Servo Drives

7/18/2019 Servo Drives

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Fundamentals of Motion Control:

Motion control consists in moving a load in a given period of time with a very high

level of accuracy.

There are dierent characteristics linked to motion.

The rst is Movement, which can e dened y the following points:

!nitial position

Final position or target position

"peed and acceleration

Type of tra#ectory

Then there is $erformance, for which the following points are important

"tatic and dynamic performance

%ccuracy

"peed

"taility

There is also $osition, which can e

%solute

&r 'elative

%ll usiness sectors are concerned y these applications, which may e simple or

relatively complicated.

(e can draw two a)es: the * a)is representing $erformance +i.e. position accuracy,

speed, acceleration and torue- and the a)is representing the comple)ity of the

movement..



"peed drives are used for simple motion applications like $umps, Fans, /oisting

devices, Compressors, etc.

They can e ualied as simple as the only signal controlled is speed.

%t the other end of the scale there is 0igital control. This is for more comple)

applications such as roots, control systems and very high performance machines.

0igital control is used for roots and machine tools for e)ample. !n this kind of

application, the comple)ity is proportional to the numer of drives to e

coordinated. For e)ample, a root with 1 a)es reuires 1 drives that need to e

controlled simultaneously.



Motion control is somewhere etween the two.

!t is used for general applications with single and multiple a)es reuiring accurate

and uick response time positioning.

For e)ample, motion control applications cover Materiel handling, Component

assemly, $alleti2ers, /andling cranes, $ackaging machines, etc.

3 signals have to e controlled in motion control: position and speed.

The following presentations focus on the eld of Motion Control.

/ere you can see the main components of a Motion Control "ystem:

The Motion Controller

The 0rive

The Motor



The $ositioning Feedack "ystem

The Mechanical 4oad

The /uman Machine !nterface

The Motion Controller co5ordinates movements and calculates the target positionand speed.

The 0rive supplies and converts electrical energy for the motor and ensures

regulation.

The Motor converts the electrical power into mechanical power.

The positioning feedack system supplies the drive with the position and speed of

the motor.

The Mechanical 4oad is in charge of performing a given task.

The human machine interface makes it possile to monitor and set up the system.

!t is an optional component, which may e used to enter data into the application,

feed ack information +aout speed, faults, etc.- and input set points.

This interface is generally installed y an automation engineer.

"ome e)amples of machines with position control are Flying shears, $ackaging

applications, Material handling systems, 'otating lades, Machine tools, etc.

/ere is a rst e)ample of *6 positioning: the transtocker. The three ), y and 2movements are coordinated to move and position tus in the right place in the

warehouse. /ere the parameter to e controlled is position.



!n this application, the o#ective is to cut a moving strip of paper into eual sections,

however fast the strip of paper is moving. /ere, the key part of the application lies

in the synchroni2ation of the two a)es.

% third e)ample of a machine tool is a 4aser cutting machine. The three ), y and 2

movements are coordinated to cut out comple) parts. This application reuires

calculation of the tra#ectory.

This last e)ample shows a handling machine that can wind and unwind products.

The machine must ensure regular winding in order to guarantee product uality. !n

this application, it is the torue and speed that need to e controlled.



/ow does a system ehave in relation to what is reuired of it +the green curve-7

Two opposing responses are possile.

8 % very fast response ut with an overrun and oscillations +in purple-

8 % slow response with no overrun +in orange-

9ach application has its own response covering its response time, overrun and

accuracy.

ood movement control provides the necessary ad#ustment ;e)iility to otain an

optimal response +in lack-.

4et<s now descrie the key components of a motion system, starting with the Motion

Controller.

The motion controller has two roles: it supervises and controls.

!ts asic function is to manage the control =seuence of the application> and to

send information to the drive.

For e)ample, this involves going to a given position, at a given speed, and then

coming ack.



!t can control one or more drives and therefore supervise and synchroni2e the

dierent movements.

The motion controller can e a $4C, a $C, a dedicated calculator, etc.

There are some systems where the motion controller +the seuence- is included in

the drive and others where positioning is performed y the controller.

The controller coordinates the seuence of single5a)is or multi5a)is movements

programmed y the operator. !t denes the path of movement, or tra#ectory, y

looking at the position in relation to time and generating the speed set point.

For e)ample:

For Movement ?: the %)is moves from the @ position to the ? position with a A?

speed set point

For Movement 3: the %)is moves ack to its previous @ position with a A3 speed

set point.



4inear a)es are a)es with limited movement. The limits are given y limit switches

or y a mechanical end stop.

!n the case of a simple linear a)is, the motion controller initiali2es movement and

then ensures travel from point % to point B

%)is motion etween a and is limited y a lower and higher limit.

!nnite a)es are a)es that are asically rotating so the general notion of limited

movement does not apply.

The drive has the #o of converting electrical energy from the electrical network

+power supply- for the motor.

!t controls voltage and freuency and regulates dierent values: torue, speed andposition.

The drive operates according to three regulation loops in a cascade formation:



The Torue loop provides the motor with the energy reuired to drive the system ata certain speed.

The speed loop provides the torue loop with the torue setpoint to reach the right

speed. !t answers the uestion: =/ow much torue is needed to get where ! want to

go at the right speed7>

The position loop provides the speed loop with the speed prole needed to reach

the position setpoint.

Motor

Because it is mechanically linked to the load, the motor acts as a converter,

converting electrical energy into mechanical energy.

There are ve main categories of motor:

"ynchronous or %C rushless

0C rushless

%synchronous

"tepping

!C4%

"ynchronous or %C rushless



There are a numer of advantages and drawacks to each type of motor.

The advantages of the synchronous motor are that it is compact and has a low level

of inertia ecause of the permanent magnets in the rotor. !t is therefore highly

dynamic and is ale to very uickly reach high speeds. !ts drawacks are that it has

a comple) ordering system, there are no e)isting standards, it is costly and cannote controlled without a sensor.

"ynchronous motor is called also %C Brushless motor

0rives for %C Brushless motors use "inusoidal Commutation device.

0C rushless

0C rushless motors have also permanent magnets in the rotor

The associated drives use a trape2oidal commutation device.

%s the drive for 0C rushless motors does not generated a very complicated signal,a low resolution encoder will suce to relay the rotor position to the drive in order

to commutate itself

The drive for 0C rushless motors does not produce a torue ripple as low as

sinusoidal one, ut the circuits are not as comple), so it is cheaper.

