The characteristics of the trapezoidal diagram of PLC control system and structural analysis

1, the characteristics of the trapezoidal diagram of PLC control system (

1) PLC control system of the input signal and output load

relay circuit diagram of ac contactor and electromagnetic valve actuator to control with PLC output relay, the coil in the output of the PLC. Button, switch, limit switch, proximity switch, etc to provide PLC controller control commands and feedback signal, their contact in the input end of the PLC.

( 2) Relay circuit diagram of the intermediate relay and time relay processing

relay circuit diagram of the function of intermediate relay and time relay with PLC internal auxiliary relays and a timer to complete, it has nothing to do with the input of PLC relay and output relay.

( 3) Set the middle unit

in the ladder diagram, if more than one coil is affected by a certain contact series/parallel circuit control, in order to simplify the circuit and can be set up in the ladder diagram with the auxiliary relay control circuit, auxiliary relay is similar to the intermediate relay relay circuit.

( 4) Time relay transient dynamic contact processing

time relay in addition to delay action contact, and the coil to electricity or snap action immediately when losing electricity contact. For a time relay with instantaneous moving contact, can be in the ladder diagram corresponding auxiliary relay coil two ends of the timer in parallel, the latter's contact is equivalent to the instantaneous moving contact of time relay.

( 5) The establishment of external interlock circuit

in order to prevent two contactor control is/reverse action at the same time, cause the three-phase power supply circuit, in addition to setting in the ladder diagram and their corresponding output relay coil series of dynamic contact broken soft interlock circuit, should also be set up outside the PLC interlock circuit.

2, the structure of ladder diagram analysis

the general programming method or using sequence function diagram programming method; Using sequence function diagram of single sequence structure or sequence structure, parallel sequence structure, use rev/security/power road, stepping sequence control instruction in programming or by setting/reset instruction in programming.

ladder diagram decomposition by the operating command circuit ( Such as buttons) Start, check to the main circuit of tracing control appliances ( Such as contactor) Action, in the middle should pass many programming components and circuit, it is difficult to find it.

no matter how complex the ladder diagram, is composed of some basic unit. According to the composition of main circuit, using the method of inverse read back, put the ladder diagram and instruction statements into with the main circuit of the electrical appliances ( Such as motor) Corresponding to a few basic unit, and then a link, a link to the analysis, finally re-use skewer the each link along the tracking method of reading.

( 1) Button, trip switch, switch configuration and function of

the trip switch, switch contact state of different working alone see circuit diagram is difficult to understand, must be combined with the schedule of equipment specifications, electrical components, clear purpose of the trip switch, switch, control switch mechanical linkage, contact under different closing or opening state work condition of the circuit, etc.

( 2) By using inverse read back to load ( If multiple motor circuit) Decomposed into single load ( Such as single motor) Circuit

according to the main circuit of control to control electrical main contact text symbols, find out in the PLC controller I/O wiring diagram to control the load of the output of the relay, contactor coil in the ladder diagram and instructions in the table statement again find out to control the output relay coil and its related circuit, this is the local control the load circuit.

in the ladder diagram and instructions in the table statement, it's easy to find the output relay coil circuit and its electric and electric condition, but the cause of the coil to electricity, loss of electricity and the related circuit is not easy to find, by using inverse read back to look for:

in the output relay coil circuit in series, in parallel in the contact closure, disconnect other programming element is the electricity, the condition of losing electricity output relay.

so keep looking, until we find the input relay ( The main electrical appliances) So far. It is worth noting: when a certain programming elements to absorb or release, after losing electricity should put all of the programming element level before and after contact by a programming element all find out the role of the state, shall not be missed.

find a programming element in other circuits move contact, contact, the contact for other programming components of electricity, electricity or provide the conditions for interlocking, interlocking conditions, other electrical components, drive to perform electrical appliances.

( 3) Single load circuit is further decomposed

local circuit control single load may still be very complex, also need to be further broken down, until the decomposition of the basic unit circuit.

( 4) Decomposition of the circuit note

if the motor speed of main shaft with the relay, the speed control principle of the motor through the parking brake circuit. If

electric machine Lord have rectifier circuit, show that the motor USES the parking brake circuit energy consumption.

( 5) Comprehensive analysis for the whole set zero,

string together the basic unit circuit, suitable reading tracking method is used to analysis the whole circuit.

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