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

A quarterly publication brought to you by Motion Designs Inc. November 2010

In this issue of Design Trends:

Technology: Closed Loop Stepper ........................................................... page 1

New Product: IDM680 EtherCAT.............................................................. page 5

Product Feature: TMC-3D G-code............................................................ page 6

Application Solution: Unmanned Underwater Vehicles.......................... page 10

Closed Loop Stepper

Stepper motors have long been thework horse of low power positioningapplications. High volumemanufacturing has constantly drivendown the cost of these fractional HP

electric motors. However, the open loopnature of these devices has alwayslabeled them as low performance andhence inferior compared to closed loopservos. This article will attempt to reviewa few techniques to use the steppermotor in a closed loop fashion.

Open Loop Stepper Operation

Stepper motors are inherent positioning

devices and are a member of thebrushless synchronous motor family.They are built in many forms andshapes, power range, step angle, phasecount But the fundamental operationremains the same. A typical motor isconstructed as follows:

A multi-phase stator generates arotating electro-magnetic field, while therotor has many salient teeth. Dependingon the rotor construction, stepper

motors are permanent magnet, variablereluctance or hybrid based. Byenergizing the stator coils in the propersequence, the rotor can be moved tospecific locations. Torque productionrelies on interaction with the motormagnetic field (PM motor), minimal

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reluctance or a combination of both(hybrid).

In full step mode, the motor coils areenergized such that every electric cycle

is sub-divided in 4 steps. So a 1.8degree 2-phase step motor thenprovides 200 steps per revolution andhas 50 electrical cycles. Steps can befurther sub-divided to provide more stepresolution (not necessarily moreprecision), leading to so-called micro-stepping.

Regardless of the step sub-divisions,some open loop torque Vs. speed curve

can be established, typically looking likethis:

What is not shown on this curve is thatthere are a couple of speeds wheretorque drops out (at so-called resonancepoints). Remember that torqueproduction relies on the angle betweenthe stator and rotor field. In open loop,this angle is not explicitly controlled andis load dependent. At each step, there is

a transient with some natural frequency.The plot below shows the motor positionmeasured with an encoder for asequence of steps:

If the motor is excited at a resonancefrequency, the instability will result inmotor stalling.

The other limiting factor is of courseavailable DC bus and motor back-EMF.

As speed increases there is no morevoltage over-head to control currenthence the rapid decline in torque athigher speed.

If the maximum motor torque is notsufficient to overcome the load torque,the rotor skips over one (or more)electrical cycle(s) and positionsynchronism is lost.

In order to avoid loss of synchronism,stepper motors are typically over-sizedsuch that during normal operation thereis sufficient margin between worst caseload torque and motor torque (often asmuch 100%).

In applications where sufficient margincan not be designed in or where loss ofsynchronism is detrimental to theapplication, position feedback has to beadded. The most common solution isaddition of an optical incrementalencoder.

End Of Move Verification

One of the simplest ways to operate astepper motor in closed loop is tocompare the theoretical position thatshould have been reached based on thenumber of steps and the actual position

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that is reached based on the positionfeedback.

If there is a difference in position afterthe move is completed, a correction

move is made.

The black line is the theoretical position

and velocity (per the open loop stepcommand). The red line is the actualposition from the feedback. Once themove is completed, when a difference isdetected an additional correction moveis made.

One important consideration is the stepresolution and encoder resolution.Typically the step resolution is muchhigher then the encoder resolution so

the final position error is governed bythe encoder resolution.

This approach is acceptable for point-to-point applications where only the finalposition is important. This allows motorsizing with less margin.

Dynamic Position Verification

The disadvantage of the previous

approach is that the move has to becompleted prior to any detection and/orcorrection of loss of positionsynchronism. A second approach toclosed loop operation is to continuouslymonitor the difference between theposition step reference and encoderfeedback.

There are many different algorithms thatcan be used to dynamically compensateonce a position error has been detected:

- increase the pulse rate

- increase the current temporarily- adjust the step angle-

In the example below, a stall conditionduring the move was correcteddynamically (red line is positionreference; the blue line is encoderbased position):

Torque Control

The third method of controlling a steppermotor closed loop has been made

possible by the availability of costeffective DSPs. As mentioned before, astepper motor is a brushlesssynchronous motor, not unlike atraditional brushless servo motor. Thekey differences are:

- high pole count- high torque constant (and back-

EMF)- high pole saliency

But otherwise it is simply an extremelylow cost brushless motor (in all fairnessit may not have the mechanicalconstruction that will provide the long lifeof a true brushless servo motor) .

