RE: [CAD_CAM_EDM_DRO] Re: Z Axis motor speed

-----Original Message-----
From: caudlet [mailto:info@...]
Sent: 10 April 2004 22:04
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: [CAD_CAM_EDM_DRO] Re: Z Axis motor speed


--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Dave Fisher" <dave@d...>
wrote:

> On the router / mill table I am building I have chosen a 24 Volt DC

motor to

> drive the Z axis ball screw. The motor is rated at 15 Amps with a

speed

> rating of 500 RPM.
>
> The motor is directly coupled to the ball screw and a 2000 line per

rev

> shaft encoder which when calculated gives me 200 steps per MM of

travel on

> the axis.
>
>

humm (thinking outloud here) you have a 10mm pitch ballscrew (based
on the math above) If you could spin the motor at 500 rpm you should
be able to move 5000mm / min or 83mm/sec (about 180 IPM for us yanks).
Obviously your motor is fast enough if you can get enough pulses
there. Your shaft encoder has 2000 lines and 8000 pulses in
quadrature. To spin it at 500 rpm you need 8000 X 500 or 4,000,000
pulses per minute. Pulses are measured per second so that's 66,666
pulses per second....woops.

So what program are you using to generate the pulses (software
controller)? If it's MACH2 you have a limit of 45,000 per second but
you need a pretty fast computer to get there. At the default
25,000pps you will only have 1,500,000 pulses per minute or about 187
rpm max (31mm/sec). Not great but still not as slow as you describe.
Make sure you have your velocity tuned up to where the math says you
should be.

If the pitch you gave is right it appears the issue may well be the
max pulse rate. There are a couple of solutons. 1.)Replace the
encoder with a 250 or 500 line encoder. Now you can get max speed
with fewer pulses. 2.)Use a multiplying servo controller like the
Gecko 340 series.

Give us the specifics of the screw pitch and the software you are
using. Don't forget the 4:1 pulse increase from the dual channel
encoders.

==

Software is DeskCnc via their dedicated 'PIC' based serial controller - not
sure
about max encoder frequency input - but Fred Smith states on their forum:

'At the present time our Serial and usb interfaces provide the same max
throughput of 300 blocks per second, and 40,000+ Hz, step rates to each
axis'

I am sure the pitch is correct, I have the setting for 'Steps per MM ' at
200 and the shaft encoder is rated at 2000 pulses per rev + the Z axis is
moving the correct distances under software control.

I must admit that I have never considered the input frequency of the system
to be a limitation ?
I would not really want to decrease the resolution of the encoder, as if my
understanding is correct I will only be sacrificing accuracy ? BUT, I would
like to get the Z axis moving faster.

Funny, now I have re read your very concise and helpful answer - the
encoders on my X any Y axis's are only 400 pulses per rev and although
different motors - the x and y move at very fast speeds.

Dave Fisher UK


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