Robotics Tech Papers
Getting Swinging Lights to Stop on a Dime
maxon precision motors, inc. Posted 02/15/2012At the Royal Shakespeare Company’s refurbished venue, lightweight lights are hung from swinging cables. A unique design stops the swinging to put the spotlight on the actors.
The Royal Shakespeare Company recently went through a $200 million refurbishment atThe RSC Lightlock system operates using Newton’s third law of motion: every action has an equal and opposite reaction. Stratford-upon-Avon. During that refurbishment, it was decided that the lighting system design needed to be approached differently than in the past, and had to be driven by health and safety concerns. For example, in the past, maintenance personnel had to climb scaffolding to where fixtures were mounted then, while strapped into a secure harness, reach to replace the bulbs or perform maintenance tasks. The reduction of bridges or trusses would eliminate the need for maintenance crews to work at such heights during routine work or when adjusting the lighting. The original idea was to be able to hang the lights from cables that could be lowered to the maintenance crew. By doing so, the RSC would be able to provide higher levels of safety, and save on maintenance times, both of which were expected to produce a cost savings.
Designing a system that would allow for the lights to be hung from cables created additional benefits. The new lighting system would be able to be used in a broader variety of theatre, TV, and film situations because the need for a heavy-duty mounting infrastructure had been eliminated. This advantage meant that the rigging would allow the lights to move virtually anywhere. No shot would be outside the capability of the moving lights. Plus, a lighting designer could choose to create the same design with smaller rigging requirements.
A Swinging Pendulum
The challenge in creating a lighting system that hangs from chains or cables is the pendulum effect. Once the light moves into place and is told to stop, the weight of the lighting apparatus causes it to continue to swing rather than stop where it’s supposed to be. This sends the spotlight all over the stage while it’s settling, which disrupts the performance at the least and, at the worst, ruins it.
RSC’s team came up with a unique method of settling the lighting system after a move. After inventing the device, they needed a company to build and perfect the design. The RSC chose to work with Total Structures (Ventura, CA). The company not only helped with the design of what was eventually labeled as the RSC Lightlock system, but they also license it for manufacture and sales.
On stage, the lighting system must settle into place quickly and accurately so that the performance can go on smoothly and on time.The final device was designed to eliminate the unwanted oscillation of the lightweight structures by using Newton’s third law of motion: for every action there is an equal and opposite reaction. The engineers decided to use use a heavy counterweight device that, mounted on an internal disc, swung in the opposite direction to the movement of the light. By accelerating and decelerating the balanced mass, or counterweight, at the appropriate moment of the swing cycle of the moving structure, the Lightlock would stop the motion. Essentially, the opposite reaction of the counterweight cancels out the movement of the light itself, stopping it in its tracks.
Nook Schoenfeld, from PLSN (Projection, Lights and Staging News) evaluated the Lightlock system independently from the company and said in a recent article, “The light swung 90 degrees and the truss barely swung. Within two seconds, the light beam and truss had settled on the focus target.” This is in opposition from moving the light without the Lightlock system, which took much longer. Later in his examination he added that, “The Lightlock really does work.”
Motion Control for Swinging Lighting Systems
The heavy counterweight, which swings in the opposite direction as the light, is rotated by a flat, brushless motor designed and manufactured by Maxon motor. The motor used is the EC 90, which is only 90mm in diameter. This motor was selected because the Lightlock device had to be small, while offering high performance and precision. The EC 90 Flat motor operates fast, and even with sudden, dramatic movements, can cancel out all unwanted motion in under a few seconds.
Maxon motor offers a full line of fractional horsepower moving coil DC motors and brushlessMaxon motor designs and manufactures the EC90 Flat motor used in the Lightlock system to swing the counterbalance into place. motors ranging in size from 6mm to 90mm, and from 30 mW to 500W. They also offer gearheads, controllers, and accessories. Particularly, flat motors manufactured by Maxon Precision Motor provide long life along with their low profile package. The entire EC series of brushless motors are electronically commutated, which enables them to have extremely long motor life since there are no mechanical brushes to wear out. The motors incorporate ball bearings or ruby bearings that also add to the longevity of the motors, especially needed under such unique conditions where maintenance of the lighting system is routine. The flat motors were designed specifically for low profile applications like the Lightlock where size and weight are important selection criteria.
The Lightlock operates autonomously and needs no data inputs from an external source. It has an integrated track for half inch or M12 fixings that can be rigged with standard clamps and couplers or direct mount plates. The device also has an internal electronic safety device that prevents it from being rigged the wrong way. Plus the Lightlock self-calibrates every time it is powered up. The calibration takes only about 1.5 seconds to complete. The Lightlock is not affected by the high heat from the lighting fixtures.
One of the additional features of using Maxon motors is that the RSC Lightlock emits such low noise levels that allow it to be used for live performances. The overall Lightlock system measures approximately 20.3-inches wide by 16.9-inches deep, and only 3-inches high. It weighs only 31 pounds. The system draws a maximum of 150 watts of power. Multiple units can be incorporated for difficult lighting arrangements as well. In fact, according to Total Structures, the RSC Lightlock will remain effective for as many moving fixtures as required, however it may take slightly longer for the structure to come to rest.