%synchronous



The advantages of the asynchronous motor are that it is roust, it is more

economical and has standardi2ed )ing mechanisms and ;anges.

!ts drawacks are its si2e , lack of dynamism due to the weight of the rotor +inertia-,

the control of the torue is not easy +slipping-.

"tepping

The stepping motor has the advantage of eing easy to control. !ts drawack is its

lack of power.

!C4%



The !C4% solution comprises a Brushless 0C motor with integrated electronics

!t oers a numer of advantages.

!t is ;e)ile thanks to a signicantly smaller amount of wiring.

!t is compact and therefore oers a /igh level of 9lectromagnetic Compatiility

+9MC- thanks to the integrated wiring

!t is modular, which means less time spent deugging, and the product range is

much wider.

4ess space is taken up in the control cainet and the demand on the cooling system

is smaller.

"afety is ensured y integrated =$ower 'emoval> technology, which does away with

the need for e)ternal contactors

!t is also advantageous in terms of price: e)pansion planning costs are lower, for

oth machines that are manufactured manually or automatically.

!ts drawacks are as follows:

8 !t needs more space than a single motor

8 "hould a failure occur, the whole component has to e changed.

8 (et area applications are restricted y the degree of protection.

The feedack system

The feedack system is a key element in the chain. !t continually provides the drive

and sometimes the controller with information aout the position of the motor rotor

or the mechanical parts.

$ositioning accuracy is strongly linked to the uality of the sensor.



!f there is no sensor, the system is said to e in an open loop, with position and

speed eing estimated ased on a calculation. !n this case, the positioning is much

less precise.

The ma#ority of motion control and digital control applications reuire feedack.

The position sensor is generally )ed to the motor a)is. !t provides the motor shaftposition.

There are D types of position sensor:

% ='esolver> is a system that sends ack analog information aout the position of

the rotor.

%n !ncremental encoder is an optical system that sends ack digital information

aout the relative position of the rotor

%n %solute encoder is a "ystem that sends ack oth digital and analog

information aout the asolute position of the rotor.

The common feedack parameters are:

'esolution, accuracy and repeataility

The asic types of sensors are introduced in the Motor E Feedack seuence.

Conclusion



(e shall conclude the rst part of this seuence with two uestions: The rst is

what is servo control7

This is the management of the dierent regulation loops in the system to reach the

target as uickly as possile with a ma)imum level of accuracy.

The second is what is a regulator 7

The regulation loop compares the feedack with the setpoint and controls the drive

in order to eliminate the error.

The correction method is most often ased on the $roportional and !ntegral +$!-.

"pecic lters can also e applied for the correction.

'egulation will e looked at more closely in the following seuences.

AC SERVO MOTOR DRIVES

The %C servomotor elongs to the $ermanent Magnet "ynchronous Motor family.



!t is made up of a stator and a rotor.

The stator has three symmetrical phase windings like an asynchronous motor. They

are internally connected in a star formation. The common point is not availale.

The rotor has permanent magnets.

There is an air gap etween the stator and the rotor.

The magnetic eld is produced in this air gap thanks to the permanent magnets.

(e shall now e)plain how ma)imum torue is otained with an %C servomotor.

The torue is proportional to the rotor eld ) stator eld ) phase shift etween

them.

The torue<s mathematical e)pression is shown: M G? . H/rH . H/sH . sin I

The torue, G?, is constant. !t depends on the motor.

The rotor eld, /r, is created y the magnets and is constant too.

The stator eld, /s, is proportional to the current in the stator windings.

The phase shift etween the rotor and the stator fields, I, increases with the torue

on the shaft.



4et us use the complete formula: M G? . H/rH . H/sH . sin I

!t is important to rememer that:

The rotor eld /r is constant +ecause it comes from the permanent magnets-.

The angle theta is maintained eual to J@K, therefore: sin theta ?.

The stator eld /s is proportional to the current in the windings.

"o, the formula to e)press the torue is simplied as M G ! where ! is the motor

current.

The J@L angle etween the stator and rotor elds ensures ma)imum torue.

"ervo 0rive

To make sure ma)imum torue is achieved, the servodrive needs to know the

position of the rotor.

This is why a position sensor is used.



The purpose of the position sensor is to allow the servodrive to maintain the stator

eld perpendicular to the rotor eld.

!t does this y controlling the phase of the motor current according to the rotor

position.

The drive can then calculate the speed and, if necessary, the a)is position.

The motor position sensor is generally a resolver or a similar type of sensor.

!n some cases, the resolver can e used as a position sensor for the application.

%C "9'A& M&T&' C&M$&9MT"

This picture shows the main components of an %C servomotor: connectors, optional

holding rake, position sensor, magnets, windings, ;ange and housing.

%C servomotor eciency is very good.

The power loss on the rotor is very low while the power loss on the stator can e

easily dissipated into the amient air.



The intrinsic performance of an %C servomotor is mainly linked to the properties of

the permanent magnet material.

!n the past, the magnets were made of steel. Nnfortunately, these had a very low

energy level and were often demagneti2ed.

Today, in order to manufacture permanent magnets, steel has een replaced with

other materials such as eodymium !ron5Boron, which is the second generation of

rare earth magnets.

%C servomotors often come with a holding rake to e supplied from the outside.

(hen the rake is not supplied with energy, the rotor is locked.

/olding rakes are designed as standstill rakes and are not suitale for repeated

operational raking.

Torue speed curve

The %C servomotor torue5speed curves are important to si2e the motor.

&nce the %C servomotor has een chosen, the level of torue mainly depends on

the associated servodrive, ecause it controls the motor current. The ma)imum

voltage delivered y the servodrive sets the peak torue according to the speed.



To si2e the %C servomotor, it is important to take into account the application<s

cycle to see whether it will work in the continuous operating area or in the

intermittent operating area.

"i2ing will e studied in greater detail in a future module.

Mechanical gear o) or $lanetary 'educers:

The gearo) and, in particular, the planetary reducers.

To have a stale system, it is often necessary to reduce the inertia of the application

re;ected to the motor shaft.

For this, mechanical gearo)es or reducers are mounted on the motor. These are

often planetary reducers.

The speed of the motor is divided at the reducer output, so the torue is higher than

that on the motor shaft.

$lanetary reducers are designed for the specic needs of %C servomotors.

They come in several si2es and several reduction ratios.

