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Lineamenti Di Storia Del Controllo Automatico Lineamenti di storia del controllo automatico Angelo Monfroglio Sommario Il controllo automatico moderno ha 400 anni di storia, anche se le sue radici risalgono al terzo secolo prima di Cristo in epoca ellenistica. L’articolo tratta i lineamenti di questa storia con i concetti e i principali scienziati ed inventori, mettendo in evidenza le tappe fondamentali per l’industria e la scuola. Abstract The history of automatic control dates back to about 400 years ago. The article presents the fundamental scientists, motivations, steps and achievements in this technology, which plays a crucial role in the modern industry and factory automation. Introduzione <Those who cannot remember the past are condemned to repeat it> (Chi non sa ricordare il passato è condannato a ripeterlo) George Augustin Nicolas Ruiz de Santayana y Borras (Madrid 1863 – Roma 1952, filosofo e poeta spagnolo), Reason in Common Sense- The Life of Reason, 1936 Il controllo automatico e, in particolare, l’applicazione della retroazione negativa (feedback), sono stati fondamentali per lo sviluppo dell’automazione e quindi dell’industria moderna. L’origine del controllo automatico risale addirittura all’antichità araba e greca: controllo di livello, orologi ad acqua, pneumatica ed idraulica. Certamente, tutti conoscono Archimede ma, purtroppo, pochi hanno studiato Ctesibio di Alessandria e Filone di Bisanzio, padri della meccanica e della pneumatica, vissuti nel terzo secolo prima di Cristo (età ellenistica); qualcuno in più ha sentito parlare degli automi di Erone, Greco del primo secolo dopo Cristo. Lo studio degli orologi ad acqua fu continuato dagli Arabi Al-Jazari e Ibn al-Sa-ati ,e dal cosiddetto Pseudo-Archimede. Regolatori di flusso si devono ai tre fratelli Banu Musa a Bagdad. Erone di Alessandria D’altra parte, la “macchina” che realizza nel modo più perfetto il controllo automatico è il nostro corpo: negli animali a sangue caldo c’è un mirabile meccanismo di regolazione (cioè mantenimento) della temperatura corporea. Gli animali a sangue freddo, come, ad esempio le tartarughe, non hanno regolazione e, quando fa freddo, devono andare in letargo. Nel corpo umano vengono stabilizzate molte altre grandezze, come la pressione sanguigna, gli zuccheri, l’umore, ecc. A partire dal diciassettesimo secolo, furono sviluppati sistemi per controllare la temperatura, i mulini, le macchine a vapore. Due secoli dopo, apparve chiaro che la retroazione può introdurre instabilità vanificando i suoi vantaggi: da negativa e fonte di stabilità, può trasformarsi in positiva e portare, in certi casi, a rottura catastrofica. La stabilità della retroazione è importantissima anche in economia: basta pensare ai mercati di borsa impazziti dei nostri giorni. Criteri di stabilità furono dapprima introdotti a fine ‘800 da Hurwitz in Svizzera e Routh in Inghilterra, oggi abbinati nel metodo di Routh-Hurwitz. Negli stessi anni nacque la parola servomeccanismo, per il timone e poi per il pilotaggio automatico delle navi. Minorsky costruisce negli anni ’20 del Novecento la teoria del controllo navale. I problemi balistici alla vigilia della seconda guerra mondiale stimolano la costruzione di quel corpus di teorie chiamate oggi Controllo Classico. In Russia (URSS) Lyapunov , seguendo la strada del grande matematico francese Poincaré, sviluppa un approccio innovativo, sconosciuto nel resto del mondo, perché pubblicato in Russo, una lingua poco nota. Ricordiamo che il problema del controllo automatico si può così riassumere: -è dato un impianto o un sistema da controllare -sono date una o più grandezze fisiche (velocità, tensione, temperatura, pressione, ecc.)a cui si vuole far assumere certi valori che si chiamano variabili controllate -sono date una o più variabili su cui si agisce per effettuare il controllo, attraverso opportuni attuatori: come motori, valvole, ecc. -è dato un segnale di riferimento, eventualmente variabile (ad esempio con un potenziometro) chiamato set point -sono dati alcuni trasduttori (o sensori) che misurano le grandezze da controllare e le riportano in ingresso con la retroazione negativa -viene progettato un regolatore a reazione negativa che si oppone cioè alle cause e disturbi che allontano l’uscita da quella desiderata. L’aggettivo automatico significa che il controllo deve essere effettuato il più possibile senza l’intervento dell’uomo. Il primo periodo d’oro del controllo automatico Dopo il medio evo, durante il quale, a parte gli Arabi, i grandiosi studi ellenistici si persero, molti controlli automatici dovettero essere reinventati in Inghilterra nel diciottesimo secolo con la rivoluzione industriale. Il primo sistema retro azionato per il controllo di temperatura di un’incubatrice per polli con termostato è accreditato a