Engineering Quantity conversion method I often see questions raised by netizens on the forum, and I would like to make a special topic here for your reference. 1. Basic Concepts We live in a material world. All things in the world contain both chemical and physical properties. We understand and express the physical and Physical Properties of things through the apparent nature of things. These apparent properties are the physical quantities expressed in mathematical languages, such as quality, temperature, speed, pressure, voltage, and current. They are called engineering quantities in the field of self-control. This statement is intuitive and easy to understand. Before the emergence of electric sensing technology, traditional measuring instruments can directly display the measured physical quantity, including mechanical electric meters. 2. Standard Signal In the age of electric sensors, central control becomes possible, which requires remote transmission of detection signals. However, the direct transmission of complex physical quantity signals will greatly reduce the applicability of the instrument. In addition, most sensors are weak-signal models, and long-distance transmission is prone to attenuation and interference problems. Therefore, a secondary transmitter and a standard electrical transmission signal appear. The secondary transmitter is used to enlarge the sensor signal to an electrical signal that complies with industrial transmission standards, such as 0-5 V, 0-10 V, or 4-20mA (the most used is 4-20mA ). The transmitter can accurately correspond the standard signal to the detected range of physical quantity through zero point migration and gain adjustment of the amplifier circuit, such as 0-100 ℃ or-10-100 ℃. This is a mathematical transformation of physical quantities using hardware circuits. The instrument in the central control room will drive these electrical signals to the mechanical voltmeter and current meter to display the measured physical quantity. For different range, you only need to replace the dial behind the pointer. Changing the dial will not affect the fundamental nature of the instrument, which will bring unlimited benefits to the standardization, versatility and large-scale production of the instrument. 3. Digital Instruments In the digital age, the dot matrix display table has become a more intuitive and accurate digital display mode. In digital instruments, this display method is actually a pure mathematical method for the inverse conversion of standard signals, becoming a regular expression of physical quantities. This transformation relies on software for mathematical operations. These operations may be linear equations, or non-linear equations, which are easy for computers. 4. Mathematical Problems in signal transformation The signal transformation must go through the following process: physical quantity-sensor signal-standard electrical signal-A/D conversion-numerical display. Declaration: For simplicity, we will discuss linear signal transformation here. It also skips the signal transformation process of the sensor. Assume that the physical quantity is A, the range is A0-Am, real-time physical quantity is X; the standard electrical signal is B0-Bm, real-time electrical signal is Y; A/D conversion value is C0-Cm, real-time numerical value is Z. Therefore, B0 corresponds to A0, BM corresponds to AM, y corresponds to X, and y = f (x ). Because of the linear relationship, the equation is Y = (Bm-B0) * (X-A0)/(Am-A0) + B0. Because of the linear relationship, the mathematical equation z = f (x) After A/D conversion can be expressed as Z = (Cm-C0) * (X-A0)/(Am-A0) + C0. Then it is easy to obtain the inverse transformation of the mathematical equation x = (Am-A0) * (Z-C0)/(Cm-C0) + a0. The X calculated in the equation can be directly expressed as the detected physical quantity on the display. 5. Calculation Method of inverse transformation in PLC Taking S7-200 and 4-20mA as an example, after A/D conversion, the obtained values are 6400-32000, and C0 = 6400, Cm = 32000. So, x = (Am-A0) * (Z-6400)/(32000-6400) + a0. For example, a temperature sensor and transmitter detect-10-60 ℃, expressed as X = 70*(Z-6400)/25600-10 with the above equation. After the calculation of the PLC mathematical operation instruction, the HMI can be read from the result register and displayed as the engineering quantity directly. With the same principle, we can input engineering quantities on the HMI and convert them from software to standardized values used by the control system. In the S7-200, the calculation result of (Z-6400)/25600 is very important. This is a real number ranging from 0 to 1.0 (100%) and can be directly sent to the detection value input of the PID command (not the command wizard. The PID command output is also a real number ranging from 0 to 6400. Through the previous computation-type Inverse Computation, it can be converted to 32000-20mA, and sent to Port D/A to 4-for output.
