[itsJustme]

I bought a hobby cnc machine as well as closed loop steppers and controllers my first question
1. Are all the controllers now some what similar as far as getting and sending commands?
2. the controllers that came with motors are a HBS57H these are all push button instead of dip switches!
3. Would these wire into the Acorn card as shown in videos of the wiring on Ytube?
I was going through the setup process of setting up and the question came up and really did not know how to answer and thought I would come here for some help! This is my 3rd machine with the first 2 being the plug! But I wanted to take this one to the next level by going with closed loop steppers and a higher end controller but have never wired one and have been seeking some help with little luck.

Thanks in advance for any help!

[joeavaerage]

Hi,
the 'loop' is closed by the drivers. The steppers have an encoder in them which is connected to the driver. The driver recieves Step/Direction signals from the motion
control, in this case an Acorn, and the stepper follows those commands. The actual position of the stepper is monitored by the encoder. If the actual position differs from
the commanded position the driver automatically inserts extra steps to have the stepper catch up.

All closed loop steppers and most modern servos work this way. That is to say that the drive closes the loop.

In earlier times this is not what closed loop meant. So we have a mixture of terminology which causes no end of confusion.

Servos had a tacho generator or sometimes a resolver or an encoder, and that output would be fed to the motion controller, and the motion controller would apply a correction
that was then communicated ( by a varying analog voltage) to the drive, which is really just an amplifier. This meant the the motion control had to have the smarts to
close the loop and make the corrections necessary to drive the servo. An Acorn motion control cannot do this, it is described as an 'open loop motion control' and
there are many such controllers, Mach, UCCNC, PlanetCNC etc.

The Centroid Oak however, which is made by the same company as the Acorn, is capable of this style of control but is over $1300USD. These are often called
'full closed loop control'....and now you can see why the confusion arises.

In your case the stepper drivers close the loop, whereas another controller (Centroid Oak) with analog servos the motion controller closes the loop.

There are many whom believe the later type is better, I however do not.

Modern AC servos are extremely tightly and accurately controlled by their matching drives. A control engineer would say the 'control loop is high
bandwidth' ie the corrections are applied fast and very accurately, and follow the motion commands to a very high degree.
A 'full closed loop controller' like a Centroid Oak, a Galil, or LinuxCNC also do a good job but cannot match the bandwidth that a dedicated and matched servo drive
manufactured as a pair.


In more recent years various 'distributed motion control' solutions have been adopted, Ethercat being a common and well regarded example. With Ethercat there
is no motion controller, the Ethercat Master (PC typically) provides numeric trajectory data for each of the servos and each individual servo is responsible for its 'slice' of the motion
control. Thus two axes are coordinated by virtue of receiving simultaneous numeric motion commands, but otherwise each servo operates independently. This might not sound
like a good idea if you are wanting to obtain tight coordination between several axes.....but is in fact exactly the solution employed by many of the worlds leading CNC manufacturers.

All-in-all this trend towards 'distributed motion control' is seeing the gradual demise of the controllers which are capable, indeed rely on 'full close loop control', and note here I'm using
the older sense of the description.

Craig

[joeavaerage]

Hi,
just answer your specific questions

1) Many of the motion controllers that will interest you are 'open loop controllers' of which the Centroid Acorn is an example. Mach4 and an Ethernet SmoothStepper is another
example. UCCNC and one of the motion controls from the same company like a UC300 are yet another example, namely 'open loop control'. That means that the control
issues commands to the servos/steppers and the servos/steppers are expected to do it. In your case the steppers can be relied on to follow the commands and they have a means
of checking and correcting, if needs be, that the stepper is accurately following the commands. These types of motion controls are relatively simple, low cost and enjoy widespread
use by hobbyists.

'Full closed loop controls' like Centroid Oak, Masso, Galil the feedback from the servo is direct connected to the motion controller and the motion controller provides the required
corrective signals if required. These controllers tend to be much more complicated and expensive than the open loop types. None-the-less they are used by hobbyists but the cost and complexity
is seeing their use diminish, except in certain applications where a central closed loop control is determined to be required.

You might note that I left LinuxCNC off that list. LinuxCNC is a realtime computing system and can therefore do both open loop control or closed loop control equally well. LinuxCNC is open source,
or free. The hardware needed to get the best out of LinuxCNC, various Mesa boards, are moderately priced. All-in-all LinuxCNC is very capable and moderately priced and enjoys a large and
enthusiastic user base. It certainly benefits those who can code in C/C++, although is usable for simple applications without coding.

2) Closed loop stepper drivers are very much more sophisticated than ordinary stepper drivers that you are maybe familiar with. They often require programming, much more so than
you are used to. AC servos are worse, having some hundreds of parameters that need to be programmed, so much so that many servos have an auxillary program that runs on a PC
just to program the firmware of the servo drive.

3) Yes, these closed loop stepper controllers would wire very nicely to the Acorn, or indeed any of the open loop control solutions I've mentioned.

The truth be told, closed loop steppers are not all they are claimed to be. The manufacturers of them claim 'more powerful, faster, never miss steps....', all pure hype and BS.
A closed loop stepper is still a stepper, and when a stepper gets overloaded it misses steps and closed loop changes that not at all. So the driver might issue some extra steps to catch up,
but guess what, the extra steps are just as likely to be missed as the ordinary ones, the stepper is marginally overloaded after all.

There are two advantages that closed loop steppers have over open loop steppers:
1) A closed loop stepper can linearly interpolate between full steps, and so has higher effective resolution than an open loop stepper.
2) If a closed loop stepper misses steps and cannot recover then it will signal a fault and stop, whereas an open loop stepper will carry on as if nothing is wrong
and get a misshapen part.

These two advantages you are paying dearly for....I personally don't think they are worth it. If you want genuine closed loop performance than get genuine AC servos....they kick arse and will
eat any stepper ever made!

My new built machine has 750W Delta servos on each axis and they absolutely humble any stepper, although they do cost a lot more.

Craig

[itsJustme]

Thanks for the response in answering my questions, and from what I gleaned from your response I should be able to wire the ones I have as others have in the video's that have been posted on line of users of the basic acorn control card! As I wasn't really sure if there might be some unknown black magic or hidden trickery form maker to maker that impost to table top user!

[joeavaerage]

Hi,
I have not used an Acorn, but I use Mach4 and an Ethernet SmoothStepper (ESS) and there are many similarities between the two.
The user manual of the Acorn is pretty straight forward and complete. The Acorn is an excellent match for your closed loop steppers.

The ESS requires another board, a breakout board, between the ESS and the external wiring to stepper drives and switches. The breakout board does not
do anything but if more about signal conditioning and amplification with screw terminations for external wires. As I posted earlier I use Delta servos in my machine, and like most industrial
controls are 24V, while the ESS is 5V. Thus the breakout board does the voltage translation. Also to use the servos superior speed and resolution requires high frequency signalling to the drives.
I use differential signalling, there again accommodated by the breakout board.

All the features I've described about the breakout board are already incorporated into the Acorn, it is not something that you have to add. For this reason the Acorn is a nearly 'plug n play'
solution, and is a reason that it is so popular. The only catch is that you need to follow Centroids wiring/logic practices closely whereas there is much greater flexibility with the ESS and breakout
board. Either will allow you to make and control a great machine.

Craig