The drive for better crane control

A Control Techniques product story
Edited by the Engineeringtalk editorial team May 21, 2004

The Unidrive SP offers a number of features particularly suited to crane control.

Crane control has come a long way in the last few years, with "intelligent" drives with onboard software giving improved low-speed and movement control and therefore the ability to limit load sway and eliminate snatch on the hook as well as giving reduced energy consumption and lower maintenance.

All of these benefits stem from more sophisticated control across the speed range, without energy wastage and without the shock loads that cause load swaying and heavy wear and tear on ropes, wheels, gearing, bearings, switchgear and motors.

For both new build crane manufacturers 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 - ie in the drives themselves.

This greatly simplifies the wiring of both new cranes and retrofits and often cuts one major cost factor out of the equation - the PLC.

In addition, the use of a fieldbus network such as Profibus, 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 at home with a new or retrofitted crane.

Traditionally, slewing control has been achieved by applying counter-torque to the slip-ring motor and operators are used to controlling the crane in this way to minimise swing of the load.

The slip-ring motor gives good motor torque for acceleration and deceleration and it is possible to coast when the controller is moved to zero.

However, this method of control 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.

Dockside, gantry and warehouse cranes 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 uneconomical and downtime is seriously affecting the operation of the dockyard or the factory.

The issues of control 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 the swaying of the load.

Without the complete confidence of the operator, it will not be possible to operate the crane to its optimum capacity, safely and predictably.

Control Techniques has this specialised experience and its Unidrive SP offers a number of features particularly suited to crane control.

A secure disable (SD) function meets all the requirements of EN954-1 category 3 or 4 for the prevention of motor operation.

Included as a no cost standard feature on the Unidrive SP, secure disable can be used as the ultimate control function in a safety interlock system; it replaces a contactor which increases reliability, decreases panel space and of course decreases cost and installation time.

S-ramp profiles on all motors prevents jerk and reduces sway and shock loads The hoist drive can be programmed, in the on-board applications module, to maximise hook speed in all conditions, by balancing motor flux to the load.

This can give light hook speeds of three times base speed, or half load at twice base speed, for example.

Again using the plug-in coprocessor module, the amount of sway (being a function of load and rope length) can be calculated and sway can be "quenched" on stopping.

Also, by using one or more of several techniques - load sharing and position monitoring using encoder feedback from non-driven wheels - crabbing can be completely eliminated, reducing wear and tear on wheels, gearboxes and tracks.

Traditional fixed torque motor control of umbilical winders accelerate cable ageing.

A sophisticated cable tension controller can, using a software option on the Control Techniques Unidrive SP - can increase the life of the cable by a factor of three.

Finally, programming within the applications module can recreate the same characteristic "feeling" on the controller.

Find out more about this article. Request a brochure, download technical specifications and request samples here.
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