System identification of the brake setup in the
TU Delft Vehicle Test Lab (VTL)
Jean-Paul Busselaar
MSc. thesis
Goals of this final MSc. presentation:
1.
Introducing the basics of ABS, tire dynamics and tire
friction models related to the Feed Forward ABS
controller
2.
Introducing the TU Delft VTL
3.
Describing the dSPACE data acquisition
4.
Presenting the results of the VTL measurement setup
5.
Presenting the conclusion
ABS introduction
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An Anti-lock Braking System (ABS) is a vehicle motion control system to
increase the passive safety of a vehicle.
ABS intends to prevent wheel lock by regulating the hydraulic brake
pressure of each wheel to increase the lateral stability.
ABS introduction
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ABS consists of mathematic models which are based on physical properties
related to tire-road conditions.
Tire friction and road conditions are affecting the control schemes of the
mathematic models.
Tire dynamics
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The dynamic behaviour of a tire is suggested in a coordinate system.
The three forces are:
The three moments are:
1. Longitudinal force Fx
1. Overturning moment Mx
2. Lateral force Fy
2. Rolling moment My
3. Normal force Fz
3. Yawing moment / self-aligning
torque Mz
Tire dynamics
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Side slip α is the angle between the tire longitudinal axis and the wheel
velocity vector
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Longitudinal slip
velocity:
is the normalized ratio of the slip velocity and vehicle
Tire dynamics
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A free rolling wheel is represented with
.
A locked wheel is indicated by
.
ABS controllers are optimized at the transition of the rising and falling slope
Identification of the algorithm for estimating the peak-value
can
provide a reliable estimation for ABS activation.
Stationary friction models
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Static friction is determined by the friction coefficient
Coulomb force
and the normal force
Quasi-physical dynamic friction model
LuGre Tire friction model
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The constant Coulomb friction force is replaced by the speed dependent
sliding friction function
.
The LuGre model is related to the bristle interpretation of friction. Friction is
modeled as the average deflection force of elastic springs.
Feed Forward ABS controller
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The Feed Forward ABS controller model is used to do brake pressure
dynamics measurements
Quarter car with LuGre tire friction
– With
– And
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is the linearized viscous damping
The term
road friction
with
is the non-linearity of the tire-
Feed Forward ABS controller
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Flatness-based model inverse
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Where
is a function of the vehicle speed, acceleration, jerk
and the third derivative of the speed
TU Delft Vehicle Test Lab
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The TU Delft VTL is used for testing tire behaviour and for prototyping ABS
strategies
TU Delft Vehicle Test Lab
Tire setup room
Close up test wheel
Control room
Close up brake caliper
Brake setup Vehicle Test Lab
• The VTL brake system consists of:
1. A hydraulic pump
2.
3.
4.
5.
A by-pass valve to (dis)connect the brake caliper hydraulically
A rubber brake hose in the original situation
A brake caliper with a brake disc, brake pads and forced air cooling
A piezo-electric brake pressure transducer
Brake setup Vehicle Test Lab
6. A charge amplifier to receive charge form the piezo-electric transducer
7. An analog proportional controller to measure and adapt the error between
the reference and measured brake pressure
8. A brake torque sensor and receiver
Brake setup Vehicle Test Lab
• The situation with design improvement:
Stainless Steel braided brake hose
dSPACE
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dSPACE is used to control and measure the parameters of the VTL setup.
dSPACE contains software as well as hardware components.
Brake pressure measurements are performed with an existing FeedForward ABS model in dSPACE.
Modifying the Feed-Forward controller by a signal generator for brake
pressure actuation purpose.
Constant average brake pressures with varying amplitudes are used. To
achieve this, a constant is added with the signal generator.
dSPACE ControlDesk
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To actuate and measure the test setup real-time, a control panel in
ControlDesk has to be made.
From the existing Feed-Forward ABS model, the specific parameters has to
be carefully chosen.
The control panel contains :
– ON-OFF buttons
– A constant pressure value slider
– graphs with measuring parameters
– settings of the actuating signal
– data acquisition
Brake pressure dynamics results
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Sinus wave excitation measurements are executed to show possible
differences in brake pressure dynamics.
These are relatively simple and quick measurements to see the effects
between frequencies, amplitudes and constant average in the original
situation.
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Two last waves are taken to compare for the repeatability result
About 3bar difference between the desired and the actual max pressure
0,07bar max difference in one 97bar case between nr77 and nr78
0,08% max difference in 60bar case between nr77 and nr78
Brake pressure dynamics results
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When the frequency increases with relative low amplitudes: little pressure
difference is observed
Brake pressure dynamics results
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When the amplitude increases 2x, the max pressure dynamics decreases
Brake pressure dynamics results
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Further increase of the frequency with relative high amplitudes results into
larger decrease in pressure dynamics
Brake pressure dynamics results
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With design improvements stainless steel braided brake hose
Shows an improved repeatability
0,75bar improved compared to the original situation
Brake pressure dynamics results
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Random noise excitations to determine transfer functions
Best result of the original settings, cross over frequency 150Hz
Resonance frequency of approx. 84Hz
Bandwidth of approx. 118Hz
Time delay to approx. 250Hz
Brake pressure dynamics results
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Cross over frequency from 150Hz to 300Hz
Resonance frequency approx. 84Hz
Bandwidth approx. 116Hz when using relative low amplitudes
Time delay to approx. 400Hz
Brake pressure dynamics results
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Amplitude 1bar
Resonance frequency of approx. 75Hz
Bandwidth of approx. 120Hz
Time delay to approx. 300Hz
Brake pressure dynamics results
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Check channel 5 input signal with channel 6 with limited freq. content
Resonance frequency of approx. 33Hz
Bandwidth of approx. 115Hz
Time delay to approx. 300Hz
Brake pressure dynamics results
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Best overall results when changing gains
Resonance frequency of approx. 90Hz
Bandwidth of approx. 112Hz
Time delay to approx. 500Hz
Brake pressure dynamics results
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Best bandwidth result when changing gains
Resonance frequency of approx. 50Hz
Bandwidth of approx. 117Hz
Time delay to approx. 300Hz
Conclusion
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Sinus wave excitation results of the original situation show clearly
decreased brake pressure dynamics when increasing the frequency.
Also relative small maximum pressure increase of 0,75bar for the stainless
steel braided brake hose situation is observed. When applying relative high
amplitudes of 20bar with a constant average 80bar @ 1Hz.
Decreasing the varying amplitude will improve the dynamical behaviour of
the measurement setup.
Random noise excitation results with design improvements when increasing
the cross over frequency to 300Hz the overall results are slightly decreased.
When changing the proportional gains, little difference in bandwidth is
noticed in most cases.
Channel 6 with a limited frequency content has a comparable bandwidth,
overall results of channel 5 show a better performance.
The upper knob gain setting of 2.00 shows the best overall results.
The upper knob gain setting of 5.00 shows the best bandwidth of 117Hz.
A cross over frequency of 150Hz and relative low varying amplitudes to 1bar
show the best results of all measurements