torque-dtrends may 2008

7/24/2019 Torque-DTrends May 2008

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WWW.MOTION-DESIGNS.COM (805)504-6177 PAGE 1

A quar ter ly publ ication brought to you by Mot ion Designs Inc. May 2008

In this issue of Design Trends:

Technology: brushless motor commutation ..............................................page 1

New Product: Technosoft IDM680.............................................................page 7

Product Feature: CTHMI named resources...............................................page 8

Application Solution: XYZ positioning with Arcus 4EX-SA.......................page 11

Brushless Motor Commutation

Not unlike current control in motor drivesand amplifiers, brushless motorcommutation is one of thosemysterious topics that can cause quitea bit of grieve when one has to tinker

with it. Hopefully, this article will removesome of the magic behind motorcommutation and will turn the tinkering abit more into deliberate engineering.

As discussed in the previous edition ofDesign Trends, torque generation inelectric motors is achieved throughmagnetic field interaction. Assuming thecurrent control portion itself has beenaddressed, the second key to optimal

torque control is field orientation. Inorder to produce optimal torque (per

Ampere of current), the 2 magneticfields that interact to produce torqueshould be at a 90 degree angle. Moreprecisely, torque is proportional to thesine of the angle (sin ) between the 2fields. That is why 90 degrees produces

optimal torque and 0 degrees producesno torque at all. It is interesting to notethat this relationship is relatively robust.For example, if the torque angle were offby 10 degrees (i.e. at 80 degrees),

torque loss is only 1.5% (heat losseswould increase a mere 3%). If thetorque angle where off by 30 degrees(i.e. at 60 degrees), torque loss is only13% (on the other hand heat losseswould increase by almost 34%).

Formally, commutation is the process ofmaintaining this optimal 90 degreetorque angle. Before we move on tobrushless motor commutation, let us

quickly discuss stepper and brushedtype motors:

Stepper motors are (typically)NOT commutated, i.e. there is notorque angle control. The rotorspins synchronous with the statorand the torque angle depends onthe load (also see Stepper vs.


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Brushless in November 2007edition of Design Trends).DC brushed motors commutationis done via the mechanicalcommutators and brushes. This

ensures that the magnetic field ofthe rotating armature is alwaysperpendicular to the permanentmagnet field of the stator.

Brushless Motors (again)

A brushless motor consists of a 3-phasestator winding (connected in either Wyeor Delta) and a permanent magnet rotor:

They are typically constructed as 2, 4, 6or 8-pole (higher pole counts arepossible). The pole count determinesthe number of electrical cycles (E.C.)per mechanical revolution. A 2-polemotor has 1 electrical cycle perrevolution; a 4-pole motor has 2, and soon. Linear brushless motors are alsopossible, and the same torque angleconcept applies (although this would

actually be a force angle, but thatterminology is never used). Oneelectrical cycle would have acorresponding linear length (pitch).

The equivalent electric circuit for a 3-phase motor (in Wye) is as follows:

Typically all 3 phases are symmetrical(Ra= Rb= Rc; La= Lb= Lc). The back-EMF voltages Ea,b,c (induced voltages bythe permanent magnet on the rotor) aresinusoidal and 120 degree phase shifted(electrical degrees). The amplitude is

proportional to the motor speed.

By (correctly) driving a set of 3-phasesinusoidal currents Ia, Ib, Ic through thestator windings the amplitude andposition of the stator current vector canbe accurately controlled. The objectiveof commutation is to maintain the angleof this field with respect to the rotor(permanent) magnetic field. Thisrequires knowledge of the rotor

magnetic field position. The type ofposition information available creates 2classes of commutation:

Trapezoidal (a.k.a. 6-step, DCbrushless)Sinusoidal (a.k.a. AC brushless)

Sinusoidal Commutation

Sinusoidal commutation is only possiblewhen there is sufficient rotor position

resolution. Because the phase currentsare sinusoidal in function of the rotorposition, one needs a minimum amountof granularity to create a sinusoid withadequate fidelity. In addition, becausethe absolute position of the rotor magnetis required, the rotor position feedbackneeds to be absolute. Because absolute

Ra La Ea

Ia

Va

Rb Lb Eb

Ib

Vb

Rc Lc Ec

Ic

Vc


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feedback is not always affordable, there exist 2 other techniques to determine absoluterotor position (or more accurately, absolute electrical rotor angle since the positionwithin one electrical cycle is all that is required):

Obtain a coarse absolute position on power-up and then use incrementalfeedback for higher resolution.

