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ECE5320 Mechatronics Sensing Using Low Cost Accelerometers

ECE5320 Mechatronics Sensing Using Low Cost Accelerometers. Prepared by: Wayne Sanderson Dept. of Electrical and Computer Engineering Utah State University T: ( 435)797-4572. 3/11/2005. Outline . Introduction Major applications Using capacitance to measure displacement

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ECE5320 Mechatronics Sensing Using Low Cost Accelerometers

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  1. ECE5320 MechatronicsSensing Using Low Cost Accelerometers Prepared by: Wayne Sanderson Dept. of Electrical and Computer Engineering Utah State University T: (435)797-4572 3/11/2005

  2. Outline • Introduction • Major applications • Using capacitance to measure displacement • Accelerometer on a chip • Use of micromachined sensors • A peek at piezo accelerometers • Examples of applications • To probe further • Reference list ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  3. Introduction to Accelerometers • Much of what we do is based on our perception of our world. We rely on some type of sensor to give use this information. • No where are sensors used more then in the field of controls. • The measurement of acceleration, vibration and position are becoming more necessary. • A new family of small low cost accelerometers are finding their way into our lives. They tell us everything from when to deploy our car’s airbag, how to adjust the ride of our suspension system, if our machinery has a worn bearing, or if we have just dropped our laptop. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  4. Major Applications • Drop Indicators. • Vibration Monitoring / Cancellation. • Airbag Deployment. • Machine Condition Monitoring and Diagnostics. • High Accuracy, Tilt Sensors. • Platform Stabilization / Leveling. • Navigation, Mapping. • Position Tracking in Small Rockets. • Alarms and Motion Detectors. • Shipping Recorder. • Automatic Sleep Mode. • Military Fuse, Safe and Arm. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  5. Accelerometer Selection • The choice of accelerometers are largely a question of quality vs. cost. The two main kinds of contender types are • Piezo • Not covered in detail here • High Absolute Accuracy • Expensive ($100 - $∞) • Large Package Size • High Reliability (Military, Space) • Differential Capacitance (Growing in popularity) • Good Relative Sensitivity • Low cost ($10 - $100) • Small Package Size • Good Reliability • This presentation will focus mainly on the low cost capacitive accelerometer. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  6. Mass-Spring system • The basic physical principle behind most accelerometers is that of a simple mass spring system. Springs are governed by a physical principle known as Hooke's law. Hooke's law states that a spring will exhibit a restoring force which is proportional to the amount it has been stretched or compressed. • The other important physical principle is that of Newton's second law of motion which states that a force operating on a mass which is accelerated will exhibit a force with a magnitude F=ma. If a system undergoes an acceleration, then by Newton's law, there will be a resultant force equal to ma. This force causes the mass to either compress or expand the spring under the constraint that F=ma=kx. Hence acceleration a will cause the mass to be displaced by x. In this way we have turned the problem of measuring acceleration into one of measuring the displacement of a mass connected to a spring. • Note that this system only responds to accelerations along the length of the spring. This is said to be a single axis accelerometer. In order to measure multiple axes of acceleration, this system needs to be duplicated along each of the required axes. [1] Accelerometer ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  7. Capacitance The problem which needs to be solved is that of measuring the displacement of the bar. To find the displacement many manufactures use the property of capacitance. The amount of capacitance that a set of plates would exhibit is dependent on the area of the plates, the distance between the plates and the dielectric material between the two plates. Therefore, if the dielectric and the plate area remain constant, and we are able to measure the change in capacitance, then the change in distance x can also be determined. Many accelerometer designs, take this technique one step further and uses two capacitors configured as shown and measure the differential capacitance. If the device is at rest, and the spacing between each of the plates is equal, then the value of each of the capacitors will also be equal. Two capacitors with a common center plate [1] Accelerometer ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  8. Capacitance of Three Plates If the middle plate is moved by a distance x, then this results in: This can then be written as: The difference between the two capacitors is given by: For small values of displacement x, the above expression