steady does it

8/3/2019 Steady Does It

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2 MONTH YEAR MOTION SYSTEM DESIGN www.motionsystemdesign.com

oaz Kramer

ontrol and application development

CS Motion Control Inc.

ymouth, Minn.

n this installment:

Passive and active isolation issues

Isolation system behaviorChallenges

Higher velocities and ac-celerations are usually re-quired to increase machine

hroughput, but orces are otenmited by the voltage and currentstraints o a systems motors and

rives. That said, even i a motorn produce enough power to de-

ver aster operation, an aggressiveotion prole does not necessarilyield higher throughput. In many

cases, practical velocity and accel-eration are well below maximumachievable values. Why?

Prole duration, the commandedmove time, is only hal o the equa-tion. The other part is settling time.Settling time is determined by dy-namic eects and can signicantlyincrease when a system makes veryaggressive moves. An example is

shown inFigure 1 .The problem is even more severe

when the settling widow is tightenedand higher resolution is required.

To illustrate, the resolution osemiconductor instruments is ap-proaching (and in some cases, goingbelow) 1 nm. Instruments with thisresolution are sensitive to even thesmallest vibration or disturbance particularly vibrations excited by themotion prole itsel.

The most common vibrationmitigation approach is to modiy the

motion prole by means o trial anderror, to keep the entire move andsettle duration as short as possible.Oten, however, this method is insu-

3MONTH YEAR MOTION SYSTEM DESIGN www.motionsystemdesign.com

cient. Lets explore more eectivesolutions to address the problem.

Passive isolationPassive isolation systems are typ-

ically used to isolate systems romdisturbances transmitted rom thefoor. They employ a seismic masssupported on a sot spring made oair, metal, or rubber. The springsdamping action absorbs vibrationsabove the springs resonance. Forthis reason, passive isolation manu-

acturers usually try to lower springresonant requency to increase theeective isolationrange. The prob-lem is that this

makes most passiveisolation systems

very sot andsotness is detri-mental on ast-moving systemssuch as stages.

When a servo

orce is applied to a load to gener-ate motion, it also acts on the isola t-

ourseAudit

Consider the plight o motion system designers in the semiconductor indus-

try, who ace constant demand or higher perormance to produce more

semiconductors, aster, and at a lower cost. This makes or signifcant chal-

lenges. To illustrate, one aspect o the semiconductor manuacturing process,

inspection, usually involves positioning silicon waers relative to optical (or oth-

er) components by placing the waers on a ast-moving XY stage. How can this

delicate operation be sped up with accuracy? Part o the answer, which we

cover here, is in mitigation o vibration. Whether you work in the semiconduc-

tor industry or another feld, controls-based damping could help your design

move more nimbly.

0 0.5 1

Time (sec)

Aggressive

profile

Less aggressive

profile movesmore slowly.

Velocity

(mm/sec)

Fig. 1a

0 0.5 1 1.5 2 2.5 3

Time (sec)

More aggressive profileactually takes longer to settle.

Amplitude

(m)

20

20

15

10

5

0

-5

-10

-15

-20

-25

Move 1

Move 2

Move and settle 1

Move and settle 2

Fig. 1b

0.5 1 1. 5 2

60

40

20

0

-20

-40

-60

-80

-100

-120

Time (sec)

Fig. 2

Vibration deteriorates settling time.

Error taking vibration into account

Servo position error with isolation

system vibration excluded.

Settling time is de-termined by dynamic

eects and it increases

when a system movesaggressively. Let aretwo velocity proflesor the same moving

distance. The graph atright shows their servo

position error.

Though the blue pro-fle has signifcantlyshorter duration, it

generates a higher po-sition error and results

in a longer settlingtime. Total move andsettle time is much

shorter with the lessaggressive profle.

When orce is applied to a load to generate motion, it alsoacts on the isolated stationary base, causing it to vibrate.Low requency and light damping contribute to isolation

system vibration, even long ater motion has ceased. Thisvibration acts as a disturbance to the servo system, intro-

duces position error, and prolongs settling time.

Vibration isolation systems are useul,but introduce issues o their own. Inthis series, well review those challenges and some control-based solutions.

