Robotics and Computer-Integrated Manufacturing 38 (2016) 82–92

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Robotics and Computer-Integrated Manufacturing

http://d0736-58

n CorrE-m

journal homepage: www.elsevier.com/locate/rcim

Review

Avoiding defects in manufacturing processes: A review for automatedCFRP production

M. Perner n, S. Algermissen, R. Keimer, H.P. MonnerGerman Aerospace Center (DLR), Institute of Composite Structures and Adaptive Systems, Lilienthalplatz 7, 38108 Braunschweig, Germany

a r t i c l e i n f o

Article history:Received 27 August 2014Received in revised form15 September 2015Accepted 8 October 2015

Keywords:Manufacturing defectsActive vibration suppressionSmart structures technologyMechatronic systemsRobot-based manufacturingCarbon fiber reinforced plastics

x.doi.org/10.1016/j.rcim.2015.10.00845/& 2015 Elsevier Ltd. All rights reserved.

esponding author. Fax: þ49 531 295 2876.ail address: marcus.perner@dlr.de (M. Perner)

a b s t r a c t

Linear robots, numerically controlled machines and articulated robots are some of the major componentsin automation. In various applications, these single systems are linked together to become advancedmechatronic systems since they can realize special and general tasks. Unfortunately, the degree of au-tomation in carbon fiber reinforced plastic (CFRP) manufacturing is currently rather low. A lot of researchstill needs to be done in order to achieve the knowledge necessary to gain a consistent, robust andreliable production chain for CFRP. The most decisive criterion in automation is the tradeoff betweenspeed in production and the attainable layup accuracy. One aspect that affects accuracy is vibrationcaused by rapid movement and interaction forces. In the case of advanced mechatronic systems, theeffect of the dynamic coupling is one key factor in accuracy. Over the last few decades, the field of activevibration suppression has made use of its excellent capability to improve the accuracy of machines.Therefore, the paper presents a comprehensive review of active vibration suppression technology for theimprovement of precision in manufacturing. The work is mainly focused on, but not limited to, theproduction of components made of CFRP. The paper concludes with a discussion of promising approachesto identify a trend for future work.

& 2015 Elsevier Ltd. All rights reserved.

Contents

1. Challenges of CFRP automation for the aviation industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 822. Error classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 843. Developments in the field of avoiding manufacturing deviations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

3.1. General automation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 853.2. Signal filtering or modifying drive control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 853.3. Machining calibration and error compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 863.4. Approaches using smart devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

3.4.1. Linear robots and machines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 873.4.2. Parallel robots and mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 883.4.3. Other solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 883.4.4. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

4. Identifying trends for the future: promising fields of research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 885. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

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1. Challenges of CFRP automation for the aviation industry

In the 1960s, carbon fiber reinforced plastics (CFRP) were firstused in the manufacture of structural components in aerospaceengineering. Later in the 1990s, the price for CFRP fell below 100Euro/kg and it was used in certain sports such as golf, tennis orracing. In the period from 1998 to 2006, the world market for CFRPparts has doubled in size [1]. The main reasons for this are theincreasing demand for lighter materials in various technical areas.A strong market growth for commercial aircraft, wind turbines andcorrosion-resistant pressure vessels leads to booming sales ofCFRP. Accordingly, the global demand in terms of volume is ex-pected to keep a compound annual growth rate (CAGR) of about15% between 2012 and 2020 [2]. That equals a CAGR of about 12%in terms of turnover [2]. Taking the upcoming demand for civilaircraft into account, Stumpp [3,4] predicts that the aircraft man-ufacturer Airbus has to raise the production of the A320 passengerjet planes to up to 42 aircrafts per month. Yet there are indicationsthat even this aspired rate will not be sufficient to meet the ex-pected future market requirements. The need for huge improve-ments in CFRP manufacturing becomes even more obviousthrough a simple calculation. With a production scenario of about480 aircrafts per year, 1920 CFRP wing covers will need to bemanufactured. Assuming that the weight per wing cover is about1300 kg, the target production rate would need to be 2,500,000 kgper year, which equals 10,000 kg per day. In the case of 16 h ofproduction per day, a layup rate of 600 kg per hour would have tobe met. Even if there were six parallel wing cover productionunits, each of them had to perform a layup rate of 100 kg per hour.This aim is highly ambitious because of additional maintenanceand material delivery service during production. Currently, themanufacturing of CFRP components is limited by manual work-steps. As the predicted layup rate seems very far from feasible inshort-term, the CFRP industry has a strong desire for compre-hensive automation in order to increase the machine efficiencyand process profitability [5]. CNC1 machines have been identifiedas a possible promising component to reach the manufacturingtargets since they support both fast synchronous and powerfultransformation functions [6]. Industrial robots have a compara-tively lower price than CNC tools. Because of their compliance,they offer less machining accuracy [7] and are more susceptible tovibration [8]. The work of Pan et al. [9] greatly investigates thechatter characteristics of a robot equipped with a spindle formilling operations. Due to mode coupling chatter, rigorous vibra-tions occur. In particular for industrial robots, this kind of chatterhappens when the structural stiffness is in the same order ofmagnitude as the processing stiffness. In the case of CNC ma-chines, mode coupling chatter rarely arises since the structuralstiffness is usually hundreds of times higher than the processingstiffness. A robotic-based design for a CFRP production plant has tochallenge this chatter phenomenon. During layup of fiber materialthe interaction forces easily provoke structural vibration and affectthe layup result. Commonly, the robot path is pre-calculated withsimulation tools and the layup process is based on open-loopcontrol. Nevertheless, industrial robots are fully developed andstandardized components. In comparison to their size, they pro-vide a large working area. Therefore, robotic approaches are con-sidered as well [10].

