a r X i v :n u c l -t h /0412114v 1 31 D e c 2004Electromagnetic and Mass Di?erence Corrections in πN Scattering
W.R.Gibbs and R.Arceo
Department of Physics,New Mexico State University
Las Cruces,New Mexico 88003,USA
Coulomb and mass di?erence corrections to low-energy pion-nucleon scattering are calculated
and compared with previous work including potential models,dispersion relation methods and
chiral perturbation theory calculations.Particular attention is paid to their role in testing isospin
breaking.
I.INTRODUCTION Electromagnetic corrections are a key element in understanding low energy pion nucleon scattering and,in turn,the fundamental elements of chiral symmetry breaking and the pion-nucleon coupling constant.They are also crucial in the important role of low-energy pion-nucleon scattering as a testing ground for isospin breaking.Tests of isospin breaking have been reported[1,2,3]using the extracted charged pion scattering ampli-tudes to predict charge exchange amplitudes.A breaking of the order of 8%in the amplitude was found.Piekarewicz[4]has suggested a possible explanation for the isospin breaking based on the e?ect of quark mass di?erences on the charged and neutral pion coupling constants.The breaking mentioned above is beyond the expected corrections due to electromagnetic interactions and mass di?erences.Corrections for these e?ects have been reported using a potential model for their calculation[5,6,7]based on pioneering work by Oades and Rache[8].In addition,an analysis was made[1](hereafter referred to as GAK)in which the potential method corrections were incorporated into the ?t itself.These last corrections have not been previously reported and it is a primary purpose of this paper to give them.For unbroken isospin one can write the charged pion scattering amplitudes in terms of the isospin ampli-tudes.f π+p →π+p =f 33(f 32);f π?p →π0n =√3(f 32)=
12 f π+p →π+p ?f π?p →π?p (1)In the potential treatment each of the pure isospin amplitudes is assumed to satisfy a wave equation with potentials corresponding to isospin (V
32).Since the π+p amplitude consists of a single isospin amplitude,only the Coulomb correction needs to be taken into account.The method for this case is covered in Section II.
The last two amplitudes,being a mixture of isospin amplitudes,pose the additional problem of the neutron-proton and π±–π0mass di?erences.By using a linear combination of the amplitudes,the solution is expressed as a pair of coupled equations corresponding to the charge states.
T cψc+
22+12 ψc+√3 V32 ψ0=?cψc(2)
T0ψ0+√
3 V32
ψc+ 12+22 ψ0=?0ψ0(3)
whereψc and?c are the charged wave function and kinetic energy andψ0and?0are the corresponding neutral pion quantities.T c and T0are the kinetic energy operators.The corrections are then included by adding a Coulomb potential to the equation describing theπ?only.The masses entering into the kinetic energy operators,T,and the kinetic energies,?,are also replaced by expressions with the true masses corresponding to the charged or neutral pions and nucleons.It is assumed that the potentials remain unchanged during this process.This assumption has been questioned by Rusetsky[9].
Although the general method used in each case is very similar,each group has used di?erent forms for the potentials,?t di?erent data sets,used di?erent relativistic prescriptions for the pion energies and reduced masses(energies)and,in some cases,di?erent wave equations.Thus,variations are to be expected among the resulting corrections.
In addition to the potential method,a dispersion approach has been used[10,11].Sauter[12]developed the basic method for s-waves only without considering mass di?erences.The model was extended in several works[13].This approach has become the most common method for making the corrections and they are often referred to as the NORDITA[10,11]results.The advantages and disadvantages of the potential and dispersion relations method are discussed in Ref.[5].
II.π+PROTON SCATTERING
In this case the corrections in a potential model are straightforward.The wave equation can be solved for each partial wave to obtain the phase shifts.While this full solution for the amplitude includes everything, the partial wave expansion diverges,as does the one for a pure Coulomb potential.Since the sum of the Coulomb series alone is known,the non-spin-?ip and spin-?ip amplitudes can be written as
f(θ)=f c(θ)+
1
2k
∞
?=0[e2iδ?+?e2iδ??]P1?(cosθ)(5)
whereσ?is the Coulomb phase shift and f c(θ)is the Coulomb amplitude.The notation?±means j=?±1
2ik
∞
?=0e2iσ?[(?+1)(e2iδn?+?1)+?(e2iδn???1)]P?(cosθ)(6)
where the nuclear phase shift is de?ned as
δn?±=δ?±?σ?.(7)
FIG.1:Comparison among the di?erent method forπ+p elastic scattering.The di?erence between the point charge (pc)and full calculation of Gashi et al.[5]is very small except for the s wave.The dotted curve represents a?t to the NORDITA results[10]to be used later and given in the appendix.
This phase shift is then compared with the hadronic phase shift,δh?±,i.e.the one that would exist if there were no Coulomb interaction.A potential model is used to calculate these two quantities in the present case. In the limit that the interactions are very weak,the hadronic and nuclear phase shifts become equal so that the di?erence may be expected to be small.The correction is de?ned as the di?erence between them.
