High-pass filter

Simple high-pass filter in form of IIR of 1st order. See more info here. In my application HMC demo I poll sensor (accelerometer) and filter Ax signal:

HPFilter hpf
hpf configure -datatype flt -datatypes {phys}
Saver saver1
saver1 listen -on
saver listen -on
listen phvals:hpf
listen hpf:saver1
serial open 4
sensor poll -loop

This will create 2 CSV files with native and with filtered signals (after saving we rename them). We can render signals with GLE tool:

begin graph
    title "Accelerations"
    xtitle "Time [sec]"
    ytitle "Ax [g]"
    axis grid
    ticks color grey40
    subticks on
    subticks lstyle 2
    set font "texcmr"

    d1 file "c:/tmp/saver.csv,0,5"
    let d2 = d1
    d1 line color red
    d2 deresolve 10 average line color red lwidth 0.1

    d3 file "c:/tmp/saver1.csv,0,5"
    let d4 = d3
    d3 line color blue
    d4 deresolve 10 average line color blue lwidth 0.1
end graph

begin key
    set fill yellow
    line color red text "Native"
    line color blue text "Filtered"
end key

Removing of slow drift is used in position estimation of sensor (from acceleration) in multicascading scheme:

HPF -> INTEGR -> HPF -> INTEGR -> HPF

Here is the C code (for SWIG) of this kind of filter:

double* filter(double *v) {
        int i;
        int vlen;

        vlen = (int)v[0];
        if (vlen != $self->n) {
            return (NULL);
        }

        for (i=1; i <= vlen; i++) {
            $self->v[i] = $self->a * ($self->v[i] + v[i] - $self->x[i]);
            $self->x[i] = v[i];
        }
        return ($self->v);
}

$self is the pointer to CHPFilter struct:

typedef struct CHPFilter {
        int n; // length of v
        double a; // alpha
        double *v; // calculated values (keep prev. before filter)
        double *x; // prev. input
} CHPFilter;