Obtain the electrical rotor angle using incremental feedback only via a specialphase finding routine.

The first approach is typically achieved with a combination of Hall sensors andincremental encoder. The second approach is only applicable if some uncontrolled motion can be allowed during start-up.

With 3-phase sinusoidal currents and back-EMF, the motor torque is:

2

3

))3

2(sin)3

2(sin)((sin*

))3

2sin()

3

2sin()

3

2sin()

3

2sin()sin()(sin(*

/)**(

,

222

,

,

=

+++=

++++=

++=

ptm

ptm

ptm

mccbbaapm

KT

KT

KT

IEIEIENT

The first equation follows from the electrical to mechanical power conversion. Kt,pis thetorque constant per phase of a multi-pole motor. The current and back-EMF aresinusoidal with 120 degree phase shift as mentioned earlier. Hence the torque is alwaysconstant.

The picture below shows the 3 phase currents while the motor accelerates. The currentreference is modulated by the rotor electrical angle.


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Sinusoidal commutation providesoptimal (constant) torque control butimposes certain position feedbackrequirements. In applications that

require position control, this positionfeedback requirement is typically not anissue (because position feedback isalready available or required).

Trapezoidal Commutation

Applications that do not require positioncontrol (e.g. velocity or torque controlapplications) may take advantage of asimpler, lower cost commutation

alternative through trapezoidalcommutation. This commutationapproach does not attempt to maintainan ideal 90 degree angle between thestator and rotor magnetic field at alltimes. Instead it takes advantage of therelative insensitivity of torque productionin the vicinity of 90 degrees. Specifically,the rotor electrical position is determinedwithin only 60 degrees (i.e. 6 positionsper 360 electrical degrees). This resultsin a torque angle variation of 90 degrees+/-30 degrees (i.e. 13% torquevariation).

This coarse position measurement isrealized with Hall sensors. Typically, 3Hall sensors are mounted inside themotor and they detect the rotor magnet.The 3 sensors are spaced to create thefollowing pattern in function of theelectrical rotor angle:

As mentioned before, this pattern isrealized per electrical cycle, so for a 4-pole motor, this pattern would repeatitself twice over one mechanical

revolution. This mechanism provides acoarse absolute angle position, sinceeach 60 degree segment is identifiedwith a unique Hall signal pattern.

Assuming the signal level variesbetween 0 and 1, the following tablesummarizes the 3-bit value for each 60degree segment:

Segment Hall States

0-60 101

60-120 100

120-180 110180-240 010

240-300 011

300-360 001

One can quickly detect a simple 3-bitGray code pattern that provides thedesired absolute position.

In order to operate around the optimal90 degree torque angle, the Hallsensors are located relative to the motor

back-EMF (which indicates the rotormagnet location) as follows:

Hall 1

Hall 2

Hall 3

0 60 120 180 240 300 360

Ea-b

Eb-c

Ec-a

Hall 1

Hall 2

Hall 3

0 60 120 180 240 300 360


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Basically, the back-EMF zero-crossingscorrespond to Hall state changes.

Note: the configuration above is often

referred to as 120-degree Hall phasing.There also exists 60-degree Hallphasing, with equivalent results.

With this Hall sensor feedback in place,trapezoidal commutation is simplyachieved by driving current into 2phases (while leaving the 3rd phaseopen). The Hall states determine whichpair of phases will be used to drivecurrent into. In the specific example

described here, the current would bedriven as follows:

Hall State Phase Pair

100 AB

101 CB

001 CA

011 BA

010 BC

110 AC

Notice that switching from one pair tothe next always maintains continuity forone phase (in order to maintain currentflow).In order to understand torqueproduction, we can use the observationthat conservation of energy (assumingno losses at the electromagneticconversion level) requires:

Eph-ph*Im= Tm* m

WithEph-phthe phase to phase back-EMF,Imthe phase current,Tmthe motor torque,mthe motor speed.

