Sensors and Transmitters
（传感器和变送器）
Sensors and transmitters perform the measurements (M) operation of control system.The sensor produces a phenomenon, mechanical, or the like related to the proce variable that it measures.The transmitter in turn converts this phenomenon into a signal that can be transmitted The signal, therefore, is related to the proce variable.
传感器和变送器在控制系统中起着测量的作用。传感器产生的现象、机制或其他与被测量的变量有关。变送器将传感器所产生的量转换成可以传输的信号。该信号因此与过程变量有关。
There are three important terms related to the sensor/transmitter combination.The range of the instrument is given by the low and high values of proce variable that is measured.That is, consider a preure sensor/transmitter that has been calibrated to measure a proce preure between the value of 20 psig and 50 psig.We say that the range of the sensor/transmitter combination is 20~50 psig.The span of the instrument is the difference between the high and low values of the range.For the mentioned preure instrument the span is 30 psi.To summarize, we must specify a low and a high value to
define the range of an instrument.That is two number must be given.The span of the instrument is the difference between the two values.Finally, the low value of range is referred to as the aero of the instrument.The value does not have to be zero in order to be called the zero of the instrument.For the above example, the “zero” of the instrument is 20 psig.
传感器/变送器有三个重要的术语。仪表的量程是所测量的过程变量的最小值和最大值。例如，考虑一个压力传感/变送器，已经校验为压力测量值为20到50psig。仪表的测量范围是最大值和最小值的差。因此，所论的压力仪表的测量范围为30psi。总结一下，我们必须定义仪表量程的最大值和最小值。这两个值必须给出。仪表的测量范围是这两个值的差。最后，仪表测量的最小值称为仪表的零点。该虽然称为零点，但不一定要等于零。例如，我们所讨论的仪表的零点是20psig。
This expreion shows that the gain is not constant but rather a function of flow.The greater the flow is , the greater the gain.
该表达式说明，增益不是常量，而是一个流量的函数。流量越大，增益也越大。
So the actual gain varies from aero to twice the nominal gain.
因此，实际的增益取值范围是从0到标称增益的2倍。
This fact result in a nonlinearly in flow control system.Nowadays most manufactures offer differential preure transmitters with built-in square root extractors yielding a linear transmitter.
该事实导致控制系统的非线性。，今天，大多数的制造商都能提供嵌入式的开方器，因而得到线性的变送器。
The dynamic response of most sensor/transmitter is much faster than the that of proce.Consequently, their time constants and dead time can often be considered negligible and thus, their transfer function is given by a pure gain.However, when the dynamics must be considered, it is usual practice to represent the transfer function of the instrument by first-order or second-order system.
传感器/变送器的动态响应要比过程的特性快很多。因此，它们的时间常数和死区时间都可以忽略不计，所以传递函数可以是一个纯增益。然而，当仪表的动态特性必须要考虑时，往往将其传递函数看成一个一阶或者二阶的系统。
