methods and principles for torque accuracy and safety methods and principles for torque accuracy and...
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hofer electric drive systems
Methods and principles for torque accuracy and safety
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Methods and principles for torque accuracy and safety
2
Introduction:
Table of Content
Effects of Torque inaccuracy for hybrid vehicles and electrical axis
Torque generation of PSM and ASM and main influence factors
Influence of position errors on the torque and corrective measures
Influence of current sensor errors on the torque and corrective
measures
Compensation of temperature variation and saturation effect
Torque safety and torque observer architectures
Summary and conclusions
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Methods and principles for torque accuracy and safety
3
Effects of torque inaccuracy for hybrid axis and EV applications
P2 Concept with impulse start
Torque accuracy needed for
Cross fading between pure electric drive and combustion engine start
Synchronization of gears
Torque ripple must be avoided due to:
Avoid excitation of natural harmonics of power train “bonanza effect”
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Methods and principles for torque accuracy and safety
4
Effects of torque inaccuracy for hybrid axis and EV applications
Electrical axle
Torque accuracy of electrical axle drive needed for
Torque vectoring
Vehicle stability on wet or iced road surface
Safety
Avoidance of oscillation and noise
Typical requirement for torque accuracy and torque safety
Torque deviation typical 5% (between request and real torque)
Safety issue if torque difference is higher than 10%
Rotor EM
Q EM
Achslager
Rad
QR
Chassis
Gw
c
M
Gü
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Methods and principles for torque accuracy and safety
5
Effects of torque inaccuracy for hybrid axis and EV applications
Electrical torque vectoring with electrical axle
understeering
understeering oversteering
Typical requirement for torque accuracy and torque safety
Torque deviation between 2 wheels should be below 10%
Safety issue if torque difference between the 2 wheels is higher than 20%
This means a torque deviation for each subsystem of ca. 5%
Electrical torque vectoring allows a better
and more efficient distribution of the torque
Acceleration of outer wheel while understeering
Acceleration of inner wheel while oversteering
Reduce the use of breaking system more efficient system
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Methods and principles for torque accuracy and safety
6
Torque generation of PSM and ASM and main influence factors
Lorentz force
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Methods and principles for torque accuracy and safety
The torque of an electrical machine (unified theory) is proportional to
the linked flux and the iq current ( component of current orthogonal
to the linked flux) independent of E-machine type (ASM, PSM)
PSM:
ASM:
7
Torque generation of PSM and ASM and main influence factors
qipT 2
3
dh iL
constp
currentCrossi
fluxedConcatenat
pairsPolep
TorqueT
q
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Methods and principles for torque accuracy and safety
8
Position sensor error (e.g. PSM)
T = i 3 p2 q
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Methods and principles for torque accuracy and safety Influence of position error to the torque and corrective measures
Error in position detection system leads to
Voltage output of inverter with angle error
d, q currents are affected by angle error
Reference system for control: d, q
Real reference system: d*, q*
Standard current controller cannot compensate the angle error
Time changing angle error leads to torque ripple
9
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Methods and principles for torque accuracy and safety Influence of position error to the torque and corrective measures
Pure iq operation ( e.g speed below edge speed)
Reference system for control: d, q
Real reference system: d*, q*
Due to the angle error, the real current in the electrical machine has also
an unintended id component (id*)
In this operation conditions the torque error is proportional to the cos of the
angle error e
T* = T cos e
Sensitivity of torque accuracy due to position error is low. If e 1°
Torque error = 0,025%
10
e
e
sini*i
cosi*i
qd
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Methods and principles for torque accuracy and safety Influence of position error to the torque and corrective measures
Pure id operation ( e.g speed over edge speed and torque request = 0, field
weakening only)
Reference system for control: d, q
Real reference system: d*, q*
Due to the angle error, the real current in the electrical
machine has also an unintended iq component (iq*)
In this operation conditions the torque error is proportional to the sin of the
angle error e.
