Class LinearDigitalFilter

  • All Implemented Interfaces:
    PIDSource

    public class LinearDigitalFilter
    extends Filter
    This class implements a linear, digital filter. All types of FIR and IIR filters are supported. Static factory methods are provided to create commonly used types of filters.

    Filters are of the form: y[n] = (b0*x[n] + b1*x[n-1] + ... + bP*x[n-P]) - (a0*y[n-1] + a2*y[n-2] + ... + aQ*y[n-Q])

    Where: y[n] is the output at time "n" x[n] is the input at time "n" y[n-1] is the output from the LAST time step ("n-1") x[n-1] is the input from the LAST time step ("n-1") b0...bP are the "feedforward" (FIR) gains a0...aQ are the "feedback" (IIR) gains IMPORTANT! Note the "-" sign in front of the feedback term! This is a common convention in signal processing.

    What can linear filters do? Basically, they can filter, or diminish, the effects of undesirable input frequencies. High frequencies, or rapid changes, can be indicative of sensor noise or be otherwise undesirable. A "low pass" filter smooths out the signal, reducing the impact of these high frequency components. Likewise, a "high pass" filter gets rid of slow-moving signal components, letting you detect large changes more easily.

    Example FRC applications of filters: - Getting rid of noise from an analog sensor input (note: the roboRIO's FPGA can do this faster in hardware) - Smoothing out joystick input to prevent the wheels from slipping or the robot from tipping - Smoothing motor commands so that unnecessary strain isn't put on electrical or mechanical components - If you use clever gains, you can make a PID controller out of this class!

    For more on filters, I highly recommend the following articles: http://en.wikipedia .org/wiki/Linear_filter http://en.wikipedia.org/wiki/Iir_filter http://en.wikipedia .org/wiki/Fir_filter

    Note 1: PIDGet() should be called by the user on a known, regular period. You can set up a Notifier to do this (look at the WPILib PIDController class), or do it "inline" with code in a periodic function.

    Note 2: For ALL filters, gains are necessarily a function of frequency. If you make a filter that works well for you at, say, 100Hz, you will most definitely need to adjust the gains if you then want to run it at 200Hz! Combining this with Note 1 - the impetus is on YOU as a developer to make sure PIDGet() gets called at the desired, constant frequency!

    • Constructor Detail

      • LinearDigitalFilter

        public LinearDigitalFilter​(PIDSource source,
                                   double[] ffGains,
                                   double[] fbGains)
        Create a linear FIR or IIR filter.
        Parameters:
        source - The PIDSource object that is used to get values
        ffGains - The "feed forward" or FIR gains
        fbGains - The "feed back" or IIR gains
    • Method Detail

      • singlePoleIIR

        public static LinearDigitalFilter singlePoleIIR​(PIDSource source,
                                                        double timeConstant,
                                                        double period)
        Creates a one-pole IIR low-pass filter of the form: y[n] = (1-gain)*x[n] + gain*y[n-1] where gain = e^(-dt / T), T is the time constant in seconds.

        This filter is stable for time constants greater than zero.

        Parameters:
        source - The PIDSource object that is used to get values
        timeConstant - The discrete-time time constant in seconds
        period - The period in seconds between samples taken by the user
      • highPass

        public static LinearDigitalFilter highPass​(PIDSource source,
                                                   double timeConstant,
                                                   double period)
        Creates a first-order high-pass filter of the form: y[n] = gain*x[n] + (-gain)*x[n-1] + gain*y[n-1] where gain = e^(-dt / T), T is the time constant in seconds.

        This filter is stable for time constants greater than zero.

        Parameters:
        source - The PIDSource object that is used to get values
        timeConstant - The discrete-time time constant in seconds
        period - The period in seconds between samples taken by the user
      • movingAverage

        public static LinearDigitalFilter movingAverage​(PIDSource source,
                                                        int taps)
        Creates a K-tap FIR moving average filter of the form: y[n] = 1/k * (x[k] + x[k-1] + ... + x[0]).

        This filter is always stable.

        Parameters:
        source - The PIDSource object that is used to get values
        taps - The number of samples to average over. Higher = smoother but slower
        Throws:
        IllegalArgumentException - if number of taps is less than 1
      • get

        public double get()
        Description copied from class: Filter
        Returns the current filter estimate without also inserting new data as pidGet() would do.
        Specified by:
        get in class Filter
        Returns:
        The current filter estimate
      • reset

        public void reset()
        Description copied from class: Filter
        Reset the filter state.
        Specified by:
        reset in class Filter
      • pidGet

        public double pidGet()
        Calculates the next value of the filter.
        Specified by:
        pidGet in interface PIDSource
        Specified by:
        pidGet in class Filter
        Returns:
        The filtered value at this step