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Author Topic: Servo Basics  (Read 6245 times)

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Offline Tef9

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Re: Servo Basics
« Reply #10 on: January 20, 2013, 03:56:05 PM »
Jvaldes - thanks for the reply, however its too late, I already have the motors, yes probably overkill but I aim to transfer them at a later stage to a large gantry router.  So there is some planning involved.  I am considering a smooth stepper addin as this might solve my problem.

Thanks,

Andy 

Offline Hood

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Re: Servo Basics
« Reply #11 on: January 20, 2013, 04:07:39 PM »
jvaldes

High resolution encoders is not all about resolution of the machine. Depending on the servo drives and the speed it can react to the encoder then it may well produce a much smoother and cleaner motion if you have a higher count encoder. I posted some pics a while back in another thread showing the difference of a motor I had with a sine/cosine encoder. I could interpolate the encoders counts and by using a higher number and no other changes the drive could keep the motor on track much easier.

Andy,
 Depends, likely your drives have electronic gearing in them, that means you can have less steps per unit in Mach but the drive will multiply that and make the motor move the correct distance. Its not a method I personally like but it can allow you to get more performance from your hardware when using the parallel port. Well when I say performance really I should just say Velocity as your motion may actually suffer slightly, especially at very low speeds.

Hood

Offline Hood

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Re: Servo Basics
« Reply #12 on: January 20, 2013, 04:20:09 PM »
jvaldes
here is the post I was referring to,
http://www.machsupport.com/forum/index.php/topic,21036.msg152440.html#msg152440
as you can see the higher count feedback from the encoder allows the drive to compensate much more quickly.
That may not be the case for all drives but will be for higher end ones. Not sure where the DMM drives fall, dont think they are basic but again not sure if they would really be classed as high end.
Hood

Offline Tef9

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Re: Servo Basics
« Reply #13 on: January 20, 2013, 05:44:38 PM »
Hi Hood,

Yes the internal gearing is set to 500, this is the lowest value.  What is the purpose of internal gearing, is this to give you options like in this situation?

Thanks,

Andy

Offline Hood

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Re: Servo Basics
« Reply #14 on: January 20, 2013, 06:22:25 PM »
It is one of the uses, ie you cant get enough steps per unit set in Mach to get the Velocity you want, you can then set a higher gearing in the drive. Not an ideal situation but may work well enough.
Hood

Offline Tef9

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Re: Servo Basics
« Reply #15 on: January 20, 2013, 06:37:38 PM »
Hi Hood,

I see, is it as simple as I change the internal gearing from 500 to 1000 and my steps per is reduced from 2000 to 1000?  This could be the solution.

Thanks,

Andy

Offline Hood

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Re: Servo Basics
« Reply #16 on: January 20, 2013, 06:43:18 PM »
If that is the way the gearing works in these drives then yes. I did look at them a long time ago and didnt think it was as simple as that but maybe I am thinking of some other drives.
Hood

Offline Tef9

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Re: Servo Basics
« Reply #17 on: January 20, 2013, 07:03:18 PM »
hmm think its more compelx, will have to study the documentation, however its not  a how to guide sadly.

Thanks for all your help.

Andy

Offline Hood

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Re: Servo Basics
« Reply #18 on: January 20, 2013, 08:42:59 PM »
Yes I remember having a quick look at the manual and seeing it was not the type of manual you can just have a quick look at ;D
Maybe an email to DMM will get you some numbers to try.
Hood
Re: Servo Basics
« Reply #19 on: January 21, 2013, 05:40:39 PM »
jvaldes

High resolution encoders is not all about resolution of the machine. Depending on the servo drives and the speed it can react to the encoder then it may well produce a much smoother and cleaner motion if you have a higher count encoder. I posted some pics a while back in another thread showing the difference of a motor I had with a sine/cosine encoder. I could interpolate the encoders counts and by using a higher number and no other changes the drive could keep the motor on track much easier.

Andy,
 Depends, likely your drives have electronic gearing in them, that means you can have less steps per unit in Mach but the drive will multiply that and make the motor move the correct distance. Its not a method I personally like but it can allow you to get more performance from your hardware when using the parallel port. Well when I say performance really I should just say Velocity as your motion may actually suffer slightly, especially at very low speeds.

Hood

Though I agree that the results is as the graphs show the reasoning is incorrect. The larger count encoders at the same kernal speed had different limits for speed and momentum, which results in different error allowance and less deceleration time, which later results in what you have on these graphs. The actual reasoning that should be considered is that if you were to cycle the increased rates at increase sample rates and where to operate them at the same velocity with the same error allowance (not in pulses but in dimensions) the outcome should be the same. But running an axis at high velocities with the expectation of  curve following is a matter of motor torque and inertia not pulse rates.