How to choose the most appropriate PLC

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How to choose and choose the most appropriate PLC

Abstract: every automation manufacturer and agent will follow the progress of technology and changes in the market to adapt to the release of new PLC when most domestic instruments are in the low-grade homogeneous competition state. Every year, new automation engineers enter the position of technical manager. Every year, junior engineers from professional schools enter the field of automation engineering design and research. Therefore, how to choose the most appropriate PLC is always the most basic and important topic. The loss caused by the failure of selection and selection is sometimes fatal to the damage of engineering projects. Although the selection and purchase of PLC discussed in this paper is based on Delta company, the quotation principle is suitable for general selection and design

key words: Technical Manager of PLC selection and selection

1 introduction

in the era when PLC is increasingly becoming the basic technology platform of factory automation, how to select a suitable PLC to achieve real needs and meet economic benefits is an important topic. The characteristics of PLC itself are: small size, high function, strong performance, simple operation, simple program design, flexible module expansion, easy connection, etc. According to the above characteristics, select the PLC system that meets the functional requirements and economic benefits. Although this paper is based on Delta series PLC, the quotation principle is suitable for general selection and design

2 reference conditions for selecting PLC

the functions of PLC produced or represented by various manufacturers are similar, and the differences are not significant. Therefore, the size of the controlled system, future scalability, and budget are considered. In terms of selection, we can first make a preliminary decision on the manufacturer's PLC directory, and the key items that may be considered are as follows:

basic capacity features provided by 2.1

(1) voltage range of input signal

(2) instruction functions provided: basic operation instructions, extensible application instructions, data processing instructions, arithmetic instructions, PID instructions, etc

(3) the instruction processing speed (i.e. execution speed) of PLC is usually calculated in USEC

(4) the internal memory capacity is usually calculated in K bytes

(5) timer and counter functions

(6) capacity of data buffer

(7) expansion of i/o points

(8) other special functions: such as floating-point operation, perpetual calendar and system clock, high-speed computing ability, etc

2.2 peripheral expandability

(L) convenience of connecting with personal computer

(2) ease of compiling software with personal computer

(3) supply of d/a or a/d conversion module

(4) PLC networking function

(5) control the functions of motion devices, such as stepping motor, servo motor and positioning controller

2.3 the power specifications of PLC

most of them use AC as the power supply, and the following matters should be paid attention to:

(1) the range of voltage

(2) how long is the allowable instantaneous time of power failure, and the PLC is still unaffected and continues to operate

(3) protective measures of power supply, and the maximum allowable voltage and current

(4) what is the power consumption of the whole machine? V, a or w

2.4 precautions for PLC input specification

(L) maximum input voltage

(2) input the maximum allowable current

(3) the minimum current when the input is 0n

(4) the maximum current when the input is off

(5) insulation type of circuit

2.5 precautions for PLC output specification

(L) type and value of external voltage

(2) is resistive load or inductive load

(3) the maximum leakage current value when the output is open circuit

(4) response time of output terminal at 0n and 0ff

(5) circuit insulation mode adopted by output circuit

2.6 environmental conditions

(1) temperature range used by PLC

(2) shock resistance

(3) impact resistance

(4) noise resistance

(5) ability to withstand electric shock

(6) the whole insulation resistance of the system

(7) setting of grounding

(8) the limitation of working environment is mainly IP protection level

3 pre use inspection and daily maintenance

3.1 pre use inspection

(L) check the power wiring (power off). Wrong wiring of the power terminal, short circuit between the DC input line and the power line, or a short circuit of the output line, etc. will seriously damage the PLC. Check the wiring of power supply and grounding as well as input/output wiring before connecting the power supply

(2) procedure and inspection: power supply 0n, dvp-plc, stop. After using the peripheral device to write the program to the host, read out the written program and use the program check function (please refer to HPP Chinese operation manual) to check whether the loop and grammar of the program are correct

(3) operation and test: power on, plc-run. If the run input terminal and com terminal are connected, or the run switch is turned to the 0n position, and it is in the operation mode. During the operation, the set value and temporary value of timer (T), counter C and buffer (d) can be changed by HPP, and the output contact can be forced to perform on/off action. Using HPP, the on/off action of each relay and the setting of T, C and D can be called out, from the override value and current value of single function to multiple control modes

(4) led indicator detection is abnormal. When the power light on the power indicator is normal, the LED turns on green when the host is powered on. If the indicator does not turn on when the host is powered on. If the +24v wire is removed, the indicator will turn on, indicating that the DC load of the PLC is too large. At this time, please prepare another DC24V power supply

(5) battery voltage indicator (batt, low light):

