Technology moves quickly. The sci-fi fantasies of a very few years ago are the unremarked realities of today. Antisway is one of them.
Josh Childers is training and technology manager at Ohio-based R&M Materials Handling, and was working in the crane industry when anti-sway was introduced to the market. “In 2005, I was working for R&M’s parent company and they were the ones who, around that time, brought sway control to the marketplace. It was what was called passive sway control. Calculated sway control is another term for it.”
Sway control comes in two types. One type is passive; it is also called sensor-less sway control, open-loop or calculated. The other type is sensor-based, also called active or closed-loop. Passive sway control is the simpler of the two.
“It works off the pendulum principle. A load on a rope or chain is essentially a pendulum, and how much it moves when you move the bridge or trolley does not depend on the weight of the load. It only depends on how long the rope is. You have to add in the length of the sling as well, but once you have that it is just a mathematical calculation to work out how the load is going to swing,” explains Childers.
“The technology at that time just used a small encoder that counted how much rope had been paid out (for example, do I have 1m or 2m?) and they added a compensation for the sling length. That gave the total pendulum length, and once that was known, the PLC did a small calculation and said, ‘Hey, the pendulum is this long. So, if we give it a horizontal movement, we need to compensate for that up top where the trolley is.’ And it told the bridge and trolley inverters how to move, how much to accelerate or decelerate to keep the pendulum from swaying,” says Childers.
It is known as passive because it was based on calculations, rather than knowing how much the load is swaying. The only input is the rope length, speed and directions of the trolley’s horizontal movements. “
“If a passer-by walks up and gives the load a push, or if a forklift bumps into it, or if the wind blows the load around – the system has no clue. That was a limitation. It is working in a perfect world of its own,” says Childers.
Reality strikes
Real life or course is not perfect. Forklifts and passers-by do exist. “For that reason, you really wanted an operator keeping an eye on it, to stop it when real life intervenes,” explains Childers.
What, then, was the advantage? “You have to remember that this was 20 years ago, back in digital prehistory before things like the smartphone were invented. Back then, it was revolutionary. Sure, you still needed an operator. But even newer operators who maybe didn’t have the experience to manually control the weights could now be entrusted with it. And if you had sensitive or delicate loads, it could really reduce the damage to them. So for the time it was phenomenal.”
But time moves on, and we now have active sway control – sensor-based or closed loop systems. “Rather than relying solely on calculations we clamp an inclinometer to the rope. It sends back the angle of the rope, so the crane intelligence device now is getting realtime information and live feedback. It ‘knows’ the position of the load, not just theoretically, but for real; even if someone has given the load a shove, or if it has bumped into something or if the wind has blown it sideways,” he continues.
“Now it is actually using the real-time measured position of the rope and it modifies the speed of the trolley and bridge machinery to keep that rope in a non-swaying state.”
Active sway control is now powered by an inclinometer attached to the rope that provides a real time measurement of the rope angle and direction of movement. The rope position and movement is actively measured rather than passively calculated. The inclinometer feeds the position measurement to the HoistMonitor Enclave crane intelligence device that communicates with and coordinates the bridge and trolley inverters to eliminate any sway detected, whether is it created by the crane movement or by an external force placed on the load.
So passive anti-sway is dead? According to Roland Najbar, it still has its place. Najbar is business development manager for cranes at Siemens. Siemens have both open-loop and closed-loop sway systems in their portfolio.
“Open-loop or sensorless systems are less expensive,” he says, “because they are based solely on a control algorithm in the drive. They are not suitable for outdoors application, where wind and the like can be a large factor: mathematical algorithms alone cannot predict ever-varying wind forces, so sensor-led systems are required for those applications. For applications where there are external forces, such as wind, closed-loop systems are required.
“Another scenario that an open-loop system cannot detect is where a person – usually someone rigging the load – introduces movement on the load, or if it swings due to the initial lift not being quite vertical. A truly openloop, sensor less system will have no way of determining if the load is swaying or moving: it always assumes that the load is starting from a standstill. So, if a load is already moving, the open-loop sway control system will have a difficult time stabilising it.”
Closed-loop systems, however, have input from sensors and closed the control loop. They use this feedback to automatically adjust their behaviour for a desired outcome.
Safety and experience
Anti-sway in one direction is easy to understand. In many cases – in an overhead bridge and trolley system, for example – loads may sway in both X and Y axes. In the case of a jib crane, slewing will make for more complex motions still.
“In a typical crane with hoist, bridge and trolley, the sway control occurs in the bridge and trolley directions,” Najbar says. “Usually, this is all controlled by the same motion control system, and if sensors are involved, it would usually only involve sensors – say, a camera – on the hoist where the cable pays out and a sensor, a reflector, on the load block.” The camera sends a pulse to the reflector, and from the time of the journey can work out the distance the pulse has travelled. “That way the system can actively calculate the angle of the load and eliminate the sway in the system based on the desired speed or positioning of the load. A slewing system is harder to control because more calculations go into it, but it can be done with the correct closedloop sway control system.”
Najbar goes through some of the advantages – obvious and less obvious – of anti-sway. Primary, of course, is safety.
“Absolutely, and especially in tight spaces that use manually controlled cranes, such as some automotive or battery assembly applications where people are involved. Reducing the sway or preventing off-centre picks can reduce injuries and fatalities because the load is no longer swaying around. In some systems, off-centre picks are automatically not allowed. This system will require the trolley to auto-centre over the load before lifting to prevent the case where a load is lifted off the ground and swung into a person.”
