From AddLab to AddFab

From AddLab to AddFab

From AddLab to AddFab. This is the step NTS took with KMWE and Machinefabriek De Valk in the development of 3D metal printing, Together they founded a 3D printing factory with the focus on industrializing 3D metal printing.

Click here for more information.

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From AddLab to AddFab

From AddLab to AddFab. This is the step NTS took with KMWE and Machinefabriek De Valk in the development of 3D metal printing, Together they founded a 3D printing factory with the focus on industrializing 3D metal printing.

here for more information.

Presentation at DSPE Conference

Large signal non-linear analysis and validation of the suspension of a transport tool
Araz Abbasi, Joep Tax, Pieter Nuij
NTS Systems Development, Eindhoven, the Netherlands

In the design of ultra-high precision positioning equipment the design effort is mainly focused on the process performance of the equipment under operating conditions. External disturbance sources like floor vibrations, temperature fluctuations, particle pollution, air pressure variations and acoustic excitation etc. have a significant influence on process quality. The magnitude of these error sources however often is relatively small and one may realistically assume linear behaviour of the system. This assumption significantly eases the complexity of modelling. A completely different set of operating conditions is encountered during transport of the system or it’s modules from the manufacturing site to the customer site. Drop heights of 5 to 10 cm, horizontal impacts of 3 to 5km/h, temperature fluctuations of 15?C, air pressure variations of 350mbar, 2G static acceleration, 35? rolling/pitching angles etc. are realistic magnitudes of external disturbances which must be accounted for. Due to these large magnitudes linear system behaviour often is a fiction. Therefore, pioneering in the field of non-linear system design is required to deal with the extreme magnitudes of external disturbances as encountered in for example transport tooling for highly sensitive (optics) modules.

One of the main external disturbances during transport is vibrational input from the floor, representing the mean of transportation, e.g. a truck or airplane. During loading and unloading incidental shock related events occur. The floor vibrations have to be isolated and shocks have to be absorbed. A vibration isolation system is needed to achieve this. The combination of the minimum required amount of vibration isolation, shock absorption properties, and other constraints, such as the maximum allowable deflection of the isolation elements results in contradictory system requirements.

The vibrations requirements are based on the Power Spectral Density (PSD) of the input floor vibration (related to transport method) and allowable machine vibration (based on character of the protected precision equipment). Most high-tech customers provide their own input and output PSD spectrum. For shock, the input specification consists of a certain maximal drop height or a certain constant velocity before hitting a hard wall. The shock requirement specifies the maximum allowable rest shock level, i.e. maximum absolute acceleration experienced by the equipment.

Two of the main elements used in transport tool suspension systems are wire rope and elastomer isolators. Wire ropes are great for absorbing shocks (due to their highly nonlinear behaviour) but they act as a rigid connection for small random vibration input. Elastomers are able to provide reasonable damping for those vibrations but they do not provide the same big amount of damping during large deflection shocks.

This paper proposes a series connection of a wire rope spring and an elastomer isolator resulting in a hybrid damper. Such a well-designed passive isolation system combines and optimally uses the properties of these separate components. First a mathematical model of each component is presented and then model parameters are identified by performing measurements on a SOCITEC BFL-12 elastomer mount and a HH8-38TCM wire rope spring. The behaviour of the new hybrid damper configuration is analysed and design considerations are discussed.

Wire rope isolators
Wire rope isolators show both compliant and frictional behaviour. The bending of the wire strands results in compliant behaviour. The rubbing between the strands results in friction forces. Both phenomena characterize the force-displacement relation. The damping properties are determined from the resulting so-called hysteretic behaviour. This also implies that the response of a wire rope isolator is highly non-linear.

Figure 1: Wire rope spring and Iwan model

Figure 2: Elastomer isolator and Maxwell model

A wire-rope can be represented by Bouc-Wen as well as Iwan models. Iwan model has the advantage of having a more physically based structure. The model parameters are directly related to the force-displacement characteristic. This results in a better understanding and a less complex identification process. In this study, a parallel-series Iwan model structure is used [1] which is shown in Figure 1. The series spring and Coulomb friction elements are so-called Jenkin elements. The force through the symmetric wire rope is expressed by:
The characteristic force through a single Jenkin element is expressed in terms of its time derivative [2]. In this form, only one equation is sufficient to describe the behaviour of both the sping and friction element:

The Iwan model in this form cannot describe non-linear softening, hardening or asymmetry behaviour. A non-linear parallel stiffness instead of the linear term is introduced to allow for these effects. The resulting modified Iwan model will then be:
The non-linear spring force consists of a linear, a quadratic, and a cubic term. Hysteresis, softening, hardening, and asymmetry are all described by this modified Iwan model.

Elastomer isolators
Viscoelastic isolators are in most cases made out of rubberlike polymer materials, called elastomers. Viscoelasticity encompasses both viscous and elastic behaviour. Elasticity ensures stiffness, whereas due to the viscosity energy is dissipated. Viscoelastic behaviour is frequency and temperature dependent. The viscoelastic behaviour can be described by a frequency dependent complex Young’s modulus, consisting of a storage modulus (real part) and a loss modulus (imaginary part). The magnitude of the complex modulus and the ratio of the imaginary and real part are measures for respectively stiffness and damping.

