Technische Universität München Vorlesung „Elektrische ......Technische Universität München...

Technische Universität München

Vorlesung „Elektrische Aktoren und Sensoren in geregelten Antrieben“

Drehzahl- und Positionsgeberals Rückführzweig in geregelten Antrieben

Prof. Dr.‐Ing. Ralph Kennel

(ralph.kennel@tum.de)

Elektrische Antriebssysteme und Leistungselektronik

Outline

Introduction

Drive Control

Absolute and Differential Accuracy

State of the Art

Standard Encoders

High Resolution Encoders

Future Encoder Technologies

Conclusions

Outline

Introduction

Drive Control

State of the Art

Standard Encoders

Conclusions

Anwendungsbeispiel :

Haupt(spindel)antrieb(e)

für die Bearbeitungsenergie

Vorschubantriebe

für die Positionierung

Werkzeugmaschinen

Synchronmaschinen mit Drehgebern !!!

Synchronmaschinemit Drehgeber

Montageorte von Positionsgebern

Linear Scales

Advantages

• exact/direct measurement

• small disturbances

Disadvantages

• high cost

• low robustness

Encoders

encoder type ERN, Heidenhain

Advantages

• low cost

• high accuracy

(by gear ratio)

Disadvantages

• elasticities

and back-lash

(in the drive train)

are neglected

Outline

Introduction

Drive Control

State of the Art

Standard Encoders

Conclusions

Feldschwächung

M-Regler Strom-Regler

Feld-Regler

Ma-schinen-modell

Encoder

n* i*q

field controller

machine

modelfor asynchronous

machines

weakening

speed controller current

controllers

for synchronous

machines

„0“

AC drive control

Drehzahl-regelung

Drehmoment-/StromRegelung

Lageregelung

Lagegeber

Kommutierungssignale

position controllerspeed

controller

torque/current

controller

commutation signals

tacho generator

position encoder

digital control : a single encoder for all feedbacks !?!

(simplified) basic structure of a drive control

Velocity Sensors

Ralph Kennel

Tacho (Huebner Berlin)

Prof. Dr. -Ing. J. M. Pacas,

Universität Siegen

TachogeneratorThe tachogenerator armature (= rotor) is

connected as torsionally rigid as possible to

the driving machine, whose speed is to be

detected.

As the armature rotates in the field of the

permanent magnets, voltages are induced in

the armature winding. These voltages are

tapped at the commutator with special

brushes (polarity dependent on direction of

rotation). Available at the terminals is a no-

load voltage U0 (n), which is proportional to

the speed.

This is physically a very linear characteristic !

For obtaining this signal, in difference to other

speed sensors an auxiliary power (voltage

supply) is not necessary.

Tachogenerator speed-voltage characteristic.

Universität Siegen

Tachogenerator

G uG(n)

equivalent circuit diagram of tachogenerator

with subsequent control electronics.

If the tachogenerator is loaded with the load resistance RL or load current IL (terminals A1 and

A2), the voltage is reduced by the voltage drop due to the armature resistance RA

U(n) = U0 (n) – I L · R A = U 0 (n) · R L/(R A + R L)

Usually the load resistance R L is significantly higher than the armature resistance R A ,

so that the following approximation applies

U(n) ≈U 0 (n) for RL >>RA

Universität Siegen

Tachogenerator

G uG(n)

equivalent circuit diagram of tachogenerator

with subsequent control electronics.

If the tachogenerator is loaded with the load resistance RL or load current IL (terminals A1 and

A2), the voltage is reduced by the voltage drop due to the armature resistance RA

U(n) = U0 (n) – I L · R A = U 0 (n) · R L/(R A + R L)

Usually the load resistance R L is significantly higher than the armature resistance R A ,

so that the following approximation applies

U(n) ≈U 0 (n) for RL >>RA

usually the rated output voltage of a tachogenerator

is specified with respect to a defined load resistor

Universität Siegen

Tacho (Huebner Berlin)

Universität Siegen

Tacho (Huebner Berlin)silver track

on commutator

good solution !

a brushed tachogenerator in a brushless drive

does that make sense ???

