Crane control has come a long way in the last few years, with ‘intelligent’ drives and on-board software giving improved low-speed control and therefore the ability to limit load sway as well as to reduce energy consumption and maintenance.
It may sound too good to be true, but all these benefits stem from more sophisticated control across the speed range, without energy wastage and without the shock loads that cause loads to sway and introduce heavy wear on bearings, switchgear and motors.
For both OEM cranes and for the very buoyant retrofit market, the trend is now towards distributed control – that means with the processing being as close as possible to the motors i.e. in the drives themselves. This greatly simplifies the wiring of both new cranes and retrofits and often cuts one major cost factor, the PLC, out of the equation. In addition, the use of a fieldbus network such as Interbus, DeviceNet or CTNet, simplifies communication between drives, reduces wiring costs and remote I/O, and makes functions such as digital lock between drives very straightforward. Software can also be used to provide a traditional ‘feel’ to the joystick controls, so that crane drivers immediately feel comfortable with a new or retrofitted crane.
Better control in Norway
Slewing control, for example, has traditionally been achieved by applying counter-torque to the slip-ring motor and operators are used to controlling the crane in this way to minimise load swing. The slip-ring type motor gives good torque for acceleration and deceleration and it is possible to coast when the controller is moved to zero.
This method of control, however, is very poor at low speeds, with sudden steps in torque between resistor steps wasting a lot of energy and causing excessive wear and tear. Nevertheless, modern systems need to mimic this method of control to give the driver immediate confidence and familiarity with the controls.
A typical example of this mimicry can be found on a 50t crane built by Stålprodukter in Molde, Norway, for the port of Tonsberg 100km south of Oslo. For container handling this crane has an operating radius of 11m to 30m and a hoist speed of 11.5m/min. All four drives for the hoist, boom control, slewing and track drives are Unidrive universal AC drives from Control Techniques. The 30kW Unidrive for the slewing control has additional on-board processing that runs the slewing control software. This provides the driver with control over both the speed and the motor torque.
Speed control is also important for accurate positioning at low speed. And it provides compensation for wind forces on the load. Torque control is crucial for controlling load sway. In this way, the driver is always able to anticipate the movement of the load and to compensate for it. By bringing the controller back to zero, the movement is effectively coasting, which gives a major dampening effect on a swaying load.
The 50t Tonsberg crane also features an energy saving design, with the 90kW boom drive running in regenerative mode as the boom is lowered, power is fed back to the mains supply and the need for large and costly resistor banks is eliminated.
Retrofitting
Dockside cranes – indeed, cranes of all types including gantry and warehouse types – all have one thing in common. The electrical systems invariably have a shorter life span than the very substantial mechanical parts. There is a point where the time and cost of maintenance of the crane’s drives becomes uneconomic and downtime is seriously affecting the operation of the dockyard or the factory. This explains the substantial market for retrofitting crane drives but, for the user, it is a potential minefield.
Control issues are very specialised. All too often, when replacing the slip-ring motors with a modern drive system, the results can be very disappointing – and, in extreme cases, using a conventional speed-controlled system, it is virtually impossible to control load sway.
Without the complete confidence of the operator, it will not be possible to operate the crane to its optimum capacity, safely and predictably. The options are to go for a PLC-controlled system or one, such as a Unidrive, where the processing is localised. There are, however, other issues to be considered.
• Has the supplier the experience to install and provide the necessary ongoing technical support?
• Are spare parts readily available from stock?
• Are there issues relating to the way in which the drives operate? And can they be improved?
• Are there changes that must be made to conform to current safety legislation? Is the supplier fully aware of what these are?
When designing a system for retrofitting, the supplier has to ensure that it is suitable for installing in situ in the existing environment, with the appropriate level of protection against damp, spray or dust. They must also demonstrate a level of understanding about how a crane is used in practice, with the need to recreate the same characteristic ‘feeling’ on the controller.
As for the maximum capacity of the crane, this is only valid provided that the controls for the crane movements are tuned in such a way that the driver can control this maximum load easily and safely. Otherwise the crane is effectively de-rated to the maximum load which is safe to handle.
When a crane is ready for retrofitting, it is worth considering other features that can be incorporated at the same time. Productivity can be considerably enhanced with the facility for high-speed empty hook operation – typically, three times base speed. A sophisticated power cable reeler tension control – again a software option that can be built in – soon pays for itself in reduced downtime by preventing cable fatigue and breaks. With a long travel gantry crane, absolute skew elimination is achieved with digital lock function for the twin motors – again a software function – and standard application macros for axis limit and brake control simplifies the commissioning of the drives.
