劉二寧 鄒任玲 姜亞斌 胡秀枋 盧旭華 王海濱 范虓杰 張安林
摘 要:為提高基于表面肌電信號(hào)的人體腰背動(dòng)作識(shí)別率,提出一種基于小波包能量與改進(jìn)NARX神經(jīng)網(wǎng)絡(luò)的分類識(shí)別新方法。利用小波包變換對(duì)動(dòng)作部位進(jìn)行表面肌電信號(hào)特征提取,并采用改進(jìn)NARX神經(jīng)網(wǎng)絡(luò)進(jìn)行分類識(shí)別。選取8名實(shí)驗(yàn)者分別在扭腰、彎腰、側(cè)彎腰3種動(dòng)作下進(jìn)行表面肌電信號(hào)數(shù)據(jù)采集,選擇db4小波包函數(shù)對(duì)信號(hào)進(jìn)行6層分解,得到第6層64個(gè)頻帶的小波包分解系數(shù),代表各個(gè)動(dòng)作信息的特征向量,作為改進(jìn)NARX神經(jīng)網(wǎng)絡(luò)的輸入進(jìn)行分類識(shí)別。對(duì)照實(shí)驗(yàn)組中,改進(jìn)NARX神經(jīng)網(wǎng)絡(luò)的識(shí)別率較高,總體識(shí)別率達(dá)到96.7%。實(shí)驗(yàn)結(jié)果表明,利用該識(shí)別方法對(duì)腰部動(dòng)作進(jìn)行分類識(shí)別,分類準(zhǔn)確,且識(shí)別率更高。
關(guān)鍵詞:表面肌電信號(hào);動(dòng)作識(shí)別;小波包變換;改進(jìn)NARX神經(jīng)網(wǎng)絡(luò)
DOI:10. 11907/rjdk. 201327????????????????????????????????????????????????????????????????? 開放科學(xué)(資源服務(wù))標(biāo)識(shí)碼(OSID):
中圖分類號(hào):TP301 ? 文獻(xiàn)標(biāo)識(shí)碼:A ??????????????? 文章編號(hào):1672-7800(2020)011-0071-04
A New Method for Recognition of Lumbar and Back Movements
Based on Surface EMG Signals
LIU Er-ning1, ZOU Ren-ling1,JIANG Ya-bin1,HU Xiu-fang1,LU Xu-hua2,WANG Hai-bin2,F(xiàn)AN Xiao-jie1,ZHANG An-lin1
(1. School of Medical Instrument and Food,University of Shanghai for Science and Technology,Shanghai 200093,China;
2. Shanghai Changzheng Hospital, Shanghai 200003, China)
Abstract: In order to improve the recognition rate of human low back motion pattern based on surface EMG, this paper proposes a new classification and recognition method based on wavelet packet energy and improved NARX neural network.The wavelet transform is used to extract the surface EMG signal features of the action part, and the improved NARX neural network is used for classification and identification. Eight experimental subjects were selected to perform surface EMG signal data acquisition under the lumbar motion of twisting, bending and side bending. The db4 wavelet packet function was used to decompose the signal in 6 layers to obtain the decomposition coefficient of the 64 wavelet band in the 6th layer. The coefficients represent the feature vector of each action information are used as an input to the improved NARX neural network for classification and recognition. In the control group, the improved NARX neural network has a higher recognition rate, and the overall recognition rate reaches 96.7%. The experimental results show that the waist movement is classified and recognized by this recognition method with accurate classification and higher recognition rate.
