Precise control of charged particles in radio-frequency (Paul) traps requires minimising excess micromotion induced by stray electric fields. We present a method to detect and compensate such fields through amplitude modulation of the radio-frequency trapping field. Modulation at frequencies close to the motional modes of the trapped particle excites coherent motion whose amplitude linearly depends on the stray field. In trapped-ion experiments, this motion can be detected by recording the arrival times of photons scattered during laser cooling. Only a single laser beam is required to resolve fields in multiple directions. In a demonstration using a $^{88}mathrm{Sr}^{+}$ ion in a surface electrode trap, we achieve a sensitivity of $0.1, mathrm{V}, mathrm{m}^{-1}, /, sqrt{mathrm{Hz}}$ and a minimal uncertainty of $0.015, mathrm{V}, mathrm{m}^{-1}$.
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