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Deadband gain: what does it multiply?


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Hi all

I have a question regarding the deadband gain Ixx64: what does it really multiply? In the Turbo SRM (p. 107) it is stated that the Ixx64 gain multiplies the proportional gain Ixx30; however, in the Turbo PMAC user manual it is stated that the gain is applied to the following error used in the servo loop.

It seems to me that it is more logical to have the gain multiplying the following error instead of servo output (keeping the integral part of the output near-constant over time, regardless of the deadband gain).

Am I correct? Is the Turbo SRM wrong in that?

 

thanks

gigi

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These two statements are equivalent, as the proportional gain is applied to the following error as well, effectively. Think of deadband gain as an adjustable proportional gain that is only adjustable over a range of following errors that is finite. It is implemented by multiplying the deadband gain by the proportional gain over a finite range of following errors, thus it creates a non-linear loop gain.
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The "TURBO PMAC USER MANUAL" has a good graphic of this on page 159 (175 electronic):

http://www.deltatau.com/manuals/pdfs/TURBO%20PMAC%20USER%20MANUAL.pdf?id=635288562230097687

 

Hi Steve

that's exactly what I was talking about. If you confirm that this is the way it is implemented, it's fine with me.

 

In reply to Brian: I don't think the PID result is the same if the deadband gain is applied to the following error or the Ix30.

E.G: let's forget for the moment that the gain is applied without discontinuity (as explained in Turbo PMAC USER MANUAL, p. 160).

I'm assuming the deadband gain is -12, for a total gain of 0.25;

if the deadband gain multiplies Ixx30, I will encounter a discountinuity and a large loss of output current when I enter the deadband, because suddenly the Ixx30 (and then the output) is multiplied by 0.25.

If the deadband gain multiplies the following error, when I enter the deadband, the 0.25 gain is applied only to the proportional and derivative part of the PID, leaving the integral part constant.

 

Considering the fact that I need the deadband to decrease the high frequency controller "noise" when I am keeping a position in close loop (with an high integral output because I need to keep the load in position etc), a deadband gain that multiplies the Ixx30 is unusable for my application.

 

I am running some tests right now, it seems that the gain multiplies only the FollowingError as explained in the TurboPMAC User Manual, so that's good for my application.

 

ciao

gg

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Yes, you are precisely correct. I was communicating qualitatively about what the deadband does to the loop gain, but you are correct, in implementation it does not affect the output of the integrator. For that matter, it doesn't affect the contribution of the velocity feedback either, so at high frequencies deadband doesn't do anything to attenuate oscillations just outside the deadband spacing.
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Yes, internally what happens is that the raw following error value FE is adjusted using the "deadband" parameters, producing a modified following error value FE' that the servo loop operates on. This has the result of changing the effective proportional gain as a function of raw following error.
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  • 2 weeks later...

Yes, internally what happens is that the raw following error value FE is adjusted using the "deadband" parameters, producing a modified following error value FE' that the servo loop operates on. This has the result of changing the effective proportional gain as a function of raw following error.

 

ok, thanks. On the same topic: when the deadband compensation is activated? It is always ON or there is another condition to start the compensation (i.e. commanded vel = 0) ?

The manual does not cite any particular condition to trigger the deadband compensation.

 

thanks

 

ciao

gg

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