I work for a drive manufacturer and a crane manufacturer has asked me to investigate anti-sway control mainly for gantry cranes working in the steel industry. The problem is the the variables that come into play when trying to even contemplate where to start on programming for trials. The axis are long travel, cross travel, hoist, slewing drive on C-hook(for picking up coil steel), all of which may or may not be operating at the same time due to a human operator. Has anyone ever looked into this? I've started looking at fuzzy logic but I'm sceptical, and it seems like many have tried and failed on this R & D. Am I waisting my time? Your input would be appreciated, go easy I'm only a young fella!!
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Anti-Sway Crane control
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Well, there are anti-sway techniques that you can employ on individual axis, but I know of no drive product line that would have the necessary interoperable intelligence to do all-encompassing anti-sway control on several at the same time. For example, you could control sway on an east-west trolley drive OR a north-south bridge drive individually. The one technique I used was to look at acceleration in a PID loop approach with motor current as the feedback, because as a load begins to swing to and fro, the drive motor current will modulate well beyond what you would expect. So you decrease or increase the accel rate on the fly to maintain a steady current rate proportional to your speed increase. When I did it on bridge cranes for moving jet aircraft, VFDs had much less processing power than they do now so we did it in a PLC, but only for each direction individually since they just knew not to try to slew a 747! Even though newer drives may now have the capability to do the individual directions internally, I think you would still need an outside controller of some sort to look at the effects of multiple simultaneous directions. Moving while hoisting adds a new dimension that I am unfamiliar with, and hoisting while slewing is just plain mind boggling!
A good PLC with high speed PID processing power could probably do it, but the programming would entail a lot of development, testing and adjustment. Is your customer willing to pay for that, or are they just dangling the VFD unit purchases as a carrot for you to do it free? I'd be careful about that!
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A good PLC with high speed PID processing power could probably do it, but the programming would entail a lot of development, testing and adjustment. Is your customer willing to pay for that, or are they just dangling the VFD unit purchases as a carrot for you to do it free? I'd be careful about that!
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A good operator will control swing by stopping short and then making a last short move.
for instance, as the crane is de-celerating the hook will be ahead of the perpendicular position. When the crane stops, the hook will start to swing like a pendulum. If, for example, a certain rate of de-celeration has the hook 2 feet ahead of the perpendicular position, the operator will drive the crane another two feet ahead, before the hook swings back. This puts the crane directly above the hook and there is no more force to cause swinging.
Put another way;
When the crane and the hook stop moving, the hook will be ahead of the crane and will swing back. In the moment when the hook has not yet started to swing, the crane is moved directly over the hook.
This may be hard to automate, but unless a crane has a rapid response, the operator doesn't have a chance.
yours
for instance, as the crane is de-celerating the hook will be ahead of the perpendicular position. When the crane stops, the hook will start to swing like a pendulum. If, for example, a certain rate of de-celeration has the hook 2 feet ahead of the perpendicular position, the operator will drive the crane another two feet ahead, before the hook swings back. This puts the crane directly above the hook and there is no more force to cause swinging.
Put another way;
When the crane and the hook stop moving, the hook will be ahead of the crane and will swing back. In the moment when the hook has not yet started to swing, the crane is moved directly over the hook.
This may be hard to automate, but unless a crane has a rapid response, the operator doesn't have a chance.
yours
With the low frequency of the swing you may be able to do this with regular AC motors with encoders and VFD's.
Something that would make life much easier would be knowing the location of the hook in space. The cheapest way to do this would be two perpendicular tilt sensors but you may want to get fancy with a local spatial positioning system.
With the tilt sensors and a known distance from hoist to block you can calculate the position of the hook relative to gravity. Since hopefully the angle will be less than 22 degrees the simple pendulum formula should be accurate enough.
Something that might help would be an “off center” display that would show the operator the two tilt sensor readings before the object is lifted.
I would think this would be done much easier with a servo controller supplying the DAC for the VFDs but any PID controller that can do square roots should be able to do it.
This all assumes that the controller has control of the two horizontal axis and is not fighting the operator. The changing pendulum length should not be a problem since it will be know.
I would do some estimates on the period of the worst case pendulum, short and long, and make sure your drives can move the crane a distance equal to half the linear error in less than .125 of the period. There may be a need for two acceleration rates.
Sounds like fun!!
Barry1961
Something that would make life much easier would be knowing the location of the hook in space. The cheapest way to do this would be two perpendicular tilt sensors but you may want to get fancy with a local spatial positioning system.
With the tilt sensors and a known distance from hoist to block you can calculate the position of the hook relative to gravity. Since hopefully the angle will be less than 22 degrees the simple pendulum formula should be accurate enough.
Something that might help would be an “off center” display that would show the operator the two tilt sensor readings before the object is lifted.
