Fixed PLC includes CPU board, I/O board, display panel, memory block, power supply, etc. These elements are combined into an inseparable whole.
PLC is a programmable memory, which is used to store programs, execute user-oriented instructions such as logical operation, sequence control, timing, counting and arithmetic operation, and control various types of machinery or production processes through digital or analog input/output. Understanding the principle of PLC is the basis for you to engage in related work.
PLC manufacturer
PLC manufacturers can be described as a hundred flowers blossom, each with its own characteristics. For people who are in contact for the first time, it can be said that there is no way to start, and they can only find it slowly in their future work and keep learning. The following table lists most PLC manufacturers and their main products in the market in detail.
my humble opinion
1, it is very important to know the manufacturer brand of PLC when troubleshooting, and it will be faster to find and solve problems according to brand characteristics.
2. At present, the small PLCs with the highest domestic market share are Nissan PLC brands: Mitsubishi, Omron, European and American series: Siemens, etc. Four brands of large-scale PLC: Siemens, Schneider, AB (Rockwell) and ge. Medium-sized PLC: Siemens, Schneider, Omron, Mitsubishi. Basic knowledge of Mitsubishi PLC, general domestic PLC can be.
How to maintain PLC?
1, CPU exception
When the CPU issues an abnormal alarm, all devices connected to the internal bus of the CPU unit should be checked. The specific method is to replace the units that may fail in turn, find out the failed units and deal with them accordingly.
2. Abnormal memory
When the memory is abnormal, if it is a problem with the program memory, the fault will reappear after reprogramming. This situation may be caused by noise interference, otherwise the memory should be replaced.
3. Abnormal input and output unit and abnormal expansion unit.
When this alarm occurs, first check the plug-in state and cable connection state of the connectors of the input/output unit and the expansion unit, and then replace the unit after determining the failed unit.
4. Don't execute the program
Generally speaking, the inspection can be carried out according to the steps of input-program execution-output.
(1) The input check is identified by the input LED indicator, or checked by an input monitor composed of a writer. When the input LED is not on, it can be preliminarily determined that the external input system is faulty, and then it can be checked with a multimeter. If the output voltage is abnormal, it can be determined that the input unit is faulty. When the LED lights up and the internal monitor does not display, it can be considered as a fault of the input unit, CPU unit or expansion unit.
(2) The program execution check is checked by the monitor on the writer. When the contact state of ladder diagram is inconsistent with the result, it is a program error (such as the double use of internal relay, etc.). ) or the operation part is faulty.
(3) The output inspection can be identified by the output LED indicator. When the operation result is correct and the output LED indicates an error, it can be considered as a fault of the CPU unit and the 1/0 interface unit. When the output LED is on and there is no output, it can be judged that the output unit is faulty or the external load system is faulty.
In addition, due to different PLC models, the connection mode between 1/0 and LED is also different (some are connected to the interface of 1/0 unit, and some are connected to 1/0 unit). Therefore, the fault range judged by LED is also different.
5. Some programs were not executed.
The inspection method is the same as the previous paragraph.
However, if you enter time counters, step controllers, etc. Too short, there will be no response failure. At this time, to check whether the input time is large enough, you can check according to the input time.
6, short-term power failure, the program content will disappear.
(1) At this time, in addition to the battery, the following inspections should be carried out.
(2) Check by repeatedly switching the power supply of PLC itself. In order to make the microprocessor start correctly, PLC is equipped with an initial reset point circuit and a program saving circuit when power is off. If this circuit is broken, you can't save the program. So it can be checked by the on-off of the power supply.
(3) If battery abnormal alarm still appears after battery replacement, it can be judged that it is caused by abnormal increase of leakage current of memory or external circuit.
(4) The power supply is always turned on and off synchronously with the machine system, so the noise influence generated by the machine system can be checked. Because the disconnection of the power supply often occurs at the same time as the operation of the machine system, it is mostly caused by the strong noise generated by the motor or winding.
7. You can't run at the dance.
Check whether the PROM is plugged in, and then determine whether the chip needs to be replaced.
8. After the power supply is restored to operation or reset, the action stops.
This fault can be considered as noise interference or poor internal contact of PLC. Generally, it is the noise caused by the decrease of small capacitor capacity in the circuit board or the poor performance of components. Tap the PLC body gently to check the cause of poor contact. Also check the plug-in status of cables and connectors.
9. Interference of frequency converter on PLC analog quantity
In the automatic control system, the application of frequency converter is more and more extensive, and the interference of frequency converter to PLC analog quantity is also prominent. The following examples illustrate the interference of frequency converter to PLC analog quantity and the solution to overcome this interference with signal isolation module.
Description of the phenomenon: The AO point in Siemens PLC sends out a 4-20mA current control signal, which is output to the Siemens inverter, and the inverter cannot be controlled to start.
