The Ontario Science Centre first opened its doors to the public in 1969 when it was heralded for its innovative hands-on approach to science, a model that was soon adopted by other science centers. Following its zenith in the 1970s, the Centre fell into a period of decline. The 1990s marked the beginning of a new era for significant change and expansion with improvements funded by a $40 million Agents of Change project. The final phase of this project was recently completed and on March 3, 2007, the unique kinetic installation Cloud, operated by some of today's most innovative motion control technology, was opened to the public.
Suspended from the ceiling in the Centre's Great Hall, Cloud by David Rokeby is one of three permanent installations selected by an international juried competition to enhance the Science Centre experience. A fusion of art and technology, this mesmerizing piece is composed of hundreds of acrylic panels rotating at slightly different speeds to create visually shifting waves of patterns as light plays across the panels.
Cloud is an arrangement of 100 identical 14 ft long hanging assemblies suspended at regular intervals from an aluminum grid that measures 42 x 42 ft. The complete installation, including roof support structure, weighs 14,000 lb. Each hanging assembly is comprised of a brushless motor motion control system from which an acrylic shaft is suspended.
Mechanical components for each of Cloud's 100 motion control systems include an MDrive™ integrated motor and driver with gearbox, a power supply and a custom controller the size of a credit card running a simple software interface. Suspended from each mechanical assembly is a long acrylic shaft fitted with six pairs of evenly spaced acrylic panels . Coordinated rotation, speeds and positions of the 100 acrylic shafts is controlled through a robust motion control CAN network run on a single remote computer.
MDrivePlus™ from Intelligent Motion Systems Inc. (Marlborough, CT) incorporates a high-torque brushless motor and microstepping driver into a single compact package that accepts a broad input voltage range from 12 to 75V dc with 20 microstep resolutions up to 51,200 steps per revolution for enhanced performance and speed.
When asked about his choice of MDrive for the project, its creator David Rokeby says he spent a lot of time investigating motion control options including servo motor systems, conventional stepper motor systems and various integrated stepper motor solutions. “But I didn't find any other that delivered what MDrive does. MDrive was the only solution that would have worked for this application,” he says.
Rokeby's original concept for controlling the motion of Cloud was to use servo motors. “My original thought was to use servo motors versus stepper motors. Where steppers are more commonly used for getting to a specific destination in an optimal time frame, Cloud's hanging shafts needed to pass through a specific point in its rotation at a specific time. This model more closely related to servo controls, but servos had a much, much higher cost,” he says.
This began a search for a budget solution and lead to Rokeby's discovery of the MDrive. “It fit so well because of the driver, indexer and motor combined into a very compact and flexible solution to keep bulk at a minimum,” he says. He went on to say a more conventional solution wouldn't have given as much power. “We needed to minimize the enclosure size and bulk for visual appeal and a more conventional solution would have required more space, at which point heat would have been a concern. Especially as all 100 assemblies are running continuously and are enclosed in cowlings to make them visually appealing. Reliability was also a huge concern when running more powerful motors located in a tight space.”
In addition to cost, factors concerning the designer included size, noise, motor power, heat and reliability.
When compared to conventional motion control solutions using separate components, the MDrive's integration of a high-torque brushless motor with a microstepping driver minimizes the space required. The interface cabling between the motor and driver also significantly reduced bulk where space was limited.
In addition to eliminating bulk, reduction in the cabling between motor and driver lowers the possibility of electrical noise interference. According to Russ Gibas, manager of engineering at Intelligent Motion Systems, “Drive to motor cabling has always been problematic because of EMC (electro-magnetic coupling). By integrating the drive and motor, low level signals aren't exposed to the EMI (electro-magnetic interference) from motor cabling.”
Potential noise problems could also have existed due to the close proximity of each MDrive to a switching power supply. This was addressed through proper wiring using twisted-shielded cabling.
Minimizing audible noise from motor resonance was also a concern for this public installation running 100 stepping motors continuously. To avoid a primary resonance zone range of 0.5 to 1.0 revolutions/sec, Rokeby worked with Bob Parente, IMS application engineering manager, who says, “Faster speeds tend to be noisier while slower speeds tend to be quieter. Working together, we were able to determine the best operating settings for this application's network of 100 MDrives.”
While Rokeby wanted to keep each motion assembly as compact as possible, sufficient torque was required to move 1,870 lb inch sq of total inertia per hanging shaft. With an inertia ratio load 3,500 times greater than the MDrive's 0.54 lb inch sq, use of a 25:1 ratio gearbox enhanced performance for a realized rotor-to-load inertia ratio of 5.5:1. This enabled the MDrive to control the load and settle out reasonably quickly when stopping, while avoiding oscillation up and down the hanging shafts.
To increase visual appeal of the installation, each of Cloud's 100 motion control assemblies is covered by a cowling. But in making the components less conspicuous, the possibility of trapped heat damaging them increased. To minimize this potential problem, MDrives with larger-than-required motors were used to enable operation at a lower current setting thus generating less heat.
As an additional safety precaution in a public venue, a UL-approved power supply was selected, in part, for its self-extinguishing materials rating. The power supply provides a continuous draw of around 0.3 A for Cloud's constant rotation with gentle slowing and accelerating.
An integrated motion control solution offers numerous benefits over conventional solutions. As mentioned above, there can be significant reductions in machine size, cost and complexity. There is also the benefit of a more reliable system realized through the reduction in cabling, the elimination of guesswork in combining components from various manufacturers and the proven track record of the MDrive.
David Rokeby Comments as an Artist
My immediate response when confronting the space of the Great Hall was that the space required and deserved something huge, yet largely transparent or diffuse ... something that substantially occupied the space but animated it rather than obscured it. This made me think of the fact that all states of matter are primarily composed of empty space, sparsely populated by particles. In classical Hindu beliefs, there are five elements: earth, water, air, fire and space. I took as a starting point this fifth element, space — both the magnificent (but challenging) space of the Great Hall and the space within the structure of matter.
Loosely mapping the classical elements into contemporary terms, I arrived at SOLID, LIQUID, GAS, ENERGY and SPACE and these are the five components of this proposed work. The solid, liquid and gaseous states of matter are largely differentiated by their manners of occupying space and reacting to and transferring energy. Solids tend to have very rigid, regular and coherent internal structures. As a solid melts into a liquid, this structural rigidity softens and breaks down but relationships between atoms within the fluid remain an important factor in their behaviour. When a liquid evaporates, the atoms shake loose of their relationships and move freely and chaotically. Energy drives these transformations which change the way the elements occupy space. In this work, Cloud, I have chosen to represent these states and transitions with a large dynamic diffuse uniform structure arrayed through the space above the visitors' heads.
In very simplistic terms, the work responds to the question: What might it be like to experience the 'space' inside a wave? Inside a crystal? Inside a gas?
I am also interested in the ambiguities in the definitions of those states. Research has shown that the differences between the states of matter are sometimes a matter of perspective. For example, cloud and glacial movements when played back at high speed start to resemble liquid wave patterns. In other words, whether something is a solid, liquid or gas depends partly on the frame of reference within which you choose to look at it. It is a natural characteristic of the particular spatial arrangement of Cloud that it offers a different experience and sense of the system's state from every viewing position. One visitor will see the chaotic movements of a gas as another in another position sees the ordered unfurling of a wave.
The MDrive with custom controller solution has been working perfectly. It enabled me to nail the project budget-wise and function-wise.