For a given planetary reducer model, the speed ratio can range from D:? to O@:?



B reducer:

9C&09' T9C/&4&!9" N"90 F&' $&"!T!& F990B%CG

There are three types of technology availale: the resolver, the incremental encoder

and the asolute encoder.

i- 'esolver% resolver is a very asic device with no electronic or optical

components. !t has no frame and is tted directly on to the

motor shaft. !t is a good solution for general purpose

applications.

!f the resolver has two poles, it gives an asolute position of the

angle within only one revolution.!t is asically a rotating transformer fed with a carrier freuency

9 +for instance P A at Q G/2-.% primary winding is located in the rotor and there are two

secondary windings in the stator.

The two secondary windings of the stator give signals ased on the carrier

freuency modulated y the angle of the rotor.% and B signals are demodulated inside the drive to otain "ine and Cosine

signals.



The position is then calculated using these two voltages.

The servodrive uses the position information to calculate the speed. The position feedack can also e used y the position loop inside the servodrive or

y the application<s e)ternal controller.

The mechanical angle position of the resolver must e set up when it is mounted orreplaced.

ii- !ncremental encoder%n incremental encoder provides a relative position./ere<s how it works:

The disc of an incremental encoder has 3 types of track.!t has one or several outside tracks +channels % and B-, comprising a

known numer of eual angular steps that are alternately opaue and

transparent. The known numer of eual angular steps is the resolution.!t also has an inside track comprising a single window, which is used as

the set point and triggers re5initialisation with each revolution.

&peration of the photosensitive components +i.e. the 490s R

photosensitive diodes- is ased on a real5time dierential optical reading:5 the photosensitive components of tracks of % and B are oset so that

each will simultaneously read only its respective slot +channels % and B

are electrically oset y J@K-,5 the electronic components operate according to the principle of real5time

dierential measurement. The rising edge of Channel B occurs efore or after the rising edge of

Channel % depending on which way the motor rotates.



This is how the servodrive is ale to determine the direction of motor

rotation.

iii- asolute encoder%n asolute encoder has a high numer of discs, which dene the

asolute position for a single revolution or for several revolutions.

4et<s now take a look at how an asolute encoder works.

The disc of an asolute encoder comprises several concentric tracks,eually divided into alternate opaue and transparent segments.

The inside track is half opaue and half transparent and delivers the Most

"ignicant Bit +M"B- of the code ased on the encoder position.

The outside track corresponds to the 4east "ignicant Bit +4"B- and

provides the nal accuracy. !t has a numer of points corresponding to the

encoder<s resolution.



9ach track has its own transmitter and receiver. The multiturn asolute

encoder, in addition to providing the digital position within the revolution,

also provides the total numer of revolutions.

For each angular position of the shaft, the disc provides a code. Following

one complete revolution of the encoder, the same coded values are

repeated. The code can either e inary or gray.

Binary coding The inary code can e used directly y processing systems

+programmale controllers for e)ample- in order to e)ecute calculations or

comparisons, ut it has the drawack of having several its which change

state etween 3 positions.



ray coding The ray code oers the advantage of changing only one it etween 3

consecutive numers.Nsing an asolute encoder to detect position oers a numer of

advantages%n asolute encoder continuously provides a code that is an image of the

actual position of the moving part eing monitored.&n power5up, or restart following a supply failure, the encoder provides

data that is directly e)ploitale y the processing system.

Motion control loops



The "ervo 0rive has three possile operating modes. (e shall focus on these during

this session:

5 The Torue Control Mode

5 The Aelocity Control Mode

5 %nd the $osition Control Mode

4et<s start with the Torue Control Mode. This mode can e used in a

windingSunwinding machine to ad#ust and regulate the we tension. !f the torue set

point increases, the force in the we increases as illustrated here.

The torue setpoint is a current that is proportional to the motor torue .

&nly the current loop is activated.

The current regulator parameters depend on the electrical parameters of the motor.

The servo drive already knows the motor parameters.

!n most cases, the current regulator is automatically set and so the user does not

need to make any ad#ustments.

ow let<s move on to the Aelocity Control Mode.

This mode can e used in machines where very high speeds are reuired

The speed setpoint is a reference to e followed y the motor.

Both the current and speed loops are activated.

The speed regulator is a $! regulator +proportional plus integral actions-.

!n practice, it is tuned or ad#usted to ensure the control system ehaves in the right

manner. !n other words, to ensure:

staility,

steady5state accuracy

transient response

freuency response.



The $! regulator ensures the motor<s speed is stale and accurate.

!t uses two input data:

8 The speed reference and the speed feedack.

!t also uses two setting parameters:

5 The proportional gain or =Gp> : used to optimise the response time.

5 %nd the !ntegral gain or =Gi> ?STi: used to stailise the system and improve

accuracy.

Ti is the integral time in milliseconds.

These two gains are used to correct, cancel or reduce errors when the feedack is

not eual to the reference.

4et<s start with the proportional action.

The proportional action is ad#ustale thanks to the Gp gain, which has no units.

4et<s have a look at how the system responds when we act on the Gp gain.



The system<s response is divided into two states: a transient state and a steady

state.

!n the transient period:

*ou can see that the higher the Gp, the shorter the system<s response time. Gp

makes it possile to increase the system<s speed of response. !f Gp increases too

much, the system ecomes unstale.

!n the steady5state period:

*ou can see from the curves that the higher Gp is, the smaller the speed error. The

speed error is never cancelled when a proportional gain only is applied.

There will always e a speed error in oth the transient and steady5state periods.

This prolem can e solved y applying an integral gain.

4et us now focus on the !ntegral action. !t is ased on the mathematical operator

=integral related to time>.

The !ntegral action is ad#usted according to the integral time +Ti-.



The system<s response time is in;uenced y the gain settings ut also y the

reference speed.

4et<s have a look at the curve:

?. /ow does the integral action setting in;uence the system<s response7

*ou can see that the shorter Ti is, the smaller the integral action.

The !ntegral action tends to cancel the static error.

3. /ow does the speed reference in;uence the system<s response7

0epending on how fast the speed reference is, the integral gain is not constant.

0uring the transient period, the integral gain drops.

!n steady5state conditions, the integral gain is innite. "o, there is no speed error in

steady5state conditions.

!t<s time to look at $roportional5!ntegral action.

The overall gain results from the comined proportional and integral gain.

0uring the transient period, it is the proportional gain that ensures system staility

and rapidity.

