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Plug-in ADAptronic MODules for real-time errors (Thermal & Vibration) compensation and superfine positioning in reconfigurable high precision machine tools

Final Report Summary - ADAMOD (Plug-in adaptronic modules for real-time errors (thermal & vibration) compensation and superfine positioning in reconfigurable high precision machine tools)

Ever increasing competition in international markets drives manufacturers to shorten design cycles and reduce manufacturing times and costs for their production. This trend leads to a growing demand for smarter, more flexible and faster machining systems, which must be easy to set up and configure and able to drastically increase both productivity and final accuracy. To deliver this performance, production research must focus on radical transformation of the fundamental processes of manufacturing so that they become adaptive: responding automatically to changes in the operating environment (MANUFUTURE - a vision for 2020).

The new generation of machine tools will be likely to aim at accelerations of 40 m / s2 with machining accuracy in microns (0.5-1 µm on a 1500 - 2000 mm workpiece). Such ambitious performances will increase dynamic problems (inertial forces, vibrations). These will jeopardise precision and machining quality, unless adequate solutions are found for: reduction of the mass of moving parts, stiffness increase and vibration / oscillation damping. Moreover, simultaneous accuracy increase and mass reduction will make thermal stability even more critical, as a temperature gradient in the machine structure of a few degrees can drastically compromise the machining precision.

It is well known that static stiffness and thermal stability can be achieved by increasing mass (e.g. wall thickness, ribs). However, this mass increment will have a negative effect on dynamic stability, beyond the obvious cost increase. On the other hand, conventional light structures could lead to a lack of static and even dynamic stiffness. They tend to vibrate at lower frequencies, generating machining errors, noise and can even lead to failure from fatigue.

Where innovative materials like carbon fibre reinforced polymer (CFRP) composite are used, problems can arise at interfaces with steel or aluminium because the different material properties (e.g. thermal expansion coefficient) can cause unpredictable component distortion (not possible to be compensated via CNC), dangerous concentration of stresses and structural damage. The ideal structure is an ultra-light structure able to match its shape and structural parameters to the current operating conditions. These adaptive structures will be the frontier of knowledge and will revolutionise machine tools and machinery construction in the 21st century.

The challenge in this area is to realise availability of 'extremely' stiff, light and well damped structures which fully and deeply integrate new adaptronic devices with electromechanical, electronic devices, measuring systems, sensors and actuators. Getting more intelligent and integrated structural solutions is essential to really introduce enormous benefits in machine tool design and development.

Therefore the primary goal is to achieve cost-effective structural solutions consisting of a new class of 'modular adaptronic devices' based on smart and multifunctional actuators / sensors capable of performing a wide array of multiple functions, ranging from high and adaptable damping and stiffness characteristics to more demanding new requirements, such as active structural measurement and control function to achieve extremely high dynamic / thermal stability required in fast and precision machining. The overall adaptronic system consists of integrated multi-functional sensors and actuators, with the associated electronics and an adaptive control system capable of acting in real time.

ADAMOD aims at designing, developing and validating novel adaptronic modules for machine tools applications, belonging to the following two main typologies:

1. adaptronic modules for Active vibration control (AVC) and superfine positioning;
2. adaptronic module for thermal compensation.

Project context and objectives:

The ADAMOD project aimed at designing, developing and validating novel adaptronic modules for machine tools applications, belonging to the following two main typologies:

1. Adaptronic modules for AVC and superfine positioning

The system consists of a 'fixed flange, a 'mobile interface' and embedded smart actuators and sensors. The main concept of this device is to orient and/or lift off (compared to a fixed flange linked with the MT structure) the mobile interface where the end-effectors (e.g. spindle, rotary head, rotary table) are mounted. The capability to orient / move the end-effectors makes it possible to suppress vibrations / disturbances and / or achieve superfine axis positioning (compatible with the stroke of smart actuators). This device is very compact, with high structural stiffness and low weight. Moreover, it includes special innovative mechanical joints to preserve the functionality and reliability of smart actuators (e.g. in the case of PZT stacks functionality and reliability by filtering the shared forces and tilting / torque effects, while keeping high stiffness). Geometrical accuracy and ultra-precision movement are obtained through the combined effects of friction-less guiding systems (flexure hinges) and smart positioning embedded sensors. Finally, a microcontroller (to drive smart actuators in closed-loop control) is integrated into the device itself.

