Re: [CAD_CAM_EDM_DRO] Constant Velocity unwinder

On Mon, 17 Jul 2006, Fernando wrote:

> Hello,
> I think this would be off topic, dont know so if you think it is,
> please add the OT on the subject on your replies.
>
> I have a client that has an unwinder, it unwinds flatstock about .25"
> thick, it has a 5HP motor with a reducer, and a VFD. The client has it
> right now as to supply the max amount of material it can, but
> obviously, as the diameter of the roll gets smaller the less material
> is being delivered. What would be a good way to supply constant
> material flow? I was kind of thinking of replacing the motor and VFD,
> with a servo operating in velocity mode, and attaching the encoder to
> a following wheel that is in contact with the material being
> delivered. Does anyone know if there are VFD that can accept a signal
> that regulates the speed, that signal being an encoder or tach
> attached to the advancing material?
> Any suggestions would be greatly appreciated.

If you were to take a typical DC servo drive (either DC input
or pulse direction input) that might be used in a CNC application and hook
it up as you are suggesting, there is a good chance you would have
serious oscillation problems because of the loose coupling
between the encoder and the motor which the typical servo
drive is expecting to be tightly coupled. Not that there aren't
servo motor controllers that can do the job or ways to fix
the problem with ones that are less well suited. But you
are likely to buy a high performance product expecting smooth
operation and end up with performance you could have gotten
just by flipping the on/off switch.

Oscillation can be a problem with just about any configuration
but your typical VFD is probably more closely tuned to your
application. Speed changes on the order of a second rather
than a millisecond. But remember to ask your vendor
about oscillation in your application and you will probably
need to configure the drive for slow response times or
low pass filter your signal input (in 0-10V or digital form).

Your existing VFD has no inputs of any sort to control the speed
other than a keypad/display? What is the make/model? Even
a $320 VFD has a 0-10V input and external run/stop signals
and modbus inputs and a remote up/down inputs.
http://www.driveswarehouse.com/Drives/AC+Drives/Compact+VFD/L200-037LFU.html
Not even a front panel or remote operator control box potentiometer that
can be disconnected and connected to your source?

Mechanical solutions:
- Move the motor from the reel to where the feed material exits
and put a capstan/pinch roller there (i.e. one wide tire on
the motor shaft and one spring loaded or weighted).

- On good reel to reel tape decks, they ran the tape around
a swing arm roller between two fixed rollers. The position of the
swing arm measured the tension and told the reel servo how fast to move.
Note that this doesn't just give you a constant feed, it gives
you the speed required by whatever machinary is processing
the stock, assuming it has motors to move the stock but not
strong enough to run the reel. Depending on the stiffness of
the material you might need several feet of space for the sensing
setup. Use the swing arm position with a potentiometer, optical
sensor, switch or switches to generate a 0-10V, 4-20ma, start/stop
signal, or speed select signals, or up/down signals.

If you know how to use microcontrollers, they can be used as
an interface between encoders and whatever inputs your system
needs. Or a PLC.

PC solution:
you might be able to use the spindle speed control features on EMC or
other milling servo controller to drive either a motor or
a 0-10V input.

Frequency to voltage converters are availible as ICs or as
industrial modules that mount on a DIN rail. google
"frequency to voltage converter DIN rail". Can be used
with a decoded output (usdigital sells converters) from an
optical encoder - don't use the raw signal as it suffers
from "bounce" when you are on the edge of line - to produce
a 0-10V output from the encoder signal. Polarity will probably
be opposite what you want but you may find some that can
invert.

Check out http://web2.automationdirect.com/. I have not used their
products, they were just the first match with an encoder input. But
their drives have a number of features that are applicable:
- Digital inputs (start/stop, speed selection, etc) standard
- 0-10V or 4-20ma analog inputs: standard
- encoder feedback: optional on some models
- modbus: standard on most models, optional on others
- ethernet optional on most models
You might find other model that have PWM or reference frequency inputs.

google "variable frequency drive "encoder input" OR "encoder feedback"
for more VFDs.

You should not need to replace the motor if you change drives; you
should be able to find a drive that will work with your motor, assuming
that your motor can turn fast enough (with the aid of a VFD that
can produce a high enough frequency).