JMP 1 Accelerometers Justin Piccirillo Texas Instruments Incorporated Sensors & Controls Division

JMP 2 Summary Definition of Acceleration Technologies Texas Instruments - Capacitive Acceleration Sensor (CAS) Terminology Effect of Tilt Typical applications Demonstration Summary Questions & Answers

JMP 3 Acceleration Fundamentals What is Acceleration? –Definition: the time rate of change of velocity –A.K.A.: the time rate of change of the time rate of change of distance What are the units? –Acceleration is measured in (ft/s)/s or (m/s)/s What is a “g”? –A “g” is a unit of acceleration equal to Earth’s gravity at sea level 1 g = 32.2 ft/s 2 or 9.81 m/s 2

JMP 4 More Notes on Acceleration What is the time rate of change of velocity? slope –When plotted on a graph, velocity is the slope of distance versus time slope –Acceleration is the slope of velocity versus time

JMP 5

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8 Acceleration in Human Terms What are some “g” reference points? Description “g” level Earth’s gravity1g Passenger car in corner2g Bumps in road2g Indy car driver in corner3g Bobsled rider in corner5g Human unconsciousness7g Space shuttle10g

JMP 9 What’s the point? Why measure acceleration? –Acceleration is a physical characteristic of a system. –The measurement of acceleration is used as an input into some types of control systems. –The control systems use the measured acceleration to correct for changing dynamic conditions

JMP 10 Common Types of Accelerometers Sensor Category Key Technologies Capacitive -Metal beam or micromachined feature produces capacitance; change in capacitance related to acceleration Piezoelectric -Piezoelectric crystal mounted to mass – voltage output converted to acceleration Piezoresistive -Beam or micromachined feature whose resistance changes with acceleration Hall Effect -Motion converted to electrical signal by sensing of changing magnetic fields Magnetoresistive -Material resistivity changes in presence of magnetic field Heat Transfer -Location of heated mass tracked during acceleration by sensing temperature

JMP 11 What Type of Acceleration Sensor Does TI Produce and why? Capacitive Acceleration Sensor –“CAS” CAS

JMP 12 CAS Conceptual Design ACCELERATION OUTPUT (%Vcc) Mechanical Deflection Acceleration Change in Capacitance Voltage Output Proportional to Mech. Input Electronic Calibration Conditioning Electronics Capacitive Sensing Element Rigid Substrate Movable Blade Acceleration Induced Load Output Voltage Bearing Pin

JMP 13 Capacitive Sensing Element (CSE) Compliant Pins Blade (welded to bearing pin) Substrate Capacitance detect Capacitance generated by electrical current through parallel plates: C = capacitance K = dielectric constant of the insulating medium e o = permittivity of free space A = effective area d = distance between plates (gap) function of acceleration

JMP 14 Conditioner Module Connection to CSE Connection to Base ASIC Application Specific Integrated Circuit (ASIC): Works in conjunction with discrete electronic components (i.e.: capacitors, resistors, thermistors, etc.) to transform the variable capacitance read at the CSE into an output voltage given an electrical potential (Vcc vs. GND = 5V)

JMP 15 Capacitive Acceleration Sensor Base Conditioner Module Compliant Pins Bracket Shell Blade Bearing Pin Substrate Protective Sleeve Cup

JMP 16 Acceleration Sensor Terminology (Typical TI Convention) +1g: Output of the sensor with the base connector pointed up 0g: Output of the sensor with the base connector horizontal -1g: Output of the sensor with the base connector pointed down Linearity: The maximum deviation of the calibration curve from a straight line.

JMP 17 Acceleration Sensor Terminology Sensitivity: A measure of how much the output of a sensor changes as the input acceleration changes. Measured in Volts/g Vcc: The voltage supplied to the input of the sensor –5.000 ± 0.25V for CAS device %Vcc: Readings are often represented as a % of the supply voltage. This allows for correction due to supply voltage variances between readings.

JMP 18 Example: Sensitivity & Linearity

JMP 19 Ratiometric: The output of the sensor changes with a change in the input voltage. Example: At Vcc = 5.000V, Vout at 0g = 1.800V In terms of %Vcc, this is 1.800Vout/5.000Vcc *100% = 36%Vcc Now suppose the input voltage changes: Vcc = 5.010V. At 0g, the ratiometric device output is still 36% Vcc. In terms of the output voltage, 36%Vcc * 5.010Vcc = 1.804Vout So a 0.010V change in Vcc will cause a 0.004V error in the 0g output if you do not evaluate the output as %Vcc Acceleration Sensor Terminology

JMP 20 DC Response DC response means that a sensor can measure 0Hz (static) events. Static position or tilt are 0Hz events Some sensor types cannot measure static events. They need motion to give an output. This type of sensor rolls off as you get closer to 0Hz. The CAS is a DC response sensor

JMP 21 Effect of Tilt DC response sensors measure tilt. Mounting errors are therefore significant a 1 o tilt in the 0g position creates an output error equivalent to a 10 o tilt in the +1g or -1g positions 0g is the most sensitive to mounting errors

