Implement of M/T measurement of speed in C2000 F2803x

Eli_jia

Hi,

I read the eQEP's reference guide, and found it can be used to measure high speed with speed_FR macro, low speed with speed_PR macro. But I want to measure both high speed and low speed in the same application, I think I need the method known as M/T. I am not sure how to achieve that in the F2803x controller. Who can give me some hints? Thanks very much!

Eli

over 11 years ago

0 ChrisClearman over 11 years ago

TI__Guru** 119500 points

Yes, we've done this. Let me see if I can get someone to give some advice.

0 Eli_jia over 11 years ago in reply to ChrisClearman

Intellectual 530 points

Wow, thanks!

0 Steve Lim over 11 years ago in reply to Eli_jia

TI__Intellectual 2875 points

Jia,

You can implement MT method if you use the following equation as below.

V(k) = (X(k) - X(k-1)) / ( T - deta_T(k) + delta_T(k-1))

V(k) : current speed out

X(k) : current position count(QPOSLAT(k))

X(k-1) : old position count(QPOSLAT(k-1))

T : Unit time (QUPRD)

delta_T(k) : the difference between Uint time and the latest pulse (you can use just QCTMRLAT(k) value for this delta_T)

delta_T(k-1) the old delta_T(k) (is equal to QCTMRLAT(k-1))

0 Eli_jia over 11 years ago in reply to Steve Lim

Intellectual 530 points

Hi Steve,

Thanks for your detailed reply. But I still have a confusion. The time of T is constant, right? If the speed is very low, there are a few pulse in the period of T, right? Actually, I can't find the difference between this method and the method of deta(X)/sample time.

0 Steve Lim over 11 years ago in reply to Eli_jia

TI__Intellectual 2875 points

Hi Jia,

Yes, the time T is constant which is the speed sampling time. In a low speed, the resolution of M method(delt(x)/T) get worse. But, MT method can be used very wide speed range except ultra low speed. In other word, MT method works like T method at low speed and works like M method at high speed range.

0 Eli_jia over 11 years ago in reply to Steve Lim

Intellectual 530 points

Hi Steve,

I got it. Thanks very much. I think I found the key point that the T is variant by the deta t(k) and deta t(k-1), in another words, it is changed due to the speed. right?

Happy holiday in advance!

Eli

0 Steve Lim over 11 years ago in reply to Eli_jia

TI__Intellectual 2875 points

Yes, you are right. The total T(= Tc - deltT(k) + deltaT(k-1)) is variant in MT method according to speed.

0 Eli_jia over 11 years ago in reply to Steve Lim

Intellectual 530 points

Steve,

Sorry, I meet another problem. If the T is variant, then I need to change the coefficient of the low-pass filter every calculation of speed, right?

Hope I don't disturb your holiday! Thanks!

Eli

0 Steve Lim over 11 years ago in reply to Eli_jia

TI__Intellectual 2875 points

No, you don't need to change the coefficient of low-pass filter because the coefficient is just depend on the speed sampling period. I think you use fixed control frequency for speed control. If so, th speed sampling or control period(Tc) is not changed by speed. The average T value which is used in MT method calculation is just changed according to the speed.

-Steve

0 Eli_jia over 11 years ago in reply to Steve Lim

Intellectual 530 points

I see now. Thanks very much！

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Implement of M/T measurement of speed in C2000 F2803x