Latching or Sample and Hold Questions

Hello friends,

I'm working on a project in which I want to use a variable frequency
drive to automatically adjust the carriage speed of a panel saw.
There is a chip load rating for the blade that I am trying to
maintain, and I have arranged an output so that when the frequency
drops to a certain limit, a warning light turns on to signal the
operator to change the blade.

We have a fixed RPM motor that draws current to maintain RPMs
depending on the condition of the blade. When the blade becomes
duller and starts to draw more current, I want the VFD to slow down
the carriage to maintain the chip load rating. This is easily done.
However, I need to find some way to "lock in" the frequency from the
previous pass as a starting frequency for the next pass.

I was wondering if anyone has any experience or ideas about this type
of setup. Would a frequency to voltage converter with some sample and
hold capability be the right direction to look?

Thanks in advance for any help or info you can provide.

btfloyd@gmail.com

 

[John Larkin]

On Thu, 9 Oct 2008 08:30:24 -0700 (PDT), btfloyd@gmail.com wrote:

Hello friends,

I'm working on a project in which I want to use a variable frequency
drive to automatically adjust the carriage speed of a panel saw.
There is a chip load rating for the blade that I am trying to
maintain, and I have arranged an output so that when the frequency
drops to a certain limit, a warning light turns on to signal the
operator to change the blade.

We have a fixed RPM motor that draws current to maintain RPMs
depending on the condition of the blade. When the blade becomes
duller and starts to draw more current, I want the VFD to slow down
the carriage to maintain the chip load rating. This is easily done.
However, I need to find some way to "lock in" the frequency from the
previous pass as a starting frequency for the next pass.

I was wondering if anyone has any experience or ideas about this type
of setup. Would a frequency to voltage converter with some sample and
hold capability be the right direction to look?

Thanks in advance for any help or info you can provide.

btfloyd@gmail.com

There are micro-PLCs for under $200, like the IDECs. They can do most
anything, are easy to program, and sure beat designing your own
electronics for stuff like this.

John

 

[Robert Monsen]

On Thu, 9 Oct 2008 08:30:24 -0700 (PDT), btfloyd@gmail.com wrote:

Hello friends,

I'm working on a project in which I want to use a variable frequency
drive to automatically adjust the carriage speed of a panel saw.
There is a chip load rating for the blade that I am trying to
maintain, and I have arranged an output so that when the frequency
drops to a certain limit, a warning light turns on to signal the
operator to change the blade.

We have a fixed RPM motor that draws current to maintain RPMs
depending on the condition of the blade. When the blade becomes
duller and starts to draw more current, I want the VFD to slow down

Why does the motor draw more current when the blade is duller? Seems
like it would draw less current, because it has less of a load at a
fixed RPM.

the carriage to maintain the chip load rating. This is easily done.
However, I need to find some way to "lock in" the frequency from the
previous pass as a starting frequency for the next pass.

I guess you need a control loop where you measure current draw over a
period of time, and then set vfd input control voltage.

So, why are you measuring frequency? Why not measure current into the
fixed RPM motor using some kind of current transformer or shunt?

I was wondering if anyone has any experience or ideas about this type
of setup. Would a frequency to voltage converter with some sample and
hold capability be the right direction to look?

Thanks in advance for any help or info you can provide.

btfloyd@gmail.com

I don't know much about this, so I'm interested in your reasoning. By
the way, google shows everal patents on controlling carriage speed of
a panel saw (from the 60s) so you might get some ideas there.

Thanks,
Bob Monsen