
IEEE/CAA Journal of Automatica Sinica
Citation: | Qiu-Yan He, Yi-Fei Pu, Bo Yu and Xiao Yuan, "Arbitrary-Order Fractance Approximation Circuits With High Order-Stability Characteristic and Wider Approximation Frequency Bandwidth," IEEE/CAA J. Autom. Sinica, vol. 7, no. 5, pp. 1425-1436, Sept. 2020. doi: 10.1109/JAS.2020.1003009 |
STUDIES show that the fractional calculus is now the most appropriate description for most phenomena and their intrinsic essence in nature, such as diffusion processes [1]–[3], electrolytic chemical analyses [4], [5], and biological systems [6], [7]. Owing to the properties of long-term memory, non-locality, and weak singularity, the fractional calculus has been widely used in various scientific fields, such as nonlinear control systems [8]–[20], fractional-order chaos systems [21]–[24], neural networks [25]–[30], and fractional-order circuits and systems [31]–[34].
As the physical realization of fractional calculus, the fractor is a burgeoning new circuit element with characteristics of the fractance which is a portmanteau of the fractional-order impedance. The ideal fractor does not currently exist and its approximate physical realization is called a fractance approximation circuit (FAC) [35].
FACs can be classified into two types. The first type is constructed directly or by modeling some phenomena of electrochemistry or other scientific fields, such as the 2h-type fractal tree FAC [35], the scaling fractal-lattice FAC [36], [37], the fractal-ladder FAC [38], , and fractal-chuan FAC [38]–[40]. The second is to design a rational function approximating to the irrational impedance function of the ideal fractor, then a circuit is synthesized to provide the corresponding rational function, such as the Oustaloup algorithm [41], the Carlson’s iterating rational approximation algorithm [42]–[44], etc. There are also some single type fractors [45], [46].
There exist the operational oscillation phenomena in the frequency-domain characteristics of the above mentioned approximate realizations of the fractor except the Carlson’s iterating rational approximation algorithm. The Carlson’s iterating rational approximation algorithm, proposed by Carlson and Halijak, is that the rational approximation of the fractance of
The paper is organized as follows. The backgrounds about fractors and FACs are introduced in Section II. The principle of the AHOSCWAFB is presented in Section III. Section IV deals with approximation performance comparisons and circuits realization. The summary is made in Section V.
The impedance function of an ideal fractor of
I(μ)(s)=F(μ)sμ,(μ∈Q) |
(1) |
where
According to (1),
The second type FAC is to design a sequence of rational functions
Zk(s)=nk∑i=0βkisidk∑i=0αkisik→∞→I(μ)(s) |
(2) |
where
The aim of the rational approximation algorithm is to seek a rational function which satisfies (2). Let us assume that
Zk(s)=s±m/n⇔x=am/n⇔xn−am=0 |
(3) |
where the operational order
f(x)=xn−am=0. |
(4) |