C?±=δn?±?δh?±(8) Figure1shows a comparison of the results the di?erent determinations.It is seen that the potential methods are in reasonable agreement.The point Coulomb result of Gashi et al.[5]included no higher order electromagnetic e?ects whereas the full calculation included a?nite charge distribution as well.Since the corrections from GAK included a?nite charge density but no no higher order electromagnetic e?ects,there is no exact comparison possible.The dispersion relation approach is seen to give only slightly di?erent results. While one can judge to some extent the size and degree of agreement from Fig.1,it is useful to see the e?ect on the cross section itself.Figure2shows such a comparison at a kinetic energy of45MeV.The hadronic phase shifts are taken for Ref.[1]for all of the cases.It is seen that the correction is small but not negligible,being of the order of two standard deviations of accurate data.The di?erences between
FIG.2:E?ect of the corrections forπ+p scattering in a typical case.It is seen that the corrections are small but not negligible.The three methods compared give very nearly the same result.The data are from Brack et al.[15].
III.π?PROTON SCATTERING AND CHARGE EXCHANGE
In this case the expression of the corrections is more complicated since the potential system consists of a pair of coupled di?erential equations.Not only is there a correction due to the?nite charge in theπ?p channel but the neutron-proton andπ0?π±mass di?erences need to be taken into account.
A.Method
To express the corrections in the coupled channel case we follow the formulation of Gashi et al.[6].The solution of the coupled channel system[1,14]produces the amplitude forπ?proton scattering,charge exchange andπ0neutron scattering.The result is expressed as a2by2symmetric S matrix with the diagonal elements representingπ?p andπ0n elastic scattering and the o?-diagonal elements charge exchange.The scattering matrix is expressed in terms of a2by2real K matrix.
S=1+iK
S+1
.(9)
These equations hold for each partial wave where we have suppressed their indices.The S11matrix element has been multiplied by e?2iσ?to remove the pure Coulomb e?ect before the calculation of the K matrix. Since the K-matrix is real and symmetric it can be transformed to a diagonal form with an orthogonal
matrix.
cosφ?sinφsinφcosφ K11K12
K21K22
cosφsinφ
?sinφcosφ
= K100K2 ≡ tanδ10
0tanδ2 .
(10) FIG.3:The solid curve gives the result with both Coulomb and mass di?erence corrections from GAK.The dashed line is from Gashi et al.[6],the solid dots are from Tromborg et al.[10]and the solid triangles are from Zimmermann[7].
The dotted lines represent a?t to the NORDITA[10]results used later.
If isospin is a good quantum number then cosφ→ 3≡cosφ0andδ1andδ2are the isospin12 phase shifts.The form expressed in Eq.10is valid independent of isospin conservation and the di?erences of the resultant phase shiftsδ1,δ2as well as the value ofφneeded to diagonalize the system,from the unbroken isospin values are used to quantify the breaking.
The corrections are expressed as
C1≡ˉδ12;C3≡ˉδ32;?φ≡ˉφ?φh=ˉφ?φ0(11)
FIG.4:Comparison with the correction for the rotation angle?φ.The meaning of the symbols is the same as in Fig.3
for each partial wave,where the barred quantities are those obtained from the model by solving forδ1,δ2and φfrom the equations above with Coulomb and mass di?erences present.The quantities with super script “h”(the hadronic values)are those obtained from the solution of the system with all pion masses equal to the charged pion mass,the neutron mass equal to the proton mass and no Coulomb interaction.
The hope is that these corrections will be nearly model independent and one can?t the data by choosing δh1andδh2,apply the corrections to obtain the full K and S matrices,add in the Coulomb amplitude where appropriate and compare with data to calculate a value ofχ2.A search on the values of the isospin pure phase shifts,δh1andδh2can then be made.
We now address the problem of calculating the corrections using the model.Since K1and K2are the eigenvalues of the K-matrix we can solve directly for them with
λ=K11+K22± 2.(12)
FIG.5:The short dashed line contains the Coulomb correction only,the dash-dot line is with the mass di?erence correction only and the solid line contains both corrections.The results of Zimmermann are given by the solid triangles(full calculation)and solid squares(Coulomb correction only).
We may now identify the plus and minus signs in this expression with the isospin1
2states.
Multiplying Eq.10from the left by the transpose of the rotation matrix we?nd four relations for the tanφ,all of which are equivalent.Two of them are:
tanφ=K11?K1
K2?K11.(13)
B.Di?culty with the P1
2scattering a problem arises.In
this case around60MeV the two hadronic phase shifts(P31and P11)cross.For the two eigenvalues,K1 and K2to be equal the radical in Eq.12must vanish.This requires that the conditions
K11=K22;K12=0(14)
FIG.6:Individual contributions to?φ.The convention for the lines and points is the same as in Fig.5
be satis?ed simultaneously.For unbroken isospin this indeed happens.In this case isospin requires
K12=
√
(K11?K22)2
2
(16)
In order forλto have a continuous derivative the±association must be changed at the zero of the radical. If one chooses the sign such that the radical has the actual sign of K11?K22then
λ+=2K11?K22=tanδ32.(17) For broken isospin,in general,both conditions in Eq.14will NOT be satis?ed at once so that the two eigenvalues can never be equal.To understand what happens in this case it is useful to look at the direct solution of Eq.10.
In the general case we can expand Eq.10to?nd that the condition for the o?-diagonal elements to be zero is
2φ=tan?12K12
FIG.7:Comparison of methods of corrections for charge exchange for forward angles at low energies with the data of Frlez et al.[16].The dotted curve represents no correction,the solid curve gives the correction from GAK[1],the chain-dash curve is from Gashi et al.[6]and the dashed curve represents the corrections from NORDITA[10]
so that2φchanges byπaround the point K22=K11.Consider a simple example in which the isospin relation is slightly broken in a simple way such that
K12=
√
2+
2?
