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Lightweight Design worldwide

, Volume 10, Issue 6, pp 22–27 | Cite as

Carbon Arm Improves Dynamics of Small Robots

  • Norbert Müller
  • Paul Zwicklhuber
  • Georg Steinbichler
Construction Robotics
  • 171 Downloads

The e-pic small robot was developed by Engel Austria with the goal to offer a flexible, powerful, dynamic yet energy efficient solution for easy part removal and depositing of small moulded parts. The key to this lies in completely new kinematics and an innovative lightweight design.

The special feature of the e-pic robot is its swivel arm, Figure 1. It moves in the direction of the x-axis, which consequently completely disappears and essentially merges with the y-axis to form a single unit. Mounted on an injection moulding machine, the robot needs considerably less space than a linear robot on both the injection and clamping side of the machine. It can be integrated within the injection moulding machine’s safety guarding, thereby keeping the entire production cell compact, Figure 2. In contrast to the x axis of a linear robot, the swivel arm can quickly and easily work around obstacles such as core pulls, slides etc.
Figure 1

The e-pic robots have a lightweight swivel arm and are thereby capable of achieving dynamics and energy efficiency (© Engel Austria)

Figure 2

The e-pic robot was designed for use on injection moulding machines; the new kinematic system allows for particularly compact production cells (© Engel Austria)

The e-pic robot makes history, not only in terms of its functionality, but also its production process. For the first time, composite technologies have been used to reduce the weight of the moving masses and thus achieve superior dynamics and energy efficiency. Early in series production, some e-pic robots were built with an aluminium swivel arm. Compared to the already very light aluminium design, the composite material reduced the weight of the robot’s rotary axis by 37 %. Particularly in the market segment of simple pick-and-place applications, the dynamics and efficiency are among the main decision-making criteria. When an injection moulding process is automated, the total cycle time must not increase compared with conventional free falling demoulding.

Thermoplastic Tapes on their Way to Series Production

A completely new process was developed for the production of the composite arm, using thermoplastic tapes with carbon fibre reinforcements, Figure 3. While the first production methods using thermoplastic fabrics have reached the series production stage in the vehicle and sports products sectors, processing of thermoplastic tapes is still at the beginning of its industrialisation process.
Figure 3

The newly developed inline-capable tape laying cell by Engel includes two fast robots as well as magazines for separating the pre-cut tape (© Engel Austria)

The prefabrication processes are easily automated and can be implemented as an integral part of the tape-laying cell.

In the field of tape laying, thermoset processing leads the way. Pre-impregnated, rolled, and unidirectionally reinforced fibre composite materials, which are offered as pre-pregs, have established themselves. Thermoplastic variants of the tapes with matrices of PA, PP, PE, PC-ABS, PPS and PEEK can be processed far faster, but require an entirely different process technology, which in turn is closely related to the processing of thermoplastic fabrics [1].

Thermoplastic fabrics contain one or more layers of fabric and are available for purchase, as water-jet pre-cut parts, tailored to the respective product. For processing, the thermoplastic fabrics simply need to be heated in a controlled manner using an infrared furnace and then inserted into the injection mould, where they are reformed, reconsolidated and functionalised by injection moulding.

In contrast to the thicker, mostly multi-layered thermoplastic fabrics, tapes with carbon or glass fibre reinforcements have a thickness of just 0.14 to 0.3 mm. Therefore, a single layer of tape is not sufficient for most applications. Instead, layups are built from multiple, sometimes up to 20, layers of tape. In this way, components made of carbon tapes can achieve a wall thickness of around 3 mm.

Tape Layups in the Processing Cycle

Layup design is part of the component design. The objective is, on the one hand, to achieve optimal performance using as little material as possible, while keeping the number of tapes manageable on the other. It is not always possible to combine lightweight design and economic feasibility. While the optimal design of the mechanical structure leads to a layup with many small tapes, their arrangement and number based on the load path must be limited to a practical level for economical production.