The root can be obtained with the classical Newton’s iterating function. The convergence order of this Newton’s iterating process is of second order. To speed up the convergence of this Newton’s iterating process, the predistortion exponent function
p(x)=x(n−1)(1+δ)/2,−1≤δ≤1 |
(5) |
is introduced to
fH(x)=f(x)/p(x)=x[n+1−(n−1)δ]/2−amx−(n−1)(1+δ)/2 |
(6) |
where
xk+1=FH(xk)=xk−fH(xk)f′H(xk)=xkr11(n,δ)xnk+r12(n,δ)amr21(n,δ)xnk+r22(n,δ)am |
(7) |
where
r11(n,δ)=(n−1)(1−δ), r12(n,δ)=n+1+(n−1)δ,r21(n,δ)=n+1−(n−1)δ, r22(n,δ)=(n−1)(1+δ). |
The corresponding iterating function is
FH(x)=xr11(n,δ)xn+r12(n,δ)amr21(n,δ)xn+r22(n,δ)am. |
(8) |
There are some special iterating functions when the predistortion exponent
FGN(x)=x(n−1)xn+amnxn |
(9) |
FGU(x)=xnamxn+(n−1)am |
(10) |
FGC(x)=x(n−1)xn+(n+1)am(n+1)xn+(n−1)am. |
(11) |
Theoretically,
xkk→∞→r=am/n. |
(12) |
If
Zk+1(s)=FH(Zk(s))=Zk(s)r11(n,δ)sm[Zk(s)]n+r12(n,δ)r21(n,δ)sm[Zk(s)]n+r22(n,δ),k∈N. |
(13) |
Ideally, the impedance function can approximate to the irrational function of the ideal fractor as
Zk(s)k→∞→I(μ)(s)=s−m/n. |
(14) |
Whether the impedance function sequence
Along with the amplitude-frequency and phase-frequency characteristics; the order-frequency and F-frequency characteristics introduced in [35], [47], all are used to represent frequency-domain characteristics of the impedance function
s=jΩ=j2πf=j2π10ϖ⇔ϖ=lg(Ω/2π),(Ω∈R+,ϖ∈R) |
(15) |
then the frequency-domain characteristics of the ideal fractance
Λ(μ)(ϖ)=lg|I(μ)(j2π10ϖ)|=μ[ϖ+lg(2π)] |
(16) |
Θ(μ)(ϖ)=πμ/2 |
(17) |
M(μ)(ϖ)=dΛ(μ)(ϖ)/dϖ=μ |
(18) |
where
In the same way, the frequency-domain characteristics of the impedance function
Λk(ϖ)=lg|Zk(j2π10ϖ)| |
(19) |
Θk(ϖ)=arg{Zk(j2π10ϖ)} |
(20) |
Mk(ϖ)=dΛk(ϖ)/dϖ |
(21) |
where
Fk(ϖ)=|Zk(j2π10ϖ)||2π10ϖ|Mk(ϖ). |
(22) |
According to the frequency-domain characteristics of the impedance function
1) Validity of the amplitude-frequency characteristic
Λk(ϖ)k→∞→Λ(μ)(ϖ). |
(23) |
2) Validity of the phase-frequency characteristic
Θk(ϖ)k→∞→Θ(μ)(ϖ). |
(24) |
3) Validity of the order-frequency characteristic
Mk(ϖ)k→∞→M(μ)(ϖ). |
(25) |
If the impedance function
For the value of the operational order in the range
Plots of the iterating function
For a given operational order
Let
x=r×10α,α∈R |
(26) |
where
lg[FH(r×10α)r]=lg[xr×r11(n,δ)rn10αn+r12(n,δ)amr21(n,δ)rn10αn+r22(n,δ)am]. |
(27) |
Considering
ηH(α)=α+lgr11(n,δ)10nα+r12(n,δ)r21(n,δ)10nα+r22(n,δ). |
(28) |
The left asymptote function of the normalized exponential function is expressed as follows:
ηL(α)={α+lgn+1+(n−1)δ(n−1)(1+δ),−1<δ≤1,(1−n)α−lg(n),δ=−1. |
(29) |
Likewise, the right asymptote function of the normalized exponential function is represented as
ηR(α)={α+lg(n−1)(1−δ)n+1−(n−1)δ,−1≤δ<1,(1−n)α+lg(n),δ=1. |
(30) |
Plots of the normalized exponential functions with different predistortion exponents
For the AHOSCWAFB, it is already known that this initial impedance function should be a rational function according to the computational rationality. The selection of initial impedances for every operational order is tested by experiments. Some frequently encountered examples are included in Table I.