Key Words: surface EMG signal;motion recognition;wavelet packet transform;improved NARX neural network
0 引言
表面肌電信號(hào)是指通過表面肌電拾取方式采集肌肉活動(dòng)產(chǎn)生的電信號(hào),可在一定程度上反映神經(jīng)肌肉的活動(dòng)。其借助于無創(chuàng)傷、使用方便等優(yōu)勢(shì),在康復(fù)訓(xùn)練中常被用來對(duì)患者動(dòng)作進(jìn)行分類與識(shí)別,從而實(shí)現(xiàn)智能控制,以進(jìn)一步提高康復(fù)效果[1-3]。
利用表面肌電信號(hào)對(duì)人體動(dòng)作進(jìn)行分類識(shí)別,關(guān)鍵在于如何提高表面肌電信號(hào)的分類識(shí)別率。目前,國(guó)內(nèi)外學(xué)者利用人工神經(jīng)網(wǎng)絡(luò)、模糊熵判別法、神經(jīng)模糊熵判別法等算法,一定程度上提高了識(shí)別率[4-8]。其中具有代表性的有楊偉健等[9]提出的基于空域相關(guān)濾波的小波熵和近似熵特征提取與分類方法,實(shí)現(xiàn)了肩頸部肌電信號(hào)識(shí)別,可對(duì)輪椅進(jìn)行智能控制;魏偉等[10]提出一種小波變換與粒子群優(yōu)化支持向量機(jī)(Particle Swarm Optimization-Support Vector Machine,PSO-SVM)相結(jié)合的模式分類方法,能夠成功識(shí)別表面肌內(nèi)翻、外翻,以及握拳、展拳動(dòng)作;王紅旗等[11-12]提出一種將播報(bào)主元分析與線性判別分析相結(jié)合的表面肌電信號(hào)動(dòng)作特征識(shí)別新方法,其將小波包系數(shù)矩陣由16維降到4維,對(duì)前臂4種動(dòng)作模式(握拳、展拳、手腕內(nèi)翻和手腕外翻)的正確識(shí)別率平均可達(dá)98%,與傳統(tǒng)小波包變換相比識(shí)別率更高;Yogendra等[13]利用原始表面肌電圖(sEMG)信號(hào)特征的一階微分得到4個(gè)新的時(shí)域特征,與其它分類器相比具有更好的分類精度;Mane等[14]提出一種新的基于單通道的表面肌電信號(hào)識(shí)別算法識(shí)別表面肌電信號(hào),利用小波變換與人工神經(jīng)網(wǎng)絡(luò)相結(jié)合的方法能更好地對(duì)表面肌電信號(hào)進(jìn)行識(shí)別與分類。
傳統(tǒng)方法利用小波變換提取表面肌電信號(hào)特征向量,無法很好地分解與表示包含大量細(xì)節(jié)信息的表面肌電信號(hào),而小波包變換是一種信號(hào)的時(shí)間—尺度分析方法,具有多分辨率分析的特點(diǎn),在時(shí)、頻域具有較強(qiáng)的表征信號(hào)局部特征的能力。因此,為提高分類與識(shí)別的準(zhǔn)確性,本文提出采用小波包變換法對(duì)動(dòng)作部位的表面肌電信號(hào)進(jìn)行特征提取,之后輸送到改進(jìn)NARX神經(jīng)網(wǎng)絡(luò)進(jìn)行分類與識(shí)別,并將該方法應(yīng)用于腰部腰方肌、豎脊肌的動(dòng)作(扭腰、彎腰、側(cè)彎腰)識(shí)別。實(shí)驗(yàn)結(jié)果表明,該方法可以顯著提高對(duì)腰背部動(dòng)作的分類識(shí)別準(zhǔn)確性,在康復(fù)訓(xùn)練中可起到指導(dǎo)訓(xùn)練、提高康復(fù)效果的作用。
1 小波包能量特征向量提取
小波變換原理:設(shè)ft是平方可積函數(shù),φa,bt被稱為基本小波或母小波,定義如下:
φa,bt=1aφt-ba ?? (1)
則信號(hào)ft連續(xù)小波變換定義為:
Wφfa,b≤ft,φa,bt≥1a-∞+∞ftφ*t-badt???? (2)
式中,a>0,b∈R,a是尺度因子,b是伸縮因子,“<>”表示內(nèi)積,“*”表示共軛。信號(hào)ft的離散小波變換定義為:
Wf2j,2jk=2-j2-∞+∞ftφ2-jt-kdt ??? (3)
其逆變換為:
ft=f=-∞+∞k=-∞+∞wf2j,2jk?2j,2jkx ??? (4)
由式(2)可知,小波變換是先選定一個(gè)基小波函數(shù)φt,然后將信號(hào)ft在φt下展開。對(duì)于一個(gè)已知信號(hào)st進(jìn)行小波變換,如式(5)所示。
