MEAS Silicon MEMS Piezoresistive Accelerometer and its Benefits

MEAS Silicon MEMS Piezoresistive Accelerometer and its Benefits

Piezoresistive Accelerometers 1. Bonded Strain Gage type (Gages bonded to metal seismic mass using epoxy) Undamped circa 1950 s Fluid (oil) damped circa 1960 s 2. Silicon MEMS type (Gages defused into single piece of silicon) Undamped circa 1980 s Gas damped circa 1990 s

Piezoresistive Accelerometers 3 mm x 3 mm Typical Bonded Strain Gage Sub-Assembly MEAS Silicon MEMS Element

Silicon MEMS Accelerometers How Silicon MEMS Accelerometers Work

Silicon MEMS Accelerometers MEMS = Micro Electro Mechanical Systems Works Like a Trampoline

Silicon MEMS Accelerometers Silicon MEMS Construction: Piezoresistive gages sense changes in stress and strain in the hinges (springs in the trampoline) under acceleration The four springs are wired in a 4-arm Wheatstone Bridge configuration

Silicon MEMS Accelerometers Silicon MEMS Construction, Cross-sectional View Direction of acceleration Top Cap MASS Bottom Cap Thin air gap: Squeeze Film Damping Piezoresistive gages are built into the hinges or springs of the trampoline Mechanical stops to prevent overstress

Silicon MEMS Accelerometers Squeeze-Film Gas Damping cross sectional view top view Direction of acceleration Extremely small airgaps between the moving seismic mass (in the middle) and the top/bottom caps facilitate squeeze film damping. Damping is obtained from the integrated pressure profile at the surface of the sensor cell. Airgap thickness controls the effective damping ratio of the device.

Silicon MEMS Accelerometers Typical Damping Ratios Using Squeeze-Film Technique AMPLITUDE RESPONSE, MODEL 64 SERIES 30 25 20 15 10 Deviation in db 5 0-5 2000g, Q=0.05 Q=0.05 2000g, Q=0.3 Q=0.3 1000g, Q=0.4 Q=0.4 500g, Q=0.4 Q=0.4 50g, Q=0.5 Q=0.5-10 -15-20 -25-30 100.00 1000.00 10000.00 100000.00 Frequency [Hz]

Silicon MEMS Accelerometers What are the Benefits?

MEAS MEMS Advantages Benefit: Low Off-Axis Sensitivity Patented double-cantilever webbed design provides low off-axis sensitivity with minimum response to cross and rotation acceleration commonly found in dynamic testing. Silicon MEMS sensor element Sensor design with high rotational & translational stiffness

MEAS MEMS Advantages Benefit: Resonance Control & Broad Response MEAS s MEMS accelerometers offers the optimal amount of damping to achieve maximum resonance control when the sensor is exposed shock impact, and still maintains the broadest frequency response required by the various industry regulations, such as SAE-J211 and ISO-6487.

MEAS MEMS Advantages Example of Resonance Control in Damped Accelerometer 9000 6000 3000 0g -3000-6000 -9000 -- Undamped -- Damped 0 5 10 15 20 ms Undamped accelerometer resonated at its natural frequency after exposed to shock impulses

MEAS MEMS Advantages Benefit: Stable Response over Temperature Compared to a fluid damped design in which damping characteristics changes dramatically with fluid viscosity at various temperature, frequency response of MEAS s gas damped accelerometer is not affected by temperature. 1 10 100 1,000 +25 C BOTH -40 C GAS -40 C FLUID +100 C GAS +100 C FLUID FREQ (HZ) 200% 175% 150% 125% 100% 75% 50% 25% 0% -25% -50% -75% AMP DEV (%)

MEAS MEMS Advantages Benefit: Excellent Dynamic Range and Linearity Compared to capacitive designs, MEAS s silicon MEMS piezoresistive accelerometers offer much broader mesurement range and unmatched amplitude linearity. Linearity of a Typical 2000g MEMS Accelerometer

MEAS MEMS Advantages Benefit: Shorter Warm-Up Time After power is applied to the sensor, the zero offset of the accelerometer stabilizes as the heat generated by the gages reaches an equilibrium. In a MEAS design, the thermal imbalance in the full bridge configuration is kept to a minimum due to the uniform heating in all 4 active silicon gages. Competitor s design using half-bridge configuration (2 active gages and 2 completion resistors) produces thermal imbalance due to uneven the heating characteristics that take much longer to reache an equilibrium.

MEAS MEMS Advantages Benefit: Better Thermal Stability With higher gage impedance, MEAS s design comsumes considerably less power than competitor s sensor, hence lower heat dissipation. Lower heat dissipation translates into higher thermal stability and faster warmup time. Zero Output Stability, One Hour after Power-On voltage, mv -5.3-5.5-5.7-5.9-6.1-6.3-6.5 0 500 1000 1500 2000 2500 3000 3500 4000 time, seconds