Z0(s) | μ | ||||||||||
−1/2 | −1/3 | −2/3 | −1/4 | −3/4 | −1/5 | −2/5 | −3/5 | −4/5 | −1/6 | −5/6 | |
R | Y | Y | N | Y | N | Y | N | N | N | Y | N |
1/(Cs) | Y | N | Y | N | Y | N | N | N | Y | N | Y |
R+1/(Cs) | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y |
R/(RCs+1) | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y |
R1+R2/(R2C2s+1) | Y | Y | Y | Y | N | Y | Y | N | N | Y | N |
1/(C1s)+R/(RC2s+1) | Y | Y | Y | N | Y | N | N | Y | Y | N | Y |
(R1R2Cs+1)/((R1+R2)Cs+1) | Y | Y | Y | Y | N | Y | Y | N | N | Y | N |
(RC2s+1)/((R1C2s+1)C2s+1) | Y | Y | Y | Y | Y | Y | N | Y | Y | Y | Y |
Note: for a certain operational order, “Y” means that Z0(s) can serve as the initial impedance, while “N” shows that Z0(s) can not be the initial impedance. |
1) High Order-Stability Characteristic: Taking
In order to show the degree of approximation of the impedance function calculated by the AHOSCWAFB to the irrational function of the ideal fractor, the relative error functions are introduced to make approximation error analysis, as defined below:
rMk(ϖ)=1−Mk(ϖ)/μ |
(31) |
rΘk(ϖ)=1−2Θk(ϖ)/(πμ) |
(32) |
where
In addition, a brief analysis is made of the high order-stability characteristic of the AHOSCWAFB. We know the impedance function can be expressed in the form of zeros and poles, that is
Zk(s)=r+ls+1cs+∏is−zis−pi |
(33) |
where
A system with the impedance function can be divided into a series of subsystems based on (33). Taking
2) Wider Approximation Frequency Bandwidth: Approximation frequency bandwidth B of an impedance function
The valid approximation frequency bandwidth B of an impedance function approximating to that of the ideal fractor is approximately equal to
B≈k×κ(n,δ) |
(34) |
where
κ(n,δ)={nlg(n+1n−1)2−δ21−δ2,−1<δ<1,nlgnn−1,δ=±1,n≠2,2nlgnn−1,δ=±1,n=2. |
(35) |
The approaching rate is determined by both the predistortion exponent and
With the operational order
Approaching rates of impedance functions with different predistortion exponents are included in Table II. Subscripts O and P correspond to the order- and phase-frequency characteristics, respectively. It is really that the approximation frequency bandwidth increases with the increase of the absolute value of the predistortion exponent.
δ | −0.75 | −0.5 | −0.25 | 0 | 0.25 | 0.5 | 0.75 |
κ(n,δ) | 2.6858 | 2.0969 | 1.8697 | 1.8062 | 1.8697 | 2.0969 | 2.6858 |
κO(n,δ) | 2.6857 | 2.0969 | 1.8698 | 1.8062 | 1.8697 | 2.0970 | 2.6857 |
κP(n,δ) | 2.6858 | 2.0969 | 1.8697 | 1.8062 | 1.8698 | 2.0969 | 2.6859 |
Considering two different predistortion exponents
μ | −1/2 | −1/3 | −2/3 | −1/4 | −3/4 | −1/5 | −4/5 |
κ(n,0) | 1.9085 | 1.8062 | 1.8062 | 1.7748 | 1.7748 | 1.7609 | 1.7609 |
κ(n,δ) | 2.0443 | 1.9802 | 1.9802 | 1.9746 | 1.9746 | 1.9793 | 1.9793 |
κO(n,δ) | 2.0444 | 1.9803 | 1.9802 | 1.9746 | 1.9747 | 1.9793 | 1.9794 |
κP(n,δ) | 2.0444 | 1.9801 | 1.9801 | 1.9746 | 1.9746 | 1.9793 | 1.9794 |
μ | −1/2 | −1/3 | −2/3 | −1/4 | −3/4 | −1/5 | −4/5 |
κ(n,0) | 1.9085 | 1.8062 | 1.8062 | 1.7748 | 1.7748 | 1.7609 | 1.7609 |
κ(n,δ) | 2.7318 | 2.9101 | 2.9101 | 3.0946 | 3.0946 | 3.2526 | 3.2526 |
κO(n,δ) | 2.7317 | 2.9101 | 2.9098 | 3.0947 | 3.0945 | 3.2523 | 3.2527 |
κP(n,δ) | 2.7317 | 2.9100 | 2.9101 | 3.0947 | 3.0946 | 3.2528 | 3.2526 |
As can be seen from these tables, approaching rates are the same as the theoretical values according to (35). Given a predistortion exponent, approaching rates are identical for the same value of
Comparisons about approximation performance between the AHOSCWAFB and other types of FACs are made in this section.
