Messages

1441

General Mach Discussion / Re: What to buy for a new project

« on: June 11, 2020, 02:47:04 AM »

Hi,

Looks like the AC servos and drives are more expensive than steppers, but perhaps just because they go up in size quickly, while steppers with more than 1 N*M torque seem rare.

Yes, good quality servos but still not 'top shelf' are close to double the price of a stepper and drive.

All steppers lose torque the faster they go. The higher the winding inductance the worse the torque degradation. Most run of the mill
cheap 23/24 size steppers with 6mH inductance have as little as 5% of their holding torque at 1000 rpm. If you choose wisely and get steppers
of 1-1.5mH they may retain 35%-50% of their holding torque at 1000 rpm.

34 Size steppers have a lot more torque, 1200 oz.in is common but have very much higher inductance also and therefore will probably
not be much good beyond 500rpm.

All steppers, irrespective of size have no overload capacity, the first time you know that you are approaching the torque limit (at the given operating
speed) is a missed step or stall. Servos on the other hand just 'dig in', and produce the extra torque, commonly three to four times rated torque
with a heat limited duration.

The torque/speed diagram you posted is normal for servos. My Delta servos have torque/speed curves exactly like that.

I suggest you look at Delta, a Taiwanese brand manufactured in China or DMM, a Canadian brand manufactured in China, both good quality
and well supported that won't break the bank. There are even cheaper no-name Chinese brands but of questionable quality, support and
documentation, none-the-less seem good value.

Well chosen (low inductance) steppers with the best highest voltage drivers and highest voltage power supply deliver great results when used within
their envelope, but the operative words here are 'within their envelope'. If you attempt to stray out of that envelope they stall, no ifs or buts, they stall.
Having said that they have a great deal of torque in a small package, better than servos pound for pound. Where servos show their strength is
their overload capacity and speed, but you buy those advantages at a hefty price.

Craig

1442

General Mach Discussion / Re: What to buy for a new project

« on: June 10, 2020, 11:11:24 PM »

Hi,

The PMDX controller I suggested is based on the assumption that I can run Mach4 on the laptop and it will direct the controller via USB, which then directs the motor drivers.  Any flaw in that simple picture?  I haven't figured out what the value in having a parallel port connection would be if I am setting up a new machine and control system from scratch.

USB is a poor choice for controller interconnection, it works but is noise prone, better Ethernet. If memory serves the only all the PMDX controllers are USB connected,
which would be their only weakness. An Ethernet SmoothStepper is a viable alternative. But, otherwise yes, the PC connects to the motion controller either USB or
Ethernet, and the controller connects, via in some cases a breakout board, to the motor drivers, limits/homes/probes and Estop as required.

Now that you mention it, an AC servo definitely will have the guts to move anything, but doesn't that complicate the motor driver and controller?

No. The AC servo is iteslf a fairly detailed and complex setup task but the connection to the motion controller is as straight forward as a stepper.

Craig

1443

General Mach Discussion / What to buy for a new project

« on: June 10, 2020, 08:39:54 PM »

Hi,
price scales exponentially with size, particularly if you require the rigidity to machine aluminum. If you want to machine
steel then factor at least a five-fold increase in rigidity, in fact a router table is the wrong design for ferrous materials, for those
you need a mill.

PMDX controllers are parallel port for Mach3. The only PMDX external motion controllers are for Mach4.

Do you wish to persue a parallel port or do you wish an external controller?

Leadshine have a range of controllers that are very capable but less costly than Geckos, the AM882 for example.

34 Size steppers have great torque but are as slow as a wet week....if you must use 34 (or bigger) then select models with
the lowest possible inductance or you will be dissapointed.

If you are of the opinion that you need the torque that you can get from 34 size steppers you should consider AC servos, an
appropriately sized servo will leave any stepper in the shade.

Craig

1444

Mach4 General Discussion / Re: Mach4 can do Arc voltage sampling for a CNC plasma table?

« on: June 07, 2020, 03:54:41 PM »

Hi,
the script based Mach4 THC solution samples the arc votlage approximately 100 times a second.
The Nyquist Sampling theorem tells us the theoretical maximum control bandwidth is half, ie 50Hz.
A Control Engineer will tell you that the 'discrimination' is rubbish at 50Hz and only starts looking like
a modest performing control loop around 5-10Hz.

Realtime THC controllers sample the arc voltage thousands of times per second.

Craig

1445

Mach4 General Discussion / Re: Mach4 can do Arc voltage sampling for a CNC plasma table?

« on: June 07, 2020, 07:25:54 AM »

Hi,
there are two means of THC.

Mach4 has introduced a script based THC, one of the first Windows based CBC solutions to do so. Because Windows is
not a realtime system in the past the communication delays have all but precluded a script THC solution. Given that
Mach4 is faster it can do a fairly slow control loop without a great deal of hardware.

If you demand really high dynamics and robust anti-dive features then you need to get a genuine realtime THC solution.
At the current time only two control boards support realtime THC, the Hicon from Vital Systems and the ESS from Warp9TD.
The ESS supports the TMC torch height control system specifically and probably has the most up-to-date anti-dive algorithms.