2. Adaptronic module for thermal compensation

Thermal stability represents a critical aspect, as the objective is to reduce mass while still preserving or enhancing accuracy. Different innovative solutions for thermal compensation have been investigated at an early stage. One of these innovative solutions consists of a reticular framework which entails a set of fibre Bragg grating (FBG) displacement sensors; the sensor arrangement have been tailored in order to be embedded into the machine structure and to measure the total axial elongation (due to combined static and thermal loads). FBGs work as optical filters by selecting a certain portion of the input spectrum of light to be reflected and the rest is transmitted. The centre wavelength of the reflected light of the FBG shifts with respect to the grating's temperature or strain state. They are integrated as sensors in optical fibres, as information carriers. The principle of the FBG sensor is based on the measurement of changes in the reflected signal (which allows high accuracy). FBGs offer the possibility of measuring in multiple points at arbitrary spacing with a single optical fibre by 'multiplexing'; one can interrogate each sensor independently from the others and obtain therefore a distributed measurement over large structures. These online 'elongation' measurements have been used as input in a structural mathematical model of the part (and related algorithm in the form of linear equations) which predicts the distortion of the structure. The signal processing systems (the optical 'reading unit' which is a Febry-Perot interferometer) of reflective light spectrum coming from the optical fibres have been developed in a very compact box in order to be placed on-board the machine structure. This avoids employing long supplementary carrying optical fibres that connect the sensors to the external post-processing device and then to the CNC. The onboard 'reading unit' through wireless protocols transmits the post-processed measurements to the CNC (in which the proper algorithms are implemented) that provide the necessary compensation in real-time to the axis motion drives.

Project results:

The ADAMOD project accomplished all the activities foreseen in the description of work, achieving 10 exploitable results that have been demonstrated thanks to the realisation of three demonstrators.

The 10 exploitable results, having different level of readiness at the end of the project, are:

- adaptronic platform for AVC and superfine positioning (AVC) (including control);
- adaptronic thermal compensation unit based on FBG;
- active struts for parallel kinematic robots;
- provide a service to design mechatronic systems and methodologies;
- low cost amplifier for piezo-electric actuators (key component FILTER);
- new sensor - NANOGAUGE - double-balanced displacement sensor with nano-metric resolution;
- new sensor - SMARTDISC - in situ displacement sensor, integrated on piezoelectric stack actuators to measure the position of the piezo tip at any time
- smart brackets for clamping FBG with fine tuning capabilities;
- PK robots with active struts;
- HSC milling machine with adaptronic module.

The three demonstrators, on which the results have been implemented, are:

1. the smart platform for active vibration compensation, installed and tested on a FIDIA DL155 high speed milling machine;
2. adaptronic system for cancelling vibrations on PK robots, installed on a two DOF TECNALIA parallel robot for pick and place application, integrating smart rods for cancelling vibration originated by the robot's arms;
3. thermal compensation of machine tools implementing FBGs sensors for distributed distortion measurement, applied to the UoH test rig for thermal compensation testing.

Potential impact:

ADAMOD results will definitely contribute to the transition towards a more knowledge-based industry and sustainable development. In fact, within the traditional machinery industry (resource intensive), the static, dynamic and thermal performance of machine tools are achieved through the over-dimensioning of structures, which often lead to heavy parts, not rationale use of resources / material and then to high energy consumption. On the contrary, ADAMOD results will promote, support and allow the exploitation of a new concept of ultra 'light' self-adaptive structures that could be able to match its structural parameters and requested performances to the current operating conditions, through the integration of the here developed adaptronic modules (smarter and smarter rather the heavier and heavier!). Therefore the proposed modules provide 'inherent' intelligence to the structural elements / components of the next generation of machine tool / manufacturing equipment, then creating new high added value products capable to meet the challenge imposed by new industrial revolution and competition at global level (which are in this case high productivity, precision and rapid configurability) through an efficient and sustainable way. In fact, the ADAMOD modules will enable the local control and compensation of errors (vibration, thermal) very close to place where they are generated, so without the introducing heavy movable structures, high power (and expensive) motors / driving systems, energy consuming auxiliary devices for thermo-conditioning of parts, big machines / equipment, which in turn means less power installed, less energy consumption, narrow size machines and then smaller areas occupied by the machining shop.

Project website: http://www.adamod.eu