Pet Shop Boys: Venetian Blinds

Project brief

Fineline Solutions was approached to design a motor, mechanism and control system to act as part of a touring set. The concept was to produce a set of four giant Venetian blinds, which were hung at the back of the stage. The blinds, when closed, could be used as a projection surface. A multi-level trussing system was rigged behind the blinds, which contained blinders and strobe lights. The blinds were to be opened at various times throughout the performance, revealing the lighting effects to the audience.

For ease of transportation, each of the four blinds was to be constructed inside a 3m length of heavy-duty truss, modified to allow the blind slats to be stored within the truss itself. Fineline Solutions was asked to provide two mechanisms:

Lift winch specification:

  • Control via standard CM motor pickle
  • Helical winding on drum to accept two wire ropes, one in each direction
  • Motor brake enabled when winch not in motion
  • Sufficient torque to lift all slats to rig height
  • Winch to be used for deploying and storage only – no use during performance

Tilt mechanism specification:

  • Control from lighting board via ArtNet
  • Tilt from 0° (fully open) to 90° (fully closed)
  • Sufficient torque to fully open and fully close blind, and to hold the slats at any position indefinitely
  • Blind to return to 0° when power is removed
  • Control system to provide detailed diagnostic information, and to allow calibration data to be modified if required
  • Control system to be robust and to transparently recover from connection faults, incorrect power up sequence etc.

Implementation

A Venetian blind typically consists of a number of horizontally-mounted slats, which are supported by a ‘ladder’ consisting of vertical wires or tapes, with horizontal wires or tapes supporting each slat. In this implementation, there were a total of four ladders per blind. In addition, two steel wires were used to lift the slats, one at each end. The wires passed through the slats and were secured to a solid aluminium support at the bottom.

If the front and back wires of the ladders are raised or lowered, the blind will open and close. When the steel lift cables are retracted, the lower aluminium support will collect the slats in a stack, eventually stowing the slats within the truss.

The lift winch mechanism was manufactured by our manufacturing company, Fineline Manufacturing. It consists of a single helically-wound drum, allowing steel cable to be wound from each direction. The drum is connected to the motor via a reduction gearbox, and a standard industrial AC motor is used to provide lifting power. The motor is equipped with a power-to-release brake, which ensures that the brake remains engaged whenever the motor is not turning.

A variable-frequency drive was employed to control the AC motor. To avoid damage to the blind mechanism, the drive was configured to provide a short acceleration and deceleration at the end of each move. The drive also ensured that maximum torque would be available from the motor at low speeds. To ensure that the blind slats were not pulled too far into the structure when being raised, a limit switch was provided to stop the motor before damage occurred.

The tilt mechanism was designed around a single shaft, which runs the entire length of the truss. Four ‘saddle arms’ were machined to support the blind ladders: these were attached to the shaft. When the shaft is rotated, the front and back wires of the ladder are raised and lowered, causing the blind to open and close.

The shaft was connected to the motor via a standard toothed belt and pulley system. The pulleys were specified to develop the necessary torque from the motor when stationary, allowing the blind to be held in a particular orientation. For safety, hard limits were added to the structure to prevent the central shaft from being over-rotated: this situation could potentially snap the ladder wires, causing the slats to be dumped on the floor.

To determine the blind position accurately, a servomotor was used rather than a conventional AC motor. This type of motor uses a shaft encoder and suitable drive to accurately control both the motor position and the motor torque. Each motor and drive combination is manually tuned to obtain the required performance under load. The motor drives used in this system allow custom code to be uploaded, which was used in this case to implement a position-based PID loop, and apply other settings to the drive.

To interface between the motor drives and the lighting board, a standalone control PC was provided. This PC runs Linux, and has two Ethernet cards. One Ethernet network was used to receive ArtNet data from the lighting board, and the other was used to connect to the four drive units. The PC was built using a solid state drive to avoid problems with rough handling while on tour. The PC does not require a monitor, and has been designed to start all software automatically.

Custom software running on the control PC translates between incoming ArtNet DMX packets and positions that each blind should achieve. Two channels were used for each blind, allowing 16-bit positional accuracy. The control system was designed to accurately track fader positions, so the speed of movement is entirely defined by the transition speed or fade time on the lighting board.

The control PC also provides a web interface for status, diagnostics and configuration. This interface allows an operator to see real-time status from each of the drives, indicating parameters such as demand position, actual position and drive temperature. The interface also allows drives to be manually moved, which is very useful for troubleshooting if the lighting board is not available.

The control system has completed one world tour, and is about to embark on a second.

Further information

For further information, or to discuss a similar project, please contact Fineline Solutions.