Messages

1471

General Mach Discussion / Re: switching

« on: September 10, 2010, 12:03:31 PM »

I was suggesting them for Tweakie's laser not my trigger Amir. Keep up 

1472

General Mach Discussion / Re: Soft stop at limit switches

« on: September 10, 2010, 11:57:37 AM »

Nicolas - If I understand you correctly, on the one hand you have concern over what I'll call the "bang" that a system suffers when the step pulses are simply "cut off" and you suggest that a ramp would be kinder. BUT on the other hand you suggest that a ramp could actually slow the machine down (deccelerate) faster than simply cutting off the step pulses.

But there surely is the contradiction. The "bang" is the result of rapid decceleration and you're wanting to INCREASE that deceleration.

Or have I missed something?

Ian

1473

General Mach Discussion / Re: Soft stop at limit switches

« on: September 10, 2010, 07:52:23 AM »

Hood - understood - thanks.
Nicolas - fair point - I can see in that case why you might want to introduce a brake on E-stop rather than just rely on the pulse train stopping  - (don't these exist for heavy machines that would benefit?). Afraid I still can't see the limit thing though - given soft limits.

Ian

1474

General Mach Discussion / Re: Soft stop at limit switches

« on: September 10, 2010, 06:06:43 AM »

Nicolas - Let's go with your method for a moment. Suppose from max rapid your machine takes 10mm to come to a controlled stop. You'd have to place your limit switch at least 10mm from where the machine absolutely must be stopped. Now suppose for whatever reason the machine finds itself heading for a position at 5mm from end stop. It'll hit the switch at 10mm and come to your controlled stop. Now suppose you go with the soft limit method. Your machine heads for the 5mm position, hits the soft limit at 10mm, slows down, gets to the 5mm position and CARRIES ON with the job. Which is better? Now suppose it's heading to a position beyond the end stop. It goes into the slow zone and then hits the hard limit - but it does this at a speed set to NOT damage the machine. It's win win either way.
Now your E-Stop - you get your arm caught - would you prefer it to stop NOW or let it chew your arm for another 10mm or so in order not to possibly damage the machine?

And now a confession - Many moons ago I used to think the same as you - that was until a younger Hood and others put me right 

1475

General Mach Discussion / Re: How to recover from power interuption?

« on: September 10, 2010, 05:52:32 AM »

Off the top of my head - how about something like this: In any job (ok - some jobs) there are places that are "ok" and "not so ok" to stop maching if you have to. How about putting a custom macro into your code at all the places where a stop would be "ok". That macro would check if the power is on/off. If "on" then just carry on else do the controlled stop thing whilst you still have UPS. Typically the macro might just turn off the spindle, mill and controller, do a feedhold and store the current line for later. Mach could be left running in power saving mode and subsequently shut down by the UPS if neccessary.

Ian

1476

General Mach Discussion / Re: Closed loop control using encoders?

« on: September 10, 2010, 04:40:10 AM »

Question: If one uses servos (not steppers) then in theory no steps should be ever lost. Is this statement correct?

Well the crux is the "in theory" bit.
Put it this way. Are servos able to produce infinite power? No. Are steppers? No.
If the servo or stepper is correctly matched to the job required of it then NIETHER will ever "in theory" miss a step. But the contrary is also true - and yes - I mean for servos too.

Ian

1477

General Mach Discussion / Re: switching

« on: September 10, 2010, 04:07:15 AM »

OK guys - thanks.

Tweakie - have you looked at logic level mosfets for your cross hair pointer - they sound like the mut's nuts for that.

Ian

1478

General Mach Discussion / switching

« on: September 08, 2010, 06:12:33 AM »

I need to switch 18V 10mA via TTL (turn on/off a plasma torch via the parallel port).

A relay seems a bit overkill for this - doesn't it? and I read somewhere (I think) that such low current is not best switched by a SSR - true? or have I made that up?

Thoughts on cheapest/bestest/nicest/properest way to do this?

Cheers

Ian

1479

Tangent Corner / Re: cnczone

« on: September 08, 2010, 04:36:34 AM »

well it's slightly better than my last one - at least I now get my crap service and crap support slightly cheaper 

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Tangent Corner / Re: cnczone

« on: September 08, 2010, 03:41:58 AM »

Nope - haven't changed anything. Used to use IE but have been on firefox for a year or so. This slowness has really just started happening over the last couple of months and seems to be getting worse. With the Mach forum for example I can usually get to the front page (forum) pretty quick but sometimes getting to a sub-forum can take an age, sometimes it's fine.

Ian