The scaling fractal-ladder FACs (SFLFACs) and the scaling fractal-chuan FACs (SFCFACs) can achieve the rational approximation of fractances of arbitrary order and the corresponding optimized circuits have been discussed in, [40]. Another optimization method of SFCFACs has been proposed by Adhikary in [39]. The corresponding circuit is denoted by SFCFACs_A. The classical negative-half-order lattice-type FAC, denoted by LATTICE, also can realize the rational approximation of fractances of arbitrary order after making scaling extension [36], [37]. The Oustaloup algorithm, a matching algorithm of recursive distribution of zeros and poles, also can realize the rational approximation of fractances of arbitrary-order [41], [48], [49].
Considering the roughly same approximation frequency bandwidth, the frequency-domain characteristics for different FACs are depicted in Fig. 10. Results of comparisons of approximation performance are summarized in Table V. In the valid approximation frequency range, the oscillation amplitudes of the frequency-domain characteristics obtained by other FACs are not equal to zero and there exist operational oscillation phenomena in band, whereas the frequency-domain characteristics of the impedance function calculated by the AHOSCWAFB are smooth and consistent with those of the irrational function of the ideal fractor and the oscillation amplitude is equal to zero. In addition, the F-frequency characteristic is a fixed value in the valid approximation frequency range, which makes the AHOSCWAFB more suitable for applications with high-order stability.
Type | Bandwidth (orders of magnitude) | Oscillation amplitude | Complexity of circuit | |||
Order-frequency | Phase-frequency | Order-frequency | Phase-frequency | |||
SFCFACs | 2.85 | 2.16 | 0.0122 | 0.0031 | 7 | |
SFCFACs_A | 2.52 | 1.48 | 0.0256 | 0.0037 | 6 | |
SFLFACs | 2.84 | 2.15 | 0.012 | 0.0030 | 7 | |
LATTICE | 1.98 | 0.51 | 0.0966 | 0.0151 | 6 | |
Oustaloup | 3.35 | 2.00 | 0.0127 | 0.0029 | 9 | |
AHOSCWAFB | 2.41 | 2.38 | 0 | 0 | 22 |
The highest power of the impedance function means the complexity of a circuit. The highest power of the impedance function
For other FACs, oscillation phenomena of frequency-domain characteristics are inherent properties. Although the oscillation amplitude can be reduced by adjusting circuit parameters, the oscillation phenomena do not disappear and the circuit stage is increased. Despite the complexity of circuit is comparatively high, the excellent approximation performance shows that the AHOSCWAFB is also a good direction of the research.
In this section, the aim is to design the FAC to provide the impedance function obtained through the AHOSCWAFB. This kind of circuit is called the FAC with high order-stability characteristic and wider approximation frequency bandwidth (FACHOSCWAFB).
Given a certain operational order
Zk(s)=dk∑i=0ris+pi |
(36) |
where
Take the operational order
Z2(s)=4s5+42s4+92s3+80s2+24s+1s6+24s5+80s4+92s3+42s2+4s. |
(37) |
Partial fraction expansion applied to (37) leads to the following relationship:
Z2(s)=2.5918s+20.2726+0.75s+2+0.0788s+0.2112s2+1.6008s+0.7790+0.3294s+0.1267+0.25s. |
(38) |
For the first term on the right-hand side of (38), it can be realized with the 1/2.5918-F capacitor in parallel with the 2.5918/20.2726-
If we use the calculation results in (38) as values of the used circuit elements directly, the approximation frequency is too small to measure. It is possible to measure circuit characteristics with resistance of each resistor magnified
The negative resistor in the FACHOSCWAFB is realized by the negative impedance converter shown in Fig. 12.
These elements in Fig. 11 can be realized with resistors, capacitors, and active elements. All these circuit elements can be purchased at the market. The type of the operational amplifier used above is OP37. The practical testing system is represented in Fig. 13. The types of the signal generator, the oscilloscope, the current probe, and the current amplifier used in this experiment are NDY EE16330, Tektronix TDS 1012C-EDU, TCP312A, and TCPA300, respectively.
