1. Introduction to Smart Systems
Accelerometers 1

2. Accelerometers – Concepts (1)
An accelerometer is a sensor for measuring acceleration (rate of change in speed).
Gravity induced reaction force (gravitational pull) is also sensed by an accelerometer.
Gravitational pull and acceleration are both expressed in units of meters per second per second (m/s2) and also in g-force.
1g is the nominal acceleration of gravity on Earth at sea level, which is defined as m/s2 ( ft/s2). 2 2
Embedded Systems Programming II Richard Anthony, Computer Science, The University of Greenwich

3. This is equivalent to a g-force of 31.25/9.8 g, (about 3.19 g).
Accelerometers – Concepts (2)
Example 1. A car decelerates from 30m/s to rest in 0.2s (hits a solid object); deceleration is 150 m/s2, it experiences a g-force of 150/9.8 g (about 15.3 g).
Example 2. A racing car traveling at 50m/s around a bend with curvature radius of 80m undergoes acceleration of 502/80 m/s2 = 31.25 m/s2.
This is equivalent to a g-force of 31.25/9.8 g, (about 3.19 g).
Example 3. Military pilots withstand more than 9g by using a ‘G-suit’ to prevent black-out (prevents to much blood flowing into the legs and depriving the brain of blood). 3 3
Embedded Systems Programming II Richard Anthony, Computer Science, The University of Greenwich

4. Example 1. A wrist-watch might be designed to withstand a shock of 7g.
Accelerometers – Concepts (3)
Short-term accelerations, lasting a few ms, are often referred to as ‘shocks’.
Example 1. A wrist-watch might be designed to withstand a shock of 7g.
Example 2. GPS units for artillery shells need to withstand 15,500g to survive the acceleration when the shell is fired. 4 4
Embedded Systems Programming II Richard Anthony, Computer Science, The University of Greenwich

5. Accelerometers – Concepts (4)
Notes:
1 An object with constant speed, experiences no acceleration (except gravity).
2 An object in freefall does not experience gravity, as the object ‘moves with’ gravity.
Because gravitational pull can be sensed anywhere on Earth, accelerometers can measure static characteristics (i.e. inclination) as well as motion effects such as acceleration, vibration, and shock. 5 5
Embedded Systems Programming II Richard Anthony, Computer Science, The University of Greenwich

6. There are many types of accelerometer, examples include:
Accelerometers – Operation and Types (1)
There are many types of accelerometer, examples include:
Microelectromechanical systems (MEMS)
MEMS comprise components measuring 1 to 100 ųm (0.001 to 0.1 mm).
MEMS devices measure from 20 ųm (20 millionths of a meter) to a millimeter.
MEMs accelerometers are the simplest of a wide-range of MEMS devices, consisting of a cantilever beam (i.e. a beam supported at one end only) with a proof mass (also known as seismic mass) attached at the free end.
Mechanically the accelerometer behaves as a mass-damper-spring system
(damping results from residual gas sealed in the device) 6 6
Embedded Systems Programming II Richard Anthony, Computer Science, The University of Greenwich

7. Accelerometers – Operation and Types (2)
Capacitive 7 7
Embedded Systems Programming II Richard Anthony, Computer Science, The University of Greenwich

8. Other types of accelerometer include:
Accelerometers – Operation and Types (3)
Other types of accelerometer include:
Laser (optical measurement of displacement of pendulous mass).
Magnetic induction
Piezoelectric - Piezoelectricity is the ability of some materials (e.g. crystals and certain ceramics, inc. bone) to generate an electric voltage when mechanical stress is applied. 8 8
Embedded Systems Programming II Richard Anthony, Computer Science, The University of Greenwich

9. Single- and multi-axis devices are available.
Accelerometers – Applications (1)
Single- and multi-axis devices are available.
E.g. X and Y, or X, Y and Z planes.
Thus can detect magnitude and direction of acceleration as a vector quantity
(e.g. by combining values in 2 or 3 dimensions). 9 9
Embedded Systems Programming II Richard Anthony, Computer Science, The University of Greenwich

10. Movement is a key form of context for many applications, examples:
Accelerometers – Applications (2)
Movement is a key form of context for many applications, examples:
- Robotics systems to detect their attitude (to prevent tipping etc).
- Mechanical systems to detect vibration (fault detection in aircraft engines).
- Adaptive suspension on trucks and sports cars.
- Attitude control in unmanned aircraft.
- Anti-shake mechanisms in digital cameras.
- Step counting (for walkers, joggers).
- Games user interfaces (such as Nintendo's Wii handheld controller).
- Phones and similar, for added-value applications (BlackBerry, iPhone).
- Free-fall detection on Apple ‘drop proof’ laptop (automatically retracts the hard disk heads if a fall is detected). 10 10

11. Accelerometers – Example
An example 3-axis accelerometer module. The accelerometer on the module is the ‘Analog Devices ADXL330’ 3-Axis ±3g MEMS technology with 10,000g shock survival .
It produces three independent analogue outputs which can be connected direct to an ADC input of a microcontroller.
The
Accelerometer 11 11

12. Accelerometers – Example
The ‘Analog Devices ADXL330’ accelerometer – internal architecture
Cantilever beam mechanism with pendulous mass. Position is detected through capacitance changes caused when the fixed mass moves
Functional block diagram
Analogue outputs 12 12
Embedded Systems Programming II Richard Anthony, Computer Science, The University of Greenwich

13. Accelerometers – Demonstration application
A digital 3-Dimensional ‘spirit level’
X-Axis orientation
Y-Axis orientation
Z-Axis orientation
Accelerometer
module 13 13
Embedded Systems Programming II Richard Anthony, Computer Science, The University of Greenwich