2K11(1+cos2φ)+
1
2(21)
K2=1
2
K22(1+cos2φ)+K12sin2φφ→φ0
→tanδ1
FIG.8:Separation of the Coulomb and mass corrections Compared with the data of Frlez et al.[16]
One can choose to change the association of the plus and minus signs at the zero of K12(that is what is done in the present work)or when K11=K22but either way there will be discontinuities in the corrections extracted at the crossing point.The actual amplitudes remain continuous,it is only this representation of the breaking which shows an anomalous behavior but the compensation only occurs for the hadronic phase shifts used in the calculation of the correction.If one uses these corrections with phase shifts other than the ones for which the corrections were derived,discontinuities in the resulting amplitudes will result.Of course,this is a small correction in a small partial wave in this case so in a practical sense the problem may not be very serious.
The di?culty comes partly from the fact that,even for good isospin,for K1=K2any value ofφis valid for the transformation soφis undetermined at that point.For unbroken isospin there is no problem,however, sinceφis determined arbitrarily close to the crossing point on either side.
https://www.doczj.com/doc/da11682552.html,parison with previous models
An obvious isospin breaking e?ect due to the mass di?erences leads to a positive Q-value forπ?p→π0n and a negative Q-value forπ0n→π?p.This fact destroys the symmetry of the amplitudes derived from the symmetric S-matrix,due to a factor k f/k i originating from the incoming and outgoing?uxes which is obtained in the charge exchange cross section obtained from the procedure above.This factor is included in the calculations of charge exchange cross sections to be shown in this section.
Figures3and4show a comparison of the corrections obtained from GAK[1]with previous work.It is seen that there is qualitative agreement among all calculations.There is rather good agreement between the present work and Zimmermann at the higher energies where he calculated.For the S11and P33partial
FIG.9:Comparison of the e?ects of the corrections with the charge exchange data of Isenhower et al.[17]at39.4 MeV.Other comparisons at10.6and20.6MeV are similar.
waves there is good agreement with the work of Gashi et al.[6]as for the case of?φfor the P3
2channel a comparison can be made and the agreement is good.The NORDITA[11]group
also gave a separation into the Coulomb and mass di?erence contributions.The mass di?erence correction dominates at low energy(see Ref.[11],Figures2,3and4).Hence,while these corrections are often called electromagnetic,it is the correction arising from mass di?erences which is the most important.
While the expression of the correction in this form is very useful,it is di?cult to appreciate the e?ect on the cross sections of the di?erences among the determinations as they may well be compensating to give similar corrections to physical observables.Hence,we consider the e?ects of these corrections on some charge exchange cross sections and theπ?p elastic amplitude.In each case the“no correction”calculation is made from the hadronic phase shifts of GAK[1]and the corrections are made relative to them.
Figure7shows a comparison of the correction with the charge exchange data of Frlez et al.[16]at27.5 MeV for the three groups who calculated in this energy range.It is seen that the resulting corrected cross sections are all very nearly the same and that the correction takes the result away from the data whereas the uncorrected calculation(dotted line)lies much closer to agreement.This last e?ect occurs only at forward angles.
Figure8gives the breakdown of the contributions.It is seen that the Coulomb correction actually moves the result slightly closer to the data while the mass di?erence correction takes it away.
Figure9shows the e?ect over the full angular range compared with the data of Isenhower et al.[17].It is
FIG.10:Comparison of the e?ect of the corrections with the data of Bagheri et al.[18]at62.2MeV.Other comparisons with data from the same group at45and92MeV are similar.
seen that for larger angles the corrections move the prediction closer to the data.
At higher energies the e?ect of the corrections is somewhat smaller.A comparison with the data of Bagheri et al.[18]is shown in Fig.10.The experimental results were given as coe?cients of Legendre polynomials in this case and the dots shown were calculated from these numbers.The e?ect of the corrections is seen to be small at this energy and the two potential models agree well.
In general,the di?erence between prediction and data can be expressed as a di?erence in normalization of around15%.
The forward charge exchange cross section is a sensitive measure of isospin breaking since it shows a deep minimum near45MeV.While most checks on isospin depend on obtaining an absolute cross section,the position of this minimum does not;only a knowledge of the beam momentum is needed.
The left part of Fig.11shows the e?ect of the corrections of the three groups compared with the data of Fitzgerald et al.[19].We see that the corrections are all very nearly the same and move the predicted cross section away from the data.While the uncorrected prediction cannot be said to give a good?t to the data, it is much better than the prediction after correction for the mass di?erences.The Coulomb potential has very little e?ect on the position of the minimum.
The right part of Figure11shows the prediction of the corrections for the amplitude forπ?p→π?p.It is seen that the mass correction is signi?cantly di?erent from the pure Coulomb e?ect.One can also note a large di?erence among the methods.
FIG.11:Left:Zero degree charge exchange with corrections from GAK[1],Gashi et al.[6]and NORDITA[10].Right: Corrections to the real part of fπ?p→π?p for GAK[1](dash:Coulomb only,solid:full correction),Gashi et al.[6] (dash-dot)and NORDITA[10](dotted).
https://www.doczj.com/doc/da11682552.html,parison with Chiral Perturbation Theory
While the results of the previous section are rather consistent,at least for the charge exchange,they may all be wrong.Thus,it is very desirable to have a comparison with an independent method calculating from an approximate QCD viewpoint.