reduces to: [1] Accelerometer ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  9. Parallel-plate Capacitors • Parallel-plate capacitors are formed between the shuttle fingers and the anchored fingers. • Upward shuttle movement increases C1 and decreases C2. Downward movement has the opposite effect. • Analog electronic circuitry is used to sense the change in capacitance. [2] Development of a MEMS Testing Methodology ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  10. Movable shuttle fingers form variable capacitors. • Potential divider is formed using modulating voltages of opposite phase. • Shuttle movement is therefore converted to a voltage output. • Output is zero if C1 = C2 [2] Development of a MEMS Testing Methodology ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  11. Implementation of Mass Spring • The Analog Devices ADXL50 is a micro-machined stand-alone accelerometer which consists of a mass spring system as well as a system to measure displacement and the appropriate signal conditioning circuitry. The mass spring system used in this device is depicted here. • The mass is a bar of silicon, and the spring system is implemented by the 4 tethers which attach to each corner of the mass. It responds to accelerations that occur in line with the length of the mass. When an acceleration occurs, the mass moves with respect to the anchored ends of the tethers. • Roughly speaking, the amount of acceleration is ideally proportional to the amount of displacement of the mass. Real nonlinearities are compensated for by sophisticated signal conditioning circuitry built into the device. [1] Accelerometer ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  12. Supported Beam with Plates (Summit Instruments’s Mass and Spring) [3] Accelerometer Theory of Operation ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  13. Supported Beam with Plates (cont.) • Summit Instruments accelerometers also employ polysilicon surface micro-machined sensors, each capable of measuring positive and negative acceleration along one axis. Each sensor consists of a tethered main beam with center plates at right angles to the main beam as shown in the previous diagram. • Each of the center plates fits between two adjacent fixed plates, forming a capacitive divider. The two fixed plates are driven with an equal amplitude but opposite polarity square wave signals. With no acceleration, the two capacitances are approximately equal and the center plate will be at approximately 0 volts. Any applied acceleration causes a mismatch in plate separation which results in greater capacitive coupling from the closer fixed plate; a voltage output can thus be detected on the center plate. The acceleration signal is contained in the phase relative to the driving signal, thus a synchronous demodulator technique is actually used to extract the relatively low frequency acceleration signal. [3] Accelerometer Theory of Operation ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  14. More Fingers Complete the Picture [2] Development of a MEMS Testing Methodology ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  15. Accelerometer “System” • The difference in capacitance is proportional to x, but only for small values of displacement. To guarantee this condition, the ADXL50 uses a negative feedback control loop to make sure that the movement of the mass is kept small, so that the above expression remains correct and linearity is improved. • The block diagram of the entire system is shown. [1] Accelerometer ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  16. Functional block diagram • The sensor is a surface-micromachined polysilicon structure built on top of the silicon wafer. Polysilicon springs suspend the structure over the surface of the wafer and provide a resistance against acceleration forces. Deflection of the structure is measured using a differential capacitor that consists of independent fixed plates and plates attached to the moving mass. The fixed plates are driven by 180° out-of-phase square waves. Acceleration will deflect the beam and unbalance the differential capacitor, resulting in an output square wave whose amplitude is proportional to acceleration. • Phase sensitive demodulation techniques are then used to rectify the signal and determine the direction of the acceleration. [8] ADXL103/ADXL203 ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  17. Sensor on a Chip-ADXL05 (50g) [2] Development of a MEMS Testing Methodology ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  18. Surface Micromachined Accelerometers • Surface micromachining has been used to make very low cost, but relatively low performance accelerometers for the Airbag and consumer products industry. The small sizes of surface micromachined structures generate tiny signals that limit the noise floor and accuracy of these devices. Advances in circuit architectures and beam structures used in integrated micro-electromechanical systems have allowed orders of magnitude improvements in resolution and accuracy. • These devices are fabricated on an