st=AIt+I=1UDt=waIaIt+I=1UwdIt (5)
式中,AIt為第I級(jí)的低頻分量,DIt為第I級(jí)的高頻分量,waI為第I級(jí)的低頻小波系數(shù),wdI為第I級(jí)的高頻小波系數(shù),aIt與dIt是小波基。
由Parse-val定理得知,頻域計(jì)算信號(hào)與時(shí)域計(jì)算信號(hào)的能量相等,由此可知,分解后各個(gè)頻帶的能量為EI,以EI為元素可構(gòu)造特征向量:R=E1,E2,E3,?,EI,其中I=2i。再令E=sqrt(i=0I|EI|2),歸一化處理后的能量為R'=E1/E,E2/E,E3/E,?,EI/E,最后以R'作為神經(jīng)網(wǎng)絡(luò)的輸入進(jìn)行動(dòng)作識(shí)別。
設(shè)φ(x)、ψ(x)分別是尺度函數(shù)和小波函數(shù),令:
ψ0(x)=φ(x)ψ1(x)=ψ(x) (6)
φ2l(x)=k=-∞+∞hkφl(2x-k)ψ2l+1(x)=k=-∞+∞gkφl(2x-k) ???????? (7)
則定義的函數(shù)ψn稱為關(guān)于尺度函數(shù)φ(x)的小波包。
利用小波包算法提取運(yùn)動(dòng)的表面肌電信號(hào)特征能量時(shí),只要選擇合適的小波包函數(shù)和分解層數(shù)[15-16],即可將進(jìn)行不同運(yùn)動(dòng)時(shí)肌電信號(hào)中的各個(gè)頻段信息分解出來,最后得到一組代表各個(gè)動(dòng)作信息的特征向量[17-19]。
2 NARX神經(jīng)網(wǎng)絡(luò)改進(jìn)算法
NARX神經(jīng)網(wǎng)絡(luò)算法在通信隨機(jī)信號(hào)獲取特征值方面已取得較大進(jìn)展,同樣也適用于處理肌電信號(hào)的隨機(jī)性,可用來識(shí)別不同的肌肉運(yùn)動(dòng)[20-21]。改進(jìn)NARX神經(jīng)網(wǎng)絡(luò)在輸入延時(shí)環(huán)節(jié)上作了改進(jìn),隱層神經(jīng)元使用動(dòng)態(tài)的激勵(lì)函數(shù),優(yōu)化后的神經(jīng)網(wǎng)絡(luò)更加簡(jiǎn)單,能更好地滿足康復(fù)訓(xùn)練對(duì)高精度控制的動(dòng)態(tài)要求。因此,在小波包處理表面肌電信號(hào)基礎(chǔ)上采用改進(jìn)NARX神經(jīng)網(wǎng)絡(luò)提高識(shí)別率。
NARX神經(jīng)網(wǎng)絡(luò)(見圖1)由輸入層、延時(shí)層、隱含層和輸出層、反饋環(huán)節(jié)構(gòu)成,是一種被廣泛應(yīng)用于動(dòng)態(tài)非線性研究的反饋神經(jīng)網(wǎng)絡(luò),也稱為實(shí)時(shí)遞歸網(wǎng)絡(luò)。
NARX 神經(jīng)網(wǎng)絡(luò)權(quán)值更新算法為:
ΔWkη?eTk?yk?w+dxykT ??? (8)
其中:
dxyk=dykdyk-1,dykdyk-2,...dykdyk-mT ? (9)
隱層神經(jīng)元激勵(lì)函數(shù)參數(shù)學(xué)習(xí)方法如下:
(1)參數(shù)α的學(xué)習(xí)算法。
Jk=12e2k;ek=dk-yk (10)
αi,k+1=αi,k-Δαi,k;Δαi,k=ηαdJkdαj,k (11)
dJkdαi,k=-ekdykdαi,k (12)
dykdαi,k=w1idzidαi,k=ziαi,kw1i (13)
zi=fivi,αi,βi=αi?eβivi-e-βivieβivi+e-βivi (14)
αi,k+1=αi,k+ηα?ek?ziαi,k?w1i (15)
其中,αi,k+1是第i個(gè)隱層神經(jīng)元激勵(lì)函數(shù)參數(shù)αi在k+1時(shí)刻的值,ηα是學(xué)習(xí)速率,w1i是第i層隱層神經(jīng)元到輸出層的連接權(quán),zi是第i個(gè)隱層神經(jīng)元的輸出,vi是第i個(gè)隱層神經(jīng)元的輸入。
(2)參數(shù)β的學(xué)習(xí)算法。
βi,k+1=βi,k-Δβi,k;Δβi,k=ηβ?dJkdβi,k (16)
dJkdβi,k=-ek?dykdβi,k ??????? (17)
