Smarter Thinking.

© MIRA Ltd 2014

Smarter Thinking. © MIRA Ltd 2014

Vehicle Data

Collection

Fundamentals

EIS Event

9th September 2014

MIRA

Smarter Thinking.

© MIRA Ltd 2014

Schedule

Introduction – Geoff Rowlands

Who needs data?

Planning and Designing a Data Collection

The Main Elements -

1. Sensors

- Strain Gauges

- Wheatstone bridge ¼ ½ and Full bridge

- Transducer Types – Load cells, Displacement, Accelerometers

- Temperature measurement

- Calibration

- Wheel Force Transducers

- CAN

- GPS

2. Recording Data

Data Recorders

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Schedule

Instrument Setup and Checks – Steve Payne

- Installation

- Transducer checks

- Polarities/vehicle co-ordinates

- Vehicle shakedown

12.30pm Lunch

1.15pm Show and tell – CARDUR

vehicle on main site – David Ensor, Geoff Rowlands and Steve Payne, MIRA

2.30pm Tea and Coffee

Data Collection – David Ensor, MIRA

- Load conditions

- Tracks

- Environmental considerations

- Sample rate and filtering

4.30pm Q&A and closing comments – David Ensor – MIRA

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The Data Process

Transducer

Analysis

Useful

Data !

Recording

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But to begin with..

What is the

Question?

How can I

get it?

Data

Collection!

What

information

do I need to

answer

this?

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Simulation

Rig Testing • Fatigue Analysis

• Fatigue Editing

FE Analysis

• Data Processing

• Data Management

Kinematic SimulationsProving Ground

Data Flow

Who are the Customers for Your Data?

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Who are the Customers for Your Data?

Need to talk to customers to fully understand needs

and wants

Need to establish:

- Accuracy required

- Type of measurement

- Location of measurements

- Timescale for data.

- Type of data transfer / presentation

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Typical User Requirements

Structural Test

- Strain gauges, accelerometers, displacements, focus on severe events and enough data to define typical conditions.

Suspension

- Wheel displacement and loads. Acceleration at hub and body shock mount. Vehicle speed, Damper temperatures. Focus on severe suspension events like potholes, speed bumps, pave, rough road, handling circuit.

Transmission

- Long test routes to accurately capture real road running. Road types are defined in terms of User Profile – Urban, Hilly, Off road, Motorway, A and B roads. Main collection channels are speeds and torques.

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The Data Chain - Sensors

These convert PHYSICAL quantities into something we

can record, an ELECTRICAL signal.

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Sensing Elements

Strain gauge – Displacement into resistance

Piezoelectric – Force into Electrical Charge

Potential Divider – Displacement into Resistance

ICP – Displacement into Capacitance

LVDT – Displacement into Inductance.

Thermocouple – Temperature into voltage

Etc. etc.

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Simple example – fuel gauge.

E F12

v

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Strain Gauges

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What is Strain?

Stress

Strain

Limit of

Proportionality

(Elastic Limit)

Yield point

(UTS)

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Strain Gauge Construction

Sensitive axis

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Rosette Strain Gauges

• 60 degree Delta rosette • Rectangular Rosette

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Changing strain into resistance- Gauge Factor

The amount of resistance

change for a given

amount of strain is

defined as the “gauge

factor” Gf

Resistance

changeStrain

∆𝑅

𝑅= 𝐺𝑓 ∗

∆𝐿

𝐿

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Measuring Strain

The resistance changes are very small; for 1000

microstrain in a 120 ohm strain gauge, the resistance

change would be 0.24 Ohms.

Conventional methods of resistance measurement are

unable to deal with these small changes. That is why a

Wheatstone Bridge is used.

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Wheatstone Bridge Circuits

V

R1 R2

R3 R4

Vs+

Vs-

Vs+

Vs-

R1 R2

R3 R4

V

Wheatstone Bridge Equivalent circuit

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How Gauge resistance is cancelled.

10v

0v

120

120120

5.000120 Ohm

Gauge

10v

10m

V

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How Gauge resistance is cancelled.

10v

0v

120

120120

5.000120 Ohm

Gauge

10v

10m

V

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How Gauge resistance is cancelled.

10v

0v

120

120120

0.000120 Ohm

Gauge

10v

10m

V

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How Gauge resistance is cancelled.

10v

0v

120

120120

5.000120 Ohm

Gauge

1000u

E10v

10mV

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Strain Gauge on Simple beam.

G V 4

1 = V so

Straingauge

Load

Vo

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Half Bridge Bending measurement

G V 2

1 = V so

A

B

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Full Bridge bending measurement

G V = V so C

D

A

B

A

B C

D

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Measuring axial load.

