A cheap PID will not work for controlling pneumatic systems if the load is a big valve and the valve must be positioned rapidly and accurately. A pneumatic system postion system is a third order system. This means that the controller should have 3 gains to place the poles. This means there should be a proportional, derivative and second derivative gains. Clearly a PID does not have a second derivative gain. If the integrator is added it will only make the steady state error zero. The integrator has its own pole so that can be counted as one of the three required poles.
In addition, the system is not linear which causes problems for cheap controller. These systems typically require a position feedback, duh. The trick is to get the second derivative for the second derivative gain. This can be done by differentiating the position twice. Good luck with out using an observer. One can use an accelerometer. This works well in the lab but will not survive in many industrial environments. The best method is to put pressure sensors on either end of the cylinder. The pressure on each end of the cylinder is mulitplied by the area on each side of the piston and a differential force is computed. The differential force is roughly proporitonal to the acceleration of the load or the second derivative.
So how big is the valve and how fast and accurate must the control be?
Peter Nachtwey