A robotic tape-laying process has to comply with variousstandards [11]. First of all, the material needs to be free of wrin-kles, creases and other deformations. Secondly, a defined gap be-tween the tape courses has to be guaranteed. The production ordeposition velocity needs to be clearly defined. It is typically

1 Computerized numerical control.

chosen in such a way that the machine tracks the laying path withsufficient accuracy (about 1 mm). During the movement along thecalculated path, singularities of the robotic system or evenmovement in its vicinity have to be avoided. The press of the tapeduring layup has to be done with a defined force. Similarly, notensions within the material are allowed to arise [6]. The layinghead should always be positioned normal in relation to the surfaceto ensure a uniform contact pressure. Most importantly, theserequirements are interdependent. For example, in order to in-crease the layup rate, an increase of the compressive force is ne-cessary. Also, a powerful data file management for large filescontaining many laying courses should exist. Finally, previous re-search [12] illustrates that the requirements in accuracy are un-compromisingly high. Therefore, the exact positioning of eachlayer in the structure is essential for the finished product to havethe desired mechanical properties.

Olsen and Craig [11] point out the advantages of automation.These are the consistency of the production, the fact that CAD datacan be assimilated directly and that expensive patterns to markplacement areas for machine operators during the productionprocess can be avoided. Furthermore, workers are less exposed totoxic resins and the process time is shortened. Chestney and Sar-hadi [12] show that in the case of hand-operated production, therestill is the risk of a poor positioning of the single layers, of theapplication of wrong material types, of missed layers and im-purities in the preform. For this reason, a thorough inspection ofeach single layer is necessary, which is both time-consuming andcost-intensive. In most applications, automation seems to be onekey factor of the cost-effective production of composite structures[13]. In addition, the accuracy can be improved in such a way thatthere is less subsequent fault management and fewer degradedparts are produced. Automation also offers promising approachesfor reliable process documentation. Sutherland et al. [14] state thatthe increasing demands on tool or manufacturing plants underconditions of an increasing production with less human controlsteps require new approaches in production technology. Theyclaim that the key factors for new approaches are the appropriatemodeling of manufacturing steps and machines, the integration ofmultiple sensors, the fault-detection and troubleshooting strate-gies. Furthermore they say that the current technological andeconomical trends require the development of new flexible, robustand automated manufacturing systems. They also emphasize thatthe machine tool, the manufacturing process, the workpiece, thecatch and the tool have to be treated as a comprehensive holisticsystem in order to understand the interactions between the in-dividual elements.

Advanced mechatronic systems are defined by flexible com-ponents, e.g. a robot mounted on a linear unit. The simultaneousmotion of the platform and the robot is affecting its performance

Fig. 1. Example of an advanced mechatronic system consisting of an individualtooling (4) attached to a standardized industrial robot (3) mounted on a mobileplatform (2) to provide supplies and to move along a linear rail system (1).

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(cf. Fig. 1). Certain coupling effects have a huge impact on theobtainable machining precision. Therefore, an excerpt of researchstudies in this field will be provided. One example of advancedmechatronic systems is given by macro–micro-manipulators. Theyare proposed for long reach tasks that require speed and precision.Bassan et al. [15] observe that controlling the joints does not ne-cessarily guarantee adequate behavior while positioning of theendeffector. In addition, Tahboub [16] shows that the motion ofthe base and the coupling forces affect the manipulator's dy-namics. The tracking error is interpreted as the base motion andcorrected by modifying the desired position. The author proposescorrections at a slow rate due to chattering effects caused by thecontinuous correction of the desired position of the endeffector.Moreover, base disturbances are induced by low-frequency oscil-lations excited by road bumps or sea waves. Therefore, Chang andShaw [17] developed an approach using an optical device forfeedback control. These examples illustrate the need for a com-prehensive knowledge of the coupled dynamics of advanced me-chatronic systems as base excitations affect the endeffector's per-formance. The general purpose of suppressing vibration is to en-hance the narrow tolerances of the production process.

It is clear that every single component of a plant has an influ-ence on possible errors in the production process. In order tounderstand which sources of error exist within automation pro-cesses, the following section proposes an error classification basedon existing studies. Following that, approaches to avoid deviationsin automated manufacturing are illustrated. The review focuses onapproaches with regard to active vibration suppression technol-ogy. Based on general approaches for automation, specific solu-tions and proposals for the modification of the drive control,machining calibration and compensation strategies are discussed.Then, special attention is given to smart structures technology.Concepts from different industrial fields will be presented as theamount of research concerning active vibration technology foraccuracy improvement in the special field of CFRP production israther small. Recent developments in the design of a CFRP pro-duction plant require tools for active vibration suppression. Theaim is to break away from huge, heavy and slow machines to goforward to a more flexible production using standardized, fast andmobile industrial robots. Building upon the review, the paperconcludes with naming trends for future work and promisingfields of research.

Fig. 2. Classification approach for understanding inaccuracy in machining.

2. Error classification

The accuracy achievable within a production process is oftendescribed as the degree of compliance to defined shapes and di-mensions. Errors may be treated as any deviation from the theo-retical value required to produce the product in the predeterminedtolerance. In literature, errors or inaccuracies are classified inseveral ways. Ramesh et al. [18] present a comprehensive reviewof the parameters affecting the attainable machine accuracy. Basedon this review, the main sources of error can be divided into fourcategories. These are geometric and kinematic errors, thermal er-rors, deformations due to boundary conditions and changes due tospecific process loads. Deviations of the basic mechanical structureand the performance of individual components as well as devia-tions due to assembly and the relative motion of multiple com-ponents belong to the geometric and kinematic error group.Thermal errors arise primarily from distortions within the ma-chine design and from by extensions of the bearings and axles.Errors due to the bearing or boundary conditions are associatedwith deformation due to the contact between the clamp and theworkpiece and its correct alignment. Process loads generally leadto bending deformations, to heat development and to thermal

expansion. Therefore, these factors partly affect and cause eachother.