Craig

1446

General Mach Discussion / Re: Pending signal managment on mach3

« on: June 05, 2020, 04:13:31 AM »

Hi,
neither Mach3 or Mach4 are feedback software, Windows all but precludes realtime operation and therefore the feedback loops have delays in them
which screw it up bigtime.

The current paradigm in industrial machine control is moving away from feedback controllers in any event. The feedback loops
are actually between the drive and the servo, and the drive is it's own motion controller as part of a distributed motion control solution.
Read up on EtherCat, its a fascinating insight into where motion control is going.

If you want to have linear scale feedback in addition to step/direction control the I suggest that you look to recent developments in AC servos.
The Delta A2 series servos for instance have a ridiculously high resolution encoder (1,280,000 count per rev) but in addition have a second
encoder channel for a 'load monitoring' encoder, aka a linear encoder. What's more these A2 series are only another $40-$50 more than
the B2 series (160,000 count per rev encoder, without load sensing).

Most of the top tier servo manufacturers offer load sensing.

Craig

1447

General Mach Discussion / Re: Pending signal managment on mach3

« on: June 04, 2020, 04:09:54 AM »

Hi,
yes Mach4 can be operated that way but why?

This question has come up before....and on the surface it seems reasonable enough...ie have Mach wait until one move is complete
BEFORE issuing the next move...right??

Well no actually. Mach issues a stream of commands without regard to whether the drive and steppers can keep up, but that's what you've got
closed loop steppers for, they will fault 'following error' should they ever 'fall' behind the commanded position. When they fault 'following error'
the machine will Estop.

What prevents Mach from issuing instructions faster than the machine can respond?....remember you program Mach with max acceleration and
velocities, which are determined by the steppers and drives, and if chosen correctly the machine will go as fast as its drivers and motors will take
it without losing steps and without Mach getting ahead of itself. This applies equally to open loop steppers as closed loop steppers or even servos.
The advantage of servos and closed loop steppers is that the drive will signal an error if Mach attempts to issue instructions faster than they can handle.

So you can see that having Mach wait it not actually necessary. In fact its worse than not necessary. If Mach waits for a move to complete before
it issues the next move the machine will stop and have to wait for Mach to receive the Pending signal and then wait while Mach sends it a new instruction.
That would be really 'jerky'. Its not dissimilar to 'Exact Stop' mode in Mach3. Try it...you probably won't like it. Mach (3 or 4) runs best in
'Constant Velocity' mode where it tries to blend the end of one move into the start of the next move. It keeps the machine moving and is (usually)
faster and smoother.

Having Mach wait until it receives a signal to proceed with the next move will make the machine slower and jerky.

Craig

1448

Mach4 General Discussion / Re: help required for Reference all Axis script

« on: May 31, 2020, 07:19:42 AM »

Hi,
so you want the machine to go to Z20 before you start.....but where is Z20?......if your machine is referenced already then yes Z20
makes sense but if its not referenced then G0 Z20 could be anywhere. I can't quite see the logic of what you are trying to achieve.

That does not change the fact that you have coded  incorrectly.

the Line:
mc.mcCntlGcodeExecute(inst, 'G00 Z20')
starts the move, but while the move is still executing the function returns. You need to use:

mc.mcCntlGcodeExecuteWait(inst,'g0z20')

As a side matter Machs Gcode interpreter converts all Gcode to lowercase, strips out leading zeros and whitespace. Thus 'G00 Z20'
becomes 'g0z20' It should, in theory, mean the same thing, but there are occasions when the translation does not work and you will
struggle to identify the fault. My suggestion is to write the Gcode exactly as the interpreter requires rather than rely on its translation
of what you have written.

Craig

1449

Mach4 General Discussion / Re: mach4 homing

« on: May 28, 2020, 03:22:03 PM »

Hi,
you are confusing home and limit switches.

When you use SoftLimits you are declaring to Mach that the extents of the machine are so many inches in the X
direction, so many inches in the Y direction etc from your Home point.

So how do you deterime your Home point? Today when you started your machine and pressed <RefAllHome>
you were 4 inches from the lefthand end of the X axis and 3 inches from the end of the Y axis.
Tomorrow when you start your machine but this time when you press <RefAllHome> you are six inches from
the lefthand end of the X axis and 6 inches from the end oy the Y axis.  Thus the data that Mach was using to determine
the 'SoftLimits' was correct on day one, but now your Home point has shifted on day two its incorrect.

The only way for SoftLimits to work properly is for the Home point to be a uniquley defined and REPEATABLE place
on your machine. That is what home switches are for.

You could use three of your limit switches to determine one 'corner' of your machine and call that Home. My preference
is to have separate home switches as you can determine the design, mounting and location of the switches to give
the best possible repeatability of the home loaction.

Craig

1450

Mach4 General Discussion / Re: mach4 homing

« on: May 27, 2020, 11:54:54 PM »

Hi,
do you have Home switches assigned and working?

I notice you say you have set up SoftLimits. SoftLimits only make any sense if you have Home switches and you reference (or home) your machine at start-up.

In absence of Home switches Mach will 'Home In Place' that is, the machine coordinate DRO's will reset to zero but without movement. If you have
home switches then the machine will move until it detects switch activation.

The 'depending on whats programmed' that Bill_O refers to is the small script within the ScreenLoad Script that determines the behaviour
of the <RefAllHome> button.

Craig