Department of Mechanical Engineering

MEMS1049

MechatronicsChapter Sensors 9-1

Department of Mechanical Engineering

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Proximity sensors and Switches

Proximity sensoro A proximity sensor is a sensor able to detect the presence of

nearby objects without any physical contact. o A proximity sensor often emits an electromagnetic or

electrostatic field, or a beam of electromagnetic radiation (infrared, for instance), and looks for changes in the field or return signal. The object being sensed is often referred to as the proximity sensor's target.

o Different proximity sensor targets demand different sensors. For example, a capacitive or photoelectric sensor might be suitable for a plastic target; an inductive proximity sensor requires a metal target.

Department of Mechanical Engineering

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Proximity sensors and Switches

Photoemitter-detector pairs

Department of Mechanical Engineering

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Proximity sensors

Ultrasonic proximity sensor

Ultrasonic proximity sensors use a transducer to send and receive high frequency sound signals. When a target enters thebeam the sound is reflected back to the switch, causing it to energize or de-energize the output circuit.

Department of Mechanical Engineering

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Proximity sensors

Ultrasonic proximity sensor

A piezoelectric ceramic disk is mounted in the sensor surface. It can transmit and receive high-frequency pulses. A high frequency voltage is applied to the disk, causing it to vibrate at the same frequency. The vibrating disk produces high-frequency sound waves. When transmitted pulses strike a sound-reflecting object, echoes are produced. The duration of the reflected pulse is evaluated at the transducer. When the target enters the preset operating range, the output of the switch changes state. When the target leaves the preset operating range, the output returns to its original state.

Department of Mechanical Engineering

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Proximity sensors

Ultrasonic proximity sensor A blind zone exists directly in front of the sensor. Depending onthe sensor the blind zone is from 6 to 80 cm. An object placedin the blind zone will produce an unstable output.

The radiation pattern of an ultrasonic sensor consists of a maincone and several neighboring cones. The approximate angle ofthe main cone is 5°.

Department of Mechanical Engineering

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Proximity Sensor

Underwater Sonar

Diver Using INSS Handheld Sonar(Integrated Navigation SensorSystem)

Sport Fishfinder

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Proximity sensor

Eddy Current proximity sensoro An eddy-current sensor produces an alternating magnetic field at the probe

tip. When this field is near a conductive material, the field creates (induces) small electrical eddy-currents in the material. These currents generate a magnetic field that opposes the field from the sensor. As the gap between the probe and target gets smaller, the field interaction changes. The sensor electronics can measure this field interaction and generate an output voltage proportional to the change in the gap..

A typical proximity sensor used for vibration monitoring has a scale factor of 200 mV/mil.

Department of Mechanical Engineering

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Proximity sensor

Capacitive proximity sensoro Capacitive proximity sensors produce an electrostatic field. The sensing surface

is formed by two concentrically shaped metal electrodes of an unwound capacitor. When an object nears the sensing surface it enters the electrostatic field of the electrodes and changes the capacitance in an oscillator circuit. As a result, the oscillator begins oscillating. The trigger circuit reads the oscillator’s amplitude and when it reaches a specific level the output state of the sensor changes. As the target moves away from the sensor the oscillator’s amplitude decreases, switching the sensor output back to its original state.

Department of Mechanical Engineering

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Proximity sensor

Capacitive proximity sensoro Capacitive proximity sensors switches will sense metal as well as

nonmetallic materials such as paper, glass, liquids, and cloth.

Capacitive sensors depend on the dielectric constant of the target. The larger the dielectric constant of a material the easier it is to detect.

Department of Mechanical Engineering

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Proximity sensors and switch

Switch

Three pushbutton switches (Tactile Switches). Major scale is inches.

Department of Mechanical Engineering

PotentiometerGibson 300kOhm Potentiometer Linear Taper/Long Shaft

PCB mount trimmer potentiometers, or "trimpots", intended for infrequent adjustment.

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Potentiometer

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RRV

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Linear variable differential transformer(LVD LVDT

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Linear variable differential transformer(LVDT) LVDT

Department of Mechanical Engineering

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Linear variable differential transformer(LVDT) LVDT demodulation

Department of Mechanical Engineering

The Full-Wave Bridge Rectifier Bridge Rectifier

Department of Mechanical Engineering

The Full-Wave Bridge Rectifier Bridge Rectifier with RC Filter and LC filter

Department of Mechanical Engineering

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Linear variable differential transformer(LVDT) LVDT demodulation