Vs

Vo

AA

B

B

ratio Poissons is Where

G )+(1 4

1 V = V so

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Torque measurement

shaft.the of radius the is r

material the of modulus shearthe is M

torque is T

strain shearthe is Where

r M

T 2 =

3

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Full Bridge Torque measurement

𝑉𝑜 = 𝑉𝑠 𝛾 𝐺

Where 𝛾 is the shear strain

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Strain Gauge Wiring

2 wire connection

Component

Rb

Ra

120Ω

Connection wires may

be subject to

temperature change

Vo

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Strain Gauge Wiring

2 wire connectionVs

+

Vs

-

R

1R

2

R

3R

4

V

Ra

RbVo

Component

Rb

Ra

120Ω

R1

R3 R4

R2

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Strain Gauge Wiring

Vo

3 wire connection

Component

Rb

Ra120Ω

Rc

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Strain Gauge Wiring

3 wire

connectionRa

Rb

Vs+

Vs -

R1 R2

R3 R4

V

o

Rc

Vo

Component

Rb

Ra 120Ω

RcR1

R3 R4

R2

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VoBalance

Control

R1

R2

Vs +

Vs -

Bridge Balancing

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Calibration resistor

Vs +

Vs -

Calibration

Resistor

switch

Vo

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Strain gauge reading instrument.

PSD704 Strain Indicator by Strainsense

Useful tool for setting up

Strain gauges. Has the

facility for bridge

completion, balancing

and accurate reading of

strain.

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Transducers Using Strain Gauges.

Many types of measurement devices are possible using strain gauges as

the sensing element. These include load cells, accelerometers, pressure

transducers and even displacement transducers.

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Load cells

Strain

Gauges

Compliant Element Stiff Element

Loa

dLoa

d

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Accelerometer

Damping Fluid Seismic Mass

Strain gauges

HousingSensitive

axis

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Pressure transducer

High Pressure Low

Pressure

Strain gauges

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Making Components into Load Cells

It is often desirable to add strain gauges to a component to turn it into

a load cell. A propshaft may be gauged to measure torque for

instance. Or a suspension component gauged to measure force.

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Making Components into Load Cells 2

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Amplification

As we have seen, the voltage output from a strain gauge transducer will be in

the order of a few millivolts.

In order to display or record this, some amplification (gain) will be needed.

In addition there may be a need for bridge completion, balancing and

calibration.

This can either be done using a separate signal conditioning unit or may be

incorporated into the recording device itself.

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Other Transducers - Displacement

Linear Potentiometer Rotary Potentiometer

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String Pot for large displacements

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Linear Variable Differential Transformer LVDT

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Vo

Non- Magnetic

Plunger

Energising supply

Ferromagnetic armature

LVDT

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Piezoelectric Force transducer.

Piezoelectric Crystal

Force

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Where to Mount Accelerometers?

If characterising the whole vehicle motion then on C of G.

Not in centre of a panel that resonates, but on something

rigid.

On engine mount, both body and engine side to derive

transfer function across mounting.

Similarly for a gearbox mounting.

For Instrument panel, or other internal body parts, mount

accelerometers for all three axes adjacent to the mounting

points. These can then be used to set up a shaker test for

these type of components.

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Rate Gyro

Used to measure angular velocity.

Mount at Centre of Gravity.

Gyroscopic type – Based on precession

forces on a spinniing mass

Solid State type – “Tuning fork” is excited

at its resonant frequency. Then it

behaves like a gyroscope with Coriolis

forces generated by “out of plane”

movements.

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Temperature measurement.

There are many types of temperature measuring device

including:

- Thermocouple

- RTD

o Thermistor

o (PRT) Platinum Resistance Thermometer.

- Pyrometer

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Thermocouple

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Resistance Temperature devices RTD

ThermistorPRT

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Infra-red Pyrometers

Great for non-contact

temperature

measurement but

require careful setting

up for accurate

readings.

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Wheel Force Transducers (WFT)

These are a versatile type of measuring device which are bolted onto the

vehicle in place of a pair, or all four wheels.

MTS SWIFT

Caesar WFT

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WFT Channels

7 Measurement channels are available per wheel

- 3 Forces X Y and Z

- 3 Moments Mx, My, Mz

- Wheel rotational angle

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Installation considerations.

The dynamics of the WFT may not match the

actual wheel usually fitted – Weight, Tyre

Diameter, stiffness, and offset.

The rotation sensor is used to calculate some

of the forces into vehicle coordinates.

- Careful restraint is key to accuracy.

Sensor restraint fixture

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CAN-Bus Measurements

CAN (Controller Area Network) systems are now

common on vehicles.

These systems consist of many devices including

sensors, that are used by the Electronic Control

Units (ECU) to monitor and control the functions of

the vehicle.

They can be a useful source of data for

development and testing.

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CAN-Bus Measurements

Data collection hardware allows CAN channels to be selected and recorded

along with other sensors.

Throttle Position

Brake actuation

Engine speed

Gear selected

Wheel speeds

Engine temp

Ambient temp etc.

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CAN Measurements limitations

Users should be aware that:

- CAN measurements are often not calibrated.

- They may have low resolution.

- Update rates are sometimes slow

- Samples are not taken at regular intervals

- Signals can exhibit noise or shut down.

Are best suited for supplementary information or slowly changing

parameters.

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GPS Data

Global Positioning System (GPS) data has become very

cheap and easy to capture.

Some recorders have this facility “built in”.

