Vehicle suspension controller

A vehicle suspension controller (15) includes a differentiator (33) for deriving an analogue relative suspension velocity signal from a relative suspension position sensor (13); and the analogue relative suspension velocity signal is digitised by digital signal processing apparatus (30) at a predetermined sampling frequency. The differentiator (33) comprises a bandpass filter having a linearly increasing gain and a 90 degree phase lead for differentiator action in a low frequency range of vehicle suspension resonances (0.5-20 Hz) in which signal differentiation is desired, a peak in gain and group delay at an intermediate frequency above the low frequency range of vehicle suspension resonances, at which vehicle suspension inputs are small, and a decreasing gain from the intermediate frequency to a higher aliasing frequency (500 Hz) related to the sampling frequency of the digital signal processing apparatus. The circuit is used in combination with a two-pole low-pass filter (32) to decrease further gain above the intermediate frequency; and, even with the low-pass filter (32), it has low group delay in the low frequency range.