dykdβi,k=w1idzidβi,k=w1i?vi?αi,k?1-z2i (18)
βi,k+1=βi,k+ηβ?ek?w1i?αi,k?v?1-z2i ? (19)
其中, βi,k+1是第i個(gè)隱層神經(jīng)元激勵(lì)函數(shù)參數(shù)βi在k+1時(shí)刻的值,ηβ是學(xué)習(xí)速率。
改進(jìn)NARX神經(jīng)網(wǎng)絡(luò)(見圖2)在輸入延時(shí)環(huán)節(jié)上作了改進(jìn),結(jié)構(gòu)上與傳統(tǒng)NARX回歸神經(jīng)網(wǎng)絡(luò)結(jié)構(gòu)基本相同。改進(jìn)NARX網(wǎng)絡(luò)在應(yīng)用時(shí)只需確定隱層神經(jīng)元數(shù)目,采用實(shí)時(shí)遞歸學(xué)習(xí)(Real Time Recurrent Learning, RTRL)算法,此時(shí)該網(wǎng)絡(luò)隱藏層具有動(dòng)態(tài)的激勵(lì)函數(shù)。
3 動(dòng)作識(shí)別實(shí)驗(yàn)與結(jié)果分析
為驗(yàn)證該方法對(duì)腰背部動(dòng)作的識(shí)別率,本文將腰背動(dòng)作分為易于識(shí)別的扭腰、彎腰、側(cè)彎腰3個(gè)動(dòng)作。實(shí)驗(yàn)采用芬蘭Mega公司開發(fā)的ME6000表面肌電測(cè)試儀,包含16通道的數(shù)據(jù)記錄器和無線遙控裝置,能夠精準(zhǔn)測(cè)量與監(jiān)控表面肌電信號(hào)。本實(shí)驗(yàn)設(shè)置的采樣頻率為1 000Hz,肌電信號(hào)高通濾波器截止頻率為20Hz,低通濾波器截止頻率為500Hz。實(shí)驗(yàn)選取8名23-26周歲的大學(xué)生,身體健康且24h內(nèi)無腰背部劇烈運(yùn)動(dòng),分別在其扭腰、彎腰、側(cè)彎腰3種動(dòng)作下進(jìn)行信號(hào)采集(見圖3)。實(shí)驗(yàn)期間保證每位測(cè)試者在非疲勞情況下測(cè)試,每個(gè)動(dòng)作時(shí)間為30s,且動(dòng)作平穩(wěn)。
3.1 特征向量提取與神經(jīng)網(wǎng)絡(luò)構(gòu)建
選取腰背部3種動(dòng)作的表面肌電信號(hào),采用db4小波包函數(shù)對(duì)信號(hào)進(jìn)行6層分解,得到第6層64個(gè)頻帶的小波包分解系數(shù),之后計(jì)算出每個(gè)頻帶的信號(hào)能量,再將得到的能量進(jìn)行歸一化處理,作為神經(jīng)網(wǎng)絡(luò)的輸入。
為區(qū)分腰背部的3個(gè)動(dòng)作(彎腰、扭腰和側(cè)彎腰),設(shè)置3個(gè)對(duì)應(yīng)的輸出層節(jié)點(diǎn):(1 0 0)表示彎腰動(dòng)作,(0 1 0)表示扭腰動(dòng)作,(0 0 1)表示側(cè)彎腰動(dòng)作。
3.2 改進(jìn)神經(jīng)網(wǎng)絡(luò)訓(xùn)練與測(cè)試
改進(jìn)NARX神經(jīng)網(wǎng)絡(luò)體系結(jié)構(gòu)包括3層前饋網(wǎng)絡(luò)與2個(gè)輸入,以及8個(gè)隱層神經(jīng)元。神經(jīng)網(wǎng)絡(luò)內(nèi)部運(yùn)算函數(shù)使用Sigmoid函數(shù),以提高計(jì)算速度。采用MATLAB神經(jīng)網(wǎng)絡(luò)工具箱進(jìn)行神經(jīng)網(wǎng)絡(luò)分析,并與經(jīng)典NARX神經(jīng)網(wǎng)絡(luò)進(jìn)行比較。測(cè)試數(shù)據(jù)來源于此次試驗(yàn)采集的數(shù)據(jù),隨機(jī)產(chǎn)生240組輸入輸出數(shù)據(jù)對(duì),其中前200組用于訓(xùn)練,后40組用于測(cè)試。經(jīng)典NARX神經(jīng)網(wǎng)絡(luò)采用三階輸入延遲與一階輸出延遲,以及8個(gè)隱層神經(jīng)元;改進(jìn)NARX神經(jīng)網(wǎng)絡(luò)采用一階輸出延時(shí),以及8個(gè)隱層神經(jīng)元。使用240個(gè)數(shù)據(jù)集進(jìn)行網(wǎng)絡(luò)訓(xùn)練,兩種神經(jīng)網(wǎng)絡(luò)學(xué)習(xí)速率均為0.001,訓(xùn)練1 000步,比較結(jié)果如圖4、圖5所示。其中,圖4為NARX神經(jīng)網(wǎng)絡(luò)MSE擬合圖,圖5為改進(jìn)NARX神經(jīng)網(wǎng)絡(luò)MSE擬合圖。MSE用來表示訓(xùn)練后輸出矩陣與目標(biāo)矩陣的擬合曲線,R(相關(guān)度)的值越接近1,表示擬合效果越好。比較兩幅圖的結(jié)果得出,改進(jìn)NARX神經(jīng)網(wǎng)絡(luò)相比傳統(tǒng)NARX神經(jīng)網(wǎng)絡(luò),擬合度與識(shí)別率更高。
