Overhead cranes are used around the world for moving cargo containers, crates and other materials to different locations in warehouses, train yards, and shipping ports. Controlling the speed and direction of the cranes is of utmost importance to prevent collisions and damage to either the cranes or the materials being moved.
The operator is stationed either in a booth mounted on the crane or on the ground with a remote control pendant. In either location, accurate feedback from the crane is essential to determine its position and speed.
This feedback is typically provided by sensors mounted on the crane to detect the distance and position relative to a reference point. Alternatively, an encoder may be used to count pulses. The outputs of the sensors or encoders are transmitted to displays or programmable logic controllers (PLC) through cables or wires.
Cable Stress Increases Maintenance Cost and Failure Risk
As the crane moves throughout the work area, these cables are continuously flexed and bent, creating a critical maintenance issue. Many overhead cranes use flexible conduit or tracks to reduce the stress of the flexing on the cables. Some use loops in the cables and wires to minimize movement during the flexing. But even with these solutions, the cables and wires are at risk of failure and eventually need to be replaced.
Wireless Communication Eliminates Long Cable Runs
Wireless I/O devices, like the SureCross DX70 and DX80 from Banner Engineering Corp., are being used increasingly in both new crane designs and retrofit installations. Eliminating the cables and wires decreases both the cost of installation of the sensors and encoders and the cost of maintenance on the overhead cranes.
In one of the many applications of wireless control, a manufacturer of sheet metal rolls and metal plates uses an overhead crane controlled from the floor of the warehouse. The location of the crane is monitored by using 2 retro-reflective laser sensors. One sensor measures the (X) location along the length of the warehouse, and the second measures the (Y) location of the crane along the width.
The two sensors provide analog outputs connected to numeric LED displays that indicate the crane position in both the X and Y axes of movement. The operator of the crane reads the information from the displays to correctly position the overhead crane to pick up and place the metal rolls or plates in the proper locations.
The retro-reflective laser sensors each give a 4-20 mA analog output, which is connected to an analog input on the wireless I/O device. The wireless device checks the analog output of the sensors as many as 8 times per second to monitor the position of the overhead crane. Since the wireless device and both sensors are mounted on the moving crane, very short runs of cable are needed to connect them and they do not bend or flex.
The wireless device transmits the analog output information to a second wireless I/O device that is installed next to the digital displays. This device has 2 analogue outputs that are connected to the digital display inputs, one showing the X location of the overhead crane, and the second showing the Y location. Based on the digital readouts, the operator uses the remote control pendant to move the crane to the correct location.
Compatible with Many I/O and Control Devices
In addition to discrete and analog I/O, industrial wireless networks also allow communication via RS485 and RS232 serial communication, and Ethernet TCP/IP communication. This allows the wireless networks to interface with many different types of I/O devices, as well as PLCs and computers.
Signal integrity is important in all wireless networks. Some concerns when using wireless signals are that objects might block the signal or that other radio sources in the area could interfere. These are valid concerns and are accounted for with the design of modern wireless I/O networks.
The use of frequency hopping spread spectrum and time division multiple access protocols prevent much of the interference from other radio sources and objects. Some wireless I/O networks also include a communication or link loss indication and an output default condition that prevents crane operation if the signal is lost. These features result in robust, reliable wireless communications.
The use of the wireless I/O devices saves money in three ways:
◦During control system installation there is no need for long runs of cable.
◦Maintenance costs and system downtime are reduced because there is virtually no risk of broken cables.
◦Reliable location information transmitted by the wireless devices reduces the risk of collisions. This reduces costs for replacing damaged materials or parts of the overhead crane.
Author :Lee Kielblock
Source : http://www.ipfonline.com/
Manufacturers : Banner Engineering India Pvt Ltd, Pune 411 016. Tel: 020-66405624. Fax: 91-20-66405623. Email: sales india@ bannerengineering.com)
1 comment:
Fantastic description for wireless communication. With the proper experience and training, jib cranes can be extremely safe and useful on any job.
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