A viscoelastic elastomer isolator is considered to behave like a viscoelastic solid. A generalized Maxwell model for viscoelastic solids consists of Maxwell elements and a single spring in parallel, as shown in Figure 2. The complex stiffness of this model is given by:
The loss factor of this viscoelastic model, the measure of dissipation, is given by:

Measurement and validation
Wire rope model
Two types of quasi-static measurements are performed with the wire rope isolator: large input global behaviour and small input local behaviour measurements. The Iwan model consists of a set of global parameters and local parameters. The global parameters describe the characteristic of the (non-linear) parallel spring, whereas the local parameters describe the Coulomb friction force and linear stiffness of the Jenkin elements. 

An optimization algorithm is used to fit the model on the measurement data and hence to identify the model parameters. The deflection vector is assumed to be known input, the force vector is output. Moreover, the objective function to be minimized for each set is defined as a relative root mean square (RMS) error of the force value.
Figure 3: Wire rope Iwan model fit for small deflections                               

Figure 4: Wire rope Iwan model fit for large deflections

The optimization for an Iwan model of order 2 is shown in Figure 3 for a small deflection set and Figure 4 for a large deflection set. The mean error over the measurement sets for small deflections is reasonable (7.70%). The most significant deviation is seen for increasing deflection, the force is slightly underestimated in this region. The mean error over the large deflection sets is 7.07 %.

Elastomer model
Quasi-static measurements are used to determine and validate the static stiffness of the elastomer mount (Figure 5). Furthermore, possible amplitude dependent behaviour can be detected. The behaviour is observed to be almost linear without significant hysteretic effects. 
 Figure 5: Elastomer quasi-static measurement
 Figure 6: Elastomer FRF measurement

For the dynamic measurements a dynamical shaker setup was used with sinusoidal and white noise excitation signals. Linearity of the elastomer is assessed by the sinusoidal input measurements. White noise inputs are used for the identification of the (linearized) model parameters.

Figure 6 shows the frequency response functions (FRF) from the base velocity to the isolated mass velocity for three different input amplitudes. The shift in eigen-frequency is negligible and consequently, a linear amplitude independent model describes the dynamic behaviour sufficiently good.

The quasi-static results and white noise shaker measurements are used to identify the generalized Maxwell model. In Figure 2 an overview of the model with parameters is shown. The (linear) static stiffness is easily derived from the quasi-static measurements. The white noise measurements are used to identify the parameters of the Maxwell elements. Using different top masses, the transmissibility FRF is determined from each of these measurements. These FRF’s are used to fit a second order Maxwell model of the elastomer used in the isolation system. In the optimization process the simulated transmissibility of the model is compared to the measured transmissibility. The result fits are shown in Figure 7. The coloured lines show the measured transmissibility as function of the varying top mass. The dotted black lines represent the fitted model. The eigen-frequencies and corresponding damping values are well described. 
Figure 7: Fit transmissibility elastomer (magnitude)

Figure 8: Schematic overview hybrid isolation system

Hybrid damper concept
The concept of this hybrid composition is the series utilization of the (large response signal) shock absorbing properties of the wire rope isolator and the (small response signal) damping properties of the elastomer isolator (Figure 8). For vibrations with low amplitudes the wire rope spring remains in its stick regime and is stiff with low damping. The damping required for damping the resonance is generated by the (less stiff) elastomer. For high amplitude input and response (i.e. shock) the wire rope spring is in its slip regime and will absorb much energy due to the high friction between the strands.

The static stiffness ratio () of the used elastomer and wire rope elements turns out to be an important parameter for tuning the isolation characteristics. Furthermore, the performance of the hybrid damper is highly dependent on the input amplitude. The shock response of the hybrid damper is bounded by that of the elastomer isolation system and wire rope isolation system. For shock absorption a hybrid damper with a dominant wire rope component is beneficial (low ). Considering the overall performance of an isolation system for vibration isolation and shock absorption, the use of hybrid isolators with respect to single wire rope or elastomer isolators is beneficial, however, the specific component selection is much more complex as the system response lies somewhere in between the two individual responses. Figure 9 shows that the output PSD level of a hybrid isolation system under white noise excitation is between the PSD levels of pure wire rope and elastomer responses. Figure 10 shows the same effect for the rest shock level after experiencing a shock.
Figure 9: Output PSD comparison for different isolation system configurations

Figure 10: Rest shock level comparison for different isolation system configurations
Conclusion and future developments
The behaviour of wire rope isolators is highly amplitude dependent. For small deformations the behaviour is stiff and close to undamped. For increasing deformations the cable strands start to slide, resulting in a decreasing stiffness and increasing damping. Further, the behaviour is characterized by softening in compression and hardening in tension. The modified Iwan model describes the amplitude dependent hysteretic loop of the wire rope isolator accurately.

