Improving Servo System Design to Optimize Coating ... - Kollmorgen

IMPROVING SERVO SYSTEM DESIGN TO OPTIMIZE COATING AND LAMINATING QUALITYImproving Servo SystemDesign to Optimize Coatingand Laminating Quality1Tom England, Market Development Director

IMPROVING SERVO SYSTEM DESIGN TO OPTIMIZE COATING AND LAMINATING QUALITYIn some cases these concerns can be adequatelyaddressed by selecting high-quality mechanicaltransmission components. However, the added expensecan become prohibitive and they will still weareventually. The ultimate solution is a direct drive rotarysystem, which eliminates the transmission altogether. Indirect drive systems, the motor directly drives the load.The accuracy of direct drive systems can be up to 60times better than that of traditional systems and audiblenoise can fall by 20 dB. Other measures such as servoresponse (bandwidth), machine parts reduction, andreliability also can improve dramatically.The motor's primary contribution to velocity ripple is itselectromagnetic cogging, which is usually at the pole orslot frequency or some multiple of it. The best way toovercome electromagnetic cogging is to begin with a lowcogging motor that requires less correction. Highperformancemotors with less than 1% pk-pk coggingare a very good start, and nearly always superiorcompared with standard motor options due to betterelectromagnetic design, and simulation of theelectromagnetic circuit.Frameless Direct Drive MotorsWhen the load is directly coupled, there is no limitationon the inertia mismatch between load and motor. Theservo loop gains can now be increased significantly toprovide the necessary servo stiffness to achieveexcellent speed regulation to optimize product quality.Web speeds can be increased in many applicationsusing direct drive technology because the accuracy ofthe mechanical transmission system is often the limitingfactor.When the transmission components such as gearboxes,belts, rack and pinion, and pulleys are eliminated, theservo system becomes free of such negative factors ascompliance, backlash, and component wear. Theaccuracy increases, inertia-matching requirements relax,acceleration and deceleration improve, maintenancebecomes unnecessary, and the product life increases bya significant degree.Direct drive rotary motor size can be based on the peaktorque required for achieving the desired accelerationtime specifications. With direct drives, inertia mismatchof 250 to 1 is common and mismatch of 800 to 1 hasbeen implemented. In many coating and laminatingapplications, the size of the motor is dictated by theinertial matching requirements. The result is that a muchsmaller and more energy-efficient DDR motor can beused in most applications.Kollmorgen’s KBM series frameless brushless motorsDirect drive rotary technology has developed in anevolutionary manner. The original frameless direct drivemotors were designed into the machine architecturealong with a feedback device and became a fullyintegrated part of the machine. This approach has theadvantage of consuming the least amount of space. Onthe other hand, frameless motors are relativelyexpensive to fully integrate as they typically requiresubstantial changes to the design of the underlyingmachine. Frameless motors are also more difficult toservice because they are embedded into the machine.While the initial development cost burden is high, thebenefits of higher performance, higher quality, and smallspace requirements justify this technology in someapplications.Full Frame SystemsKollmorgen’s Housed DDR motorsDDR motors deliver increased throughputThe next generation of direct drive rotary technology,sometimes referred to as full frame systems, integratesall of the components of a complete motor including the3

IMPROVING SERVO SYSTEM DESIGN TO OPTIMIZE COATING AND LAMINATING QUALITYrotor, stator, bearings and feedback device within ahousing. The machine shaft slips through the bore in themotor and attaches to the rotor. This approachsubstantially reduces development costs since the motorno longer needs to be integrated with the coating roll.The disadvantage of this approach is that the motor andmachine bearings must be precisely aligned, which is acomplex and time-consuming task. The bearings in themotor and the load are directly coupled in a linearfashion making it nearly impossible to align the systemcomponents properly without causing premature bearingfailure due to uneven loading.Installation typically takes less than five minutes. Themotor slides over the shaft until a motor pilot engages amachine pilot. The housing is secured with bolts. Themotor rotor is then secured to the machine roller shaft bymeans of a compression coupling tightened to aspecified torque. The rotor is now rigidly connected tothe machine shaft. The encoder alignment is pre-set sothat no adjustments need to be made. Cables areconnected and the motor is ready to run.Kollmorgen Cartridge DDR® motorsKollmorgen Cartridge DDR® motorsThe most recent approach to DDR systems is theKollmorgen Cartridge DDR motor which is fully housedand ready for mounting to the machine. However it hasno bearings in the motor, but instead uses the hostmachine to support the motor’s rotor. This approachmakes it easy to use direct drive technology onmachinery that already has bearings, particularly inapplications such as coating, laminating and printingwhere rollers already use heavy-duty, precisionbearings. The motor has a hole in the middle which slipsover the shaft extension of the roll and the motorhousing bolts to the machine frame.Proprietary electromagnetic design givesKollmorgen Cartridge DDR® motors more torque per volumethan conventional DDR technology.A servo system equipped with a Kollmorgen CartridgeDDR servomotor is expected to work for ten yearswithout any maintenance. Although the initial systemcost might be higher compared to a conventional gearedsystem, over a period of several years, eliminating thecost of repairs and periodic maintenance makes theoverall cost of purchasing and operating a cartridgesystem lower. Even with the slightly higher initial cost,over a five-year period, Kollmorgen Cartridge DDRmotors can reduce operating costs up to $10,000 permotion axis compared to conventional geared servosystems.Press-feed machine builtwith a conventionalservomotor, gearhead, beltand pulleys.4

