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  #1  
Old Fri 25 May 2012, 01:06
Leadshine-Eric
Just call me: Eric
 
Shenzhen
China
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Closed Loop Stepper Compared with Open Loop Stepper

Traditional open loop stepper systems operate in open loop providing position control without feedback. When stepper motors move loads in open loop, a potential loss of synchronization called "Stall" or "Loss of Steps", between commanded steps and actual steps may occur. In the past, this has been solved by adding a feedback device such as an optical encoder and feedback the "current" position for monitoring or verification. But this implemenation can only "alarm" postion loss and can't correct it at the end of the move. When an application requires real-time closed-loop performance throughout the move, engineers are forced to choose much higher cost and complexity of servo systems. But in many applications, because traditional servo systems lack of high torque feature of stepper systems, expensive planetary gearboxes have to be added to to achieve desired torque.

Leadshine closed loop stepper is named as hybrid servo system, by implementting high resolution encoders to update the motor position every 25 micro-seconds, closed loop stepper drives ensure controled real-time position accuracy without lose of steps in open loop stepper system.

Different from the constant current output of open loop stepper systems, output current in closed loop stepper systems is optimized and changes based on moved loads. Therefore, closed-loop stepper systems generate higher torques, lower motor heating, lower vibration, and lower noise than open-loop stepper systems.

When compared with brushless or brush servo systems, cost for closed-loop stepper systems are much lower. In addition, much higher torque, simple tuning and configuration, no overshooting, and no hunting also make closed loop stepper systems better solutions over servo systems in many low to middle applications.

Visit website www.leadshine.com to get more product information.
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  #2  
Old Fri 25 May 2012, 13:51
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
A closed-loop motor that self-adjusts (i.e., similar to the Oriental Motor Alpha series that Shopbot has used for more than six-years) is totally worthless unless all axes are affected when one axis "compensates". Without having all axes inter-connected, you'll end up with "divots" in the workpiece.

I had a PRT-Alpha that used Oriental Motors Alpha stepper motors. There was no inter-connection between the axes, so I ruined a lot of parts before I slowed the feed speed/depth of cut so that "compensation" would never be needed.

I personally think that buying a "closed loop" stepper is a total waste of money. A controller to handle it efficiently is not available for use with Mach 3.
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  #3  
Old Sun 27 May 2012, 20:25
Leadshine-Eric
Just call me: Eric
 
Shenzhen
China
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Quote:
Originally Posted by Richards View Post
A closed-loop motor that self-adjusts (i.e., similar to the Oriental Motor Alpha series that Shopbot has used for more than six-years) is totally worthless unless all axes are affected when one axis "compensates". Without having all axes inter-connected, you'll end up with "divots" in the workpiece.

I had a PRT-Alpha that used Oriental Motors Alpha stepper motors. There was no inter-connection between the axes, so I ruined a lot of parts before I slowed the feed speed/depth of cut so that "compensation" would never be needed.

I personally think that buying a "closed loop" stepper is a total waste of money. A controller to handle it efficiently is not available for use with Mach 3.
In my opinion, a closed loop system is to avoid "Loss of Steps" between commanded steps and actual steps , no matter woked with a controller or Mach3. And the closed loop stepper combines the advantages of both open loop stepper and traditaional servo. It's a cost effective choice.

Talking about the inter-connection between the axes, I think it is about the interpolation motion. So if the machine have a high requirement of cutting, you need a controller to handle it as you said previous.
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  #4  
Old Mon 28 May 2012, 01:51
danilom
Just call me: Danilo #64
 
Novi Sad
Serbia
Without all the axis doing this synchronous its really worthless just like Richards said, when a stepper loses its step its already too late and nothing can save your work as it will show on material.
It says nothing in the manual about rs232 port, can it alarm when its making this correctional move? or make an alarm about Positional Error limit overflow?
If it had these functions, someone might use it.

In our application when it loses steps its only two things, mechanical jam or two deep in the material. None of these situations can't be repaired by making the motor push further.
Just imagine this video situation on one of the gantry motors
http://www.youtube.com/watch?v=2SozZ7af3wg

Eric it would be more helpful if you explained us the use of 3 phase motors and drives Leadshine makes, to whom Tormach switched to and had a nice performance test.
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  #5  
Old Mon 28 May 2012, 02:30
Leadshine-Eric
Just call me: Eric
 
Shenzhen
China
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Danilo, I agree with what you said. And the performance of the mahine is depend on many parts including motion controller, servo or stepper system, actuator and mechanical structure etc. The collection and calculate ability of controller have directly effect with the synchronous of axes. So the encoder resolution.

