Options Extend Temperature Controls
Basic PID temperature controllers are not so basic any more. Learn about features that may help you with your control needs.
Virtually all manufacturers of temperature controllers today offer a means of autotuning. Generally, you turn autotuning on, and then allow the temperature controller to run the temperature up and down for a few cycles. This gives the controller a feel for how the system responds when heat is applied, and how the system cools once the heat is removed. From those initial measurements, an autotuning temperature controller can set the PID parameters for you.
But, PID temperature controllers are reactive by design. They need to see an error before they begin to adjust the output. An error means that the process value
"... PID temperature controllers are reactive by design. They need to see an error before they begin to adjust the output."
(PV) does not equal the setpoint (SP). When an error is measured, the controller begins to calculate the error and adjusts the output accordingly. So, by design, the PID controller reacts once the error occurs.
Many manufacturers offer another means of tuning. Called fuzzy logic or continuous tune, with this method, the controller learns from experience and begins to anticipate changes and upsets to the system. The system does not react to errors; instead, it learns from history and attempts to anticipate the changes. These controllers work well for processes with conditions that change frequently, such as those with varying ambient air temperature, frequent changes to the material being heated, and changes in line speed.
Another feature worth exploring is the ability to have more than one setpoint. Multiple setpoint is particularly useful if the process needs to stay at an idle temperature for a period of time, then change to another setpoint when the process is running. The ability to change the setpoint from an idle point to a working point can save money.
Profiling controllers allow users to set up temperature ramps and soaks. For instance, a user would say, “I’d like to ramp the temperature from 70 to 500°F, and take two hours to reach 500°F. Then hold [soak] the temperature at 500°F for three hours.” Some profiling controllers will control two PID processes at the same time like temperature and humidity. And, many profiling controllers offer digital inputs and outputs that allow the user to turn on things like fans or lights at specific points in the profile.
Temperature controllers with an LCD display that displays text and graphics can be used to run profiles and log data. Some displays even can show a graph, much like a chart recorder.
Yet, another new feature offered by some manufacturers integrates a temperature controller and a limit device in one package. Integrated multi-function controllers are a new class of instruments that combine many functions into a single package:
- Process and machine control
- Over- and under-temperature limit protection
- High-amperage power switching
- Safety disconnect and field communications
This provides both a temperature controller and a limit device in one small package.
One of the most interesting controls offered is an integrated solid-state controller that offers a complete thermal system control solution in a single package. Features include a PID temperature controller connected to a high-amperage, solid-state relay, with the option of adding a properly sized heat sink, an over- and under-temperature limit, a power shutdown contactor and digital communications in one complete product. The main advantage of this controller is the reduced wiring needed for installation.
Certain features may be particularly attractive for specific applications. For instance, current measuring is a popular option. A small donut transformer is placed on one of the leads on the heater. The controller measures the current and alarms if the current falls below a certain point, informing users that the heater may be defective.
Password protection is another feature used in many industries. Most controllers offer a method to lock out a user and prevent unauthorized personnel from making changes to the controller.
Some manufacturers even offer a software package to set up the controller and even save the configuration.
The best way to know how much control you need is to begin with your process. But once, you know, rest assured that there is a controller out there that can give you complete mastery over your process.
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