Part 1 o 2 Two key factors

Extended ringing

Steadydoesit


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Circle XXX


d stationary base, causing it to vi-rate. Because the requency is lowusually below 1 Hz, to 10 Hz) andamping is very light, the isolationystem continues vibrating long a-er the motion prole has ended.eeFigure 2 on the previous page.This vibration acts as disturbanc e

o the servo system, introduces er-

or, and extends the settling time reventing inspection instruments,o use our example application,rom taking measurements.

Active dampingVibration problems are much less

evere with active isolation systems.

These, like passive systems, isolatesystems rom disturbances transmit-ted to the system rom the foor. Ac-tive systems detect vibration levels

with sensors that send signals into aeedback or eedorward controller,

which in turn sends signals to actua-tors to counteract the orces.

In other words,

within the eedbackloop, a sensor mea-sures vibrations a-ecting the isolatedbase and then theactuator reacts toreduce the levelo vibration. So,

the active isolation system not onlyabsorbs energy entering rom thefoor, but also eectively absorbs

vibrations generated by the movingstage.

In addition, typical active systemsare inherently stier than air tables.

Their resonant requency dependson the tuning o their servo, but itis typically higher (greater than 10Hz) than that o the passive systems and actively isolated systems aremuch better damped. As a result, vi-bration is suppressed more quickly

and eectively.Here is the catch: Most activevibration isolation systems are rela-tively complex and costly. They arealso more dicult to install andtheir support electronics oten re-quire adjustment.

Command feedforwardA simpler and less expensive ap-

proach or active damping is thecommand eedorward method. Itis most useul where a known orceis applied to an isolated base, and asignal proportional to that orce isavailable.

More sophisticated motion con-trollers are necessary with this meth-od, as the applied orce depends onthe stage acceleration determinedby the motion controller.


Heres how it works: An analogsignal (proportional to the com-manded acceleration in a certain di-rection) is sent by the motion con-troller to an actuator, which in turnproduces orce equal in magnitudebut opposite in direction. Typically,additional analog signals (indicatingthe commanded position) are alsosent. They account or the eectso coupling twist: An isolated basetwists clockwise i there is X ac-celeration when the stage is in ull-Y position, and counterclockwise

when the stage is in ull +Y posi-tion.

Besides being more cost eectivethan sensor-based isolation systems,command eedorward cannot be-

come unstable, and signicantly im-proves settling perormance. Onecaveat: Command eedorward alsorequires setup and proper tuning othe eedorward gains.

Modeling isolation systemvibration

Active isolation systems, even when well tuned, cannot eliminate vibration entirely and to reiter-ate, semiconductor inspection ma-chines can use very high-resolutioneedback devices and are extremelysensitive to even the slightest dis-

turbances, which can extend settlingtime to nanometer or sub-nanome-ter settling windows.

Lets explore another option. Con-sider isolation-system vibration othe model shown in Figure 4, whichdepicts a direct-drive system on a

M

MB

K

xL

xB

F

xE

d

K

d

Fig.4

Control

laws-

+

Command

Machine

Feedback

Drive

Acceloremeter

K.ML

Filter

Fig.5

CourseAudit

The eect o the isolation system vibration can bedescribed using a basic model. A direct drive system

is mounted on a base with limited mass and stiness.Force is applied on the load at the stationary base.

Base acceleration is measured ineach direction and a compensa-tion orce M

Lx a is applied to the

drive command, such that result-ing position error is signifcantlyreduced (M

L- moving load mass).

This does not prevent isolationsystem vibration, but minimizes

relative movement between thebase and load.

0 0.5 1 1.5 2 2.5 3Time (sec)

Position error

Amplitude

(m)

20

20

15

10

5

0

-5

-10

-15

-20

-25

In this example, the settling

window is one count.

Settling time

Velocity profile

Fig. 3

Settling time can last longerthan move time.

Move time

Move time is the duration o the motion profle. It is dominated bycommanded motion parameters such as velocity and acceleration.Settling time is the duration o time rom when the profle ends un-til the system reaches and stays within a certain target window. Thesum o move plus settle times is what determines a systems agility.That is why or applications with point-to-point moves, machinethroughput is aected and expressed in terms o move and settle.

