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AccelerometersAccelerometers

Justin PiccirilloTexas Instruments Incorporated

Sensors & Controls Division

jpiccirillo@ti.com

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SummarySummary

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

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Acceleration FundamentalsAcceleration Fundamentals

• What is Acceleration?– Definition: the time rate of

change of velocity

– A.K.A.: the time rate of changeof 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/s2 or 9.81 m/s2

2

2

t

x

t

va

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More Notes on AccelerationMore Notes on Acceleration

• What is the time rate of change of velocity?– When plotted on a graph, velocity is the slopeslope of distance versus time

– Acceleration is the slopeslope of velocity versus time

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How to find velocity from distance traveled

0.000

0.200

0.400

0.600

0.800

1.000

1.200

0.000 0.200 0.400 0.600 0.800 1.000 1.200 1.400

Time (seconds)

Dis

tan

ce T

rave

led

(m

eter

s)

V(t=1.160) = 0 m/s

V(t=0.640) = 1 m/s

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How to find acceleration from velocity

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-6.000

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0.000

1.000

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3.000

0.000 0.200 0.400 0.600 0.800 1.000 1.200 1.400

Time (seconds)

Vel

oci

ty (

met

ers/

seco

nd

)

V(t=1.160) = 0 m/s

V(t=0.640) = 1 m/s

a(t=0.960) = 0 m/s2

a(t=1.040) = -10 m/s 2

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Acceleration vs. Time

-45.000

-40.000

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0.000

5.000

10.000

0.000 0.200 0.400 0.600 0.800 1.000 1.200 1.400

Time (seconds)

Acc

ele

rati

on

(m

eter

s/s

ec^

2)

a(t=1.040) = -10 m/s2

a(t=0.960) = 0 m/s2

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Acceleration in Human TermsAcceleration in Human Terms

• What are some “g” reference points?

Description “g” levelEarth’s gravity 1gPassenger car in corner 2gBumps in road 2gIndy car driver in corner 3gBobsled rider in corner 5gHuman unconsciousness 7gSpace shuttle 10g

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What’s the point?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

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Common Types of AccelerometersCommon 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

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What Type of Acceleration Sensor What Type of Acceleration Sensor Does TI Produce and why?Does TI Produce and why?

• Capacitive Acceleration Sensor– “CAS”

CAS

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CAS Conceptual DesignCAS Conceptual Design

0

25

50

75

100

ACCELERATION

OU

TP

UT

(%

Vcc

)

Mechanical Deflection

Acceleration

Change in

Capacitance

Voltage Output Proportional to

Mech. Input

Electronic Calibration

Conditioning

Electronics

CapacitiveSensingElement

Rigid Substrate

MovableBlade

AccelerationInduced Load

Output Voltage

BearingPin

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Capacitive Sensing Element (CSE)Capacitive Sensing Element (CSE)

Compliant Pins

Blade(welded to

bearing pin)

Substrate

Capacitancedetect

Capacitance generated by electrical current through parallel plates:

d

AKeC o

C = capacitanceK = dielectric constant of the insulating mediumeo = permittivity of free spaceA = effective aread = distance between plates (gap) function of acceleration

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Conditioner ModuleConditioner 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)

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Capacitive Acceleration SensorCapacitive Acceleration Sensor

Base

Conditioner Module

CompliantPins

Bracket

Shell

Blade

BearingPin

Substrate

ProtectiveSleeve

Cup

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Acceleration Sensor TerminologyAcceleration 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.

goutgoutgout VVVLinearity 1,1,0, 2

1

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Acceleration Sensor TerminologyAcceleration 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.

g

VV

g

VySensitivit goutgoutout

21,1,

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1.800

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2.800

-1g 0g +1gg Position

CA

S o

utp

ut,

Vo

ut,

(V

)

Theoretical

Actual

Example: Sensitivity & LinearityExample: Sensitivity & Linearity

VoltsVVVLinearity goutgoutgout 3.05.21.12

11.2

2

11,1,0,

g

Volts

g

VV

g

VV

g

VV

g

VySensitivit goutgoutout 7.0

2

5.21.1

2

5.21.1

21,1,

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• 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 TerminologyAcceleration Sensor Terminology

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DC ResponseDC 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

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Effect of TiltEffect of Tilt

• DC response sensors measure tilt. Mounting errors are therefore significant

• a 1o tilt in the 0g position creates an output error equivalent to a 10o tilt in the +1g or -1g positions

• 0g is the most sensitive to mounting errors

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0g Orientation

= 1° → Gn = 1.7x10-2*G

+1g Position

(-1g Position uses same equation)

= 1° → Gn = 0.9998*G

G G

Why is device sensitive to Why is device sensitive to tilt in the 0g orientation?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 1Conclusion: at 0g orientation, change in 1°° tilt causes 57x tilt causes 57x bigger change in sensor output versus -1g or +1g orientationbigger change in sensor output versus -1g or +1g orientation

blade

pinCSE substrate

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Effect of Tilt on DC AccelerometerEffect of Tilt on DC Accelerometer

CAS Output Voltage Change with Position

Cha

nge

in O

utpu

t (m

V)

500mV/g Device

Degrees

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Typical Accelerometer ApplicationsTypical 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

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Important Setup Requirements Important Setup Requirements for your CAS Devicefor 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

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DemonstrationDemonstration

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Electronics SetupElectronics Setup

1 channel12 bit A/DConverter“LTC1286”

CAS Sensor

Serial connection to PC

Piezoelectricspeaker9V

Microprocessor, RAM, EEPROM

Vcc

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Tilt from Horizontal

-100

-80

-60

-40

-20

0

20

40

60

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100

0 10 20 30 40 50 60 70

Time (sec)

Tilt

(d

egre

es)

Example Output: Tilt DataExample Output: Tilt Data

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Acceleration

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0.500

1.000

1.500

0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00

Time (seconds)

Acc

eler

atio

n (

g)

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

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Average Acceleration

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1.00

0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00

Time (second)

Ave

rag

ed A

ccel

erat

ion

(g

)

Acceleration – 1 second averagesAcceleration – 1 second averages

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SummarySummary

• 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 1o tilt in 0g = ΔVout for 10o 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

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Questions?Questions?

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EXTRA SLIDESEXTRA SLIDES

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Frequency Response (FR)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.)

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Components of FRComponents of FR

• Magnitude or Amplitude– Ratio of the output (CAS) / input (reference accel. output)

– Calculated in Decibels (dB) = 20 * log10(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

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Measurement of FRMeasurement 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.

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• 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 GraphsKey Information on FR Graphs

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• 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

• 10o at 5Hz

• 10o at 10 Hz

• 70o at 15Hz

Key Information on FR GraphsKey Information on FR Graphs

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FR of Your CAS SensorFR of Your CAS Sensor3LG20-15A

Offset

-3db Frequency: 18.2Hz 180° Phase Shift Frequency: 231.25Hz

0

0.1

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0.9

1

1.1

1 10 100 1000Frequency (Hz)

g's

(CA

S)/

g's

(R

ef)

-300

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-100

-50

0

50

Pha

se L

ag (

Deg

rees

)

FrequencyResponse

Phase Lag 3LG20-15

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CAS Design Characteristics Effecting FRCAS 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.

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• Effect of Gap on Frequency Response, -3dB

New Metallization, # 1 Blade S2G evaluation

1.00

1.25

1.50

1.75

2.00

2.25

2.50

2.75

3.00

40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

-3dB Frequency (Hz)

0g

Ga

p (

mil

s)CAS Design Characteristics Effecting FRCAS Design Characteristics Effecting FR

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• 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 FRCAS Design Characteristics Effecting FR