Fault finding:
1, suspected analog output board problem, use multimeter to measure 4-20mA output signal, and the signal is normal.
2. At first, it was suspected that there was a problem with the control signal input end of the frequency converter, and a frequency converter of the same model was replaced, but the problem remained.
3. Use a hand-held signal transmitter as a 4-20mA output signal source, output a standard current signal to the frequency converter, and start the down converter, so that we can eliminate the faults of the analog output board and the frequency converter.
4. It is inferred that the interference signal of the frequency converter is conducted to the analog channel.
5. For verification, the signal isolation module TA30 12 is installed in the analog output channel of PLC. Input terminals 5 and 6 of TA30 12 are connected to analog output module, output terminals 1 2 are connected to frequency converter, and terminals 3 and 4 are connected to external 24VDC power supply. The frequency converter starts normally.
6. Based on this, it is concluded that the root of the problem lies in the interference of the inverter to the analog channel.
10 and troubleshooting of Mitsubishi PLC
How does Mitsubishi PLC investigate the cause of PLC failure according to the working conditions of LED indicators (power indicator, EPROR indicator, input indicator and output indicator)?
0 1 power indicator
The power indicator lamp is the power indicator lamp, which first supplies power to the basic unit of Mitsubishi plc. Under normal circumstances, the power indicator light will be on, and no adjustment is needed. If it doesn't light:
(1) Check whether there is any problem with the power supply wiring. On the other hand, when there is a driving sensor on the power supply, it is necessary to check whether there is a load short circuit or overcurrent and adjust it according to the actual situation.
(2) Check whether there are other conducted interferences or other abnormal conditions in the PLC, which leads to the fuse in the basic unit being blown. If so, the interference and abnormal conditions must be eliminated and replaced with corresponding good fuses.
02e pressure indicator lamp
EPROR indicator light, also known as error indicator light, is not lit under normal circumstances. EPROR indicator lamp fault indications are:
(1) The flashing of EPROR indicator light is caused by the following reasons:
(1) program syntax error (such as forgetting to set timer or counter constant, etc.). ).
(2) When the contents of the program memory are changed due to abnormal noise, conductive foreign matter, etc. If the running state of Mitsubishi plc is "Stop" and all outputs are turned off, first check whether there are any errors in the program, and then check whether there are conductive foreign objects mixed in and high-intensity noise sources.
(2) The reason why the 2)EPROR indicator lights up is as follows:
① In the first case, the CPU of Mitsubishi plc is disturbed by mixed conductive foreign matter or abnormal external noise, which makes the CPU out of control or the running cycle exceeds 200ms, resulting in WDT error and EPROR indicator light.
② Check whether the grounding of plc meets the requirements. If the [[EPROR]LED lights up and flashes during the inspection, please check the program.
③ If the EPROR indicator light is on all the time, check whether the running cycle of the program is too long (monitor D80 12 knows the maximum scanning time).
④ There is an internal fault in ④plc, please contact the technical service provider. If the running status of plc is "Stop" and all outputs are turned off, power-off reset can be performed. If the plc returns to normal, please check whether there are abnormal noise sources and conductive foreign objects mixed in.
03 input indicator light
Input LEDs usually indicate a fault:
(1) The input signal switch is turned off, but the circuit is turned on:
① The rated current capacity of the input switch is too large or it is prone to poor contact due to oil invasion and other reasons.
② When the input switch is connected in parallel with the LED lighting resistor, even if the input switch is turned off, the parallel circuit is still connected, and Mitsubishi FX series PLC can still input. The above reasons not only affect the input of OFF, but also affect the input of on.
(2) The input signal switch is in the on state, but the circuit cannot be connected:
① The light-emitting/light-receiving parts of input devices such as optical sensors are disturbed by the outside world (such as impurities, dust or damage, etc.). ), greatly reducing the sensitivity, resulting in the operation state that the loop cannot be formed.
② Different voltages applied to the input terminals will damage the input circuit. For example, it will cause Mitsubishi FX2N series plc to fail to receive ON and OFF inputs in a short time.
04 output indicator light
The output indicator indicates a fault: the load cannot be turned on or off. Reason:
(1) overload, load short circuit or impulse current of capacitive load, etc. , causing the relay output contact to get stuck.
② Poor contact surface of contact leads to poor contact.
Matters needing attention
1, working environment
Temperature PLC requires the ambient temperature to be 0-55℃, so it should not be placed under high heat value components during installation. The surrounding ventilation and heat dissipation space should be large enough, and there should be an interval of more than 3Omm between the basic unit and the expansion unit. The upper and lower parts of the switchgear should have ventilation shutters to prevent direct sunlight; If the surrounding environment exceeds 55℃, an electric wind brush should be installed for forced ventilation.
