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Etch-a-sketch artwork is up to scratch

04 July, 2013

Visitors to the renowned Venice Biennale art festival this year can see a striking exhibit in which a sketch drawn on an iPad is transferred in real time into a version scratched onto a glass surface by a diamond ring. Developing the mechatronic positioning system that turned artist Ron Arad’s vision into a practical reality was a daunting task with tough time limits.

The artwork, called Last Train, was commissioned by Steinmetz Diamonds, and is intended to portray the strength of diamonds. A composite cast of Arad’s clenched fist wearing a diamond ring is used to scratch an enlarged replica of the iPad sketch onto a lead-glass panel which is illuminated along its edges by LEDs.

The work (above) is on show at the Biennale until 24 November. Already, it has been used to convert iPad sketches by artists including Ai Weiwei, Anthony Gormley and Francesco Clemente, into glass engravings.

An app running on the iPad outputs details of the sketch via Wi-Fi to a PC which feeds the information to a two-axis mechanical positioning system. A third axis moves the fist and its diamond ring towards, and away from, a glass panel measuring 875mm by 1155mm.

The first prototype system, based on stepper motors, proved erratic and did not transfer the image onto the glass as quickly, smoothly and accurately as Arad wanted. The stepper motors were also noisy.

In the prototype, a solenoid was used to position the diamond against the glass but it proved difficult to control and caused a shock to the glass when it was “fired” into the etching position. Furthermore, the H-frame gantry positioning system did not allow the diamond to cover the whole area of glass, limiting the visual impact of the artwork.

Also, it was not possible to fit all of the drive and control electronics inside one cabinet, necessitating a separate control enclosure which would not be ideal for use in an art exhibition.

The UK motion systems integrator Heason Technology was therefore called in to design and build a system that would fit inside the restricted dimensions of a presentation cabinet. The delays caused by the attempts to build the original prototype meant that Heason had only around 12 weeks to design, build and deliver the system in time for the opening of the Biennale in early June 2013. The company would have preferred twice this time to design the system, perform the necessary reviews, and allow sufficient lead-times for components.

The company based its design on an extruded aluminium profile base frame, and Thomson Movopart belt-driven linear positioning slides in an I-frame (or rotated H-frame) arrangement. Two synchronised parallel horizontal axes support a single vertical traversing axis, allowing the fist and diamond to reach all parts of the glass.

The I-frame also needed less power than an H-frame with two parallel axes acting horizontally. The linear slides had to be modified by machining material away from their covers and using inverted parallel belt drive adapter plates to make the extruded aluminium profiled beams fit into place.

The slides were specified with stainless-steel cover bands to prevent glass dust from the etching process from entering the ball-guided bearings.

With the main mechanical system design completed, the drives, EMC filters, motion controller, computer, Wi-Fi components and an interfacing panel were placed around the cabinet (above). To achieve the speeds and dynamic performance required, a servomotor-based system was chosen.

Multi-axis motion control is provided by an ABB NextMove e100 multitasking controller which synchronises the motion via its integrated Ethernet Powerlink network. By connecting the distributed MicroFlex e100 drives via Powerlink, the amount of wiring needed was minimised compared to traditional servo systems.

A C++ program running on a PC interprets data from the iPad app (which was commissioned separately) and sends real-time linear position co-ordinate and etching axis on/off information to a Mint program running on the motion controller. This information is stored in the Mint program’s move buffer and can be advanced and started on demand – streaming the positions to create the smooth, co-ordinated and contoured motion needed to recreate the original iPad sketch.

The Mint program synchronises the parallel axes. During start-up, the axes align themselves using a modified datum command, thus ensuring orthogonality with the vertical axis from the beginning.

The third axis of motion – placing the diamond ring against the glass and later removing it – is taken care of by a separate brushless DC motor and drive using the NextMove controller’s torque-limiting function. This enhances the simple on/off input command to soften the impact when placing the diamond against the glass. A combination of hard and soft limits, and torque control, ensures that all axes stay safely inside the cabinet.

Arad is pleased with the end results. “It was a pleasure to work with a company [Heason] that was more of a perfectionist than we ever imaged,” he says. “When things were good enough for us, they weren’t good enough for them and in the end we appreciated and benefited from Heason’s professional attitude in a time-frame that has challenged everyone involved in the project.”

Arad hopes to collaborate with the UK integrator again in the future. “Working on the Last Train project has given us the appetite for other projects that we wish to follow up on,” he says.

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