Accelerometers Principles and Operation

1. AccelerometersPrinciples and Operation Paschal Meehan Keith Moloney

2. The Accelerometer. • An accelerometer is a device for detecting and measuring acceleration. It produces an output, usually electrical, which is proportional to the rate of acceleration. • There are many different scientific and engineering principles used leading to many different types available. Plenty of choice for different applications.

3. Acceleration. • When considering the operation of any measurement device it is essential to understand what’s being measured. • Acceleration is the rate of change of velocity • Velocity is speed and direction • So any time there is a change in speed or the direction an object is moving in there is acceleration • Must be a change – if something is moving at a constant speed in a straight line there is no acceleration no matter how fast it is moving • Acceleration has nothing to do with going fast. An object can be moving very fast and still not be accelerating. If an object is not changing its velocity, then it is not accelerating

4. Acceleration examples • Car driving at 120 km/h on a straight level road • Constant speed and direction - no acceleration • Push down on accelerator • Car speeds up so there is acceleration • Harder you push the pedal the greater the acceleration • Brakes applied • Car slows down, change in speed so there is deceleration • Harder you brake the faster the car slows the greater the deceleration • Go round a bend • Direction is changed so there is acceleration • The sharper the bend the greater the acceleration Acceleration Examples

5. Force and Acceleration • An object’s velocity will only change, that is accelerate, if there a net force acting on it. • Thus a change in the forces acting on an object will produce acceleration and an accelerometer can be used to measure not just acceleration, but any change in force such as vibration, shock and tilting. • These forces may be static, like the constant force of gravity pulling at your feet, or they could be dynamic - caused by moving or vibrating the accelerometer

6. Types of Acceleration • Linear Acceleration • Object moves in a straight line with speed steadily changing • Vibration • Speed and direction are changing continuously and repeatedly • Shock • object undergoes a sudden rapid deceleration • Tilt • objects orientation changes so there is a change in force due to gravity • Rotation • Object is turning so direction of motion is continuously changing

7. Units The SI unit for acceleration is m.s-2 (meters per second squared). Another unit that is sometimes used is the g-force which is the acceleration experienced by an object in freefall and is equal to 9.81 m.s-2 (or 9.80665 m. s-2 to be exact). Some examples are • Saturn V moon rocket just after launch, 11.2 m. s-2 / 1.14g • Space Shuttle, maximum during launch and re-entry, 29.4 m. s-2 / 3g • High-g roller coasters, 49 m. s-2 / 5g • Typical maximum turn in an aerobatic plane or fighter jet, 118 m. s-2 / 12g • Death or serious injury likely > 490 m. s-2 / > 50g.

8. Measurement Transducer • A Measurement Transducer and a signal conditioning circuit converts a physical quantity into an output we can use • Output can be a needle moving on a dial • Larger the movement the bigger the value measured • Examples are weighing scales, speedometers • An electrical output is more useful • For example produce a voltage that is proportional to value measured • Value can be displayed, stored or processed to make a decision • Examples are electronic weighing scales, speedometers and DVMs

9. Simple Accelerometer • Diagram below shows a simple spring mass accelerometer • Mass resting on a low friction base and attached to it by a spring • If the system is at rest the spring will be relaxed and won’t applied any force to the mass • If the system is moving at constant speed the spring will be relaxed and won’t applied any force to the mass

10. Simple Accelerometer • If a force is applied to the base it will accelerate • The mass will stay where it is as there is initially no force acting on it • The spring will be stretched until the force it exerts on the mass gives it the same acceleration as the base • Larger force on the base will stretch the spring more • Extension is proportion to the force applied • Extension of spring can be read off scale and calibrated to give acceleration

11. Car Analogy • Spring mass accelerometer similar to a passenger in a car • Car seat is the base, Passenger is the mass, Seat belt is the spring • If the car is travelling at speed and the brakes are suddenly applied • Force from brakes will cause acceleration on car and car seat • Car seat will slow down • Passenger will continue moving at original speed and move forward from seat • Seat belt will extend like spring stretching • Hopefully force from seat belt will slow the passenger

12. Measuring Acceleration Measurement involve converting a physical quantity into something we can use Traditional measurements give a value on a scale Ruler, weighing scales, spring mass accelerometer An electrical output is more useful Can display, record or process to take actions Commercial Accelerometers produce electrical outputs proportional to the value of the acceleration Can be voltage or digital number Many different conversion methods Heat Transfer Piezoelectric Capacitive

13. Heat Transfer Heat transfer device use the distribution of hot gases within a chamber gas chamber with heating element in centre and four temperature sensors When chamber is level all 4 sensor measure the same temperature When it is tilted or accelerated hot gases rise to highest point and sensors read different temperatures This difference can be interpreted to give the acceleration

14. Piezoelectric Some crystals produce a voltage when they are compressed Call the piezoelectric effect Size of voltage is proportional to stress Common use is spark igniters for gas fire/cooker Accelerometers use a piezoelectric beam supported at one end When a force is applied the beam bends This creates stress and as a result a voltage is produced The size of voltage is proportional to the acceleration Voltage is read off and usually amplified and filtered

