Floating Web Accumulator

[balex2323]

How is floating web accumulator works ? I have a project where I will control a floating accumulator without speed trim. Input and Output rolls speed are controlled by another plc. I control the hydraulic pump of the accumulator and set the tension. There is no interface with input and output rolls. I dont understand how the control works without speed trim to one of input or output drive. Shouldnt it be like when I increase tension from 100N to 150N , the output speed is to increase due to elongation ?

 

[Manglemender]

I'm sue there are people here that can help you need to provide some more details: What do you mean by "Floating Web Accumulator". If you are not controlling input or output rolls, how can you hope to control fill level? Where does your speed trim go? maybe draw a picture to describe the system

 

[balex2323]

To be honest I dont know. customer told me it is working this way for years and What I understood is once output roll is stopped for some reason, The tension on the accumulator which is set 100N (example) pulls the web upward. After output drive is restarted, operator manually increase the output speed from another hmi controlled by another plc and bring the accumulator to down position. After it reaches down position, operator is done until next stop. What I mean by floating accumulator was that it was not closed loop system there is no control for both side(input and output rolls) during normal operation.
From my experiences with these kind of systems, I believe if I increase tension from 100N to 150N, The output speed must be increased as well due to elongation etc.

 

[Manglemender]

Accumulators are normally used between a continuous process and a stop start type process. An acumulator before the continuous process is normally full and empties as the input stops to keep the process going. Downstream from the continuous process an accumilator runs empty and fills when the output stops. In both cases the aim is to keep meterial feed to/from the process at a constant tension to avoid disturbance. The speed of the discontinousous process is normally modulated to empty of fill the accumulator. This would normally be automatic but could be manually controlled.

You definately need to get more details of what the system looks like and how everything is controlled.

 

[Puddle]

Input roller should run relative to line speed.
Output roller should run relative to carriage position and/or re-roll speed.
If the web stretches, this will be seen in the carriage position.

I haven't seen one with hydraulic control before since you don't normally need that amount of force applying. Some are simply countweighed and I've seen them fitted with pneumatic balancing cylinders or driven carriages with load cell feedback.

 

[lfe]

It seems that this accumulator is used to maintain tension on the web even with the pump stopped, it acts like a spring.

I made time ago a hydraulic design in which the door of a large kiln was closed hydraulically and an accumulator maintained a certain closing pressure.

It seems that in what you explain is the operator that maintains the pressure, in my case there was a pressure switch that if the pressure dropped too much, the pump would turn on again for a few seconds to raise it.

 

[drbitboy]

How is floating web accumulator works ? I have a project where I will control a floating accumulator without speed trim. Input and Output rolls speed are controlled by another plc. I control the hydraulic pump of the accumulator and set the tension. There is no interface with input and output rolls. I dont understand how the control works without speed trim to one of input or output drive. Shouldnt it be like when I increase tension from 100N to 150N , the output speed is to increase due to elongation ?

An accumulator between two processes with independent web movement rates is like a tank with independent inflow and outflow rates. The tank cannot control the inflow and outflow rates and only accumulates material when the outflow is less than the inflow; the amount of accumulation is (inflow - outflow) x time, which is a simple conservation of mass law (in = out + accumulation).

Likewise the web accumulator accumulates web material length (inspeed - outspeed) x time.

That said, the analogy does break down because there is no equivalent in the tank analogy for tension.

Because it is a function of the input and output webs, that speed of the accumulator cannot be "set" directly or independently by the accumulator controller; the accumulator speeds are instead "found" empirically by the accumulator controller by maintaining a constant tension:

• if the accumulator is accumulating material slower than the net (input - output) web material is accumulating, then the tension will drop, and the accumulator controller will detect that drop in tension and in response speed up its accumulation rate to restore the tension to setpoint.
• if the accumulator is accumulating material faster than the net (input - output) web material is accumulating, then the tension will rise, and the accumulator controller will detect that increase in tension and in response slow down its accumulation rate to restore the tension to setpoint.

The details of how the accumulator controller measures tension and converts any error between measured and setpoint tensions to a change in accumulator speed is implementation-dependent. It may be analogous to a VFD in torque control. It could controlled by gravity, or by hydraulic pressure, etc.

 

[drforsythe]

These accumulators can be quite tricky. When the measured tension is above the setpoint, the accumulator needs to collapse and when it is below setpoint, the accumulator needs to move away. The key to these accumulators is fast response, and accurate force measurement.

Search "web accumulator design" and you will see some images and some resources to learn about them.

We had one for a while on a film winding line, but the system was not specified properly and the accumulator would either not keep up or break our film.

 

[kamenges]

Assuming this is an accululator that is loaded with a constant pressure there is no need for any speed information. The basic hydraulic system will supply flow as necessary to maintain the desired pressure. Assuming the mechanical system is somewhat carefully designed (yea, right) that pressure will translate to a specific tension. Maintain pressure, maintain tension. You can augment this if a controller is maintaining pressure (as opposed to a mechanaical regulator) by adding flow based on velocity or modifying pressure during acceleration or deceleration. But assuming neither of these values are "large" simply maintaining a set pressure will get you where you need to be.

The problem with this method is quite often the force required to maintain a given tension is completely swamped by the force required to statically hold or accelerate the system. In those cases you do need closed loop pressure/flow control that monitors the tension in the accumulator zone and modifies pressure to maintain the desired tension. this is why we have transitioned to screw/belt driven accumulators with drives. But, as stated, in that case you do need to know input and output speed.

