Sway away

The principle of anti-sway is, in its basics, simple. A load suspended from a rope will act like a pendulum. If the support-point of the pendulum—that is, the trolley on a hoist or on the jib of a tower crane—starts to move, the pendulum and the load on the end of it will start to sway. Slowing or stopping the trolley will cause the rope and load to sway. Anti-sway systems modify the acceleration and deceleration of the trolley in such a way so as to make the two swayings cancel each other out. The rope stays effectively vertical while the load is being moved.

A very skilled crane operator can get close to such a result manually. Computers modifying the operator’s accelerations and decelerations of the crane trolley can achieve it with near perfection. “With a modern anti-sway system the departure of the lifting rope from vertical is reduced to virtually zero,” says Dieter Feldhinkel, technical director of Mannheim-based SWF Krantechnik.

SmartCrane are another company that have been delivering sway control systems since 2001. They explain the principle they use in more detail: “If an operator moves the control stick to full speed, the SmartCrane Anti-sway Control accelerates initially according to the operator’s demand, inducing an initial load sway. When about half the reference velocity has been reached, the anti-sway ‘coasts’, i.e. maintains constant velocity, for a short time. Then the trolley is accelerated again, this time to the full operator demand velocity.

This second acceleration removes the sway induced by the first acceleration, so the trolley is now travelling at the operator reference velocity with the load hanging directly below the trolley. When the operator releases the stick demanding zero velocity, the same process is repeated in reverse to bring the load to a stop without sway.”

All this requires precise timing. The timing depends on the natural frequency of the pendulum motion, which in turn depends principally on the length of the rope above the load. Anti-sway software takes into account changes in hoist cable length as the load is raised or lowered, feeds its modifications of the operator’s instructions back to the trolley, and does so in real time.

Stefan Elspass is in charge of product management for HMI—Human-Machine Interfaces—and control systems at Demag cranes. Demag has been in the lifting business for many years—it will celebrate its 200th anniversary in 2019—and so can claim first-hand expertise in past and present developments. In 2003 the company launched the first processor controlled rope hoist—a milestone, they say, and a first step towards anti-sway.

“We have a long tradition of serial hoist systems,” says Elspass. “We started sway control in 2004. Since then it has gone through several evolutions.

“Over the past ten years waves of embedded control technology have more and more been entering lifting products, especially for users involved in serial production of high volumes of products.”

There are, says Elspass, two philosophies, or approaches, in anti-sway technology. “The first approach is based on parameterisation. The control systems are set into operation by mathematical parameters or algorithms—a mathematical model of the way in which the load will move when the rope lifts it and the trolley accelerates. The mathematics inside the computer predicts the amount of sway, and sends instructions on trolley movements to reduce this to zero.”

SmartCrane uses just such a system to control the primary causes of sway.

“The key feature is that the SmartCrane Anti-sway Control uses precise timing of accelerations to control the sway, rather than real-time sway measurement and control feedback. It does not require a camera or other sway-sensing device to control sway induced by moving the crane,” says the company.

Ari Lehtinen, manager, industrial cranes automation for Konecranes, describes these as “open-hook” systems. “The open- hook system is controlled by mathematical models,” he says. “The mathematics needs the rope length, and the acceleration and speed of the trolley in order to send appropriate instructions to the trolley.”

Sensors mounted on the drum can detect how much rope has been paid out, and this is the primary data that the algorithm needs. “It is a fairly accurate system,” says Lehtinen. “When it is working in good co-operation with the motor control it gives practically no sway.”

He qualifies that, for accuracy: “It gives practically no observable sway. Because of course if there was no sway at all then the load could not be moved except vertically up or down. It is the angle of the rope that moves it along or sideways. But the sway is so small that operator does not see it at all.”

This first approach uses mathematics to predict the angle that the rope will make with the vertical at any time. The second approach, says Elspass, is less theoretical: it uses sensors to detect the angle of the lifting rope, and feeds back that information to the processor that is controlling the crane. He believes that this approach has advantages: “The parameterisation approach assumes that all the surrounding conditions are ideal: for example that the load really is exactly vertically beneath the hook when lifting begins. But in reality there is often a slight angle, perhaps from the operator being in a hurry, which will induce sway. Or wind conditions can change.

Any number of events that are not in the mathematical model can in real life affect the sway.”

Sensors, he says, give information based on the actual, rather than the theoretical, situation at the crane. As well as detectors mounted on the drum, lasers pointed at detection marks on the rope or at the hook can measure distances to great accuracy.

Perhaps the simplest solution of this type is a rope angle sensor positioned at the point where the rope leaves the trolley.

“A disadvantage is that as more rope is paid out, the more imprecise is the measurement. In practice, the useful limit is about 16m of rope.”

