Four essential differences between electrical control and PLCs

Thread Content

The star-delta starting circuit for motors is certainly familiar to anyone working in electrical and industrial control fields. Generally, such circuits are built using time relays, intermediate relays, and AC contactors. Four essential differences between electrical control and PLCs: yunrun.com.cn/tech/2550.html. The following image shows a motor star-delta starting control circuit built using a PLC: http://yunrun.com.cn/upload/201906/17/201906172358125713.png. Let’s take a look at its main program as well: http://yunrun.com.cn/upload/201906/18/201906180006395068.png. In the above image, TON is a timing module for delaying power activation within the PLC. In the upper left corner of each module is the digital input, while in the lower left corner is the delay time; in this diagram, it is 10 milliseconds ； The top right shows the corresponding register, while the bottom right indicates termination by time delay. There are many digital input terminals in the above diagram, such as the circuit breaker status digital signal, control button digital signals, AC contactor status digital signals, and so on. The reason for the delay is to eliminate jitter when digital inputs are applied. The following image shows the control program: http://yunrun.com.cn/upload/201906/18/201906180014199012.png The working principle of the program shown in the image will not be explained in this article. From the above diagrams, we can see that the control of a PLC is very different from that of a circuit built with relays. 1. Everything that a relay control circuit can do can also be achieved by a PLC ； Conversely, functions that PLCs can perform may not necessarily be achievable with relay control circuits. For example, in the contact debouncing routine shown in Figure 2, although we could also use time relays to achieve this, dealing with such a large number of digital switches would require a large number of time relays, resulting in high costs and low reliability. This is the first advantage of PLCs: they offer many special functions. 2. Compared to relay circuits, PLCs offer more precise control functions and better performance. However, not many people can understand PLC programs, so it is difficult to troubleshoot PLCs when problems arise. 3. Compared to relay circuits, PLCs enable easy instant data acquisition as well as data exchange. Let’s take a look at the image below: http://yunrun.com.cn/upload/201906/18/201906180014354307.png. The image shows the SOE program for the time tags of digital signal changes and faults. The measurement accuracy of the SOE program is 1 millisecond. It is difficult to implement SOE functions using relays. Take a look at the following image: http://yunrun.com.cn/upload/201906/18/201906180023072165.png Functions similar to those in the above image cannot be achieved by relays. 4. Compared to relay circuits, PLCs are of course more expensive. However, its high reliability and good stability are also incomparable to those of relay circuits. That’s about it. Another point is that some people believe microcontrollers will gradually replace PLCs in certain fields, but this view is incorrect. A PLC is essentially a microcontroller-based application system, but its technical specifications are many times higher than those of a microcontroller. What we see in industrial control environments are various types of PLCs, rather than specially developed microcontrollers. Why is that? The reason is that PLCs have extremely strong interference resistance, that is, EMC resistance to electromagnetic interference. The so-called EMC, or electromagnetic interference, has two aspects: first, electrical products should have a high capacity to resist electromagnetic interference; second, such products should not generate strong electromagnetic interference that could affect the surrounding electromagnetic environment. In electromagnetic disturbance testing, the most challenging ones are static electric disturbance and EFT electrical fast transient pulse group electromagnetic disturbance. Below is an article from Baidu titled “EFT Electrical Fast Transient Pulse Group Testing”, which explains the testing related to electrical fast transient pulse group electromagnetic interference: Excerpts from the article “EFT Electrical Fast Transient Pulse Group Testing”: Most electronic products need to pass standard tests such as those for Electrical Fast Transient Pulse Groups (EFT) and Electrostatic Discharge (ESD). EFT and ESD are two typical types of burst disturbances; the peak voltage of a single pulse in an EFT signal can reach up to 4 kV, with a rise time of 5 ns. The peak voltage of the ESD signal during contact discharge testing can reach up to 8 kV, with a rise time of less than 1 ns. Both types of sudden disturbances are characterized by suddenness, high voltage, and wide frequency bands. Electrical fast transient pulses are transient disturbances that occur at a disconnection point, caused by inductive loads such as relays and contactors (which generate conductive interference), or by radiation interference resulting from the switching of high-voltage switches; these disturbances arise due to insulation breakdown in the switch contacts or contact bounce. When an inductive load is switched on and off repeatedly, the pulse train will appear again multiple times with corresponding time intervals. This type of transient disturbance has low energy and generally does not cause damage to equipment; however, due to its wide spectral distribution, it can affect the reliable operation of electronic and electrical devices. The purpose of the electrical fast transient burst test is to evaluate the performance of electronic and electrical equipment under the influence of such transient disturbances. The repeated fast transient test is a test in which a pulse train composed of many fast transient pulses is coupled to the power, signal, and control ports of electrical and electronic devices. The key aspects of the experiment are a short rise time of the transient, repetition rate, and low energy. This type of test is a pulse train test consisting of many rapid transient pulses, which is coupled to the power supply lines, control lines, and signal lines. This waveform is not the actual waveform that occurs when an inductive load is disconnected (the amplitude of the interference generated when an inductive load is disconnected increases), and the waveform used in the experiment makes the testing conditions more stringent. A electrical fast pulse train consists of consecutive pulse sequences separated by 300 ms, with each sequence lasting 15 ms. It is made up of several unpolarized single pulses; the rise time of each pulse is 5 ns, its duration is 50 ns, and the repetition frequency is 2.5 KHz for the 4 KV test level or 5 KHz for other levels. According to the Fourier transform, its spectrum consists of discrete spectral lines ranging from 5K to 100M, with the distance between each line corresponding to the repetition frequency of the pulse. Yes, interference is applied to the coupling/decoupling network selected for the power supply terminal, with a coupling capacitor of 33nF. Interference is introduced to the I/O signals, data, and control ports using dedicated capacitive coupling clips, with an equivalent coupling capacitance of approximately 50–200 pF. The former involves connecting the reference ground of the electronic device to the ground of the instrument, and then applying a high voltage to the casing of the electronic device to determine its resistance to such voltage. Many times, even if the power supply filtering capacitor is slightly too high, the electronic device will arc and explode on the spot. As for the electrical fast pulse group test, industrial products must pass the test using three levels of pulse groups. If it’s a regular microcontroller system, it will get damaged on the spot, while the PLC remains unaffected. To improve the anti-interference capability of the PLC, the basic program of the PLC operating system follows the principle of executing one instruction at a time, without waiting in place. In this way, it is difficult for the basic program of the PLC to enter an infinite loop, thereby improving the PLC’s resistance to interference. As for ladder diagrams and modular programming languages, they are both standard advanced programming languages under IEC61131, and are independent of the basic language of the PLC operating system. The above are the basic performance requirements that a PLC must meet; they have nothing to do with programmers! As for using a microcontroller to replace a PLC, it is nothing but a naive idea and misunderstanding on the part of laypeople. Author: Zhang Baifan