Flow controllers are electric devices which monitor and maintain flow-rate variables, typically in process applications. They can be used in conjunction with pumps and valves in fluid flow systems, in order to provide better control of flow variables. Flow controllers are also commonly seen in manufacturing to count items on a conveyor belt or used to monitor air speed or quantify powder flowing through air. The electric device reads a sensor and opens or closes the gates or valves in response to the signal input.

Flow controllers can control flow in six main ways:

Limit control - Limit control established set points or limits that when reached a signal is set to stop or start a process variable. This "on-off" or "bang-bang" control is a fairly simple type of control.

Linear 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. They match a variable input signal with a corresponding variable control signal. Signal conditioning, filtering, and amplification may occur to produce the proper output control signal.

Proportional, integral, and derivative control - Proportional, integral, and derivative (PID) control requires real-time system feedback. PID is a sophisticated control technique which monitors the error between a desired variable value and the actual value, and adjusts the control accordingly (proportional). PID uses an intelligent I/O module or program instruction, which provides automatic closed-loop operation of process control loops. The action dampens the system response; integral corrects the outset; and derivative prevents over- and undershoot. The three mode action control is described below.

• Proportional control alone - The control signal is proportional to the error between the reference and feedback signals.

• Proportional plus derivative control - The error signal is differentiated to get the rate of change. This type of control is used to increase the controller’s speed of response, but can be noisy and make the system less stable.
• 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.

Feedforward - Feedforward control is a direct control or compensation from the reference signal. It may be open loop or in conjunction with PID control and does not require a sensor.

Fuzzy logic - Fuzzy logic is a type of logic in which variables can have imprecise values (as in partial truth) rather than a binary status (completely true or completely false). Information and rules can be inexact, and calculations can be made on these imprecise values.

Advanced or nonlinear control- 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.

Specifications

Specification for flow controllers include controller functions, number of channels, type of output and user interfaces.

Controller Functions

Common functionalities for flow controllers are:

• Rate indication and control- This functionality monitors, indicates, and/or controls the rate of the material or media throughout the system.

• Batch or totalizer indication and control- This functionality totalizes and indicates the amount of the material, media, or process variable.

Input and Control Channels

To choose a flow controller, one important piece of information is the number of inputs, control outputs, and number of zones.

• Number of inputs- The number of inputs is the number of signals sent to the controller. This number includes process variable signals such as temperature, pressure, level, speed, etc., and status input such as on/off.
• Control outputs- The number of control outputs is the total number of outputs used to control, compensate or correct the process. This number includes analog control outputs as well as relays and switches which activate or deactivate control devices.
• Number of zones- The number of zones or areas monitored and maintained by the controller.

Flow controllers can have multiple control modes or functions, which may or may not use different inputs and outputs. Also, multiple control loops may be linked to improve controller performance and/or stability.

Controller Output

• Switch or relay- Some controllers are able to handle high power switching such as relays and optoisolators.
• Update rate- 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.

User Interface

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.

Resources

Glossary of Process Control Terminology

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Kaeser Compressors | Intersoll Rand | Dwyer | Unitronics