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Following Error

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lsgreen:
I’m hoping someone can help me understand my following error. My system uses a Hicon 7766 with the closed loop option. I understand that the error is the difference between the commanded position and the actual position of a motor, (in my case it is steppers). My problem is that I repeatedly get a following error limit stoppage on motor 2 (Z axis) even at very slow speeds overall. The limit is set at 500 for that axis and i guess my question is what is a recommended allowance as well as other approaches to correct the error, i.e. motor settings such as acceleration ect. Any suggestions are welcome.
Thanks, Lou

joeaverage:
Hi,
I assume that you have encoders on the steppers?

If your stepper lags behind the commanded position the stepper driver will have to 'add some additional steps in the pulse stream' to have it catch
up. That would require an intelligent driver. What drivers are you using?

Note the HiCon can 'close the loop' and exert PID dynamics on servos but cannot insert steps into a pulse stream.

Craig

lsgreen:
Craig,
Thanks for the reply, My drivers are 2HSS86H Chinese closed loop drivers, and yes on encoders. I’m doubtful that they are “intelligent”, documentation is not very comprehensive. I understand it is a function of the Hicon to re-sync lost steps. What I don’t know is what is an acceptable range to set for a following error on the Hicon setup page. I don’t want to set the range so wide that the work gets sloppy but last night it stalled where the Z had to retract leave a simple tab on a straight cut. Like I said the machine was not moving very fast, only ~ 60 ipm. Seems really basic.
Thanks, Lou
Sorry about your friend

joeaverage:
Hi,
if your drivers are closed loop that means the loop is closed by the Driver and the Stepper,
the HiCon has nothing to do with it. The HiCon provides pulses to the driver at the behest of
Mach and then the driver/stepper moves to the correct position. If the stepper fails to arrive
at the correct location the driver will add or subtract steps to the stepper to bring it closer.
If it fails to do so because of overload or it gets just too far behind the it faults and produces an
alarm. (Section 3.1 page 7).

I am sorry to have to tell you that closed loop steppers are really very little better than ordinary open loop steppers.
They are still a stepper motor and their torque falls off very rapidly with speed. Most steppers will have 20%
or less of their rated torque at 1000 rpm. Closed loop steppers are exactly the same. If some manufacturer
tries to tell you otherwise they are lying....to get you to part with your money.

Another,if not a lie, a half truth is that the closed loop stepper will catch up lost steps. If your stepper is operating
at speed and it loses a step because its torque is no longer enough to keep up then a closed loop stepper driver
will insert another step in the pulse stream but because the stepper is overloaded it WONT catch up! If this is repeated
the driver will recognise that the stepper is lagging the commanded position by a too great a margin and 'fault'.
The truth is that a well designed, driven and specified open loop stepper will NOT lose steps, and if its overloaded
(speed/acceleration) to the extent it does start to lose steps a closed loop stepper will also lose steps....and for exactly
the same reason.

The manufacturers of closed loop steppers play the marketing card very well, they capitalise on your fear of losing steps.
One claim that they can make and is justified is that their devices can give you much higher resolution (4000 steps per rev
verses 200 step per rev openloop).

I personally am of the opinion that the premium paid for closed loop steppers bring too few advantages to justify the extra cost.
Just a little more and you can have genuine AC servos and they will KICK STEPPER ANUS ALL DAY LONG!!!

If you use proper AC servos then you don't need the 7766 extension board. All AC servos and their matching drive
close the loop, the HiCon controller is not required to do so. Typically the servo manufacturer, having control of the
design of the servo drive, have a closed loop performance better than you or I could achieve by programming
the HiCon.

Craig

joeaverage:
Hi,
I started reading up on the 7766 board, thinking it was the expansion board that made the HiCon
capable of closed loop control.....sorry. The 7766 IS the HiCon. The HiCon is not closed loop capable
without  software activation, an optional extra.

The HiCon unit passes commands to the drier in the form of Step/Direction pulses and the  driver/stepper take it
from there. The HiCon is not involed in closing the loop and has no control or even awareness that the
stepper cant keep up. If the stepper cant keep up, as measured by the difference between its actual encoder
position and its commanded position, the 2HSS86H  faults.

I guess that is the problem you are having....the Z axis driver signals a fault?

I noted in the manual  that there are a number of conditions that cause the fault. I suspect you will
have to experiment with them until you find which feature is causing you grief. It could be Position Error
but it could also be Over Current or Over Voltage.

I will do a little more reading and maybe we can come up with a plan to diagnose whats going on.

Craig

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