integrated surface micromachining process that combines the mechanical sensor and the electronic signal conditioning circuitry on a single IC chip. Mechanical structures are fabricated using a surface micromachining approach. • The sensor elements, built of 2um thick polysilicon, have 1um feature size, a mass of only 0.4 ugram, and are 400um by 400um in size. With a source capacitance in the neighborhood of 60fF, the signal output by these devices is easily lost due to parasitic capacitance or noise. Only the presence of the signal conditioning on-chip allows the minute signals to be read without interference. [5] High Performance Surface Micromachined Accelerometer ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  19. MICROMACHINED SENSOR [2] Development of a MEMS Testing Methodology ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  20. Manufacturing Limitations • The size and mass of the structural beam place a great constraint on the performance of the devices. To make a precision accelerometer, it is helpful to have as much mass as possible and also to have a large output signal from the sensor. • Both parameters are limited by the size of the sensor element, which in turn is limited by the ability to control the curvature, or out of plane deviation of the structure due to internal polysilicon stress. Limits on size and thus mass set a lower noise limit on the sensor. • An additional constraint is the stiffness of the springs in the accelerometer. Weaker springs allow the sensor to flex more under acceleration, increasing distance moved and the delta capacitance read by the circuit. Conversely, there is a direct trade-off between the weakness of the springs and the robustness of the structure to shock and overload. Sensors that are too weak can cause the sensor elements to touch, resulting in a stiction failure. [5] High Performance Surface Micromachined Accelerometer ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  21. Example-Intigrated Accelorometer-ADXL 203 • Single/Dual Axis Accelerometer • The ADXL103/ADXL203 are high precision, low power, complete single and dual axis accelerometers with signal conditioned voltage outputs, all on a single monolithic IC. • Measures acceleration with a full-scale range of ±1.7 g . • The ADXL103/ADXL203 can measure both dynamic acceleration (e.g., vibration) and static acceleration (e.g., gravity). 1 G Calibration [8] ADXL103/ADXL203 ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  22. Example-High G Accelerometer-ADXL190 • Key Specifications • Single Axis • +/- 100-g range • 40 mG Resolution • 400 Hz Bandwidth • 2000-g survivability • Low Power-2 mA • Single Chip. • No external part required. • Cost < $20 [8] ADXL190 ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  23. MEMSIC Accelerometer Layout • Dual axis Accelerometer • Produced by MEMSIC • CMOS Capacitive Sensor • Thermal Accelerometer Principle • Micro Fabrication • On Chip Mix Signal Processing [7] MEMSIC ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  24. A Piezo-electric Accelerometer-Quick Look • Relies on piezo electric effect. Force squeezes the crystal and a voltage is produced. • A single ended compression, (pictured), accelerometer is where the crystal is mounted to the base of the accelerometer and the mass is mounted to the crystal by a setscrew, bolt or fastener. [10] Honeywell International Inc. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  25. IEPE Accelerometer • An IEPE accelerometer is a two-wire sensor that requires a constant current supply and outputs an AC voltage on a DC voltage bias. • The DC bias is often removed by the use of a decoupling capacitor. [10] Honeywell International Inc. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  26. Typical Piezo-electric Frequency Response • Accelerometers, whether electrical or mechanical have a resonant frequency that limits the operating range. • For an accelerometer to be useful, the output needs to be directly proportional to the acceleration that it is measuring. This fixed ratio of output to input is only true for a range of frequencies as described by the frequency response curve with flat response. • The usable frequency response is the flat area of the frequency response curve and extends to approximately 1/3 to ½ of the natural frequency. • Many types of capacitive accelerometer do not have a lower frequency limit and can measure constant DC forces [10] Honeywell International Inc. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  27. Piezo Accelerometer Comparison [10] Honeywell International Inc. