Re: Gecko differences

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "turbulatordude"
<davemucha@j...> wrote:

>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, Stephen Wille Padnos
> <spadnos@s...> wrote:
> > turbulatordude wrote:
> >
> > >Hi all,
> > >
> > >I am interested in the recent discount of Gecko drives and was
> > >pondering the idea of using servos on a machine.
> > >
> > >I did not see a detail listing the differences and similartitys

of

> > >the different drives.
> > >
> > >
> > >What makes a 340 better/different than a 320 ?
> > >
> > >dittto for the 201/210
> > >
> > >
> > The 340 and 210 have the G901 pulse multiplier on their inputs.
> This
> > allows them to multiply incoming pulses by 2, 5, or 10 (or 1), so
> the
> > motor moves faster. It also gives the ability to work with a
> common
> > ground or a common supply for the optoisolators.
>
> I'm looking at a wood router and a plasma table.
>
> Both will probably be either rack or open belt, more likely the
> plasma will be rack and the router will be open belt.
>
> Any idea on what type of machine the multiplier is needed ? Or is

it

> more for higher resolution or faster rapids....
>
> I'm trying to see the added benifit for my applications.
>
>
> Dave
>
>
>
> Any ide

Dave you need to do a little math to decide is a multiplier is
needed. There are a couple of givens: The max pulse rate you can
get (depending on the PC used) is between 25,000 and 45,000 pulses
per second. The next number you need is the maximum speed in IPS you
expect to need. Now you have to calculate the gear/belt ratios. The
final number is the line count of the encoders in the case of servos
or the microstepping ratio in the case of steppers.

Lets' work through a scenero.

1. The max pulse rate of our PC and software is 35,000 pps (pulses
per second) If we are spinning a stepper we know most take 200 steps
to turn one revolution. Now if we are using a Gecko 201 it
microsteps at a 1/10 increment meaning that if we send it a pulse it
moves the motor 1/10 the normal distance so to get the same single
revolution we have to send ten times as many pulses or 2000 to turn
it one time.

Lets assume that your belt/gear/pinion ratio is 2:1. You have to
calculate how far the load will move given one revolution of the
motor shaft. If one rev will move the load .5 in then it will take
4000 pulses to move the load 1 inch. We now have a normalized number
to use. Now lets say our target speed is 200IPM (3.33 ips) then all
we have to supply to get there with the given ratios is 13,333 pps;
well within our 35,000 max rate.

If your numbers came out that you need to supply 65,000 pulses per
second to get your target speed then you would need to use a pulse
multiplier so that one pulse from the computer puts 2 or 5 or 10
pulses into the circuit.


The problem starts to be amplified when there are servo's involved.
First it's a good idea to operate servo's at higher RPM rather
because of the torque characteristics. Then usually means your
gearing ratio needs to be higher. A 3 or 4:1 ratio is often used.
The good news is that the speed is now converted to increased torque
(like dropping the old pickup into a lower gear). You have to know
the line count of the encoder used for feedback.

Lets use the same basic setup as before but add in a 3:1 belt
reduction before the rack reduction of 2:1 and the fact we have an
encoder of 500 lines.

We first need to multiply the 500 line encoder by four since we get
four distinct transitions (counts) per line. Called Quadrature it
increases the resolution but forces us to deal with the higher
number. Since a pulse from our PC will move the servo on count we
now need 2000 pulses to move one complete revolution on the motor.
Apply the reduction ration of 2:1 (rack) and 3:1 (toothed belt) for a
total of 6:1 speed reduction and now we need 12,000 pulses to move
the load 1 inch. to move the same 3.33 ips as before we need 39,960
pulses per second.....woops our PC will only do 35,000 maxed out. A
multiplier is the best way to get hit our top speed.

What you find in real life is that you have to play with the
numbers. Every change effects something else. The torque vs speed
issue is one. That's why you see a lot of motors that are oversized
since you would have to change the entire design if you were trying
for a given force at the tool. Often in our world of HSCNC you
design around "found objects" so that might dictate the other numbers.

I found some nice ballscrew assemblies with belt driven servos
attached. The encoders were 500 line. With a 5 TPI screw ratio, a
3:1 belt ratio and 500 line encoders and my target speed of 150 IPM,
I needed 2000 * 3 * 5 to get an inch or 30,000 pulses to move ips.
The target speed is 2.5 times that so I would have needed 75,000
pps. The Gecko 340 was called for. I ran the multiplier at 1/5 so I
only need to supply 15,000 from MACH2.

As for the gain or loss of resolution that is in the numbers as well
but as we have noted true accuracy is not the resolution of your
machine but its ability to be in the exact positon commanded. MAny
things effect that number.

My advice is to first decide on your drive method and motors then
start running numbers to see how fast you will have to spin the motor
shaft to hit your target speed. Once you have that and if the
numbers are reasonable then you can determine which model of Gecko
you would need.