Cornelius Drebbel agli inizi del 1600. Il suo regolatore è completo di vite per regolare il segnale di riferimento d’ingresso (set point). Un settore importante del 1700 è il controllo automatico del mulino a vento, sia per variare l’angolo delle pale secondo la direzione del vento, sia per regolare la velocità in modo che non fosse eccessiva quando il vento era forte. Inoltre, un altro controllo automatico regolava la distanza fra le mole. Il più noto dispositivo è quello di Thomas Mead, basato sulla forza centrifuga di un pendolo. Intorno al 1780 compare il famosissimo regolatore di James Watt per una macchina a vapore che serviva per una pompa. Quasi tutti i testi sui controlli automatici accreditano Watt come il pioniere del moderno regolatore. James Watt Cornelius Drebbel Macchina a vapore con regolatore di James Watt Il problema della stabilità nel secolo decimo nono Il successo dei regolatori basati sulla forza centrifuga fu ben presto accompagnato dai tipici problemi dei controlli automatici. L’assenza di un’azione integrativa porta l’offest: un errore a regime. In altri termini, il sistema modera l’uscita ma non la regola: non si ottiene cioè l’uscita esattamente desiderata. Il secondo problema è la lentezza dell’azione regolatrice al variare del carico. I due problemi portano alla necessità di due studi matematici correlati: l’analisi statica e quella dinamica, oggi affrontate anche dai nostri studenti di Elettronica all’Omar. Intanto, in Inghilterra, nasce una nuova applicazione: il controllo automatico del movimento di un telescopio che è puntato su un oggetto celeste. Appare subito chiaro che la reazione può diventare instabile. Nel 1868 James Clerk Maxwell, il celebre autore delle leggi sull’elettromagnetismo, analizza il problema e racchiude, con la sua straordinaria capacità di sintesi, in modello matematico con un’equazione differenziale del terzo ordine, il modello del controllore. Negli stessi anni, a San Pietroburgo, il Russo I.A. Vyshnegradskij trasforma l’equazione in un diagramma con cui si può analizzare comodamente la stabilità per via grafica. James Clerk Maxwell Navi, aerei e controlli industriali fino alla seconda guerra mondiale Verso la metà del diciannovesimo secolo compare il primo controllo retro azionato di posizione del timone di una nave. Poco dopo viene controllata la torretta dei cannoni delle navi da guerra. La retro azione viene poi usata per controllare la profondità delle torpediniere e fanno il loro ingresso i giroscopi. All’inizio del ventesimo secolo si realizzano autopiloti a giroscopio per le navi. Elmer Sperry brevetta uno stabilizzatore chiamato gyrocompass (bussola giroscopica). Si tratta di un controllo automatico a doppia reazione: una interna regola il timone, un’altra esterna sfrutta il giroscopio per la direzione. Lo stesso Sperry, con l’aiuto del figlio, progetta un pilota automatico per aerei con tripla retro azione. Nel 1933 Mason brevetta Stabilog, un sistema che comprende per la prima volta quello che oggi è chiamato regolatore PI: con azione proporzionale e integrativa. Poco dopo, viene aggiunta l’azione derivativa e si arriva al moderno controllore completo denominato PID. Inoltre l’industria, con l’avvento dell’energia elettrica, richiede controlli di tensione e frequenza. Elmer Ambrose Sperry col gyrocompass Il controllo automatico ha raggiunto la maturità, ma la perfezione arriva solo con l’avvento dell’elettronica e delle telecomunicazioni. Bode: chi era costui? Il controllo automatico elettronico Ho studiato per la prima volta i controlli automatici nel corso del quarto anno di Ingegneria Elettronica al Politecnico di Milano e ben presto è comparso il nome di Bode: diagrammi di Bode, criterio di Bode, ecc. Ma chi era Bode? Il cognome, anche in assenza del nome di battesimo, lascia nel dubbio: Italiano? O Tedesco? O Inglese? Laureato e arrivato all’Omar come docente in due quinte elettroniche, ho subito riscontrato la mia stessa curiosità negli studenti: Bode, chi era? Nessuno dei libri di testo adottati spiegava l’enigma. Se avrete la pazienza di continuare la lettura di questo articolo conoscerete finalmente la storia. Già, la storia. Anche la storia della scienza e della tecnologia è importante e spesso trascurata. Ho sempre notato che riempire gli alunni di formule astruse, senza mai dire quando e perché sono state introdotte, non è molto produttivo. Il contesto storico, le notizie su scienziati e tecnologi, le motivazioni delle scoperte e invenzioni sono fondamentali e aiutano a memorizzare e a capire. Torniamo al controllo automatico. Dalla metà del diciannovesimo secolo la diffusione pervasiva dell’industria telegrafica e telefonica porta allo sviluppo della teoria dei circuiti elettrici. Il primo nome che vogliamo citare è quello di Oliver Heaviside che sviluppò
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