POCPA Conference DESY Michele Martino (TE-EPC-HPM) CURRENT MEASUREMENT DIGITAL FILTERING - TUTORIAL - 1 POPCA 2012 FIR or IIR?

CURRENT MEASUREMENT FOR CONTROL 2 voltage/current signal transmission Converter Control Current Transducer Power Circuit ADC Anti Aliasing / Signal Conditioning  Control systems can tolerate some delay in the measurement chain but certainly don’t like it ! Fundamental trade-off of the measurement chain: accuracy vs speed !  Properly designed digital controllers can “easily” handle delays (RST), so what’s the problem?  Delays increase model order (Z transf) → Model mismatch become rapidly critical for stability!

SAMPLING BASICS 3  Ideal Sampling Time DomainFrequency Domain Spectrum of a critically sampled strictly band-limited signal: no alias!!!  Noise is always present so there are no strictly band-limited signals!  Anti-aliasing filtering always needed!

WHY OVERSAMPLING? 4  Standard Nyquist Sampling  Oversampling Analog filter Anti-aliasing can be dealt with only analogically! Digital filter Analog filter Anti-aliasing filtering can be shared between analog and digital! Re-configurability now possible

DECIMATION 5 Replicas still occur due to decimation! SNR not improved due to lack of filtering!

THE NEVER-ENDING DISPUTE: IIR VS FIR 6 Charles M. Rader : The Rise and the Fall of Recursive Digital Filters – IEEE Signal Processing Magazine, Nov 2006 (non-causal)

7  Yes, 1-bit only! It is the digital filter that actually determines the ADC “precision”!

HOW TO SPECIFY DIGITAL FILTERS 8 “alias-free” bandwidth

MINIMUM-PHASE FIR  Actually not if the filter is part of the measurement chain of a control loop!!!

MINIMUM-PHASE FIR DECIMATION

IIR IMPLEMENTATION 11  Implementation on DSP hints  Hints based on Texas Instruments TMS320C28x with CCS  The hints clearly work with floating point DSP such as the TMS320F28335 _IQmpy ↔ *

IIR IMPLEMENTATION 12  Implementation hints on DSP : Break up Structure and Combine Terms

IIR IMPLEMENTATION 13  Implementation hints on DSP : Inline  Inline is automatic with –O3 Optimization mode  Source must be visible to calling file  Make data allocation/definitions visible to calling file  Compiler can make use of Direct Addressing Mode

“INTERACTIVE” SESSION 14

MATLAB FDATOOL 15  Different algorithms are available for minimum-phase design  Generalized Equiripple calculates minimum (even) order filters!  Other algorithms can be successfully used once the filter order is approximately known  What about very long filters?  For filter orders larger than a thousand taps the algorithms fail  But the problem can be decomposed and the FDATOOL let you the “cascade”  You can also quantize coefficients and then create a.coe file

SOME SUBTLE PHENOMENA 16  What can go wrong?

SOME SUBTLE PHENOMENA 17  It looks very nice isn’t it?  It looks very “white” !!!!

SOME SUBTLE PHENOMENA 18  Let’s have a look at the histogram:

SOME SUBTLE PHENOMENA 19  Let’s have a look at the impulse response:

CASE STUDY

 DC performance

CASE STUDY  AC performance

CASE STUDY  AC performance  Making hardware or perform measurements as flat as the digital filter may turn out to be unfeasible or unworthy!

CASE STUDY  Sine-fit Amplitude Estimation

REFERENCES 30   IEEE Std Standard for Terminology and Test Methods for Analog-to-Digital Converters  A. Tessarolo, Getting the Most from Your C Code on the TMS320C28x™ Controller Using Code Composer Studio™  Delta-Sigma Data Converters Theory, Design, and Simulation. Norworthy, Schreier, Temes, IEEE – Wiley-Interscience 1992  K. Steiglitz, T. W. Parks, and J. K. Kaiser, ”METEOR: a constrained-based FIR Filter design program,” IEEE Trans. Signal Proc., vol 40, no. 8, pp , Aug  Digital Filters with MATLAB ® : Ricardo A. Losada, 2009, The MathWorks, Inc.  New class of recursive digital filters for decimation: Horacio G. Martinez and Thomas W. Parks, 1978, Rice University Electrical Engineering Dept. Huston  M. Martino, et al. “Low emission, self-tunable DSP based Stepping Motor Drive for use with arbitrarily long cables,” IFAC Large Scale Systems Symposium, Villeneuve d’Ascq, France, 2010

BACKUP SLIDES 31

MINIMUM PHASE FIR FILTERS 32  Delay is minimized, but there is a lot more overshoot! Is that a problem? 100A 10A

HOW TO SPECIFY DIGITAL FILTERS? 1 33

HOW TO SPECIFY DIGITAL FILTERS? 2 34  From Precision to Filter Specs 1.74 x x x 10 -3

HOW TO SPECIFY DIGITAL FILTERS? 3 35  From Precision to Filter Specs  Now we are done! Or maybe not?

WHY OVERSAMPLING? 1 36  A “bit” of theory  Quantization Process  Quantization is a non linear process - vast and tricky subject  Fortunately an approximated model works very well almost every time!  Ok but what does that mean?

WHY OVERSAMPLING? 2 37 a) b)  Easy analog filter design, reduced delay, re-configurability, Ok!  Is that all? What about PQN power? Sampling FrequencyBandwidth