Temperature control basics
A temperature controller is merely a part of a bigger system that depends on feedback (Fig. 1). If you apply heat to a process without knowing the actual temperature of the process, eventually you would have too much heat, or too little. The temperature of a process must be known in order to control it accurately.
A temperature controller is merely a part of a bigger system that depends on feedback (Fig. 1). If you apply heat to a process without knowing the actual temperature of the process, eventually you would have too much heat, or too little. The temperature of a process must be known in order to control it accurately. A sensor must be used to provide temperature measurement and process feedback to any controller.
Sensors used for modern temperature controllers are thermocouples and resistance temperature detectors (RTDs). A thermocouple produces a voltage in the mV range directly proportional to the temperature it senses. It operates on the galvanic action principle, which makes use of predictable and measurable voltage when two dissimilar metals are bonded (Fig. 2).
Fig. 2. A thermocouple is made of two different metals, the ends of which are welded together. A cold junction, or reference junction, is formed where the thermocouple lead wires attach to the controller. A voltage called thermoelectromotive force is developed when the measuring junction and the reference junction are at different temperatures.
The electrical resistance of an RTD changes according to the temperature it senses. The sensing element can be a wire-wound conductor or deposited thin film. RTDs are usually constructed of platinum. However, other materials, such as nickel, have been used to sense temperature and temperature changes.
Closing the loop
Just as a sensor is necessary for temperature control, so is controller output. It does no good to accurately measure a process if there is no way to control the amount of heat or cooling applied to that process.
The process itself ties the system together. The process materials absorb the energy applied to the process. The sensor detects the temperature of the process and feeds this information back to the temperature controller. The controller affects its output to apply more or less heating or cooling to the process. Thus we have closed loop temperature control.
Of all the different temperature controllers on the market, there are still only three basic types — on-off, proportional, and PID. When to use which controller depends on the application and the other equipment in the system.
On-off control is the simplest form of temperature control. All temperature controllers use a setpoint. The setpoint establishes the temperature at which a process is maintained (Fig. 3). For example, setting a temperature controller that maintains a food mixture in a vat at 195 F should ensure that the temperature of ingredients in that vat is around that temperature.
Fig. 3a-b. The output of an on-off temperature controller is either on or off. In (a), the output relay is actuated (on) when the temperature in the furnace is less than the setpoint. It is not actuated (off) when the temperature reaches setpoint. The resulting temperature response is illustrated in (b). On-off temperature control is cyclical for most process applications.
A controller that uses on-off control supplies an output to increase heat when the process temperature is lower than the setpoint and no output when the process temperature is higher than the setpoint. It is 100% on when heating is called for; it is off when the process temperature is at or above the setpoint. This arrangement is reversed for cooling control.
Theoretically, the controller switches on-off states exactly at the setpoint. However, in reality this is not practical. If this condition were allowed to exist, the output device would switch on and off so quickly that it would either make the process unstable or ineffective. Another reason is because rapid state changes would quickly wear out the output actuation device.
Rather than have an on-off temperature controller switch on and off exactly at the setpoint, manufacturers provide for an adjustable range around the setpoint. Introducing a small range above, and/or below the setpoint effectively desensitizes the controller to rapid on-off cycling around the setpoint. Some manufacturers call this adjustable range deadband. Others refer to it as hysteresis. Regardless of the name, it can be effective in stabilizing the operation of an on-off controller if adjusted properly.
In some applications, on-off control produces a cyclical temperature response. The actual temperature of a process could vary from a minimum temperature to a maximum temperature. If the process can tolerate this, an on-off controller may be a simple, inexpensive solution to a temperature control need.
In other applications, the thermal mass of the process may be large enough to resist rapid thermal changes. An example of this is die casting. Some die casting machines maintain a reservoir of molten metal at an optimum temperature to allow the machine to operate efficiently. Because of the amount of material and its resistance to thermal change, an on-off controller is adequate for maintaining precise temperature control over this process.
Proportional control takes on-off control a step further. A temperature controller can be proportional with respect to time, or it can be analog proportional.
