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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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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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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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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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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Lorentz force

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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


qipT 2

3

dh iL

constp

currentCrossi

fluxedConcatenat

pairsPolep

TorqueT

q

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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

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Pure iq operation ( e.g speed below edge speed)



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

qq

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Pure id operation ( e.g speed over edge speed and torque request = 0, field

weakening only)



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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Corrective measures:

Active compensation of position failure through position observer

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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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Results for ASM with position observer:

Oscillation in DC current and phase current oscillation in torque and power

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Influence of position error to the torque and corrective measures

Filtered

Observer active

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Influence of current sensor error and corrective measures

T = i3 p2 q

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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


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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


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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)

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Advantages:

Increase of torque accuracy

Reduction of torque ripple

Disadvantages:

Increase of power electronic cost

Increase of SW effort

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Compensation of Temperature variation and saturation effect

T = i 3 p2 q

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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)

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Temperature compensation is mandatory for rotor and magnets to avoid

bad torque accuracy

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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.

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Corrective action:

Compensation of non linearity by means of matrix dependencies

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0100

200300

400500

600

0

100

200

300

400

500

100

120

140

160

180

200

Querstrom iq [A]

Längsinduktivität Ld

Längsstrom id [A]

Induktivität

[µH

]

0100

200300

400500

600

0

100

200

300

400

500

100

150

200

250

300

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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Saturation effects:

For an ASM machine the main inductance Lh will decrease due to

saturation (especially at high torque demand):

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Corrective action:

Compensation of non linearity by means of one dimensional table

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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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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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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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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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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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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%

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Thank you for your attention

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