For my bandsaw feed I used simple timer like IC and driver for the stepper:
http://www.ajawamnet.com/ajawam3/swarf/bandsaw.html
For z axis of the sieg mill I used an arduino with an Anaheim automation
stepper with integral drive built in:
http://www.ajawamnet.com/ajawam3/swarf/mill_mods.html
For my CNC I went with Gecko. One of the reasons I went with Gecko is
that Mariss has dealt with the dynamic of stepper motors for long time.
One issue that you can run into is resonance. Typically you'll never see
it - but if you run enough jobs you'll come across it; where it just so
happens a line of G code will cause one or more of the steppers to hit
it and stall:
http://homepage.divms.uiowa.edu/~jones/step/physics.html#resonance
"In practice, this oscillation can cause significant problems when the
stepping rate is anywhere near a resonant frequency of the system; the
result frequently appears as random and uncontrollable motion. "
and this
http://www.haydonkerkpittman.com/learningzone/technicaldocuments/stepper-motor-theory
"Resonance
Stepper motors have a natural resonant frequency as a result of the
motor being a spring-mass system. When the step rate equals the motor's
natural frequency, there may be an audible change in noise made by the
motor, as well as an increase in vibration. The resonant point will vary
with the application and load, but typically occurs somewhere between 70
and 120 steps per second. In severe cases the motor may lose steps at
the resonant frequency. Changing the step rate is the simplest means of
avoiding many problems related to resonance in a system. Also, half
stepping or micro stepping usually reduces resonance problems. When
accelerating to speed, the resonance zone should be passed through as
quickly as possible."
I talked with Mariss about how he handles this in the older, all analog
drives as well as the newer ones that use an FPGA. On the older 201's he
does it using a slick comparator circuit that detects it - most probably
like Jones mentions:
"Controlling resonance in the low-level drive circuitry
A resonating motor rotor will induce an alternating current voltage in
the motor windings. If some motor winding is not currently being driven,
shorting this winding will impose a drag on the motor rotor that is
exactly equivalent to using a magnetic eddy current damper.
If some motor winding is currently being driven, the AC voltage induced
by the resonance will tend to modulate the current through the winding.
Clamping the motor current with an external inductor will counteract the
resonance. Schemes based on this idea are incorporated into some of the
drive circuits illustrated in later sections of this tutorial. "
On 3/21/2018 1:44 PM, Chris Albertson albertson.chris@gmail.com
[7x12minilathe] wrote:
> Yes, you can make a pulse generator. But when you have a lab bench full
> of test instruments you tend to use the instruments for testing. The
> signal generator can do "sweeps". this is where it changes the pulse rate
> per time. I can sweep the step rate and in that way measure
> acceleration. I get measurements good to more decimal points than I need
> and I don't need to build anything.
>
> In the end the motor will be controlled by some kind of computer, likely
> one running Linux and LCNC or MK.
>
> I think if I were building a simple power feed I'd want at least a tiny LCD
> screen that reads out the feed rate in units like mm per second or inches
> per minute and some stop/start and direction buttons. The 555 timer can't
> drive the LCD. And you'd want to be able to make it stop EXACTLY at
> some point (because maybe you are milling an internal pocket.)
>
> The hard part of building a pulse generator for a power feed is getting the
> user interface right. Generating pulses is trivial. But I think the
> user wants to set the feed rate BEFORE he presses "start". and likely
> wants to be able to set a limit. I would like to be able toes and verify
> those numbers before I press "go".
>
> Cost for a controller of any kind is very low, so that is not the issue,
> the hard part is figuring out what it should do.
>
> One use case of the CNC mill will be to design parts using Autodesk Fusion
> 360 then make parts. But this is a long process. For simple things,
> like boring a hole or pocket or face mining or dozen other easy jobs I'd
> like to just tell the machine what I want done. Where the current state
> of the art needs to be "pushed" is right there -- how to tell a machine
> what you want. Current methods are primitive and limited
>
> On Wed, Mar 21, 2018 at 9:52 AM, mark.kimball2@frontier.com [7x12minilathe]
> <7x12minilathe@yahoogroups.com> wrote:
>
>>
>> RE: pulser for simple stepper control. I made one for zero dollars -- I
>> already had a 555 timer chip, and my electronics scrap box had the perf
>> board, resistors and capacitors I needed to make a simple clock generator..
>> A slide switch allows me to select the direction, and I can adjust the
>> clock rate with a pot.
>>
>> I used the thing to run a stepper as a low-speed diamond grinder for
>> shaping and sharpening carbide scrapers. I bought a set of 6" diamond
>> lapping disks from a lapidary supply outfit, 150 grit up to 3,000. The low
>> speed keeps everything cool (particularly since I run the bottom portion of
>> the disk through a pool of water). It does a good job for me. I made a
>> simple arbor to mount the disks using my lathe.
>>
>> The ebay pulse generator looks like a good value if you're not able or
>> willing to DIY one.
>>
>> Mark
>>
>>
>
>
Posted by: WAM <ajawam2@comcast.net>
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