MEMS Accelerometers. Agenda. Introduction. Sourabh Sthapak April 13, 2010

Transcription

1 MEMS Accelerometers Sourabh Sthapak April 13, 2010 Agenda Introduction to MEMS Types of accelerometers Capacitive accelerometers Electrical circuit and operation Multiple-axis accelerometers Important parameters Applications Advantages Conclusion Introduction MEMS stands for Micro-ElectroMechanical Systems MEMS Accelerometer is an electromechanical device that measures acceleration forces Static forces: like the constant force of gravity pulling at our feet Dynamic forces: caused by moving or vibrating the accelerometer First MEMS accelerometer was developed in 1979 at Stanford University 1

2 Types of Accelerometers Piezoelectric: The piezoelectric crystals produce an electric charge when a force is exerted by the seismic mass under some acceleration. The quartz plates (two or more) are preloaded so that a positive or negative change in the applied force on the crystals results in a change in the electric charge. Strain Gauge: often called as "piezoresistive" accelerometers, use strain gauges acting as arms of a Wheatstone bridge to convert mechanical strain to a DC output voltage. Capacitive: A change in acceleration causes a change in the space between the moving and fixed electrodes of a capacitive accelerometer. Others: Resonant, optical, Magnetic Capacitive Accelerometers Basics Senses the change in Capacitance The parallel-plate capacitance is: Where and A is the area of the electrodes, d the distance between them and the permittivity of the material separating them A change in any of these parameters will be measured as a change of capacitance and variation of each of the three variables has been used in MEMS sensing Chemical or humidity sensor may be based on a change of epsilon, accelerometers have been based on a change in d or in A Capacitive Accelerometer Typical MEMS accelerometer is composed of movable proof mass with plates that is attached through a mechanical suspension system to a reference frame. Movable plates and fixed outer plates represent capacitors The deflection of proof mass is measured using the capacitance difference. The freespace (air) capacitances between the movable plate and two stationary outer plates C 1 and C 2 are functions of the corresponding displacements x 1 and x 2 Every sensor has a lot of capacitor sets. All upper capacitors are wired parallel for an overall capacitance C 1 and like wise all lower ones for overall capacitance C 2, otherwise capacitance difference would be negligible to detect 2

3 Mathematical Description If the acceleration is zero, the capacitances C 1 and C 2 are equal because x 1 =x 2. The proof mass displacement x results due to acceleration. If x 0, the capacitance difference is found to be Measuring C, one finds the displacement x by solving the nonlinear algebraic equation (For small displacements, the term Cx 2 is negligible) We can conclude that the displacement is approximately proportional to the capacitance difference C Electrical Circuit and Operation Analog Devices accelerometer ADXL05, that has 46 pairs of capacitors. Sensor s fixed plates are driven by 1MHz square waves with voltage amplitude V0 coming out of oscillator. Phases of the square waves that drives upper and lower fixed plates differs for 180 Simple voltage divider whose output goes forward through buffer and demodulator Voltage output V x, that is actually the voltage of the proof mass. It holds true that Electrical Circuit and Operation When there is no acceleration (a1 = 0), the proof mass doesn t move, and therefore, the voltage output is zero If we accelerate the sensor (a1 > 0), the voltage output V x changes proportional to alternating voltage input V 0 If we inverse the acceleration (a1 < 0), signals V x and V y get negative sign Demodulator gives the sign of acceleration, because it multiplies the input signal V y with the square waves V 0 coming from oscillator 3

4 Acceleration and displacement For an ideal spring, according to Hook s law, the spring exhibit a restoring force F S which is proportional to the displacement x. Thus, F S =k S x, where k S is the spring constant From Newton s second law of motion, neglecting the air friction (which is negligibly small), the following differential equation results ma = md 2 x/dt 2 =k S x Thus, the acceleration, as a function of the displacement, is The acceleration is found to be proportional to voltage output Multiple Axis Accelerometers In the previous example, we saw the example of a single axis accelerometer. It s capacitance changes due to a change of distance d between capacitor plates If we include sets of capacitors turned in perpendicular directions, we can get two axis or even three axis accelerometer Important Parameters Analog and digital output Number of axis and measurement range Sensitivity and bandwidth: Frequency of the oscillator has to be a lot bigger than bandwidth frequency, because electronic circuit must read changes in capacitance faster than acceleration changes Noise characteristics: Analog Devices ADXL05 has voltage noise density typically around 500µg/ Hz, newer ADXL202E 200µg/ Hz The noise characteristics will influence the performance of the accelerometers especially when operating at lower g conditions, since there is smaller output signal Types of noise sources: mechanical vibration of the springs, from the signal conditioning circuitry, and from the measurement system itself 4

5 Applications Airbag deployment in automobiles Earthquake monitoring/ structural monitoring Medical applications Navigation Transport Gravimetry Consumer Electronics 1) Motion input in video games 2) Orientation sensing in PDA, cell-phone, notebooks etc. 3) Image stabilization in cameras 4) Device integrity Advantages Small size; Easier integration; Greater sensitivity Precision 1) The homogeneity of materials used for the sensitive part of the sensor (Silicon and silicon oxide), which minimise all the mechanical constraints 2) The use of monocrystalline silicon, a material with excellent mechanical proprieties, in particular a for the spring of the inertial mass Robustness 1) The fact that the spring of the sensitive part acts as low pass mechanical filter and removes the high frequency components of the shocks Life time and cost 1) The choice of the Silicon technology and industrial approaches and associated processes which are well established and well controlled. 2) The use of 4 to 6 silicon wafers linked to MEMS batch processing, ensuring potentially low unit costs Conclusion Accelerometers find a wide variety of applications in automotive, aviation, seismic, consumer electronics industry MEMS helped in realizing small scale devices with greater sensitivities Well known fabrication processes ensures robustness high precision and Mass production of devices helps in lowering down the unit cost In regards to MEMS accelerometers, indeed, There s plenty of room at the bottom!" 5

6 Thank You! Questions? 6