Taking into account the consideration of Aggogeri et al. [19], thecombined group of geometric and kinematic errors can be dividedinto static geometric errors and errors due to the dynamical be-havior. Static geometric errors have the largest impact on theprocess accuracy [18,20,21]. Although Chen [22] proposes to de-scribe dynamic errors through thermal variation, in the authorsopinion they should be looked at separately. Aggogeri et al. [19]follow the discrete classification of the error caused by thermalmaterial characteristics. The dynamical behavior, which de-termines the vibration arising through processing, is another causefor significant variations in production. Vibrations can be causedby process-induced movements, by environmental conditions orby drive engines. They affect the system's behavior, as well as theworkpiece, and the tool. According to Zhang [23], the main sourceof position errors in the case of robotic machining is caused byinteraction force and motion behavior, with deterioration as theresult. In this case, the achievable accuracy and the speed of pro-duction are limited and become two contradictory outcomes. Theaim to maximize both means that the vibration behavior becomesone of the most important modifiable properties [24].

Another classification of machining errors is proposed by Shinand Wei [21]. They define random and systematic error classes. Asrandom errors, the authors mention temperature dependence,varying frictional stress and vibrations. These factors, then, gen-erally have no Gaussian distribution. Gossel [25] uses the sameclassification and notes that random errors influence pose re-peatability, whereas systematic ones account for pose accuracy.

As the classification strategy first explained is also the onenearest to practice, it will be used in this review paper as the basisfor understanding the huge field of describing machine errors.Fig. 2 illustrates this classification. Basically, three groups of ma-chining errors can be described: stationary geometric, dynamicand thermal errors. The inaccuracy in the case of stationary geo-metrical variations is caused by geometric variations and couplingeffects due to joints [22], design-dependent positioning limits ofthe industrial robot or misalignments of components [26]. Thegroup of dynamic errors defines the vibration arising throughmovements or interactions. Errors through process loads areconsidered within this group of interaction forces. The dynamicbehavior is mainly determined by the kinematic structure and thebearing conditions. The vibration and damping behavior influ-ences both product quality and production speed [27]. Inaccuracythrough thermal characteristics arises primarily from distortionwithin the machine design and from extensions of the bearingsand axles [18]. These are considered in the thermal error group.

Once sources of inaccuracy have been identified, another issuehas to be considered. Even if the impact of the three defined error

Fig. 3. Plain automation approach for CFRP layup: 1 – layup-head, 2 – mold, 3 –

industrial robot.

M. Perner et al. / Robotics and Computer-Integrated Manufacturing 38 (2016) 82–92 85

groups on the accuracy of a system can be numbered, one scalarvalue is not enough for its description [28]. In addition, the be-havior of the machine varies throughout the working area [22]. Inparticular, the dynamic behavior, or the eigenfrequencies respec-tively, vary while changing the operating position [27]. Moreover,machine errors are time-varying [22].

Last but not least, another decisive factor for the achievableaccuracy is the measurement device. As described above, accuracyis the degree of compliance to a defined shape and dimensional.This is why the imperfection of the actual measuring device has aninfluence on the calculated error or misalignment as well. Lee et al.[29] examine approaches for the analytical description of un-certainties in the context of high-precision machine calibrationsand measurements. They assume that several factors influence themeasurement uncertainty and thus strongly affect the quality andreliability of the results. These are the imperfection of the mea-suring instrument, the geometric variations and the intrinsiccharacteristics of the measurable physical quantities. The goal ofthe approach is to identify a more appropriate error range thatillustrates the significance of the absolute value. The authors dis-cuss the impact of machine- and workpiece-related uncertainties.The results of their work show that all geometric errors areminimized when the measurement direction is perpendicular tothe object's surface.

The previous introduction into the sources of errors and theirclassification shows that a comprehensive knowledge of the au-tomation system is necessary to obtain a robust process in terms ofhigh precision and speed. Nevertheless, the imperfection of themeasurement device becomes an essential criterion. In order to beable to face the demands in future aviation industry, the produc-tion rate needs to be increased and machines have to be lighterand faster. Yet designing machines by lightweight aspects andincreasing the production speed causes even more vibration [30].In most cases, these vibrations lead to damages. Therefore, Genta[30] argues that dynamic design will become one of the mostimportant features during the design process of machines.Nowadays, studies have to cover the dynamical behavior of futuremachines in detail.

The next section presents approaches for avoiding deviations inproduction and for achieving higher accuracy. The review focuseson, but is not limited to, the manufacturing of CFRP components.Since the degree of automation in this branch of industry is low incomparison to others, general concepts are considered as well.

3. Developments in the field of avoiding manufacturingdeviations

Basically, repeatability and processing time form the base ofautomation. Therefore, manual tasks have to be reduced becauseof human imperfection. However, this requires a lot of researchand development. The goal is to standardize manual tasks intomachining language and sequences. Automation depends on theestimated economic benefit. Further improvement leads to accu-racy within single process steps. In general, deviations of singleprocesses are summed up and form the manufacturing accuracy.Due to this, different systems and approaches have been devel-oped within the last few decades with the aim to minimize de-viations and to maximize accuracy. Yet, these approaches werealways linked to specific requirements. This section presents areview of practice-driven approaches to avoid deviations in pro-duction processes. The review focuses on CFRP production. Sinceautomation and, in particular, approaches to increase accuracy arerather sparse in this field of industry, the review also considersmachining industries like milling, cutting, turning and handling.The approaches to increase accuracy found in the literature can be

mostly classified into four groups:

� general automation through robot-based systems,� signal filtering or modifying drive control,� machining calibration and error compensation, and� integration of smart-structure systems.

In the following, the improvements and drawbacks of these fourgroups are discussed.