Therefore, considerations relating to the productivity of the entire system are made at an early stage of process development. The expected component weight, and an estimate of the extent to which the finished part will be made up of tapes, Table 1, provide some direction on the performance requirements for the tape-laying cell. The production speed of the tape-laying cell must be based on the production cycle of the processing machine. If tape laying requires far more time than processing, this step becomes the limiting factor for the entire chain of production. For the overall system to be able to operate economically, the number of laying operations must be sensibly limited and minimum formats must be defined for the tapes. It must also be noted that a major part of the overall structure must consist of tapes to achieve a meaningful lightweight design effect.
Table 1

The number of laying operations required to create a tape layup can be estimated based on the component weight, the tape fraction and the tape formats (tape: PA-CF, thickness 0.14 mm, density 1.46 g/cm3) (© Engel Austria)

Component weight [g]

100

250

500

1000

Tape ratio

40 %

60 %

80 %

40 %

60 %

80 %

40 %

60 %

80 %

40 %

60 %

80 %

Format 300 x 450 [mm]

1.4

2.2

2.9

4

5

7

7

11

14

14

22

29

Format 200 x 300 [mm]

3.3

5

7

8

12

16

16

24

33

33

49

65

Format 140 x 210 [mm]

7

10

13

17

25

33

33

50

67

67

100

133

Format 100 x 150 [mm]

13

20

26

33

49

65

65

98

130

130

196

261

Format 60 x 100 [mm]

33

49

65

82

122

163

163

245

326

326

489

652

Laying process (3 s per laying operation): green: up to 1.0 min | yellow: 1.0 up to 3.0 min | orange: 3.0 up to 5.0 min | red: more than 5.0 min

For a component weight of 250 g, for example, the tape sections should have an average area of at least 300 cm2. Consequently, around 20 to 30 laying operations are sufficient to create a layup with a weight of around 100 to 200 g, which corresponds to a tape weight of 40 to 80 % compared to the total component.

With a cycle time of approximately 60 s for the injection moulding process, the main requirement for a tape laying cell is that it needs to lay tapes, and spot weld them together, at intervals of 3 s for tape dimensions of up to approximately 300 x 450 mm. The new conceptual approach to meet this requirement is based on the pick-and-place principle.

The way the tapes are prefabricated, that is, the preparation of the tapes for the laying process, is based on the technical and economic criteria. The most important design parameters are the least possible waste, maximising the size of the tape cuts and the best possible arrangement of the tapes in the layup in terms of the mechanical structure. The prefabrication processes are easily automated and can be implemented as an integral part of the tape-laying cell. Also, the tapes can be pre-cut before feeding into the tape-laying cell.

An upstream prefabrication step — such as straight or oblique cutting from the roll, die cutting with curved end contours, punching of almost any shape, and cutting tapes using ultrasonic knives — produces blanks that can be stored in magazines, or directly processed without waiting.

Component Design Reflects the Load

The design of the rotary axis on the e-pic robot was driven by structural-mechanical performance (lightweight design effect) and low tooling costs. A solution was selected that involved creating the component from two identical half-shells, which are joined to create a closed profile during final assembly. Only in the area of the connection to the servomotor an aluminium die-cast part is used; the component geometry is highly complex and the rotational speeds in this area are low due to the distance from the centre of rotation, Figure 4.
Figure 4

The rotary axis was designed to use welded carbon tape profiles and a die-cast aluminium stub for connecting the servomotor (© Engel Austria)

The majority of tapes used are inserted longitudinally to the rotary axis in order to meet the high stiffness requirements. In addition, several layers of tape are laid at angles of ±45° and 90° to the longitudinal axis. The most favourable design for the required load cases was determined by means of FEM calculations, Figure 5. The results of this design work were stored in the plybook, which serves as a guide to the layer structure of the tape layups.
Figure 5

The load-bearing design of the rotary axis was based on FEM calculations (© Prime aerostructures)

In the multiple-stage production process of the rotary axes, tape laying is followed with a consolidation step, with heating and shaping of the consolidated tape layup, and the trimming and joining steps, Figure 6.
Figure 6

The process for manufacturing the rotary axes consists of six steps: building the layup (tape laying cell), consolidating the layup (heating/cooling press), heating the layup (IR heater), forming (reforming station), trimming (water jet cutting machine) and assembly (gluing station) (© Engel Austria)

Precision in the Laying Process

Four different tape formats are required for the rotary axis. A half-shell consists of 32 individual, unidirectional reinforced tapes. A fast articulated-arm robot is used to achieve the required high dynamics in the laying process. The tapes are retrieved from the magazines as needed, regardless of the actual laying process, so that the deposit speed of the robot is not reduced by slow removal operations.

