Flow Controllers

Flow controllers monitor and maintain flow-rate variables, typically in process applications. They can have three main ways of controlling low: limit control, linear control and advanced or nonlinear control. Limit control interrupts power through the load circuit when flow exceeds or falls below the limit set point. A limit controller can protect equipment and people when it is correctly installed with its own power supply, power lines, switch and sensor. Advanced or nonlinear control uses process control strategies beyond PID loop control, such as dead-time compensation, lead/lag, adaptive gain, neural networks, and fuzzy logic. Common functionalities for flow controllers are rate indication and control as well as batch or totalizer indication and control.

Flow controllers with linear control use a classical type of control and can incorporate linear regulation, proportional, integral and derivative (PID), and feedforward methods. Proportional, Integral, and derivative control use an intelligent I/O module or program instruction, which provides automatic closed-loop operation of process control, loops. With Proportional plus integral control the error signal is integrated and is for eliminating steady state or offset errors. This may also be called automatic reset/bias/offset control. Proportional plus derivative control has the error signal differentiated to get the rate of change. This type pf control is used to increase the controller’s speed of response, but can be noisy and make the system less stable. Proportional control by itself has a control signal that is proportional to the error between the reference and feedback signals. Feedforward control is a direct control or compensation from the reference signal. It may be open loop or in conjunction with PID control.

To choose a flow controller, one important piece of information is the number of inputs and control outputs and control or feedback loops desired. These controllers can have multiple controls modes or functions, which may or may not use different inputs and outputs. Also, multiple control loops may be linked to improve control performance and/or stability. Typical control signals for flow controllers are analog voltage or current or else a switch turning on or off. Update rate is also an important specification. This is the frequency with which devices take readings and adjust their output. Flow controllers can have PLC and discrete control and can be compatible with TTL type I/O. Some controllers are able to handle high power switching such as relays and optoisolators.

Displays for flow controllers can be simple analog indicators, numeric or alphanumeric digital readouts, or video terminal displays. User interfaces are similar. Analog interfaces can have switches, dials and potentiometers. Digital user controls are typically keypads, menus and other digital interfaces. A remote computer can also program these controllers. Common computer interfaces are serial and parallel, but other options such as SCSI or network connections may be specified.

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Part Numbers for Flow Controllers