A calculation of isospin breaking in Chiral Perturbation Theory(ChPT)has been made by Fettes and Mei?ner[20].They calculated6ratios which are measures of isospin breaking.Most of the ratios(3-6)involve quantities di?cult to measure.Ratio number one involves isoscalar quantities which vanish at threshold so that the ratio will depend sensitively on the phase shift?t used to compute them.
Their second ratio,
R2=2fπ+p→π+p?fπ?p→π?p?√fπ+p→π+p?fπ?p→π?p+√
comparison.As stated by the authors,a direct comparison can not be made between their work and data but one can calculate the ratio with the hadronic model under the same conditions.
If one were to attempt to compare with actual charge exchange amplitudes obtained directly from the data they would include the?ux factor
2waves
the pure Coulomb correction agrees rather well with the ChPT(full)calculations.The P1
2wave,even the ratio calculated with Coulomb corrections alone is more than a factor of2larger than
the ChPT result,perhaps due to the fact that the amplitude is larger and the perturbation series has not converged.
Without the?ux factor,only the P3
2 partial wave,especially when the mass correction is included.This may indicate that there is a general error
in the hadronic corrections or that the Delta degree of freedom needs to be treated di?erently in the ChPT calculation.
Since the mass correction in the form currently used has been questioned it is possible that the assumption about the isospin potentials remaining unchanged is incorrect.
If one assumes that the hadronic corrections are too large then a closer agreement can be achieved in the case of forward angle charge exchange at low energy and the position of the forward minimum.However, this conjecture does not correct the discrepancy at larger angles.So even this rather drastic possibility does not solve the problem.
We thank G.C.Oades and Ulf Meissner for comments on the manuscript.
This work was supported by the National Science Foundation under contract PHY-0099729.
FIG.12:The left?gure shows the ratio R2calculated with GAK corrections for Coulomb only(dash-dot)and Coulomb plus mass di?erences(solid)compared with that from Fettes and Mei?ner[20](solid squares).The dashed line gives the result with the corrections of Gashi et al.[6]and the dotted line gives that of NORDITA[10].The?ux factor k f/k i is not included in the hadronic calculations.
APPENDIX A:FITS
Following are a set of?ts to the phase shifts and corrections which are given as an aide.The form of the ?ts has no physical signi?cance and should not be used outside the range5-100MeV.In what follows,T is the laboratory kinetic energy in MeV andδis in degrees.
1.GAK Hadronic Phase Shifts
The?t to the GAK hadronic phase shifts for the S waves is in the form
δ=
a
√
1+bT+cT2
.(A1)
For S1a=1.0319;b=2.865×10?3;c=?8.0×10?7(A2)
For S3a=?0.5197;b=?5.37×10?3;c=1.939×10?5(A3) For the p-waves
δ13=?0.001806T3
1+0.001T
;δ33=
0.01084T3
1?0.000436T
(A4)
δ11=?0.00636T3
1+0.0131T
;δ31=?
0.00196T3
1+0.00011T
(A5)
2.π+p Corrections
The?ts to theπ+p corrections by GAK are
C0+=0.1+0.000049T;C1?=?0.009+0.00045T;C1+=?0.0185?0.000397T1.5(A6) The?ts to theπ+p corrections by Gashi et al.[5]are
C0+=0.075+0.0005T;C1?=0.005+0.0002T;C1+=?0.033?0.0000195T2.2(A7) For NORDITA
C0+=0.095;C1?=0.00045T;C1+=?0.035?0.000067T2(A8)
3.Coupled Channel Corrections
The?t to the GAK corrections for the s wave is
C1=0.753T?12
(1?0.0051T)
T1.3
+0.0011T?0.026(A10) For the P1
1.+.0008T2
+0.01745tan?1 0.02
C 3=0.019?0.01137T 0.728
T ?60.1
(A12)?φ=?1.2+
148
T ?60.1 (A13)
For the P 3
100 4;C 3=0.1+0.0048T ?0.23 T
T +0.37(A14)
The ?t to the Gashi et
al.[6]corrections used in this work is
For s C 1=1.15
T 0.3?0.04(A15)
For p 3
2C 1=?0.045+0.000016T 1.9;C 3=?0.05+0.00025T +0.046e ?0.08T (A17)?φs =10.3
2=1602=?45
T 0.4?0.095;C 3=?0.55
2C 1=?0.00065T ;C 3=0.008T +0.06(A20)?φs =16
2=?
482partial wave.