3.1. General automation

General automation approaches are characterized by the ap-plication of machines instead of humans. A robot-based approachfor rapid fabrication of composite structures is presented byKaufman et al. [31]. The structure does not require a mold, butconsists of layers. A variable and rapidly changing tool shapes thesurface layer. The robot performs the positioning. An automationapproach to manufacture CFRP components is exemplified inFig. 3. The manual task of material layup in a mold (2) becomesfully automated by the industrial robot (3), which is equipped withthe layup-head (1). Ahrens et al. [32] present an automation ap-proach for the fiber placement process with thermoplastic mate-rial. A 6-axis industrial robot was used. With a carrying capacity of200 kg at speeds of up to 5 m/s, a motion accuracy of more than0.1 mm and advanced equipment with new controllers similar toCNC machines, these robot systems are suitable for complex taskssuch as fiber placement. Kordi et al. [33] present a multi-functionalendeffector for the automated manufacture of textile preforms. Itconsists of various systems for gripping, altering and dropping thematerial. The endeffector can be adapted to fit the layup shape.The authors assume that thanks to this system, various manualprocesses could be automated. Consequently, the quality wouldincrease and the manufacturing cost would decrease. Never-theless, with certain tasks, human abilities like tactile and opticsenses are not yet replaceable by machines. To overcome thisdrawback, new sensing and actuation technologies are part ofcurrent and future research and development. So in order toqualify robot-based automated structures for sophisticated tasks,further approaches have to be implemented. One such approach isto filter or model the control signal of the drive engines.

3.2. Signal filtering or modifying drive control

A lot of research has been done in the last few decades toimprove the accuracy of automation tasks by modifying the con-trol signal of the drive engines. To give an example, the work ofEconomou et al. [34], McConnel and Bouton [35], Kapucu et al.[36], Park and Chang [37], Tao et al. [38], Hu [39], Ahmad et al.[40], Bachmayer et al. [41] and Huang [42] can be mentioned here.

2 Tool center point.

M. Perner et al. / Robotics and Computer-Integrated Manufacturing 38 (2016) 82–9286

Lee et al. [43] develop a polishing system that consists of a three-axis machining center and a two-axis robot. Replacing the com-mon proportional derivative control by a sliding mode controlwith velocity compensation the tracking error is significantly re-duced. Özer et al. [44] propose a semi-active approach to controlchatter during turning. The elbow stiffness changes with the nat-ural frequency of the two-link robotic arm and the spindle speed.This change is realized by the application of a counteracting torqueto the oscillations of the joint position error. As a result the chatterfeedback is damped. As a benefit, the method requires no addi-tional sensing device, since intervals are pre-determined by thespeed of the workpiece. With regard to the production of largeCFRP parts this approach does not seem practicable as the inter-action conditions of the robotic system and the workpiece changewith every placed fiber material. Mei [45] addresses the circum-stance with regard to boring manufacture. Because of differingglobal structural properties and the occurring backlash of machinethey proposes a feedback wave control design. A collocated ar-rangement of the sensor and actuator pair is used. The appliedforce controls the transmitted and/or reflected vibration modes.The authors show that damping is added to several mode shapesand that chatter is greatly reduced. Özer et al. [46] illustrate anapproach to suppress oscillations for metal spinning devices. Anindustrial robot is equipped with the spinning tool. The control ismodified by cascaded trajectory tracking algorithms and an addi-tional vibration suppressor for laboratory. The results show thatthe oscillation is reduced. Although huge improvements have beenachieved, the results of the approach also illustrate its limits. In thecase of work [46] wrinkles cannot be completely avoided. Forfurther improvement an optimal trajectory profile needs to beselected. For vibration suppression of robotic systems, Lopez-Li-nares et al. [47] study a scheme for the control of a multi-degree-of-freedom system. Good results are achieved by using the controlstrategy in a limited work space of the robot system, although theresults are not specified in detail. Besides using the scheme forvibration suppression, a smooth movement along the robot path ispossible. This is why the results show a significant decrease inspeed. The limit in increasing speed is due to the fact that thepositional deviation rises. Mohammadiasl [48] illustrates that byusing servo motors the mechanical backlash always generatesnon-linear behavior and unwanted vibration. It is often the causeof manufacturing deviations and vibrations occurring duringmovement and decay phase, i.e., after reaching the end point. Thehugest field of research and development approaches to increasethe performance of machining systems is the class of calibrationand error compensation methods.

3.3. Machining calibration and error compensation

Using machine calibration and pre-processing compensationmethods, the volumetric error is improvable to up to one order ofmagnitude [18]. These methods are most effective when the de-viations in the process are systematic and repeatable [18,25]. A lotof research concerning these approaches has been done in the lastfew years. To give an insight into this group of methods, its effortsand drawbacks are illustrated in the following by using someexamples.

Within the group of machining calibration and error compen-sation, the aim is to generate a corrected or modified motiontrajectory based on previous measurements of deviations withregard to the system's behavior. Therefore, concepts for modelingquasi-static errors have been developed in the work of Kiridenaand Ferreira [28,49,50]. Simulations show that the approach isvery promising since the error after compensation in this parti-cular case is less than the resolution capability of the system.Another approach is proposed in the work of Abele et al. [7] for the

case of industrial robots in milling applications. A mathematicalmodel reflects the static and dynamic interactions between robotsand milling structure. Abele et al. [7] point out that the staticdeviations can be computed in preparation by simulation and canbe reduced significantly through modification of the milling pro-gram. In general, after the pre-measurements, a new “compen-sated” motion track is programmed using several correction values[12,26,51]. Thus, path planning appears to have a huge impact onthe ability to perform a vibration-free motion [52].