I would think this would be done much easier with a servo controller supplying the DAC for the VFDs but any PID controller that can do square roots should be able to do it.
This all assumes that the controller has control of the two horizontal axis and is not fighting the operator. The changing pendulum length should not be a problem since it will be know.
I would do some estimates on the period of the worst case pendulum, short and long, and make sure your drives can move the crane a distance equal to half the linear error in less than .125 of the period. There may be a need for two acceleration rates.
Sounds like fun!!
Barry1961
There has been some interesting and underappreciated work in generating position trajectories so as not to excite a system natural frequency of oscillation, basically by removing that frequency content from the commanded trajectory through a specialized digital filter. This has mostly been applied when the oscillation is due to a system structural resonance, but I'm pretty sure that it has been applied to hoists/cranes before (fueling nuclear reactors IIRC!).
To do this, your system would need to be a full-fledged positioning servo system on all axes. The crane operator would be acting much as a CNC operator would be, in "jogging" the axes -- starting and stopping closed-loop positioning trajectories.
One of the biggest problems I see is that your frequency of oscillation would change with the effective length of the hoist cable, which means that the filter coefficients would have to change with the height. I don't know if it would be feasible to allow the height to change while the horizontal axes are in motion.
Also, such trajectory filtering does add delays in completing the commanded trajectory -- at least one-half the period of oscillation. But I think this is unavoidable in any technique, and certainly less than waiting for multiple periods of the true oscillation to damp out.
If you contact me off-line, I can put you in touch with people who provide these types of algorithms. (In the interest of full disclosure, they embed these algorithms in a variety of controllers, including ours.)
Curt Wilson
Delta Tau Data Systems
To do this, your system would need to be a full-fledged positioning servo system on all axes. The crane operator would be acting much as a CNC operator would be, in "jogging" the axes -- starting and stopping closed-loop positioning trajectories.
One of the biggest problems I see is that your frequency of oscillation would change with the effective length of the hoist cable, which means that the filter coefficients would have to change with the height. I don't know if it would be feasible to allow the height to change while the horizontal axes are in motion.
Also, such trajectory filtering does add delays in completing the commanded trajectory -- at least one-half the period of oscillation. But I think this is unavoidable in any technique, and certainly less than waiting for multiple periods of the true oscillation to damp out.
If you contact me off-line, I can put you in touch with people who provide these types of algorithms. (In the interest of full disclosure, they embed these algorithms in a variety of controllers, including ours.)
Curt Wilson
Delta Tau Data Systems
tmcgett
Electrical
- Jan 3, 2004
- 13
Waross
Being a crane operator for over 30 years I am impressed with your description of how a crane operator catches the drift in the hook.You must have some seat time in you past.
Your statement, “unless a crane has a rapid response, the operator doesn't have a chance.” Is again, Right On !
Old school cranes have a good response due to their over engineered design. The newer cranes now can’t take the stress as well so the response is getting longer. Making a hard job harder !!
TERRY
Being a crane operator for over 30 years I am impressed with your description of how a crane operator catches the drift in the hook.You must have some seat time in you past.
Your statement, “unless a crane has a rapid response, the operator doesn't have a chance.” Is again, Right On !
Old school cranes have a good response due to their over engineered design. The newer cranes now can’t take the stress as well so the response is getting longer. Making a hard job harder !!
TERRY
MikeHalloran
Mechanical
At least one company makes soft- start controls specifically for the crane industry. They accept 2-speed pushbutton inputs, and ramp a 3 phase motor nicely, with no feedback mechanism at all. Of course, some operator skill is required, just not as much as with the pushbuttons alone.
Since human operators with primitive controls and a little practice can usually do just fine, a crane manufacturer would fund development of an advanced controller only for the purpose of removing the human, and his paycheck, from the loop entirely.
Don't underestimate the commercial significance, and the difficulty, of solving the problem.
Mike Halloran
Pembroke Pines, FL, USA
Since human operators with primitive controls and a little practice can usually do just fine, a crane manufacturer would fund development of an advanced controller only for the purpose of removing the human, and his paycheck, from the loop entirely.
Don't underestimate the commercial significance, and the difficulty, of solving the problem.
Mike Halloran
Pembroke Pines, FL, USA
If it is an AC crane, there is a drive manufacturer that has had this technology for many years and is very reliable.
Steel mill cranes are often DC, if so, I know of no anti-sway technology electrically.
Mechanical means by reeving is also an option, but normally only when starting from scratch as it is very difficult to modify existing hoists due to the lack of real estate on the hoist deck.
Steel mill cranes are often DC, if so, I know of no anti-sway technology electrically.
Mechanical means by reeving is also an option, but normally only when starting from scratch as it is very difficult to modify existing hoists due to the lack of real estate on the hoist deck.
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