Last revised: December 3, 2024

Reviewed by: Scott Orlosky, consulting engineer

Pressure controllers are used to regulate positive or negative (vacuum) pressure. They receive pressure sensor inputs, provide control functions, and output control signals. Pressure controllers use several control types. Limit controls protect personnel and equipment by interrupting power through a load circuit when pressure exceeds or falls below a set point. Advanced controls use non-linear control strategies such as adaptive gain, dead-time compensation, and feed-forward control. Linear controls use proportional, integral, and derivative (PID) control; proportional and integral (PI) control; proportional and derivative (PD) control; or proportional (P) control. PID control uses an intelligent input/output (I/O) module or program instruction for automatic closed-loop operation. PI control integrates error signaling for steady-state or offset errors. By contrast, PD control differentiates error signals to derive the rate of change. PD control increases the speed of controller response, but can be noisy and decrease system stability.

Specifications

Pressure controllers differ in terms of performance specifications, control channels, control signal outputs, and sensor excitation supply. Performance specifications include:

• adjustable dead-band or hysteresis
• minimum and maximum set points
• update rate or bandwidth
• percentage accuracy

Hysteresis or switching differential is the range through which an input can be changed without causing an observable response. Hysteresis is usually set around the minimum and maximum end points.

Control channel specifications for pressure controllers include the number of inputs, outputs, and feedback loops. Multi-function controllers and devices with multiple, linked loops are commonly available. Control signal outputs include analog voltages, current loops, and switched outputs. Some controllers power sensors with voltage levels such as 0–5 V or 0–10 mV. Others power sensors with current loops such as 0–20 mA, 4–20 mA, or 10–50 mA.

Selecting pressure controllers requires an analysis of discrete I/O specifications, user interface options, and special features. Devices differ in terms of the total number of inputs, total number of outputs, and total number of discrete or digital channels. Some pressure controllers provide alarm outputs or are designed to handle high power. Others are compatible with transistor-transistor logic (TTL). Analog user interfaces provide inputs such as potentiometers, dials, and switches. Digital user interfaces are set up or programmed with a digital keypad or menus. Pressure controllers with a graphical or video display are commonly available. Devices that include an integral chart recorder can plot data on a strip chart, in a circular pattern, or on a video display.

Features

Special features for pressure controllers include:

• self-tuning
• programmable set points
• signal computation or filters
• built-in alarms or indicators

Pressure controllers vary in terms of communications and networking. Both serial and parallel interfaces are available. Common protocols include attached resource computer network (ARCNET), the AS-interface (AS-i), Beckhoff I/O, controller area network bus (CANbus), DeviceNet, Ethernet, FOUNDATION Fieldbus, general-purpose interface bus (GPIB), Seriplex, smart distributed system (SDS), small computer system interface (SCSI), INTERBUS-S^®, process fieldbus (PROFIBUS^®), and Sensoplex^®.

INTERBUS-S is a registered trademark of Phoenix Contact GmbH & Co. ROFIBUS is a registered trademark of PROFIBUS International. Sensoplex is a registered trademark of Hans Turck GmbH & Co.

Pressure Controllers FAQs

What are the differences between PID, PI, and PD control strategies?

PID controls feature proportional (P), integral (I), and derivative (D) components. They use an intelligent input/output (I/O) module or program instruction for automatic closed-loop operation.

PID controls are ideal for processes requiring precise control and stability, such as in manufacturing and automation systems. They also provide comprehensive control by addressing both the magnitude of the error (P), the accumulation of past errors (I), and the rate of change of the error (D).

Potential downside: PID controls can be complex to tune and may require more computational resources.

PI controls feature proportional (P) and Integral (I) components and they integrate error signaling for steady-state or offset errors.

PI controls are suitable for systems where eliminating steady-state error is crucial and they are simpler than PID controls and effective in eliminating steady-state errors.

Potential Downside: PI controls may not respond as quickly to changes as PID control because they lack the derivative component.

PD controls are made up of proportional (P) and derivative (D) components and they differentiate error signals to derive the rate of change, increasing the speed of controller response.

PD controls are used in systems where quick response is needed and they increase the speed of controller response.

Potential Downside: PD controls can be noisy and decrease system stability due to the absence of the integral component, which helps in eliminating steady-state errors.

What are the advantages of self-tuning pressure controllers?

They can significantly enhance the performance and reliability of pressure control systems.

Self-tuning pressure controllers automatically adjust control parameters to maintain optimal performance. This means they can adapt to changing conditions without manual intervention, ensuring consistent and accurate pressure control.

By automatically tuning themselves, these controllers reduce the need for manual adjustments and fine-tuning, which can be time-consuming and require specialized knowledge. This makes them user-friendly and less dependent on operator expertise.

