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PID & Process Temperature Controllers

Introduction to Temperature Control: On-Off, Proportional, and PID Controllers

A temperature controller is an instrument, which is used to control the temperature of a process. A temperature controller is part of a system, which is connected to two components: an input, such as a temperature sensor (thermocouple or RTD) and an output, such as a control element (heater or fan). The temperature controller receives an input signal from the temperature sensor and based on the signal received, it will send an output signal to the control element.

In order to control the process temperature accurately without operator intervention, the temperature controller comparers the actual temperature, which it receives from the temperature sensor to the desired control temperature or setpoint of the process. The temperature controller will then send an output to the control element to either increase or decrease the temperature to meet the setpoint of the process.

What Are the Different Types of Temperature Controllers and How Does a Temperature Controller Work?

There are three types of controllers: On-Off, Proportional and Proportional Integral Derivative (PID) and depending upon the system to be controlled, the operator is able to use one type or another to control the process and these three different types are described below.

On-Off Temperature Controllers

The first and simplest type of a temperature controller is the On-Off controller. The output signal for On-Off controllers is either on or off, which means there is no middle state. This type of controller will send an output signal to turn on the control element when the temperature of the process exceeds the setpoint.

When controlling heating, the output is “on” when the temperature is below the setpoint and the output is “off” when the temperature is above the setpoint and as the temperature crosses the setpoint, the process temperature will cycle continuously to above and below the setpoint. For processes, in which cycling occurs rapidly or to prevent damage from occurring to contactors and valves, an on-off differential, or "hysteresis," is added to the controller. This differential requires that the temperature exceed setpoint by a certain amount before the output will turn off or on again. On-off differential prevents the output from "chattering" or making fast, continual switches if the cycling above and below the setpoint occurs very rapidly. On-off controllers are used usually when precise control is not necessary in systems which cannot handle having the energy turned on and off frequently or when the mass of the system is so great that temperatures changes are extremely slow, and for a temperature alarm.

One special type of on-off controller, which is used to alarm the operator, is a safety limit controller. A safety limit controller is an off-off temperature controller with a latching output, which means when the output exceeds the setpoint, the operator is required to reset the controller manually. Safety limit controllers are used as redundant controllers typically to shut off a process when an undesirable temperature is reached.

Proportional Temperature Controllers

The second type of controller is the Proportional Controller. To eliminate the cycling associated with On-Off controllers, proportional controllers are designed to decrease the average power supplied to the heater as the temperature approaches setpoint, which results in slowing down the heater so that the temperature does overshoot the setpoint but rather approaches the setpoint slowly and maintains a stable temperature. This action is refered to as proportioning.

This proportioning action can be accomplished by turning the output on and off for short time intervals. This "time proportioning" varies the ratio of "on" time to "off" time to control the temperature. The proportioning action occurs within a "proportional band" around the setpoint temperature. Outside this band, the controller functions as an on-off unit, with the output either fully on (below the band) or fully off (above the band). However, within the band, the output is turned on and off in the ratio of the measurement difference from the setpoint. At the setpoint (the midpoint of the proportional band), the output on:off ratio is 1:1; that is, the on-time and off-time are equal. if the temperature is further from the setpoint, the on-off-times vary in proportion to the temperature difference. If the temperature is below setpoint, the output will be on longer; if the temperature is too high, the output will be off longer.

Proportional-Integral-Derivative (PID) Temperature Controllers

The third type of controller is the proportional-integral-derivative controller (PID). PID controllers are the most accurate and stable controller of the three types and are best used in systems or processes with relatively small masses, which react quickly to changes when energy is added to the process. This type of controller combines proportional control with two adjustments, which helps compensate automatically for changes in the system. These two adjustments, known as integral and derivative, are expressed in time-based units and are also referred to by their reciprocals, Reset and Rate, respectively. The proportional, integral, and derivative terms must be individually adjusted or "tuned" to a particular system using trial and error and are recommended in systems, in which the load changes often and the controller is expected to compensate automatically due to frequent changes in setpoint, the amount of energy available, or the mass to be controlled. OMEGA Engineering offers a number of controllers, which tune themselves automatically and these are referred to as auto tuning controllers. To see a complete line of PID Controllers in Singapore, visit OMEGA Engineering’s Singapore website.

Selecting the Appropriate Temperature Controller

The controller is one part of the entire control system and it is recommended to review the entire system should be prior to selecting a controller. The following points will help guide you through the selection process of a controller:

  1. Type of input sensor (thermocouple, RTD) and temperature range
  2. Type of output required (electromechanical relay, SSR, analogue output)
  3. Control algorithm required (on-off, proportional, PID)
  4. Number and type of outputs (heat, cool, alarm, limit)
Temperature Controller: Cutouts of Standard DIN for Panel Meters

Temperature Controller Standard Sizes

Temperature controllers are generally mounted inside instrument panels, which requires the panels to be cut to accommodate the appropriate size of temperature controller and in order to provide interchangeability, most temperature controllers are designed to standard DIN sizes. The most common DIN sizes are shown below. To see a complete line of Temperature Controllers in Singapore, visit OMEGA Engineering’s Singapore website.

  1. 1/4 DIN
  2. 1/8 DIN
  3. 1/16 DIN
  4. 1/32 DIN

Choose the PID controller for your application

Top
Autotune PID Controllers Autotune PID Controllers
PID controllers are the most accurate and stable controllers but the PID algorithm requires tuning. OMEGA Engineering offers a number of PID controllers, which tune themselves automatically and these are referred to as auto tuning controllers.
Multiloop Controllers Multiloop Controllers
Each control loop consists normally of one input and at least one output. OMEGA offers numerous multi loop controllers which can handle more than a single control loop, such as OMEGA's CN1507, which can handle up to seven control loops.
On-Off Controllers On-Off Controllers
On-Off controllers are the simplest type of temperature controllers and they feature on-off control, which provide PID functionality but at a price suited for on-off applications.
Safety Limit Controllers Safety Limit Controllers
A safety limit controller is an off-off temperature controller with a latching output, which means when the output exceeds the setpoint, the operator is required to reset the controller manually. Safety limit controllers are used as redundant controllers typically to shut off a process when an undesirable temperature is reached.
Temperature Switches Temperature Switches
An adjustable temperature switch is suited for applications which require an economical solution to temperature control. Temperature switches are less complicated and easier to set up than more sophisticated electronic controllers.