König et al. [27] propose the use of previous measurement runsfor the evaluation of an optimized process motion. The main dif-ference to the method described above is that the driving tracksare pre-calculated so that the track with the best performance, orwith the lowest error between the programmed and the actualpath respectively, is chosen for processing. The simplest way tocompare the measured errors is to examine mean deviation forcertain defined points. Different driving tracks are evaluated for aportal robot [27] in order to optimize the processing-time in theproduction of large carbon-fiber-reinforced structures. They con-clude that the main deviations are caused by the varying vibrationand damping behavior of the structure within its working area.

Currently, calibration methods reach their limits in the com-pensation of errors generated by the rotary axis. This might ex-plain why nowadays only a few approaches are presented in theliterature [20]. According to them, the error parameters of eachrotary axis can be determined separately at individual positions inthe workspace, thus yielding a modification of the traversingprogram. The authors emphasize that this method is suitable forthe compensation of geometrical deviations. Still, deviationscaused by to processing loads require a different approach. Theresults of the work of Kiridena and Ferreira [28] show that theamount of required compensation depends upon the position ofthe machine, which is why a simple correction measure such as aconstant offset factor is not feasible. By a recursive subdivision ofthe path motion into finer sections, the number of program blocksincreases that have to be loaded into the machine. Consequently,the program is longer. With regard to the feasibility and the ac-curacy of the compensation, the authors recommend the in-tegration of additional sensors at the TCP2 to measure vibrations.

To suppress disturbances, approaches using smart structurestechnology focus on industrial operating conditions. Based on thedevelopments in the field of multifunctional materials, electronicsand control devices, the class of smart structures systems is ap-plied in research platforms. Furthermore, research attempts tocomply with the need for application in practice, i.e. in the in-dustry. Since an increasing amount of research work focuses onthe use of smart systems in automation, certain approaches arediscussed next.

3.4. Approaches using smart devices

Devices become smart through a comprehensive system con-sisting of sensing elements, a control unit and actuation elementsintegrated into the conventional structure in a high degree. Due tothese components, the dynamical behavior of the structure can beadapted by means of actuating forces. Traditional approaches suchas the addition of damping or stiffening methods to the structurealways lead to an increase of mass. Using appropriate smart de-vices can lead to the same results, but with less mass and lowercosts of the structure [30]. However, the use of additional sensingelements for the detection of the interaction force seems difficultand ineffective in practice. Tungpataratanawong et al. [53] claimthat one major cause is the need for an essential conceptual

M. Perner et al. / Robotics and Computer-Integrated Manufacturing 38 (2016) 82–92 87

change of current industrial robots in order to make them com-binable with all actuator systems. Hesselbach and Helm [24] givean insight into smart structures approaches to influence the vi-brational behavior of technical systems. Multifunctional actuatorsand sensors can be integrated into the mechanical structure. Dri-ven by a robust control strategy and using fast digital signal pro-cessors, vibrations can be reduced. Hesselbach and Helm [24]claim that discrete actuators or encapsulated active groups aremost effective at improving existing systems in terms of their vi-brational behavior. The largest gain in performance in terms of thecontrollability of the entire system can be achieved by using dis-tributed active functional components already included in thedesign of the technical system. To realize this approach, knowl-edge of the dynamic behavior of the purely passive system is ne-cessary. Furthermore, the interaction between the multifunctionalsmart structure component and the conventional kinematic en-gine comes into focus. Multi-functionality is a key advantage ofsmart structures systems. Different applications are illustrated inthis section to show their flexibility under operating conditions.The review mainly focuses, but is not limited to, the application inrobotic devices and manufacturing.

3.4.1. Linear robots and machinesMostly, the piezoelectric effect is used to measure strain or to

apply bending moments into a structure. In general, piezoelectricmaterial is encapsulated and bonded to or mounted on thestructure. Using the inverse piezoelectric effect, the structure'srigidity or stiffness can be adapted. In the paper of Chang et al.[54], a piezoelectric actuator is used to control vibrations of alinear robot. Due to rapid motions, structural resonances of therobot are generated. The vertical forces exceed the tolerances. Theactive suppression satisfactorily covers up to seven eigenmodes byusing PZT3 stacks, high voltage power amplifiers, accelerometersand a control device. Sahin et al. [55] carry out theoretical andexperimental studies with regard to the vibration control of smartstructures. The test setup consists of a smart cantilever beam and asmart fin, which is a plate-like part. Both structures comprisesurface-bonded PZT patches. Free and forced vibration experi-ments are examined, considering open-loop and closed-loopcontrol. The better performance of the closed-loop control incomparison to the open-loop control is shown to illustrate theefficiency of PZT patches for usage in vibration control. Reis andCosta [56] present a study of an actively bendable link for use inrobotic manipulators. Piezoelectric film actuators (MFC4) are em-bedded in this element, which is made of polycarbonate. Move-ment-excited vibrations are damped and controlled. The simula-tion results show that the settling time of the controlled system isdecidedly shorter than that of the uncontrolled system. Laux [57]developed a high-dynamic actuator for an active bearing. Thissystem consists of a hydraulic lever transmission, a piezo-stackactuator, a control gain and a displacement sensor. A prototype isintegrated into a passive power engine bearing to optimize theacoustic properties. Using the proposed active vibration isolationstrategy, the introduced interference force is reduced.

As mentioned in Section 1, robotic approaches are focused onfor an automated CFRP layup. Typically, current approaches tocompensate for operational vibrations of similar technologies arereviewed for the feasibility of their application. Milling, turning,painting, picking-and-placing and welding are industrial fields inwhich robots, in particular articulated robots, are used for motiontasks. To affect the accuracy of a robotic system in a positive way,external sensors, actuators or special innovative calibration

3 Plumbum zirconate titanate.4 Macro-fiber composite.

methods are required [58,59]. In the case of force closure betweenthe robot and the workpiece, only a limited amount of approacheshas been investigated in prior work. Most optimization approacheswith regard to the mechanical structure consider specific solu-tions. A standard solution that is independent of any specific robotcontroller has not been developed so far.

