Trutegra was hired by a cement manufacturing company to semi-automate a bucket crane in one of their facilities. This type of crane has a bucket at the end of two separate cable systems requiring two separate hoists, one a holding hoist that lifts and lowers the actual load and the other a closing hoist that opens and closes the bucket. Software controls both hoist motors to ensure synchronous operation on the way up and down, avoiding the bucket opening or closing in transit. A sister plant in the same company, learning of this project, requested the installation of full automation at their facility. Trutegra completed the two projects in parallel, building on the one application to complete the task.
As may be imagined the manufacture and handling of cement is an extremely dusty operation. In earlier times an operator sat in the cab on the crane to control its position, the cab was fitted with very heavy filtration to minimize the dust. It became obvious that this mode of operation was both hazardous to the operator and inefficient so Trutegra was approached to semi-automate the bucket crane at one plant. Trutegra’s engineers realized that because the visibility in the plant was reduced to some 10-15 feet, optical laser feedback devices could not be used to position the crane. Therefore, radar-positioning technology was selected by Trutegra to accomplish that operation. Radar is able to see through the dust, but it has a very slow update rate, requiring the feedback to be augmented with an encoder to match the rate required by the processor in the control system.
The basic control system provided by Trutegra for a single bucket crane included provisions for distance measurements for the bridge position, the trolley position, and the encoder for the position of the hoist. Trutegra’s No-Sway technology was also included to essentially eliminate load swing, avoiding loss of cycle time and hazards to operators, structures, and equipment. The basic system controls also included TruMotion Semi-Automated Positioning which allows the operator to input desired locations via the HMI. In addition, continuous skew control was incorporated allowing the PLC to monitor and control skew by varying the speed commands to the bridge motors so that wheel alignment is maintained. Skew is monitored in real-time and excessive skew triggers an alarm and the crane is stopped.
The rate at which materials enter the area is calculated and upon reaching a specific value the material is transferred to a storage bin. A similar method is used with the hopper that feeds the furnace. When the level in a feed hopper drops below a predetermined value, the material is picked up from a high spot in a particular bin to be added to the feed hopper. A warehouse management system, known as TruStor effectively manages all the inventory levels as well as monitoring the drips (the feed to the crane hall) coming in. TruStor can age material as required. For example, hot material is left to cool for a specific amount of time before it is being fed into the next available hopper and moved into the system. It is important to pull from the coolest bin first to avoid overheating the process downstream.
The inventory of material in the bins below the crane must be measured and a powerful LIDAR device was utilized to sweep the area below the crane as it passes and moves up and down that area to create a three-dimensional map. The crane PLC uses that data to identify low and high spots in each of the bins while the HMI generates a 3D graphic for the operator. A series of “picks” (referring to the picking of material from a high spot) and “dumps” (referring to dropping material at a low spot) are prioritized by TruStor to maintain an even level below the crane.
A major challenge in implementing both the semi-automated and fully automated system was the very limited visibility in the plant. In addition, the way the material flow works proved difficult. This latter challenge arises from the fact that the bucket contacts the material. To close on a full bucket of material it is necessary to pay out a little more cable than necessary because the bucket tips and sinks as it digs. This extra amount of cable is calculated by “some neat math”, but it is important to avoid the cable jumping the groove on the drum above or having cables come off the drum. In either case considerable downtime results. One other problem was caused by the temperature of the material since the tilt sensor on the bucket had to survive at temperatures between 300 and 400 degrees Celsius. It must be recognized that Trutegra was unable to refer to other semi or fully automated cement-handling crane projects so that the system introduced was designed and proven out entirely in-house. It may be said that Trutegra “invented the solution to the problem.”
Future projects could be to install the automatic system at other locations. However, some of the cranes in those other locations would require mechanical upgrades or possibly replacement due to their age and the abuse typically experienced by the cranes. It is a very harsh environment leading to expensive mechanical wear and maintenance.
The benefit realized by both automation and semi-automation of the bucket cranes is a reduction in maintenance costs and downtime as well as improved safety, with better material throughput and reduced production time.