IMPROVING SERVO SYSTEM DESIGN TO OPTIMIZE COATING AND LAMINATING QUALITYImproving Feedback Device PerformanceSince the feedback device is in the control loop, anyerror of the device itself is "corrected" by the drive in theservo loop, and ends up as a velocity ripple on thecoating roll. As the servo loop gain is increased theerror in the feedback device is amplified and transferredinto the coating roll. So, starting with as precise afeedback device as possible is very important tominimize velocity ripple. There are several ways thataccuracy can be improved. The most straightforward isto use a more accurate device. For example, mostencoder manufacturers offer encoders in a variety ofresolutions. Resolution is usually measured in lines perrevolution. Encoders with higher line counts usuallyhave greater accuracy. However, this is not always thecase, so users must check the specifications of theencoder carefully. In some cases, highly resolveddevices can still be relatively inaccurate.Users can also change the type of feedback device.Sine encoders are the most accurate motor-mountedfeedback devices commonly used in industrial servosystems. Sine encoders are typically accurate to +/-25arc-sec, compared with a resolver at +/- 10 arc-min.High-line-count incremental encoders (above 10,000lines-per-revolution) are also often highly accurate.Medium line-count encoders (2,000 to 10,000 lines-perrevolution)are fairly accurate, usually holding 3 arcminutesor less. Resolvers are the least accuratesensors, typically holding from 5 to 20 arc minutes.Effect of the Servo DriveSame machine with aKollmorgen Cartridge DDR®motor installed.Here, the shaft of the driven rollis extended into the CartridgeDDR motor and the motorapplies torque directly to thedriven roll.The servo drive such as Kollmorgen’s AKD® can add toand correct various velocity ripple components. It addsto the ripple by having uneven current loop gains in eachof the phases, i.e. phase A might be 1% higher thanphase B or C and this causes a torque error in themotor. Likewise, the amplitudes of each phase might beslightly different adding an error to the torque as well.The drive can correct these by having digitaladjustments in the current loops to take out these offseterrors.One can also reduce the velocity ripple of the system byusing harmonic correction in the current waveform. Forexample, when the characteristic of the cogging in themotor (frequency and amplitude) is known, this can becorrected by applying an anti-phase ripple in the 3 phasecurrent from the drive. The same can be done with thefeedback device to correct a repeatable error. Both themotor and feedback error frequencies are well knownand repeatable so all we are left to do is adjust phaseand amplitude.With low inertia construction being inherent to the designof most permanent magnet servomotors, mismatchesbetween the high inertial loads of the roll and the lowloads of the motor need to be accounted for. Servomotorcontrol systems can be tuned to handle inertiamismatches that are inherent in coating and laminatingapplications. But the gain in the servo amplifiers needsto be optimized to maximize response while avoidinginstability and oscillations.The traditional approach to compensate for inertialmismatches and compliant loads is to use low-pass,band-pass and high-pass filters to eliminate theunwanted frequencies. The problem with this approachis that the multiple filters that are required to eliminatethe resonances introduce calculation delays and phaseshifts which have a tendency to throw the system out ofcontrol.Recently, substantial improvements in performance havebeen achieved with the use of bi-quadratic filters thatcan emulate nearly any combination of simpler filterswithout introducing significant delays. The bi-quad filtertunes out problematic frequencies, making it possible tooptimize servo system performance. For example, if themechanical system has a 200 Hz resonance, the bi-quadfilter can be configured to remove 200 Hz while providinghigh levels of gain and bandwidth.Kollmorgen’s Ethernet-based AKD® servo drives deliverbest-in-class performance with industry-leading flexibility,scalability and power range to meet the unique performancerequirements of nearly any application.5

IMPROVING SERVO SYSTEM DESIGN TO OPTIMIZE COATING AND LAMINATING QUALITYConclusionCoating and laminating operators and equipment manufacturers are striving to increase machine speed while at thesame time maintaining the highest possible levels of quality. Achieving this goal requires accurately maintainingconstant web velocity in order to avoid banding and other problems. Improvements can be made by reducing thecompliance of the mechanical transmission system, or eliminating it entirely through the use of Kollmorgen CartridgeDDR technology that connects the motor directly to the roll. The latest generations of servo drives, motors andfeedback devices all provide advantages that can help minimize velocity ripple in order to deliver product that meetsthe requirements of the most demanding customer applications..ABOUT KOLLMORGENKollmorgen is a leading provider of motion systems and components for machine builders around the globe, withover 70 years of motion control design and application expertise.Through world-class knowledge in motion, industry-leading quality and deep expertise in linking and integratingstandard and custom products, Kollmorgen delivers breakthrough solutions unmatched in performance, reliability andease-of-use, giving machine builders an irrefutable marketplace advantage.For more information visit www.kollmorgen.com, emailsupport@kollmorgen.com or call 1-540-633-3545.KM_WP_000175_RevA_EN6