15. Capacitive Capacitive systems are similar in principle to the spring mass but the movement is converted into a change in capacitance Accelerometer uses two capacitors C1 & C2 made between two fixed plates and a moveable plate held by springs The value of the capacitors is set by the separation of the plates When system is at rest or moving at constant sped the separation of the plate is the same and the two capacitors are identical Moveable Plate SPRING MASS C1 C2 Fixed Plates

16. Acceleration When an acceleration is applied there is movement between the fixed and free section similar to the spring mass system Size of movement set by acceleration, mass and spring Free plate is now closer to one fixed plate than the other Two capacitors are no longer equal C2 > C1 Capacitance difference is detected amplified and converted to a voltage ACCELERATION Movement C1 C2

17. MEMS Accelerometers Original capacitive accelerometers were made using standard componets Make them relative bulky and expensive During the 1990s a new technology called Micro Electro Mechanical Systems (MEMS) was developed All components are made in a single piece of silicon using similar processing to integrated circuit (IC) chip manufacturing. Electronic circuitry can be built into the same piece of silicon This allows very small cheap high performance accelerometers to be manufactured. MEMS accelerometers are now the most common type Many manufactures but two of the leading ones are Analog Devices and Bosch

18. MEMS Accelerometer MEMS really is mirco Pictures below are of an early Analog Device accelerometer Dimensions are for one capacitor finger set Actual device have multiple parallel fingers 1mm is one thousandth of a millimetre ADXL78 Sensor Structure 1.3mm GAP “MASS” = 0.22mGRAM 3mmTHICK 56mm OVERLAP

19. MEMS Pictures MEMS can be used for a lot more than accelerometers Actuator, switches, microphones, moveable mirrors, oscillators, mircofluidics Picture shows an insect’s legs on a MEMS drive

20. Accelerometer Applications The key specs in describing an accelerometer’s performance are Range – how big an acceleration it can measure Sensitivity – how small an acceleration it can detect Response – how quickly the output changes Axis – how many acceleration directions it can detect 1,2,3 Initial MEMS sensors where greater than 50g single axis devices Could only measure very large acceleration in 1 direction Latest are 3 axis 2g and cost less than €3 each Excellent sensitivity and response and various ranges This higher performance and low cost has opened up a vast range of applications Can use anywhere you want to measure shock, linear acceleration, tilt, vibration or rotation Automotive, Consumer electronics, industrial, health security applications

21. Shock The original application for MEMS accelerometer was car airbags The accelerometer detects the shock of a collision and deploys the airbag Very large but demanding market System must decide if collision is severe enough to fire airbag in <30ms Must be reliable for at least 10 years in a very harsh environment Many cars contain multiple accelerometers and a central control unit to deploy different air bags and seatbelt pretensioners

22. Airbag The Airbag Control Unit must Get the signal from the accelerometer that a collision has occurred decide if it severe enough to require the airbag Release the airbag All before the driver’s head hits the steering wheel ~30mS What the airbag is supposed to do http://www.youtube.com/watch?v=gzFOP2t_blo What’s not supposed to happen http://www.youtube.com/watch?v=0ZCoIege8oM

23. Linear Acceleration The most common everyday use to detect linear acceleration is the Nintendo Wii A 3 axis accelerometer in the Wii Remote measures the acceleration caused by the hand movements and transmits the information to the console. Fast wrist flicks give the highest acceleration and game response Optical sensors detects position Another common use is freefall detection in PCs Accelerometer detects the acceleration if PC is dropped Parks the disk drive head before the impact to minimize damage Accelerometers can detect when portable devices are pick up to automatically turn them on and off to prolong battery life. Pedometers can use accelerometers to detect the up down motion of each step

24. Wii Remote 6 forms of acceleration can be detected

25. Tilt The Apple iPhone and its competitors use accelerometers to detect the orientation of the phone and adjust the display accordingly Can also use for scrolling and other control functions Lots of game applications Can use tilt to accurately level iPhone has a spirit level application

26. Vibration Accelerometers can be used to detect the vibration caused by camera shake and the information is then used to stabilize the image Accelerometers can measure the vibration of expensive equipment and shut it down or issue warnings to schedule maintenance ADXL345 accelerometer demo http://www.youtube.com/watch?v=8o_b2DO3d3E

27. Websites Analog Devices MEMS http://www.analog.com/en/mems/products/index.html Bosch MEMS http://www.bosch-sensortec.com/content/language1/html/4377.htm Parralax Heat Transfer accelerometer http://forums.parallax.com/forums/default.aspx?f=6&m=55816 Somat Piezoelectric Accelerometer http://www.somat.com/products/sensors/piezoelectric_accelerometer.html Sandia Labs – MEMS information & pictures - http://mems.sandia.gov/gallery/ YouTube for lots of videos of Wii and iPhone accelerometer hacks