Universität Siegen

Ripple of tachogeneratorsThe tacho generator DC voltage is

superposed with small ripple

voltages upp, the frequency and

amplitude of which depends on

the speed, number of poles

(number of magnetic poles),

number of armature slots and

number of commutator segments.

Incorrect installation of the tacho

generator on the driving machine

can increase the ripple. For this

reason, special attention must be

paid to correct installation

ripple

incorrect mounting

(simplified) basic structure of a drive control

Drehzahl-regelung

Drehmoment-/StromRegelung

Lageregelung

Lagegeber

Kommutierungssignale

controller

torque/current

controller

commutation signals

tacho generator

position encoder

digital control : a single encoder for all feedbacks !?!

controller

torque/current

controller

commutation signals

tacho generator

position encoder

speed signal

position signal

current distribution

encoder

the encoder has to meet the requirements

with respect to all 3 (!) signal types

Requirements for Encoders

replacing 3 different systems

by a single encoder

has serious impact

on the requirements for the single encoders !!!

(because the needs of all feedback loops have to be fulfilled)

• commutation encoder

• tacho generator

• position encoder

Outline

Introduction

Drive Control

State of the Art

Standard Encoders

Conclusions

Genauigkeit

Auflösung

differentielle

Genauigkeit

Zusammen-

J ! ?Zusammen-

teilweise

was ist das ???

Zusammenhang

unklar

Accuracy of Position/Speed Encoders

Position

difference between

the actual (real)

position (angle)

and the position (angle)

measured by the encoder

(e. g. position of lines)

difference between

the actual (real) speed

and the speed

measured by the encoder

(e. g. voltage of tacho

generator)

Position

number of different

positions (angles)

the encoder

is able to distinguish

(e. g. number of lines)

number of different

speeds

the encoder

is able to distinguish

(e. g. minimum voltage

of tacho generator)

Resolution of Position/Speed Encoders

Resolution and Accuracy of Incremental Encoders

(mathematical description)

• resolution

ireal,iref,

xxa• (absolute) accuracy

1-ireal,ireal,

real1-ireal,ireal,

a• (differential) accuracy

only absolute !!!a differential resolution

does not make any sense !

by any calcution it is not possible to make these equations identical

the respective characteristics are physically really different !!!

feedback sensor 1980‘s 1990‘s 2000‘saccuracy resolution accuracy resolution accuracy resolution

position sensorfor position control

medium medium

(10.000)

medium high

(100.000)

speed sensorfor speed control

low high medium high

position sensorfor current control

high low

high medium

(1.000)

very high

?very high

Development of Accuracy and Resolutionduring the recent decades

Requirements forSimultaneous Position and Speed Measurement(for being competitve to an analogue tacho generator)

of course, higher differential accuracy

can be gained by increasing the resolution

this means, however,

an extraordinary high resolution !

Requirements forSimultaneous Position and Speed Measurement(for being competitve to an analogue tacho generator)

position :

speed :

• accuracy of 0,001°

• speed range : 0,01 ... 20.000 rpm

• resolution : 0,001 rpm at speeds < 0,1 rpm

in combination with a controller cycle time of 50 µs

this results in a resolution demand for the encoder of 30 Bit !!

Resolution of position/speed encoders

during the recent decades

resolution

this point describes

the extraordinary

requirements (30 bits resolution)

mentioned before

resolution

Resolution of position/speed encodersduring the recent decades

incremental encoders were

the industrial standard

in the 1980‘s

Outline

Introduction

Drive Control

State of the Art

Standard Encoders

Conclusions

Resolver

Statorue

RotoruR

Statoru2

Statoru1

Tamagawa

injection of a

stationary (sinusoidal)

high frequency signal

sensing of a two-

dimensional

signal response

Resolver

Statorue

RotoruR

Statoru2

Statoru1

injection of a

sensing of a two-

dimensional

signal responseTamagawa

Resolver

Statorue

RotoruR

Statoru2

Statoru1

injection of a

sensing of a two-

dimensional

signal responseTamagawa

resolution

resolvers

are an industrial standard

in low performance

servo drives

Tooth wheel encoder (reluctance resolver)

resolution :