· when the battery voltage is insufficient, this indicator light is on, and the special auxiliary relay is on O. when this light starts to light up, about a month later, the program (when ram is used) automatically disappears, and the memory area of power failure memory based on the battery is all set to 0

· when this light is on, please exchange a new battery (it is recommended to replace it every five years)

· the battery can be replaced when the cover is removed

· if the special auxiliary relay is driven, even if the battery voltage has been reduced batt, the VLED will not be lit, but the special auxiliary relay will be set to on

· if the battery voltage drops, the data in the data buffer used to set timers, counters or other purposes will be unreliable

(6) program error indication (prog.e light flashing):

· when the timer/counter forgets to set and uses this contact, the program circuit is unreasonable, the battery power is insufficient, or the program content changes due to external noise interference, there is no place for the operation of the experimental machine to flash more professionally than us. At this time, please check whether the program is correct, whether there is a strong noise interference source, and whether the battery voltage is insufficient

· when an error occurs, you can press the [other] key twice by HPP to display the debug number, and you can also display the value by d1004. Please refer to the function description for the debugging number

· CPU error indication: the error light is on

· when the CPU is out of control due to foreign noise or foreign object intrusion, or the program execution time exceeds 0.1s, CPU, error will light up. Under the condition of PLC power supply 0n, plugging in and out the memory cartridge will also make the cpu-led light up

· turn off the power for a while, then turn on the power, and then make the PLC operate. If the PLC can operate normally at this time, please check the possible noise source or whether there is foreign matter intrusion inside the PLC

· PLC grounding wire shall be at least 2 square meters (awg14) and the length of grounding wire shall be shortened as far as possible, It is recommended to use class grounding (the grounding impedance is lower than 100. If the PLC function is abnormal due to incorrect grounding, please remove the grounding wire from the terminal. At this time, it is necessary to connect the grounding terminal of the host and the expansion machine together.

· when the grounding wire is removed from the PLC, if the error indication changes from cpu-led to prog.e, please check whether the execution cycle time of the program is too long (more than 100ms)? The execution cycle time is stored in the data buffer d1012

(7) input signal display:

· the on/off state of the input signal is displayed by the input indicator light, and can also be called out for monitoring by the online screen of HPP or computer

· when the on grid current of the input switch is too large, the input switch will have poor contact. Please pay attention

· when the input switch is connected with the indicator light for input display, please note that when the switch is off, the PLC will still cause malfunction because of the residual current of the indicator light, which will cause the PLC input point to not be off

· for external input points on/off that are shorter than the program execution time, the CPU cannot grasp them effectively

· when the DC24V power supply end on the PLC host or i/0 expansion base is overloaded or short circuited, the internal protection circuit will automatically reduce the voltage and cause PLC shutdown. In this case, please remove all the wiring of 24+ and then check the wiring

· never apply voltage to the input terminal, especially AC 110v/220v will cause the input circuit to burn

· the input terminal block of the host and i/o expansion base can be freely inserted/unplugged

(8) output signal display. If the output load fails to act on/off according to the LED status, the possible reasons are:

· the output contact is damaged or poor due to overload or short circuit

· the terminal block of the host or expander is removable, which may be in poor contact. Re lock it

· there is a problem with the external power line

3.2 daily maintenance and inspection

(1) the service life of lithium battery is about 5 years and needs to be replaced

(2) if the output contact current is large or O n/off is frequent, pay attention to the contact life, check and replace

(3) when PLC is used on vibrating machinery, pay attention to the looseness of terminals

(4) pay attention to the peripheral temperature (o-55 Square) humidity (%rh does not condense) and dust of PLC

(5) the lithium battery voltage is too low, and the w light on the panel will be on, although the program can be maintained for more than one month

(6) battery replacement method steps: cut off the power supply, remove the upper cover plate, unplug the old battery, plug in the new battery within 30 seconds, fix the new battery and cover the upper cover plate

4 conclusion

p the price of recycled waste plastics is much cheaper than the rising price of plastic raw materials in recent years. The selection and selection of LC is a multi-objective comprehensive optimization process, including both engineering and technical problems and economic cost problems. Therefore, how to choose the most appropriate PLC is often difficult for engineering managers to grasp. The loss caused by the failure of selection is fatal to the project. For example, does the instruction speed meet the real-time performance of the system? For example, does the data structure meet the project requirements? For example, does the capacity of the data buffer meet the length requirements of the user program? Some technical details in this paper are inevitably limited to specific PLC brand characteristics, but almost all PLCs have corresponding function definitions, which can be transformed into universally applicable principles through appropriate comparison. (end)

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