The characteristic behaviour of an elastomer isolator is frequency, temperature and geometry dependent. For low frequencies, the stiffness and damping are low, for high frequencies the stiffness is high and the damping is low. The intermediate frequency region is interesting: the stiffness is increasing and there is significant damping. The linear generalized Maxwell model is able to properly describe the elastomer behaviour both in the time and frequency domain.

The concept of the hybrid damper is introduced which aims at utilization of the (large deformation) shock absorbing capabilities of the wire rope isolator and the small signal (vibrational) damping properties of the elastomer isolator. For small deformations the wire rope is stiff and consequently the elastomer component dominates the overall stiffness and damping. For increasing input, the cable strands start to slide and the wire rope stiffness decreases, resulting in beneficial shock absorption properties. The use of a hybrid system is beneficial to optimize both the vibration isolation and shock absorption performance. However, the non-linear behaviour is dependent on the stiffness ratio () of its wire rope and elastomer components. In contrast to linear passive systems, an input amplitude dependent trade-off between vibration isolation and shock absorption performance can be made, to optimally suit the application requirements. 

W. Iwan, "The distributed-element concept of hysteretic modelling and its application to transient response problems," International Journal of Non-linear Mechanics, vol. 4, pp. 45-57, 1969.

D. S. a. P. Hagedorn, "On the hysteresis of wire cables in Stockbridge dampers," Journal of Non-linear Mechanics, vol. 37, pp. 1453-1459, 2002.

Substantial growth NTS with acquisition of Norma

Significant boost of competence in machining

NTS-Group and Norma have signed an agreement for Norma to become part of the NTS-Group. With this acquisition NTS becomes a group of high-tech industry suppliers with globally 1,500 employees and an annual revenues of over 210 million Euros. Norma will continue to operate under its current name. Norma CEO, Frank Biemans, will report directly to the Board of Directors of NTS-Group.

Marc Hendrikse, CEO of the NTS-Group comments: "Norma's core competences in the area of precision machining perfectly complement those of NTS. What's more, the companies' cultures are a great fit. All in all, together with Norma, we represent a powerful and ambitious first-tier supplier to high-tech industry machine builders, whom we are able to serve optimally thanks to our scope, international position and complete portfolio of knowledge and expertise. The acquisition of Norma perfectly fits our growth strategy to support our customers on a higher level by developing, producing and assembling systems, modules and components."

As a high-tech supplier of mechatronic systems, Norma (400 employees) has a wealth of knowledge and experience in precision machining and the assembly of ultra precise modules. Norma also boasts key knowledge in the field of value engineering. The company operates out of three sites: Hengelo, Drachten and Doetinchem (NL). Norma CEO Frank Biemans, comments: "Joining forces with NTS - in regard to both production and engineering - will enable us to meet the needs of a great number of customers. These customers are looking to do business with larger suppliers, capable of a higher level of support and taking on bigger projects."

Stan Oude Mulders, Norma majority shareholder: "We view this step as a logical extension of our successful strategy of previous years. We know NTS very well and have already worked intensively together. Our businesses are a good fit, there is obvious synergy. We are pleased to be a part of this family-owned company."

The transaction is expected to close in the third quarter of 2016, subject to fulfilment of customary conditions including where applicable completion of the concentration notification with the competition authorities.


NTS-Group opens second factory in Singapore

The Eindhoven NTS-Group recently opened a second factory in Singapore: NTS Mechatronics Singapore. This company will be responsible for engineering and assembling modules and systems in the Singapore region and will work closely with NTS Components Singapore, an existing company that specialises in the production of high-precision frames and sheet metalwork.

Marc Hendrikse, CEO of the NTS-Group: We already had a significant knowledge and experience of co-design and assembly, but were being forced to relocate mechanical assembly activities because of growth in the current sheet metalwork factory. The need to launch the new company was also confirmed by demand from clients wanting the possibility to outsource in a cleanroom environment at module level.

Possibilities for further growth
The new building covers a surface area of more than 1,000 m2 and has now been put into operation. It has the space necessary to successfully assemble and test clients' modules and machines. The factory features a metrological department with a brand new Mitutoyo measuring machine. The 350-m2 cleanroom in class 10,000 and special boothfor class 100 are state-of-the-artfor module assembly. Although the company currently employs some 20 people, this number will grow to approximately 80 employees in time.

Marc Hendrikse: All of our systems and processes have been aligned to the other NTS companies. Added to this, an exchange programme helps us to work together efficiently, gain an insight into each other's cultures and that know-how is kept up-to-date at all times. The expansion represents an important next step in our strategy, which involves the further development of our leading position as an international system supplier.”

Footprint in the Asian market
The Asian NTS companies have shown a substantial increase in turnover in recent years and further growth is expected for 2016. This is the result of continued investment in skill, quality and speed. The growing footprint of NTS in the Asian market is helping too. "We gain extra strength from the cooperation with the NTS companies based in China and Singapore. They enable us to respond fully to the wishes of our clients” Hendrikse says. “The challenges that we are taking on might be big, but they will enable us to continually improve in everything we do.”