Humidity In order to ensure the insulation performance of PLC, the relative humidity of air should be less than 85% (no condensation).
Vibration should keep PLC away from strong vibration sources to prevent complex or continuous vibration with vibration frequency of 10-55Hz. When the environment inevitably vibrates, sincere vibration measures must be taken, such as using damping glue.
There are corrosive and flammable gases in the air, such as hydrogen chloride and hydrogen sulfide. For the environment with a lot of dust or corrosive gas in the air, PLC can be installed in a better closed control room or control cabinet, and an air purification device can be installed.
Power supply PLC power supply is 50Hz, 220( 1+ 10V) alternating current, which interferes with the power line. PLC itself has enough resistance. For occasions with high reliability requirements or environments where power supply interference is particularly serious, a shielding isolation transformer with a transformation ratio of 1: 1 can be installed to reduce the interference of equipment to the ground, and the power supply input can be connected in series with an LC filter circuit. FX series PLC has DC 24V output terminal, which can provide DC 24V power supply for input sensors (such as photoelectric switch or proximity switch). When external DC power supply is used at the input end, DC regulated power supply should be selected. Due to the common power supply of rectifier and filter, PLC can easily receive error information due to the influence of ripple.
2. Installation and wiring
The power line, control line, power line and I/O line of PLC should be wired separately, and the isolation transformer should be connected with PLC and I/O with double glue lines.
PLC should be far away from strong interference sources such as welding machines, high-power silicon rectifier devices and high-power equipment, and cannot be installed in the same switchgear with high-voltage appliances.
The input and output of PLC should be wired separately, and the switching value and analog value should also be laid separately. The analog signal shall be transmitted by shielded wire, and one or both ends of the shielding layer shall be grounded, and the grounding resistance shall be less than110 of the shielding layer resistance.
The basic unit and expansion unit of PLC and the connecting cable of your module should be laid separately to prevent the interference of external signals.
The AC input line and DC output line should not be in the same cable, and the output line should be as far away from the high voltage line and power line as possible to avoid parallelism.
Don't use a cable for AC output line and DC output line, and the output line should be as far away from the high voltage line and power line as possible to avoid parallelism.
3, I/O terminal wiring
In general, the input wiring should not exceed 30m. However, if the environmental interference is small and the voltage drop is not large, the input wiring can be appropriately longer. The input and output lines cannot use the same cable, and the input and output lines should be separated. Connect it to the input terminal in the form of normally open contact as far as possible, so that the ladder diagram is consistent with the relay schematic diagram and easy to read.
Output terminal wiring is divided into independent output and common output. Different types and voltage levels of output voltages can be used in different groups, but only the same type and voltage level of power supply can be used for outputs in the same group.
Because the output components of PLC are packaged on the printed circuit board and connected with the terminal board, if the load connected with the output components is short-circuited, the printed circuit board will be burned. Therefore, fuses are used to protect the output elements.
4. PLC and frequency converter * * *
In the automatic control system where PLC and frequency converter are used at the same time, the following matters should be noted:
1.The PLC power supply is configured separately from the power system power supply (inverter power supply), and the PLC power supply should use an isolation transformer;
2. The power line should be separated from the signal line as far as possible, and the signal line should be shielded;
3. Signal isolation module should be used in analog channel whether it is analog signal input or analog signal output;
4.4. Do software filtering design. PLC program;
5. The signal ground and power ground are designed separately.
Do the above five points well, and the problem of frequency converter interfering with PLC analog quantity can be solved easily.
On the sharing of PLC
How does Siemens PLC shorten the program scanning cycle?
1, when you don't need * * * to enjoy the signal, putting multiple instructions in the same network will produce additional stack entry and exit operations (specifically, it can be converted into STL for analysis). If it is not a logical requirement, you should avoid horizontal series connection, so that at least one AND instruction can be reduced. The advantage is that it happens to be placed in a network, and it feels a bit compact.
2. Reasonable use of the immediate IO instruction (as little as possible) saves the conversion time for PLC to process the immediate instruction.
3. Try to use the calculation result memory instead of the transitional memory in the calculation.
4. Those who can use "word" should try to avoid using "double word", and those who can use integer should try to avoid using real number.
5. Try to avoid data type conversion. If necessary, try to store intermediate variables with AC to reduce the number of conversions. In other words, when programming, you must first reserve storage space. For example, when VW2 is used to store integers, VW0 is not used, so the data in VW2 can be directly accessed in the form of VD0.
6. Reduce unnecessary network scanning, and classify networks that can be executed conditionally (especially AIW and AQW) into subroutines that can be called conditionally (such as timed interrupt).
7. Under the premise of ensuring the process requirements, appropriately reduce the interruption frequency.