How to use PLC control to realize positioning function of the encoder - - programmable logic

by:Coolmay     2020-06-21
Strictly speaking, the encoder will only tell you how to locate and how to perform, it is need to rely on numerical control system ( Or a PLC controller) Control servo and stepper motor to achieve positioning, encoder is like people's eyes, know the motor shaft and load, at a certain position in the current industry in general is photoelectric type encoder.

photoelectric encoder is in a very thin very light round plate, through close instruments to corrosion carved many fine seam, is equivalent to a 360 - degree, subdivided into many parts, such as 1024 group, so that the Angle difference between each group is 360/1024 = 0. 3515625 degrees.

and then have a sophisticated light source, mounted on the encoder side, the other side of the encoder, there will be a receiver, such as the use of the photoresistor these components of amplifying and shaping circuit, the encoder rotation, there is gap will pervious to light in the past, the receiver will instantly receive light pulse, after dealing with the circuit, and output an electrical signal, this code rotating a week, 1024 will correspond to the output pulse, the first pulse position if it is 0, the second pulse position is 0. 3515625 °, the third pulse position is 0. 3515625 ° * 2, and so on, such as long as have instrument can read pulse number, the encoder is known corresponding in what position, if the encoder mounted to the motor shaft, the motor shaft and encoder is rigid connection, both the location of the relationship will be one to one correspondence, by reading the encoder pulse, the motor shaft position is known.

and the motor shaft, such as through the synchronous belt, gear, chain drive some load, such as control of lead screw, so there will be a so-called electronic gear ratio, the relationship between motor turns a circle, how many mm screw coming, so read the corresponding output to the pulse encoder, through the pulse number can be launched against the screw position.

but the encoder is round, if unlimited rotation, Angle will be infinity, so designed a kind of incremental encoder, turn round, can output ABZ three sets of signals, the AB is the same pulse, such as a circle above said there are 1024 pulse, AB pulse corresponds to a circle circle Angle, and it is two kinds of pulse in orthogonal state, if it is positive &negative, judging by AB pulse rising and falling down the order, you can know the encoder of the current is clockwise or counterclockwise.

in addition a Z pulse, though because the circumference of a circle will not halt, the Angle will be endless, but they are all a week a week of repetition, fixed at a certain position in the circumference of a circle, zero phase pulse encoder per round, only one zero phase pulse output, so that if a to Z pulse as the benchmark, so every time to read the pulse time, the system will reset once, can let a maximum Angle control within 360 °, the equivalent of a zero datum.

so even if the system is breaking down, again with electricity, as long as you can find the reference point, you can know the screw of the initial position in what place. Above this positioning is called incremental coordinate system, so the encoder is incremental encoder, apply more extensive, because flexible and cheap.

if only devices only need to turn a circle, which is within 360 ° Angle, the encoder can segmentation precision, such as 13, equivalent to 2 ^ 13 power pulse circuit, corresponds to 360 °, the pulse number and the Angle of the one-to-one correspondence does not fear the power system need to readjust the zero, this encoder is called single ring absolute encoder. If the load need to turn more circle, but the number of turns can not very much, such as five laps, equivalent to 1800 °, 360 ° = 5 * this pulse and 1800 ° one-to-one correspondence, the application in some high-end CNC machine, screw or some rotation work can know the current precision position, and don't have to worry about power system to zero.

in addition, the encoder and magnetoelectric way, such as processing a lot of the encoder with a north and south between small magnets, through the hole to read small magnet signal, the output signal of the same are amplified and plastic into electrical impulses, this and photoelectric encoder is similar, but the price will be cheaper, reliability is high, but the accuracy is almost than photoelectric.

PLC can input switch, which is held high level voltage, and the encoder pulse signal, can understand a certain period of time, complete set of switch with great speed. But because this kind of switch frequency is too high, so the ordinary I/O port of PLC controller is unable to read the number of these pulses accurately, because exists in the working process of PLC controller scan cycle, need each time to refresh the common I/O data of mouth, and the precision of the encoder is too high, the output per unit time of pulse number is too much, common I/O is insufficient.

general PLC has the high speed counter port design, the essence is to use the underlying microcontroller hardware logic to complete these encoders count, avoids the problem of scan cycle, the PLC controller have dedicated high-speed count instruction design, use, direct call these instructions can read the current pulse value. But on the calculation and output pulse, because scanning period, often there is a lag effect, if used to control some actuators, such as cylinder to action cutting action, so want to consider the compensation of nc.

as a reminder, if you want to use PLC to control the servo or stepping system, often do not need through the encoder feedback to determine position, through some PLS order to issue a position such as pulse in the servo drive, position loop in the servo drive internal composition is good, and the PLC here is just an instruction organization, did not constitute a closed loop position, of course, if it is a special positioning module, using the NC control mode, such as, can be in the building of a position closed loop inside.

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