With fast and high-resolution camera technology, the positioning accuracy of the individual tapes can be significantly improved.

The quality of the tape layup depends greatly on the precision of positioning the tape. Where tapes overlap, pronounced fibre displacement occurs during the consolidation phase; for example, the fibres shift sideways to compensate for local thickness differences in the tape layup. If the tapes are laid with a spacing, linear areas without fibres occur during consolidation that have a reduced stiffness and strength; these are referred to as matrix gaps. In contrast to thermoset tape layups, the gap between the tapes often does not fully close during consolidation because the viscosity of the thermoplastic matrix is several factors of ten higher than that of, for example, epoxy resin. It is therefore essential to lay the tapes precisely despite a high deposit speed. Often positioning accuracies with gap widths or overlaps of 1.0 mm are required, and in some cases even less than 0.5 mm.

The dimensional deviations when depositing the tapes partially result from inaccuracies in cutting or punching the tape. The tape stacks may also not be aligned very accurately in the magazines, and the mechanical system for separating and picking up with the laying head can lead to other positional deviations. In particular, shape variations which already exist in the pre-cut parts cannot be eliminated through high precision. Instead, the system must detect and compensate for errors as much as possible to ensure that the tapes are stored precisely and with optimal alignment. Achieving minimum gap widths or overlaps has a higher priority than a precise outer contour of the tape layup.

Optical Metrology for High Precision

With fast and high-resolution camera technology, the positioning accuracy of the individual tapes can be significantly improved, Figure 7, by determining the tape position either at the pick-up position, for example after separation, or on the laying head. In order to precisely determine the position of the tapes, reference elements are required at the pick-up position or on the laying head, the position of which is precisely known to the robot coordinate system. A digital image of the tape is captured and, using a threshold value method, converted into an image that divides the image content into segments, using the Otsu method [2]. Further calculations provide information on the edges and centroid of the tape with respect to the reference coordinate system.
Figure 7

An optical video measurement system was integrated into the pilot system to determine the actual position of the tape on the laying head (© Engel Austria)

The measurement results in correction values for the position and angular position, which are determined separately for each tape; these results are transferred to the robot control and taken into account during laying. One of the key factors for accurate edge detection is the lighting when capturing the image, Figure 8.
Figure 8

Testing direct and indirect lighting for image capture: Lighting is a key factor in accurate edge detection (© Engel Austria)

The first tape layer is held on the depositing table by a vacuum. The further layers are each spot welded onto the underlying layer of tape. Electrical heating cartridges help to bond the tapes together in just 0.2 s.

Conclusions

The laying technology, with optical image processing and depositing based on the pick-and-place method, is highly flexible. There are no restrictions to specific tape widths. On the contrary, the pre-cut tape parts can have almost any contour, shape and size. Deviations resulting, for example, from tape production or the cutting process are detected using image processing and effectively corrected, ensuring a high-quality tape layup for downstream processing. Because the layups are given the shape required for the component during laying, functionalisation can occur after consolidation in the injection moulding step without the need for intermediate trimming.

The new technology for creating tape layups is a milestone on the way to more widespread use of thermoplastic tapes in industrial lightweight design.

References

  1. [1]
    Rettenwander, T.; Zwicklhuber, P.; Fischlschweiger, M.; Steinbichler, G.: Maßgeschneiderte Composites für hohe Stückzahlen (Tailored Composites for High Volumes). Kunststoffe 104 (2014), No. 10, p. 138–143Google Scholar
  2. [2]
    Burger, W.; Burge, M.J.: Digitale Bildverarbeitung (Digital Image Processing), chapter 11: Automatische Schwelloperationen (Automatic Swelling Operations). Berlin/Heidelberg: Springer, 2015Google Scholar

Copyright information

© Springer Fachmedien Wiesbaden GmbH, part of Springer Nature 2017

Authors and Affiliations

  • Norbert Müller
    • 1
  • Paul Zwicklhuber
    • 2
  • Georg Steinbichler
    • 3
  1. 1.Engel Austria’s Technology Centre for Lightweight Composite TechnologiesSt. Valentin Austria
  2. 2.Development Projects in the Field of Thermoplastic Fibre Composites for Engel St. ValentinAustria
  3. 3.Technologies Research and Development of Engel AustriaSchwertbergAustria

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