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Mobile controller transforms cranes into intelligent machines

Fig.1: Indusign builds 10 to 14 transhipment cranes per year. They are sold worldwide under the brand name E-Crane.

Faster loading and unloading of ships becomes more and more important due to the increasing demand and the increasing number of bulk carriers. This means that bigger transhipment cranes are needed for bulk processing. The loaders, however, have difficulties to find experienced crane operators. This is why the industry is looking for more powerful cranes which are as easy to operate as possible.

In the last years, these trends provoked an evolution at Indusign, a Belgian manufacturer of transhipment cranes based in Adegem. The company was founded in 1990 as an engineering office, but after some years and on request of several customers they changed over to manufacture. Today the company builds 10 to 14 cranes per year which are used worldwide under the brand name E-Crane.

Some impressive figures: the biggest crane has a height of 25 m, a range of 45 m, can move 45 tons per cycle and is equipped with a drive motor of 600 kW. The use of these giants has become common for a long time now. Just 10 years ago the transhipment crane had an average capacity of 5 to 10 tons; today it is 10 to 30 tons. It is the only possibility to speed up the logistic processes in the harbours. These cranes cannot be compared with hoisting cranes. The latter ones are actually designed for a limited number of movements per day, the transhipment cranes however can be used permanently. Moreover, these machines are expected to be especially flexible as the cranes must be capable to move containers, lift a bob cat on board, etc.

With the E-Crane, Indusign has integrated an electronic control of the cranes in the last years. The first reason was to replace the initial joysticks with hydraulic valves by an electronic version, given that the hydraulic version is used less and less. A controller ensures then the conversion of the electronic control to a control of the hydraulic valves installed on the crane. Once the controller has been put into place, more and more functions were added so that today it is an intelligent machine with a control, visualisation and modern diagnostic possibilities.

A balanced Project

The letter E in E-Crane stands for equilibrium and relates to the crane-specific concept of balance. The boom (main arm) is a parallelogram. Its second arm is a connecting rod that links the front arm and the counter-weight of the crane. The movement of the front arm is made by swinging the counterweight by means of a hydraulic cylinder, which is transferred to the front arm by the connecting rod. Result: the counterweight swings back when the front arm is moving forwards so that the gravitational centre of the whole structure remains in place.

Thanks to this equilibrium energy can be saved due to the fact that the power of the motor is only used for lifting the actual load and, of course, for moving the crane, but not for compensating a disequilibrium. In contrast to cable cranes this construction offers another advantage: the load cannot swing so that it is possible to work faster and more independently of the experien-ce of the crane operator. The cranes are operated via two joysticks. Further to the movement of the boom and the front arm they offer the possibility to swivel the crane and to open, close or rotate the grab. All these movements are driven hydraulically, therefore the crane has an electric motor and a hydraulic pump on board.

The changeover to an electronic control is due to the fact that the previously-used joysticks with hydraulic valves are no longer available. They were to be replaced by electronic joysticks and a new controller for the conversion of electrical input signals for the control of the hydraulic valves. Soon it became obvious that the electronic control offered many additional possibilities such as control of acceleration and deceleration at the end of the work zone. The behaviour of the crane is less abrupt and has thus a positive effect on safety and durability.

At the same time it was possible to connect all kinds of sensors to the control so as to adapt the speed to the load weight and to integrate the protection against overload of the control.

Another logic development is that the operator of the crane has now a graphical interface with a numerical reading of the crane status. The load of the crane is the most important parameter for the operator, but also a whole series of error messages are displayed graphically on the interface. The interface features among other a high number of diagnostic screens enabling the access to a series of status data of the crane. A flash compact memory card in the graphical interface saves about every other second the values of about 60 variables.

All this has considerably changed the working methods of the operator and the supplier's maintenance staff. In the past, for example the connection cable of the dynamometric sensor (load cell) of the crane arm deteriorated and led to the immobilisation of the crane for reasons of safety, and the safety system was bridged in the relay control cabinet.

Today this is done on the software level which means that you can decide together with the supplier to temporarily bridge the safety system. This procedure is recorded, can trigger an alarm, etc. and thus allows to temporarily reduce the speed and program a maintenance intervention. This implementation improves the efficiency, reliability and lifetime of the machine.

A mobile controller for extreme conditions

After careful consideration, the mobile controllers of the R360 series from ifm electronic were chosen for this application. A typical advantage of "mobile" PLCs over "industrial" controllers is their increased robustness. In this case the controllers feature a shock resistance with certificate 1; they function in a temperature range from -40 à +60 °C, have the protection IP 67 and are highly resistant to electromagnetic interference. Another interesting aspect of this application is the low supply voltage which allows supply by battery. Thus the crane can function independently.

Another important criterion for the crane constructor was the open character of the system. Indusign has an engineering know-how and wants to develop and master the technology of their machines. The PLCs from ifm are programmed via CodeSys, an open standard for which exists a complete library with software modules for the different components of the installa-tion. Thus, the company does not depend on a single-supplier of systems as regards sensors, valves and other components. With regard to this know-how they applied a rather spontaneous but efficient method to involve all employees in the changeover to an electronic control. A test installation with PLC, human-machine interface and a simulation of all applicable inputs / outputs was provided in the canteen. Each operator had the opportunity to practise in his free time. Further, the engineering department has a similar installation for test purposes and implementation of customer requests.

The automation of the crane is based on CAN bus for which the mobile PLC has a standard interface. There are five I/O islands on the bus: two in the "power unit" with the motor and the hydraulic pump, two in the driver cab and a last one on the crane chassis near the valves. Even the display, also developed by ifm, is connected to CAN bus and provides access to all system parameters. On the interface level are also all variables periodically saved on a memory card. This history is useful when it comes to diagnosis and maintenance for which specific interfaces were again programmed on the display level.

Remote diagnosis

Automation of the cranes has brought with it another possibility: remote diagnosis. For this purpose a special GSM modem was integrated in the system on the CAN bus. This modem allows to call each crane wherever it is in the world, from the plant in Adegem. It is a triband modem that adapts itself to the network of its location. Remote connection is technically not difficult. The plant has access to the variables on the bus just like the display on the crane. Using a similar software it is thus possible to view in real time the same cranes as the crane operator. The previous example of a faulty sensor cable can be solved by the modem together with the user and the supplier. Different user levels determine who can take part in which intervention. The data received via the modem can also be transmitted to the test installation of the engineering department.

A problem resulting from such a concept with CAN bus is that the PLC will play a central but also crucial role for the functioning of the crane. Whereas experienced maintenance staff used to handle many individual adjustments, today all modifications are started with the software. For these giants it is a rather strange situation as this mobile PLC seems at first sight to be a negligible element of the whole system, with regard to dimensions or investments. Indusign applies the same open approach to this problem as for selecting the controller. In other words, each customer who wants to carry out adaptations on the control receives the source code from the supplier. In praxis, such a demand has not yet been made. The cranes are part of a market that the manufacturer knows to the core and where all functions of the system are developed to meet the demands of the industry. A last opening: from the crane cab an interface can be implemented with other software systems from the user.