3.3 結(jié)果分析
將改進(jìn)NARX神經(jīng)網(wǎng)絡(luò)與BP、Elman神經(jīng)網(wǎng)絡(luò)進(jìn)行對(duì)比實(shí)驗(yàn)。BP神經(jīng)網(wǎng)絡(luò)隱層神經(jīng)元為10,輸出神經(jīng)元為3,inputs輸入數(shù)據(jù)中70%用于訓(xùn)練,15%用于擬合,15%用于測(cè)試,總體識(shí)別率為88.3%,說明其只能基本識(shí)別測(cè)試動(dòng)作;Elman神經(jīng)網(wǎng)絡(luò)隱層神經(jīng)元為14,輸出神經(jīng)元為3,inputs輸入數(shù)據(jù)中70%用于訓(xùn)練,15%用于擬合,15%用于測(cè)試,總體識(shí)別率為90%,表明Elman神經(jīng)網(wǎng)絡(luò)模式識(shí)別結(jié)果優(yōu)于BP神經(jīng)網(wǎng)絡(luò);NARX神經(jīng)網(wǎng)絡(luò)隱層神經(jīng)元為8,輸入延時(shí)為2,輸出反饋延時(shí)為2,輸出神經(jīng)元為3,inputs輸入數(shù)據(jù)中70%用于訓(xùn)練,15%用于擬合,15%用于測(cè)試,總體識(shí)別率為93.3%;改進(jìn)NARX神經(jīng)網(wǎng)絡(luò)隱層神經(jīng)元為8,輸入延時(shí)節(jié)點(diǎn)數(shù)為0,輸出延時(shí)節(jié)點(diǎn)數(shù)為2,輸出神經(jīng)元為3,inputs輸入數(shù)據(jù)中70%用于訓(xùn)練,15%用于擬合,15%用于測(cè)試,總體識(shí)別率為96.7%。根據(jù)幾組數(shù)據(jù)比較結(jié)果,改進(jìn)NARX神經(jīng)網(wǎng)絡(luò)模式識(shí)別結(jié)果優(yōu)于傳統(tǒng)NARX神經(jīng)網(wǎng)絡(luò)。
從表1中可以看出,各組針對(duì)扭腰模式的識(shí)別效果相對(duì)較好,而針對(duì)彎腰與側(cè)彎腰的識(shí)別效果較差。其中,BP神經(jīng)網(wǎng)絡(luò)的識(shí)別結(jié)果最差,NARX神經(jīng)網(wǎng)絡(luò)識(shí)別結(jié)果最優(yōu),改進(jìn)NARX神經(jīng)網(wǎng)絡(luò)通過對(duì)輸入隱層神經(jīng)元的優(yōu)化,識(shí)別率得到了很大提升。
4 結(jié)語
本文提出一種基于小波包能量與改進(jìn)NARX神經(jīng)網(wǎng)絡(luò)的分類識(shí)別新方法,利用小波包多分辨率的特點(diǎn)對(duì)表面肌電信號(hào)進(jìn)行多維度分解與重構(gòu),提取各個(gè)頻段歸一化能量作為腰背動(dòng)作特征向量。實(shí)驗(yàn)結(jié)果表明,改進(jìn)NARX神經(jīng)網(wǎng)絡(luò)能夠?qū)ρ秤?xùn)練中的3個(gè)動(dòng)作進(jìn)行有效分類,相比BP神經(jīng)網(wǎng)絡(luò)、ELMAN神經(jīng)網(wǎng)絡(luò)與NARX神經(jīng)網(wǎng)絡(luò),分類結(jié)果更準(zhǔn)確,識(shí)別率更高。但不足之處是該方法對(duì)彎腰和側(cè)彎腰動(dòng)作識(shí)別率相對(duì)較低,未來研究方向是進(jìn)一步提高每個(gè)動(dòng)作的識(shí)別率,并測(cè)試將該方法應(yīng)用于下肢運(yùn)動(dòng)的動(dòng)作識(shí)別中。
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(責(zé)任編輯:黃 ?。?/p>