There is also ease of operation. “The beauty of sway control systems now is that you can have an on-the-fly active damping factor that determines how much sway control is offered. So, for example, if you have a new operator, you can increase the damping factor which will increase the gain on the sway control system, most likely making the new operator’s job easier. But if during the next shift you have an operator that has been doing this for 40 years, they are the sway control, and may want to reduce the system’s contribution to the bare minimum or completely turn it off. So, you really have a system that appeals to the majority.
“Speed is an advantage, especially in semi-automated and fully automated cranes. There are of course mechanical and electrical restrictions, but especially in a closed-loop system the crane knows exactly where it’s going and automatically eliminates the sway along the way to its position.”
And there are some benefits that are less immediately apparent. Anti-sway can give more accurate positioning of set-down, down to millimetre-level if required. It also gives longer lifetime to your lifting machinery. “The number of start-stops and control alterations in each lift is much less than under operator control – that, and the fact that there is no sway in the system also reduces the mechanical stress on the system.”
Collison avoidance is another digital-led technology that, in fact, pre-dates sway control but can be integrated with it into a single cranecontrol system. It does, though, require another set of sensors.
“Collision avoidance has been out for some 30 years or more and is generally used to keep cranes that are operating on a shared runway apart, or to create a zone where the crane is not permitted to operate,” says Childers. “And what you need to avoid collisions is some way of knowing how far away the object you might collide with is. At R&M Materials Handling, we offer multiple types of collision avoidance sensors, and the type used will depend on the environment where the crane is installed. Collison avoidance can be provided for cranes with or without sway control. Collison avoidance is a key feature for customers looking to increase operation efficiency and reduce the risk of an incident caused by a crane or load striking a machine or structure.
“R&M has combined both collision avoidance and sway control technology into the crane package offerings. At the heart of the system is fieldbus technology,” he adds. “In the past each of those commands – of anti-sway, of anti-collision, of overload protection and the like – were sent on individual wires, with one set of wires for each function and virtually no data sharing between devices. Fieldbus simplifies all of that: a single four-wire cable connects all the electrical controls together and to the HoistMonitor Enclave which is the brain of the system.
“And the huge advantage that it gives is that it allows all the commands to be sent very quickly over a network, creating one communication system that allows each device to communicate with all other devices that enable the sway control and other operator assist features. The sum of the parts is greater when connected together than when operating separately.
“This has been available on high-end, high-cost, highly engineered cranes for some time, but now it is available to everyone. It is offthe- shelf, it is high-volume, it is affordable. You could say it is egalitarian. It has brought this great technology to the masses. All you have to do is buy it, install it and turn it on.”
So, given that sway control and its stablemate anti-collision systems are so easily available, and are so clearly useful and cost-effective, does anyone still buy hoists without them?
“People do still purchase cranes that are not equipped with sway control. The decision to add sway control or not to a crane depends on its intended use and the value of the crane in the overall production environment. In a small repair shop where the crane is not directly impacting production, sway control may be more of a luxury as opposed to an application where the crane directly impacts production and a cycle time. Additionally, the end-users level of comfort with digital control also plays a role in the decision to add sway control to a crane,” says Childers.
“The capital costs are not large when compared to the lifetime operating costs of a crane, especially when viewed in light of the value received from avoiding damages that are caused by load sway. If you are buying, say, a $50m crane package, sway control might add $5,000 to that. But if you are moving, say, a coil of aluminium or steel and you save just one damaged coil through having anti-sway, how much is that worth? And if you prevent just one or two safety incidents, then you have paid for the system and it is basically free money after that. The value over the crane’s lifetime is the key to deciding to purchase this technology.”
It is also, he says, a generational thing. “Many of the current decision-makers are accustomed to cranes that are void of advanced features and are hesitant to take up a new style of crane controls. However, the next generation coming up through the ranks have grown up with the digital age and expect machinery to offer features powered by technology. As they move into senior roles, sway control will move from a premium option item to a standard expected item. To us, anti-sway still seems a marvel. To them, it is simply a given.”
PAR EXPERTOPERATOR
Passive anti-sway systems are often described as having no sensors. To be strictly accurate, they typically include one component that could be considered a sensor: the device that measures the length of rope paid out. This measurement is essential because rope length directly affects the sway frequency of a suspended load.
For any given load and rope length, there are multiple ways to move it from point A to point B without sway. One option is to travel at a very slow, constant speed – an approach that will always work, but is tedious and inefficient. Another is to accelerate quickly, then decelerate the trolley to counteract the sway, and finally reverse the process to stop. In fact, there are an infinite number of such carefully timed procedures that can achieve sway-free motion.
There is, however, one system on the market that avoids the need for these procedures altogether. Unlike conventional passive systems, the ExpertOperator anti-sway system from PAR uses no sensors whatsoever – not even rope-length feedback – and requires no operator input, such as rigging length or tuning, beyond initial setup. Instead, it transforms the operator’s raw crane commands into ‘expert’ commands. Unlike most other passive systems on the market, which use rope length to attenuate a single frequency, ExpertOperator’s core algorithm attenuates sway regardless of frequency, meaning it adapts automatically to any rope length.
The result, according to PAR, is a system that delivers exceptional performance at low cost with minimal setup. Importantly, it remains a passive system – it does not actively compensate for wind loads, off-vertical lifting or sway introduced by riggers. But it is unique in being a truly sensorless anti-sway solution.
