Real Time Systems

FPGA-Based Control

FPGAs implement control logic as parallel hardware, giving nanosecond, jitter-free timing that software on a CPU cannot match for the fastest loops.

Logic Instead of Instructions

A field-programmable gate array (FPGA) is a chip whose digital logic is configured after manufacture. Instead of executing instructions sequentially like a processor, an FPGA implements a control algorithm as a network of gates and registers that operate in parallel, clocked by a fixed oscillator. Timing is set by the hardware, not by software scheduling, so it is deterministic to the clock period.

Why It Suits Fast Control

Kronos motion — control room

For the fastest control loops, an FPGA offers latency measured in tens of nanoseconds and jitter set by the clock, orders of magnitude tighter than a CPU running an operating system. Because many operations run concurrently in hardware, a control law that would be a sequence of instructions on a CPU can be a single-cycle datapath on an FPGA. There is no cache, no scheduler, and no interrupt latency to bound.

The Trade-Offs

Deterministic, parallel, low-latency: excellent for inner loops and protection
Harder to develop: hardware description languages and timing closure, not ordinary programming
Less flexible: complex, evolving logic and floating-point math are costly in gates
Reconfigurable but not at runtime like software

The Common Architecture

Real systems partition work by timescale. The FPGA handles the fastest, most safety-critical functions: fast digital signal processing, protection interlocks, and the innermost feedback loop. A CPU handles slower supervisory control, setpoint generation, communication, and logging, where flexibility matters more than nanosecond timing. The two exchange data through a well-defined interface.

Application to Fusion Control

Fast plasma control is a natural fit. In a design like the Hyperion breeder, the innermost magnetic-control and protection functions require reaction far faster and more repeatably than a general-purpose computer can guarantee, so such logic is naturally implemented in FPGA hardware while higher-level scenario control runs on conventional computers. The machines are at the design and simulation stage; the architecture reflects standard practice for fast fusion-plasma control rather than an operating device.