Delayed excitation timers, TON, are an essential component for any PLC program.
The IEC standard for programming foresees the existence of three types of 32-bit Timers, these are TON, TOFF, and TP, different from the old 5 types available in the obsolete simatic manager.
Note: PT and ET is a Doble INT format, here generically shown like Double Word.
Between the input IN and the output Q there is a switch controlled by the internal comparator, as shown in the image.
In a functional schematic these is the right symbols.
SawTooth generation:
The exercise consists of generating a sawtooth waveform that can be intercepted in many levels with comparators.
The exercise is useful for developing cyclic drums or sequencers such as traffic lights, valve assemblies, etc.
The TON timer generates a ramp, at the ET output (elapsed time) with 1ms resolution, and timeout in the value provided at the PT input (preset time).
The Boolean input and output IN and Q are used to generate the maintenance signal on and the restart trigger.
The internal switch is kept closed always closed except for a scan cycle, about 1ms, sufficient to reset the value reached by the ramp.
The internal comparator, when ET becomes equal to PT, generates the opening of the internal switch that issues the trigger on Q used for the restart.
The generated signal is periodic.
Using “in Range” comparators it is possible to release each digital output independently from the state of the previous ones.
A strategy that allows to intercept the ramp on several levels is to set Merker flags to be placed in parallel logic.
The PT and ET signals are DINT type and are placed in a global Data Block so that the value can be changed from the operator panel.
It follows that the period of the signal tooth wave can be set by HMI.
Note: Using a single timer it is possible to generate a square wave at the desired frequency as long as the frequency is less than half of the cyclical times in respect of the Shannon rule.
Proposed exercise, the solution is on the textbook. Make the square wave shown in the diagram. Try using Twoo TON timers.
Recommended bibliography for the United States and English-speaking countries.
Book optimized for college level and industrial automation laboratories, self-taught students, robotics or automation engineering students.
Download and zoom images for Zoom the book index. It si available in Amazon.
Per studenti Italiani:
Il libro in inglese mostrato sopra è disponibile per l’italia in lingua Italiano. E’ pensato per l’anno 2020 ma in anteprima è disponibile fin da settembre 2019.
Amazon offre la possibilità di averlo con il bonus cultura e/o carta del docente , quindi gratis (Novembre 2019).
Il libro di testo consigliato per le scuole ITIS e IPSIA è mostrato nell’immagine:
Questo è l’indice del testo. Scarica l’immagine e fai lo zoom per una più agevole lettura:
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Corsi di programmazione dei PLC siemens
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