Actuator Sharing and Control Arbitration
Many control goals compete for the same limited actuators, so an arbitration layer allocates them by priority and respects hard limits.
The competition problem
Several controllers want the same actuators at once: heating power is asked for by the temperature loop, the current-drive loop, and the mode-stabilization loop; gas valves serve both density and detachment control. Since actuators have finite capacity, something must decide who gets how much when the total request exceeds what the hardware can deliver.
Priority and reservation
Arbitration assigns priorities so that safety and stability tasks win over performance tasks. A stabilization loop may hold a reserved allocation of electron-cyclotron power that cannot be spent on heating, because losing stability is worse than losing a little temperature. When requests exceed capacity, lower-priority demands are scaled back first, keeping the plant within limits.
Respecting limits
The arbitration layer also enforces hard limits that no controller may exceed: total heating power bounded by plasma and divertor limits, coil currents bounded by their supplies, fueling bounded by the density limit. It converts high-level requests into commands that are simultaneously feasible for every shared resource, a constrained allocation solved each control cycle.
- Multiple loops compete for shared actuators
- Priorities let safety and stability win
- Reserve allocations protect critical tasks
- Enforce hard limits across all resources
Why it matters
Without arbitration, independent loops can collectively command more than the hardware or the plasma can take, causing a trip or an instability. A clean arbitration layer makes the whole control system predictable under stress. In the Kronos control architecture, described as a design and simulation model, arbitration mediates between heating, fueling, and stabilization on the breeder and between heating and confinement on the burner.
Arbitration is how many well-meaning controllers add up to one safe machine.