An actuator comprising: a static component; a moveable component moveable relative to the static component; at least two shape memory alloy actuator wires, each SMA actuator wire having a first portion coupled to the moveable component and a second portion coupled to the static component, where contraction of the SMA actuator wires drives movement of the moveable component; and a controller, coupled to the at least two SMA actuator wires, for generating pulse width modulated control signals with a fixed PWM frequency to power the at least two SMA actuator wires, where the PWM control signals are applied in a series of time slots having a duration defined by the PWM frequency, by the controller being configured to: divide each time slot into at least two sub-slots; generate a first PWM signal to apply power to a first SMA actuator wire, the first PWM signal comprising pulses at the PWM frequency; generate a second PWM signal to apply power to a second SMA actuator wire, the second PWM signal comprising pulses at the PWM frequency; and apply the first and second PWM signals such that in each time slot, a pulse of the first PWM signal occurs during a first sub-slot of the PWM cycle and a pulse of the second PWM signal occurs during a second sub-slot of the PWM cycle, wherein the duration of the pulse of the first or second PWM signal is less than the duration of the respective first or second sub-slot, characterized in that the duration of each sub-slot is adjustable to alter the maximum power deliverable in the sub-slot to the SMA actuator wire. a static component; a moveable component moveable relative to the static component; at least two shape memory alloy actuator wires, each SMA actuator wire having a first portion coupled to the moveable component and a second portion coupled to the static component, where contraction of the SMA actuator wires drives movement of the moveable component; and a controller, coupled to the at least two SMA actuator wires, for generating pulse width modulated control signals with a fixed PWM frequency to power the at least two SMA actuator wires, where the PWM control signals are applied in a series of time slots having a duration defined by the PWM frequency, by the controller being configured to: divide each time slot into at least two sub-slots; generate a first PWM signal to apply power to a first SMA actuator wire, the first PWM signal comprising pulses at the PWM frequency; generate a second PWM signal to apply power to a second SMA actuator wire, the second PWM signal comprising pulses at the PWM frequency; and apply the first and second PWM signals such that in each time slot, a pulse of the first PWM signal occurs during a first sub-slot of the PWM cycle and a pulse of the second PWM signal occurs during a second sub-slot of the PWM cycle, wherein the duration of the pulse of the first or second PWM signal is less than the duration of the respective first or second sub-slot, characterized in that the duration of each sub-slot is adjustable to alter the maximum power deliverable in the sub-slot to the SMA actuator wire. divide each time slot into at least two sub-slots; generate a first PWM signal to apply power to a first SMA actuator wire, the first PWM signal comprising pulses at the PWM frequency; generate a second PWM signal to apply power to a second SMA actuator wire, the second PWM signal comprising pulses at the PWM frequency; and apply the first and second PWM signals such that in each time slot, a pulse of the first PWM signal occurs during a first sub-slot of the PWM cycle and a pulse of the second PWM signal occurs during a second sub-slot of the PWM cycle, wherein the duration of the pulse of the first or second PWM signal is less than the duration of the respective first or second sub-slot, characterized in that the duration of each sub-slot is adjustable to alter the maximum power deliverable in the sub-slot to the SMA actuator wire.