Heave compensation/Constant tension

Does any one have any experience using a plc to control a hoist and enable it to pay off wire and take up wire at various rates depending on the error of the load. I have tried using Allen Bradleys Fuzzy Logic generator with limited success.

Bruce Flett said:

Does any one have any experience using a plc to control a hoist and enable it to pay off wire and take up wire at various rates depending on the error of the load. I have tried using Allen Bradleys Fuzzy Logic generator with limited success.

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What do you mean be error of load?? Do you mean different load weights??

One thing I would consider is what are you using wire rope or chain or?? If anything other than chain then I think you have to account or allow for the "springiness" stretch of wire rope (or??).

The other consideration is are you using winch or capstain??

If your winch has the ability to lift or pull the load with adequate safety factor then provided you do not overload the winch I do not think I would change line speed based solely on changing load but would leave it alone.

I would tend to let a VFD do the speed control and let it control the acceleration as the winch starts and allow for more accel as the load starts moving. I would tend to slow line speed as you add wraps on line on the drum.

Dan Bentler

this is a marine application requiring a weight to sit on the seabed and the wire to remain tight at all times with wire spolling on and off the drum as required. load is sensed by a sheer pin load cell.
Bruce

Sounds like an application where you should use a PI or PID loop, with feed forward, controlling the drum motor TORQUE (NOT Speed).

I am curious as to why the current system isn't working.

rdrast said:

Sounds like an application where you should use a PI or PID loop, with feed forward, controlling the drum motor TORQUE (NOT Speed).

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So how do you use feed forwards when there is no target generator? Yes, torque control the way to go but the speed must always be monitored. Should a cable break you don't want the motor spinning wildly trying to maintain a torque. The speed must be limited.

Have you considered hydraulics for this? Torque in an electric motor requires plenty of current and cooling can be a problem when just holding a strain. An electric motor would be more efficient while paying out or reeling in. A hydraulic motor uses little oil to maintain a torque. Ships often have hydraulic systems already so they aren't something new.

I would use a hydraulic motor with two pressure sensors. One on the A port and one one the B port. The difference between them is proportional to the torque. A servo valve would meter oil in and out to maintain a desired torque and/or speed limit.

How would i use torque for the control. Could I hold a constant torque and an increase in the load would draw wire from the drum and a decrease would cause the drive to rotate to draw wire back in. We are monitering speed at present with an encoder.The vsd controls the speed relative to the rate of change on the load cell.
Bruce

You mereley started this off if we had any experience. If you lay all the cards on the table face up we are better able to deal with it.

If you are measuring line tension (which is proportional to torque on the drum) and controlling for that as you described then I think that should work well. I agree with Peter but will paraphrase - is it working well for you and doing what you want?

Dan Bentler

Dan Bentler

Bruce Flett said:

How would i use torque for the control. Could I hold a constant torque and an increase in the load would draw wire from the drum and a decrease would cause the drive to rotate to draw wire back in.

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Yes, is that what you want? The problem arises when the load gets too high or too load and the speed goes too fast.

We are monitering speed at present with an encoder.The vsd controls the speed relative to the rate of change on the load cell.
Bruce

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You really should be using a torque PI or PID and using the speeds as an override or sanity check. Your algorithm for changing the speed as a function of the rate of change in the load is probably not right.

First off all sorry for not giving all the information at the start off this thread its the first i have posted.

High speed of retrival is not so mush of an issue as I have worked out that 10Htz would be the worst case and in most conditions 0-5 Htz would suffice.As you have said If weight loss happened we can not allow the wire to retrive at an uncontrolled rate. Torque seems to provide a way forward to this problem but I will let you know how I get on. I have used VSDs for years but never used torque control. if you have any other thoughts on this problem let me know
thanks
Bruce

Speed control can be advantageous if the torque required to produce the tension is similar to the friction levels and they are down at the bottom of the motor torque rating.

What sort of levels of tension/torque/friction/inertia do you have ?

I would personally use speed control with a torque limit once you are at the bottom. When the load reaches the bottom set a torque limit slightly greater than the cable weight and a retract speed command high enough to compensate for the crane end motion relative to the load. If you have the cable strain gage you can tweak the torque limit to reach the actual tension you want. This method will make sure you don't go ludicrous speed if the cable breaks.

The downfall to this is that it doesn't compensate for system friction or inertia. So if the drivetrain has significant friction you will see a significant difference in tension as the ship changes direction relative to the load. If you have significant cable drum inertia this will also add and sutract to/from the cable force. The system may be able to keep up with acceleration just from the cable strain gage since this is a reasonably linear effect. The friction is more problematic since it makes a step change on change in direction and, as a result, is highly non-linear.