T* ~ sin e
Sensitivity of torque accuracy due to position error is high. If e 1° torque
error = 1,7%
11
e
e
cosi*i
sini*i
dd
dq
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Methods and principles for torque accuracy and safety
Corrective measures:
Active compensation of position failure through position observer
12
Influence of position error to the torque and corrective measures
Advantages:
Increase of torque accuracy
Reduction of torque ripple
Reduction of accuracy
requirement
of the position sensor cost
Model works very well
especially at high speed
where the failure sensitivity is
more critical
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Methods and principles for torque accuracy and safety
Results for ASM with position observer:
Oscillation in DC current and phase current oscillation in torque and power
13
Influence of position error to the torque and corrective measures
Filtered
Observer active
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Methods and principles for torque accuracy and safety
14
Influence of current sensor error and corrective measures
T = i3 p2 q
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Methods and principles for torque accuracy and safety
In the filed oriented control the phase currents i1 and i2 are measured
and transformed in field oriented coordinates (d,q). In this coordinate
system the current is ideally a stationary vector
This current vector is used by the controller to control the torque
Possible errors of phase current sensor error (i1 and i2):
1. Amplitude error (gain or offset)
2. Error due to delay between i1 and i2
15
Influence of current sensor error and corrective measures
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Methods and principles for torque accuracy and safety
1. Phase current gain error:
The current vector in field oriented coordinates is no more constant but moves
around the set point
Consequence are low torque accuracy and torque ripple:
16
Influence of current sensor error and corrective measures
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Methods and principles for torque accuracy and safety
Corrective measures:
Active compensation offset error (by zero current conditions)
End of line compensation of gain error
Online current sensor temperature compensation
Use of compensated current sensor (low hysteresis)
Implementation of learning algorithms and adaptive models in order to compensate
long time effect (aging)
Use of low tolerance HW components (e.g resistors, capacitors, Op-Amps)
17
Influence of current sensor error and corrective measures
Advantages:
Increase of torque accuracy
Reduction of torque ripple
Disadvantages:
Increase of power electronic cost
Increase of SW effort
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Methods and principles for torque accuracy and safety
18
Compensation of Temperature variation and saturation effect
T = i 3 p2 q
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Methods and principles for torque accuracy and safety
Temperature effects:
For all machines the temperature has an influence to the winding
resistance which can change about a factor of 2 between -40°C and 160°C
For a PSM the temperature has an influence to the flux generated by the
magnets (approx. 20%)
For an ASM the temperature has an influence also on the rotor resistance
(squirrel cage)
19
Compensation of temperature variation and saturation effect
Temperature compensation is mandatory for rotor and magnets to avoid
bad torque accuracy
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Methods and principles for torque accuracy and safety
Saturation effects:
For a PSM machine the magnetical circuit will be partially saturated
(especially in the teeth and near the magnets):
This leads to a strong dependency of the Ld and Lq inductance from the
actual current.
20
Compensation of temperature variation and saturation effect
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Methods and principles for torque accuracy and safety
Corrective action:
Compensation of non linearity by means of matrix dependencies
21
Compensation of temperature variation and saturation effect
0100
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0
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Querstrom iq [A]
Längsinduktivität Ld
Längsstrom id [A]
Induktivität
[µH
]
0100
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400500
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0
100
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Querstrom iq [A]
Querinduktivität Lq
Längsstrom id [A]
Induktivität[
µH
]
Easy to implement Determination of EM parameter only
with special algorithm possible
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Methods and principles for torque accuracy and safety
Saturation effects:
For an ASM machine the main inductance Lh will decrease due to
saturation (especially at high torque demand):
22
Compensation of temperature variation and saturation effect
Corrective action:
Compensation of non linearity by means of one dimensional table
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Methods and principles for torque accuracy and safety
23
Torque safety and torque observer architectures
Needed torque deviation and safety operating area
Save operation can only be
guaranteed when torque
information is fully
independent of torque
control (ideally independent
algorithm and sensors)
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Methods and principles for torque accuracy and safety
24
Torque safety and Torque observer architectures
Direct torque measurement
Examples from
MAGTROL
Examples from
LORENZ
Precise
Fully independent
Large
Expensive
Not automotive
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Methods and principles for torque accuracy and safety
25
Torque safety and torque observer architectures
Calculation of the torque by means of flux and current
qipT 2
3
Well known algorithm
Well known compensation
strategies (e.g for temperature,
current offest and gain, position error)
Similar precision as torque control
Same algorithm for torque
control and torque observer
Same sensors are used
twice for torque control and
torque observer plausibility
needed (redundancy of sensors)
y cannot be measured
directly
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Methods and principles for torque accuracy and safety
26
Torque safety and torque observer architectures
Calculation of the torque by means of power approch
Power-flow from the battery to the drive shaft
Losses in EM-Stator
Output Power
Losses in Power Electronics
Me
ch
an
ica
l P
ow
er
Airg
ap
of E
M
Ele
ctr
ica
l P
ow
er
AC
Po
we
r E
lectr
on
ics O
utp
ut
Ele
ctr
ica
l P
ow
er
DC
Ba
tte
ry O
utp
ut
Ele
ktr
ica
l P
ow
er
DC
(co
mp
lete
)
Ba
tte
ry-in
tern
al
Losses in Battery
Heating of
EM
-Rotor
Losses in EM-Rotor
Me
ch
an
ica
l P
ow
er
EM
-Sh
aft
Heating of
EM
-Stator
Heating of
Pow
er Electr.
Heating of
Battery
Drive
Me
ch
. P
ow
er
Drive
-Sh
aft
Heating of
Gearbox
Losses in Gearbox
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Methods and principles for torque accuracy and safety
27
Torque safety and torque observer architectures
Calculation of the torque by electrical power and speed of the shaft
Shaft
Rotor,LossStator,LossEM,el PPPT
323121, LLLLLLEMel iuiup
Simplified calculation of electrical power
Algorithm is independent of torque control algorithm
Calculation is independent of type of machine (ASM / PSM / IPM / …)
Calculation is based on new and independent information (e.g. Phase
Voltage)
Third current (iL2) can be used for plausibility check of the other currents
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Methods and principles for torque accuracy and safety
28
Summary
Using electric drives in electric and hybrid vehicles
means
heavy demands in tolerances
regarding generating torque and observing torque
to reach these demands with low cost measures
a lot of know how and experience is necessary
Applying the measures shown in this presentation
it is possible to implement
a torque control with a tolerance better than 5%
and a torque observer with detection of and reaction on
toque deviations of 10%