Note: There are other ways to derivetorque, for example by considering the

harmonics of the square currentwaveforms.

With a constant phase current andassuming speed is constant; the motor

torque has the following shape:

Basically, the Hall sensor statedetermines which pair of phases will bedriven and hence which section of theback-EMF portion will be used in thefinal torque production (taking intoaccount the direction of current flow).

This shows the resulting torque ripple(due to the torque angle variation).Further analysis shows that the averagetorque is roughly 95% of the peaktorque and the torque varies by 13%. Inlow performance applications, thistorque ripple may be of no consequence(e.g. in a high inertia system theresulting velocity variation may beunnoticeable). In addition to simplifyingthe feedback requirements, trapezoidal

commutation also simplifies the driverequirements because the switchinglogic and current control can beimplemented with relatively simpleanalog circuitry.

Practically speaking, setup andconfiguration of trapezoidal

0 60 120 180 240 300 360

Ea-b

Eb-c

Ec-a

Tm


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commutation typically requirespermutation of either the motor powerleads or the Hall sensors. In theory, fora given motor connection, there are 6different ways to connect the Hall

sensors (or vice versa). Of those 6possible ways to connect either themotor or the Halls, only one providesproper operation. To understand whathappens with the wrong connection,consider the resulting torque as derivedabove, but this time with 2 Hallconnections interchanged (e.g. Hall 1and 2):

The resulting average torque is zero andoscillates (this typically results in alocked or jumpy motor). There are 3combinations that result in this torqueprofile. Unfortunately, there are 2combinations that result in a non-zeronet torque (but it is sub-optimal). Forexample, when interchanging Hall 1 to

2, and 2 to 3 (and hence 3 to 1), thefollowing torque profile results:

Although there is a net, non-zero torque,this combination can sometimes be

mistaken as functional. Unfortunately, itcan lead to drive and/or motor over-heating or reduced performance. Hence,always make sure to have tried ALL 6possible connections.

There are additional commutationmethods that can be considered, butwhich require more applicationdependent considerations such assensor-less control. Either trapezoidal or

sinusoidal commutation is possibleusing no motor feedback. Also, althoughit was mentioned that stepper motorsare not commutated, it is possible toapply commutation to the stepper motor(i.e. operate as a 50-pole brushlessmotor), but the pull-out torque curve stillapplies for maximum torque.

In conclusion, as with current control,brushless motor commutation is at the

basis of proper torque (or force)generation. Both feedback and amplifierselection needs to be consideredcarefully depending on the application.Once the proper selection is madebased on performance and cost, properconfiguration is critical to obtain thedesired results.

0 60 120 180 240 300 360

Ea-b

Eb-c

Ec-a

Tm

0 60 120 180 240 300 360

Ea-b

Eb-c

Ec-a

Tm


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IDM680 universal drivefrom Technosoft

Based on MotionChip III DSP

Universal drive (brushless, brush,stepper) with embedded motioncontrollerPosition, speed or torque controlS-curve, 1stand 3rdorder interpolation,gearing and camming

Axes synchronization between alldrives12-80VDC bus, 8A continuous, 16.5Apeak12-48VDC logic

5 digital in, 6 digital out 24VDC, opto-coupledRS232, RS485 and CAN communication channelsFeedback support for (model dependent): incremental encoder, digitalHall, SSI encoder, linear Hall, sine/cosine encoder, resolver and BiSSSmall size: 136x84.5x26 mm

Complete configuration and programming with TML instruction set throughEasyMotion Studio:


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Product Feature: Named Resources With CTHMI

Traditionally, machine builders using a separate HMI(s) in conjunction with theirPLC/PAC have not only had to accept using two different software development

environments one for the HMI and one for the PLC to develop the HMI screens andPLC code respectively, but also have had no easy way to pull resource tags (which hadalready been created and named in the PLC/PAC) into the HMI software environment.Having globally viewable tags is very desirable since it would:

