FieldPoint Debuts on the London Stage

Author(s):
Andy Penny - Systemation Software Ltd.

Industry:
Industrial Controls/ Devices/ Systems

Products:
Compact FieldPoint, LabWindows/CVI, FieldPoint,

The Challenge:
Building a stage elevator control desk (ECD) for the Royal National Theatre, London

The Solution:
A touch screen PC based system, built with FieldPointI/O hardware and LabWindows/CVI.

Introduction
The stage of the Lyttelton Theatre, part of the Royal National Theatre complex on the south bank of the River Thames, comprises four elevator platforms. Stage operators can raise, lower, or rake forward the main part of the stage. They also can independently raise and lower three smaller elevators at the front of the stage. The main elevator can lift complete stage sets weighing many tons. However, successful elevator operation requires not only brute force, but accuracy. The operator must position the elevator perfectly, placing it at the correct millimeter mark.

The new system replaces one built in the 1970s using custom electronics. It is more compact and yields the following advantages:

•  Takes advantage of commercial, modular products
• Easier and more cost effective maintenance
• Greater flexibility
• Better performance
• Easy upgrades for future needs
  
  LabWindows/CVI Controls - Stage Movement in Real Time
  We installed the ECD in the " perch," a small balcony overlooking the stage, where a member of the stage crew often sits during performances. The ECD housed in a wall-mounted cabinet. It also holds an industrial panel PC with touch-screen interface. With LabWindows/CVI software, the stage operator controls all elevator systems via the touch-screen of the PC. The operator can move whatever elevator he chooses after activating the necessary hydraulic pumps. If the operator selects more than one elevator (such as front elevators A and B), they are automatically clutched together. The operator can then move the elevators either manually, using a joystick, or automatically, moving the elevators to a set position under software control. National Instruments LabWindows/CVI automatically controls the brake. The PC screen updates in real time showing elevator positions.
  
  NI Software and Hardware Simplify System Development
  Proportional hydraulic servo valves control the power to each elevator. The system provides drive signals for the servo valves, it also switches the three-phase hydraulic power pack. A combination of hydraulic rams, scissor lifts, and suspension wires move the elevators. Shaft encoders communicate the position of the various stages with an accuracy of less than one millimeter for stage precise position. In addition, 24 and 240 V relays automatically switch brake and clutch systems. The system also drives an auxiliary hydraulic motor under joystick control.
  

The ECD cabinet contains the panel PC and control electronics, based on the FieldPoint I/O system from National Instruments. FieldPoint has numerous benefits, including modular design and built-in terminal blocks, that simplify wiring and reduce costs. The FieldPoint Explorer software also proved extremely useful during commissioning, and gave us low-level control of the hardware without programming. Interfacing to the PC via RS-232, FieldPoint provides analog input (for joystick and power supply monitoring), current-loop output (for servo valve control), relay drive capability and switching (for clutch, brake, and other control), plus high voltage monitoring up to 240 V.

We wrote the application in LabWindows/CVI. The user interface tools lend themselves easily to this kind of event-driven user interface. Also, as a C-based development environment, it is straightforward enough to incorporate hardware drivers for equipment such as the PCI-based shaft-encoder interfaces. We wrote the control algorithms in C, using a "dual-slope ramp" approach that slows the elevators as they reach their destination. We tuned each elevator drive for speed, because the dynamic characteristics are all quite different.

Safety
We incorporated an emergency stop system into the ECD for safety and also designed the system so that it can only power-up with the use of a key. In addition, we built a number of protection features and interlocks into the system that limits the speed and position of the elevators. Additional features include online help and a cue list. Using the cue list, operators can store elevator positions in the order they are needed throughout a performance.

The new ECD is now in use and performing very well. "We accomplished the installation of the new system with very little upheaval," said Frank Stevens, Systems Engineer at the Royal National Theatre and commissioner of the project. "We are all very pleased with the result."

For more information, contact:

 Andy Penney

Systemation Software Ltd.

98 Saxonhurst Road

Bournemouth, BH10 6JJ, England

Tel: 01202 265558

Fax: 01202 775535

E-mail: info@systemation.co.uk

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