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Jun
27

Increase-Decrease Control

Motorized actuators may be used for applications that require larger valves, such as those used for the regulation of wastewater flow. Also, a motorized actuator may be preferred in some applications, such as the basis weight control of a paper machine, that require precise positioning of the valve. In such an application, these actuators typically have some gearing mechanism and the motor is designed to run in a forward or reverse direction. This allows a valve to be opened or closed by running the motor in a forward or reverse direction. The length of time the motor remains on, and its direction, determines how much a valve is closed or opened. In these types of applications, it is possible to use a discrete output that supports duty cycle control (see in my recent blog on Duty Cycle Control ) to tell the motor when to turn on and off. However, since the motor can run in a forward or reverse direction, it is necessary to use two discrete outputs to interface to a motorized actuator. This type of regulation is known as increase-decrease control. In Chapter 11 of Control Loop Foundation – Batch and Continuous Processes, we address use of motorized actuators in control applications.

The implementation of increase-decrease control is simplified when a measurement of the actuator position, such as valve stem position, is available for use in the discrete output interface. In this case, the regulation of the discrete outputs for forward and reverse operation can be based on the error between the position setpoint and the measured position. For example, if the error is positive then the motor may be run in the reverse direction for a period of time based on the magnitude of the error. Similarly, if the error is negative, then the motor may be run in a forward direction for a period of time based on the magnitude of the error. The calculation of the error and the coordination of the two discrete outputs can be implemented using a calculation and splitter block as illustrated below.

Since there is some minimum on-time that the motor will respond to, it is important that the interface accumulate any changes that cannot be acted on by the motor.

At Emerson Exchange 2011 I will be co-hosting a workshop, “ Interfacing DeltaV to Motorized Actuators”. This workshop will address the use motorized actuators in control applications. Details will be provided on the design and implementation of a control system interface to motorized actuators that provide stem position feedback and to motorized actuators that only provide limit switch feedback. If you attend Emerson Exchange this fall then I hope you will be able to attend this workshop. After Emerson Exchange I will be blogging more on this workshop and testing in Fisher Controls flow lab that was part of this interface development.