At the cinema, each film is preceded by trailers of forthcoming attractions and advertisements, each of which is relevant to the target audience and is changed weekly at each screen and each cinema throughout the UK.

This is the sole responsibility of Bucks Laboratories of Slough, which designs and builds special purpose machines for the editing task.

Engineering manager Graham Greenfield felt confident that, using the capabilities of Control Techniques Commander SK AC drives, they could build the machines – White Winders – which provide fast, reliable and easy-to-control editing of the heavy filmreels; and meet the initial deadline of three weeks. The first machines were operated manually with foot-pedal control, however as time went on he was able to introduce more sophisticated control, including automatic recognition of film joins using ultrasonic attenuation sensors that allow the film to be stopped with millimetre accuracy and very precise tension control.

Greenfield explains that as there is a lot of inertia in the reels, stopping a film from 1000ft/min quickly requires very precise control. The take-up drive determines the speed, in closed loop mode with feedback from a diameter sensor, using PID control to keep film speed constant between edits. The feed drive controls the torque, with a spring-loaded dancer control feeding back a 1-10V signal to the drive.  For each edit, the operator determines which join on which to stop. The take-up drive counts the signals from the ultrasonic thickness sensor and initiates a stop signal that simultaneously ramps down both drives whilst maintaining precise tension control and stops the film to within one millimetre accuracy. Since film has 16 frames/ft, 64 perforations/ft, this effectively gives exact position control of the film to allow the operator to carry out the appropriate action specified on the worksheet. Emerson Leroy Somer motors ensure that slippage is eliminated.

The winders can be used in four basic modes, feeding from the top or bottom of either reel and with a third reel that can be designated to run as either feed or take-up if required.  This means that each drive has to be capable of performing different roles as needed and is programmed accordingly with numerous ‘recipes’.  The operator’s control simply designates duty to each drive by code number that selects the appropriate programme.

Bucks Laboratories now has a total of 18 White Winders. Each has one 0.37kW, one 0.75kW and one 1.1kW Commander SK drive, complete with LogicStick, and no additional I/O or special plug-in modules are needed. 

With the cinema contract, there are two reels for each screen, one at the cinema and the second at Bucks for preparation – cutting, inserting and changing content order. Thousands of reels have to be completed in time for the weekly changeover.