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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
Smarter Thinking.
© MIRA Ltd 2014
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
Smarter Thinking.
© MIRA Ltd 2014
The Data Process
Transducer
Analysis
Useful
Data !
Recording
Smarter Thinking.
© MIRA Ltd 2014
But to begin with..
What is the
Question?
How can I
get it?
Data
Collection!
What
information
do I need to
answer
this?
Smarter Thinking.
© MIRA Ltd 2014
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?
Smarter Thinking.
© MIRA Ltd 2014
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
Smarter Thinking.
© MIRA Ltd 2014
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.
Smarter Thinking.
© MIRA Ltd 2014
The Data Chain - Sensors
These convert PHYSICAL quantities into something we
can record, an ELECTRICAL signal.
Smarter Thinking.
© MIRA Ltd 2014
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.
Smarter Thinking.
© MIRA Ltd 2014
Simple example – fuel gauge.
E F12
v
Smarter Thinking.
© MIRA Ltd 2014
Strain Gauges
Smarter Thinking.
© MIRA Ltd 2014
What is Strain?
Stress
Strain
Limit of
Proportionality
(Elastic Limit)
Yield point
(UTS)
Smarter Thinking.
© MIRA Ltd 2014
Strain Gauge Construction
Sensitive axis
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© MIRA Ltd 2014
Rosette Strain Gauges
• 60 degree Delta rosette • Rectangular Rosette
Smarter Thinking.
© MIRA Ltd 2014
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
Smarter Thinking.
© MIRA Ltd 2014
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.
Smarter Thinking.
© MIRA Ltd 2014
Wheatstone Bridge Circuits
V
R1 R2
R3 R4
Vs+
Vs-
Vs+
Vs-
R1 R2
R3 R4
V
Wheatstone Bridge Equivalent circuit
Smarter Thinking.
© MIRA Ltd 2014
How Gauge resistance is cancelled.
10v
0v
120
120120
5.000120 Ohm
Gauge
10v
10m
V
Smarter Thinking.
© MIRA Ltd 2014
How Gauge resistance is cancelled.
10v
0v
120
120120
5.000120 Ohm
Gauge
10v
10m
V
Smarter Thinking.
© MIRA Ltd 2014
How Gauge resistance is cancelled.
10v
0v
120
120120
0.000120 Ohm
Gauge
10v
10m
V
Smarter Thinking.
© MIRA Ltd 2014
How Gauge resistance is cancelled.
10v
0v
120
120120
5.000120 Ohm
Gauge
1000u
E10v
10mV
Smarter Thinking.
© MIRA Ltd 2014
Strain Gauge on Simple beam.
G V 4
1 = V so
Straingauge
Load
Vo
Smarter Thinking.
© MIRA Ltd 2014
Half Bridge Bending measurement
G V 2
1 = V so
A
B
Smarter Thinking.
© MIRA Ltd 2014
Full Bridge bending measurement
G V = V so C
D
A
B
A
B C
D
Smarter Thinking.
© MIRA Ltd 2014
Measuring axial load.
Vs
Vo
AA
B
B
ratio Poissons is Where
G )+(1 4
1 V = V so
Smarter Thinking.
© MIRA Ltd 2014
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
Smarter Thinking.
© MIRA Ltd 2014
Full Bridge Torque measurement
𝑉𝑜 = 𝑉𝑠 𝛾 𝐺
Where 𝛾 is the shear strain
Smarter Thinking.
© MIRA Ltd 2014
Strain Gauge Wiring
2 wire connection
Component
Rb
Ra
120Ω
Connection wires may
be subject to
temperature change
Vo
Smarter Thinking.
© MIRA Ltd 2014
Strain Gauge Wiring
2 wire connectionVs
+
Vs
-
R
1R
2
R
3R
4
V
Ra
RbVo
Component
Rb
Ra
120Ω
R1
R3 R4
R2
Smarter Thinking.
© MIRA Ltd 2014
Strain Gauge Wiring
Vo
3 wire connection
Component
Rb
Ra120Ω
Rc
Smarter Thinking.
© MIRA Ltd 2014
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
Smarter Thinking.
© MIRA Ltd 2014
VoBalance
Control
R1
R2
Vs +
Vs -
Bridge Balancing
Smarter Thinking.
© MIRA Ltd 2014
Calibration resistor
Vs +
Vs -
Calibration
Resistor
switch
Vo
Smarter Thinking.
© MIRA Ltd 2014
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.
Smarter Thinking.
© MIRA Ltd 2014
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.
Smarter Thinking.