!n steady5state conditions, it is the integral gain that ensures system staility andaccuracy.

This is a lock diagram showing the velocity regulator.

The acceleration and deceleration ramps reduce speed reference dynamics.

This also helps to limit overshooting and improve staility y increasing the front

rise time of the speed reference.



The desired speed response is when the set5point is reached with acceptale:

5 rise5time

5 overshoot

5 settling time

5 accuracy +static error-

The est possile response is ased on a compromise etween these four values.

The rise time is the time taken to rise from ?@ to J@ of the response to a step

input.

The peak overshoot is the ma)imum amount y which the output e)ceeds the input

in response to a step, e)pressed as a proportion of the step height.



The settling time is the time taken to settle to within P of the new steady output

after a step applied to the input.

The accuracy +or static error- is the dierence etween the output and the input in

steady5state conditions.

'emark: The well5tuned servo system should not have any static error for a step

change in the speed reference or in the load torue.

To implement the position loop, the current loop and speed loop must rstly e

correctly tuned.

!t is essential to tune the regulation loops to the servo motor and its load. Tuning

sets the gains of each loop. !t can e done automatically or manually.

Many digital %C servo drives also have an auto5tune mode that estimates the value

of load inertia and initially sets the tuning parameters to reasonale values for user

specied targets such as low, medium or high response.

The auto5tune values usually provide a stale system that is often sucient for the

application or at least serves as a starting point for ne5tuning y the user.



Tuning has a direct eect on the system<s andwidth.

The andwidth is the aility of a speed drive, comined with a motor and its load, to

follow a uickly varying setpoint: in our case, this is the speed reference.

This diagram shows the andwidth characteristic: the freuency value read when

the gain is 5DdB.

Furthermore, the andwidth increases in accordance with the proportional gain +Gp-

of the speed loop.

enerally, the andwidth of the speed loop must e at least D times the andwidth

of the position loop. This prevents the position loop from overshooting.

Nnfortunately, real loads connected to the servo system rarely ehave like perfectlyinert loads. 'eal loads can e su#ect to friction, damping, compliance, acklash,

variale inertia and other possile non5linearities.

This is why tuning the %C "ervo "ystem gives the est results when the actual load

is linked to the %C "ervo "ystem.

&lder control systems reuired a tachometer on the motor to provide speed

feedack as well as a resolver or encoder for position feedack.

!n modern systems with powerful microprocessors, it is possile to have only one

feedack device for position and speed.

To calculate the speed, the position is derived.



Motor si2ing with respect to the servo drive and the motion prole

!f the motor and drive are incorrectly si2ed for the desired motion prole no amount

of tuning will yield the desired results.

9arthing and cale shielding connections

reat care must e taken when following the earthing and shielding procedures in

the servo drive product installation manual.

!f there is e)cessive system noise, the system must e detuned +low andwidth- so

that it is not e)cited y high freuency noise +cause of permanent instailities-.

The application<s mechanical parts

9nsure that there is minimum ;e)iility in the mechanical system and that

couplings are tight. !f the system is not well5designed from a mechanical point of

view, this will produce resonance reuiring the system to e detuned.

4et us now turn to the position control mode.

The position setpoint denes the position to e reached or followed y the

application. !t is a target position or tra#ectory.

This is the most widely used mode and it applies to all applications with a)is

positioning.



!n position control mode, the D loops are activated.

The regulation loops are organised in a cascade formation.

The position loop delivers the speed setpoint to the speed loop that, in turn, delivers

the current reference to the current loop.

The motor current generates the necessary torue to reach the desired speed.

$osition control applications fall into 3 asic categories:

5 $oint5to5point

5 Contouring

$oint5to5point applications are not usually concerned with path control ut with

move time, setting time and the velocity prole.

!n general, contouring applications follow a path. They reuire the actual position to

follow the command position in a very predictale manner and a high level of

mechanical rigidity to re#ect the eect of any load torue disturances.



Tuning of motion system

/ello and welcome to this seuence on =Tuning a Motion "ystem>.

This module e)plains why and how a motion system is tuned so that it ehaves in

the right way, i.e. so that it operates in a stale manner.

% motion system<s instaility is characterised y oscillations that can e oserved

and corrected.

The tuning operation is necessary to ad#ust the speed loop. This ensures that the

motion system operates in a fast, stale and accurate manner.

!t is essential for the current loop to e properly set efore tuning the speed loop.

The commonly used reference signal for tuning the speed loop is a suare wave.

Freuencies can e e)cited in the motion system. !f there is a staility prolem, this

is usually shown y a suare wave.

This method is ased on a temporal analysis.

Temporal analysis using a suare wave reference is the main method used to tune

motion systems ecause it covers a very large spectrum of freuencies ranging

from low to high freuencies.



% motion system must work properly with the entire application. !t must take into

account the application<s needs and minimi2e the conseuences of interference.

%n application<s needs are generally:

'esponsiveness

"taility

%nd 'igidity

'esponsiveness descries the aility of a system to react to a reference change.

% system<s responsiveness is usually measured using the settling time.

/igh gains make systems more responsive.

"taility descries the margin according to which a servosystem does not fall intoself5sustained oscillation.

% system must have a reasonale margin of staility.

"taility is commonly measured y overshoot and settling time in response to a

suare wave.

/igh gains often make systems unstale.



'igidity descries the aility of a servo system to overcome load torue

disturance.

The way the system reacts in terms of speed makes it possile to estimate system

rigidity.

/igh servo gains make systems more rigid.

Mechanical resonance can e provoked y the oscillation freuency of the speed.

This happens when the speed oscillation freuency matches the resonance

freuency of the mechanical parts.

/igh gains increase the risk of amplifying the resonance freuency.

oise sensitivity descries the e)tent to which a servo system amplies a source of

noise.

Common noise sources are feedack resolution and 9lectromagnetic !nterference

+9M!- received through the command.

oise sources generate heat, viration, torue disturance and acoustic noise.

/igh gains increase noise sensitivity.

oisy systems reuire some kind of compromise.



/ere is a general method to e followed y users when tuning the speed loop:

Nse step commands such as a suare waveform and do not saturate the current

loop

?- "et the proportional action:

Cancel !ntegral action and apply a low $roportional gain

!ncrease the proportional gain in steps of aout 3@ to otain a suare response.

Monitor staility

(hen you see overshoot, reduce the gain.

