On/Off Control. On/off is a simple, economical control method. For heating applications, the heater being controlled is either completely on (if the process temperature is below the setpoint) or completely off (if the process temperature is at or above the setpoint). On/off control does not account for how far the actual temperature is from the setpoint. Because the process temperature must cross the setpoint for the output to change, frequent cycling between on and off is common. Rapid switching is known as chatter.
Proportional Control. When a system requires a higher level of accuracy or experiences frequent load changes, a proportional temperature controller should be selected. Multiple types of proportional control exist. Depending on the application, some types are more appropriate than others.
In heating applications employing time-proportioning control, the heater is on as long as the measured temperature is below the proportional band. Once the temperature enters the band, the heater is cycled on and off. As the process temperature approaches the setpoint, the heater is switched on for decreasing amounts of time and off for increasing amounts of time. At the midpoint of the proportional band, cycle time for both on and off is 50%. Timed cycling between on and off within the proportional band helps prevent setpoint overshoot.
True proportional control employs the same proportional band as time-proportioning control, but this method utilizes linear outputs such as current or voltage to vary the degree of on or off rather than varying the amount of time. For example, a solenoid valve that ac-cepts a 4 to 20 mA signal allows the controller to proportionally vary current supplied to the valve based on the de-viation from setpoint once the process temperature enters the proportional band. This situation allows the valve to be 0 to 100% on, or open, instead of completely on (open) or off (closed).
The primary problem with both time-proportioning and true proportional control is that stabilization usually occurs slightly above or below the actual setpoint. This phenomenon is known as offset or droop (figure 2). On many controllers, a manual reset is available to return the process temperature to setpoint. This manual adjustment must be made by an operator and may prove to be inconvenient. Proportional-integral (PI) control addresses this problem by incorporating an automatic reset to compensate for droop before it occurs (figure 3).
Fuzzy logic PID control is a development in machine intelligence that utilizes true or false facts composed of a series of ones and zeros, giving the controller a range of response possibilities. Fuzzy logic allows the controller to create a temporary setpoint that continuously changes as the process temperature approaches setpoint. This essentially eliminates overshoot during startup or load changes. Fuzzy logic is too complex to be discussed in detail in this article, but numerous books on the topic are available.
Consider Your OptionsAs important as the control action being used, additional features and options help customize a controller to meet the specific needs of a thermal control system. When selecting a controller, remember to consider external factors and performance features including:
- Number of sensor inputs, their type and placement.
- Number of outputs and types (mechanical relay, solid-state relay, voltage pulse, current or voltage).
- Output action (direct or reverse acting).
- Ramp and soak profiling.
- High/low alarms.
- Analog retransmission.
- Digital communication.
- Power rating.
- Agency approvals.
- Operator lockout.
- Housing size and type.
By taking advantage of select features, a controller can be configured to meet the needs of your specific application.
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