Electrically operated traction cranes

Pressure from the market has compelled industries to seek new avenues for improving productivity and safety through the application of higher-level automation. Cranes form one of the major items in all kinds of industry. In many places productivity depends on how efficiently the cranes are managed.

Wireless remote control systems have come to acquire a special place in this environment. A wireless remote control system has been commissioned in one of the existing EOT (electrically operated traction) cranes in the billet & bloom mill of one of SAIL’s (Steel Authority of India Ltd) integrated steel plants. The system installed has greatly increased the operational efficiency of the crane and ensured ‘accident free’ operation. The benefits reaped from the system have compelled other production shops to adopt the new technology/

The innovation relates to the introduction of a wireless radio control system to control the existing EOT overhead crane in steel plants. In the conventional system, during loading, unloading and shunting in shop transportation, the crane operator is required to be stationed inside the driver’s cabin to operate the crane movements until completion of the task. A second person is required to be stationed on the shop floor to hook the sling to the material to be transported by the crane. Since the cabin, which houses all the joysticks and switches for manually driving the crane is typically 10 meters above the shop floor, the person stationed on the ground has to give directions to the cabin operator when moving the load. During such modes of operation the crane operator in inefficiently used and the cycle time for completing a function becomes high for the following reasons:

The operator cannot perform any other job. The operator remains idle for a considerable time as driving is dependent on the completion of other ground jobs.

The visibility of the operator from the operator’s cabin is restricted. Most of the time the operator is completely dependant on the signals from his coworkers. This creates a poor safety condition. Accidents can take place as a result of incorrect signaling or interpretation due to human error.

The poor visibility reduces the speed of operation and thus lowers productivity. A team of people is required to complete a material handling task (as separate operators are required for operating the loading and unloading).

Conventional EOT cranes have three motions: Long travel (LT); Cross travel (CT) and Hoist travel (HT). Additionally, cranes are provided with another hoist called Auxiliary Hoist Travel for lifting small loads in conjunction with hoist travel. There are four drives powered by 3-phase, 415V AC supply, the speed of each being controllers in the motor drivers through cables.

In a steel plant, the crane motions are operated bi-directionally by a reversible air break cam operated master controller in 4/5/6 step speed. These master controllers are provided with a crank handle or joystick handle. There is no position for ‘emergency stop’ for manual intervention as such, however there is an emergency switch, which when pressed cuts all power thereby bringing the system to a halt. Conventionally, the operator operates the individual master controller located in the cabin.

The objective of introduction of a wireless radio remote control system in the existing crane was to enhance operational efficiencies, reduce dependency on crane operators and improve safety. A wireless radio remote control system gives the following benefits:

- Faster operation and therefore greater productivity;

- Improved safety;

- Lower manpower requirements for completing a particular task.

CONTROL REQUIREMENTS

The four motors for each motion operate on three phase 415V, 50Htz AC supply. However, the drive control circuit is operated on a 220V DC supply. Four 4-0-4 step master controllers are located in the operator cabin and are provided with 12 contacts. The closing sequence of these contacts depends on the position of the joystick lever at 1 of 9 notches of the 4-0-4 step of master controller. Each contact corresponds to a particular speed for a particular type of motion. To introduce the new system demanded insertion of a parallel path that could use the existing control sequence and the controls.

The control parameters:

-The closing sequence of master controller;

- The crane operates as long as the radio communication link is active and healthy;

- ‘Remote” and “Local” operation are mutually interlocked;

- Simultaneous operation of two kinds of motion is provided.

Design of control system

The system was designed to provide reliable, compact and user-friendly operation of the crane in a way that readily integrated with the existing controls and selected for continuous use in heavy industrial service. The main components of the system are 1) Wireless radio interface 2) Microprocessor based controller 3) Visual motion display unit and 4) Electrical interfaces.

This successful implementation of the radio control system opens up scope for using the technology in wide ranging plant operations, where conditions due to tough environmental conditions and poor visibility. The system is easily adaptable to both AC and DC crane control circuits. This is just the beginning of technical and cultural changes within SAIL.