3- "et the integral action with the previous proportional gain:

"tart with a high integral time to ensure staility.

'educe the integral time in steps of aout 3@

Monitor staility

"top when the overshoot is aout ?@

&nce you have followed these steps your speed loop will e tunedU

Many digital %C servo drives have an auto5tune mode that estimates the value of

load inertia and initially sets the tuning parameters to reasonale values for user5

specied targets such as low, medium or high response time. The auto5tune values

usually provide a stale system that is often sucient for the application or at least

serves as a starting point for ne5tuning y the user.

!f the auto5tuning step does not provide satisfactory results, the user can then

optimi2e tuning himself.

The rst %uto5Tuning step involves the entire application including the mechanical

parts.

0on<t forget that the current loop must e set rst.



The current loop regulator parameters are automatically ad#usted according to the

following motor characteristics: !nductance 4 and resistance '.

This is how auto5tuning works:

Firstly, the current is measured at constant speed in order for the servodrive tocalculate the load torue + M.load-.

e)t, the servo drive applies a constant current so that the motor provides the load

with a constant torue +M-.

The acceleration torue is the dierence etween the motor torue +M- and the load

torue +M load-.

This results in a linear increase in speed +w-.

The servo drive then calculates the inertia +V- of the system.

!n the second step the user optimi2es the speed loop of the the servo drive.

%s seen previously, the temporal analysis is very helpful ecause it shows up any

prolems immediately.

To perform this operation, a software tool is used with the servo drive.



The software also includes an oscilloscope function that provides the results of

tuning y displaying curves.

9ach time the user changes one parameter, a gain for instance, he can see the

result immediately.

Before starting a measurement:

Check that the current loop is set properly +this usually involves checking that the

right motor has een selected-.

Check that the speed set point used is compatile and not dangerous for the

application: waveform, level, polarity, freuency and moving distance.

The user should proceed as follows:

First, tune the speed loop proportional gain.

"econd, tune the speed loop integral time.

Finally, tune the position loop gain.

For each gain:

5 "tart with a low gain to ensure staility.

5 and increase this gain until it causes a prolem such as instaility.

%fter measuring the parameters:

Check that the motor current does not reach the servo drive current limitation.

Geep in mind that it is acceptale for the current loop to e saturated for a few

milliseconds.



The upper limit of the proportional gain depends on the system<s components, i.e.:

5 The drive,

5 The feedack,

5 The drive may have certain limits with respect to:

The sampling time

The lters used

The slow current loops

and sensitivity to noised the mechanical parts.

The same applies to the feedack system:

5 'esolver5to5digital conversion is like a lter

5 %nd low resolution encoders generate noise.

The temporal analysis reveals any prolems relating to the system<s response.

/owever, if there are several causes of instaility, it is dicult to locate each one.

This is the case, for instance, when several resonance freuencies are e)cited in the

motion system, as shown in the picture.



To perform a more in5depth investigation, freuency analyses must e carried out

using the freuency analysis with a software tool.

The aim of freuency analysis is to oserve the motion system<s response in

relation to the freuency of the speed set point signal.

% sine signal is sent as a speed set point and is compared to the sine feedack.

The amplitudes of each sine signal and the phase shift =φ> etween them are

measured.

These measurements, from low freuencies up to high freuencies, are made step

y step y the software tool.

&nce the step5y5step freuency measurements are nished +starting with low

freuencies and moving up to high freuencies-, the software tool displays the ode

diagrams.

% ode diagram shows, depending on the freuency, the evolution of:

8 the magnitude of the system in deciels: the feedack compared with the

reference

8 and the phase etween the feedack and the reference in degrees.

The system<s ehaviour is shown clearly and, at this stage, it is important to check

the staility criteria.



(ith a closed speed loop conguration:

The staility criteria depend mainly on a safety margin, also called the phase

margin φm+phi5m-, at a specic value of magnitude: 5DdB +minus D 0eciels-,

φ m must e located etween 5?Q@K and @K.

To achieve this, the speed loop must rst e ad#usted with the proportional gain.

The freuency at 5D dB is the andwidth of the system.

% 5?Q@K value corresponds to a completely unstale system.

The most typical e)ample of which is an oscillatorU

(ith an open speed loop conguration:

The phase margin at @ dB must also e checked. /owever, it is physically

impossile to open the loop in a real application.

To overcome this, the software tool generally calculates the results for an open loop

conguration too.

!n practice, a phase margin of at least OPK has to e kept.



This is necessary ecause the system must have good staility with an open loop

conguration, B9F&'9 the loop is closed. &therwise, it will e impossile to ensure

staility with a closed loop congurationU

Tuning a motion system

Temporal analysis using a suare wave reference is the main method used to tune

motion systems ecause it covers a very large spectrum of freuencies rangingfrom low to high freuencies.

% motion system must work properly with the entire application. !t must take into

account the application<s needs and minimi2e the conseuences of interference.

%n application<s needs are generally:

'esponsiveness

"taility

%nd 'igidity

!nterference usually comes from:

Mechanical resonance

%nd sensitivity to noise

'esponsiveness descries the aility of a system to react to a reference change.



% system<s responsiveness is usually measured using the settling time.

/igh gains make systems more responsive.

"taility descries the margin according to which a servosystem does not fall into

self5sustained oscillation.

% system must have a reasonale margin of staility.

"taility is commonly measured y overshoot and settling time in response to a

suare wave.

/igh gains often make systems unstale.

'igidity descries the aility of a servo system to overcome load torue

disturance.

The way the system reacts in terms of speed makes it possile to estimate system

rigidity.

/igh servo gains make systems more rigid.



Mechanical resonance can e provoked y the oscillation freuency of the speed.

This happens when the speed oscillation freuency matches the resonance

freuency of the mechanical parts.

/igh gains increase the risk of amplifying the resonance freuency.

oise sensitivity descries the e)tent to which a servo system amplies a source of

noise.

Common noise sources are feedack resolution and 9lectromagnetic !nterference

+9M!- received through the command.

oise sources generate heat, viration, torue disturance and acoustic noise.

/igh gains increase noise sensitivity.

oisy systems reuire some kind of compromise.



/ere is a general method to e followed y users when tuning the speed loop:

Nse step commands such as a suare waveform and do not saturate the current

loop.

?- "et the proportional action:

Cancel !ntegral action and apply a low $roportional gain

!ncrease the proportional gain in steps of aout 3@ to otain a suare response.