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金黄色葡萄球菌 金黄色葡萄球菌革兰氏染色显微照片 金黄色葡萄球菌 (Staphyloccocus aureus Rosenbach) 是人类的一种重要病原菌，隶属于葡萄球菌属（Staphylococcus），有“嗜肉菌"的别称，是革兰氏阳性菌的代表，可引起许多严重感染。而对于金黄色葡萄球菌在速冻食品中的存在量，卫生部于2011年11月24日公布食品安全国家标准《速冻面米制品》，允许金葡菌限量存在。 目录 简介 流行病学 引发病症 球菌检验 球菌控制 感染处理 限量存在 简介 金黄色葡萄球菌细胞壁含90%的肽聚糖和10%的磷壁酸。其肽聚糖的网状结构比革兰氏阴性菌致密，染色时结晶紫附着后不被酒精脱色故而呈现紫色，相反，阴性菌没有细胞壁结构，所以紫色被酒精冲掉然后附着了沙黄的红色。金黄色葡萄球菌与青霉素的发现有很大的渊源。当年弗莱明就是在他的金黄色葡萄球菌的培养皿中发现有些球菌被杀死了，于是发现了青霉素。而研究也表明青霉素只对以金黄色葡萄球菌为代表的革兰氏阳性菌作用明显。这也是由肽聚糖层的厚度和结构造成的。新出现的耐甲氧西林金黄色葡萄球菌，被称作超级细菌，几乎能抵抗人类现在所有的药物，但是万古霉素可以对付它。典型的金黄色葡萄球菌为球型，直径0.8μm 左右，显微镜下排列成葡萄串状。
显微图像 金黄色葡萄球菌无芽胞、鞭毛，大多数无荚膜，革兰氏染色阳性。金黄色葡萄球菌营养要求不高，在普通培养基上生长良好，需氧或兼性厌氧，最适生长温度37°C，最适生长pH7.4,干燥环境下可存活数周。平板上菌落厚、有光泽、圆形凸起，直径1~2mm。血平板菌落周围形成透明的溶血环。金黄色葡萄球菌有高度的耐盐性，可在10~15%NaCl肉汤中生长。可分解葡萄糖、麦芽糖、乳糖、蔗糖，产酸不产气。甲基红反应阳性，VP反应弱阳性。许多菌株可分解精氨酸，水解尿素，还原硝酸盐，液化明胶。金黄色葡萄球菌具有较强的抵抗力，对磺胺类药物敏感性低，但对青霉素、红霉素等高度敏感。对碱性染料敏感，十万分之一的龙胆紫液即可抑制其生长。 流行病学 金黄色葡萄球菌在自然界中无处不在，空气、水、灰尘及人和动物的排泄物中都可找到。因而，食品受其污染的机会很多。美国疾病控制中心报告，由金黄色葡萄球菌引起的感染占第二位，仅次于大肠杆菌。金黄色葡萄球菌肠毒素是个世界性卫生难题，在美国由金黄色葡萄球菌肠毒素引起的食物中毒，占整个细菌性食物中毒的33%，加拿大则更多，占到45%，我国每年发生的此类中毒事件也非常多。 金黄色葡萄球菌的流行病学一般有如下特点：季节分布，多见于春夏季；中毒食品种类多，如奶、肉、蛋、鱼及其制品。此外，剩饭、油煎蛋、糯米糕及凉粉等引起的中毒事件也有报道。上呼吸道感染患者鼻腔带菌率83%，所以人畜化脓性感染部位，常成为污染源。 一般说，金黄色葡萄球菌可通过以下途径污染食品：食品加工人员、炊事员或销售人员带菌，造成食品污染；食品在加工前本身带菌，或在加工过程中受到了污染，产生了肠毒素，引起食物中毒；熟食制品包装不密封，运输过程中受到污染；奶牛患化脓性乳腺炎或禽畜局部化脓时，对肉体其他部位的污染。金黄色葡萄球菌是人类化脓感染中最常见的病原菌，可引起局部化脓感染，也可引起肺炎、伪膜性肠炎、心包炎等，甚至败血症、脓毒症等全身感染。金黄色葡萄球菌的致病力强弱主要取决于其产生的毒素和侵袭性酶：
典型电---气比例阀、伺服阀的工作原理 电---气比例阀和伺服阀按其功能可分为压力式和流量式两种。压力式比例/伺服阀将输给的电信号线性地转换为气体压力；流量式比例/伺服阀将输给的电信号转换为气体流量。由于气体的可压缩性，使气缸或气马达等执行元件的运动速度不仅取决于气体流量。还取决于执行元件的负载大小。因此精确地控制气体流量往往是不必要的。单纯的压力式或流量式比例/伺服阀应用不多，往往是压力和流量结合在一起应用更为广泛。 电---气比例阀和伺服阀主要由电---机械转换器和气动放大器组成。但随着近年来廉价的电子集成电路和各种检测器件的大量出现，在1电---气比例/伺服阀中越来越多地采用了电反馈方法，这也大大提高了比例/伺服阀的性能。电---气比例/伺服阀可采用的反馈控制方式，阀内就增加了位移或压力检测器件，有的还集成有控制放大器。 一、滑阀式电---气方向比例阀 流量式四通或五通比例控制阀可以控制气动执行元件在两个方向上的运动速度，这类阀也称方向比例阀。图示即为这类阀的结构原理图。它由直流比例电磁铁1、阀芯2、阀套3、阀体4、位移传感器5和控制放大器6等赞成。位移传感器采用电感式原理，它的作用是将比例电磁铁的衔铁位移线性地转换为电压信号输出。控制放大器的主要作用是： 1）将位移传感器的输出信号进行放大； 2）比较指令信号Ue和位移反馈信号U f U； 3）放大，转换为电流信号I输出。