Theoretical and experimental work on the vertical position control system on the COMPASS-D tokamak is described. An analog proportional + derivative (P + D) system is currently used, and two important sources of disturbance are observed in the system. One source is 600-Hz noise from thyristor power supplies, and the other is impulselike disturbances due to edge-localized modes (ELMs). A high-order controller is developed using the H-infinity technique to reduce the effect of the 600-Hz noise. This initial design is based on a model of the plasma position system obtained from system identification. The controller is implemented on a digital signal processor and tested on COMPASS-D. The controller synthesis procedure and the experimental results are presented. Large, separated ELMs on COMPASS-D cause impulselike responses to be observed in the power amplifier and position signals. Closed formulas are given for the minimum possible peak of the impulse response of the system, which is used to find the limit of performance.