Leadshine hybrid servo product do have a Positional Error limit overflow setted via the PC-based software.

In some simple CNC Router machines which is used traditional stepper system face some issues like noise, motor heating, loss of step and bad high speed performance. Leadshine's 3 phase motors and drives fix these issues by by implementting high resolution encoders to update the control between the drive and motor. The motor would run more smoothing at the low spped. The output current is optimized and changes based on moved loads, so there is lower heating of motor.
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  #6  
Old Mon 28 May 2012, 03:28
Quadro
Just call me: Anthony #77
 
South Australia
Australia
EvaRobot also makes a closed loop stepper driver. I asked if they could use one of the 'spare' out pins on the driver to send a signal to Mach/LinuxCNC when a preset torque/amps was met it would slow down. The reply was:

'Unfortunately not.

The outpus can be used to signal a collision error (i.e. the motor has come against a hard stop and can't maintain speed), but this may be signalled later than you need it to and usually indicates that the obstacle cannot be overcome (at any speed).'

In the videos it shows torque via the serial port so I can't see why it isn't possible for the controller to output a signal say at 90% of max torque.

Proactive is better than Reactive.
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  #7  
Old Mon 28 May 2012, 03:46
Leadshine-Eric
Just call me: Eric
 
Shenzhen
China
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Quote:
Originally Posted by Quadro View Post
EvaRobot also makes a closed loop stepper driver. I asked if they could use one of the 'spare' out pins on the driver to send a signal to Mach/LinuxCNC when a preset torque/amps was met it would slow down. The reply was:

'Unfortunately not.

The outpus can be used to signal a collision error (i.e. the motor has come against a hard stop and can't maintain speed), but this may be signalled later than you need it to and usually indicates that the obstacle cannot be overcome (at any speed).'

In the videos it shows torque via the serial port so I can't see why it isn't possible for the controller to output a signal say at 90% of max torque.

Proactive is better than Reactive.
Actually, one of our customer has mentioned this idea before. But there is one problem. When the drive stall or meet a obstacle, the feedback current is a dynamic peak current depend on the load, not a continuous current. So it's hard to indicate.
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  #8  
Old Mon 28 May 2012, 08:58
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
Having feedback work depends on having a controller that can handle all axes simultaneously. Mach 3 cannot handle feedback for even one axis, let alone three axes.

It seems to me that buying a stepper motor with feedback will give you a sense of false security, just like buying a PRT-Alpha did for me. The PRT-Alpha could not slow all the axes down when one or more axis had a problem. Mach 3 cannot slow down all the axes when one or more axis has a problem.

Until someone designs an affordable controller that can handle "slowdown events" on all axes simultaneously, I would strongly advise you to save your money and stick with the open-loop configurations that everyone is currently using.

Just because someone has designed a "better mousetrap" doesn't mean that we need that mousetrap or that we could even use that mousetrap. At this time, the closed-loop motors seem to be a solution looking for a problem. That's not how I design machines. I look for a problem and then design a solution.

Designing a simultaneous axes tracking system would be extremely complex for a machine whose total cost is about $5,000, not $250,000. Those of you who are familiar with ramping (acceleration/deceleration) curves for stepper motors will understand what happens if three axes had to be coaxed through a ramp in the middle of a move. It would be a nightmare. It is not something that the people at Mach 3 are going to waste a lot of time worrying about.

Perhaps the best solution would be for Leadshine to tell us where we could buy a license for the necessary controller software for Mach 3's price of $150.
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  #9  
Old Mon 28 May 2012, 10:00
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
I agree with Mike in that a closed loop stepper system is not going to give our community any benefit today.

However, I do appreciate that Eric of Leadshine has bravely come to this forum and told us of their new developments. (closed loop steppers are not really new). Leadshine does appear to be doing their own designs and developments these days and coming up with good products at competitive prices. While Gecko prides itself on supposedly being made in USA (it is soldered there), Leadshine is the clear leader in the rest of the world.
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  #10  
Old Mon 28 May 2012, 19:41
Leadshine-Eric
Just call me: Eric
 
Shenzhen
China
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Thanks for all you guys. It's very important for Leadshine to get more applicaiton and product design information. And that would be what we working for next.
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  #11  
Old Thu 14 June 2012, 06:15
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Eric, can you please take a look at this discussion and give your opinion:
http://www.mechmate.com/forums/showt...&postcount=270
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