此外，为了改善比例阀的性能，控制放大器还含有对反馈信号 Uf的处理环节。比如状态反馈控制和PID调节等。 带位置反馈的滑阀式方向比例阀，其工作原理是：在初始状态，控制放大器的指令信号UF=0，阀芯处于零位，此时气源口P与A、B两端输出口同时被切断，A、B两口与排气口也切断，无流量输出；同时位移传Uf=0。若阀芯受到某种干扰而偏离调定的零位时，位移传感器将输出一定的电压Uf，控制放 放大后输出给电流比例电磁铁，电磁铁产生的推力迫使阀芯回到零位。若指令Ue>0，则 电压差U增大，使控制放大器的输出电流增大，比例电磁铁的输出推力也增大，推动阀芯右移。而阀芯的右移又引起反馈电压Uf的增大，直至Uf与指令电压Ue基本相等，阀芯达到力平衡。此时。
金黄色葡萄球菌是人类化脓感染中最常见的病原菌，可引起局部化脓感染，也可引起肺炎、伪膜性肠炎、心包炎等，甚至败血症、脓毒症等全身感染。它在自然界中无处不在，空气、水、灰尘及人和动物的排泄物中都可找到。因此，它很容易就能够污染一些食物来源，从而引发疾病。特别是金黄色葡萄球菌肠毒素，它是个世界性卫生难题，在美国由金黄色葡萄球菌肠毒素引起的食物中毒，占整个细菌性食物中毒的33%，加拿大则更多，占到45%，中国金黄色葡萄球菌引起的食物中毒事件也时有发生。误食金葡菌污染的食品,可引起呕吐和腹泻等症状。因此，在本篇文献中，我们选取了关于金黄色葡萄球菌肠毒素的一部分内容进行了较为细致的思考。 文中提到，肠毒素能够引起人和哺乳动物肠胃道毒性反应。但是，金葡菌肠毒素同时也是一种典型的超级抗原。在免疫反应中，只需极微量，就能通过一种独特的机制，使之产生大量细胞因子和细胞毒性物质。从而抑制异常分裂的癌症细胞生长而可能起到治疗癌症的效果。 那么，肠毒素治疗肿瘤的机制是什么呢？ 首先，肠毒素是一种超级抗原。超抗原是一类只需极低浓度就能激活大量T细胞克隆或B细胞克隆、产生极强免疫效应的物质。它远超普通抗原的多克隆激活能力，可视其为具非特异性免疫原性但无免疫反应性的抗原。它在体内能够活化CD4 + T细胞，这种细胞能分泌多种细胞因子。它们不仅能够直接或间接地杀伤肿瘤细胞，而且可以增加肿瘤细胞表达MHC 抗原分子，增强肿瘤细胞刺激宿主免疫系统的能力; 另一方面，这些细胞因子又刺激T细胞进一步增殖分化，而增殖分化的T细胞又将产生更多的细胞因子与细胞毒作用，共同导致肿瘤细胞的破坏溶解，从而形成级联效应，达到对肿瘤的治疗作用。 除了对肿瘤的治疗作用，肠毒素在一定浓度和不同途径给予时，还具备着非特异性促进人和哺乳动物细胞的有丝分裂效果。特别是对损伤部位的组织有促进分裂和生长作用，能够致使损伤组织快速愈合。故有利于对损伤组织的治疗。但这种反应作用的机制我们小组尚且还无法解释，希望在之后的课程中能够有所启发。
伺服驱动器的工作原理 随着全数字式交流伺服系统的出现，交流伺服电机也越来越多地应用于数字控制系统中。为了适应数字控制的发展趋势，运动控制系统中大多采用全数字式交流伺服电机作为执行电动机。在控制方式上用脉冲串和方向信号实现。 一般伺服都有三种控制方式：速度控制方式，转矩控制方式，位置控制方式。 速度控制和转矩控制都是用模拟量来控制的。位置控制是通过发脉冲来控制的。具体采用什么控制方式要根据客户的要求，满足何种运动功能来选择。 如果您对电机的速度、位置都没有要求，只要输出一个恒转矩，当然是用转矩模式。 如果对位置和速度有一定的精度要求，而对实时转矩不是很关心，用转矩模式不太方便，用速度或位置模式比较好。如果上位控制器有比较好的死循环控制功能，用速度控制效果会好一点。如果本身要求不是很高，或者，基本没有实时性的要求，用位置控制方式对上位控制器没有很高的要求。就伺服驱动器的响应速度来看，转矩模式运算量最小，驱动器对控制信号的响应最快；位置模式运算量最大，驱动器对控制信号的响应最慢。 对运动中的动态性能有比较高的要求时，需要实时对电机进行调整。那么如果控制器本身的运算速度很慢（比如PLC，或低端运动控制器），就用位置方式控制。如果控制器运算速度比较快，可以用速度
方式，把位置环从驱动器移到控制器上，减少驱动器的工作量，提高效率（比如大部分中高端运动控制器）；如果有更好的上位控制器，还可以用转矩方式控制，把速度环也从驱动器上移开，这一般只是高端专用控制器才能这么干，而且，这时完全不需要使用伺服电机。换一种说法是： 1、转矩控制：转矩控制方式是通过外部模拟量的输入或直接的地址的赋值来设定电机轴对外的输出转矩的大小，具体表现为例如10V 对应5Nm的话，当外部模拟量设定为5V时电机轴输出为2.5Nm:如果电机轴负载低于2.5Nm时电机正转，外部负载等于2.5Nm时电机不转，大于2.5Nm时电机反转（通常在有重力负载情况下产生）。可以通过实时的改变模拟量的设定来改变设定的力矩大小，也可通过通讯方式改变对应的地址的数值来实现。应用主要在对材质的受力有严格要求的缠绕和放卷的装置中，例如饶线装置或拉光纤设备，转矩的设定要根据缠绕的半径的变化随时更改以确保材质的受力不会随着缠绕半径的变化而改变。 2、位置控制：位置控制模式一般是通过外部输入的脉冲的频率来确定转动速度的大小，通过脉冲的个数来确定转动的角度，也有些伺服可以通过通讯方式直接对速度和位移进行赋值。由于位置模式可以对速度和位置都有很严格的控制，所以一般应用于定位装置。应用领域如数控机床、印刷机械等等。 3、速度模式：通过模拟量的输入或脉冲的频率都可以进行转动速度的控制，在有上位控制装置的外环PID控制时速度模式也可以进行
