What is probably the world’s largest kinetic sculpture was realised in ‘Kinetic Rain’ at Changi Airport in Singapore. The artistic concept of the installation and the calculatory design of the choreography originate from the Berlin-based Art+Com. In allusion to the tropical rain, the installation consists of 1216 brilliantly sparkling, copper-plated aluminium droplets. These are suspended from the ceiling on thin steel wires and each one is moved by a small servomotor. During a 15-minute show, the droplets are formed into different pictures connected with the subject of flying. MKT AG, experts in kinetic installations, took care of the complete technical implementation, including the software. The challenge of moving 1216 servo axes synchronously was solved on the basis of EtherCAT, TwinCAT and the compact servo drives in bus terminal format.
Passengers and visitors who enter Terminal One at the airport are stopped in their tracks by the kinetic sculpture. “The harmonious interaction of more than a thousand droplets also symbolises the many people at the airport who ensure day after day that passengers are positively surprised and have fond memories of it,” says Yeo Kia Thye, vice-director of airport operations.
The demands on the control of ‘Kinetic Rain’ are extraordinarily high, with the precise movement of 1216 axes. In addition to that, the project demanded high availability, compact design of the components and the replacement of components without addressing. “One of the paramount specifications of our customer, Changi Airport, was that the system must run 24 hours a day. Even if an individual axis were to fail, the show must go on,” says MKT director, Peter Haschkamp.
How do you control 1216 axes synchronously?
“The synchronous movement of 1216 axes is one of the absolute highlights of this project,” stresses Raphik Shahmirian from the Beckhoff office in Munich. Beyond that, requirements had to be met where the dynamics, precision and speed of the motion sequences were concerned. The droplets move with a speed of 1,5 m/s and the movement must be dynamic, but at the same time flowing and absolutely free of jerks. MKT found the solution to this task in the PC and EtherCAT-based Beckhoff control platform with compact servo drive technology.
A central C6525 Industrial PC is responsible for the control. It communicates by TwinCAT ADS with the special GUI computer from MKT. At the same time, the PC centrally controls the 1216 axes via TwinCAT NC PTP and acts as the master.
Via the TwinCAT cam table function the master PC coordinates the distribution of the position data to the six slave PCs, to each of which 192 or 208 axes are assigned, and ensures the synchronicity of all axes according to a master axis as reference. Communication takes place in real-time over EtherCAT up to the drives.
The GUI computer from MKT serves for visualisation, but also contains the show in the form of a table containing the position data for each droplet at time intervals of 200 ms. This corresponds to five pictures per second. A flowing movement perceptible by the human eye without jerks is possible only through the interpolation in TwinCAT NC PTP. Here, 100 intermediate positions are calculated for each droplet using a spline algorithm in a 2 ms NC task. These calculations take place on each slave PC for the local axes assigned to it.
The artistic intention to have the synchronous movements of the droplets run like a kind of 3D film is made possible through the use of EtherCAT and TwinCAT NC PTP. While the master keeps all the axes synchronous to one other, the slave PCs calculate the positions of the axes assigned to them every 2 ms and communicate them over EtherCAT in real-time to the servo drives.
Compact servo drive in a 12 mm terminal
The movement of the individual axes is extremely precise and lies in the range of 1 mm for an overall length of 7,6 m.
The maximum offset between two droplets is 0,25 mm. Each droplet is controlled via an EL7201 EtherCAT servomotor terminal and a servomotor of the type AM3121. “In the servo terminals, which fit seamlessly into the EtherCAT terminal row, we found an extremely compact solution that fits the structural conditions perfectly, with limited installation space in the ceiling of the building,” says Haschkamp. In addition, servo drive technology offers dynamic advantages and allows flowing transitions.
Modularity of the controller simplifies commissioning
“The modularity of the control solution was important to us and the fact that an individual axis can be exchanged without addressing. That made partial commissioning possible, for example we were able to work in parallel on software, hardware and the mechanism, allowing us to keep within the narrow timeframe that we had for this project,” explains Haschkamp.
With TwinCAT NC PTP it is additionally possible to ‘jog’ the whole show, i.e. one can fast forward and rewind like a film. That very much simplified the commissioning for MKT. If an individual passage of the sequence was not yet 100% satisfactory, the engineers could repeat it continuously. “A big advantage,” stresses Haschkamp. “With other solutions this is not possible and you are forced to start the show from the beginning each time until you reach the desired position, it is very time consuming.”
In contrast to comparable kinetic sculptures realised in the past over decentralised hardware controllers, ‘Kinetic Rain’ is controlled via a central PC and software and transmitted over EtherCAT. The cam table function, which MKT accommodated locally in the drive amplifier in earlier control solutions, is now executed by the software. In this way, the position data can be managed centrally and distributed in real-time to the axes over EtherCAT.
View the official airport video at http://tinyurl.com/pxyghlg
For more information contact Kenneth McPherson, Beckhoff Automation, +27 (0)11 795 2898, k.mcpherson@beckhoff.com, www.beckhoff.co.za
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