Move time vs. settling time

Frame acceleration compensation control scheme

Isolation system model


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Circle XXXCircle XXX

ase with limited mass and stiness.The eedback reading is relative

the base:

The orce is applied both on thead at the stationary base.The transer unction describing

he relation between the applied

orce and eedback reading is:The part in the parentheses rep-

resents isolation system dynamics,with parameters:M

L= Total mass o load

MB = Isolated stationary base massF= Applied orceX

L= Load displacement relative to

groundX

B= Base displacement

K= Stiness o isolation systemd= Damping o isolation system

Two possible solutions exist: Increased disturbance rejection

o the servo system Special motion profles that do

not excite disturbing vibrationsOne method to improve the servo

disturbance rejection isframe acceler-ation compensation. The perormance

o a stage with passive isolation canbe upgraded by using two acceler-ometers in XY directions. The ac-celeration o the base is measured ineach direction, and a compensationorce (M

L a) is applied to the drive

command such that the resultingposition error is signicantly re-duced as shown inFigure 5.

This method does not preventisolation-system vibration, but doesminimize the relative move betweenbases and loads benecial becauseinspection instruments are installedon bases.

Tune in for the second part of thistwo-part series next month, when wellreview frequency domain behavior,disturbance-rejection algorithms, andanother approach using input shaping.

Well also study a new class of con-trols, executed as special functions onmotion controllers, that minimizes theeffect of isolation-system vibrations on

settling time. For more information, email

[email protected] or visitacsmotioncontrol.com.

Xencoder

F

=1

MLs2+

1

MBs2 + ds +K

=1

MLs2

MB+ M

L( )s2 + ds+K

MBs2 + ds+ K

Xencoder

= XL X

B

100 101 102 103

Frequency (Hz)

Magnitude

(dB)

Phase

(degrees)

60

40

20

0

-20

-40

-60

0

-90

-180

-270

-360

Fig.6


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oaz Kramer

ontrol and application development

CS Motion Control Inc.

ymouth, Minn.

n this installment:

Isolation resonance andme-domain response

Vibration mitigationunctions

Disturbance-rejectionlgorithms, input shaping

In the rst part o this series (see

August 2008 issue) we reviewedhow although higher velocities

nd accelerations are usually re-uired to increase machine through-ut, even i a motor can producenough power to deliver aster op-ation, an aggressive motion pro-e does not necessarily yield higher

hroughput.Prole duration, the commanded

move time, is only hal o the equa-tion. The other part is settling time.Settling time is determined by dy-namic eects and can signicantlyincrease when a system makes veryaggressive moves. The most com-mon vibration mitigation approachis to modiy the motion prole bymeans o trial and error, to keep the

entire move and settle duration asshort as possible. Oten, however,this method is insucient. Lets ex-plore some more suitable and eec-tive solutions to address the prob-lem.

Frequency domain behaviorIsolation system dynamic mod-

els show that a second-order pole(resonance) and a second-orderzero (anti-resonance) are added.

Their requencies are R

and A

re-spectively.

Typically, MB

>> ML,

so thetwo

requencies are very close. Thatsaid,

A>

R.

Figure 6(see above, acing page)

shows the open-loop Bode plot oa system controlled by a PIV lter.

The blue plot is o a system withoutisolation system dynamics; the redplot shows the eect o isolation sys-tem dynamics. Notice the addition-al resonance and anti-resonance.

A phase lead is introduced in thephase: The eect in the requencydomain is not signicant, and yet,the eect on time-domain peror-mance can be crucial.

Frame acceleration compensationis successully implemented withcertain equipped motion controllers


on systems with passive isolation.Conside a system with 0.0625-mresolution and a 1-m settling win-dow. The isolation system in this

case does not use air, and its reso-nance is relatively high, at greaterthan 10 Hz.

SeeFigure 7below. As with active

isolation systems, rame accelerationcompensation is less eective whenthe required settling window is aew nanometers or less.

CourseAudit

Vibration isolation systems are useful, but introduceissues of their own. In this series conclusion, we reviewthose challenges and some control-based solutions.

SteadydoesitPart 2 of 2

fR=

1

2K M

B

fA =

1

2K M

B +M

L( )

100 101 102 103

Frequency (Hz)

Magnitude

(dB)

Phase

(degrees)

60

40

20

0

-20

-40

-600

-90

-180

-270

-360

Fig.6

A Bode plot o a system controlled bya PIV lter shows the system withoutthe isolation system dynamics (blue)and under the infuence o isolationsystem dynamics (red.)