500 000 positions

per revolution

(ca. 19 Bit)

inductive (magnetic) encoderHeidenhain

resolution :

18 Bit(262 144 positions

per revolution)

inductive (magnetic) encoder

Heidenhain

(Optical) Incremental Encoder

photo elements

optical grid

ball bearing

reference

marker

encoder disc

Heidenhain

next slide

Measuring Principle of Optical Incremental Encoders

Incremental encoder (with rectangular signals)

Spur A

Spur B

4-fachAuswertung

Null-Impuls

LogikSpur B

Spur A

Drehrichtung

360°/p

4-fachAuswertung

track A

track B

multiplication

of resolution

marker

4x resolution signal

direction

signal

processing

Encoder Discs (Optical Toothwheels)by nature : better differential accuracy good for speed control

a real incremental encoder

with rectangular output signal

has a characteristic more like right side !

Outline

Introduction

Drive Control

State of the Art

Standard Encoders

Conclusions

Incremental encoder (with sinusoidal signals)

Spur A

Reference mark

Dig itale LogikSpur B

4-fach-Auswertung

Drehrichtung

360°/p

4-fach-Auswertung

Spur B

Spur A

direction

Spur A

Spur B

4-fachAuswertung

Null-Impuls

LogikSpur B

Spur A

Drehrichtung

360°/p

4-fachAuswertung

track A

track B

multiplication

of resolution

marker

4x resolution signalsignal

processingdirection

resolution

high resolution optical encoders

are an industrial standard

in high performance

servo drives

Position Interpolation(Incremental Encoders with Sinusoidal Signals)

360° / line /pulse number (= 1 increment)

Track A

High Resolution Encoders with Sinusoidal Output Signals

in x-y Projection - Real Measurements

ROD 486-5000 RON 251-1000

by nature : better absolute accuracy good for position control

a real high resolution encoder

with sinusoidal output signal

has a characteristic more like left side !!

Accuracy of Resolver and Optical Encoder

resolution

resolvers as well as high resolution optical encoders

lie well below the „cost-line“ accepted by the market

Reasons for Good Control Behaviour

of Servo Drives with Digital Control

Commutation Effects of Brushless Tachogenerators

Effect of a Superposed Position Control Loop

Significance of Differential Accuracy

see explanations above

Outline

Introduction

Drive Control

State of the Art

Standard Encoders

Conclusions

resolution

which attempts have been made

to get further improvements ??

(Actual) Publications

• Wen-Hong Zhu, Tom Lamarche :

Velocity Estimation by Using Position and Acceleration SensorsIEEE-IES Transactions, vol. 54, No. 5, September/October 2007

(the need for a real speed signalleads to an additional acceleration sensor)

• Several years ago there was the proposal to usean additional analogue tacho generator

(with silver commutation track) as speed sensor

the problem still is not solved sufficiently

encoders

optical capacitivemagnetic

incremental

encoder

absolute

encoder

interferometric

encodervariable

electrodes

variable

dielectricum

Resolvers

with high

pole number

pseudo

absolute

encoder homodyne

interferometric

encoder

heterodyne

interferometric

encoderPRC coded

encoder

reflecting

encoder with CCD

resolver tooth wheel

Resolvers

with low

pole number

Encoder Technologies

Possible Encoder Technologies

Tacoder Principle

invented and introduced to the market some 15 years ago

in difference to incremental encoders with sinusoidal outputs,

where the position information

is “coded” in the amplitude ratio of 2 orthogonal sine waves,

the Tacoder uses the phase angle between two digital signals

to code the position information

200 kHz

Tacoder – Basic Idea

optical disc

optical

receiver

reference

oscillator

(standard)

incremental

encoder

phase/

frequency

modulation

1 line

interpolation of

resolution by factor 4

interpolation of

resolution by factor 256

the Tacoder principle was invented and introduced to the market some 15 years ago

in difference to the version with sinusoidal outputs, where the position information is

included in the amplitudes of sine waves, the Tacoder uses the phase angle between

two digital signals to code the position

the improvement of the Tacoder concept can be understood when listening to radio

programs. The quality of the sound is significantly better, when frequency modulation

(FM) or phase modulation is used for transmission instead of amplitude modulation

(AM). Electromagnetic disturbances do impact the phase or frequency of a signal

much less than its amplitude. This effect was used by the Tacoder.