Time-proportional controllers apply power to the output as a percentage of a cycle time. If cycle time is adjustable, the time proportional control divides this cycle time into a percentage of that time. If the cycle time is 10 sec, and the controller output is at 45%, the outputs are energized for 4.5 sec of the cycle time. Obviously, for the remaining 5.5 sec the outputs are deenergized. Time proportional controller outputs can be relay, triac, solid-state relay (SSR), or dc pulse, which drives an external SSR.
Analog proportional controllers can have voltage or current outputs. Popular output ranges are 0-5 Vdc and 4-20 mA. Analog proportional controllers are used with SCR power controllers or valve positioning motors (Fig. 4).
Fig. 5. Proportional band is a region above and below the setpoint within which the output of the controller is neither full on nor full off, but somewhere in between. When the proportional band is too narrow, it acts more like an on-off controller with a cyclical temperature response. When the proportional band is too wide, the oscillations disappear, but the controller is sluggish and may never reach setpoint. When proportional band is correct, some initial oscillation may occur, but the temperature will stabilize. If offset occurs, use integral or reset to correct.
To set proportional control, the user selects a proportional band. Proportional band is a region above and below the setpoint within which the output of the controller is neither full on nor full off, but somewhere in between (Fig. 5). The direction and deviation between the setpoint and process temperatures determines the exact output level.
Proportional-integral-derivative (PID) control combines proportional control with two other actions. Integral action is also referred to as reset. It is introduced when a stable process does not coincide with the setpoint. Derivative action is also referred to as rate. It is introduced when abrupt or rapid changes in the load affect controller response.
Reset and rate are intended to compensate for temperature offsets and shifts. More often than not, heaters and burners do not match the application. Typically, systems are designed using the "if enough BTUs are good, then more are better" concept. Not true. In a perfect world, heater or burner output would be 50% when the process and the controller are at setpoint. In real life, there are usually many more BTUs available than are actually needed. Reset helps to minimize this mismatch.
Rate is used when the process or load changes. Extreme variances in load size and thermal mass necessitate the use of the rate parameter. Since the process behaves differently with different loads, the controller must compensate for this difference as if there had been no load change. When used correctly, rate is effective only when there are rapid changes in process physics.
From food processing to HVAC, and from chemical processing to heat treating, there is a plethora of applications for which temperature controllers can be used.
Some plants use embedded temperature control inherent in automated manufacturing lines. Others use temperature controls to maintain the temperature of molten metal in die-casting machines. Then there are injection and blow molding machines, paint drying ovens, industrial/food processing ovens and freezers, and many custom-designed temperature control applications. Regardless of the application, all temperature controllers work in basically the same way.
PLANT ENGINEERING magazine extends its appreciation to Honeywell, Omron Electronics, and Watlow Electric for the use of their materials in the preparation of this article.
Troubleshooting temperature control systems
Most modern temperature control systems are quite reliable. However, problems can be encountered from time to time. Listed are a few things to look for:
Controller shows maximum temperature indication, but other sources show that temperature is ok.
Thermocouple is open. Most controllers have upscale TC burnout, or it can be programmed.
Temperature indication is accurate and controller is calling for heat.
Output section of controller is not operating properly; power controller, valve controller, SCR, or other external control device is not operating.
Check controller output. Check external control devices. Replace defective component.
Temperature indication is accurate, controller outputs and external devices working properly.
Heaters or burners defective.
Replace heating elements. Repair or replace burners.
Tendency toward runaway heat
Temperature indication is ambient, controller is calling for heat, actual temperature is excessive.
Thermocouple leads are shorted outside of the process area.
Locate short. Check for TC wires touching grounded objects such as TC well head or cover; lead wires with bare insulation that could be touching a cabinet or each other.
Temperature indication inaccurate
Temperature controller calibration, faulty sensor
Temperature controller is not calibrated correctly affecting accuracy. Thermocouple is deteriorating. Type J rusts; Type K corrodes with "green rot."
Calibrate temperature controller. Replace thermocouples
Erratic operation; erratic display
Loose thermocouple connections
Loose connections at the controller terminals, junction points, well head, any place along the thermocouple lead path.
Tighten connections wherever they are encountered. Don't use solid lead wire to connect to controller; use stranded. Don't ever crimp lugs to thermocouple lead wire to connect to the controller.
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