Anderton升艇机中的DeviceNet
Source: CAN Newsletter December 2002
Located on the banks of the River Weaver, at Northwich in Cheshire, the Anderton Boat Lift is one of the greatest monuments to Britain's last canal age and known as the "Cathedral of the Canals". Built in 1875 it was the world's first boat lift and proved to be the prototype for others across the world.
The lift was designed by Edwin Clark and built in 1875 to speed up the movement of cargoes between the River Weaver and the Trent and Mersey Canal. The 50 feet height difference between the two waterways was a constant problem to the salt barges that transported along the navigations. Clark's solution was to use a revolutionary new system of hydraulics, where the tanks or caissons are supported by a single central hydraulic ram. A short interconnected tunnel was constructed which allowed flow of the hydraulic medium 'water' between the cylinders. However after a decade of pollution within the river, corrosion of the pistons and seals resulted in closure of the lift in 1906. After this, the decision was made to change the lift to an electrically driven mechanical operation. The lift continued to work like this until in 1983, when during routine maintenance work, serious corrosion was discovered. Discovery of this resulted in the lift being closed, for what many thought would be the last time. In 1998 a successful application to the Heritage Lottery Fund secured £ 3,3 Million towards the £ 7 Million total cost of restoration. It was decided to restore the lift back to its 1875 hydraulic operation.
Fairfield Control Systems were awarded the contract to design and install the electrical system. As well as integrating the new hydraulic system into the original structure, there was the challenge of integrating the legacy of the gates and wedges into the modern day control system. The original system operated with water hydraulics and relied on a difference of water levels between the two caissons to make the descending caisson heavier. The other caisson was filled from the aqueduct to a level 150 mm higher than that of the lower river caisson. A valve connecting the two rams was then manually operated to allow the caissons to change positions. Problems in achieving the differential pressure between the two caissons led to a 'fully pumped' design system having to be implemented with the hydraulic medium now becoming mineral oil. Two methods of operation were designed in to the new system: A 'balanced mode', where both caissons are operated simultaneously, and an 'isolated mode', which operates either caisson in isolation. With the new balanced mode, both caissons have an equal amount of water in them. Thus with the caissons balanced, oil has to be pumped from one side into the other to achieve movement. This is done with two main 45-kW pumps providing up to 1300 l/min of flow. Control and acceleration of this flow is provided by a 'S' curve algorithm, designed by Fairfield Controls. Smaller 18,5-kW pilot pumps can be used to overcome natural leakage within the system and maintain the caisson positions. When operating a caisson raise in isolated mode, two 90-kW fixed displacement pumps are required. These provide the hydraulic power that would normally be provided by the lowering cylinder when in it is balanced mode. This can also be referred to as a 'virtual caisson'. When lowering in an isolated mode, just the main pumps are required to push the oil back to the main reservoir. Position measurement of the rams is achieved by a highly accurate position measurement system known as 'Cims'. Each ram has been finely serrated and covered in a protective 'Ceramax' coating. Cylinder position is determined by an induced pulse count from the serrated ram, giving an accuracy of 1024 counts per cm.
Fairfield's have used their expertise to design an interface card to the non-standard pulse train, which provides interfacing to a standard high speed encoder allowing a high cylinder position accuracy. The control system consists of a six tear, PLC-controlled MCC panel located in the meter building, and a series of distributed I/O systems located at the river and aqueduct areas. Operational control of the lift is designed to be remote from the structure itself. Thus the main lift operator can control the sequence of the lift from the soon to be built £ 1 Million operations center, utilizing the CCTV (closed circuit television) and public address systems provided by Fairfield Controls. Operation of the gates, wedges and flooding valves are performed by the local operators, via interaction of local panel view interfaces. The PLC rack consists of a power supply, the CPU and three DeviceNet network scanner cards. All three DeviceNet networks emanate from this point. The PLC contains the three scanner cards, which are used to interface the three networks to the PLC. The scanner cards run in order from left to right - networks A, B and C. The DeviceNet power taps provide each network with 24-VDC power supply, while the open style taps provide the outgoing termination points for each of the networks. The E3 overload relay replaces the normal overload, which would be seen in a traditional starter circuit. The E3 overload-relay is a multifunction solid-state micro-processor based electronic overload relay for the protection of squirrel-cage induction motors rated from 1 to 2250 Amps. The E3 overloads communicate with the PLC via to the DeviceNet network. The main PLC rack consists of the rack, power supply, CPU and three DeviceNet scanner cards, one for each network. The CPU is an SLC 5/05 with 32 KiB memory. The three DeviceNet scanner cards allow connection of the three DeviceNet networks to the PLC.
The system uses a distributed I/O scheme, consisting of the main PLC and the Flex nodes attached to it via the DeviceNet network. The Flex I/O nodes perform the function of the more traditional rack mounted I/O cards, except that the equipment is hardwired to the I/O point (Flex nodes) locally and the data relayed back to the main PLC via the network. Each of the Flex nodes has its own individual address on the network. Each of the scanner cards contains configuration information, which must be re-downloaded, should the unit be replaced. The configuration information should also be modified if any nodes are added to any of the networks. Scanner card configuration is carried out by Rockwell's (www.ab.com) DeviceNet configuration Software - RSNetworx. Under normal circumstances, no network configuration is necessary.