Amplitude and phase spectra of the above FACHOSCWAFB through simulation by the Multisim are pictured in Fig. 14 and in accordance with those of the modified impedance function simulated by the MATLAB. As can be seen from this figure, the modified impedance function can approximate to the fractance of
The input voltage
I(s)=U(s)Zk(s)≈U(s)F(μ)s(−μ). |
(39) |
The input current can be viewed as the
The AHOSCWAFB is researched in detail in this paper. The predistortion exponent is not limited to be zero and has a value between –1 and 1. The AHOSCWAFB not only realizes the rational approximation of fractances with values of the operational order in the range
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Z0(s) | μ | ||||||||||
−1/2 | −1/3 | −2/3 | −1/4 | −3/4 | −1/5 | −2/5 | −3/5 | −4/5 | −1/6 | −5/6 | |
R | Y | Y | N | Y | N | Y | N | N | N | Y | N |
1/(Cs) | Y | N | Y | N | Y | N | N | N | Y | N | Y |
R+1/(Cs) | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y |
R/(RCs+1) | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y |
R1+R2/(R2C2s+1) | Y | Y | Y | Y | N | Y | Y | N | N | Y | N |
1/(C1s)+R/(RC2s+1) | Y | Y | Y | N | Y | N | N | Y | Y | N | Y |
(R1R2Cs+1)/((R1+R2)Cs+1) | Y | Y | Y | Y | N | Y | Y | N | N | Y | N |
(RC2s+1)/((R1C2s+1)C2s+1) | Y | Y | Y | Y | Y | Y | N | Y | Y | Y | Y |
Note: for a certain operational order, “Y” means that Z0(s) can serve as the initial impedance, while “N” shows that Z0(s) can not be the initial impedance. |
δ | −0.75 | −0.5 | −0.25 | 0 | 0.25 | 0.5 | 0.75 |
κ(n,δ) | 2.6858 | 2.0969 | 1.8697 | 1.8062 | 1.8697 | 2.0969 | 2.6858 |
κO(n,δ) | 2.6857 | 2.0969 | 1.8698 | 1.8062 | 1.8697 | 2.0970 | 2.6857 |
κP(n,δ) | 2.6858 | 2.0969 | 1.8697 | 1.8062 | 1.8698 | 2.0969 | 2.6859 |
μ | −1/2 | −1/3 | −2/3 | −1/4 | −3/4 | −1/5 | −4/5 |
κ(n,0) | 1.9085 | 1.8062 | 1.8062 | 1.7748 | 1.7748 | 1.7609 | 1.7609 |
κ(n,δ) | 2.0443 | 1.9802 | 1.9802 | 1.9746 | 1.9746 | 1.9793 | 1.9793 |
κO(n,δ) | 2.0444 | 1.9803 | 1.9802 | 1.9746 | 1.9747 | 1.9793 | 1.9794 |
κP(n,δ) | 2.0444 | 1.9801 | 1.9801 | 1.9746 | 1.9746 | 1.9793 | 1.9794 |
μ | −1/2 | −1/3 | −2/3 | −1/4 | −3/4 | −1/5 | −4/5 |
κ(n,0) | 1.9085 | 1.8062 | 1.8062 | 1.7748 | 1.7748 | 1.7609 | 1.7609 |
κ(n,δ) | 2.7318 | 2.9101 | 2.9101 | 3.0946 | 3.0946 | 3.2526 | 3.2526 |
κO(n,δ) | 2.7317 | 2.9101 | 2.9098 | 3.0947 | 3.0945 | 3.2523 | 3.2527 |
κP(n,δ) | 2.7317 | 2.9100 | 2.9101 | 3.0947 | 3.0946 | 3.2528 | 3.2526 |
Type | Bandwidth (orders of magnitude) | Oscillation amplitude | Complexity of circuit | |||