金黄色葡萄球菌的危害程度评估报告 一、生物学特性 金黄色葡萄球菌是人类的一种重要病原菌，隶属于葡萄球菌属，可引起多种严重感染。金黄色葡萄球菌为球型，直径0.8μm左右，显微镜下排列成葡萄串状。金黄色葡萄球菌无芽胞、鞭毛，大多数无荚膜，革兰氏染色阳性。金黄色葡萄球菌营养要求不高，在普通培养基上生长良好，需氧或兼性厌氧，最适生长温度37°C，最适生长pH 7.4。平板上菌落厚、有光泽、圆形凸起，直径1-2mm。血平板菌落周围形成透明的溶血环。金黄色葡萄球菌有高度的耐盐性，可在10-15%NaCl肉汤中生长。可分解葡萄糖、麦芽糖、乳糖、蔗糖，产酸不产气。甲基红反应阳性，VP反应弱阳性。许多菌株可分解精氨酸，水解尿素，还原硝酸盐，液化明胶。 二、危害程度分类 根据中华人民共和国卫生部制定《人间传染的病原微生物名录》该菌危害程度为第三类。 三、致病性和感染剂量 金黄色葡萄球菌是人类化脓感染中最常见的病原菌，可引起局部化脓感染，也可引起肺炎、伪膜性肠炎、心包炎等，甚至败血症、脓毒症等全身感染。金黄色葡萄球菌的致病力强弱主要取决于其产生的毒素和侵袭性酶，有报道目前出现越来越多的耐药菌株，MRSA即耐甲氧西林金黄色葡萄球，菌致病性也随着变强。 四、暴露的潜在后果 暴露后可能引起感染，菌量大时可使实验人员出现皮肤软组织感染、全身性感染、呼吸道感染、中毒、肠炎等。被感染后，成为传染源，可能对周围及环境造成污染，应及时得到治疗和控制。 五、感染途径 通过污染食品和水源经口传播，也可通过呼吸道和接触传播。 六、微生物在环境中的稳定性 葡萄球菌是无芽胞菌中抵抗力最强者，而干燥可达数月，加热80℃30min才被杀死。5％石炭酸，0.1％升汞10～15min死亡。1:100000～1:200000龙胆紫溶液能抑制其生长。对磺胺增效剂、青霉素、红霉素等较敏感，但耐药株逐年增多，MRSA即为耐甲氧西林金黄色葡萄球菌。 七、浓度和浓缩标本的容量
伺服电机工作原理及和步进电机の区别 2010-03-30 17:14 伺服电机内部の转子是永磁铁，驱动器控制のU/V/W三相电形成电磁场，转子在此磁场の作用下转动，同时电机自带の编码器反馈信号给驱动器，驱动器根据反馈值与目标值进行比较，调整转子转动の角度。伺服电机の精度决定于编码器の精度（线数）。 什么是伺服电机？有几种类型？工作特点是什么？ 答：伺服电动机又称执行电动机，在自动控制系统中，用作执行元件，把所收到の电信号转换成电动机轴上の角位移或角速度输出。分为直流和交流伺服电动机两大类，其主要特点是，当信号电压为零时无自转现象，转速随着转矩の增加而匀速下降.。 请问交流伺服电机和无刷直流伺服电机在功能上有什么区别？ 答：交流伺服要好一些，因为是正弦波控制滚珠丝杆，转矩脉动小。直流伺服是梯形波。但直流伺服比较简单，便宜。永磁交流伺服电动机20世纪80年代以来，随着集成电路、电力电子技术和交流可变速驱动技术の发展，永磁交流伺服驱动技术有了突出の发展，各国著名电气厂商相继推出各自の交流伺服电动机和伺服驱动器系列产品并不断完善和更新。交流伺服系统已成为当代高性能伺服系统の主要发展方向，使原来の直流伺服面临被淘汰の危机。90年代以后，世界各国已经商品化了の交流伺服系统是采用全数字控制の正弦波电动机伺服驱动。交流伺服驱动装置在传动领域の发展日新月异。 永磁交流伺服电动机同直流伺服电动机比较，主要优点有：⑴无电刷和换向器，因此工作可靠，对维护和保养要求低。⑵定子绕组散热比较方便。⑶惯量小，易于提高系统の快速性波纹管联轴器。⑷适应于高速大力矩工作状态。⑸同功率下有较小の体积和重量。 伺服和步进电机 伺服主要靠脉冲来定位，基本上可以这样理解，伺服电机接收到1个脉冲，就会旋转1个脉冲对应の角度，从而实现位移，因为，伺服电机本身具备发出脉冲の功能，所以伺服电机每旋转一个角度，都会发出对应数量の脉冲，这样，和伺服电机接受の脉冲形成了呼应，或者叫闭环，如此一来，系统就会知道发了多少脉冲给伺服电机，同时又收了多少脉冲回来，这样，就能够很精确の控制电机の转动，从而实现精确の定位，可以达到0.001mm。 步进电机是一种离散运动の装置，它和现代数字控制技术有着本质の联系。在目前国内の数字控制系统中，步进电机の应用十分广泛。随着全数字式交流伺服系统の出现，交流伺服电机也越来越多地应用于数字控制系统中。为了适应数字控制の发展趋势，运动控制系统中大多采用步进电机或全数字式交流伺服电机作为执行电动机。虽然两者在控制方式上相似（脉冲串和方向信号）弹性联轴器，但在使用性能和应用场合上存在着较大の差异。现就二者の使用性能作一比较。 一、控制精度不同 两相混合式步进电机步距角一般为 3.6°、 1.8°，五相混合式步进电机步距角一般为
