technical presentation accelerometers · technical presentation accelerometers seal team seven ....
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Technical Presentation Accelerometers
Seal Team Seven
Agenda
• Introduction
• History of accelerometers
• How they work
• What are MEMS
• Digital vs. Analog
• Applications
• Conclusion
What is a “g”? • One g is equal to the acceleration due to gravity (9.81m/s2)
g references
Vertical
• Loss of consciousness 4.5-6.3
• Limit of sustained human tolerance 5
• Roller coaster (max at bottom of first dip) 4.5
• Surface of Moon 1/6
• Free fall 0
Horizontal
• Max acceleration of typical car 0.4
• Max acceleration of F-1 race car 1.7
• Extreme Launch roller coaster takeoff 2
• Limit of sustained human tolerance 8
• Space shuttle launch 10
History
• First accelerometer (1923)
• Credited to McCollum and Peters
• Resistance Bridge type
• Dimensions: 0.75” X 1.875” X 8.5” (~1cubic foot)
• Commercialized by 1927 in the US through Southwark
• Reported resonant frequency was less than 2000 Hz
• Various applications
• Major revision (1936)
• 2-axis with up to 100g range
• Applications vastly increased
• Price: $420 ($6275 at todays rate)
Capacitive Accelerometers
• The sensing element consists of two parallel plate capacitors acting in a differential mode.
• The accelerometer senses the capacitance change between a static condition and the dynamic state.
• Detection circuits capture the peak voltage, which is then fed to a summing amplifier that processes the final output signal.
Piezoelectric Accelerometers
• Inside a piezoelectric version, the sensing element is a crystal which has the property of emitting a charge when subjected to a compressive force.
• This crystal is bonded to a mass such that when the accelerometer is subjected to a 'g' force, the mass compresses the crystal which emits a signal. This signal value can be related to the imposed 'g' force.
Piezoresistive Accelerometers
• Piezoresistive accelerometers (strain gauge accelerometers) work by measuring the electrical resistance of a material when mechanical stress is applied. The resistors are normally configured into a Wheatstone bridge circuit, which provides a change in output voltage that is proportional to acceleration.
• Advantages of Piezoresistive accelerometer
• Piezo resistive accelerometers are preferred in high shock applications.
• They can measure accelerations down to zero Hertz.
• Disadvantages of Piezoresistive accelerometer
• Limited high frequency response
Use piezoresistors to convert stress in suspension beam > change in resistance > change in voltage
Hall Effect Accelerometers
• Hall Effect accelerometers measure voltage variations stemming from a change in the magnetic field around the accelerometer.
Heat Transfer Accelerometers
• Heat transfer accelerometers measure internal changes in heat transfer due to acceleration.
• A single heat source is centered in a substrate and suspended across a cavity.
• Thermoresistors are spaced equally on all four sides of the suspended heat source. Under zero acceleration the heat gradient will be symmetrical.
• Acceleration in any direction causes the heat gradient to become asymmetrical due to convection heat transfer.
MEMS Technology • Micro-Electro-Mechanical Systems
• Extremely small mechanical and electro-mechanical devices
• Typically 1um to a few mm in size
• Made by developing moving parts on a substrate, similar to ICs
• Can be used to create transducers, both for sensing and actuation
MEMS Manufacturing • Basic Processes:
• Etching
• Lithography
• Deposition
• Manufacturing Process:
• Bulk
• Surface
• High Aspect Ratio (HAR)
MEMS Applications
• Motors
• Pumps, fluid control
• Accelerometers
• Gyroscopes
• Micromirror controllers
• Resonant Structures
• Microflaps on plane wing
Accelerometer Output: Digital vs Analog
Analog Output - Output a continuous voltage that is proportional to acceleration - Output needs an offset, amplification and filtering - Simple communication with microcontrollers “acceleration =
read_adc( )” - Resolution of the Data dependent on ADC, not accelerometer
Digital Output - Uses Pulse Width Modulation and Practical with digital input
microcontrollers - Require timing resources of microcontroller to measure the duty cycle,
as well as performing intensive division operation - Requires communication protocol between accelerometer and
microcontroller (I2C, SPI)
SPI – Serial Peripheral Interface
www.byteparadigm.com
- Synchronous data link named by Motorola
- 4 wires (SCLK, MOSI, MISO, SS)
- Simple, Low cost - Supports full duplex
communication - No pull up resistors =
less power consumption
I2C – Inter-Integrated Circuit
- Multi-master protocol utilizing 2 bidirectional signal lines (SDA, SCL)
- No need for “SS” - 8 bit data bytes - Requires a few control bits for
start , end, direction and acknowledgement
- Data rate of 100kbps, 400kbps and 3.4Mbps.
www.byteparadigm.com
www.byteparadigm.com
I2C and SPI – Pros and Cons
• I2C spares more pins, board routing and easier to build a large network (only requires 2 line)
• SPI is ideal at very high speeds. (no limitation on speed, often over 10Mbps)
• I2C though more complex offers advance features – multi-master conflict handling and addressing management, however somewhat lacks performance.
• SPI is simple and offer flexibility for extensions (ideal for custom communication between ICs). Offers increased data transfer and high freedom.
Applications
• Consumer electronics
• Motion input
• Orientation sensing
• Image stabilization
• Free-fall sensor
• Navigation
• Altitude
• Speed
• Distance
Applications
• Medical
• Exercise
• Surgery
• Biology
• Animal behavioral patterns
• Structural
• Measure dynamic loads
• Industrial
Applications (G-Switch)
• Machinery shutdown • Excessive vibration
failure
• Fan monitoring
• Transportation monitoring
• Seismic active warning • Earth quake
preparation
• Bridge closure
Conclusion
• History
• Types of accelerometers
• MEMS technology
• Digital vs. Analog
• Application
Questions??