Centroid Community CNC Support Forum

Hey Folks,

I have an Acorn on Hardinge OmniTurn lathe. The motors are Nema34 hybrid steppers from stepperonline. Every time I hit reset button on VCP the machine loses positioning. Is it a known problem? Where to start troubleshooting?

Thanks, M

Yup, quite common! The E-stop is for EMERGENCY use only and not intended to be used to stop for any other reason. You need to re-home after each Estop use! (On Acorn)

Dave C.

Dave_C wrote: ↑Thu Jun 24, 2021 9:29 am Yup, quite common! The E-stop is for EMERGENCY use only and not intended to be used to stop for any other reason. You need to re-home after each Estop use! (On Acorn)

Dave C.

yep! unless you have closed loop axis motor drives, like Estun, Yaskawa, Delta etc.. you can hit estop with these drives and the drive still knows where its at since the logic power was never removed from the drive and the motors have an encoder, yet another reason to go hybrid closed loop with Acorn and use a closed loop axis motor drive.)

cnckeith wrote: ↑Thu Jun 24, 2021 10:08 am

Dave_C wrote: ↑Thu Jun 24, 2021 9:29 am Yup, quite common! The E-stop is for EMERGENCY use only and not intended to be used to stop for any other reason. You need to re-home after each Estop use! (On Acorn)

Dave C.

yep! unless you have closed loop axis motor drives, like Estun, Yaskawa, Delta etc.. you can hit estop with these drives and the drive still knows where its at since the logic power was never removed from the drive and the motors have an encoder, yet another reason to go hybrid closed loop with Acorn and use a closed loop axis motor drive.)

This is confusing… I am using hybrid close loop drives and motors with encoders from stepperonline.

Our Mazak 8G with Acorn and DMM motors doesn’t have this problem. To make sure—I am talking not about hitting mechanical E-Stop, but Reset on VPC.

Best, Mark

It's simple. If the drives power down, you need to rehome the machine. There is a difference between steppers and servos on this regard.

You also may have the estop actions and/or the NoFault output setup differently on the 2 machines.

Word of advice... A stepper system should require a rehome after an estop event. And estop events should be very rare, if ever. Don't confuse it's function with that of the feed hold

And don’t confuse its function with the “reset” button you now say you are pushing. The title of this topic is “Acorn Losing Position After Emergency Stop . Are you pressing the physical e stop or the reset on the VCP? They are two totally different functions.

ShawnM wrote: ↑Thu Jun 24, 2021 11:10 pm And don’t confuse its function with the “reset” button you now say you are pushing. The title of this topic is “Acorn Losing Position After Emergency Stop . Are you pressing the physical e stop or the reset on the VCP? They are two totally different functions.

Sorry for the confusion! I meant reset button on VCP and edited the title and first message.

Thanks, Mark

So my next question is "why are you pressing the reset button"? Are you pressing this while the machine is running? This is to reset the machine at startup and reset any faults. I think you need to be pressing FEED HOLD or TOOL CHECK instead. It might be advisable to read the manual to determine what each button does and find the correct one to press for your situation.

What are you trying to do? Why are you pressing the "reset" button?

You have hybrid closed loop steppers. What you don't have is true closed loop system. One that keeps the encoders powered up (so they retain position during an Estop/Reset) More importantly, on a lathe you need accurate way to home the machine each time. If you are content on taking a skim cut each time you power up to set X/Z then that's fine. (And is usually the trade off for using an open loop system or a system that does not provide an index pulse for homing very accurately.)

In a true closed loop motion control system, The axis motors have encoders. Those encoders are connected back to the Axis drives whose LOGIC power comes on with the motion controller and is never turned off unless the machine is powered down. Those encoders have an index pulse. One pulse per revolution of the encoder. So when you home the machine, each axis moves to a mechanical switch, hits the switch to coarsely adjust the home position, then backs off and the control continues to move the axis until the index pulse is found, stops and sets home at that point. Unless you have crappy mechanical switches (Long arms, cheap quality) this is pretty repeatable. The motion controller is monitoring the encoder via a cable OUT from the drives to encoder inputs on the motion controller. So, as long as the motion controller and the drive LOGIC (not motor power) is powered on, the motion controller knows the position of the machine. You hit Estop or reset, you simply remove MOTOR power via the ESTOP contactor (You have not told us how your drives are wired in the cabinet either) but the encoders are still powered up providing feedback to the motion controller.

In a HYBRID system like yours, most inexpensive closed loop drives like your stepper on line drives/motors do not have a separate LOGIC power input. So if power is removed from the drive, the drive no longer knows the encoder position. Further, these drives generally do not bring out the index pulse so you can use it as another input into Acorn for accurate homing. The only "Inexpensive" drives I know of are DMM DYN2 and DMM DYN4 and of course AC Servo drives. However, (and DMM addressed this) you have to ensure that the Z index pulse output from the drive, can sink the Acorn input without more logic. Otherwise the drive (and perhaps the encoder depending on how the manufacturer did the Index pulse output circuitry) can be damaged.

Acorn is open loop. It has no way to accept axis encoder inputs like Oak, All in One DC or MPU11. SO with a hybrid closed loop system like yours, you rely on the drive to close the loop. The drive receives the step/dir signals from Acorn and commands the axis motor to move, all the while it monitors the motor encoder to see if has moved and is in the commanded position. As I previously mentioned, if you power off the drive, by removing power, it no longer knows if the motor is in position.

This is kind of an overview. As others have said, if you remove power from your drives, hit the Estop, you will have to rehome. And since you don't have an index pulse to home to, you need to remeasure/set your X and Z axis again.

I hope this makes some sense. If you need to temporarily stop machining, use Feed Hold (I smack the keyboard space bar which is feed hold) If you hit EMERGENCY stop, best for you to reset the machine.

You have been asked why you are hitting Estop. Please explain.
Marty