Re: Servos drivers

--- In CAD_CAM_EDM_DRO@yahoogroups.com, R Rogers <rogersmach@y...>
wrote:

>
>
> caudlet <thom@t...> wrote:
> > [Non-text portions of this message have been removed]
>
> The lowest cost for that application is the DeskCNC servo card at
> $79 bucks sold by Fred
>
>
> They do not have step multiplication. You would need step
> multipliers if the line count of the motor encoders times 4 (in
> quadrature) times gear/belt/leadscrew reductions are greater than
> your max pulses per second to move the target distance in that one
> second.
>
> Try this formula:
> V1 = target speed = max speed in IPS
> Pr = pulses/rev = Encoder line count * 4
> Pi = pulses/inch = Pr * (belt-gear ratio) * (leadscrew pitch)
>
> PPS = pulses per second needed from PC = V1 * Pi
>
> IF PPS > 45,000 THEN implement step multiplication
>
> The line count of the encoder is a very big factor here because you
> have to supply four times the line count to rotate the motor one
> revolution (Pr). Then you have to multiple that by any gear rations
> like belt reduction or leadscrews (Pi). If you do all of that and
> find you need 45,000 pps or less then you DON't need a step
> multiplier. As you can see you need to have all of the numbers to
> make an accurate prediction.
>
> Hi Tom,
>
> Always enjoy your posts. The way I address this is issue is: Find

suitable motors,calculate the high speed/torque in relation to the
supply voltage and amperage, install encoders that allow the use of
drives void of step multipliers, Calculate ratio between servo and
ballscrew to attain the traverse rate desired. Is this just another
path in which to arrive at the same point? Or am I missing something
here?

>
> Ron
>
>
>
>
>
>Ron, In our varied world of Home Shop CNC (HSCNC) we are often

working with "found" objects. A lot of the people don't realize the
signifance of being able to exchange encoders or the fact you can
often use PWM rated DC motors and add your own encoders.

I presented the concept formula to try and point out that there are
several distinct variables involved. I agree your approach is the
best however I can tell you from experience that a "deal" is hard to
turn down. I found 5 7' linear assemblies complete with ballscrews,
motors, reduction belts and hardened rails with cam bearing
carriages. All 5 cost me less than one normal servo motor. The motors
had 500 line encoders. The pitch was 5 TPI and the belt reduction
was 3:1 If you plug the numbers in at my target rapids of 200 IPM
(actually ended up limited by "screw whipping" to about 160) You come
up with (500*4)(5)(3)= 30,000 then multiply the target speed of 3.33
inches per second and you see the number of 99,990 pop up. It was
clear I could live with a lot less resolution so a Gecko 340 at X5
saved my grits.

As scroungers there are some things we have control over and some
not. Most often we get a post from someone that starts out " I just
bought this [put your desc here] on EBAY and I plan on building a 12
axis carrot peeler. Can you help?" Part of the fun and challenge in
this hobby is trying to make something work that someone says won't!

It appears that the datum point of most CNC projects starts out with
some found or purchased motors and goes from there. The power supply
needs to be selected based on the motor specs and then the drives
from both. Sometimes you have to backup and let the limit of one
drive the choice of the other and compromise.