What is Digital Temperature Controller?

A Digital Temperature Controller is a device that is used to control temperature.The role of the temperature controller is to measure the temperature thru the thermocouple, compare it to the set point and to calculate the amount of time the heater should remain switched on to maintain a constant temperature.

Working

It does this by first measuring the temperature (process variable), it then compares to the desired value (set value). The difference between these values is known as the error (Deviation).

Digital Temperature Controller use this error to decide how much heating or cooling is required to bring the process temperature back to the desired value. Once this calculation is complete the controller will produce an output signal that effects the change required.

This output signal is known as the (manipulated value) and is normally connected to a heater, control valve, fan or some other “final control element” which actually injects or removes heat from the process.

Temperature control applications in industry:

 1. Heat Treat/Oven. Temperature controllers are used in ovens and in heat

      Treating applications within furnaces, ceramic kilns, boilers, and heat

      Exchangers.

 2. Packaging.

       3. Plastics.

       4. Healthcare.

       5. Food & Beverage.

Types of Digital temperature controller

1. On/off  temperature Controller.

2. Proportional Control.

3. Proportional integral derivative control (PID)

On/off temperature Controller

An on-off Digital Temperature Controller is the simplest form of control device. The output from the device is either on or off, with no middle state.An on-off controller will switch the output only when the temperature crosses the set point.

Working

For heating control, the output is on when the temperature is below the set point, and off above set point.

Since the temperature crosses the set point to change the output state, the process temperature will be cycling continually, going from below set point to above, and back below.

 In cases where this cycling occurs rapidly, and to prevent damage to contactors and valves, an on-off differential, or “hysteresis,” is added to the controller operation.

This differential requires that the temperature exceed set point 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 set point occurs very rapidly.

On-off control is usually used where a precise control is not necessary, in systems which cannot handle having the energy turned on and off frequently,

where the mass of the system is so great that temperatures change extremely slowly, or for a temperature alarm.

One special type of on-off control used for alarm is a limit controller. This controller uses a latching relay, which must be manually reset, and is used to shut down a process when a certain temperature is reached.

Applications

Digital Temperature Controller systems for houses (heating and cooling), freezers and   other home appliances are using on-off control.

Proportional Control

Proportional controls are designed to eliminate the cycling associated with on-off control.

WORKING
 A proportional controller decreases the average power supplied to the heater as the temperature approaches set point. This has the effect of slowing down the heater so that it will not overshoot the set point, but will approach the set point and maintain a stable temperature. 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 set point temperature.

Outside this band, the temperature 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 set point.

At the set point (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 set point, the on- and off-times vary in proportion to the temperature difference.

If the temperature is below set point, the output will be on longer; if the temperature is too high, the output will be off longer.

Application
1. It is used to precisely control the temperature of optics, lasers, and biological samples.

2. It is used sensitive devices. One is a thermoelectric or Peltier device.

Types of Proportional Control Temperature controllers

1. Precisely control
2. sensitive control

PID Control

Proportional with integral and derivative control (PID). This controller combines proportional control with two additional adjustments, which helps the unit automatically compensate for changes in the system.

A proportional integral derivative (PID) controller can be used as a means of controlling temperature, pressure, flow and other process variables.

As its name implies, a PID controller combines proportional control with additional integral and derivative adjustments which help the unit automatically compensate for changes in the system.

PID Controller Working Principle

The working principle behind a PID controller is that the proportional, integral and derivative terms must be individually adjusted or “tuned.” Based on the difference between these values a correction factor is calculated and applied to the input.

For example, if an oven is cooler than required, the heat will be increased. Here are the three steps:-

1. Proportional tuning: involves correcting a target proportional to the difference. Thus, the target value is never achieved because as the difference approaches zero, so too does the applied correction.

 2. Integral tuning: attempts to remedy this by effectively cumulating the error result from the “P” action to increase the correction factor.

For example, if the oven remained below temperature, “I” would act to increase the head delivered. However, rather than stop heating when the target is reached, “I” attempts to drive the cumulative error to zero, resulting in an overshoot.

3.Derivative tuning: attempts to minimize this overshoot by slowing the correction factor applied as the target is approached.

Application

The best PID controller application is temperature control where the controller uses an input of a temperature sensor & its output can be allied to a control element like a fan or heater.

1. Temperature Control of Furnace

2. Heat treatment of metals: “Ramp & Soak” sequences need precise control to ensure desired metallurgical properties are achieved.

Types of PID Controller

1. ON/OFF Control

2. Proportional Control

3. Standard Type PID Controller

4. Real-Time PID Controllers