Tacoder

position information encoded in phase between two digital signals

MTTacoder

Tacoder

Bus to the

(Micro-)Controller

Tacoder – Signal Processing by a simple ASIC

provided the following characteristics

the Tacoder

high speed range

• digital output signals (good disturbance ratio)

• high „differential“ accuracy (like incremental encoders)

low speed range

• digital output signals (good disturbance ratio)

• high resolution (like „sinusoidal“ encoders)

resolution

was not really successful in the market of servo drives !!!

the Tacoder

• phase modulated signals cannot be synchronized

with the cycle period of the controller

• market leader for encoder systems has decided

to promote encoder systems with sinusoidal outputs

encoders

incremental

encoder

absolute

encoder

interferometric

encodervariable

electrodes

variable

dielectricum

Resolvers

with high

pole number

pseudo

absolute

encoder homodyne

interferometric

encoder

heterodyne

interferometric

encoderPRC coded

encoder

reflecting

encoder with CCD

Resolvers

with low

pole number

Encoder Technologies

have the capability to provide

high resolution

as well as robustness

Capacitive Encoders

shaped form of electrode(s)

similar design to optical encoders

• drum or disc with shaped electrode

mounted on the shaft

• tube or 2nd disc with shaped electrode

fixed at the stator (housing)

the size of the electrode areas

in direct oppostion changes with motion

shaped form of dielectricum

• drum or disc with shaped electrode

fixed at the stator (housing)

• tube or 2nd disc with non-shaped electrode

also fixed at the stator (housing)

• shaped dielectric tube or disc

mounted on the shaft

the dielectric charactersitic

between the electrodes changes with motion

detection of varying capacitance by a high frequency signal

Universität Siegen

Prof. Dr. -Ing. R. M. Kennel,

Technische Universität München

Capacitive position sensor

Quelle : SICK/Stegmann

neu am Markt : ein kapazitiver Geber

2-Plate

Capacitive Encoder

(shaped form of electrode)

3-Plate

Capacitive Encoder

(shaped form of dielectricum)

Capacitive Encoder Topologies

Multi-Electrode

Transmitter

Rotor(shaped form of dielectricum)

Holistic

Receiver

Components of Capacitive Sensing Element

Feinspur (16

Signalperioden/Umdr.)

Grobspur (3 Signalperioden/Umdr.)

leitende

Empfänger

Fläche

Dielektrischer

Excitation

GeneratorSensor

Charge

Amplifier

Synchronous

Demodulator

Low Pass

FilterDriver

RotorTx PCB Rx PCB

Cosine

Signal Electronics of Capacitive Encoder(1 channel only)

Position Detection: Sine/Cosine, Coarse/Fine Track

Coarse

Sine & Cosine Signals

of Fine Track

(16 Periods per Revolution)

Sine & Cosine Signals

of Coarse Track

(3 Periods per Revolution)

Multi-Electrode

Transmitter PCB

Dielectric

Prinzip: Sinus/Cosinus, Grob-/Feinspur

Experimental

Encoder under Test

Precision Index Table

Mechanical Jig for

Controlled Manipulation

of Axial as well as Radial

Displacements

(EUT vs. Shaft)

Large Hollow Shaft Encoder

Sensor Electronics

Ø ~ 300mm

Experimental Results –

Large Hollow Shaft Encoder

Position Angular Error

angular position [°]

r [“

Absolute Accuracy(non-holistic design !)

230”

Meßbereich

measuring area

Auflösung

resolution

Robustheit

robustness

Massenprodukti

mass production

magnetisch

magnetic

Resolver :

am Umfang/

circularZahnrad/tooth wheel :

punktförmig/point

niedrig

low(multipole Resolver:

20 Bit)

schlecht

optisch

optical punktförmig

(CCD: > 30 Bit)

problematisch(temperatur- und

stoßempfindlich)

problematic(sensitive to shock

and temperature)

sehr gut(fotografische

Herstellung)

very good(photographic

production)

kapazitive

capacitive

am Umfang

circular

Zu wenig Erfahrung, um konkrete Aussagen zu machen.