1) help unify the code development environment2) shorten code development3) allow for virtually instantaneous recognition of new tags in the HMI environment4) make HMI modification easier

It has always been assumed that these controller resource tags would have to be

manually re-created in the HMI development environment from scratch. With the adventof the CTC 5300 BlueFusion controller combined with the CTHMI screen developmentsoftware, this is no longer the case. Control Technology Corp's CTHMI developmentenvironment allows the HMI screen developer to use any and all resource tags in thePAC just by simply calling out their name. Control Technology Corp has labeled thisfunctionality QuickView (QV) and it runs between CTHMI and CTC's QuickBuilderIDE (QB) for the 5300 series BlueFusion controllers and the controllers themselves. QVinterrogates a running program to find out the named resources within the controller.Before delving into QuickView functionality, it is important to understand the basicstructure of CTHMI, particularly how screen objects are created and then how controllerresources are assigned to them.

CT HMI is part of the CTC Enterprise Software suite, which was designed to exchangereal-time information between real-world industrial devices andenterprise systems in a bi-directional manner. CTHMI is astandalone Human MachineInterface or a window into aprocess designed to monitor andcontrol. Since CTHMI is based onthe Java Virtual Machine, it runs on

almost any hardware and operatingsystem. CTHMI project files arecreated from individual screens orpanels, which contain the fielddevice elements or objects. Thesedynamic, JAVA based objects inCTHMI are called widgets. Some


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examples of widgets are : pushbuttons, potentiometers, analog panel meters, alarmlights, digital data displays, pumps and motors just to name a few.

In Fig.1 an actual, simple CTHMI screen is shown. As can be seen the analog, rotarygauge widget is highlighted. By clicking on a widget and selecting it, the user can edit

and change its properties in the Property Inspector Listing window as shown in Fig. 2:

In Fig. 2, some of the properties for theanalog gauge can be viewed. Threeproperties for the Analog meter widget inparticular are pertinent to our discussion.They are:

CTdatabase: the IP address ofthe controller from which the datawill be read.

CTfqn: the actual tag name

already created and in use on thecontroller side.

CTServer: the pointer name forthe connection and server typeback to the controller.

As can be seen above, the CTServerfield is set to @QV, which designatesthe use of the QuickView Server functionto make available all of the previouslydefined controller tags. The CTfqn fieldcontains the controller resource tag

speed, which is an actual variablealready created and in use by thecontroller. By using the conventionsshown above, the HMI designer now hasa simple method to access previouslycreated resource tags on the controllerside using the functionality of CTC'sQuickView. Let us now turn to CTC'sgraphical QuickBuilder IDE to understand some of the specifics of QuickView and itsimplementation.

Upon the successful connection with a CTC 5300 controller, the CTC QuickBuilder (QB)program causes QuickView (when launched) to interrogate a controller's runningprogram to determine the named resources in the controller. Once this is accomplished,QuickView presents the data to be viewed and edited in a table format. Since thisprocess is accomplished every time QuickView connects with a controller, the datapresented is always synchronized with the current running project. QuickView can berun from within the QuickBuilder environment or as a standalone application for control


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and diagnostic purposes. Figure 3 below shows the results of a QuickView query andlisting:

As illustrated above, the QuickView screen corresponds to a unique controller at aunique IP address. On the left side of the QV window are all of the resources namedand recognized by the program. Any one of these tags can then be used by the CTHMIprogram simply by calling out its name in the proper CTHMI field as shown earlier. Thehighlighted tag speed was in fact used in Fig. 2 earlier, illustrating the Property Fieldentries. To access the QuickView window from within CTC 's QuickBuilder environment,simply right mouse click on a highlighted controller found in the Resources Section of

the Main screen. This will cause a listing of functions available to the controller toappear. Scroll down to QuickView and click it to instantaneously establish a connectionwith the controller.

CTC's QuickView functionality, which is built into the QuickBuilder environment,provides a simple but powerful method to quickly make controller resources easilyavailable to the HMI designer.


