Convert a force in uN/mm to voltage.

        Parameters
        ----------
        force : float
            Force in **uN/mm**.
        frequency : float
            Actuation frequency.

        Returns
        -------
        float
            Actuation voltage to apply :data:`force` at an actuation frequency
            of :data:`frequency`.

def force_to_voltage(self, force, frequency):
        '''
        Convert a force in uN/mm to voltage.

        Parameters
        ----------
        force : float
            Force in **uN/mm**.
        frequency : float
            Actuation frequency.

        Returns
        -------
        float
            Actuation voltage to apply :data:`force` at an actuation frequency
            of :data:`frequency`.
        '''
        c_drop = self.calibration.c_drop(frequency)

        # if c_filler hasn't been set, assume c_filler = 0
        if self.calibration._c_filler:
            c_filler = self.calibration.c_filler(frequency)
        else:
            c_filler = 0

        return np.sqrt(force * 1e-9/ (0.5 * (c_drop - c_filler)))