1B30023H01模块备件

1B30023H01

从Ovation系统到电厂的数字输出信号是机电的
或与控制器电隔离的半导体输出。
从Ovation系统到电厂的模拟输出信号为电压或
电流输出也与控制器电隔离。
2-6.4.噪声敏感电路噪声抑制
所有传输、低电平模拟和数字电路必须假设为
对噪声敏感，并且需要针对噪声的特殊保护。来自
过程传感器（热电偶、RTD等）特别敏感
噪音。噪声可以通过三种方式耦合到这些敏感电路中：
•通过分布式电容进行静电耦合。
•通过分布式电感进行电磁耦合。
•导电耦合，例如共享公共回路的电路。
这些噪声敏感电路的噪声抑制涉及以下一项或多项：
以下是基本措施：
•噪声产生电路和噪声敏感电路之间的物理分离。
•用于工厂内信号连接的双绞线。
•正确接地，特别是避免电缆屏蔽的多重接地。
噪声小化技术
2/03 2-7 R3-1150（第3版）
艾默生过程管理专有2C类
•适当的屏蔽，尤其是电缆屏蔽。
•浪涌保护。
物理电路分离
电路分离是一种简单有效的静电和
电磁场感应噪声控制。这是因为静电和
电磁场随着距离的增加而衰减，产生较低的振幅
并且保持良好的信噪比。
双绞线
双绞线通过消除以下电路回路来抑制噪声：
对杂散电磁场敏感。因此，建议所有
模拟信号电路连接应采用双绞线。数字的
信号连接应在同一电缆中带有一组回路（或公共）
作为信号线。
在噪声异常的数字电路中也建议使用双绞线
环境是存在的。信号线及其返回导体的扭曲变为：
随着两者的长度越来越大，以及距离
来自噪声源的噪声变得更少。双绞线或小型电缆（小于1/2英寸
外导体圆直径），每英尺至少一到两次扭转的扭转率为
推荐。
正确接地和屏蔽
适当的接地和屏蔽会导致噪声感应电流流入
屏蔽，并从屏蔽接地，而不是在相应的信号
导体。屏蔽本身有助于避免电容耦合噪声。这个
屏蔽的一功能是减少内部导体的有效电容
屏蔽外部导体。为了实现这一点，屏蔽应是连续的
尽可能配备一根“加蔽线”，以确保单点接地。
只有在以下情况下，导体和相应的回路才可在屏蔽内分组：
它们之间的电容耦合是可以接受的。避免

1B30023H01

1B30023H01

Digital output signals from the Ovation System to the plant are electromechanical or semiconductor outputs which are electrically isolated from the Controller. Analog output signals from the Ovation system to the plant are either voltage or current outputs which are also electrically isolated from the Controller. 2-6.4. Noise-Sensitive Circuit Noise Rejection All transmitting, low-level analog and digital circuits must be assumed to be noise-sensitive and to require special protection against noise. Field signals from process transducers (thermocouples, RTDs, and so forth) are especially susceptible to noise. Noise can be coupled into these sensitive circuits in three ways: • Electrostatic coupling via distributed capacitances. • Electromagnetic coupling via distributed inductances. • Conductive coupling, such as circuits sharing a common return. Noise suppression for these noise sensitive circuits involves one or more of the following basic measures: • Physical separation between noise-producing and noise-sensitive circuits. • Twisted-pair wiring for signal connection within plant. • Proper grounding, especially avoiding multiple grounding of cable shields. Noise Minimization Techniques 2/03 2-7 R3-1150 (Rev 3) Emerson Process Management Proprietary Class 2C • Proper shielding, especially cable shielding. • Surge protection. Physical Circuit Separation Circuit separation is a simple and effective means of electrostatic and electromagnetic field induced noise control. This is because electrostatic and electromagnetic fields decay with increasing distance, producing lower amplitude noise and maintaining a good signal-to-noise ratio. Twisted-pair Wiring Twisted-pair wiring suppresses noise by acting to eliminate circuit loops which are sensitive to stray electromagnetic fields. For this reason, it is recommended that all analog signal circuit connections should be made with twisted-pair wire. Digital signal connections should carry a group return (or common) wired in the same cable as the signal wires. Twisted pairs are also recommended in digital circuits where unusually noisy environments exist. Twisting of the signal wire and its return conductor becomes increasingly important as the length of the two becomes greater, and as the distance from noise sources becomes less. In twisted pairs or small cables (less than 1/2 inch outer conductor circle diameter), a twist rate of at least one to two twists per foot is recommended. Proper Grounding and Shielding Proper grounding, along with shielding, causes noise-induced currents to flow in the shield, and from the shield to ground, rather than in the corresponding signal conductors. Shielding itself is useful in avoiding capacitively coupled noise. The shield’s sole function is to decrease effective capacitance from conductors inside the shield to conductors outside. To accomplish this, the shield should be as continuous as possible and equipped with a “drain wire” for secure single-point grounding. Conductors and corresponding returns may be grouped within a shield only if capacitive coupling between them is acceptable. Avoid