I am using a AB SLC 5/03 PLC. What I want to accomplish is to control a position thru the PLC. I am sure if I should use the PID functions or compare statements to accomplish this task. I am just looking for a little guidance as to how I am able to proceed with it. We are using pneumatic brake pads, w/tension control to the dancer and a potentiometer from the dancer position to control the brake pressure. There are no load cells installed. I just need to be pointed in the right direction.
dancer control
- Thread starter davepie
- Start date
dcooper33
Lifetime Supporting Member + Moderator
Hi Dave,
What kind of tension control to the dancer do you mean?
Is the potentiometer signal currently going to an analog input on the PLC? Straight to a proportional valve?
What kind of machine, and exactly what are you trying to accomplish?
We'd need to know more about your current set-up to offer any useful advice.
Cheers,
Dustin
This is a web handling machine, mainly used for rework, not a full production. Actually there is no tension control at all. The entire system is based on air pressure, both to the unwind brakes and the dancer cylinder. What I am looking to accomplish is rather have the operators the air pressure to the brakes and the dancer cylinder by way of pressure regulators, I would like to set it up to balance the dancer in relation to the set point of brake pressure. This way all the operators have to do is set the required brake pressure and let the PLC do the rest of the work.
OkiePC
Lifetime Supporting Member
Unwind brakes are the tension control, the dancer is the feedback.
What is the line speed like? What are the accel and decel rates?
PID control may be applicable, but if you want to taper the setpoint it can be tricky.
You need to be able to scale this dancer feedback accurately into useful linear units for best fit with the PID instruction.
What are your tolerances for this dancer position? I have done a lot of winding and tension contols with all sorts of brakes. One of my favorites was a disc brake controller that you sent an analog signal (4-20mA) and it converted to pounds of tension to the 0.1 using a simple easy to maintain pneumatic disc brake. It was extremely accurate, with nice load cell feedback of course.
What is the line speed? If it's not to wickedly fast, a PLC may be able to manage this with PID but that depends on the mechanics of the system as much as anything else...but if the process is intermittent, it is worhtwhile to take into account the line speed (when the line is stopped, the PID can do whatever the heck it wants to brake pressure and that damn dancer ain't gonna move) This can be as simple as an auto/manual mode in the PID...which can by itself be complex.
What is the line speed like? What are the accel and decel rates?
PID control may be applicable, but if you want to taper the setpoint it can be tricky.
You need to be able to scale this dancer feedback accurately into useful linear units for best fit with the PID instruction.
What are your tolerances for this dancer position? I have done a lot of winding and tension contols with all sorts of brakes. One of my favorites was a disc brake controller that you sent an analog signal (4-20mA) and it converted to pounds of tension to the 0.1 using a simple easy to maintain pneumatic disc brake. It was extremely accurate, with nice load cell feedback of course.
What is the line speed? If it's not to wickedly fast, a PLC may be able to manage this with PID but that depends on the mechanics of the system as much as anything else...but if the process is intermittent, it is worhtwhile to take into account the line speed (when the line is stopped, the PID can do whatever the heck it wants to brake pressure and that damn dancer ain't gonna move) This can be as simple as an auto/manual mode in the PID...which can by itself be complex.
dcooper33
Lifetime Supporting Member + Moderator
Seems to me like you could use your dancer position as the PV into your PID loop, then control the output to your brakes, based on how far off you are from the desired position of your dancer. The point is to maintain a uniform tension on the web as the machine stops?
I suppose you could leave the brake pressure constant, and then control air pressure to the dancer to try to maintain dancer position, it just seems like that would be less effective.
We have a similar machine (rewinding) that we have been having problems maintaining even tension on the unwind end. On it the unwind (mill roll) is driven off the same shaft as the rest of the machine, only with a v-belt and variable speed pulley. The pulley is controlled mechanically with a dancer roll that, as it moves up and down, opens and closes a linear regulator that feeds air pressure into something called a "pneumatic tension regulator" The incoming air from the dancer works on a diaphragm that operates ANOTHER regulator that will vary the position of an air cylinder attached to this monstrosity that in turn will move the tension roller for the unwind drive belt, thereby changing where the belt rides on the variable pulley. This allows the unwind roll to keep a (somewhat) constant surface speed matching the drive speed of the machine. I guess you could call it "air over air", pretty amazing, really that it works. I guess that is what guys used back in the 40's before VFD's existed. (I'm not kidding, we have mechanical drawings for these things from PCMC dating from 1947!)
Anyway, hats off to the genius that invented that thing, but it's beyond obsolete. I'm wanting to put an ABB drive with an independent motor and timing belt drive to it, using one AI from the main drive with another AI for trim from a dancer pot, and a diameter calc in the drive for the speed reference.
Not sure if this kind of set-up would do you any good in your application, just thought I'd put it out there.
Cheers,
Dustin
I suppose you could leave the brake pressure constant, and then control air pressure to the dancer to try to maintain dancer position, it just seems like that would be less effective.