Effect of isolation system dynamics

Frame acceleration compensation

Velocity prole (yellow), position error (green), and in-position bit (red) plots beore and ater implemen-tation show settling time reduced almost to zero. A: Perormance beore rame acceleration compensationis implemented shows that base vibration signicantly infuences settling. B: Perormance ater rame ac-celeration compensation shows that oscillation in position error is almost completely eliminated.


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Circle XXX4 MONTH YEAR MOTION SYSTEM DESIGN www.motionsystemdesign.com

Again, it is impossible to com-ensate or vibration entirely andttling time is always aected. Sys-ms may also be sensitive to noisesulting rom accelerometers, so

ppropriate ltration should be ap-

plied. Increasing servo bandwidthcan help, especially on air-table sys-tems where resonance requency isrelatively low in other words, wellinside the bandwidth o the servo.

In many cases, a standard PID

or PIV control algorithm is insu-cient, and additional high-orderlters (and special nonlinear algo-rithms) urther enhance disturbancerejection o the servo.

The example inFigure 8 depictssettling perormance in a system

where the target move is a ew mil-limeters and the settling window isonly 1 nm. Here, a controller withan onboard SIN-COS multiplierenhances output rom a 2-m reso-lution encoder, to give one example,or a nal resolution o 0.49 nm.

Improved settling time

Another example is in Figure 9, which shows the perormance oa large gantry stage used or fat-panel display inspection. For thisparticular setup, total moving stagemass is about 500 kg, and it mustbe moved at velocity o 500 mm/sec with acceleration o 0.5 g, andthen settle into 1-m window. Ba-sic encoder resolution is 40 m andis multiplied by an on-board SIN-COS multiplier or a nal resolu-tion o 4.9 nm. The stage is mount-

ettling in a 1-nm window

n black is the position error when using a standard PIV algorithm.n red is the position error rom a system driven by an ACS SPiiPlus-eries motion controller using a special algorithm with enhanced

disturbance rejection.

s a velocity prole (yellow), position error (blue) and an in-position bit (green) with and without a disturbanceion algorithm. A: Move and settle perormance o a large gantry stage with standard PIV control is less nimble and

e than that in B o move and settle perormance o a large gantry stage using a disturbance rejection algorithm.

isturbance rejection works

ourseAudit


6/66 MONTH YEAR MOTION SYSTEM DESIGN www.motionsystemdesign.com

Circle XXX


ed on a massive granite base with vibration requency o2.5 Hz. The gure showsthe resulting velocity pro-le, position error, and the

in-position bit without and with adisturbance-rejection algorithm the latter o which shows signicantimprovement.

Input shapingAnother eective approach is to

use special motion proles designedto avoid excitation o vibrations. In-put shaping, a technique developedby Convolve Inc., Armonk, N.Y.,is a method or generating mo-tion proles that minimizes vibra-

tions excitation in moving systems.Sotware calculates sequences oimpulses that do not excite reso-nant system modes. The impulsesequence is then convolved withtarget command signals. How isthis eective? Because the impulsesequence causes no vibration, theconvolution product also causes novibration.

Input shaping is a eedorwardtechnique that requires no eedbackand has no aect on servo stability.In addition, it can handle multiplemodes o vibration so it can beused to eliminate not only isolationsystem vibration, but other distur-bances as well, such as vibrationscaused by system cables. It can alsohelp eliminate vibrations unobserv-able by eedback transducers, butstill aecting points o interest.

The disadvantage o using input

shaping is that or very low requen-cies, it can signicantly increasethe prole time by at least hala cycle o the vibration. Preventingthe isolation system rom vibratingis key to a great design, but only iit does not severely degrade movetime. Otherwise, isolation system vibration prevention is suitablewherever its eect on position erroris minimized as much as possible.

For more information, email [email protected] or visit acsmo-

tioncontrol.com.

Time (msec)

Position(mm)

A1 response

A2 response

Total response is quickly smoothed.Fig. 10

Input shaping principleWith input shaping, i a rst impulse starts a vibration (bluegraph), a second impulse (red graph) can cancel it. The totalresponse has only hal a cycle o vibrations. Timing andmagnitude o the second impulse need to be accurate, so capablecontrollers must be used with this method. Additional pulsesmay be added to improve robustness.

steady does it

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