Difference between the hottest safety PLC and ordi

2022-10-14
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The difference between safety PLC and ordinary PLC

with the increasing maturity of industrial automation, modern chemical plants have abandoned the original simple equipment, single function and lack of safety. Therefore, in the industrial automation field we are engaged in, emergency stop buttons, safety door locks, and safety systems are increasingly applied to our automated production equipment and production lines. Their use adds safety assurance to the originally dangerous equipment. It enables us to monitor our work process safely, which not only enables the on-site operators to have a better safety production environment, but also contributes to the efficient production and safety management of the enterprise

as the most important part of the safety family, safety PLC has been recognized by more and more people, but in the process of use, there are still many users confused, why a set of PLC similar to the previous use is crowned with safety PLC, and what are the differences between safety PLC and ordinary PLC? I would like to share with you here

as we all know, we must remember three words about the concept of safety design: 1 Redundancy; 2. Differences; 3. Self test

only products designed with the above three safety concepts can be considered as safety products, while ordinary PLC products do not have safety design. Then let's take a look at how the safety PLC realizes these three concepts through design

1. Redundancy

there are one or more CPUs in ordinary PLC, but the program usually carries out one processing. The function of multiple CPUs is to share the logical operation, arithmetic operation and communication functions in the program, that is, collaborative processing

there are at least two or more CPUs in the safety PLC. The function of the two CPUs is to execute the same program once respectively, and then compare the demerits together. If the results are consistent, they will be output. If they are inconsistent, the scope of application will select the safe results to be output and Honeywell will announce them (no output or shutdown in the general sense)

therefore, only CPUs with redundant design can be called safety PLC

in addition, the detection of CPU in safety PLC includes clock detection, monitoring clock, sequence check and memory check due to the research results of nano precision machinery, modern chemistry research results at molecular level and biological research results at gene level

clock measurement: in the processor circuit, two different oscillators cross check their behavior, and each processor uses one clock to check whether the other is running. If it is detected that the other party is not running in a certain cycle, the CPU will enter a safe state. The firmware checks the accuracy of both oscillators every second

monitoring clock: the monitoring clock of one hardware and one firmware checks the activity of PLC and the execution time of executing user logic. This is the same as the conventional PLC system

sequence check: sequence check monitors the execution of different parts of the CPU operating system

memory check: all static memory areas, including flash memory and ram, are detected with cyclic redundancy code (CRC), and double code execution. The dynamic memory area is protected by double codes and detected periodically. At cold start, these tests are reinitialized

from the above analysis, it can be seen that the diagnosis and detection of safety PLC are much more than that of conventional PLC, so the design of hardware and software is relatively more complex. Of course, the scope of detection and diagnosis is also broader and more detailed

2. Dissimilarity

Security PLCs usually have two processors, which are usually provided by two different manufacturers, such as Motorola and Intel, for decoding and execution at the same time. This difference provides the following advantages of failure detection:

1 The two executable codes are generated independently, and the difference of compilation makes it easy to detect system failure when the code is generated

2. The two generated codes are executed by different processors, so the CPU can detect system failure and random failure of PLC during code execution

3. Two independent memory areas are used for two processors, so the CPU can detect the random failure of ram, which cannot be detected during all ram checks in each scan cycle

3. Self detection

self detection of safety PLC is reflected in all aspects, including self detection of CPU processing, self detection of power monitoring, and self detection of circuit board status at safety input and output points

here we introduce how the design of safety input and output reflects the safety concept of self-test

safety digital input

the yellow part is the unique circuit design of the safety input point, and the ordinary input point does not have it

internal diagnosis: each input channel uses a common input circuit and two independent acquisition links, and each microprocessor drives a digital input serializer (DIS) to sample the input information. In addition, the microprocessor also drives a digital input restorer (did), and then drives the diagnostic function block to diagnose, so as to realize the synchronous comparison between the restored data and the input data

input channel error detection: digital input monitors the power supply on the site side, and uses external wiring to detect the leakage current. The minimum leakage current is 1mA. If there is no leakage current, it means that the external circuit has an open circuit fault. In the case of dry contact, connect a 10k Ω pull-up resistor at both ends of the contact in parallel for the disconnection detection of external lines. Each input circuit is equipped with a switch, which is periodically forced to 1 or 0 to detect whether the circuit is healthy. Each input circuit is detected independently. If a problem is found, diagnose position 1 and declare that the channel is in an unhealthy state

safe digital output

the yellow part is the unique circuit design of the safe input point, and the ordinary input point does not have it

internal diagnosis: in order to check whether the switch can be opened and closed, a pulse test should be carried out in the output module (insert a periodic diagnostic cycle in the internal circuit of the module)

the diagnosis sequence includes:

change the switch command. This time is very short and will not affect the actuator. The maximum time is no more than 1ms; Verify the test results and restore the correct switch command

power monitoring: each output circuit includes two switches in series, which are controlled by two processors respectively. The first microprocessor uses a digital output restorer (DoD) to drive its switch, while the second microprocessor drives its switch after the restorer. In each cycle, the midpoint voltage of the two microprocessor systems should be compared with a threshold value, and then their IFS should be exchanged to evaluate the state of the midpoint and diagnose the state of the switch. If an error behavior is detected in a channel, it will be shut down immediately, and the diagnostic bit will be set to notify the CPU, and the CPU will have fault information

to sum up, I hope you have a further understanding of the difference between safety PLC and ordinary PLC, and understand the three important concepts of safety product design through the above introduction. When using safety related products in the future, we can understand these safety products in combination with the content shared today. Through their design, they are different from standard control products

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