sensors

Bimetalic Strip

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Temperature Sensors

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The magnetic flux created by the primary current I_P is concentrated in a magnetic circuit and

measured in the air gap using a Hall device. The output from the Hall device is then signal

conditioned to provide an exact representation of the primary current at the output.

The magnetic flux created by the primary current l_pis balanced by a complementary flux produced by driving a current through the secondary

windings. A hall device and associated electronic circuit are used to generate the secondary

(compensating) current that is an exact

Current Sensors

Open Loop Technology

Closed Loop Technology

Hall Sensor

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Torque Transducers

Principles:

• Strain Gauge Torque Sensor • Deflection Torque Sensor • Reaction torque Sensors

Drive Element (Motor) Driven Element (Load) Torque Sensing Element

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Deflection Torque Transducers

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Keypads