Universal process controllers constitute a broad area of control devices that may be employed in the monitoring and control of many different process variables, including temperature, pressure, flow and other values.

Universal process controllers monitor various parameters and initiate controller functions based on measurements. They receive sensor inputs, provide control functions, and output control signals. Universal process controllers use several control types.

Limit controls protect personnel and equipment by interrupting power through a load circuit when a variable 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

Universal process 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
• Accuracy

Hysteresis or switching differential is the range through which an input can be changed without causing an observable response. Typically, hysteresis is set around the minimum and maximum end points. Control channel specifications for universal process controllers include the number of inputs, outputs, and feedback loops. Multi-function controllers and devices with multiple, linked looped 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 universal process controllers requires an analysis of discrete I/O specifications, user interface options, and special features. Devices differ in terms of total number of:

• Input channels
• Output channels
• Discrete or digital channels

Some universal process 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. Universal process 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. Special features for universal process controllers include self-tuning, programmable set points, signal computations or filters, and built-in alarms indicators.

Universal process controllers vary in terms of communications and networking. Both serial and parallel interfaces are available. Common protocols include:

• Attached resource computer network (ARCNET)
• 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^®)
• Sensoplex^®

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

For detailed universal process controller specifications, including controller type, form factor, inputs, outputs, and standards compliance, please see the Temperature Controllers Selection Guide.

Types

Please visit these specification guides for more information on different types of process controllers:

• Temperature
• Level
• Pressure
• Humidity
• Motion

Additionally, environmental controllers may control multiple variables, including temperature, pressure, and humidity, to maintain a stable environment for comfort or testing purposes.

Image Credits:

Endress+Hauser | Extech Instruments Corp.