In order to properly commutate astepper motor like a traditional brushless

velocity position

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servo motor, a few restrictions must beobserved:

- sufficient feedback resolution inorder to have sufficientgranularity in the electrical angle

- sufficient current loop bandwidth- sufficiently rigid coupling betweenthe motor and feedback

Lastly, commutation start-up is typicallyrestricted to a phase-align method asHall sensors are not an available optionfor stepper motors.

The first picture is an open loopconfiguration with velocity 80 rev/s and

acceleration is 400 rev/s/s. This moveprofile leads to stalling.

The same move profile for a closed loopconfiguration (with torque control) doesnot pose a problem.

One physical limitation that can not beovercome is maximum obtainablespeed. Stepper motors are specificallydesigned to provide high torque with

limited current, resulting in a high back-EMF. Hence the DC bus of the drive willalways impose a speed limitation.

On the other hand, motor current hasmore room for adjustment. Continuouscurrent ratings for stepper motors arebased on continuous operation. As abrushless servo motor, intermittentregimes are allowed during which thepeak currents can far exceed

continuous current, for short timeduration. Unfortunately, current ratingsbeyond continuous are not available, sopeak regimes are not well defined. If theRMS current does not exceed thecontinuous rating, there should not be athermal concern.

Conclusion

Stepper motors can be turned into verycost effective closed loop actuators bythe addition of a simple, relatively lowcost encoder. Moreover, they can evenbe transformed into powerful brushlessservo motors that provide high torque ina small package. This always the motorto be used more efficiently and hencemore power is available at the outputshaft.

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Technosoft EtherCAT Drive

UNIVERSAL INTELLIGENT DRIVE FOR ROTARY AND LINEAR BRUSHLESS, DCBRUSH AND STEP MOTORS

The IDM680 drive is a member ofTechnosofts new family of digitalintelligent servo drives equipped with anEtherCAT communication interface. Thishigh-performance intelligent servo drivecombines motion controller, drive andPLC functionality into a single compactunit. It can operate either as a standardEtherCAT slave using CANopen overEtherCAT (CoE) protocol or it can be

programmed to execute complex motionprograms directly at drive level, using thehigh level Technosoft motion language(TML). This enables the user to reduce a master task by calling complex motionfunctions, pre-stored in the drive memory, or by triggering their execution via I/Osignals. Compatible with EasySetUp and EasyMotion Studio for quick configuration andmotion programming at the drive level, the IDM680 drive offers a flexible and easy toimplement solution for a wide range of applications.

FEATURES Suitable for control of DC brush, stepper and brushless rotary or linear motors CANopen over EtherCAT (CoE) with full support of CiA402 Various modes of operation: position or speed profiles (trapezoidal, S-curve), 3rd order PVT (Position,

Velocity, Time) and 1st order PT interpolation, electronic gearing and camming, external reference:analog or digital, open / closed loop and microstepping (up to 256 steps/step) for step motors

Powerful Technosoft Motion Language (TML) instruction set including motion commands, program flowcontrol, I/O handling, arithmetic and logic operations, axis synchronization

Stand-alone operation with stored motion program RS-232 serial communication for setup and motion programming Opto-isolated digital I/O (6 outputs / 5 inputs) 2 Analog inputs +/- 10V differential (reference and tacho) Digital reference inputs: 2nd encoder / step and

direction, RS422 differential Position feedback: RS422 differential encoder and

digital Halls or absolute SSI encoders (IDM680-8EI-

ET), linear Halls, sine / cosine incremental encoder orsine / cosine absolute encoder with EnDAT protocol(IDM680-8LI-ET), resolver (IDM680-8RI-ET), BiSSencoder (IDM680-8BI-ET)

Power supplies for logic (12-48V) and motor (12-80V) 8A continuous, 16.5A peak current Protection for over current, short circuit, over

temperature, over- and under-voltage, I2t, control error

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Product Feature: TMC-3D G-code

Introduction

G-code is a popular programming language used in Numerical Control. Although it isnot a genuine programming language (like ANSI-C), it does provide a somewhatstandard method for multi-axis path definition and is mostly used in machine toolapplications. G-code is often the intermediary between high level CAD/CAM softwarewhere parts are designed and the actual machine that cuts the metal. Standardizationaround G-code is not very strict, leading to much user interpretation. Nevertheless, itdoes provide some framework for a common platform

Technosoft TMC-3D

The TMC-3D is a multi-axis motion controller, capable of generating 3-D paths and

commanding up to 8 axes. The TMC-3D can power a single motor, the other motors inthe system need to be controlled by other Technosoft drives. All programming is donevia TML (Technosoft Motion Language). In order to accommodate standard pathplanning, a G-code interpreter is made available within EasyMotion Studio (the IDE forTML programming). This G-code interpreter can translate a G-code sequence into theproper TML commands, eliminating almost all motion specific programming.