Monitor staility

(hen you see overshoot, reduce the gain.

3- "et the integral action with the previous proportional gain:

"tart with a high integral time to ensure staility.

'educe the integral time in steps of aout 3@

Monitor staility

"top when the overshoot is aout ?@

&nce you have followed these steps your speed loop will e tunedU

Many digital %C servo drives have an auto5tune mode that estimates the value of

load inertia and initially sets the tuning parameters to reasonale values for user5

specied targets such as low, medium or high response time. The auto5tune values

usually provide a stale system that is often sucient for the application or at leastserves as a starting point for ne5tuning y the user.

!f the auto5tuning step does not provide satisfactory results, the user can then

optimi2e tuning himself.

This is how auto5tuning works:

Firstly, the current is measured at constant speed in order for the servodrive to

calculate the load torue + M.load-.

e)t, the servo drive applies a constant current so that the motor provides the load

with a constant torue +M-.

The acceleration torue is the dierence etween the motor torue +M- and the load

torue +M load-.

This results in a linear increase in speed +w-.

The servo drive then calculates the inertia +V- of the system.



!n the second step the user optimi2es the speed loop of the the servo drive.

%s seen previously, the temporal analysis is very helpful ecause it shows up any

prolems immediately.

To perform this operation, a software tool is used with the servo drive.

The software tool makes it possile to:

5 generate the speed set point signal,

5 and optimi2e the speed loop.

The software also includes an oscilloscope function that provides the results of

tuning y displaying curves.

9ach time the user changes one parameter, a gain for instance, he can see the

result immediately.

Before starting a measurement:

Check that the current loop is set properly +this usually involves checking that the

right motor has een selected-.

Check that the speed set point used is compatile and not dangerous for the

application: waveform, level, polarity, freuency and moving distance.

The user should proceed as follows:



First, tune the speed loop proportional gain.

"econd, tune the speed loop integral time.

Finally, tune the position loop gain.

For each gain:

5 "tart with a low gain to ensure staility.

5 and increase this gain until it causes a prolem such as instaility.

%fter measuring the parameters:

Check that the motor current does not reach the servo drive current limitation.

Geep in mind that it is acceptale for the current loop to e saturated for a few

milliseconds.

The upper limit of the proportional gain depends on the system<s components, i.e.:

5 The drive,

5 The feedack,

5 %nd the mechanical part

The temporal analysis reveals any prolems relating to the system<s response.

/owever, if there are several causes of instaility, it is dicult to locate each one.

This is the case, for instance, when several resonance freuencies are e)cited in the

motion system, as shown in the picture.

To perform a more in5depth investigation, freuency analyses must e carried out

using the freuency analysis with a software tool.



The aim of freuency analysis is to oserve the motion system<s response in

relation to the freuency of the speed set point signal.

% sine signal is sent as a speed set point and is compared to the sine feedack.

The amplitudes of each sine signal and the phase shift =φ> etween them are

measured.

These measurements, from low freuencies up to high freuencies, are made step

y step y the software tool.

&nce the step5y5step freuency measurements are nished +starting with low

freuencies and moving up to high freuencies-, the software tool displays the ode

diagrams.

% ode diagram shows, depending on the freuency, the evolution of:

8 the magnitude of the system in deciels: the feedack compared with the

reference

8 and the phase etween the feedack and the reference in degrees.

The system<s ehaviour is shown clearly and, at this stage, it is important to check

the staility criteria.



(ith a closed speed loop conguration:

The staility criteria depend mainly on a safety margin, also called the phase

margin φm+phi5m-, at a specic value of magnitude: 5DdB +minus D 0eciels-,

φ m must e located etween 5?Q@K and @K.

To achieve this, the speed loop must rst e ad#usted with the proportional gain.

The freuency at 5D dB is the andwidth of the system.

% 5?Q@K value corresponds to a completely unstale system.

The most typical e)ample of which is an oscillatorU

(ith an open speed loop conguration:

The phase margin at @ dB must also e checked. /owever, it is physically

impossile to open the loop in a real application.

To overcome this, the software tool generally calculates the results for an open loop

conguration too.

!n practice, a phase margin of at least OPK has to e kept.



This is necessary ecause the system must have good staility with an open loop

conguration, B9F&'9 the loop is closed. &therwise, it will e impossile to ensure

staility with a closed loop congurationU

/ere is an e)ample of typical ode diagram plotting in two cases:

8 for a no5load system

8 and for an on5load system.

"ystem staility must rst e studied with an open loop conguration.

4et us rst look at the result for a motor with no load:

The rst magnitude spike at around 3 k/2 is caused y shaft resonance, i.e. torsion

oscillation etween the motor shaft and the feedack system.

The phase drops through the critical ?Q@K line.

This means that the loop magnitude at this freuency must e less than @ dB,

otherwise the system oscillates.

"econdly, let<s add an e)ternal load which degrades oth the magnitude and phase

characteristics.

The overall magnitude is reduced due to the higher level of inertia



The amplitude of the second spike depends on the compliance or rigidity of the

coupling etween the motor and the load.

% springy coupling will produce a large spike.

"o, it is necessary to reduce the magnitude to prevent oscillation, resulting in

poorer system stiness and slower response.

This is why it is important to use a rigid coupling etween the motor and the load.

4et us now consider some ad#ustments with a closed loop conguration and oserve

how tuning with a $! regulator inside the servo drive cancels oscillation and

overshoot.

?- The proportional gain Gp is too high and there are permanent speed oscillations.

The ode diagram shows two peaks corresponding to these oscillations.

3- Gp is reduced to prevent instaility. There is still an overshoot on speed.

The ode diagram shows that the andwidth is now lower.

The overall magnitude is lower too.



D- The integral time Ti is increased to have less magnitude at lower freuencies.

This can e seen on the ode diagram.

%s the speed loop gain is reduced at low and medium freuencies, there is no moreovershoot shown on the time curves.

%cceleration and deceleration ramps prevent higher system freuencies from eing

provoked and thus help to prevent instaility. (hen the servo drive is connected to

an e)ternal position controller, these ramps are not used. The e)ternal controller

manages them.



This low pass lter is ad#usted y the user.

!t is helpful when there is noise on the speed set point.

Geep in mind that the low pass lter will introduce additional phase shift in the

system.

This lter is used when the resonance freuency of the motor and load is amplied.

The freuency of this lter can e programmed to cancel or minimise gain at the

resonance freuency.