GPS receivers have a facility for interfacing with data

recorders.

Vehicle location is a very useful supplement to the normal

data.

Routes can be mapped and reported.

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GPS Positional Data - Uses

Can be used at analysis time to identify location of extreme events.

Assures user that route taken was correct.

Helps understanding of data for polarity – left hand bends can be clearly

seen, for instance.

Vehicle speed (every second) can be used as check.

Time from GPS helps to identify test conditions.

Altitude can show relative severity of hilly routes.

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GPS Plot of Figure of 8

Easy to see in

which direction the

car was moving

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GPS + Analysis example

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Optical speed and slip sensors

Widely Used for

Brake testing and

Vehicle Dynamics

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Sensor Calibration

Calibration is most important with any measurement.

It is the means whereby we can be confident that the measurements we

take are accurate.

Bought-in transducers should have a calibration certificate.

Custom made load cells need to be calibrated against a known

standard.

Calibration data should be recorded with all tests

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Errors - Linearity

Linearity is an

expression of the

closeness of the

response of a

system to a

straight line.

Load Cell calibration

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

4

0 0.5 1 1.5 2 2.5 3 3.5 4

Load

Vo

lts

0.1

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Errors - Hysteresis

Hysteresis is a

measure of how

different the

calibration is if

the load is

applied in

different

directions.

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Errors - Accuracy

Accuracy is the closeness of the measured value to the actual value of the

measurement. (It is worth pointing out that ultimately, we have no way of

knowing what this value actually is!)

The best we can do is compare with a known standard.

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Errors - Resolution

Resolution is the smallest

step change that can be

detected by the

measurement system. This

is often limited by the data

recording system alone.

Load Cell calibration

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

4

0 0.5 1 1.5 2 2.5 3 3.5 4

Load

Vo

lts

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Errors – Repeatability

Load Cell Calibration

0

0.1

0.2

0.3

0.4

0.5

0.6

0 1 2 3 4 5 6

Load Kg

Vo

lts

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Sensitivity

Load Cell Calibration

y = 0.1003x + 0.0003

0

0.1

0.2

0.3

0.4

0.5

0.6

0 1 2 3 4 5 6

Load Kg

Vo

lts

y=mx+c

X is the slope

of the graph.

Which is the

sensitivity of

the transducer.

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Noise in signals.

What is Noise?

Noise is unwanted signal

There are many sources but the main ones are:

- Inductive coupling

- Capacitance coupling

- Earth or ground looping

- Random noise

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How to minimise noise

Inductive

- Use good quality connection cables with twisted pair signal wires. Keep bare untwisted lengths to a minimum.

Capacitive

- Use connection cables that shielding around the signal wires.

Earth loops

- Connect shields to ground at one end of the cable only. Connect all grounds together at one point.

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Signal to Noise ratio

Signal to noise ratio gives a

measure of the significance of

noise in the signal. It is defined

as the signal power divided by

the noise power and expressed

in decibels.

VoltageNoise

VoltageSignal 20

10Log

•60 =20 * 𝑙𝑜𝑔10 ∗𝑆

𝑁

•60/20 = 𝑙𝑜𝑔10 ∗𝑆

𝑁

•𝑙𝑜𝑔10 3 = S/N

•S/N = 1000

For example 60 dB

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Data Recorders

Many types of recorder are available.

When selecting the recorder, we need to consider what is important

for our type of work.

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Logging or Recording?

Data Logging generally means

- Low sample rate – One data point per second

- Looking for trends over long time period

- Can be rugged for unattended monitoring.

- Often relatively low cost.

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Selecting the recorder

Mobile use

- 10-30v power supply.

- Stand alone Independently of setup PC

- Vibration and temperature specification

- Size and weight

Versatility

- Different types of transducer

- Sample rates available

- Synchronous samples

- Data storage capacity

- Number of Channels

- Vehicle Can-Bus Interface

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Selecting the recorder

Usability

- Unattended operation

- Remote operation

- Run-time display

- Ease of setup

Data collection modes

- Triggers and gates

- Time histories and reduced data

- Online computation of new channels

Data transfer rate

Compatible with data analysis software

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Types of transducer input

Strain gauge

- Require differential input

- Excitation voltage

- Choice of bridge resistance

- Shunt Calibration signal

- Bridge balancing (zero).

Voltage input

- Can be single ended or differential

- Common mode rejection

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Single ended versus Differential

Single ended inputs have

one common “earth”

connection for a number of

voltage channels. This is

OK for measurements that

are referenced to a known

zero volts point.

1

2

3

4

0v

1hi

1lo

2hi

2lo

3hi

3lo

4hi

4lo

Differential inputs have one

connection for signal high

and one for signal low. It is

the difference in voltage

between these two that is

amplified to the output.

0v

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Setting up to Record Data

Choosing Sample rates

Setting up Filters

How Much data to collect?

How long?

How many repeats?

How much data storage space will it take?

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Choosing Sample rates

Sample rate defines maximum frequency.

Body

Wheel Hop

Pitch

10 20 30 40Frequency Hz