G-Code Support

The following G and M codes are supported by the interpreter:

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Remarks:1. The home position parameters are set via #5161 - #5186. For details see below

the table G-code parameters supported.2. The coordinate system parameters are set via #5211 - #5386. For details see

below the table G-code parameters supported.3. If Stop program or End program M-words are used, and no cycle start button is

defined the program restarts from the beginning of the G-code program imported.

4. G-code block M30 is converted to M2. G-code block M60 is converted toM0.Pallet shuttle commands are not supported.

5. Coolant valve can be set from the Import G-Code file dialog. You can set theaxis to which the button is connected, the input line and the polarity.

6. The override percent can be changed from Import G-Code file dialog. By defaultthe override percent is set to 100%.

7. The block delete switch can be enabled from the Import G-Code file dialog. Bydefault this switch is disabled.

8. The optional program stop switch can be enabled from the Import G-Code filedialog. By default this switch is disabled.

9. In Import G-Code file dialog you can set values: traverse rate, spindle rate, feed

rate, and the rotation sense. By default the positive sense for the spindle speedis clockwise.

10. The default units are millimeters. In the Import G-Code file dialog you canchoose between inch and millimeter.

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Configuration

A G-code file can be imported into a TMC-3D project in EasyMotion Studio:

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Within the configuration window the axes can be selected, as well as feed rates andspindle speeds and all other auxiliary functions.

An example drawing, G-code list and resulting TML code are shown below:

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Application Solution: Unmanned Underwater Vehicles

Although land and air robotic vehicles have increasingly been used by civilian andmilitary agencies for border surveillance, intelligence gathering and offensiveoperations, the deployment of Unmanned Underwater Vehicles (UUV) is still in itsinfancy. In 2000 and 2004, the U.S. Navy published a master plan detailing size, weight,and mission for the two main types of UUVs. The two types were tethered (UUV power and control via an umbilical) and autonomous (AUV battery powered with on-board intelligence). Since then, with the creation of the Department of HomelandSecurity and two ongoing wars, research and development with UUVs has forgedahead worldwide. Two examples appear below, from the simple, tethered torpedo tothe futuristic and combat-oriented:

A C'Inspector UUV with side-scan sonar for seabed and ship hull inspection.

Artist's concept: Trident World Systems

Generally, UUV designers have looked to incorporate components and systems alreadyused commercially and by the scientific community not only to keep costs down, but tospeed development for ever-increasing types of missions. One of the main areas ofconcern for UUV engineers is motion.This involves depth control, back and forth propulsion and turning. The motion can befully active, meaning motors are used to drive propellers, vanes and thrusters orsemi-passive, meaning the UUV rides underwater currents and changes depth by

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vanes and ballast pumps. Although brushed DC motors have been used, it is morecommon to see both brushless DC servomotors and stepping motors being used. Withthe use of COTS (Commercial Off-the-Shelf) motors, comes the need to match therequisite drive(s) for the stepper and servomotor. In doing so, most UUV engineers lookfor the following criteria in down selecting a drive solution:

1) universality in powering both servos and steppers2) good power density3) if battery powered , a range of low and high current on low voltage DC bus

offerings4) easy to program(for AUV apps), with multiple protocol options, even mixing some5) standalone or networked slave operations, even the ability to do both

simultaneously6) ability to use one drive as a bus gateway to the other drives in the UUV7) proven bus options such as RS485 and/or CAN8) ability to be quickly modified down to the PCB

9) packaged versions for quick testing as well as OEM board level versions10) broad power range for various size motors11) drives easily integrated with a higher level, system controller if needed.

There is in fact a company, Technosoft S.A. ( www.technosoftmotion.com ) , thatprovides intelligent drives meeting the criteria above, making them an excellent choicefor incorporation in UUV designs. It will be the focus of the remainder of this articleusing a generic example of an active UUV motion layout to show how the Technosoftintelligent drives can accomplish the motion control system (hardware and software) ofa UUV.