© MIRA Ltd 2014
Load cells
Strain
Gauges
Compliant Element Stiff Element
Loa
dLoa
d
Smarter Thinking.
© MIRA Ltd 2014
Accelerometer
Damping Fluid Seismic Mass
Strain gauges
HousingSensitive
axis
Smarter Thinking.
© MIRA Ltd 2014
Pressure transducer
High Pressure Low
Pressure
Strain gauges
Smarter Thinking.
© MIRA Ltd 2014
Smarter Thinking.
© MIRA Ltd 2014
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.
Smarter Thinking.
© MIRA Ltd 2014
Making Components into Load Cells 2
Smarter Thinking.
© MIRA Ltd 2014
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.
Smarter Thinking.
© MIRA Ltd 2014
Other Transducers - Displacement
Linear Potentiometer Rotary Potentiometer
Smarter Thinking.
© MIRA Ltd 2014
String Pot for large displacements
Smarter Thinking.
© MIRA Ltd 2014
Linear Variable Differential Transformer LVDT
Smarter Thinking.
© MIRA Ltd 2014
Vo
Non- Magnetic
Plunger
Energising supply
Ferromagnetic armature
LVDT
Smarter Thinking.
© MIRA Ltd 2014
Piezoelectric Force transducer.
Piezoelectric Crystal
Force
Smarter Thinking.
© MIRA Ltd 2014
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.
Smarter Thinking.
© MIRA Ltd 2014
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.
Smarter Thinking.
© MIRA Ltd 2014
Temperature measurement.
There are many types of temperature measuring device
including:
- Thermocouple
- RTD
o Thermistor
o (PRT) Platinum Resistance Thermometer.
- Pyrometer
Smarter Thinking.
© MIRA Ltd 2014
Thermocouple
Smarter Thinking.
© MIRA Ltd 2014
Resistance Temperature devices RTD
ThermistorPRT
Smarter Thinking.
© MIRA Ltd 2014
Infra-red Pyrometers
Great for non-contact
temperature
measurement but
require careful setting
up for accurate
readings.
Smarter Thinking.
© MIRA Ltd 2014
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
Smarter Thinking.
© MIRA Ltd 2014
WFT Channels
7 Measurement channels are available per wheel
- 3 Forces X Y and Z
- 3 Moments Mx, My, Mz
- Wheel rotational angle
Smarter Thinking.
© MIRA Ltd 2014
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
Smarter Thinking.
© MIRA Ltd 2014
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.
Smarter Thinking.
© MIRA Ltd 2014
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.
Smarter Thinking.
© MIRA Ltd 2014
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.
Smarter Thinking.
© MIRA Ltd 2014
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.
Smarter Thinking.
© MIRA Ltd 2014
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.
Smarter Thinking.
© MIRA Ltd 2014
GPS Plot of Figure of 8
Easy to see in
which direction the
car was moving
Smarter Thinking.
© MIRA Ltd 2014
GPS + Analysis example
Smarter Thinking.
© MIRA Ltd 2014
Optical speed and slip sensors
Widely Used for
Brake testing and
Vehicle Dynamics
Smarter Thinking.
© MIRA Ltd 2014
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
Smarter Thinking.
© MIRA Ltd 2014
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
Smarter Thinking.
© MIRA Ltd 2014
Errors - Hysteresis
Hysteresis is a
measure of how
different the
calibration is if
the load is
applied in
different
directions.
Smarter Thinking.
© MIRA Ltd 2014
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.
Smarter Thinking.
© MIRA Ltd 2014
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
Smarter Thinking.
© MIRA Ltd 2014
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
Smarter Thinking.
© MIRA Ltd 2014
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.
Smarter Thinking.
© MIRA Ltd 2014
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
Smarter Thinking.
© MIRA Ltd 2014
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.
Smarter Thinking.
© MIRA Ltd 2014
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
Smarter Thinking.
© MIRA Ltd 2014
Data Recorders
Many types of recorder are available.
When selecting the recorder, we need to consider what is important
for our type of work.
Smarter Thinking.
© MIRA Ltd 2014
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.
Smarter Thinking.
© MIRA Ltd 2014
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
Smarter Thinking.
© MIRA Ltd 2014
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
Smarter Thinking.
© MIRA Ltd 2014
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
Smarter Thinking.
© MIRA Ltd 2014
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
Smarter Thinking.
© MIRA Ltd 2014
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?
Smarter Thinking.
© MIRA Ltd 2014
Choosing Sample rates
Sample rate defines maximum frequency.
Body
Wheel Hop
Pitch
10 20 30 40Frequency Hz