Acorn system "open loop" operating question?

[marshallexcavating]

I am considering buying the Acorne controller and accessories but am confused about how it can have real DRO capabilities without hardware/encoder for all axis(X, Y, and Z)? I am upgrading a knee mill with stepper motors from a company called "AhAH" that is open-loop configured. My current stepper motors stall and there is no way the software can know causing me big problems.
1. If I use/buy the ClearPath_SD motors and use the Acorn controller is the system a truly closed-loop system that will read the true position even if there is an overload condition?
2. Is there a better way to go utilizing my current stepper motors with a CENTROID controller to get real DRO and stall corrections on the fly?

Thank you for any information/advice.

[CNCMaryland]

I can't answer all the questions but, in an open loop system your positional accuracy between shutdown and the next startup is contingent on the accuracy of your homing switches/sensors. When you start the acorn, the first everyday step is to home the machine, this registers X0Y0Z0.

I don't know how this works with a closed loop on shutdown and restart. I would think its very close to the same as above.

Open loop versus closed loop is to me the reporting of any loss in steps during operation. Open loop has no clue if you crash and are now out of wack or if your motor stalled, etc. Closed loop would be aware of this, and the drive would fault out.

[slodat]

If your stepper drive has a fault output (most I’ve seen do), that fault output is connected to Acorn and configured as such. This would cause the machine to stop and open the e-stop contactor which would cut power to the motors.

[Caleb]

You don't actually need closed loop feedback if the setup is correct, motors correctly sized, mechanical setup of sideways etc correct and tool loads not excessive.

You can get closed loop stepper motors with encoders and drives to match the drive outputs a fault signal if the motor hasn't moved to the commanded position, the drive will also account for motor slip and therefore can produce more torque.

If stalling is an issue for you then the implementation isn't correct for your task.

If your system is setup correctly the controller doesn't require "DRO" feedback as the motors should move the prescribed amount every command and not move when they shouldn't.

If you want to go down a full positional feedback route then the complexity and expense will increase massively as you require so many more inputs per axis.

I can quite happily run my acorn machines all day with no position discrepancy.

[tblough]

With an open loop system like the Acorn, you cannot recover from a stall without re-homing the machine and/or re-indicating the part. If you have closed-loop steppers AND the fault output from those steppers correctly hooked up, you will ,at least, know when a stall has occurred. With standard steppers, you may not know a stall has occurred.

If you want to manually move the machine snd use the readouts as a DRO, there is no way to do this with an Acorn.

[Caleb]

Basically I was trying to say, work out why you're getting stalls firstly before trying to overcome them with added complexity rather than removing the cause.

[eng199]

I am considering buying the Acorne controller and accessories but am confused about how it can have real DRO capabilities without hardware/encoder for all axis(X, Y, and Z)? I am upgrading a knee mill with stepper motors from a company called "AhAH" that is open-loop configured. My current stepper motors stall and there is no way the software can know causing me big problems.
1. If I use/buy the ClearPath_SD motors and use the Acorn controller is the system a truly closed-loop system that will read the true position even if there is an overload condition?
2. Is there a better way to go utilizing my current stepper motors with a CENTROID controller to get real DRO and stall corrections on the fly?

Thank you for any information/advice.

The DRO shows where the control asked the motors to be. Without feedback to the control, this may not be where the position actually is.

1. There are some variations on closed loop, based on where the loop is closed.
a. Steppers are (most often) open loop. There is no feedback to tell if position is lost.
b. DC or brushless motors are closed loop. This is an improvement over open loop, but the position is still not reported to the control on an ACORN system. The chance of position loss is much lower, but you still can not see the error in position on the screen.
c. The most robust systems have encoder input to the controller so that actual position is shown on the DRO.

2. If you have standard stepper motors without encoders, the best option is an upgrade to a closed loop motor and drive package. To get true position on the DRO, the drives need to accept A/B quadrature position request to allow an upgrade to the OAK controller.

[martyscncgarage]

I am considering buying the Acorne controller and accessories but am confused about how it can have real DRO capabilities without hardware/encoder for all axis(X, Y, and Z)? I am upgrading a knee mill with stepper motors from a company called "AhAH" that is open-loop configured. My current stepper motors stall and there is no way the software can know causing me big problems.
1. If I use/buy the ClearPath_SD motors and use the Acorn controller is the system a truly closed-loop system that will read the true position even if there is an overload condition?
2. Is there a better way to go utilizing my current stepper motors with a CENTROID controller to get real DRO and stall corrections on the fly?

Thank you for any information/advice.

You are correct. THERE is no feedback with Acorn other than for the spindle encoder. So no, "real DRO capabilities"
You mention Clearpath SDSK. Good choice. They have the drive AND an encoder internally. This is a "Hybrid Closed Loop System" That is, Acorn sends step and direction pulses to the drive, the drive then commands the motor to turn, and the encoder tells the drive that it did or did NOT turn based on pulses sent. (Over simplifying) As others have mentioned, this is better than a stepper system with NO feedback to the drive.

Other options are AC Servo drives/motors that take step and direction signals. There are many.
It boils down to your budget and DO have a realistic budget.

Marty

[marshallexcavating]

Thank you all for the information. I now think I understand the Acorn "hybrid closed loop" designation; the fact that the Acorn controller can:
1. Take falts from a motor with the ability to sense a problem but there is no continuous position handshaking makes it a "hybrid closed loop".
2. Some step and direction motors like the ClearPath have built-in smart encores and software to locally (at the motor not in communication with the Acorn) to close loop the position internally. The smart motor makes this a "hybrid closed loop" configuration.

With this information, I am leaning in the direction of:
1. Starting with the Acorn controller configured in step and direction control with the ability to upgrade to the OKE controller if I want a truly "closed-loop system".
2. ClearPath CPM_SDSK-N05619-ELN motors for the X and Y and are "Step and Direction" and "A/B quadrature" compatible.
3. ClearPath CPM_SDSK-N05629-ELN motors for the Z knee with a hydraulic asist.

My logic is the ClearPath motor can be configured in both "Step and Direction" and "A/B quadrature" position. This will get me started with the "hybrid closed loop" system and a smaller price. If anybody sees a problem with this logic please let me know. Thank you all.

Note: I am not considering using my current stepper motors because the type of work I do has too much time invested to have a possibility for any missed steps without warnings. The possibility of an unknown position is too costly.

[martyscncgarage]

Do not believe you will be able to use Clearpath SDSK with Oak.
Something like Delta, Estun and DMM yes if you purchase the correct drives.
Telling us about the machine, providing pictures would be helpful.