The diagram below provides a typical scenario, common in rudimentary UUV designs. Itconsists of a main propeller (NEMA34 servo), two smaller thrusters (NEMA17 servos)which are each connected independently to a stepper motor (NEMA23 stepper) fordepth control and turningthe UUV. Also there is anembedded, single boardcomputer running a LinuxOS. The embeddedcomputer providessupervisory control to themotors via the drives asneeded.

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As we review the layout above, some of the criteria previously listed become apparent.First, there is one drive type, the PIM2403, which is controlling both the NEMA23steppers and the NEMA17 brushless servos. This function exists on almost all drivefamilies from Technosoft. In addition, the software development environment is thesame for all drive families, independent of motor type. This makes programming easy

regardless of drive or motor type/size. The PIM2403 drives provide 3 amps continuous,6 amps peak @ 24VDC in a small package and also are available in a packagedversion with screw terminals for quick testing/prototyping. Secondly, the Linux SBC isconnected serially only to the ISD720 drive which acts as a gateway to the PIM2403drives via a separate CAN bus connection, relaying commands and data. See below:

Not only does this simplify wiring, but it also provides for different programmingarchitecture options:

Linux SBC acts as a true master with no local code on the drives

Linux SBC acts as a master, but drives have some local code running

independently Some/all of the drives act as intelligent network nodes running code

independently and sharing data with each other. The Linux SBC acts only in ahousekeeping mode.

Also, the ISD720 drive is able to use the same bus voltage (24VDC) as the smallerservos and steppers while providing 20 amps continuous and 49.5 amps peak current.

One critical issue that is not readily apparent from the layout diagram is whether or notCOTS drives can handle the uniqueness of enclosure pressurization in oil at relativelyhigh pressures of greater than 1500psi to insure watertight sealing of the electronics.

Generally, board level components such as power capacitors and clock crystals aresusceptible and must be replaced with MIL-SPEC equivalent. To date, these changeshave been relatively easy to make in-the-field on Technosoft's open frame drives. Letus now take a quick look at how programming is handled in our example.

As seen from the layout, a Linux SBC has been incorporated and used as a higher levelcontroller. It passes velocity information to the ISD720 to regulate main propeller speed,starts and stops the thrusters (PIM2403s) and adjusts the position of the steppers

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(PIM2403s) to adjust depth of the UUV. All of this is accomplished using Technosoft'sMotion Language (TML) commands sent as binary code in HEX format over the serialconnection. As stated previously, the ISD720 acts as a gateway. As commands come inover the serial connection from the Linux SBC, the node ID is sampled and thecommand is relayed to the appropriate PIM2403 drive. In our example, determining the

correct binary code string to send is accomplished using the Binary Code Viewer fromwithin Technosoft's EasyMotion Studio programming environment. We will use theexample of setting the velocity of Node 3 (one of the NEMA17 servos PIM2403 drive)to 10 rps. The code to run at constant velocity is shown below from the Main screen ofTechnosoft's EasyMotion Studio programming environment:

The variable of interest is CSPD, which is Commanded Speed , seen on the fourth lineof the Main program section. To see the actual Binary code sent out, EasyMotion Studioincludes a Binary Code Viewer, which helps you to quickly find how to send TMLcommands using one of the communication channels and protocols supported by the

drives/motors. Using this tool, you can get the exact contents of the messages to sendas well as of those expected to be received as answers.

One can select it from the Application tab, the sub-menu Binary Code Viewer:

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In this case, sending the HEX string 08 00 30 24 A0 CC CD 00 CC 61 from the LinuxSBC results in Axis #3 (NEMA17 servomotor) running at 10 rev/sec until commanded todo otherwise. In summation, the Linux SBC packs the binary code of each TMLcommand string into a message which is sent. The low-level simplicity of the binarycode allows many different external platforms to communicate with the drives. This is

one example of how a supervisory host can communicate with the Technosoft drives.An alternate way to exchange data with the Technosoft drives/motors is via theTML_LIB libraries. A TML_LIB library is a collection of high-level functions for motionprogramming which you can integrate in the host/master application, typically found onan industrial PC or PLC.

One can see that Technosoft's intelligent drives meet the prerequisites of COTS driveslooked for by designers and UUV engineers.

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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.amosin.com: AMO manufactures induction based precision linear and anglemeasurement encoders.

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

www.nanotec.com: Nanotec provides a comprehensive range of stepper and servo motorsolutions.

www.nipponpulse.com: Nippon Pulse manufactures the unique linear shaft motor, a directdrive linear brushless servo motor.

www.stegmann.com : Stegmann is a leader in high performance motor feedback solutions.

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.

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