This current control lter is programmed y the user and can e very useful when

the speed loop gain has to e high causing a certain amount of instaility. 0o not

lower the freuency of this lter more than necessary as this will introduce

additional phase shift into the speed loop and hence reduce staility.



Positioning Tra!ectory "oming

/ello and welcome to the =$ositioning W Tra#ectory W /oming> session.

This module e)plains how a tra#ectory can e generated to drive a movement andensure good positioning. (e will descrie the main types of dierent movements,

from simple to comple).

/ere is a summary of the dierent points we shall e looking out during this

session:

Movements

$ositioning5tra#ectory

/oming.

4et<s start with movements according to a cam prole.

Mechanical cams are used to ensure that a movement is implemented according to

a prole tra#ectory ased on the =cam prole>.

The prole tra#ectory is determined in relation to the position of an a)is.

The master is the a)is.

The slave is the ruler.

The ruler follows the prole tra#ectory.



Mechanical cams have a certain numer of drawacks owing to:

5 Their in;e)ile system

5 %nd wear on mechanical components.

Today, most tra#ectories are driven y programmale logic controllers or $4C<s.

4et<s have a look at this e)ample:

'egarding the master:

The cam is replaced y the $4C.

'egarding the slave:

The ruler is replaced y the servodrive R the motor R the load

&ne controller can manage several movements.

This is a multi5a)is conguration.

ow let<s see the asic architecture of a positioning motion system.

$role tra#ectories can e congured and stored in the controller y the user.

The controller generates the setpoint according to the prole tra#ectory.



The servodrive controls the speed of the motor. The tra#ectory of the load follows

the prole tra#ectory.

The position of the load is sent ack to the controller for the position loop.

The servodrive controls the movement with its position loop.

The controller manages the seuential mode of the machine +runSstop, safety

features, faults, etc.-.

The tra#ectories can e either inside the servodrive or the 4e)ium Controller.

!n this architecture, the controller is master, calculates the tra#ectories and

transmits the setpoints to the servodrives.



!nstead of a controller, it is possile to use a $C with a control card inside it. This

oers the same functions as those of the controller.

There are two types of motion application:

8 The single5a)is motion application

and the multi5a)is motion application

4et<s start with the single a)is movement.

(ith single a)is movements, the tra#ectory is dened y the speed prole and the

position target.

The a)is accelerates from position @ to reach the steady speed "?.

Close to the position target ?, the a)is decelerates to stop at ?.

!n the multi5a)is family, there are dierent types of motion.

*ou can nd movement with interpolation, without interpolation and contouring.

!n multi5a)is motion with interpolation, the movements are:

5 either linear,



5 or circular.

4inear interpolation is reuired for multi5a)is movement from one point to another

following a straight line.

The controller determines the speeds on each a)is so that the movements are

coordinated.

True linear interpolation necessitates the aility to modify acceleration.

"everal controllers use pseudo linear interpolation ased on pre5calculated

acceleration proles.



Circular interpolation is the aility to move a load around a circular tra#ectory. !t

means that the controller has to change the acceleration in real time and in the

most continuous manner possile.

!n multi5a)is motion without interpolation, you can have synchroni2ed or

simultaneous movements.

!n the case of synchroni2ed movements, the a)es can move together or not.

/ere, the a)es do not move together

/ere, the a)es move together.

The dierence etween synchroni2ed movement and simultaneous movement is

that simultaneous movement is unsynchroni2ed.

!n this case, the a)es can also move together or not.



(ith contouring, the controller changes the speeds on the dierent a)es so that the

tra#ectories pass smoothly via pre5determined points.

!n contouring, the speed is dened along the tra#ectory and can e constant, e)cept

during starting and stopping.

/ome is a reference point that denes the position 2ero.

/ome is indicated y closing or opening a switch.

By opening a switch, it is possile to detect wire reakage.

The controller seeks the switch after power5on in a homing routine.

The actual 2ero reference point is typically oset from the switch so that the switch

is not constantly activated.

The position of a switch is not accurate and can vary from machine to machine

resulting in inconsistent home reference positions.

!t is often not desirale for the 2ero set point to e physically located on the home

switch. %s motion often occurs around the 2ero position, the home switch would e

su#ect to wear and tear.



% home oset can e used to dene the 2ero set point point at a point oset from

the actual home switch.

The homing process can e summari2ed as follows:

The home command is activated and the controller initiates motion in the

programmed direction at programmed speeds.

The programmed directions are 4!M +negative direction limit- or 4!M$ +positive

direction limit-.

(hen the home switch is detected, the controller continues to move at a slow speed

to feedack null.

/oming y feed5to5positive5stop reuires the machine to move slowly in the

programmed direction until a physical hard stop is detected. ormally the current

that the sevodrive can deliver to the motor is limited to protect the machine.

Feed5to5positive5stop is clearly a slow process and is typically done #ust once during

commissioning of the machine.

!t is typical for the motor to e euipped with an asolute feedack device that cantrack position over multiple revolutions even after power is removed from the

system. &nce the 2ero coordinate position is dened, it is not necessary to re5home

the machine after each power5on.

/ome switches generate additional cost and reduce system reliaility.



/oming is also a waste of time as the system must e homed after every power

cycle.

Clearly it is advisale to dene limits eyond which the machine cannot go.

There are two kinds of end5of5travel limits: hardware and software.

/ardware limits are physical switches +4!M and 4!M$-, which typically initiate an

9mergency "top routine.

"oftware limit switches are only active once the 2ero position has een dened.

% distance from the 2ero position is programmed in each direction of motion eyond

which the machine should not travel

!t is preferale for the controller not to pass through a software limit efore

decelerating, ut actually to anticipate the limit and decelerate to a stop on the

limit.

%t this point further motion in that direction would e inhiited.



Motion Si#ing

0uring this session, we shall look at the following points:

5 Motor and drive si2ing,

5 Mechanical transmissions,

5 %nd raking resistor si2ing.

/ere, we can see the model of a motion control system.

The denition of a servo motor starts with the correct si2ing. This includes:

5 dening the application : its mechanical parts and reuired performance and

functional features,

5 and choosing the servo motor and servo drive.



&nly once si2ing has een carried out can the regulation loops e set and the servo

system integrated in the automatic control system of the installation.

To descrie the asic approach, let<s have a look at how the servo motor controls

the load directly.

