Mechatronics Chapter Sensors 9-1

Transcription

1 MEMS1049 Mechatronics Chapter Sensors 9-1

2 Proximity sensors and Switches Proximity sensor o o o A proximity sensor is a sensor able to detect the presence of nearby objects without any physical contact. 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. 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.

3 Proximity sensors and Switches Photoemitter-detector pairs

4 Proximity sensors Ultrasonic proximity sensor Ultrasonic proximity sensors use a transducer to send and receive high frequency sound signals. When a target enters the beam the sound is reflected back to the switch causing it to energize or deenergize the output circuit.

5 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.

6 Proximity sensors Ultrasonic proximity sensor A blind zone exists directly in front of the sensor. Depending on the sensor the blind zone is from 6 to 80 cm. An object placed in the blind zone will produce an unstable output. The radiation pattern of an ultrasonic sensor consists of a main cone and several neighboring cones. The approximate angle of the main cone is 5.

7 Proximity Sensor Underwater Sonar Sport Fishfinder Diver Using INSS Handheld Sonar (Integrated Navigation Sensor System)

8 Proximity sensor Eddy Current proximity sensor o 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.

9 Proximity sensor Capacitive proximity sensor o 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.

10 Proximity sensor Capacitive proximity sensor o 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.

11 Proximity sensors and switch Switch Three pushbutton switches (Tactile Switches). Major scale is inches.

12 Potentiometer Gibson 300kOhm Potentiometer Linear Taper/Long Shaft PCB mount trimmer potentiometers or "trimpots" intended for infrequent adjustment.

13 Potentiometer V out = R R x p V s

14 Linear variable differential transformer (LVD LVDT

15 Linear variable differential transformer (LVDT) LVDT

16 Linear variable differential transformer (LVDT) LVDT demodulation

17 The Full-Wave Bridge Rectifier Bridge Rectifier

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

19 Linear variable differential transformer (LVDT) LVDT demodulation