Hi Richard,

Any idiot can IMPLEMENT a PLD algorithm, the trick is tuning the B$%^&rds ! We use a dual servo system in our instruments, with an analogue servo tweaked by a digital one, and a pure digital servo too.

The dual loop system is great, if the CPU falls over we don't toast £1000 worth of temperature cntrol gear.

We invented a kind of Ziegler-Nichols tuning method, as well as trying to find its impulse response and tune it from there, but that wasn't successful. In the end our senior engineer sat down, and tuned it by hand ! Its absolutely beautifully tuned now.....

We use an incremental filter to drive the thing.

What do you want it for ? How fast do you want to sample ?

Steve