The aim of the work [60–62] is to develop an actuator tocompensate for positioning and tracking errors. Static errors inpositioning and dynamic path errors are considered. The articlesfocus on production machining such as milling. The three-di-mensional compensation actuator is based on piezoelectric ac-tuators and flexure hinges for displacement amplification. Refer-ring to prior studies, the author notes that the position of the robotin the work space only has a minor influence on the structuralbehavior. In experimental studies, the industrial robot covers thecomponent handling. The spindle is placed at the compensationmechanism and both are mounted on an additional stand. Theperformance is checked during milling tests along a 60 mm profile.The static feed precision is improved and the profile errors de-crease [63]. A detailed description of the control development forthe configuration is presented in the work of Schneider et al. [64].

Wittstock [65] and Neugebauer et al. [66] examine a piezo-electric actuator-sensor unit (ASE) to compensate for vibrations inthe drive chain of machine tools. For this purpose, they introduceconcepts for uniaxial vibration compensation and the compensa-tion of torsional vibration. An important aim of the work is todescribe a structured approach to develop piezo-ceramic actuatorelements. The results of the experiments show that the devicesoperate well at very low disturbance amplitudes. With increasingdisplacement ranges, the units reach their limits.

Tian et al. [67] present a manipulator that consists of two frontpiezo-actuators and a lever mechanism for displacement amplifi-cation. The lever is constructed with flexure hinges and doublesthe maximum displacement range. It is observed that the accuracyand resolution of the manipulator mainly depend on the me-chanical design, the used actuator and sensor technology as wellas the control strategy. The authors follow the approach of repla-cing conventional kinematic elements by flexure hinges. Apartfrom the possible backlash- and friction-free operation, they seethe negligible hysteresis behavior as a decisive advantage. Fur-thermore, assembly errors can be reduced by designing a mono-lithic actuation device. Studies with regard to the frequency rangeare addressed in the work of [68]. The first and second naturalfrequencies of the system are determined to be at 443 Hz and532 Hz. Hence, very dynamic positioning operations are feasiblewith this system. The force that can be provided by the fully in-tegrated system is studied in [69]. Experiments show that thestiffness of the mechanism is in the range of 0.8 N/μm.

Rudolf et al. [70] present a concept of a strut with piezo-basedactuators to compensate for geometric errors in parallel-kinematicendeffectors. The strut is divided into two parts and in the middlepart there are two piezo-electric units. One is used as a sensor andthe other one as an actuator. In addition to the basic analyticalmodel of the compensation unit, the authors show first simulationresults that successfully demonstrate the sensing and actuatingeffect. Due to the actuation effect, an adaption of the strut's lengthis possible. Therefore, it can react on deviations due to geometricerrors or process loads. The work of [71,72] expands these studieswith an adequate control concept. The control task is to com-pensate for the rotational deviations of the parallel-kinematicendeffector. The authors recognize that the performance of theproposed mechanism directly depends on the number and posi-tion of the integrated active struts. Studies of the system's beha-vior with an integrated strut show that the natural frequencies aresmaller due to the stiffness losses caused by an active strut.However, any additional integrated active strut reduces the

M. Perner et al. / Robotics and Computer-Integrated Manufacturing 38 (2016) 82–9288

eigenfrequencies of the system only to a very small degree. Theauthors confirm that the arrangement is suitable for the com-pensation of static and quasi-static processes. In the work ofFleischer et al. [73], the active strut is implemented in a test rig.The authors emphasize that the assembly and manufacturingtolerances and geometric machine errors cause quasi-static de-viations. Low frequency deviations can be attributed to stresscaused by the gravitational force or process loads. Dynamic factorsare influenced by the distribution of mass and the stiffness of thesystem. The studies show an improvement of the accuracy. As afurther application, the authors suggest the use in industrial robotsto improve system rigidity.

3.4.2. Parallel robots and mechanismBesides serial kinematics, parallel robots are focused on for

handling and assembly tasks. In contrast to serial kinematics, notall joints carry drive engines. The drive engines are usually locatedat the platform. Therefore, the moved mass is smaller and high-dynamic tasks can be addressed. As described in the work [74,75],smart structures’ technology is used to improve the structuralbehavior of parallel kinematics. In detail, residual vibrations dur-ing the acceleration and the deceleration phase of the handlingtask are suppressed. Passive rods are replaced by piezo-patch in-tegrated rods. Finally, an adaption of the length of the rod is in-troduced into the structure to compensate vibrations.

In the case of high precision turning operations of a mobileplatform, Tian et al. [76] propose a piezo-based device. A parallelmechanism that consists of flexure hinges pre-stresses the ac-tuator and directs its forces to guide the platform. No backlash,negligible friction and low maintenance in operation are some keyadvantages of the piezo-driven mechanism mentioned by theauthors. Besides, the first natural frequency equals 1122 Hz, thecontrol loop has a low damping rate of 0.05, which qualifies thedevice for high-dynamic processes.

3.4.3. Other solutionsAnother idea is to use additional dampers for vibration re-

duction. In the work of Olaru [77], the dynamic behavior of aportal gantry robot is examined. First of all, the most importantcomponent in the dynamic frequencies is established by de-termining the transfer function among the endeffector, the portalstructure and the stable plate. Thus, a model for the calculation ofthe robot's behavior can be established. Varying damping com-ponents are introduced into the model to change the rigidity be-tween the single components. The endeffector's vibration magni-tude becomes very small in the low frequency domain through theidentification of the optimum vibrational behavior through simu-lation. Zhang et al. [78] show the potential of a variable viscousdamper system based on electro-rheological fluids for the use as amethod of vibration suppression for a robot arm. They found outthat both resonant and anti-resonant phenomena can be effec-tively reduced and that, in addition, the reaction to disturbanceshas been improved overall. Robinson et al. [79] develop and im-plement a method for pneumatic-powered artificial muscles forrobotic manipulators. Apply pressure to the interior of a softbladder causes the diameter to increase, while the length of theactuator simultaneously contracts. Thus, contractile lifting andpulling, similar to human muscle, is made possible. Muthalif andLangley [80] present a hybrid approach for the active control ofhigh-frequency oscillations with Skyhook dampers. The metho-dology gains efficient predictions for the behavior from the middleto high-frequency range. By applying an active concept, the vi-brational energies are reduced by approximately 2 dB.

