Cascade control is most effective when the lower loop is 4 times or more faster than the upper loop (cascade rule). If the lower loop is too slow, the upper PID must be prevented from changing the lower PID setpoint faster than the lower loop can respond to prevent a burst of oscillations for large and fast disturbances or setpoint changes.
Cascade control can greatly reduce the effect of disturbances entering the lower loop. If the cascade rule is not violated, disturbances are also reduced that enter the upper loop. The ultimate period of the cascade loop is less than the ultimate period of a single loop with the same dynamics.
Lower loops can also isolate nonlinearities from the upper loop and better enforce limits on lower process variables (e.g. coolant temperature). Lower flow loops enable flow feedforward and process simulation, metrics, and analysis.
- Use valve positioners (digital valve controllers) on all control valves.
- If the valve stroking time (time for 100% stroke) is significantly greater than the reset time of the PID manipulating the valve, add volume booster(s) on valve positioner output(s). Open the booster bypass enough when stroke testing the valve to prevent high frequency cycling of valve position (e.g. 1 cps).
- Tune valve positioner for fast response avoiding the use of integral action.
- Use lower flow loops wherever possible to compensate for nonlinear installed flow characteristics and to provide the measurements needed for mass, mole and energy balances and cost analysis.
- Use jacket and coil temperature loops for bioreactor, crystallizer, and chemical reactor temperature control.
- Tune the lowest loop first, then the next lowest loop, and ending up tuning the upper most loop last. For example, tune the valve positioners first, then the flow loop, then the jacket temperature loop, and finally the reactor temperature loop.
- Tune lower loops to be as fast as possible.
- Do not use setpoint filters on lower loops. Use lambda tuning for coordination of flow loops (e.g. maintaining stoichiometry for inline blending and reaction control). Set the lambda (closed loop time constant) of each loop to match the lambda of the slowest loop.
- If the lower loop cannot be made 4 times faster than the upper loop, tune the upper loop slower by making the upper loop lambda 4 times larger than the lower loop lambda. Consider abandoning cascade control using either the lower or upper loop process variable for single loop control depending upon whether the disturbance size and speed is more problematic in the lower or upper process. The temperature control of some bioreactors is best done by simple jacket temperature control because the jacket volume is comparable to the process volume and process disturbances from cell growth are incredibly small and slow.
- Use external reset feedback of lower loop PV to upper loop PID so the upper loop PID output does not try to change faster than the lower loop can respond.
- If flow measurement rangeability is insufficient, there may need to be a switch to direct throttling of the control valve, a common practice in boiler drum level control at low steam rates and start up. A better solution is a computed flow from the installed valve characteristic and a bumpless transition to an inferential flow measurement maintaining cascade control.