Too less experience to make knowledge-based statements.

Meßbereich

measuring area

Auflösung

resolution

Robustheit

robustness

Massenproduktion

mass production

magnetisch

magnetic

optisch

optical

kapazitive

capacitive

encoders

incremental

encoder

absolute

encoder

interferometric

encodervariable

electrodes

variable

dielectricum

Resolvers

with high

pole number

pseudo

absolute

encoder homodyne

interferometric

encoder

heterodyne

interferometric

encoderPRC coded

encoder

reflecting

encoder with CCD

Resolvers

with low

pole number

have the capability to provide

ultra high resolution

why are interferometric encoders promising ?

• actual encoder concepts physically measure position

and then gain speed by mathematical derivation

• in interferometric encoders two laser beams

– one as reference, the other reflected by the moving object –

are compared with respect to their phase shift

• interferometric encoders

provide the capability to physically measure speed

• first attempts have been made (see next slides)

and basically confirm technical expectations

but concepts are not yet mature to be used in industry

encoders

incremental

encoder

absolute

encoder

interferometric

encodervariable

electrodes

variable

dielectricum

Resolvers

with high

pole number

pseudo

absolute

encoder homodyne

interferometric

encoder

heterodyne

interferometric

encoderPRC coded

encoder

reflecting

encoder with CCD

Resolvers

with low

pole number

Homodyne Interferometric Encoder

some years ago a concept was developped and tested

– technical description in :

P. Drabarek, W. Meister, R. Kennel, M. van Keulen,Offenlegungsschrift DE 196 37 615 A1,Deutsches Patentamt München

– measurements (see next slide) showedtechnical capabilities to be very convincing

– cost estimation/projection was a serious problem

Interferometric Sensor for Very High Resolution

with measuring grid

Comparison of Signal Noise (Position Accuracy)

between a Homodyne Interferometric Encoder

and a High Resolution Optical Encoder

Interferometric

Encoder

High Resolution

Incremental

Encoder

resolution

homodyne interferometric encoders provide

significant improvement in resolution -

estimated cost (based on realistic piece number) to high

encoders

incremental

encoder

absolute

encoder

interferometric

encodervariable

electrodes

variable

dielectricum

Resolvers

with high

pole number

pseudo

absolute

encoder homodyne

interferometric

encoder

heterodyne

interferometric

encoderPRC coded

encoder

reflecting

encoder with CCD

Resolvers

with low

pole number

laser diode

acousto-optical

mode converters

beam splitter

photo diodes

optical lenses

motion

measured object

beam splitter

Integrated (i. e. on-chip) Interferometric Sensora concept idea

integrated opto-electononic chip

laser diode

any non-ideal

metallic surface

Low Cost (Heterodyne) Interferometric Sensoranother concept idea

tolerance band of incremental encoder error

position in degrees cost !!

overall tolerances

not really better

Comparison of Angle (Position) Accuracy

between a Heterodyne Interferometric Encoder

and a High Resolution Optical Encoder

what might be

the advantage ??

resolution

heterodyne interferometric encoders provide

low resolution (slightly better than resolvers) -

resolution

heterodyne interferometric encoders provide

low resolution (slightly better than resolvers) -

estimated cost lower than high resolution optical encoders

Outline

Introduction

Drive Control

State of the Art

Standard Encoders

Conclusions

Conclusions accuracy is decisive for position control

resolution (or better : differential accuracy) is decisive for speed control

in cascaded control structures speed control

must be much faster than position control

for encoders in digitally controlled drives

resolution is much more important than accuracy

actual encoders measure position and gain speed by derivation

encoders measuring directly physical speed would be advantageous

speed/position encoders still are the technical „narrow gap“ in a drive

without solving that problem „new“ control schemes do not make sense

with respect to a future increase of requirements

investigations should be done

Questions ?

Technische Universität München Vorlesung „Elektrische ......Technische Universität München...

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