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Following Error
« on: June 22, 2018, 01:34:43 PM »
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
Re: Following Error
« Reply #1 on: June 22, 2018, 03:18:16 PM »
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
My wife left with my best friend...
     and I miss him!
Re: Following Error
« Reply #2 on: June 22, 2018, 04:13:50 PM »
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
Re: Following Error
« Reply #3 on: June 22, 2018, 05:34:39 PM »
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
« Last Edit: June 22, 2018, 05:36:13 PM by joeaverage »
My wife left with my best friend...
     and I miss him!
Re: Following Error
« Reply #4 on: June 22, 2018, 05:59:30 PM »
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
My wife left with my best friend...
     and I miss him!
Re: Following Error
« Reply #5 on: June 22, 2018, 06:08:48 PM »
Hi,

Quote
10.2 Have a red alarm after running a small angle
 In the parameter of the driver, phase motor phase
sequence is properly connected. Refer incorrectly identifies
the drive motor phase sequence corresponding
connection.
(section 10.1 page 34)

Is it possible that you have hooked up the phases in reverse. Easy enough to swap (just one winding) and try again.

Craig
My wife left with my best friend...
     and I miss him!
Re: Following Error
« Reply #6 on: June 22, 2018, 08:39:30 PM »
Craig,
Thanks again, yes, I am using the Vital Systems Hicon Integra 7766 with closed loop activation. It is the actual motion controller and not an addon board as you correctly concluded. In conjunction with it is a 7737 encoder board, also Vital Systems. The encoders report directly to the 7766 thru the 7737 making the 7766 aware of the precise motor positions at all times. All calculations with regard to encoder feedback, step generation, command position, and following error are handled internally by the 7766.  I am not having any faults arising within the drives themselves as their setting perimiters are intentionally much wider than 7766 thus avoiding driver faults. I am having some success with adjusting motor settings within Mach4, acceleration in particular so it looks like a path to follow. I totally agree that servos are far superior to steppers in an arrangment, but my best friend knows my wife far to well and is not about to leave with her, so there I am stuck steppers.

Thanks, Lou
Re: Following Error
« Reply #7 on: June 22, 2018, 09:35:44 PM »
Hi,
closed loop steppers REQUIRE that the encoder be connected to the matching drives. Having the 7766 included in the loop is redundant.
The 7766 cannot arbitrarily decide to add or subtract a step, that is done by the stepper driver. The wiring diagram of the 2HSS86H clearly shows the
encoder hooked to the driver.  You are trying to insert the HiCon in the loop and yet the HiCon CANNOT control the stepper....servos yes....steppers no.

Additionally the manufacturer of the stepper also designed the driver. I rather think a manufacturer has the resources to do a much much better job than you or
even Vital Systems.

I use an ESS from Warp9 and it has never made any claim to being able to close a servo loop. I bought a second hand Allen Bradley AC servo of 1.8kW for a spindle motor.
Its the first time I've had a chance to play with an AC servo, very very impressed.  It beats the hell out of the servos we experimented with at University all those years ago.
The ESS provides the commanded position in the form of step/direction pulses just like the HiCon. Those pulses are feed to the Allen Bradley servo drive and the encoder
is hooked to the servo drive. Thus its the servo drive that closes the loop, not the ESS. This is the same arrangement you'll have to use because the HiCon cannot control the stepper
drive directly, that requires the programming and circuitry built into the drive and its not available for you to fiddle with. Both my ESS and your HiCon are being utilized as open loop
command generators and the driver/servo in my case or the driver/stepper in your case are closed loop. If the closed loop fails to accurately follow the commanded input it will signal
the ESS/HiCon with a 'following error' fault.

Craig
My wife left with my best friend...
     and I miss him!
Re: Following Error
« Reply #8 on: June 22, 2018, 09:55:52 PM »
Hi,

Quote
The 7766 cannot arbitrarily decide to add or subtract a step, that is done by the stepper driver. The wiring diagram of the 2HSS86H clearly shows the
encoder hooked to the driver.  You are trying to insert the HiCon in the loop and yet the HiCon CANNOT control the stepper....servos yes....steppers no.
I have the HiCon manual open in front of me and my previous statement may be wrong, maybe the HiCon can introduce extra steps as required.

However even if that is the case the 2HSS86H REQUIRES the encoder be plugged in, if you don't the drive has no way of knowing that the stepper is trying to follow the commanded
input and will produce a 'following error'.

You have steppers and drivers that form a closed loop AND a HiCon which can also close the loop. While you might possibly arrange to have both operating if you don't have the encoder
feedback to 2HSS86H it will fault no matter what the HiCon does.

Craig
My wife left with my best friend...
     and I miss him!
Re: Following Error
« Reply #9 on: June 22, 2018, 10:31:47 PM »
What you are saying is correct, the encoders also report to the drives and the drives have preset perimeters (some user settable and others not) and handle their business accordingly. All I am saying is that with the 7766 sharing the encoder positions the pulse train is already modified before it is ever sent to the drives and if an out of tolerance position is detected (greater than the user set allowance which in my case is 500 pulses) the 7766 issues an out of tolerance following error followed by a stop command to Mach4. This happens before the drives ever exceed their presets. Thus the drives never enter a fault state. Short story made long, the adjustments in the motor tuning solved my problem. Everything is running fine, and as always if the machine makes a mistake I can always trace it back to the operator (that's me) error.

Thanks Craig, your input is valuable.

Lou