While upgrading the receiving, storage and loadout systems for a large coal handling facility, it became necessary to modify the traveling shiploader to allow it to follow a traveling tripper/conveyor combination on the shore conveyor that runs along the loading dock. Normally the shiploader would be mechanically tied to the traveling tripper, but it was made difficult because of the limited capacity of the wheel drives on the shiploader and a long distance between the shiploader and the traveling tripper. It was decided by the Owner’s mechanical designer that the traveling tripper on the dock conveyor would include a short transfer conveyor to deliver product to the receiving hopper of the shiploader. This traveling tripper/conveyor combination would have an independent variable speed drive that was part of the shore controls, and would follow the speed of the shiploader wheel drives, which were located on the shiploader. However, power to the shiploader was provided by an existing monospiral 4kV cable reel with only power connections and no control connections. A limited capacity wireless provided the controls between the landside facilities feeding the shiploader and the shiploader itself.
Several options were considered, including physically tying the shiploader to the traveling tripper/conveyor combination, adding a control cable reel to the shore conveyor to tie the shiploader VFD wheel drives to the traveling tripper/conveyor VFD drive, and upgrading the existing wireless system to provide real time Ethernet interface between the shiploader VFD wheel drives to the traveling tripper/conveyor VFD drive. In the end, it was decided to have preset speeds for running slow and running fast speeds for each VFD drive. Two limit switches would be added to the end of the discharge of the traveling tripper/conveyor to allow minor adjustments of the traveling tripper/conveyor VFD drive to be made during operation.
The dock conveyor and the traveling tripper/conveyor each had separate VFD drives and were controlled by two separate PLC systems: one on the shiploader itself and one on the landside system. Calculations were made using the drive wheel sizes, gear reducer ratios, acceleration and deceleration times based on equipment loads and motor torque to set target values in each drive system for slow speed and fast speed movement. These target settings in Hertz were set in both the PLC for the shiploader and the PLC for the traveling tripper/conveyor. Limit switches were added on the discharge end of the traveling tripper/conveyor; these limit switches were tripped by targets mounted to the shiploader receiving hopper, which was part of the shiploader. The receiving hopper was slightly larger than the traveling tripper/conveyor discharge hood by about six inches. Because no physical connection between the traveling tripper/conveyor and the shiploader receiving hopper existed, it was up to the VFD drives to keep the two in alignment.
The two limit switches on the upstream side of the receiving hopper worked as follows:
- If both of the switches remained untripped, it was assumed that the travel of the equipment was coordinated by preset speed values in the PLC and therefore no correction of speed was needed.
- If the innermost switch was tripped and the outermost switch was not, a slight correction in the travel speed for the traveling tripper/conveyor was made. A few Hertz were added to the set point, and if the innermost switch remained tripped after a preset amount of time, another slight correction was made. This sequence continued until the innermost switch returned to its normal position.
- Likewise, if both the innermost and the outermost switch were tripped, a larger correction in the travel speed set point was made. Essentially, a larger preset value of Hertz was added to the set point, and if both switches remain tripped after a period of time, another correction was made. If during this time the outermost switch returned to its normal position and the innermost switch remained tripped, attempts to correct reverted to ‘Step Two’. This sequence continued until the innermost switch returned to its normal position.
For downstream correction, we used similar logic but instead had to decrease the set point in the VFD drive for the traveling tripper/conveyor. After all adjustments were made and the system was running fully coordinated, the adjusted set points were loaded into the PLC’s memory to be used for the next operation. As such, the system self-corrected without operator intervention. We found that because of varying operating conditions we needed to provide maximum and minimum correction limits to the preset values.
During commissioning of the system, the operating speeds of slow speed, which was used for small movements within the hold of a ship, and fast speed, which was used to travel between holds or to move the shiploader to its parked position, required different preset speeds and different correction values. This system worked well despite initial concerns with mostly trial and error modifications to the ramp times for speed changes and correction step values for the set points in the system.
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