We have a similar machine (rewinding) that we have been having problems maintaining even tension on the unwind end. On it the unwind (mill roll) is driven off the same shaft as the rest of the machine, only with a v-belt and variable speed pulley. The pulley is controlled mechanically with a dancer roll that, as it moves up and down, opens and closes a linear regulator that feeds air pressure into something called a "pneumatic tension regulator" The incoming air from the dancer works on a diaphragm that operates ANOTHER regulator that will vary the position of an air cylinder attached to this monstrosity that in turn will move the tension roller for the unwind drive belt, thereby changing where the belt rides on the variable pulley. This allows the unwind roll to keep a (somewhat) constant surface speed matching the drive speed of the machine. I guess you could call it "air over air", pretty amazing, really that it works. I guess that is what guys used back in the 40's before VFD's existed. (I'm not kidding, we have mechanical drawings for these things from PCMC dating from 1947!)
Anyway, hats off to the genius that invented that thing, but it's beyond obsolete. I'm wanting to put an ABB drive with an independent motor and timing belt drive to it, using one AI from the main drive with another AI for trim from a dancer pot, and a diameter calc in the drive for the speed reference.
Not sure if this kind of set-up would do you any good in your application, just thought I'd put it out there.
Cheers,
Dustin
Last edited:
Christine1914
Member
Just read this thread..May I know what is dancer control? And what does it do/perform?
dcooper33
Lifetime Supporting Member + Moderator
Christine,
A dancer roll is usually an idler roll on a web-handling machine that the film is threaded around. It will be mounted on a pivoting mechanism of some kind so it can move up/down with higher/lower web tension. It can be used as a means of regulating tension by itself, using an adjustable air regulator, much like a belt tensioner, or it can be used as a feedback system with a potentiometer coupled to the pivoting shaft of the dancer roll.
So, for example, on a machine the web is threaded under the dancer roll. When the downstream motor pulls harder on the film, causing the dancer to raise. This will in turn rotate the potentiometer, outputting a greater analog signal of some kind. This signal can then be used to control a device that can adjust the web tension accordingly.
HTH
Cheers,
Dustin
dcooper33
Lifetime Supporting Member + Moderator
on second thought...
Or you could just ask bce123 for his definition of "dancer control"
He may have an entirely different answer...
Or you could just ask bce123 for his definition of "dancer control"
He may have an entirely different answer...
Mickey
Lifetime Supporting Member
We built a similar machine with 4 heads for 4 reels of material. Used PID control on it. Dancer had the analog input to PLC and brake had analog output from PLC. Dancer pressure controlled with pneumatic valve. Object is to make dancer stay in the middle of top and bottom points. Pneumatic valve adjustable through HMI with options for different pressures depending on the material type.
OK somewhat late in the game. ASSuMING the roll is between free travel limits.
Doesn't the dancer weight set the tension of the wound roll? Regardless of where the dancer position is. The weight of the roll will be the same unless you counterbalance it. Then you are just adding mass to the system. This effectively reduces the weight of the dancer but adds mass to the system that has to be absorbed somewhere right? The web will absorb it for a short time then release that energy back into the dancer causing the position to change?
Doesn't the dancer weight set the tension of the wound roll? Regardless of where the dancer position is. The weight of the roll will be the same unless you counterbalance it. Then you are just adding mass to the system. This effectively reduces the weight of the dancer but adds mass to the system that has to be absorbed somewhere right? The web will absorb it for a short time then release that energy back into the dancer causing the position to change?
OkiePC
Lifetime Supporting Member
For strictly let-off stand brake tension, at low speeds, you probably just want a lever operated relieving regulator, and a dumping ball valve to release it to exchange the cores. The dancer operates a relieving regulator and you are done. No electrons involved. The dancer will dance, mind you, as the line velocity changes, but when steady, the pressure will stabilize according to your adjustment if the dancer and linkage is designed right.
If you want to use a PLC to control brake pressure based on dancer position, I would start with straight P control and see what happens. In other words, take your target dancer position, say 50%, and mathematically "wrap your range of brake pressure control around it", so that if the higher dancer position call for less brake pressure, that at 25% you get 0% brake pressure, and at 75% you get 100% brake pressure. This is a few multiply instructions, and then run it and see where the dancer position end up. Any offset can then be summed in, wound in, or dealt with however you see fit. This offset will be useful if you decide to use a PID instruction too.
If you want to use a PLC to control brake pressure based on dancer position, I would start with straight P control and see what happens. In other words, take your target dancer position, say 50%, and mathematically "wrap your range of brake pressure control around it", so that if the higher dancer position call for less brake pressure, that at 25% you get 0% brake pressure, and at 75% you get 100% brake pressure. This is a few multiply instructions, and then run it and see where the dancer position end up. Any offset can then be summed in, wound in, or dealt with however you see fit. This offset will be useful if you decide to use a PID instruction too.
Last edited:
Christine1914
Member
thanks for the response..but still can't understand what a "dancer" do either looks like.
do you have any sample picture for this one? because just read this here..so got no idea what this is and what it looks like
do you have any sample picture for this one? because just read this here..so got no idea what this is and what it looks like
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