Lehtinen confirms this: “The method works well for short rope lengths,” he says, “but for really long rope lengths it is inherently inaccurate. You are measuring a very small angle at the trolley, so a small error in that measurement would translate to a much larger position error at the end of a long length of rope.”

Nevertheless, Lehtinen says, it is a useful system, particularly for the safety-assisting features that sway control can offer as byproduct— features which can be as valuable as its primary function. One such feature is snag prevention: “If the load gets caught or snags some object while it is in mid-air, the sensor will detect the change in angle of the rope, and will stop the crane.”

A related safety feature is hook centring, says Lehtinen: “When the load is on the ground and about to be lifted, if the hook is not quite vertically above it when it starts to lift, the load will drag sideways along the ground, and then swing considerably once it is a few inches up. This of course can be extremely dangerous if the operator does not detect it. These assisting features of sway control prevent the user accidentally doing something that he shouldn’t.”

Given those safety benefits, anti-sway becomes still more attractive. And the benefits are as valid for light-duty cranes as for heavier ones. “Even for small cranes the cost of sway control is now not prohibitive,” says Lehtinen. “So we are looking to provide sway control at the smaller end of the market.”

When sway control was first introduced, five or so years ago, it was expensive, says Findhinkel of SWF, but now costs have come down considerably. “Sway control can be fitted to small hoists of 1t up to the biggest 500t machine—all are possible.”

Process cranes and hoists are a typical market where, as Findhinkel explains, the benefits of sway control are both wide and various. “One benefit is that you can drive the trolley faster, but also further, to the very end of the runway. On a hoist in a factory the end stop becomes a real end stop.

Sway control calculates how fast you can drive your load and stops it smoothly, with the rope vertical, right at the buffer. Even if the buffer is right against the wall of your factory the load will not hit the wall. So you get more useable space: in effect you are enlarging your factory building.”

Findhinkel says, “It allows increases in speed. In applications where the same movement is made many times in succession—a factory repeatedly carrying a load between two places—the time scale for repeat movements becomes quicker. Sway control makes sense for customers who need fast movements of heavy loads. It is very applicable for customers who need speeds of over 40m/ minute. In steel warehouses speeds can be 100m/minute.”

Elspass says: “Target groups at present for our anti-sway systems are logistics companies, companies producing quality & sensitive goods, and who need to transport and position bulky or heavy loads.

“Another group are companies or situations where there are many users of each crane, or where one employee needs to operate many different cranes.”

The reasoning here is that intimate familiarity with the quirks of an individual crane becomes redundant. As Lehtinen puts it: “Another benefit is that it helps the less-skilled operator. With sway control he can concentrate on the load—which is after all the important thing—rather than on the position of the trolley. A first-time user can easily drive a crane which otherwise only an experienced operator could use safely.”

Jari Myyrylainen, vice-president for marketing and sales at Konecranes, says: “Sway control is particularly useful when handling heavy or delicate loads. It is also especially useful in automated crane operations.

“Many of our process crane customers are requiring sway control systems as standard and it is becoming gradually more and more common even in standard industrial crane applications as well.”

Feldhinkel says: “SWF has a training room at our headquarters in Mannheim where we demonstrate sway control to customers. It is not fake but real life. The customer can drive the crane himself, even if he is inexperienced. Videos are one thing but nothing compares to the real feel of actually driving such a machine.

“And when they try it, they are amazed. Their faces say it all. It is always nice to see their faces change. No words are needed after that.”

Just how effective modern systems can be is shown in a promotional video from Konecranes. It shows a fully set out dining table, laid for a romantic dinner for two, being delivered to the waiting couple by crane. The soup is unspilled and the wineglasses are still full. It may not be a standard use for sway control, but it does make its point.

More practical uses are given by Elspass: “A client of ours makes wood panels for houses; each panel can be up to 12m long. Without anti-sway, moving a panel took two people: one on the crane, one on the ground to guide him. When they installed a V-girder crane with sway control the crane operator could safely control everything himself.

“Similarly, a Cologne machinery company needed to transport sensitive loads weighing 2t. Sway control not only minimises risk of accident but it supports moving the heavy parts smoothly, so that the sensitive components of the load are not damaged.” Elspass believes that anti-sway systems are but the first step in the evolution of intelligent and autonomous cranes.

“What we have now is just the beginning,” he says. “Sway control systems used to be only for bigger cranes; now they are cheap enough for every crane. When the crane becomes fully intelligent the operators need no longer worry about how the load is transported: the crane will do that for him. All he has to decide is to where loads should be moved. That turns operators into managers. It is a new era we are entering, an exciting time, a very different world.”