| Uncontrolled system If powered without regulation, a heater will rise in temperature until heat losses (increas- = ing with temperature) equal heat input. This may be acceptable in rare situations, but " normally is avoided because the equilibrium temperature is highly unpredictable. In most cases the heater temperature needs to be controlled. This allows the heater to ramp up to setpoint faster without fear of overshooting and burning out the heater. On/off control On/off is the most basic form of control: Full power on below setpoint, power off above setpoint. Electronic on/off controllers offer faster reaction time and tighter control than thermostats. All on/off controllers have a differential (hysteresis or dead band) between the on and off points to reduce rapid cycling and prolong switch life. With on/off control, temperature never stabilizes but always oscillates around the setpoint. Proportional control A proportional controller reduces power as the heater approaches setpoint. This reduces oscillation for steadier control. Note that most controllers are "time proportioning," where they scale power by rapid on/off switching. Short cycle times usually require a solid state relay for power switching. Simple proportional controllers can experience "droop" where the temperature settles at a point near the setpoint but not exactly on it. Proportional/Integral/Derivative controllers solve the problem of droop and otherwise improve control accuracy through advanced digital algorithms. They have various tuning parameters for best control, but typically have some preset modes suitable for most situations. |