IS IT MATH, INSIGHT OR MAGIC?
Industrial cranes are everywhere. They come in all shapes and sizes and serve a wide variety of purposes. From monstrous 500T production cranes to small 3T maintenance cranes. Yet they all have one thing in common. Their loads sway.
Sway is a direct consequence of Newton's First Law: "A body at rest tends to stay at rest; a body in motion tends to stay in motion." When a crane picks a load and starts to move, the load lags behind, a body at rest. When a crane reaches its destination and starts to stop, the load keeps going, a body in motion. The frequency of the sway is determined by the distance between the hoist and the load's center of gravity. The greater the distance, the greater the sway. The greater the sway, the longer it takes for the load to settle and safely set down. And the longer it takes to settle, the greater the overall cycle time and lost productivity.
The True Physics of Sway
In its simplest form, sway in cranes is a product of the delta speed of the crane and the length of their cable. The faster the crane accelerates, the more prominent the sway. The longer the cable, the lower the frequency but the greater amplitude of the sway. Ironically, the weight of the hook or the load doesn’t affect amplitude or frequency, although it definitely adds to momentum of the load in the event of a collision.
Most sensor based anti-sway systems measure the distance from the hoist to the block using an encoder, laser or camera. If you know that distance, you can derive the frequency of the sway. Knowing the frequency allows you to shape the acceleration and deacceleration profiles to cancel that frequency, there by cancelling the sway on an empty hook. If preventing sway on an empty hook is all you need, these anti-sway solutions will get the job done.
In the real world, the load is suspended below the hook expending the center of gravity and negating the effect of the antisway to the hook. Further, the load itself can introduce secondary oscillation. A sensor based anti -sway system has no visibility to the extension to the center of gravity or another oscillation or the secondary sway. Expert Operators quickly recognize this and insist the anti-sway system doesn’t work and needs to be turned off.
Rigging only further exacerbates the problem. Rigging enables cranes to transfer a variety of loads, each with its own unique dynamics. These dynamics need to be accounted for when compensating for sway. In effect, rigging is a secondary pendulum that introduces a secondary or compounding frequency to the dynamic. The load suspended below the rigging can in fact be a tertiary frequency. Rigging is not merely an extension of the overall length of the load but introduces a separate dynamic that needs to be addressed to insure the stability of the load. Sensor based anti-sway systems have no chance. Failing to address this dynamic will negate efforts to deter sway.
Most anti sway products on the market today do not automatically account for secondary oscillation. They depend on expensive sensors, including encoders and/or cameras to measure the cable length, and then rely on the operators to input real world rigging and load center of gravity data, which more often than not doesn’t happen. More fundamentally, however, even if the data is uploaded, the algorithms employed do not accurately account for the dynamics of secondary and/or tertiary oscillations. At best, they only approximate the real-world dynamics of cranes, their loads and their rigging.
The InVekTek difference
In 1996, Dr. William Singhose, then at MIT, was part of a team that NASA contracted to address unwanted oscillation in the Hubble telescope. Every time the Hubble’s solar array panels were repositioned to optimize energy collection, they would cause the satellite to oscillate, making picture taking of galaxies millions of miles virtually impossible. As a practical matter, the only option was to modify the control algorithms of the electric motors used to manipulate the solar panels. It was too late to add sensors or modify the satellite, and in any event, it was unlikely that after the fact remedies would work in a weightless environment.
After significant effort, Dr. Singhose and his team succeeded. 25 years later the Hubble telescope still explores the visible universe free from oscillation. And 25 years later, after significant additional research as a tenured faculty member at Georgia Tech, Dr. Singhose has refined the insights first gathered during his work for NASA and developed proprietary control algorithms that prevent the stimulation of unwanted motion, including vibration, oscillation and sway, in a wide variety of commercial applications.
InVekTek has the exclusive license to Singhose’s refined algorithms, as well as sophisticated proprietary software. As our initial product, we have integrated his algorithms into an industrialized product called SwayMaster™ to address sway in industrial cranes. It doesn’t need sensors, encoders or cameras. It doesn’t require operator input. It is simply configured at the time of installation, and the algorithms do the rest.
As a consequence, cranes employing InVekTek's SwayMaster™ technology run faster, smoother, safer and require less maintenance, all at the lowest installed cost.
Lots of very bright engineers ask us how we do what we do. The answer is quite simple. It’s part math, part insight, and part magic. SwayMaster™ is the new standard in Advanced Crane Controls. It has the potential to dramatically improve the productivity, safety and affordability of cranes employed in a wide variety of material handling and industrial applications.