!n this simple e)ample, the motor is directly coupled to the load, symoli2ed y a

friction torue and inertia.

The torue and speed are therefore the same for the load and the motor.

The load<s reuired speed prole is shown on this diagram.

The rst thing to do is determine the torue reuired on the motor shaft for the load

to follow the speed prole.



!t is then possile to determine some very important information aout the motor

shaft:

5 The peak torue

5 %nd the root mean suare, or r.m.s., torue, also called the euivalent thermal

torue.

The cycle time, ti, is the time in seconds.

%nd a cycle time eual to t?Rt3 R tD R tO

The following information can also e determined:

5 The peak speed 5 %nd the average speed +"avg-

e)t, it is necessary to check that these operating points are compatile with the

torueSspeed curves of the motor connected to the servo drive.

This is ecause the torue5speed characteristics also depend on the servo drive.



&nce the servo motor has een chosen, a servo drive that is compatile with this

motor is selected from the list provided in the catalogue. %t this stage, the main

servodrive selection parameters are as follows:

5 The supply network,

5 The continuous torue,

5 %nd the peak torue.

ow let<s have a look at how to dene motor si2e.

? 5 The average speed +"avg- and the thermal torue +Me- must e in the area

delimited y curve ?

3 5 Check that the peak torue and speed do not e)ceed the area delimited y curve

3.

!t is important to keep in mind that the physical si2e and cost of a motor mainly

depend on the motor<s rating torue.

(e shall now turn our attention to mec$anical transmissions.

Most applications are more comple) than #ust a direct link etween the motor and

the load.



Conventional transmission systems usually comprise the following:

8 Couplings,

8 Belt pulleys,

8 ears,

8 Conveyors,

8 4eadscrews

8 &r racks and pinions.

% reducing gear such as a transmission system ased on elt pulleys or a gearo) is

used to:

5 'educe re;ected inertia,

5 !ncrease torue and decrease speed,

5 Nse smaller motors or optimi2e the way they are used,

5 'educe load disturance.

There are dierent types of reducing gears, notaly for parallel mechanical links

+elt pulleys- aligned with the motor, as shown in the diagram.

!t must e rememered that the total inertia, torues and speeds must e

considered with respect to the motor shaft.

To calculate the torue, we need to know the inertia of all of the mechanical

components re;ected to the motor shaft. This provides results that can e used to

dene the si2e of the servo motor.

For a high5performance system, the general rule is to dene a si2e that ensures that

the inertia re;ected to the motor shaft is eual to that of the motor.



%s well as the inertia of the application itself, the inertia of the other components

such as the couplings, elt pulleys, and gearo)es, must also e added in the case

shown.

Total inertia reaching up to ?@ times that of the motor is suitale as long as the

mechanical transmission is rigid enough.

4et us now have a look at how the dierent types of mechanical transmission

in;uence speed, torue and inertia.

!n the case of a gearo):

'atio: ' radius r3 S radius r?

The load speed motor speed S '

The motor torue load torue S '

The inertia re;ected to the motor shaft total inertia S '



!n the case of a leadscrew:

The pitch represents the numer of turns per millimeter and the lead the numer of

millimeters per revolution.

4oad velocity is inversely proportional to the pitch, i.e. G?Sp

4oad torue seen y the motor is inversely proportional to the pitch, i.e. G3Sp

4oad inertia re;ected on the motor shaft is inversely proportional to the suare of

the pitch, i.e. GDSpX

For elt drives and rack5and5pinions:

The key parameter is the radius r of the pulley or pinion.

The load velocity is proportional to the radius, Ga.r

(ithout any change to linear speed, the load torue seen y the motor is

proportional to the radius, G.r

The load inertia seen y the motor is proportional to the suare of the radius, Gc

rX



"oftware such as 4e)ium "i2er is availale to help the user.

!t is very useful.

?. To select the mechanical systems used and associated characteristics.

3. To dene the prole of the speed, torue and tra#ectory.

D. To display the torueSspeed curves.



O. To select the optimum servo drive with the servo motor.

%ra&ing resistor

To conclude this overview of si2ing methodology, let<s see when a raking resistor is

used and how this resistor should e dened.

0uring raking or deceleration, kinetic energy is asored y the drive thanks to 0C

capacitors. The mechanical energy +?S3 V wX- is converted in electrical energy +?S3

CNX- across the diodes of the power ridge. This energy is stored y the capacitors

and induces an increase of the 0C us voltage according to this formula: where

5 9 generated is the mechanical energy

5 C, the capacitors value

5 and +N3X5N?X-, the dierence of 0C voltage suare etween the eginning and theend of the deceleration.

!f the kinetic energy is too high, the limit value of the 0C voltage is reached and an

e)ternal raking resistor dissipates the e)ceeded energy.



The technical characteristics of a raking resistor are as follows

Aalue in &hms

The minimum acceptale ohm value depends on the si2e of the drive.

$eak power

This depends on the ohm value and the 0C us voltage on the drive.

Continuous power

The duty cycle of the application has an in;uence on this.

To dene the raking resistor, we need to know:

5 the total inertia of the system,

5 the speed prole of the application.

'esistor calculations can e performed according to the following guideline. This is

ased on a single a)is, i.e. one servo drive with a servo motor that drives an

application.

The calculation consists of si) steps that we are going to develop right now.

!n "tep ?:$lot the speed torue curves versus time for the entire motion cycle.



!n "tep 3:

!dentify each generator mode section from the plot, where the drive is decelerating

the load, or where the speed and torue have opposite signs.

!n "tep D:

Calculate the energy of the deceleration in each generator mode section. 9

generated ?S3 V +ω3X5 ω?X-



!n "tep O:

Calculate the energy dissipated y the raking resistor.

For this, sutract the servo drive energy asorption capaility from the energy of

the deceleration.

9 dissipated 9 generated 5 9 asored y capacitors

!n "tep P:

Calculate the $ma) power of each deceleration y dividing the dissipated energy y

the deceleration time.

$ ma) 9 dissipated S t deceleration.

"tep 1:

Calculate the continuous power dissipated y the raking resistor.

$ continuous +9?dissipated R 93dissipated RY- S M cycle

For step 1, refer to the servo drive characteristics tale if you have to use an

e)ternal raking resistor.



Applications of ser'o dri'es

0uring this session, we shall look at the following e)amples of applications:

5 rouping and ungrouping,

5 and a rotary knife.

Servo Drives

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Transcript of Servo Drives