Keith

kamenges said:

I would personally use speed control with a torque limit once you are at the bottom

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How can you do speed control when you don't know the speed at which the line must be reeled in or let out? The only way to know is to use the torque. Or perhaps a 'dancer' mechanism.

If the goal is to control a constant tension then controlling torque first is the way to go and limit the speed.

If something pulls on the cable quite hard the torque will go up and the speed will go up. At that point a decision needs to be made as to which limit to exceed and by how much. If the cable goes slack the winder will go to the maximum speed. In between the normal torque control would be in operation.

Originally posted by Peter Nachtwey:

How can you do speed control when you don't know the speed at which the line must be reeled in or let out?

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The user must have some idea what the maximum heave/roll rate of the platform is. This will be long-term variable based on surface conditions but should be reasonably stable on a minute-by-minute basis. This becomes the retract speed reference for the drive. Conceiveably this is something less than the maximum speed of the drive or any type of speed limiting is useless. You run into the same issue if you do direct torque control with a speed monitor. What is your trip speed? Or are you proposing adding a three-axis orthogonal acclerometer and doing disturbance rejection at the torque level and basing the speed limits off the derivatives of the resultant acceleration?

The reason I proposed speed control with torque limit is that torque limiting occurs at the torque loop closure rate of the drive. However, it can't be used for bipolar compensation if that is needed, thus the inertia/friction comment in my previous post. Internal drive torque limiting is easily the fastest torque limit scheme achieveable with a drive as it happens at the torque loop closure rate, bypassing A/D conversion delays in a direct torque control scheme.

Keith

See SNK's thread

kamenges said:

The user must have some idea what the maximum heave/roll rate of the platform is. This will be long-term variable based on surface conditions but should be reasonably stable on a minute-by-minute basis. This becomes the retract speed reference for the drive.

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If this is a see going application then how do you know what the target speed is to control speed? This doesn't make sense.

The reason I proposed speed control with torque limit is that torque limiting occurs at the torque loop closure rate of the drive.

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So?

However, it can't be used for bipolar compensation if that is needed, thus the inertia/friction comment in my previous post.

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The idea is maintain a constant tension. Unless the speed goes to high. The OP has said anything, yet, about what happens if the tension is too great because the line is being pulled to hard or too fast..

Internal drive torque limiting is easily the fastest torque limit scheme achieveable with a drive as it happens at the torque loop closure rate, bypassing A/D conversion delays in a direct torque control scheme.

Keith

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If this is a water application then you don't need sub millisecond control.
[/quote]
Would you believe that we have a command that gets used for applications like this?
It is a MoveVel command where the max velocity of the spool will rotate is specified. However, a torque limit is also specified. When the torque limit is enabled and the MoveVel command is issued the spool will try to pull in the cable at the max velocity but the torque limit is reach first. The controller will then maintain the torque as specified by the torque limit. If the cable breaks the torque will no longer be limiting the output and the spool will accelerate to the maximum velocity. What hasn't been defined yet is what happens if something pulls on the cable so the applied torque is more than the torque of the motor. In this case the spool will unwind at a faster and faster rate and the MoveVel command specified a maximum velocity and not a minimum(negative) velocity ( unwinding ).

Hopefully the motor and spool are selected to the maximum velocity is never exceeded except for a cable break in which case a decent controller can easily see the rate of change in torque that exceed some limit and shut down.

On a motor system where one controls in torque mode the control output can just be set to a constant 1 volt or 2 volts because the current will be roughtly proportional to the voltage. That should be easy to do. Then one just monitors the velocities and as long as they aren't exceeded you just keep a constant control signal proportional to the desired torque. Simple.

The goal is to maintain a constant tension not a constant speed.

BTW, grinding applications are similar to this one.

Peter-

We just said the same thing. I just didn't explain it since I assumed you knew what I was talking about.

You would lower the 'device' toward the seabed under normal velocity control until the load cell on the sheat pin detects that the load is on the seabed. At this point you would set the drive torque limit to the desired level and give the drive a constant speed command in the up direction. You can use any speed as long as the speed is higher than the expected 'heave rate'. The drive will then cause the cable to retract until the torque limit is reached, at which point it will apply torque in the speed command direction (retracting in this case) limited to the torque limit.

The speed command you use is not important. The only reason I brought it up is that if you do get a cable break limiting the speed to the lowest reasonable level is the safest possible course of action. That is the only reason not to use full retract speed.

After thinking about this the response speed doesn't really play into this. The drive will respond as quickly to a fixed torque command as it will to a torque limit. They apply at the same point.

But as I said before this is only a reasonable course if the system friction is low enough that you don't get a significant force change between pay-out and retract when the ship motion changes direction.

Keith