Order-frequency | Phase-frequency | Order-frequency | Phase-frequency | |||
SFCFACs | 2.85 | 2.16 | 0.0122 | 0.0031 | 7 | |
SFCFACs_A | 2.52 | 1.48 | 0.0256 | 0.0037 | 6 | |
SFLFACs | 2.84 | 2.15 | 0.012 | 0.0030 | 7 | |
LATTICE | 1.98 | 0.51 | 0.0966 | 0.0151 | 6 | |
Oustaloup | 3.35 | 2.00 | 0.0127 | 0.0029 | 9 | |
AHOSCWAFB | 2.41 | 2.38 | 0 | 0 | 22 |
Z0(s) | μ | ||||||||||
−1/2 | −1/3 | −2/3 | −1/4 | −3/4 | −1/5 | −2/5 | −3/5 | −4/5 | −1/6 | −5/6 | |
R | Y | Y | N | Y | N | Y | N | N | N | Y | N |
1/(Cs) | Y | N | Y | N | Y | N | N | N | Y | N | Y |
R+1/(Cs) | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y |
R/(RCs+1) | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y |
R1+R2/(R2C2s+1) | Y | Y | Y | Y | N | Y | Y | N | N | Y | N |
1/(C1s)+R/(RC2s+1) | Y | Y | Y | N | Y | N | N | Y | Y | N | Y |
(R1R2Cs+1)/((R1+R2)Cs+1) | Y | Y | Y | Y | N | Y | Y | N | N | Y | N |
(RC2s+1)/((R1C2s+1)C2s+1) | Y | Y | Y | Y | Y | Y | N | Y | Y | Y | Y |
Note: for a certain operational order, “Y” means that Z0(s) can serve as the initial impedance, while “N” shows that Z0(s) can not be the initial impedance. |
δ | −0.75 | −0.5 | −0.25 | 0 | 0.25 | 0.5 | 0.75 |
κ(n,δ) | 2.6858 | 2.0969 | 1.8697 | 1.8062 | 1.8697 | 2.0969 | 2.6858 |
κO(n,δ) | 2.6857 | 2.0969 | 1.8698 | 1.8062 | 1.8697 | 2.0970 | 2.6857 |
κP(n,δ) | 2.6858 | 2.0969 | 1.8697 | 1.8062 | 1.8698 | 2.0969 | 2.6859 |
μ | −1/2 | −1/3 | −2/3 | −1/4 | −3/4 | −1/5 | −4/5 |
κ(n,0) | 1.9085 | 1.8062 | 1.8062 | 1.7748 | 1.7748 | 1.7609 | 1.7609 |
κ(n,δ) | 2.0443 | 1.9802 | 1.9802 | 1.9746 | 1.9746 | 1.9793 | 1.9793 |
κO(n,δ) | 2.0444 | 1.9803 | 1.9802 | 1.9746 | 1.9747 | 1.9793 | 1.9794 |
κP(n,δ) | 2.0444 | 1.9801 | 1.9801 | 1.9746 | 1.9746 | 1.9793 | 1.9794 |
μ | −1/2 | −1/3 | −2/3 | −1/4 | −3/4 | −1/5 | −4/5 |
κ(n,0) | 1.9085 | 1.8062 | 1.8062 | 1.7748 | 1.7748 | 1.7609 | 1.7609 |
κ(n,δ) | 2.7318 | 2.9101 | 2.9101 | 3.0946 | 3.0946 | 3.2526 | 3.2526 |
κO(n,δ) | 2.7317 | 2.9101 | 2.9098 | 3.0947 | 3.0945 | 3.2523 | 3.2527 |
κP(n,δ) | 2.7317 | 2.9100 | 2.9101 | 3.0947 | 3.0946 | 3.2528 | 3.2526 |
Type | Bandwidth (orders of magnitude) | Oscillation amplitude | Complexity of circuit | |||
Order-frequency | Phase-frequency | Order-frequency | Phase-frequency | |||
SFCFACs | 2.85 | 2.16 | 0.0122 | 0.0031 | 7 | |
SFCFACs_A | 2.52 | 1.48 | 0.0256 | 0.0037 | 6 | |
SFLFACs | 2.84 | 2.15 | 0.012 | 0.0030 | 7 | |
LATTICE | 1.98 | 0.51 | 0.0966 | 0.0151 | 6 | |
Oustaloup | 3.35 | 2.00 | 0.0127 | 0.0029 | 9 | |
AHOSCWAFB | 2.41 | 2.38 | 0 | 0 | 22 |