design of controls

[hooligan]

hi everyone,

i have a problem designing a control system. i have two webs of material that have holes punched in the material. the holes in both of the materials are punched the same distance apart. the materials are then wound on two seperate rolls. i am feeding the two rolls of material into a machine. i want to align (register) the holes of the two materials as they run into the machine. i have brakes on both of the unwind shafts so that i can control the individual drag on the materials. i also have sensors that can measure the the amount of mis-registration of the two materials (mismatch). i have a plc that can take the data from the sensors and manipulate the tension on the brakes. my trouble is coming up with a method to calculate how much brake i need to apply. for simplicity i was starting with manipulating only one unwind brake, i am setting the other one manually for now. what i am doing now is i am taking the mismatch data and calculating a new tension value by the following equation:

T new = [T old(MM old)]/ MM delta

T new is the new tension value
T old is the previous tension value
MM delta is the change in missmatch from the one previous to the most recent.

essentially i am using a proportonal method of changing the tension to try to minimize the mismatch (MM). my trouble arise when i have a large tension value and a small change in the mismatch value (there is a reaction time from the time you make a change to the time you actually see a difference) the above equation tends to blow up (divide by zero).

Any suggestion on how to better control the system? Any help is greatly appreciated!!

 

[millah]

I hate to be the naysayer here but I don't think a PLC will be able to 'keep up' with the calculation and adjustments required for your application. Web motion control equipment tends to be very specific and solid state in nature. I have not heard of trying to control speed using brakes instead of motion control of motors. Did this thing work before or is it a new design?

 

[hacksaw]

unusual control scheme given that web control is such a well defined (and proprietary) area of control. your use of brakes is not clear - are you using dynamic brakes(variable speed motors).

that said believe the mismatch is related to the tension difference (assuming identical sheets) not the ratio.

 

[andycapp]

Is this machine a devlopment project or is it to be a production machine.

If it is the latter, you will need something considerably more precise than a controlled speed system.

I would propose a pair of capstans with buttons to engage the holes in the webs, one for each web, driven by a pair of spur gears in constant mesh.

 

[gcaudill]

hooligan,

Assuming the machine is pulling the two material webs at the same (and constant) rate, your analysis is missing the roll diameter at the payoff. The roll diameter will affect the net tension in the material with a given brake setting. There must be some slip at the machine pulling surface for the two web registers to become misaligned, or the registers have some tolerance that causes error to build up with time.

Regarding the brake, is it a magnetic particle brake in which the torque output is linear with excitation and independant of speed? Or, is it something else that is more dynamic (i.e. speed dependant or non-linear torque/excitation curve)? Using a motor and drive operating in regenerative mode is one method, as mentioned by hacksaw, but not preferable.

The right PLC could be fast enough depending on the web speed, PLC scan rate, etc.

More information is needed.
What is the pulling source?
Can the pulling source slip?
What is the brake comprised of?
What is the speed?
What is the hole center-to-center distance?
What are typical full and emtpy roll diameters?