http://www.controlguru.com/ Proven Methods and Best Practices for PID Process Control en-us Sun, 24 Aug 2008 00:00:01 EST doug.cooper@controlguru.com doug.cooper@controlguru.com http://www.controlguru.com/2008/082408.html We build on the foundation of the previous feed forward articles as we explore the popular and surprisingly powerful static feed forward computational element for improved disturbance rejection. We explore the application and benefit of static feed forward with feedback trim on disturbance rejection performance for the jacketed stirred reactor process. Sun, 24 Aug 2008 00:00:01 EST http://www.controlguru.com/2008/072008.html The "feed forward with feedback trim" architecture is constructed by coupling a feed-forward-only controller to a traditional feedback controller. The feed forward controller output signal, COfeedforward, is computed by combining a process model and disturbance model. The process model describes or predicts how each change in CO will impact PV. The disturbance model describes or predicts how each change in D will impact PV. Sun, 20 Jul 2008 00:00:01 EST http://www.controlguru.com/2008/050408.html A feed forward controller receives a measured disturbance signal, uses it to predict an impact on the process variable, and then computes preemptive control actions that counteract the predicted impact as the disturbance arrives to maintain the process variable at set point (PV = SP) throughout the disturbance event. Sun, 04 May 2008 00:00:01 EST http://www.controlguru.com/2008/041308.html Here we explore the addition of a second select override element to create a cross-limiting architecture that prevents the air/fuel ratio fed to the burner from becoming overly rich (too much fuel) or lean (too much air) as operating conditions change. Variations on this cross-limiting architecture are widely employed within the air/fuel ratio logic of a broad range of industrial combustion control systems. Sun, 13 Apr 2008 00:00:01 EST http://www.controlguru.com/2008/012708.html A ratio control strategy can play a fundamental role in the safe and profitable operation of fired heaters, boilers, furnaces and similar fuel burning processes. This is because the air-to-fuel ratio in the combustion zone of these processes directly impacts fuel combustion efficiency and environmental emissions. A requirement for ratio control implementation is that both the fuel feed rate and combustion air feed rate are measured and available as process variable signals. Sun, 27 Jan 2008 00:00:01 EST http://www.controlguru.com/2008/021008.html A valve cannot open more than all the way. A pump cannot go slower than stopped. Yet an improperly programmed control algorithm can issue such commands. Herein lies the problem of integral windup (also referred to as reset windup or integral saturation). It is a problem that has been around for decades and was solved long ago. We discuss why it occurs and how to prevent it to help those who choose to write their own control algorithm. Sun, 10 Feb 2008 00:00:01 EST http://www.controlguru.com/2007/110407.html Like most control room displays, the Controlguru.com e-book presents PV values in engineering units. These are used directly in tuning correlations to compute controller gains, Kc. As a result, the Kc values also carry engineering units. The benefit of this approach is that Kc maintains the intuitive familiarity that engineering units provide. The difficulty is that commercial controllers are normally configured to use a dimensionless values. This article explores how to convert a Kc with engineering units into the standard dimensionless (%/%) form. Sun, 04 Nov 2007 00:00:01 EST http://www.controlguru.com/2007/090207.html When improved disturbance rejection is our goal, one benefit of cascade control over a feed forward strategy is that implementing a cascade builds upon our existing skills. A cascade has two controllers, and the proecedure is a familiar task because it essentially is to employ our controller design and tuning recipe twice in sequence. Sun, 2 Sep 2007 00:00:01 EST http://www.controlguru.com/2007/070607.html Cascade control is an architecture capable of reducing the impact of disturbances on our measured process variable (PV). To construct a cascade architecture, we literally nest a secondary control loop inside a primary loop as shown in the block diagram. Fri, 6 Jul 2007 00:00:01 EST http://www.controlguru.com/2007/042507.html The control objective for the pumped tank process is to maintain liquid level at set point by adjusting the discharge flow rate out of the bottom of the tank. This process displays the distinctive integrating (or non-self regulating) behavior, and as such, presents an interesting control challenge. Wed, 25 Apr 2007 00:00:01 EST