It is desired to control the level in a process tank where the flow out of the tank is measured and manipulated by varying the speed of its discharge pump using a variable speed motor drive. Which of the following control strategies could be used?
A. Feedback
B. Feedforward
C. Cascade
D. Feedforward and cascade
A feedback controller could be applied where the level is the process variable and the speed of the pump is the manipulated variable. Answer A is applicable.
Feedforward control is used to modify the controller output in response to a measured disturbance. There is no measured disturbance so Answers B and D are not applicable.
Cascade control can be used where a level controller (master) provides the set point for an outflow controller (slave). Using a master controller for the slower level process and a slave controller for the faster flow loop generally results in superior control because the user can tune the relatively slow-acting level controller and the relatively fast-acting flow controller independently. Answer C is applicable.
Additional complicating factors
A measurement of the flow coming into the process tank allows the possibility of using feedforward control to adjust the outflow to improve control of the tank level.
David W. Spitzer is a principal at Spitzer and Boyes, LLC, which offers engineering, focused market research, writing/editing white papers, strategic marketing consulting, distribution consulting, seminars and expert witness services for manufacturing and automation companies. Spitzer has written more than 400 technical articles and 10 books about flow measurement, instrumentation and process control. He can be reached at 845-623-1830 or via spitzerandboyes.com.
