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            在工厂中,怎么解决PLC信号受到的干扰?

            日期:2018-09-26 13:41
            在工厂中,PLC主要的作用是接收现场信号,在集中处理后,按照运行逻辑控制现场设备,而PLC最大的好处是用24V控制380V,因此,PLC接受的信号大多数是弱电信号,包括数字量、模拟量。
            而工厂的现场环境一般都很复杂,有弱电,有强电,有变频器,有伺服驱动器,总之,电磁环境很复杂,电网的纯净性很低。那么这就使得PLC的抗干扰能力受到极大的挑战。但PLC本身也是一个电子设备,它虽然有处理干扰的能力,但是仅依靠PLC本身的抗干扰是不够的,我们还需要使用一些外部措施提高PLC的抗干扰能力,使得控制更加完美。
            一:使用PLC的抗干扰能力—滤波,什么是PLC的滤波?
            可以根据现场情况选择滤波时间
            PLC的滤波可以应对的干扰信号是变化频率太快的信号,有一些PLC本身就有这个功能,只需要从设置里更改就可以,这个功能理解起来很容易,就是PLC会过滤掉小于设定时间的小信号,比如I0.0的滤波时间设置为12ms,那么小于这个时间的信号,PLC不会接受它,也就是无效的,还有一些高级的PLC在滤波的控制上有一些算法,但大体功能类似。
            根据这个原理,这个功能就有了一个很大的缺陷,那就是有延迟,也就是时效性不好。
            对于不太重要的信号,有一点延迟无关紧要,但是,对于定位的传感器、编码器或者是需要精确称量的称重传感器来说就是致命的缺陷了,所以应对这些情况时不可以使用PLC的滤波,只能依靠其他措施。
            二;工厂中应用最多的、成本最低的也是最有效的消除干扰的措施,大家应该都知道,那就是屏蔽。那么屏蔽有几种?到底应该怎么接?
            现场使用的屏蔽主要有两种,第一种屏蔽是使用金属的线槽或者其他类似的密封金属盒将强弱电电缆分开并包裹起来,从源头上减小强电的电磁辐射以及噪声源的干扰等。
             
            第二种是电缆本身带的屏蔽,屏蔽电缆主要用于精密测量的仪表或者传感器等与PLC输入的连接,可消除静电信号,将无用的电磁信号从屏蔽层导出去,以保证测量的准确性,这是外部电缆层的最后一层保护,因此非常重要。我们一般说电缆的屏蔽就得说一个专业术语“单端接地”。那么单端需要怎么接?
            一般的工厂都有PLC专用的地线,这个地线理论来说只是给弱电系统使用。单端接地,我们一般都把屏蔽层接到电气柜内的屏蔽地线上,现场那头我们只要悬空就可以。那么如果现场侧不悬空或者也接地了会发生什么?那就是现场的那头因为电磁干扰,很可能会有一个相对于PLC地线的电势差,这个电势差有时会使得信号干扰更为严重。所以对于PLC控制系统,我们将屏蔽层单端接地,而且接到应用于PLC的地线上,坚决杜绝双端接地。
            电气柜内地线
            三:电气隔离。电气隔离也分为两种,一种是电源隔离,另一种是信号隔离。如果怀疑电源本身的纯净度不高(比如说靠近变频器和伺服控制的电源),那么我们可以使用隔离变压器,使得电源能纯净一些。信号隔离是使用信号隔离器将变送器或者仪表的信号,通过电子器件调制变换,并且隔离转换到需要的信号。这也是一种非常有效的消除干扰的方式。
            四:以上是工厂里常见的消除干扰的措施,但除此之外我们还有一些小技巧也可以用来消除干扰;
            1 电缆交叉缠绕在一起,比如我们所说的屏蔽双绞线,还有大家看到的网线都是这么接的,这种方式可以最大程度的消除信号电缆本身的磁场,对于微弱信号的传输有很大优势。
            2 使用磁环,将信号电缆以变压器线圈的形式缠绕在磁环的一侧,这种方式也可使有效的减小干扰,并且将干扰磁场转为热能消耗出去。
            3 通讯隔离模块等一些其他保护装置,还有一些PLC以232,485进行通讯时也受到干扰,那么我们则使用通讯隔离模块,这种模块类似于信号隔离器,使用电子器件将信号转换。对于复杂的工厂现状我们要合理分析可能的干扰原因,找出干扰源,并给出合理的解决方案。
            In the factory, the main function of PLC is to receive the field signal. After centralized processing, the field equipment is controlled according to the operation logic. The greatest advantage of PLC is to use 24V to control 380V. Therefore, most of the signals accepted by PLC are weak electrical signals, including digital and analog signals.
             
