Changing PID parameters on the fly.

halcmd setp pid.0.Pgain 1024.5

loadrt lincurve count=3
...
setp lincurve.0.x1 0
setp lincurve.0.x2 20
setp lincurve.0.x3 100
....
setp lincurve.0.y1 30
setp lincurve.0.y2 20
...
setp lincurve.1.x1 0
...
...
net part-mass pyvcp.part-mass =>  lincurve.0.x-val 
net computed-P lincurve.0.out-rw =>  pid.0.Pgain

component lincurve "one-dimensional lookup table";
description """Performs a 1-dimensional lookup and interpolation. The x-val
parameters must be monotonic, though identical adjacent values are allowed.
(for example 0,0,0,10) for a 4-element curve. For input x less than the lowest
xval the y-val of the lowest is returned. For x greater than the largest x-val 
the yval corresponding to x-max is returned (ie, no extrapolation is performed.
""";

param rw float y-val##[16 : personality] "output values";
param rw float x-val##[16 : personality] "input values";

pin in float x "The input value";
pin out float y "The output value";
pin io float y-rw "The output value, compatible with PID gains";

variable unsigned i = 0;

author "andy pugh";
license "GPL";

function _;

;;

FUNCTION(_){
    double f;
    if (x >= x_val(personality-1)) {
        y = y_val(personality-1);
        return;
    }
    if (x <= x_val(0)) {
        y = y_val(0);
        return;
    }
    while (x > x_val(i+1)) { i++;}
    while (x < (x_val(i))) { i--;}
    
    f = (x - x_val(i))/(x_val(i+1)-x_val(i));
    y = y_val(i) + f * (y_val(i+1) - y_val(i));
    y_rw = y;
}