Robot Sensing System - 5
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Ultrasonic sensors operate by emitting sound pulses that reflect off nearby objects. The sensor then detects the echo to determine the distance to the object. There are four main components: a transducer that emits and receives sound, a comparator that calculates distance from time of flight, a detector circuit, and a solid-state output. Ultrasonic sensors can detect most materials and are less affected by moisture than optical sensors, but have difficulty detecting soft absorbing materials. Their sensing range depends on factors like target size, material, temperature, and environmental noise. Common transducer types include piezoelectric crystals and electrostatic foils.
Robot Sensing System - 5
- 1. Ultrasonic Sensors Operating Principle • Ultrasonic sensors emit a sound pulse that reflects off of objects entering the wave field. • The reflected sound, or “echo” is then received by the sensor. • Detection of the sound generates an output signal for use by an actuator, controller, or computer. • The output signal can be analog or digital.
- 2. • Ultrasonic sensing technology is based on the principle that sound has a relatively constant velocity. • The time for an ultrasonic sensor’s beam to strike the target and return is directly proportional to the distance to the object. • Consequently, ultrasonic sensors are used frequently for distance measurement applications such as level control.
- 3. • Advantage of ultrasonic sensors are capable of detecting most objects — metal or non- metal, clear or opaque, liquid, solid, or granular — that have sufficient acoustic reflectivity. • Another advantage of ultrasonic sensors is that they are less affected by condensing moisture than photoelectric sensors. • Disadvantage to ultrasonic sensors is that sound absorbing materials, such as cloth, soft rubber, flour and foam, make poor target objects.
- 4. Ultrasonic Sensor Construction There are four basic components of an ultrasonic proximity sensor: Transducer / Receiver Comparator Detector Circuit Solid-State Output
- 5. Basic Components Transducer/Receiver The ultrasonic transducer pulses, sending sound waves outward from the face of the sensor. The transducer also receives echoes of those waves as reflected off an object. Comparator and Detector Circuit When the sensor receives the reflected echo, the comparator calculates the distance by comparing the emit-to-receive time frames to the speed of sound. Solid-State Output Switching Device The solid state output generates an electrical signal to be interpreted by an interface device like a programmable logic controller (PLC).
- 7. Sensing Range The sensing range of an ultrasonic sensor is the area between the minimum and the maximum sensing limits. Minimum Sensing Distance Ultrasonic proximity sensors have a small unusable area near the face of the sensor. This unusable area is known as the blind zone. The outer edge of the blind zone is the minimum distance an object can be from the sensor without returning echoes that will be ignored or misread by the sensor. Maximum Sensing Distance Target size and material determine the maximum distance at which the sensor is capable of seeing the object. The harder an object is to detect, the shorter the maximum sensing distance can be.
- 8. Materials that absorb sound — foam, cotton, rubber, etc. — are more difficult to detect than acoustically reflective materials, like steel, plastic, or glass.
- 9. Effective Beam • When the transducer vibrates, it emits ultrasonic pulses that propagate in a cone-shaped beam. • This cone can be adjusted, usually via potentiometer, to widen or extend the sensing range.
- 10. Spacing Considerations Spacing between sensors is determined by their beam angles. The sensors must be spaced so they do not interfere with each other. This interference is sometimes called “crosstalk.” When more than one ultrasonic sensor is in use, some experimentation may be needed to determine optimal spacing for the application. This is done by the engineers working at an industry.
- 11. Sensor Alignment • Aim the sensor at the target. • Slowly turn the potentiometer until the LED illuminates, indicating target presence. • Adjust the angle of the sensor to maximize the brightness of the LED. Alignment for Analog Sensor If an analog sensor detects objects behind the desired target, turn the potentiometer to suppress the background objects, but not so far that the sensor no longer detects the target. Alignment for Discrete Sensor To set the sensing distance of a discrete sensor, adjust the potentiometer until the LED turns off while the target is not present. Next replace the target, and slowly turn the potentiometer until the LED turns back on.
- 12. Ultrasonic proximity sensors are affected less by target surface characteristics. Target Consideration The surface temperature of a target can also influence the sensing range. Radiated heat from high temperature targets distorts the sound beam, leading to shortened sensing range and inaccurate readings.
- 13. Target Size The smaller the target the more difficult to detect. Target-to-Sensor Distance The further a target is away from the sensor, the longer it takes the sensor to receive the echo.
- 14. Environmental Considerations Ambient Noise Ultrasonic sensors have noise suppression circuitry that allows them to function reliably in noisy environments. Air Pressure Normal atmospheric pressure changes have little effect on measurement accuracy but extreme pressure conditions might damage the sensor face. Air Temperature The velocity of sound in air is temperature dependent. An increase in temperature causes a slowing of the speed of sound and, therefore, increases the sensing distance. Air Turbulence Air currents, turbulence and layers of different densities cause refraction of the sound wave. An echo may be weakened or diverted to the extent that it is not received at all. Sensing range, accuracy, and stability can deteriorate under these conditions.
- 16. Different adoption methods of Ultrasonic Transducer • Piezoelectric Transducer • Electrostatic Ultrasonic Transducer • Bending Oscillator • Membrane Oscillator
- 17. Piezoelectric Transducer • Piezoelectric crystals have the property of changing their geometric dimensions when voltage is applied i.e. electrical energy can be converted into mechanical energy. • And the converse is also true i.e. when pressure is applied to outer surface, a charge is produced on the upper surface which can be measured as voltage in order of 100 V. • Piezoelectric crystals include: Lead Titanium Oxide (PbTiO₃) and Lead Zirconium Trioxide (PbZrO₃). • As growing macro crystals is difficult, piezo ceramics are used.
- 18. Manufacturing Process of Piezo ceramics • Piezo ceramics are obtained form sintering of piezoelectric micro crystal with additives (binding agent). • The ceramic obtained is polarized for aligning the dipoles in a specific manner. • This is done by applying a high polarization voltage at high temperatures.
- 19. Electrostatic Ultrasonic Transducer • It consists of a thin metallized plastic foil and grooved plate which forms a capacitor. • When voltage is applied, an electrostatic force acts on the foil. • Foil and plate attract each other. • An alternating voltage is superimposed on a DC voltage is applied causing the foil to oscillate at same frequency. • The foil is under constant mechanical pressure because of flat spring. • Frequency tuning upto 500kHz is possible through the air cushion which is trapped between the foil and grooves in metal plate.
- 20. Methods of Operation Distance Measurement sensors: • Direct detection with transceiver. • Direct detection with a two head system. Sensors in through beam operation: • Retro reflective operation. • Through beam detection with two beam sensor heads.
- 21. Direct detection with transceiver • In this case, the ultrasonic transducer is used as transmitter and receiver. • Also known as “echo ranging”. • The pulse bounces off a target and returns to the receiver after a time interval t. • The receiver records the length of this time interval, and calculates the distance travelled r based on the speed of sound c: r = c * t • It takes time for the transducer to change modes, presenting a challenge to short-distance measurement.
- 22. Direct detection with a two head system • In this there are two heads, transmitting head and receiving head. • The problem of short-distance measurement is overcome. • Frequency sensitivity of Transmitter = Frequency sensitivity of Receiver.
- 23. Retro reflective operation • A retroreflective sensor contains both the emitter and receiver in one housing. • The light beam from the emitter is bounced off a reflector (or a special reflective material) and detected by the receiver. • The object is detected when it breaks this light beam.
Editor's Notes
- #20: The DC voltage is necessary because the force on the foil is proportional to the square of the applied voltage and with AC voltage the frequency would become twice the applied voltage.