1, basic concepts

We are living in a material world. All substances contain chemical and physical properties, we are the apparent nature of the material to understand and own property of matter and motion characteristics. These apparent nature is what we used to say mass, temperature, velocity, pressure, voltage, current, the physical quantities expressed in mathematical language, in the area referred to as analog control. The advantage of this representation is intuitive and easy to understand. Before the advent of electronic sensing technology, traditional instrumentation can be directly measured physical quantities, which also includes mechanical electric instruments.

2, the standard signal

In the era of electric sensors, central control becomes possible, it needs to detect signals over long distances. But complicated physical signals directly will greatly reduce the applicability of the instrument. And most of the sensors is a weak signal, telematics is prone to decay and interference problems. So there was a second transmitter and standard electrical signals. Secondary transmitter function is to transfer sensor signal amplification to be consistent with industry standards for electrical signals, such as 0-5V, 0-10V or 4-20mA (one of the most used is 4-20mA). Transmitter through the zero point of the amplifier circuit migration as well as gain adjustment, can accurately correspond to the physical quantity of standard signal is detected, such as 0-100 ° c or-10-100 ° c, and so on. This is hardware the physical quantities are mathematical transformations. Central control room of these signal–driven mechanical voltage instrument gauge, ammeter will show the physical quantity being measured. For different measuring range, just replace the pointer behind the dial can be. Replace the dial does not affect the fundamental nature of the instrument, that instrument standardization and commonality benefits immeasurably and large-scale production.

3, digital meter

To the digital age, analog display meter becomes more intuitive, more accurate digital display. Digital meter, this way in fact is purely mathematical way to transform the standard signal, become accustomed to physical expression. This transformation is to rely on software to make mathematical operations. These operations may be linear equation may be nonlinear equations, computers for these operations is now as easy as pie.

4 mathematical problems, signal conversion

Signal transformation requires the following procedure:–physical quantity sensor–standard electrical signal-A/D convert–numerical display.

Disclaimer: for the sake of simplicity, we are here talking about linear signal transformation. Also bypass the sensor signal process.

Assume that physical quantity as a range is A0-Am, real-time physical quantity x standard electrical signal is B0-Bm, electrical signals in real-time to Y;A/D convert numerical values for the C0-Cm, real time value for z.

, The B0 is corresponding to the A0,Bm corresponding to Am,Y corresponding to x, and Y=f (x). Because it is a linear relationship between the obtained equation for Y= (Bm-B0) * (X-A0)/(Am-A0) +B0. And because it is a linear relationship, after a/d conversion of mathematical equations Z=f (x) can be expressed as z = (Cm-C0) * (X-A0)/(Am-A0) +C0. Then it is easy to draw inverse mathematical equation is x = (Am-A0) * (Z-C0)/(Cm-C0) +A0. Calculated by the equation x can be expressed directly on the monitor for detecting physical quantity.

5, the method for calculation of the inverse transform in PLC

A case study of S7-200 and 4-20mA, a/d conversion, the values we get are 6400-32000, and C0=6400,Cm=32000. So, x = (Am-A0) * (Z-6400)/(32000-6400) +A0.

For example, detection of a temperature sensor and transmitter are–10-60, using the above equations are expressed as X=70* (Z-6400)/25600-10. After math instruction computing by the PLC, HMI can read from the result registers and shows directly to analogue.

Using the same principle, we can input analog at HMI, and then converted to a standardized numerical control system used by the software.

S7-200 (Z-6400)/25600 calculations are very important values. This is a 0-1.0 (100%) of real numbers, can be directly sent to the PID command (not the command Wizard) measurement input terminal. PID command output to the real number is 0-1.0, through the front‘s new formula calculations, can be converted to 6400-32000, send to 4-20mA output D/A port.

Above is about the basic methods in PLC analog conversion, program for people, things vary. But change is not out of the compromise.