1. Advanced Mechatronics
Instructor: Professor I. Charles Ume
Lecture #1
Ultrasonic Sensors (Sonic Distance Sensors)
Photo Interrupt
Pressure Sensors
Accelerometers
Hall effect Sensors
Variable Reluctance Sensors

2. Ultrasonic transducer (piezoelectric transducer) is device that converts electrical energy into ultrasound
Upon receiving sound echo (pressure wave) back from surface, ultrasound transducer will turn sound waves into electrical energy which can be measured and displayed
Ultrasound are sound waves above normal range of human hearing (greater than 20K hertz).

3. Since piezoelectric crystal generates voltage when force is applied to it, same crystal can be used as an ultrasonic detector
Some systems use separate transmitter and receiver components while others combine both in single piezoelectric transceiver
Alternative methods for creating and detecting ultrasound include magnetostriction and capacitive actuation.
Pulse echo sensor
Transmit-Receive sensor

4. Sound is transmitted through propagation of pressure in air
Speed of sound in air is normally 331 m/sec at 0oC and 343 m/sec at 20oC for dry air
Digital signal processor embedded in sensor calculates distance between sensor and object

5. Range of sensor varies between 5 cm to 20 m
x = vsound . t
Where Vsound is known, t = 0.5 (time of flight), x is distance between sensor head and object
Range of sensor varies between 5 cm to 20 m
Sensor is not appropriate for very short distance measurements
Frequency response (distance measurement update rate) varies with distance measured
In general, it is about 100 Hz

6. Piezoelectric crystals have property of changing size when voltage is applied
Applying alternating current (AC) across them causes them to oscillate at very high frequencies, thus producing very high frequency sound waves.
Ultrasonic sensors work on principle similar to radar or sonar
Evaluate attributes of target by interpreting echoes from radio or sound waves respectively

7. Ultrasonic sensors generate high frequency sound waves and evaluate echo which is received back by the sensor
Sensors calculate time interval between sending signal and receiving echo to determine distance to object.

8. Applications
Medical:
Medical ultrasonic transducers (probes) come in variety of different shapes and sizes for use in making pictures of different parts of body.
Transducer may be passed over surface of body or inserted into body opening such as rectum or woman’s reproductive organ
Clinicians who perform ultrasound-guided procedures often use a probe positioning system to hold the ultrasonic transducer.

9. Technology can be used for measuring:
Often used in robots for obstacle avoidance
Wind speed and direction (anemometer),
Fullness of tank, and speed through air or water
Measuring amount of liquid in tank, sensor measures distance to surface of fluid.
Other applications include: burglar alarms, non-destructive testing, and etc

10. Photo Interrupt Uses emitter and detector photo diode pair
With no obstruction detector is high
When an object blocks the light the detector is low
Advantages
Simple to interface
Inexpensive
Reliable

11. Photo Interrupt Types Wide variety of packages and orientations Types
Logic (digital ±5 volts)
Transistor/diode (analog)
Manufacturers
Fairchild
Honeywell

12. Photo Interrupt Applications
Encoder wheel for angular measurements.
Computer mouse with a ball

13. Photo Interrupt Applications
Detect holes or slots for positioning of liner slides
Elevators
Detect the location of products on and assembly line

14. Pressure Sensors Used to detect pressure of fluids or gasses.
Technologies (many)
Strain gage
Piezoresistive
Microelectromechanical systems (MEMS)
Each sensor has a pressure range that it works in.
Most have analog outputs that need amplification
Some have built-in amplifiers for direct connection into microcontroller

15. Pressure Sensors Types
Differential Pressure
Difference between two or more pressures introduced as inputs to the sensing unit
2 input
Absolute/Gage Pressure
The pressure relative to perfect vacuum pressure or set pressure (like pressure at sea level)
1 input

16. Pressure Sensors Applications
Measure pressure of gas or fluids
Measure altitude
For plains or weather balloons
Measure flow
pressure sensors in conjunction with the venturi effect to measure flow
Measure depth of water
When measuring liquids, most sensors are not rated to have unclean liquids contact the sensor components. A small amount of air in the tube right before the sensor will create a barrier from the liquid.

