1. Field of the Invention
The present invention relates to an adaptive control unit with feedback compensation.
2. Description of the Related Art
Active control systems for controlling a parameter in a physical system (a plant) detect the parameter to be controlled and generate a control signal. In order to adapt control behaviour, a residual parameter is detected, i.e. an error detection is made, and this is used to modify the control behaviour in order to achieve better control. One problem in such control systems is the coupling that occurs between the output of the control system and the input of this control system i.e. undesirable feedback or pollution of the reference signal.
In one class of control systems, termed virtual earth feedback systems, the input to the control system, i.e. the measurement of the parameter, is made close to the point of control. The system thus acts in a feedback manner to drive the parameter to a desirable value such as to zero when the parameter is undesirable. However, in such systems stability of the feedback loop is a problem since the detections made are detections both of the undesired parameter and feedback from the output of the control system.
In another class of control systems, termed feed forward control systems, a reference signal is obtained which is indicative of the parameter to be controlled. This reference signal provides an indication of the degree of success of the control system in achieving the desired control. In a perfect feed forward control system, the reference signal is unpolluted by the control output of the control system. However, in practice often the reference signal is polluted by the output control signal. This can present difficulties in achieving stable control.
Control systems can be analog, digital, or a hybrid of analog and digital. For example, GB 2142091, the content of which is hereby incorporated by reference, discloses a feed forward control system for the attenuation of sound. In this system an analog feed forward controller is provided with a digital feedback compensation filter to compensate for coupling between the sound generating and detection systems. In this system the gain of the analog amplifier is fixed. Thus, this system is highly limited and does not provide for adaptive control.
GB 2222733, the content of which is hereby incorporated by reference, describes a hybrid analog and digital filter. The system disclosed overcomes the problem of the latency of digital controllers by providing an analog bypass filter in parallel with the digital filter. Digital filters have a latency problem due to analog to digital conversion delays, processing delays, and digital to analog conversion delays. In a wholly digital system this problem can only be overcome by increasing the sample rate, i.e. processing speed, thus increasing complexity and cost of the digital system. The provision of the analog bypass filter in parallel to the digital filter overcomes the latency problem of the digital controller. However, this document does not disclose any method of adaption of the hybrid analog and digital filter.
U.S. Pat. No. 6,278,786, the content of which is hereby incorporated by reference, discloses a hybrid analog and digital control system for an active noise cancellation headset system. The system is illustrated schematically in FIG. 1. A microphone 1 and loudspeaker 6 are mounted in an earcup of a headset. Thus because of the close proximity of the microphone 1 and the loudspeaker 6, the system comprises a virtual earth system. The output of the microphone 1 is digitised by an analog to digital converter 2 and the digitized signal is input to a digital signal processor 3. The digital signal processor includes a digital filter for generating a digital filtered signal for conversion to an analog control signal by a digital to analog converter. The input from the microphone 1 is also input through a compensation filter 7 and through an amplifier 8 having a gain G to generate an analog control signal. The analog control signal from the amplifier 8 and the analog control signal from digital to analog converter 4 are summed in a summing amplifier 5 before being output to the loudspeaker 6. Thus, the system comprises a digital control path in parallel with an analog control path. The digital signal processor 3 also controls the gain of the amplifier 8. The method disclosed for control is to increase the loop gain until the system is on the verge of instability, thus obtaining maximum noise reduction on the non-adaptive analog component of the system under all conditions. Thus this system does not provide a fully adaptive analog gain control loop. Further, this system makes no provision for pollution of the input signal by feedback coupling with the output signal.
In digital control systems a well known feed forward control system is the filtered X LMS algorithm. This is illustrated in FIG. 2 and reference to it can be found in “Adaptive Signal Processing” by Bernard Widrow and Samuel B. Stearns, the content of which is hereby incorporated by reference. A reference signal is detected in an active vibration system using a vibration sensor such as a microphone 10. The input signal is digitised using an analog to digital converter. In this example a feedback signal is subtracted from the input signal using a subtractor 12. A feedback signal is obtained by filtering the output of the adaptive filter 13 through a plant model C. Thus in this example, pollution of the input reference signal by the output signal is accounted for. Thus the reconstructed (unpolluted) references input into the adaptive filter 13 and the output of the adapted filter is input to a digital to analog converter 14 for output to a loudspeaker 15 for noise cancellation.
In order to perform adaption of the filter characteristics of the filter W 13, the reconstructed reference is input through a model of the plant Ĉ 18 to provide a time aligned reference r which is input to an LMS algorithm 17 together with the input error signal e from the analog to digital converter 11. The LMS algorithm 17 determines updated coefficients for the adaptive filter W 13.
Thus in the example given in FIG. 2, although a reconstructed reference is provided in a feed forward control system using a model Ĉ of the plant, i.e. the acoustic response of the loudspeaker, the acoustic environment, and the microphone 10, the system suffers from the latency problem of a digital control system.