How did Charlemagne ensure that Louis the Pious would succeed him as emperor?

In 813, Charlemagne ensured that his son Louis I (the Pious) would succeed him as emperor by formally associating him with the exercise of imperial power.1

Following the deaths of his two elder sons, Charlemagne took deliberate steps to distinguish this transition from his own coronation in 800, which had been initiated by the papacy. To assert a Frankish rather than papal theory of the empire, Charlemagne held the ceremony in the imperial chapel at Aachen rather than in Rome. The Pope was not present, and the legitimacy of the act was established through the acclamation of the gathered Frankish nobility. During the ceremony, Louis either received the diadem directly from his father or took it from the altar with his own hands. This process was conducted without papal consultation or approval, effectively securing the succession under Charlemagne's authority.12