17. Accelerometers Used to measure acceleration
Common SI units meters/second2 (m/s2) or popularly in terms of g-force (1 g is earth’s gravity)
At rest an acceleration will measure 1 g in the vertical direction
They can come in 1, 2 or 3 axis configurations
With 3 axis it gives a vector of the accelerations direction (after accounting for gravity)

18. Accelerometers
Because of earth’s gravity, the sensor will read 1 to 0 g as the sensor is rotated from being vertical to horizontal.
This can be used to measure angle the of tilt
Each sensor has a range that it works in.
Most have analog outputs that need amplification
Some have built-in amplifiers for direct connection into microcontroller

19. Accelerometers How they work
Mechanically the accelerometer behaves as a mass-damper-spring system
Many use Microelectromechanical systems (MEMS). Which use very small cantilever beams with masses on them
Under the influence of gravity or acceleration, the proof mass deflects from its neutral position.
This deflection is measured in an analog or digital manner
Commonly the capacitance between a set of fixed beams and a set of beams attached to the proof mass is measured.
Integrating piezoresistors in the springs to detect spring deformation is another method

20. Accelerometers Applications
Can be used to sense orientation, vibration and shocks.
Used in electronics like the Wii and iPhone for user input.
Acceleration integrated once gives velocity, integrated a second time gives position.
The integration process is not precise and introduces error into the velocity and position.

21. Hall Effect Sensor Sensing a Shaft Speed
Used to provide a noncontact means to detect and measure a magnetic field
Named based on their use of the Hall Effect, discovered by Edwin Hall in 1879
Hall Effect Sensor Sensing a Shaft Speed

22. Depiction of the Hall Effect
How they work
Presence of magnetic field deflects electrons flowing through a conductive material
As electrons move to one end of a conductive material, a potential is developed in the direction perpendicular to gross current flow
This potential indicates the strength of the magnetic field
Depiction of the Hall Effect

23. Applications IC Engine Electronic Ignition Systems
Used to determine position of cam shaft
Brushless DC Motor Control
Sensors determine position of permanent magnet rotor
Assembly Lines
Shaft position and velocity sensors
Contactless limit switches
Current Sensing ICs
Electrically isolated alternative to shunt resistors

24. Hall Effect Sensor Types
Linear Hall Effect Sensors
Output is proportional to magnetic field strength
Hall Effect Digital Switches
Presence of field above threshold turns switch on
Presence of field below threshold turns switch off
Hall Effect Digital Latches
North field turns latch on
South field turns latch off

25. Packaging and Manufacturers
ICs
Analog Devices:
AD22151G from Analog Devices
Allegro MicroSystems, Inc.
Wide range of linear, latching and switching sensors
Great sampling policy
Many, many more
Packaged units
Honeywell
SOT23
SIP
Hall Effect Sensor Module

26. Implementation and Words of Warning
Sensors may be affected by temperature variation. Some sensors incorporate circuitry to reduce this error.
Sensors may be directional, in which case care must be taken with respect to orientations of sensor and magnet
Some Hall Effect sensors detect presence of ferromagnetic materials, not magnetic fields

27. Variable Reluctance Sensors
Used to measure speed and/or position of a moving metallic object
Sense the change of magnetic reluctance (analogous to electrical resistance) near the sensing element
Require conditioning circuitry to yield a useful signal (e.g. LM1815 from National Semi.)
Industrial Variable Reluctance Sensor

28. How Variable Reluctance Sensors Work
A magnet in the sensor creates a magnetic field
As a ferrous object moves by the sensor, the resulting change in the magnetic flux induces an emf in the pickup coil
Variable Reluctance Sensor Construction
Typical Configuration

29. Typical Application Shaft velocity sensor for ABS/traction control
Crank and cam shaft position sensors
Installed on CV axle
Sensor Schematic

30. Interfacing Concerns
The emf is proportional to the rate of change of the magnetic flux. This dictates the ferrous material must be moving for the sensor to generate a signal. Output voltage dependent on velocity of toothed wheel - performance may be reduced at slow speeds