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Applicat ion Solution: XYZ posi tioning wi th Arcus 4EX-SA

A manufacturer of metrology systems

had been developing a new productfamily using a networked integratedstepper solution. The metrology systemconsists of:

An X, Y and Z axis driven by leadscrews. The X and Y axes movethe part under inspection and theZ axis moves a camera up anddown.Linear encoders on each axis forprecise positioning.

A PC for overall system controland video processing.A joystick for manual operation ofeach axis.

Adjustable lighting for theinspection area.

Because of cost and speed criteria, themotion was implemented using stepper

motors with integrated drive and

controller. Each stepper is networkedinto a serial network back to the PC.The linear encoder and analog joysticksignals are connected directly to thesteppers. Various problems aroseduring implementation, the mostsignificant one being a drastic reductionin serial communication response timeduring certain operating modes.

An alternative implementation was

pursued using the following Arcuscomponents:

4EX-SA controller with USBconnection to the PC

DriveMax A integrated micro-stepper motor and driver

Performax analog I/O module

The system was then wired as follows:

LIM+

ENC

LIM-

HOM

USBUSB

LIM+

ENC

LIM-

HOM

LIM+

ENC

LIM-

HOM

Joystick

STEP&DIRSTEP&DIRSTEP&DIR


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The Performax 4EX-SA controller is connected to the PC via USB, and controls each ofthe 3 DriveMax A stepper motor/drives in micro-step mode (50,000 steps/rev). Inaddition, the 4EX-SA receives the linear encoder pulses from each axis, as well as limitand home switches. Also, the joystick is connected to the 4EX-SA via analog signals, aswell as a few digital signals for axis and speed selection. Also, a Performax AIO module

with 8 analog output channels is connected via USB to the PC. The analog outputs areused to control light intensity.

All system wiring is direct; no additional interface boards are required, makingintegration very efficient and simple.

The Performax 4EX-SA is commanded via USB from the application running on the PC.The PC application functions as the machine and vision controller. The 4EX-SAcontroller offloads the following tasks from the PC:

Closed loop stepper operation: the 4EX-SA outputs step and direction signals tothe stepper motor/drives and receives the linear encoder signals for proper move

verification. This closed loop operation (Step-N-loop) ensures that allcommanded moves are properly made and can also detect any mechanical stall.The following parameters can be set for closed loop operation:

o Factor: ratio between step counts and encoder counts.o Tolerance: allowed error without correction.o Max Attempt: number of correction tries prior to system error set.o Error Range: maximum error within which correction is tried; outside this range a

system error is set.

Joystick operation: the 4EX-SA receives analog and digital signals from a joystickand controls the axes accordingly. The following parameters can be set for

joystick operation:

o Max speed: maximum allowed speed.


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o Speed delta: maximum allowed speed change (to slow-down rapidjoystick movement).

o Inner/Outer limits: for both positive and negative directions, when the innerlimit is reached, the speed is reduced, when the outer limit is reached,speed is set to zero.

o

Max/min volts: maximum and minimum voltages corresponding tomaximum and minimum speed.

In addition to being commanded via the host PC, the 4EX-SA is also capable of runninga local program. This was utilized to allow speed selection via a digital I/O line.

Implementation of both the hardware and software was done quickly, and although theprevious (non-working) implementation had taken well over 6 months, the 4EX-SAbased solution was completely functional within a few months. In addition, the 4 thaxison the 4EX-SA will be used in the future for precise focus control on the camera.


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For more information about any of the above topics or general questions or comments,please contact us:

Motion [email protected]

Tel 805.504.6177

Motion Designs is a technical sales and engineering company with extensive machine and motioncontrol experience. We work with some of the best manufacturers in the industry as witnessed byour present line card:

www.arcus-technology.com: Arcus Technology manufactures stepper motor, drive andcontroller technology, providing USB, Ethernet and Mod-Bus connectivity.

www.ctc-control.com: Control Technology is a leader in automation control technology withextensive network connectivity capabilities.

www.magmotor.com: MagMotor is a leading servo motor manufacturer.

www.technosoftmotion.com: TSM is a leading DSP motion control technology companyspecialized in the development, design and manufacture of digital motor drive products and

custom motion systems.

torque-dtrends may 2008

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