Self-tuning controllers can dynamically adjust to maintain system stability, even in the face of disturbances or changes in system dynamics. This leads to more reliable and stable pressure control.

By maintaining optimal control parameters, self-tuning pressure controllers can improve the efficiency of the system, potentially leading to energy savings and reduced wear and tear on equipment.

These controllers are versatile and can be used in a variety of applications, from simple systems to complex processes requiring precise control. Their ability to self-tune makes them suitable for diverse industrial environments.

With self-tuning capabilities, the need for system shutdowns for manual tuning is minimized, leading to reduced downtime and increased productivity.

How do downstream and upstream pressure controllers differ?

Downstream and upstream pressure controllers differ primarily in their configuration and the way they maintain pressure relative to the device itself.

Downstream Pressure Controllers

Function: Maintain the pressure downstream of the device.

Operation: Increase flow to increase the downstream pressure and decrease flow to decrease the downstream pressure.

Configuration: Known as direct acting.

Common Name: Often referred to as a standard pressure regulator.

Applications: Suitable for applications where maintaining a constant downstream pressure is critical, such as in gas distribution systems.

Upstream Pressure Controllers

Function: Maintain the pressure upstream of the device.

Operation: Increase flow to reduce the upstream pressure and decrease flow to increase the upstream pressure.

Configuration: Known as reverse acting.

Common Name: Often referred to as a back pressure regulator.

Applications: Suitable for applications where maintaining a constant upstream pressure is necessary, such as in chemical processing or fluid handling systems.

What are the different types of alarms used in pressure controllers?

Visual Alarms

LED indicators: These are simple light-emitting diodes that change color or blink to indicate a deviation from the set pressure point. They provide a quick visual cue to operators.

Digital displays: These can show numerical values or messages indicating the current pressure status and any deviations. They are often part of the user interface and provide detailed information.

Graphical interfaces: Some advanced pressure controllers feature graphical displays that can show trends, charts, or other visual representations of pressure data, making it easier to diagnose issues.

Auditory Alarms

Buzzers: These emit a sound when the pressure deviates from the set point. The sound can vary in intensity or pattern to indicate different types of deviations or faults.

Alarms: Similar to buzzers but can be more sophisticated, with different tones or volumes to signify various levels of urgency or types of issues.

Integrated Alarms with Control Functions

Automatic responses: In some advanced systems, alarms are integrated with control functions that can automatically adjust the system to correct deviations. For example, if the pressure exceeds a certain limit, the controller might reduce the flow to bring the pressure back to the desired level.

Manual intervention prompts: In other cases, the alarms prompt manual intervention by the operator to adjust the system settings or perform maintenance.

Programmable Alarms

Customizable set points: Operators can set specific pressure points and thresholds for alarms, customizing the system to meet the specific needs of their application. This allows for tailored monitoring and alerting based on the unique requirements of the process.

How do advanced control strategies work in pressure controllers?

Advanced control strategies like adaptive gain and dead-time compensation are used in pressure controllers to enhance their performance and stability.

Adaptive gain

This control strategy adjusts the gain of the controller in real-time based on the current operating conditions. This helps in maintaining optimal performance even when the system dynamics change.

The controller continuously monitors critical performance parameters and builds individual internal models for each segment of the gain scheduler. These models are used to update the appropriate set of PID parameters depending on where in the process curve the system is operating.

The adaptive algorithm automatically calculates new PID values for different segments of the process, ensuring that the controller adapts to changes in the system dynamics.

This strategy helps in controlling processes with changing dynamics, provides initial process knowledge through auto-tuning, and creates and updates an internal model of the process it is trying to control.

Dead-time compensation

This is used to mitigate the effects of dead time in control loops. Dead time is the delay between the application of a control action and the observed effect on the process variable.

Dead time can make a control loop more vulnerable to nonlinearities, periodic upsets, and poor tuning. It prevents the PID control algorithm from reacting promptly to an upset.

Dead-time compensation techniques involve predicting the future behavior of the process variable based on current and past data. This allows the controller to take corrective actions in advance, reducing the impact of dead time.

Some advanced controllers use model-based control strategies to compensate for dead time. These models predict the future values of the process variable, allowing the controller to adjust its output accordingly.

Effective dead-time compensation can significantly improve the stability and performance of the control loop, especially in systems with large dead times.

Pressure Controllers Media Gallery

References

GlobalSpec—Process/Industrial Instruments and Controls Handbook, 5th Edition

GlobalSpec—Basic and Advanced Regulatory Control: System Design and Application, 2nd Edition

Image credit:

First Sensor AG