The excavation machine was developed by the Tractators to dig tunnels beneath Frontios. It required a mind to power it, and so the Tractators took Captain Revere as a prisoner and attached him to the machine.
After his usefulness came to an end, they decided to replace him with his son, Plantagenet, who they had also taken prisoner. Shortly after being detached from the machine, Revere died. Before Plantagenet could be fully attached, the Fifth Doctor used one of the machine's wires to stun the Gravis with the effects, immobilising all of the Tractators.