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Servo controllers synchronise high-speed switching

Edited by the Engineeringtalk editorial team Jun 24, 2009

When a challenging project for a large illuminated sign for a building required complex and high-speed synchronised switching, Focus International requested Red Drive servo controllers from Rotalink.

Focus International is a fibre-optic lighting company specialising in large projects such as museums, shopping centres and corporate buildings.

Fibre optic technology lighting creates spectacular effects and is becoming increasingly popular with proven benefits of high efficiency and low energy consumption, as well as guaranteed intrinsic safety with no electrical hazard as only transmitted light reaches the target area.

For Focus International, each project is unique and, as the industry does not generally have standard equipment, involves the careful and specialist design of three main components to achieve the desired balance of illumination and intensity: the light source containing the lamp with optical modifying components and controls; fibre optic bundles to transmit the light; and the light fittings that direct the light onto the subject and its surrounding space.

For this project, the synchronised high-speed switching pattern was produced using 12 identical light sources, each with a motorised aperture disc that is rotated to allow or restrict the light path into its corresponding fibre optic bundle.

Each light source disc was controlled by a Red Drive and driven by a Rotalink supplied brushed DC servomotor fitted with a high ratio spur gearbox with position feedback from a rear motor mounted, 48 pulse-per-rev, dual-channel optical encoder.

Synchronisation between each axis was achieved by interconnecting the Red Drive's built-in and fully programmable I/O to trigger preprogrammed move sequences, with a master Red Drive interconnected to the main switch.

Focus International had considered using DMX controllers for this application but the stage lighting industry's standard protocol is found to be overly complex to program and expensive for such customised multi-axis motion-control-based lighting systems.

Red Drive, with its low cost and straightforward drag-and-drop flowchart programming, was an attractive alterative that could be quickly integrated into the application.

The credit-card-sized controller was installed in the light source housing and its EEPROM memory allowed the complex motion sequences to be stored for a fit-and-forget and stand-alone solution.

Furthermore, as Rotalink also designs and manufactures a wide range of motor technologies, gearboxes and ancillary components, it could provide the complete power train system.

In addition, the company also helped with motor sizing and application calculations - further reducing development time and design costs.

As the Windows programmable Red Drive scales the encoder and gearbox resolution into meaningful units at set-up, actual move distances and speeds matched the fixed 90deg angular aperture rotation.

PID servo tuning was taken care of by entering catalogue values for motor stall torque and no-load speed.

The set-up also included assigning each of Red Drives' nine user-definable digital input and outputs, and linking the homing and reset inputs that were called from subroutines in the flowchart-based program.

Red Drives' graphical drag-and-drop programming methodology allowed Focus International to quickly program acceleration, move and dwell parameters that created the desired lighting effect - and at this point, the subroutine feature allowed a variety of subtlety different move profiles to be evaluated with easy retrieval for final use.

It was also possible to select alternative move and lighting sequences using Red Drives' variable and loop commands to provide different lighting effects from input states to highlight various areas of the sign for different purposes.

Since its recent launch, the Red Drive has flattened the entry-level costs for integrated digital servo amplifiers or intelligent drives.

For this application, each light source was fitted with 1A Red Drive and power transmission components for less than GBP90.

Together with Rotalink's range of DC motors and gearboxes, and with a 10A current capability, Red Drive is said to allow OEMs to create mechatronic positioning systems with up to 10Nm torque economically and with an improved time to market.

Other features such as newly introduced analogue inputs that can be used with potentiometers to give positional feedback make the Red Drive an even more flexible motion controller for DC servomotors.

Following this project, Focus International is now using Red Drive for other lighting projects involving fully synchronised colour filter discs for even more spectacular effects.