Temperature control is crucial for chemical reactions and material production. Analog controllers react to the current environment, while digital controllers use algorithms like PID to anticipate future temperature changes. PID controllers use proportional, integral, and derivative calculations to maintain a constant temperature.
Temperature control is a prerequisite for essentially every chemical reaction people are interested in. Temperature affects the rate of reaction and often the completeness of the reaction. The human body incorporates a biological temperature control system to maintain a narrow range of body temperature. Processes designed to produce various materials also require temperature control. The engineer can choose between an analog and a digital temperature controller.
Some analog home thermostats consist of a copper coil. As the strip expands with heat, the spiral expands by moving a mechanical lever. The furnace or air conditioner responds accordingly. Analog controllers only react to the current environment.
The microprocessor in a digital temperature controller takes numerical input from the environment and manipulates it to allow for a greater degree of control. If a system heats up quickly, the analog system will only react when the controller reaches the desired temperature, called the setpoint (SP). The heat source can be turned off, but the system will outperform the SP because it draws energy from the hot radiating surfaces surrounding the system. A digital temperature controller calculates the rate at which the temperature is rising and activates the response of the appliance before the SP is reached. The controller used past data to predict and change future results.
There are many algorithms or calculation schemes that a digital temperature controller could employ. One of the most common is the proportional-integral-derivative or PID controller. Use three separate calculations to maintain a constant temperature.
The error (e) is the difference between the actual temperature (T) and the setpoint temperature (SP). Proportional calculus transforms an input stream into a process based on the magnitude of E. An E of 2 would require twice as much energy input as an E of 1.
Proportional control prevents the system from exceeding the SP, but response may be slow. The integral method predicts that future data trends will last. In the example above, if T increases by an E by 2 and then by an E by 4, the system might anticipate that the next E will be 8, so instead of doubling the response, it might triple the response and not wait for the next measurement.
A proportional and integral (PI) controller can oscillate around the SP, bouncing between too hot and too cold. A derivative control method will dampen the oscillation. The rate of change of E is used in the calculation.
The PID controller uses a weighted average of the three calculations to determine what action should be taken at any given time. This digital temperature controller is the most common and effective, as it uses current, historical and forward data. Other control schemes require information about the nature of the system. This knowledge increases the controller’s ability to anticipate future system response.
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