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what's the relationship between Controller DAC resolution and mechanical precision


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Dear sir,

I am just wondering the relation ship between PMAC's DAC resolution and the mechanical platform precision.

Our controller have DACs of 12bit, 16bit, 18bit resolution, the higher the more expensive, Does it have any relationship between the resolution and precisioness.

Such as I want to build a platform have Repeat positioning accuracy of 1μm, which controler should I choose, the one with 12bit DAC or 16bit.

Thank you very much, I am new guy, so this question may be seemed a little stupid :)



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The choice of DAC resolution will have very little effect on the ultimate resolution of the controller. The feedback resolution will have the most effect on this. But do note that if this is using the filtered PWM DAC of the Clipper – what I believe you are referring to when you mention 12 bit DAC – there will be some high frequency ripple on this at about 30 KHz. This could be an issue with high resolution systems that have little or no mechanical damping and/or systems with very small inductances. In this case you will want the True DAC option: ACC-8ES.
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In a digital position control system you are discretizing both the force you can apply (DAC counts) and the time interval over which you can apply it (sampling rate). The more DAC resolution you have, the finer increment of force you can apply. Looking at DAC resolution as delta Force and time resolution as delta Time, the "resolution" of the acceleration control capability of your system is limited by:


delta Acceleration = delta Force / delta Time ~ sample time / DAC bits


Of course, resolution is not everything, as noise limits the effective resolution of both of these parameters.


If you work through the physics, it turns out that the finer the resolution on acceleration control the stiffer your system can be and the smaller your following error, within the limitations of noise and mechanical hysteresis.


A practical answer to your question: if you are working on a high-resolution, low-friction system (air bearing/linear motors/nanometer resolution) then make sure your DAC resolution is finer than the analog noise resolution of your amplifiers/motors, i.e. 16 to 18 bits typically. If you are working on an "average" system with lots of friction and coarse resolution, then 12 bits is fine.


I am attaching a paper that discusses some of this; also see Precison Machine Design, by Alex Slocum, 1992


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Thank you very much, thank you, Steve, you are right, I was just considering about to use Clipper in this case. thank you, Tahoe, thank you for you patient and kindness, I will read the paper and that book carefully.

In fact I am a sales, a new guy in this field, I am lucky to met PMAC, an open controller, and guys like you, who are willing to help others. Thank you!

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