You intuitively understand there is an issue with this system, however. One of the two sides should be at least loosely monitoring the accumulator position to make sure it doesn't end up on one of its ends of travel. That side (input our output) will need to modify its speed to keep the accumulator floating.

Hydraulics tend to address one of the scariest aspects of floating accumulators. What happens when the web breaks. Most accumulators of this conceptual design that I have seen use long throw pneumatic cylinders or cable cylinders to load the moving carriage. That makes them basically compressed long-throw springs. You can expect a significant jump in carriage position when the web breaks due to the significant difference in compressed air on each side of the cylinder when in the loaded state. And there is very little that can be done to truly mitigate this case. The limited compressibility of hydraulic fluid really decreases this issue.

Keith

 

[Peter Nachtwey]

I haven't have controlled any but I have seen a few in steel and aluminum plants. There are two sets of rollers one low and one high. There may be 10 or so rollers per set. The steel or aluminum go over the first top roller and down below the first low roller then up over the second top roller. The one of the two sets of rollers can move up and down to accumulate more steel between the two sets of rollers. Moving the rollers up requires a lot of force.

Yes, this can be done by tension alone but a difference between in and out speed can be used as a feed forwards. Also, the sets of rollers are big and they are kept level.

I didn't see the accumulators ( sets of rollers ) get threaded but it make sense that the top set can be lowered to below the bottom set so the steel can be threaded over the top set easily.

I didn't ask what was moving the the rollers up and down but I doubt it was air because of the required force.

 

[Manglemender]

To be honest I dont know. customer told me it is working this way for years elongation etc.

In my experience, what customers think they know often bears no resemblance to reality.

@balex2323 I hope that the responses so far have given you some food for thought on the subject so let us know how you get on.

If it were a water tank with feed in and out, you would need to control either feed in or feed out to control the fill level. A web accumulator is essentially the same and web tension is set to suit the product and not as a control measure.

Nick

 

[bb76]

We use accumulators like this. A hydraulic motor drives a screw for the carriage to move up and down and maintains a preset pressure.

The input side of an infeed accumulator or the output side of an outfeed accumulator needs to know where the accumulator position is in order to control properly. We measure the accumulator height from 0-100% and apply this position as trim to the drives. Infeeds run full and outfeeds run empty so trim is reversed.

 

[kamenges]

Based on the two posted from balex2323 this may all just informational. As stated, the OP is not responsible for making sure the accumulator carriage is in the right location. They are simply responsible for making sure the accumulator carriage is loaded correctly to maintain the desired tension. From the OP:

Input and Output rolls speed are controlled by another plc. I control the hydraulic pump of the accumulator and set the tension.

If the mechanical design is such that the accumulator will move freely when out of force balance then it doesn't matter what is happening on the infeed or outfeed side, as far as the OP is concerned.

Keith

 

[GaryS]

I think that would be an interesting little project to do
I have seen and worked with high speed web presses that have about 10 to 15 sets of roller that expand to 10 to 15 feet or more to allow feed roll and or take up rolls to allow for a feed roll or take up role to be changed out without stopping the press. As well as other materials Steel rolls Aluminum rolls plastic rolls ect.
First you didn’t say if you have any tension feedback to work with it wous be better if you did.
The key here is controlling and regulating the hydraulic pressure on the accelerator rolls. As the web goes slack (Web Tension ) the pressure on the cylinder will go down and of course as the web tightens the pressure on the cylinder will go up. I would install a pressure transducer on the cylinder feed and us that pressure feedback to control a hydraulic servo valve to maintain the desired pressure on the cylinder. The pressure on the cylinder would end up being the web tension .
While it would be great if you could look at both the feed and discharge speeds it would make the control much better but I think you should be able to do it without them a simple PID control loop should do it.
The one thing I would add is a position feedback from the accumulator to know when it nearing the min and max positions so you can pass a signal to the other system so they can prevent web breaks, providing the signals is your choice using them is their choice that would also mean that a web break would fall on them.

 

[kekrahulik]

A Hydraulic Accumulator the second simplest type of accumulator. The simplest type is purely mechanical weight - the accumulator rolls/carriage(frame) is counterbalanced to provide the desired downward force (tension) on the web. If the infeed runs faster than the outfeed, the carriage moves down. Outfeed faster - moves up. Same speed - carriage holds position. The web is what physically moves the carriage.

The hydraulic accumulator is doing the same thing, except instead of counterbalancing the rolls/frame with weight, hydraulic pressure provides the counterbalance. Since you can adjust pressure, you can easily change web tension. The hydraulics also allow you to move/jog the carriage up manually.

Although position feedback is nice to have, many of these will just have three-four limit switches:
Above Full Alarm
Full
Empty
Below Empty Alarm

If it is on the infeed side of the line, you desire to keep the accumulator full except when changing rolls at the unwind (the accumulator then feeds the line while the unwind is stopped). Control for this is:
Outfeed Drive: Always runs at line speed
Infeed Drive:
If unwind is stopped, infeed is stopped
If Unwind is running, Infeed runs at high speed until Full Switch True.
If Infeed is running and Full Switch True, Infeed Speed = Outfeed Speed