Four will lift 2 x 12.5t over 15m spans, while a fifth will lift the same load to a 28.6m radius, rotating through 270°.
Aleris’s customers want the sheets to be delivered scratch-free. To ensure this, it is important the sheets are not dragged across each other when lifted. They must be kept perfectly level throughout the lifting process, particularly when first picked up. The sheets are lifted using a vacuum attachment, with a hook at either end of the attachment. To keep the attachment and the load level, the drives for each hook must be carefully synchronised.
A similar twin hook technique is sometimes used in container cranes. Here, if a container is loaded unevenly, the two hooks at each end need to adjust for their differing loads, while lifting the container symmetrically.
In a master/follower application, such as this, the two drives controlling the speed of each hook are interlinked by fibre optics. To maintain the precision needed, an encoder device is used to feedback the position of each hook to the respective variable speed drive. This function of the encoder is essential in synchronising the drive, based on the position of the master as measured by the encoder. The follower continuously compares its own position with the master, and adjusts its speed accordingly.
The positions are measured by motor incremental encoders. Synchro-control includes position error calculation for synchro-correction of the follower hook and also supervision of synchro fault if the difference between the hooks exceeds the user adjustable limit.
For non-symmetrical situations, the crane control stops the motors and closes the mechanical brake and the operator needs to re-synchronise the hooks or restart the operation.
The system can be designed for a maximum of five motors, either with a common drum and separate motors with load sharing, or with separate drums and separate motors with shaft synchro control.
TCS applied the same principle at Aleris, although the weight of the load, unlike in a container, remains uniform. TCS engineered for each hook a 40kW geared motor, with a 40kW ABB industrial drive used to vary the speed of each hook as it approaches the plate.
The solution to the accurate, synchronised control lies within ABB’s crane control program, a special application program for ABB industrial drives. The program features synchro-control, which, when operating together with the Direct Torque Control (DTC) motor control platform, provides the required level of synchronisation.
DTC technology allows accurate control of both speed and torque with or without pulse encoder feedback from the motor shaft. “The most critical time is at start-up, as both motors need to be magnetised simultaneously and therefore need to be synchronised. DTC provides very accurate synchronisation together with a synchro-control application,” says Frans Busschots from ABB Belgium.
The crane control program eliminates the need for a PLC. PLCs need to be programmed and their processing time can add significantly to the reaction time expected by the crane operator. PLCs need extra hardware and software at additional cost.
“A crane operator expects his crane to react immediately. Any delay leads to frustration and possible accidents,” says Leo Cazaerck, project manager for the Aleris expansion including the crane installation. “But using fibre optics to communicate directly between the two drives gives a faster response without the need for any external hardware. The crane operator gets instant results without any frustrating delays in positioning or lifting. And there will be fewer maintenance problems making the solution more cost efficient.”
The arrival of the crane control program coincided with a challenge faced by the crane operators and the remote control device that was used to position the crane and initiate lifting.
When switching between one and two hook control there was a delay of up to five seconds, resulting in intermittent operation of the drives.
ABB proposed to install a separated supply to solve this problem. TCS installed this on one of the cranes and carried out all testing. It was found to be a satisfying solution. The changing rate was shortened to 100ms. This solution was implemented on all the cranes.