JMP 0g Orientation  = 1° → Gn = 1.7x10 -2 *G  +1g Position (-1g Position uses same equation)  = 1° → Gn = *G  G G Why is device sensitive to tilt in the 0g orientation?  Gn Gx Gn=G*Cos(  g level going from 1g to some % of 1g  Gn Gx Gn = G*Sin(  g level going from 0g to some value Conclusion: at 0g orientation, change in 1° tilt causes 57x bigger change in sensor output versus -1g or +1g orientation blade pin CSE substrate

JMP 23 Effect of Tilt on DC Accelerometer CAS Output Voltage Change with Position Change in Output (mV) 500mV/g Device Degrees

JMP 24 Typical Accelerometer Applications Tilt / Roll Vibration / “Rough-road” detection –Can be used to isolate vibration of mechanical system from outside sources Vehicle skid detection –Often used with systems that deploy “smart” braking to regain control of vehicle Impact detection –To determine the severity of impact, or to log when an impact has occurred Input / feedback for active suspension control systems –Keeps vehicle level

JMP 25 Important Setup Requirements for your CAS Device Rigid Mounting –Bees Wax –Double Sided tape –Bolt(s) No Loose Wires –Loose wires can create false signals –Secure wires firmly to mounting body Weight of Sensor –Should be approximately an order of magnitude less than object being measured Example: CAS = 47g; accelerating object should be more than 470g Don’t drop the sensor! –Extreme jarring accelerations can cause permanent errors in device output

JMP 26 Demonstration

JMP 27 Electronics Setup 1 channel 12 bit A/D Converter “LTC1286” CAS Sensor Serial connection to PC Piezoelectric speaker 9V Microprocessor, RAM, EEPROM Vcc

JMP 28 Example Output: Tilt Data

JMP 29 Vehicle Acceleration on Road: Raw Data – 4 points / second

JMP 30 Acceleration – 1 second averages

JMP 31 Summary Acceleration is a measure of how fast the speed of something is changing It is used as an input to control systems Sensor voltage output should be determined as a percentage of voltage input for consistency The device is sensitive to tilt in the 0g position –ΔVout for 1 o tilt in 0g = ΔVout for 10 o of tilt in the +1g and -1g positions Application of the device must be done in a secure fashion and to bodies having an appropriate sensor-to-body weight ratio Think about how you can best use the data –Sample rates (if sensor output is to be converted to digital) –Averaging schemes –Control limits

JMP 32 Questions?

JMP 33 EXTRA SLIDES

JMP 34 Frequency Response (FR) Definition of FR –A sensors ability to track a given input acceleration in both magnitude and time Why is FR important? –Tells us how well a sensor will measure acceleration over a wide range of frequencies. –Allows us to design sensors to measure only the quantities of interest: (i.e. want to measure a braking event but not vibrations in the mounting panel.)

JMP 35 Components of FR Magnitude or Amplitude –Ratio of the output (CAS) / input (reference accel. output) –Calculated in Decibels (dB) = 20 * log 10 (CAS Output Voltage / reference acceleration output voltage) Phase Angle –A measure of the time delay between the input and the output (CAS) –No time delay would have a Phase angle = 0 degrees Frequency –A measure of the rate at which an event occurs –1Hz = 1 cycle/second –The higher the frequency the faster the event occurs

JMP 36 Measurement of FR Equipment –Shaker Table –Signal Analyzer –Reference Accelerometer –CAS Setup –Both the reference accelerometer and the device being measured (CAS) are mounted to the shaker table. –The voltage outputs of both sensors are sent to the signal analyzer –The signal analyzer compares the reference’s output to the CAS output and produces several graphs.

JMP 37 Magnitude Graph –-3dB Point (Often referred to as the roll off point): This is the frequency (Hz) point where the measured output of the sensor is equal to 70% of the input acceleration. –The output is attenuated beyond the -3dB point in order to filter out unwanted inputs –Ex. Input = 10g but the output = 7g –Calculated 20*log(7/10) = -3dB –Typical -3dB points for a CAS 10 to 60 Hz Key Information on FR Graphs

JMP 38 Phase Graph –The phase angle must be less than a specified amount at a specified frequency. Each customer will specify different values. –Examples of phase specifications 10 o at 5Hz 10 o at 10 Hz 70 o at 15Hz Key Information on FR Graphs

JMP 39 FR of Your CAS Sensor

JMP 40 CAS Design Characteristics Effecting FR Air Damping of the blade –The CAS metal beam is critically damped. –The higher the frequency of the event the more the air in the gap reduces the movement of the blade. –The 0g gap has set limits to make sure that the sensor remains critically damped. –If the gap is too small the sensor becomes over-damped and will give a reduced output. –If the gap is too large the sensor becomes under-damped and will give an amplified output.

JMP 41 Effect of Gap on Frequency Response, -3dB CAS Design Characteristics Effecting FR

JMP 42 Electrical Filters –Capacitors and resistors on the circuit electrically filter the output of the CAS –Different capacitor and resistor values in combination with the mechanical air damping of the blade create different -3dB points One design needs -3dB < 15Hz Another design needs -3dB ~ 50Hz CAS Design Characteristics Effecting FR