Aguiar et al. [81] describe an investigation on vibration re-duction using another class of smart materials. Shape memoryalloys are known to produce high forces and displacements. The

authors point out that this class of material has a complex beha-vior. The tests show that the useful range for operation is de-termined by a compromise between stiffness and hysteretic be-havior changes. Thus, the control for suppressing vibrationthrough temperature induces both a change of stiffness and achange of hysteretic behavior. Changing hysteretic behavior leadsto a reduction of vibration because the degree of energy dissipa-tion is modified. Due to shifting resonant frequencies, changingthe stiffness of the system also yields to vibration suppression.Therefore, Aguiar et al. [81] recommend the comprehensive con-sideration of both issues in further research.

3.4.4. SummaryMost realized approaches are on the research level and have

not been applied in practice, yet. The limiting factors are additionalcosts due to the necessary system development, e.g. for a robustand stable control and for fail-safe mechanisms. Casciati et al. [82]review the publications on active and semi-active control ofstructures from the years 2009 to 2011. They point out that theexisting technology in the civilian and infrastructural sector existsonly on a theoretical level. The concepts are only haltingly utilizedfor a serial production or for everyday use because of the need forthe implementation of intensive maintenance and due to actuatorswith a lack of performance. This is why research needs to focus ona simplification in the modeling phase of the control loop. Anothercomprehensive review is provided by Datta [83]. The work espe-cially focuses on active control schemes for structures excitedthrough earthquakes. Therefore, several examples of active-vi-bration-suppression approaches as well as the limitations for im-plementation in practice are given. For instance, the modelingerror, the time delay, the limited amount of sensors and con-trollers, and the parameter uncertainties in system identificationare identified as obstacles for real-time application. These chal-lenges have to be mastered in order to allow for a more wide-spread acceptance and a growth in industrial application.

4. Identifying trends for the future: promising fields ofresearch

Increasing the accuracy in automation is a widespread fieldthat concerns a huge amount of disciplines. Nevertheless, accuracyin processing is not the key factor for future developments. Theperformance actually is a compromise between speed in produc-tion and achievable accuracy. Only manufacturing plants with thehighest performance are able to fulfill future demands. In addition,degraded parts have to be avoided in order to minimize materialand energy costs.

In order to find the most reasonable compromise, various ex-perts are studying the topic of increasing precision and speed. Dueto the increasing performance, the focus for the future is to de-velop highly specific robotic systems. The idea of a system forgeneral purposes with the highest performance in terms of pre-cision and speed seems to be ambitious. There will always be acompromise due to uncertainties of the plant, to boundary con-ditions, to geometric errors, to kinematic errors, to thermal be-havior and to process loads. In the case of manufacturing com-posite structures, future machines need to provide a higher flex-ibility to deal with components of different geometry and size[84]. Hence, the geometrical dimension of the workplace is an-other key factor. Large workspaces lead to variations in the dy-namical behavior of the system. Yet, the imperfection of themeasuring device depends on the operating area as well.

The machines need to be profitable after only a few years. Ac-cording to Biermann et al. [84], processes should be developedwith research machines and improved regarding their capability

Active Vibration Suppression Technology

Modifying Drive Control

Modifying Motion

Trajectory

Integration of Smart Structures

Systems

Fig. 4. Promising approaches for the integration of active vibration suppressiontechnology in manufacturing processes.

M. Perner et al. / Robotics and Computer-Integrated Manufacturing 38 (2016) 82–92 89

for series production. Apart from the high lightweight potential offiber composites, the authors predict a promising advance inknowledge and technology, if research processes will be industrialstandardized. Another issue is to link these single productionprocesses to a consistent process chain likewise described in thework of [10,85,86]. Because the process planning level is highlyspecific with regard to the process requirements, manufacturingsystems adaptable by means of the production planning andcontrol methods form the largest part of research in the paper ofWiendahl et al. [87]. In addition, the control of the compactionforce in cases of non-orthogonal alignment of the layup head tothe surface is another research topic for future composite manu-facturing. Path-tracking and vibration control have to provide theprecise motions of the system in production.

Regarding the trends of future automation, active vibrationsuppression technology is one key factor to push the compromisebetween the speed in production and the precision of the technicalsystems to higher levels. Based on the review given in this paper,the approaches for active vibration suppression with regard tofuture industrial application can be divided into three sections:modifying drive control, modifying motion trajectory and in-tegrating smart-structures systems (cf. Fig. 4).

In the case of modifying the drive control, a huge amount of theresearch work is focused on input shaping. The input signal for thedrive engine is modified appropriately. As a desired result, vibra-tions occurring during and after the movement are reduced. Hu[39] recommends the combined use of input shaping and structurefeedback controller with some active vibration suppression tech-niques. Thus, velocity feedback control strategies are limited totwo-axis robotic systems. Besides, Lee et al. [43] advise to usesliding mode control instead of the commonly used derivateproportional method. For a further reduction of vibration duringand after a motion, Hu [39] proposes to study the use of piezo-electric materials.

Currently, research focuses on more general methods, e.g. inthe work of Lopez-Linares [47]. This approach could very well leadto appropriate applications with smart-structures technology.Puzik [60] proposes the separation of low- and high-frequencyerror components. An expansion of the drive control in order tosuppress vibration with a low frequency is pointed out for futurework. To indicate and finally prevent backlash in the joints, the useof additional sensing elements is advised in order to enlarge theresolution of the control. To verify appropriate sensor signals,Puzik [60] refers to the integration of an optical sensor system thatenables the system to detect the position of robot movement in avery precise way and even for high frequency. The integration of aposition-force visional control is another major topic in cases oflayup and transport robots for composite manufacturing.

