Industrial process control systems range from modular, panel-mounted controllers to large, interconnected, interactive, distributed control systems. All receive data from sensors that measure process variables, compare those variables against setpoints and derive command functions.

Basic controller function

Controllers govern, or hold, some variable or parameter at a set value. Two required variables are the actual input signal (also known as the process value) and the desired setpoint value. Controller inputs may be sampled many times each second, depending on the device.

Figure 1: Control panels from Wattco. Source: Wattco

In practice, the input value is compared with the setpoint value. If the measured value does not match the setpoint, the controller generates an output signal. This initiates a response to correct the actual value so that it matches the setpoint.

Motor controllers

Motor controllers receive supply voltages and provide signals to drives that are connected to motors. The controllers start, stop and run motors in a programmed manner, and can be used to start or increase motor speed, adjust torque or reverse the motor’s rotational direction. Components typically include a power supply, amplifier, user interface and position control circuitry.

Two types of controllers are common: electronic and electromechanical. Electronic units include features such as soft starting and variable frequency drives. These can be programmed to respond to system inputs and preset conditions. Electromechanical units use contractors or relays to control the motor.

Temperature controllers

Temperature controllers are used in industrial processes where a given temperature must be maintained.

Two basic types of temperature control exist. Open-loop control refers to continuous heating or cooling and operates independently of the actual temperature output. By contrast, closed-loop control measures and adjusts the output temperature to maintain a constant temperature.

The type of input sensor and signal needed for a temperature controller may vary depending on the controlled process. Typical input sensors include thermocouples and resistive thermal devices (RTDs). Controllers may also be set to accept an RTD as a temperature-sensing input.

Controllers also can be set to accept voltage or current signals in the millivolt, volt or milliamp range from pressure, level or flow sensors, among other types.

On/off controls are among the simplest controllers and work by setting a hysteresis band. For instance, if the setpoint is 68° F and the actual temperature falls to 67° F, an error signal would show a -1° F difference. The controller then sends a signal to increase the applied heat to regain the setpoint.

Every controller also has an output, which controls a process, initiates an alarm or transmits the process value to a programmable logic controller or recorder.

Typical outputs include relay outputs, solid-state relay drivers and linear analog outputs. A relay output is usually a single-pole double-throw relay with a DC voltage coil. The controller energizes the relay coil, providing isolation for the contacts. This lets the contacts control an external voltage source to power a heating contactor coil.

Analog controllers are among the simplest and are well suited for harsh industrial environments including those with significant electrical noise. Controller display is typically a knob dial.

Single-loop controllers

Limit controllers provide safety-limit controls over process temperature and have no ability to control temperature on their own. Instead, they are independent safety devices and are used alongside an existing control loop. They are capable of accepting thermocouple, RTD or process inputs with limits set for high or low temperature like a regular controller.

General-purpose temperature controllers are used to control most typical industrial processes. They come in a range of DIN sizes, have multiple outputs and programmable output functions. These controllers can also perform PID control for general control situations. They are traditionally placed in the front panel for easy accessibility.

Multi-loop controllers

In addition to these single-loop controllers, multi-loop controllers can accept more than one input variable and therefore control more than one loop. These systems tend not to have a traditional user interface but, instead, use a dedicated communications link.

Applications

Selecting motor controllers requires an understanding of application categories. Motor speed controllers are application-specific devices that are used to control machines such as conveyors. Temperature controllers may be used in a variety of heating and cooling applications, including boilers, furnaces, hot runners, HVAC and laboratory control.

Some of the most common motor applications include multi-axis controllers to control and monitor multiple, independent axes of motion; robotic motion controllers; microcontrollers, which are computer systems where real-time manipulation of large amounts of digital data is required to improve or modify it; silicon controlled rectifiers, which are used in DC motor drives to rectify the AC input and produce DC current; digital signal processors, which are microprocessors that use real-time data to improve or modify them; and pulse width modulation (PWM). A PWM drive converts AC voltage and frequency to DC and simulates a sine wave.

Regulations and standards

Controller manufacturers follow industry standards for uniform quality and safety. The most common approval, UL and CUL registration, applies to all controllers used in the U.S. and Canada. For controllers that are used in European Union countries, CE approval is necessary. A third type of approval, FM, applies to limit devices and controllers in the U.S. and Canada.

An important controller characteristic is the front panel enclosure rating. These ratings can be in the form of an ingress protection (IP) rating or a NEMA rating. IP ratings apply to all controllers and are usually IP65 or higher. This means that from the front panel only, the controller is protected from dust and against low pressure jets of water from all directions with only limited ingress permitted. IP ratings are used in the U.S., Canada and Europe.

A NEMA rating is parallel to an IP rating. Most controllers have a NEMA 4 or 4X rating, which means they can be used in applications requiring water washdown only. NEMA ratings are used primarily in the U.S. and Canada.

For more on motor controllers and temperature controllers, contact Wattco.