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Author Topic: HiCON 7766 Linear Encoder vs. Easy Servo (Leadshine)  (Read 1655 times)

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HiCON 7766 Linear Encoder vs. Easy Servo (Leadshine)
« on: April 15, 2018, 03:43:19 PM »
Hi

I'm weighing the pros and cons of closed loop operation with the 7766. Can you guys check my assumptions here?
I am planning Leadshine 3-phase AC Easy Servos in my machine setup, and am converting a Weiss 32.

Case 1: Employ absolute linear scales, feeding back to the 7766.
  - If I understand correctly, this should be the tightest loop possible, with the most accuracy and repeatability.
  - It should also effectively negate any ballscrew non-linearity, since the ballscrew would be "inside" the motion loop.
  - I suspect as a con that the resolution of the steppers is the limiting factor for accuracy, effectively degrading the scales.
  - I *think I lose the force vectoring function of the Easy Servos, since I presume I can't employ both loops simultaneously.

Case 2: Direct the Easy Servo Motor encoder feedback to the 7766.
  - This appears to be a HiCon supported configuration, and it looks like the breakout boards are compatible with the Leadshine drives.
  - This will require external drive mapping to correct non-linearity.
  - I'm confused about whether this allows the closed-loop force vectoring functions of the Leadshine to be used simultaneously to the 7766 closed loop functions.

Can you guys give me any advice?

Thanks
P
Re: HiCON 7766 Linear Encoder vs. Easy Servo (Leadshine)
« Reply #1 on: April 15, 2018, 05:25:16 PM »
Here's a diagram showing my "Case 1" wiring from the above post.
Re: HiCON 7766 Linear Encoder vs. Easy Servo (Leadshine)
« Reply #2 on: April 15, 2018, 05:48:14 PM »
Here's another diagram showing my "case 2" wiring and loops
Re: HiCON 7766 Linear Encoder vs. Easy Servo (Leadshine)
« Reply #3 on: April 15, 2018, 08:09:27 PM »
Hi,
great looking project.

Option one which includes enclosing the servo/stepper and ballscrew within the feedback loop has a lot to reccommend it.
You will require that the controller be 'closed loop activated', at a cost and you will have to do some low level
programing to make it work.

Option two is pretty straight forward.

I note the you intend on using steppers albeit closed loop steppers. To be honest I wouldn't do that. The steppers are still
steppers, no amount of feedback makes them any more accurate, faster or more powerful. The manufacturers
of them are playing on your fears about losing steps. If open loop steppers are going to lose steps due to overload
or not having the required torque then closed loop steppers will lose steps for EXACTLY the same reason. The closed loop
will try to correct it but fail because the steppers are OVERLOADED closed loop or not. Given the premium you pay
you are not getting any advantage.

Look instead at genuine AC servos, now you are talking real a real solution and not those Clearpath things, they are way
to expensive for what they are.

Craig
My wife left with my best friend...
     and I miss him!
Re: HiCON 7766 Linear Encoder vs. Easy Servo (Leadshine)
« Reply #4 on: April 15, 2018, 08:42:04 PM »
Hi,
with option two you've got two loops..why?

Just use step/dir to the servo drives and let that loop do its thing, theres no need for the extra expense
of the intermediate board.

Next question is what ballscrews are you using, you seem to be hung up on trying to linearise them?.
Even rolled ballscrews are OK to 50um over 300mm, is that not enough?
You could use ground ballscrews, C5 are good to about 18um per 300.
You could use the ballscrew map in Mach...no dollars to spend!

Craig
My wife left with my best friend...
     and I miss him!
Re: HiCON 7766 Linear Encoder vs. Easy Servo (Leadshine)
« Reply #5 on: April 15, 2018, 10:15:55 PM »
With closed-loop option, HiCON controller corrects any position error (eg ballscrew non-linearity error) at the start of the next move.  The correction does not happen on every control-loop cycle.  

you can still take advantage of all the features your drive provides like the one you mentioned closed-loop force vectoring functions etc.  Integra will not interfere with these operations.

HiCON Integra uses hardware FPGA to generate pulse train for fast and smooth motion profile.

regards,
VSI Support
Re: HiCON 7766 Linear Encoder vs. Easy Servo (Leadshine)
« Reply #6 on: April 15, 2018, 10:21:08 PM »
Hi,

Quote
The correction does not happen on every control-loop cycle. 

Sort of buggers up option one.

Craig
My wife left with my best friend...
     and I miss him!
Re: HiCON 7766 Linear Encoder vs. Easy Servo (Leadshine)
« Reply #7 on: April 17, 2018, 06:52:47 PM »
Thanks for the replies.

The reason for two loops is reliable coordinated motion. The outer loop manages the axis relative to desired position, and the inner loop manages the motor relative to commanded position. I realize that most conversions don't employ this.

In the case of the Leadshine drives, they're more like servos than open-loop steppers. The power-to-output is significantly more efficient as well. And cost per power (at accuracy) is significantly less than any servo system. So I'm going to stick with this.

VSI Support:
Can you expand on your statement? I'm not sure I fully understand this:
Quote
With closed-loop option, HiCON controller corrects any position error (eg ballscrew non-linearity error) at the start of the next move.  The correction does not happen on every control-loop cycle.
Can you please define the terms "Move" and "Control Cycle"? I want to be sure I'm interpreting correctly.

Thanks!
Re: HiCON 7766 Linear Encoder vs. Easy Servo (Leadshine)
« Reply #8 on: April 17, 2018, 07:12:53 PM »
Hi,

Quote
they're more like servos than open-loop steppers. The power-to-output is significantly more efficient as well.

Thats rubbish! They are steppers with all the limitations of steppers...feedback does not make faster, more powerful
or anything else. You are paying extra for no reason, spend the extra on securing the best open loop stepers/drives
or towards the extra for genuine AC servos.

Craig
My wife left with my best friend...
     and I miss him!
Re: HiCON 7766 Linear Encoder vs. Easy Servo (Leadshine)
« Reply #9 on: April 19, 2018, 03:48:53 AM »
Hi,
these are from Leadshines website.

There is a two phase open loop stepper 496oz.in.
A two phase closed loop stepper of the same size 496oz,in
A 400W servo.

Notice the two steppers start with the same high torque but lose it as they get faster. At 1500 rpm the closed loop stepper is about 1.2Nm and the open loop a little
less, maybe 1-1.1 Nm. So the open and closed loop steppers have very similar characteristics. Beyond 1000-1500 rpm they have lost that much torque that it is likely they
will stall. The closed loop stepper will try with additional steps in the pulse stream try to keep up but if the load is too great it cant do it.

The servo, a 400w unit from leadshine has about the same torque as the steppers at 1500 rpm, 1.27Nm to be precise, but maintain that torque right up to its rated speed
at 3000 rpm. Note how both the steppers outperform the servo at low speeds but the servo is way better when going fast.

While servos are good for the power output they are quite expensive, if you really have to go fast then you just have to pay. The steppers on the other hand are pretty good value.

Even with microstepping open loop steppers seldom achieve resolution of better than 400 steps per rev whereas the torque vectoring of the closed loop stepper allows much better,
maybe 2000 steps per rev, a definite win.

I found on EBay the open loop stepper for $70 and a AM882 driver for $80 or $150 for the open loop solution.
On Ebay the closed loop stepper and driver were $247, about $100 more than the open loop set.

Is there enough advantages in the closed loop set to be worth the extra?

Craig
My wife left with my best friend...
     and I miss him!