Drives and servo motors automate scenery changes

As part of a four-year, 112million transformation, the Royal Shakespeare Theatre in Stratford-upon-Avon has benefited from a major project to automate the movements of back-drops, scenery and lighting arrays. At the heart of the project, designed and installed by Dutch theatre automation company Trekwerk, there are around 100 AC drives and servo motors from Control Techniques. Trekwerk awarded the contract to Control Techniques' Rotterdam Drive Centre.

Trekwerk was responsible for the renovation of the over-stage installation, including the development, design, manufacture and installation of 60 winches, plus hoists for 30 light arrays. Trekwerk has used Control Techniques' drives exclusively for many years in mechanised rigging systems for theatres across Europe. Moreover, the company has developed and refined its TNM control system and the advanced SynchroDisk winch systems used at the RSC, which provide virtually silent operation.

The original theatre featured a proscenium-arch stage and a seating capacity of approximately 1400 people. However, the renovated theatre has a 1040-seat thrust stage auditorium that brings actors and audiences closer together, with the distance of the furthest seat from the stage being reduced from 27m to just 15m. The experience is much closer to how it would have been when Shakespeare's plays were first produced - but with stage remodelling and lighting effects that can only be achieved with the cutting-edge electronics offered by Trekwerk and Control Techniques.

Fast production changes

Often different productions are performed in the matinee and the evening, giving the RSC just two hours to complete the changeover. With the new automation system in place, this is now achieved with ease.

Adam Harvey, the RSC's Head of Automation, states: "The winch system is very advanced and gives us much greater flexibility in set design and operation. Safety is paramount, of course, and this is inherent in the whole project. It is very important to maintain the theatrical illusion and the reliability of the many winches involved in each production is crucial. We have not had a single drive fail, which is a great result."

All of the winches are fitted with Control Techniques 15kW Unidrive SP AC drives operating in servo mode and matched with Unimotor 190 fm servo motors fitted with double encoders for precise positioning and speed control.

A total of 46 drives are fitted to 60 winches with at least half of these being positioned above the thrust stage. Any of these can be configured for different duties, from lifting of scenery to controlling the 'flying' of actors. Sixteen of these winches are positioned in the 'slot area' specifically for stage reconfiguring, and 14 Trekwerk SynchroDisc winches give silent five-line lifting of 'flybars' for rapid changeover of scenery during productions.

Application modules

Each winch drive is fitted with an SM-Applications Plus module that is programmed to control all of the winch motions, with the load being calculated internally, based on the current being drawn by the motor and checked against a load cell signal for additional safety. Each motor is fitted with both an incremental encoder for speed control and a sin/cos absolute encoder for position feedback. All winches are capable of lifting at 2m/s. Furthermore, 60 of the winches are of the SynchroTwin type that has one drive controlling two winch cable drums either independently or synchronised together. This versatile arrangement saves valuable space in the busy plant area above the stage.

The SynchroDisc theatre winch design is extremely compact, being just 395mm wide, with lifting capacities of up to 400kg in total. The winches can be fitted with two identical drums with 6mm steel wire rope (SWR) (ie 200kg lift per rope), or one drum with 8mm SWR for lifting up to 24m. With permanent-magnet servo motors and high-frequency 18kHz switching, no braking noise and damped magnetic braking, the devices are virtually silent in operation, even when lifting at 1.8m/s.

There are also 30 light clusters, which are controlled by 1.5kW Unidrive SP drives and size 115 Unimotor fm servo motors with double brakes. This facilitates the flying of lights in and out at a speed of 1m/s, using the Light-Lock anti-swing device, patented by RSC's head of lighting, Vince Herbert. The movement of individual lights can be synchronised with the movement of the cluster, each of which has four-cable lifting on two drums.

Energy efficiency

All drives are fed from a single DC bus. During braking, recovered energy is recycled via the drives to the DC bus, producing some energy savings.

All drives that communicate with each other using Control Techniques' own high-speed network, CT Net, also communicate via CT Net with the Trekwerk control system. Three TNM control desks are preprogrammed with all critical movements for each performance and over-ride joy-stick control can be used to provide manual speed control to maintain synchronicity. The TNM control system executes instructions to the drives. The detailed motor movements are stored as programs within the SM-Applications Plus module in each drive. All programmed movements can be reviewed in the 3D graphics within the control system to identify any potential problems, thereby eliminating any chance of collisions.

The Unidrive SP 'Solutions Platform' AC variable-speed drive range spans 0.37kW to 1.9MW. It is configurable in five operating modes: open loop, RFC (rotor flux control), closed-loop vector, servo and regenerating. With a range of plug-in option modules, its onboard PLC can be supplemented with programmable, specialist feedback and communication modules.

Control Techniques explains that the SM-Applications Plus module transforms the Unidrive SP drive into a powerful automation controller that adds PLC functionality and can connect to the CT Net drive-to-drive network. This gives all the benefits of a fully distributed control system, including better performance, reduced cost and smaller electrical panel sizes.

Unimotor fm motors are available in seven frame sizes from 55mm through to 250mm and torque ratings from 0.72Nm to 136Nm. With IP65 protection, a wide range of shaft variants and flange dimensions, including IEC and Nema configurations, a choice of feedback options and other options, Control Techniques claims that the fm range is probably the most flexible in the market.

Trekwerk, established in 2000, has worked closely with Control Techniques Netherlands to develop customised lifting and control systems for theatres. Around 30 TNM control systems and over 1000 SynchroDisk winch systems have been installed in The Netherlands alone. The TNM System is also popular in other countries.

Follow the links for more information about Unidrive SP AC drives and Unimotor fm servo motors.