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  28. Applications - Tri-Axis Tilt Calculations • The Z-axis of the KXM52-1050 can be combined with the X- and/or Y-axes to maintain constant sensitivity through all 360° of tilt. The fig. shows the expected tilt sensitivity through the first 90° of tilt when using just the pitch or roll axis, the Z-axis and the combination of the two. • As you can see, approximately 17.45 mg/° of sensitivity can be maintained at any tilt orientation when combining the pitch or roll axis with the Z-axis. This method allows tilt angles greater that 45° to be sensed accurately and precisely. Both pitch (ø) and roll (r) can be sensed simultaneously using the outputs of all three axes. [11] Kionix ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  29. Tri-Axis Tilt Sensing [11] Kionix ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  30. Applications - Free Fall Sensing • When a tri-axis accelerometer is stationary, its total acceleration is 1g (9.8m/s2), regardless of orientation. This total acceleration can be calculated from the X, Y and Z outputs of the accelerometer using the equation below. • When a tri-axis accelerometer is dropped in any orientation, it is in free fall and the acceleration on all three axes is 0g, therefore, the total acceleration is zero as well. • Note: free fall cannot be detected accurately when using a dual-axis accelerometer because, when horizontally oriented, the X-axis and Y-axis outputs are the same (0g), whether the accelerometer is stationary or in free fall. [12] Kionix ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  31. Free Fall Detection Time-KXM52-1050 Accelerometer A free fall test was done using a three axis accelerometer as described. The test involved dropping an object (laptop) from a height of 30 inches to see if the fall could be detected and the computer protected. Testing show that a free fall condition can be detected in approximately 200 ms. This is equal to a distance of ~ 8 in. This allowed the computer ~ 200 ms, enough time to “safe” the hard drive and turn the power off. [12] Kionix ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  32. Summary • Through the use of Micromachining and advances in CMOS circuit integration, new low cost accelerometer “systems on a chip” are now being produced. • Although these low cost versions often do not have the accuracy as the much more expensive piezo accelerometers it is often adequate for most applications. • These capacitive accelerometers also have the advantage of small size, low power, simple interfaces, and a wide variety of measurements ranges and sensitivities. • With these characteristic they have found wide use in automobile diagnostics, airbag deployment, machine vibration analysis, tilt and drop indicators, and movement recording. • All indications are that the present trend of smaller, cheaper and better will continue into the future. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  33. To Explore Further • There are many companies that produce accelerometers, each has it’s own niche for type, specifications, or applications. A internet search for a specific application will yield ample information and examples. Four recommendations include: • For a detailed discussion on the different types of piezo accelerometers refer to reference [10] Honeywell International Inc., SENSOTEC, “Frequently Asked Questions”. • Analog Devices Inc.(R), Have a wide selection of low cost accelerometer (<$10). Their data sheets and application notes contain a wealth of ideas and good design practices. • An important area in the recent advancements of accelerometers has been in the area of mico-machining and MEMS technologies. Reference [2] Development of a MEMS Testing Methodology, Shawn Blanton, Nimoni Deb, ECE Dept, Carnegei Mellon Univ. had a impressive set of electron microscope photos. • With the development of ultra sensitive micro-g sensors the use of accelerometers for uses as position and orientation indicators has greatly expanded. References [7], [11], [12] are a good starting points for tilt and drop sensors. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

  34. Reference list • [1] Accelerometer (Analog Devices ADXL50), Tim Stilson, http://ccrma.stanford.edu/CCRMA/courses/252/sensors. • [2] Development of a MEMS Testing Methodology, Shawn Blanton, Nimoni Deb, ECE Dep, Carnegei Mellon Univ. • [3] Summit Instruments, Inc., Tech Note 402, Accelerometer Theory of Operation • [4] Kionix, KXE00 Technical Notes, Rev.0, “Analog Accelerometers”, Sept.22, 2004. • [5] Analog Devices inc., Technical Note: “A High Performance Surface Micromachined Accelerometer for Machine Health and Tactical Inertial Applications”, James Doscher, 1995-2005. • [6] Analog Deviced Inc, “New Generation of Micromachined Accelerometers and DSP Offer Low Cost Alternative to Machine Condition Monitoring”, Christophe Lemaire, 1995-2005. • [7] MEMSIC Inc, “Low-g Accelerometer Tilt Sensor”, http://www.memsic.com/memsic/products. • [8] Analog Devices, Inc., ADXL103/ADXL203, Data sheet, Rev 0, 2004 • [9] Analog Devices, Inc., ADXL190 Low Cost +/- 100 g Single Axis Accelerometer with Analog Output, Data sheet, Rev 0, 2004. • [10] Honeywell International Inc., SENSOTEC, “Frequently Asked Questions”. • [11] Kionix, “Tilt-Sensing with Kionix MEMS Accelerometers“, PN:AN005-040915, Sept. 15, 2004. • [12] Kionix, “Using the Kionix KXM52-1050 Tri-Axis Accelerometer for Hard Drive Shock Protection”, 2004. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

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