In the field of active vibration suppression, the integration ofsensing and actuation approaches for force-controlled processeswith industrial robots is another important research aim. Theobjective is to provide a vibration-free path-tracking system usingappropriate smart structures technology. The amount of work inthis particular field of research is limited so far. Impeding factorsfor an industrial application are the lack of its integration intocurrent mechatronic devices such as robotic systems. In addition,

the robustness and reliability of the control device is affected byvarying operating conditions and the influence of a large work-space. The dynamical behavior of a robotic system changes withina certain workspace region. As has also been described in the workof Algermissen et al. [74], switchable control parameters forvarying machining behavior show the suitability of the controlstrategy for its application in practice. Agnes and Mall [88] con-sider existing literature dealing with the modeling and applicationof vibration suppression methods using piezoelectric material.They note that knowledge about fatigue and crack behavior inadaptive composite structures is scarce. Some topics, like themodeling of non-linear material behavior or the influence ofvarying environmental and load conditions, ought to be studied inmore detail.

Puzik [60] proves that a three-dimensional actuation element issuitable to compensate for position and path errors in millingapplications. Therefore, future tasks are its preparation for in-dustrial application, safety concerns for continuous operation andhigh voltage issues. In addition, concepts with regard to the stiff-ness and the dynamic robot model need to be studied. As a result,an accuracy improvement of the actual position and posture of therobot is to be expected.

As the maximal stroke is the limiting value for the performanceof the piezoelectric compensation mechanism, the application ofvibration suppression is commonly restricted to small amplitudesof disturbance. Neugebauer et al. [66] sum up various methodsand requirements for the maximal stroke in the piezoelectriccompensation mechanism. So there is a need for novel approachesto amplify stroke in future works.

5. Conclusion

Advanced automation in the manufacturing technology of CFRPis a major topic because the demands for the new-generationaircraft have increased during the last few decades. Currently,restrictions call for a low-emission, less power-consuming andhighly cost-efficient production. The use of CFRP parts in aircraftdesign is seen as a key factor to reach future demands. Nowadays,a tradeoff between the scale of future aircraft orders and theachievable production rate of CFRP components looms ahead.Therefore, innovative concepts for future production plants are abig issue in research in this field of industry [89,90]. Another as-pect in research is providing the necessary accuracy of the process,since the layup result determines the mechanical characteristics ofthe final part. Yet, increasing the speed in production is not sup-posed to affect the quality of the part. In contrary, the precision ofthe process needs to be improved even more. A first step to reachthis aim would be to increase the resolution of existing systems byusing additional sensing elements. Systems that detect deviationsfrom specifications are part of a huge research field. Based on aliterature review, a classification of machining errors in automa-tion is proposed in this paper. Apart from stationary geometricerrors, there are also dynamical and thermal errors. Stationarygeometric errors are caused by geometric variations, misalign-ments of components or coupling effects through joints. Dyna-mical errors are described by vibrations or forces caused bymovements or interactions. Thermal errors are provoked by ex-tensions of the bearings and axles. It becomes clear that a com-prehensive knowledge of the system's behavior is necessary. Thisis why so many studies with various approaches have been illu-strated and discussed in this paper. The approaches for active vi-bration suppression are mainly classified into three groups –

modification of the drive control, modification of the motion tra-jectory and integration of smart structures systems. To achieveprecision improvements, highly integrated actuating and sensing

M. Perner et al. / Robotics and Computer-Integrated Manufacturing 38 (2016) 82–9290

elements are one of the most promising research fields. Accord-ingly, smart structures technology is considered more closely,particularly for CFRP automation. A smart system consists of asensor, an actuator and a control unit that are ideally integrated orbonded on the structure itself. Therefore, the actuator is integrateddirectly into the load path and takes advantage of the elasto-me-chanical properties of the used material. Currently, the amount ofwork considering smart systems for automation is small. Studiesare mainly limited to laboratory demonstrators, to simulation re-sults and to system-modeling approaches [91,74,60,66]. In order torealize a smart system, a capable integration strategy in the ma-chine structure under operating conditions needs to be addressedfirst. Future studies ought to be based on the knowledge fromprevious works. In particular, integration methods for actuatingelements and placement strategies of sensing elements need to bequalified, adapted or established for the use in CFRP automatedmanufacturing. Also, concepts for the detection of disturbancesand deviations from target values need to be addressed, as in thework of Lammering et al. [92].

Still, there are objectives that limit the performance of smartstructures systems. For instance, the maximum stroke of about0.1% in the case of piezoelectric material defines the displacementlimits. Hence, only small amplitudes in the practical range of 1 mmcan be realized. Piezo-based actuators find their limits in thecompromise of stroke and applicable force. Thus, the resolution ofabout 10 nm is much better than that of macro-systems. Macro-amplitudes in terms of deviation compensation can be achieved,for instance, by using process and motion planning algorithms aswell as drive control adaption (as described in Section 3). Never-theless, the integration of piezoelectric elements for sensing andactuating tasks is promising with regard to dynamic response,applicable force and resolution. The review shows that active vi-bration suppression is a primary research topic regarding possibleincrease in the productivity and reliability of technical systems,particularly with regard to ambitious quality requirements. It isthese quality requirements that push the compromise betweenspeed in production and attainable precision to ever higher limits.

Acknowledgments

The review about the use of active vibration technology in CFRPproduction is part of the project GroFi, which is supported by thestate of Lower Saxony, Germany. GroFi is a research project toimprove the automated fiber placement process, especially forlarge composite structures, in the fields of productivity, costs andquality.

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