Basics of Temperature Control

For accurate control of the process temperature without constant operator involvement industrial thermal processes rely upon a controller. The controller gets is inputs from contact temperature sensors such as a thermocouple, RTD or from a non-contact temperature sensor like an infrared temperature sensor. The controller transmits an electrical signal (current or voltage) to a power switch device which can be a simple relay, a solid state relay or a SCR (Silicone Controlled Rectifier).

Remember: The temperature controller is just one part of the entire control system, so it must be selected accordingly.

The basic types of temperature controllers are: 

1. On / Off control These units turn the power on and off when the set point is crossed. This type of controller is adequate when the process temperature is not very critical and/or larger masses need to be heated (like immersion tanks). Thermostatic controls are On/Off controllers.
2. Microprocessor control (PID control) This is a generic term for a control feedback mechanism widely used in thermal control systems. A PID controller attempts to correct the error between a measured process temperature and a desired set point by calculating and then outputting a corrective action that can adjust the process accordingly. PID is shorthand for Proportional, Integral, and Derivative calculations.

On / Off Controls and Thermostats

Thermostats or ON/Off controls are relatively simple and economic switches widely used for switching power for electric heaters. These controls are found in most residential HVAC systems.  The output will be on or off when the process temperature crosses the set point. This constant crossing of the set point means the temperature is constantly cycling around the set point. This constant cycling around the set point will also reduce heater life because full heat is applied whether the process temperature is 5 degrees or 100 degrees below set point. ON/Off controls increase thermal fatigue and oxidation rate on heating elements by causing wide temperature swings of the internal heating element (Imagine a light bulb turned on and off constantly).

To minimize stress for the heaters and avoid damage to the contactors by rapid cycling, a so called "hysteresis loop" can be added to the on/off control. To maximize heater performance, use a solid state relay (SSR). Such a solid state zero cross contactor can cost as little as $100 but will save you money. Another option is a silicone controller rectifier (SCR) (single phase or 3-phase).

A thermostat can be built from a mercury switch (a glass tube with a small amount of Mercury), a bimetallic strip (lamination of two different metals together) or other temperature sensing devices. Sometimes they also have a heat anticipator, which shuts down the power to the heater before the heat actually reaches the set temperature.

On / Off controls can be used for:

• high temperature alarms
• Where maintaining accurate process temperature is not necessary
• where the mass of the system is so great that temperature changes slowly

PID (Proportional-Integral-Derivative) Microprocessor Temperature Control

This very popular type of controls provides a proportional temperature control combined with integral and derivative temperature control. Proportioning the heat means less power to the heater is supplied if the heat is closer to the set point. This is achieved by the derivative and integral operating modes. The controllers come with set parameters which may be adjusted to your application.

PID controllers can be a very simple set point and auto-tune (adjusts the parameters to match the heating application) or very complicated with temperature and duration programs (ramp and soak) or multiple inputs and outputs.

General Considerations
 1.  Select PID Controller panel size.  Most popular sizes:

• 1/32 DIN (48 x 24 mm)
• 1/16 DIN (48 x 48 mm)
• 1/4 DIN (96 x 96 mm)

2.  Determine what Inputs will feed the PID controller.

The signal input (T/C, RTD or voltage) on most standard controllers may need to be selected at the time of ordering.

3. Decide which control operation is required for the PID control.

Most PID controllers come standard with two main control functions.

• on-off control (you still can set the hysteresis or "deadband")
• PID control (Proportional, Integral, Derivative) For various applications, the PID controller can be used as P control only, PI control (no offset=higher overshoot), PD control (steady state in shortest time) or PID controller. PID control is essentially a compromise between the advantages of PI and PD control ("fuzzy-logic" controllers is just another marketing term for this type of controllers.).
• Programmable ramp and soak
• Communications requirements

4. Ensuring you have enough outputs from the PID controller.

If only required for heating (as opposed to heating and cooling), one output is sufficient.  Outputs can be a relay, SSR, SCR, pulsed voltage, linear voltage, or linear current.

To further protect equipment, a high temperature alarm output may be employed.  This is an independent monitor that shuts down operations when a set temperature is exceeded.

5. Programming the PID controller.  Order and names of parameters vary among control manufacturers, but the parameter names are usually very similar. Programming tools are useful if sets of saved parameters need to be downloaded through an interface (RS 485, RS 232) into different controls or for integrating several controllers into a higher control.