金黄色葡萄球菌的概况 摘要：本文旨在讲述金黄色葡萄球菌的目前现状，以及其主要检测方法，代谢物肠毒素检测方法，耐药检测和幼儿园发病原因，这些对金葡菌的检测、治疗和预防起到了很好的帮助。关键词：金黄色葡萄球菌；检测；肠毒素；耐药；发病 金黄色葡萄球菌（Staphylococcus aureus，SA）是食品卫生标准中规定不得检出的常见食物中毒致病菌，是食品卫生微生物常规检测项目之一[1]。SA是葡萄球菌属，革兰染色阳性，呈葡萄状排列。当衰老、死亡、被吞噬后常转为阴性。无鞭毛、无芽胞、体外培养一般不形成荚膜。在浓汁及液体培养基中常单个、成对、或短链状排列。在普通培养基上即可生长，当加入血液或其他营养物质生长的更好。需氧或兼性厌氧。最适pH7.4，最适温度28-38℃，致病菌在37℃生长最好。某些菌株耐盐性特强，在100-150g/L氯化钠培养基中都能生长。在某些影响细胞壁形成物质的作用下可形成L型。触酶试验阳性。多数菌株分解葡萄糖、麦芽糖、蔗糖，产酸不产气，致病菌株可分解甘露醇。SA能产生大量核酸酶，该酶可耐受100℃30min不破坏，降解DNA和RNA的能力较强。此酶对检测葡萄球菌的致病性与血浆凝固酶具有同等价值。 由于致病金黄葡萄球菌能分泌肠毒素，因此一旦细菌污染食品，并在合适的温度环境下，细菌可以大量繁殖并产生肠毒素，从而引起消费者食物中毒[2]。金黄色葡萄球菌是一种能引起食物中毒的重要细菌。根据美国疾病控制中心的一些报告，由SA引起的食物中毒居第二位，仅次于大肠杆菌，在细菌性食物中毒中的比例为33％。加拿大的发生率更高，占细菌性食物中毒的45％[3]。中国每年发生SA中毒事件也屡见不鲜，因此造成每年的经济损失相当惨重，目前世界各国都把SA定为重要的食品卫生法定检测项目。在1960年，人们将一种半合成的青霉素-甲氧西林第一次应用于临床，而且仅仅一年之后，在英国就发现了首例耐甲氧西林金黄色葡萄球菌（Methicillin-resistance Staphylococcus aureus,MRSA），再后来，在世界范围内MRSA便以惊人的速度蔓延开去，继而发展成为医院内最最常见的微生物病原菌，与乙型肝炎、艾滋病同为当今世界三大感染顽疾[4]。耐甲氧西林金黄色葡萄球菌是医院内重要感染的致病菌，其发病率和病死率在世界各地均很高，美国疾病控制中心在2003年做了大量工作，根据统计了解，每年因为耐甲氧西林金黄色葡萄球菌感染，大约有数十万人住院接受治疗，在医院内由SA引起耐甲氧西林金黄色葡萄球菌感染，其分离程度已经高达80%以上，而且呈向社区扩散的趋势[5]。1996年，日本报告了第一例对万古霉素敏感性下降的金葡菌[6] ，人们把万古霉素认为是治疗SA的最后防线，最为经济有效的药物，不过在20世纪90年代后期又出现了耐万古霉素的SA，开始表现为万古霉素中介金黄色葡萄球菌（vancomycin intermediate-susceptible staphylococcus aureus ，VISA），美国、英国、德国、意大利、韩国等相继报道检出了VISA[7]，；1997年美国分离了2例VISA[8]同期，在欧洲与亚洲多个国家均有类似报到[9]。 国内外对SA的检测方法，主要有传统分离培养及生化鉴定、显色培养基鉴定、酶联免疫(ELISA)、PCR、美国3M公司的petrifilm培养片等方法。现行国标检测食品中的SA主要是采用传统的分离培养之后进行生化鉴定，整个检测过程获得最终结果需时5天左右。而且免疫学的ELISA法所用的抗体基本上全部依赖国外进口，试剂也相对昂贵；但传统的PCR法却需要提取DNA，加大了人力、物力的消耗，成本提高。对于SA的检测，很多研究者都来检测致使其食物中毒的肠毒素基因[10]，尤其是用于食物中毒事故的调查，所以对于研发一种快速、便捷的检测方法是目前食品安全检测的当务之急。 SA的常规检验检验程序：检样处理→增菌→分离→分纯→溶血试验→革兰染色镜检→血浆凝固酶试验→肠毒素检验[11]。常规检验具有操作简便，所需设备简单，成本相对较低的优点，但其操作繁琐，检测时间较长，且灵敏度较低[12]。
如何在EXCEL表格中批量插入对应图片学生照片用身份证号命名后，核对是个麻烦事儿，有了这个表格就直观多了。 制作过程如下： 1、粘贴学生信息、输入公式并向下填充 在D2单元格输入公式（复制下面的公式并粘贴到D2单元格中）： ="![](@d:\学生照片\"&C2&".jpg@)