The PLCs behind container cranes

HMA Power Systems designs electrical systems to operate cranes and the GE Fanuc Series 90T-30 PLC has allowed the company to implement a full range of application solutions for its customers.

In the expanding business of transport by container ships, PLCs are being increasingly used for controlling the cranes that load them.

The trend in the market is to replace existing DC electrical systems with AC systems using PLC control because this technology provides greater flexibility and better performance.

Less downtime and less maintenance increases the port's competitve edge.

Rotterdam based HMA Power Systems has more than a hundred years of experience in developing, building, installing and maintaining electrical systems and machinery.

The company's activities cover three specific application areas: cranes, maritime products, and CHP and power plants.

HMA Power Systems designs systems to operate cranes according to the manufacturer's concept and design.

Its most important applications are in grab cranes, luffing cranes, container cranes and automatic stacking cranes, rubber-tyred gantries and straddle carriers, not only in new-construction projects but also for refurbishing existing cranes.

When it comes to the shipment of merchandise, both by containers and in bulk, the demand to increase the size of the load carried and the speed at which the goods are transferred to and from the ships, continues to grow.

"Time is money" is a very valid motto.

There is now a trend towards twinlift cranes for transferring containers between ship and shore.

These are equipped with a special spreader frame with twistlocks.

The spreader hoists two 20-foot containers at a time, instead of the traditional one, with a speed of 90 to 180 m/min, depending on the weight of the containers.

To cope with the crane control requirements, and to provide the reliability demanded by users, HMA Power Systems turned to GE Fanuc for PLCs.

The GE Fanuc Series 90T-30 PLC has allowed the company to implement a full range of application solutions for its customers.

When used in conjunction with the GE Fanuc Genius(r) for distributed I/O, their advanced configuration and diagnostics enables HMA to deliver efficient, durable and safe systems offering a favourable price/performance ratio.

"Our systems have to withstand harsh harbour environments.

Downtime is very expensive for the port operators, so we have to reduce any risk of this happening to an absolute minimum," commented Wim Rijsdijk, Manager Engineering Department, at HMA Power Systems.

HMA is currently retrofitting a container crane for the port authority at Felixstowe on the East Coast of England.

All motors for activating movement, for example hoisting, moving the trolley, the gantry or the boomhoist, are AC powered.

The crane controls are divided into several parts: The E-house contains the control systems and the motor drives and acts as the control centre for the whole crane.

The communication co-processor module in the main-rack of the E-house communicates via Modbus RTU with the anti-sway-control, cabin control, boom control, etc.

Depending on the complexity of the process, and the consequent amount of I/O, the main rack can be increased up to 7 racks.

From the main PLC, signals go to the control system developed by HMA.

The signals are translated into revolutions per minute and torque for the motors.

Besides the basic functions that the PLC has to perform, a number of sensors provide continuous feedback into the system.

These signals are processed and may cause a movement to stop, or initiation of a motor to be delayed.

The PLC is also used to display errors, and it is even possible to display the exact co-ordinates of the stack, and locate the position of the container on the dockside.

The Operator Cabin contains the last expansion rack or remote I/O rack for data-collection.

It is from here that the cabin operator controls the loading and unloading of the containers.

The driver at the top of the crane controls the movement by means of a joystick.

Depending on the deflection of the joystick the movement will have a certain speed.

Because of the large distances involved between different parts of a crane, and the quantity of signals which are transmitted to and from the PLC, HMA uses PLCs with remote I/O at several locations.

For example, there is a PLC in the operator's cabin which processes the signals from the joystick and pushbuttons.

The signals from the joystick go to the main PLC in the E-house through this PLC with remote I/O.

"In modern cranes, PLCs with their advanced technology represent an important part of the control system of the crane.

GE Fanuc's Series 90T-30 and Genius bus technology offer the flexibility and reliability for such critical applications.

The PLC can communicate directly with a PC for early warning of threatening faults and near-breakdowns, and allows the necessary action to be taken straight away.

Thus several cranes can be connected to a service building.

It even becomes possible to carry out remote fault diagnosis," added van Egmond, of HMA.

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