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??