            The factory site environment is generally very complex, there are weak electricity, strong electricity, inverter, servo drive, in short, the electromagnetic environment is very complex, the purity of the grid is very low. This makes the anti-jamming capability of PLC greatly challenged. But PLC itself is also an electronic equipment, although it has the ability to deal with interference, but only rely on the anti-interference of PLC itself is not enough, we need to use some external measures to improve the anti-interference ability of PLC, making the control more perfect.
             
            One: the use of PLC anti-interference capability - filtering, what is the filtering of PLC?
             
            Filter time can be selected according to the site conditions.
             
            PLC filter can deal with the interference signal is too fast change frequency signal, some PLC itself has this function, only need to change from the settings, this function is easy to understand, that is, PLC will filter out the small signal less than the set time, such as I 0.0 filter time set to 12 ms, then less than this function. Time signal, PLC will not accept it, that is, invalid, there are some advanced PLC filter control algorithm, but the general function is similar.
             
            According to this principle, this function has a big drawback, that is, there is a delay, that is, poor timeliness.
             
            For less important signals, a little delay is irrelevant, but for positioning sensors, encoders or weighing sensors that require precise weighing is a fatal flaw, so to deal with these situations can not use PLC filtering, can only rely on other measures.
             
            Second, the most widely used, the lowest cost and the most effective measures to eliminate interference, we should all know that is shielding. How many kinds of shielding are there? How should I answer it?
             
            There are mainly two kinds of shielding used in the field. The first kind of shielding is to use metal grooves or other similar sealed metal boxes to separate and wrap the strong and weak electric cables, so as to reduce the strong electromagnetic radiation and noise interference from the source.
             
            The second kind is the shield of the cable itself. The shielded cable is mainly used for the connection between the instrument or sensor of precision measurement and PLC input. It can eliminate the static electricity signal and export the useless electromagnetic signal from the shielding layer to ensure the accuracy of measurement. This is the last layer of protection of the external cable layer, so it is very important. We generally say that the shielding of cables requires a technical term "single ended grounding". How do I pick up the single end?
             
            The general factory has PLC dedicated ground wire, which is only used for weak current system. Single-end grounding, we usually connect the shielding layer to the shielding ground wire in the electrical cabinet, the scene that we just suspended. What happens if the field is not suspended or grounded? That's the end of the scene because of electromagnetic interference, there is likely to be a potential difference relative to the PLC ground wire, which sometimes makes the signal interference more serious. So for the PLC control system, we will shield layer single-ended grounding, and connected to the ground used in PLC, resolutely eliminate double-ended grounding.
             
            Ground wire in electrical cabinet
             
            Three: electrical isolation. Electrical isolation is also divided into two types, one is power isolation, the other is signal isolation. If it is suspected that the purity of the power supply itself is not high (for example, close to the inverter and servo control power), then we can use an isolation transformer to make the power supply more pure. Signal isolation is the use of signal isolator transmitter or instrument signals, modulation and transformation through electronic devices, and isolation of the conversion to the desired signal. This is also a very effective way to eliminate interference.
             
            Fourth: The above is a common factory interference elimination measures, but in addition to this we have some small techniques can also be used to eliminate interference;
             
            1 Cables intertwined, such as what we call shielded twisted pair, as well as the network we see are connected in this way, which can maximize the elimination of the signal cable itself magnetic field, for weak signal transmission has a great advantage.
             
            2. The signal cable is wound on the side of the magnetic ring in the form of a transformer coil by using a magnetic ring. This method can also effectively reduce the interference and convert the interference magnetic field into heat energy.
             
            Communication isolation module and other protective devices, and some PLC 232,485 communication is also interfered with, then we use communication isolation module, which is similar to the signal isolator, using electronic devices to convert the signal. For the complex factory status quo, we must reasonably analyze the possible interference reasons, find out the interference sources, and give a reasonable solution。
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