Experiments

ACTIve FIXtures for high precision positioning of large parts for the windmill sector

Main technical issue

The machining of components such as the planet carrier for the windmills is a very demanding process. It is becoming more and more complex as larger and larger parts are being demanded which at the same time requires the machining of lighter materials/high added value added types of cast iron, like in this case Cast Iron EN-GJs-700-2. The main problems occur during the positioning of the part in the fixture before loading on the machine and the later positioning of the part within the machine reference system. The current process includes manual interventions which should be limited, since they are a source of variability depending on the operator’s skills. Moreover, the need of high-precision positioning when centering the workpiece is crucial.

Proposed technical solution

Workpiece clamping: the most critical action performed by the fixture was the centering of the planet carrier with a maximum tolerance of 10 ?m. Therefore, electromechanical actuators were used because of their accuracy, high load capacity and sufficient stroke length. No feed-drive on the market was suitable for this case study, as the requirements of moving a workpiece of 3000 kg with a precision of 10 ?m by means of electromechanical feed-drives are not usual. Therefore, a specific feed-drive has been developed for the application.
Lateral feed-drive: in order to adjust the planet carrier laterally with a tolerance of 10 ?m in diameter, very accurate and reliable feed-drive has been designed. The feed-drive can be divided basically in three different sections: driving components, transmission system and scales. From the point of view of the control of the position, the best option was to use a synchronous motor with an encoder. The fixturing has three actuators, placed in circular shape with an angle of 120º among them. They are independent modules and can be clamped and unclamped from the base of the fixturing. Therefore the modular system can be easily adapted to workpieces with different diameters, changing radially the position.
Control of the centering process: to control the centering movement of the planet carrier a PID control is used in position. For the control loop, the signals of the motor encoder and the scale of each actuator, along with the signal provided by the sensor placed in the lathe, are collected and cross-checked until having placed the workpiece in the required tolerance.

Main advantages of the solution

The proposed solution is based on the development of a smart and active fixture valid for a family of workpieces to position them very precisely in a semiautomatic or possibly automatic way without producing any damage to the workpiece. This solution allows the positioning of the workpiece controlled by a PC with a closed loop control unit commanding the fixture. The technological development is based on the design and development of new hardware and software solutions that is based on real time data gathering by means of sensors’ integration, and the development of the control system for the management of the new intelligent actuators taking also into account the machine. The obtained results have proved that the proposed fixture is able to center the workpiece within the tolerance of 10?m, assuring the quality requirements. This is possible thanks to the specific mechanical design of the actuators and their control by a PLC and a software program. Moreover, this new fixturing system allows GAMESA to increase its productivity, as the adjustment process is performed in an automatic way, reducing the setup time of 30% and the amount of defective parts. In other words, this solution increases the quality rate of the global process.