Actuators, Drives & Embedded State Machines

Switching a heater

The smoker’s actuator is a resistance element: give it 120 V and it makes 1,500 W of heat. It can’t be “half on” through a simple switch, and dimming it with a big resistor would just waste power as heat in the resistor. Instead, a solid-state relay switches the element fully on or fully off from a low-voltage signal, with no moving contacts to wear out.

A zero-crossing SSR waits for the AC voltage to pass through zero before turning on, so the element switches on gently without a burst of electrical noise each time, which helps the thermocouple’s microvolts from the last two lessons.

Time-proportioning

To get partial power from an on-off switch, turn it on for part of a fixed window. With a 2-second window:

• 100 percent: on the whole 2 s;
• 35 percent: on 0.7 s, off 1.3 s;
• 0 percent: off.

The smoker’s thermal mass is huge compared to 2 seconds (its time constant is around 25 minutes), so the chamber only “sees” the average power. The PID’s output, 0 to 100 percent, becomes the fraction of each window the element is on. Industrial temperature controllers drive heaters the same way.

The same idea for motors

A charcoal smoker controls heat differently: a small DC blower feeds air to the coals, and more air means more heat. The controller drives the fan with pulse-width modulation (PWM), the same on-off averaging but switched thousands of times a second so the motor just sees an average voltage and runs at a steady partial speed. The control logic, states, and tuning in the rest of the course are the same; only the actuator and its time constants change.

The states

A controller that only runs a PID loop handles the easy case and nothing else. Write the whole behavior as a state machine:

• OFF: element off; waiting for start.
• PREHEAT: element at full power until the chamber is within 20 °F of setpoint. Starting the PID far from setpoint would just saturate it at 100 percent while its integral term winds up; lesson 6 shows the overshoot that causes.
• HOLD: the PID drives the element through time-proportioning. Watch the meat probe.
• DONE: the meat probe reached its target (160 °F inside for bacon smoked at 225–250 °F ¹); drop to a low warm-hold setpoint and signal.
• FAULT: element off, alarm on, stay there until someone resets it.

Every state needs a timeout and an exit. PREHEAT that never reaches setpoint isn’t a state, it’s a stuck controller.

Faults

Any state can fall into FAULT. Decide the triggers in advance:

• Probe open or shorted: the converter flags it (lesson 2).
• Chamber over 300 °F: something is wrong; the independent high-limit thermostat (lesson 1) is the backup if this check fails too.
• No rise: 20 minutes at full power with no temperature increase means a dead element, a failed relay, or a probe that has fallen out and is reading the outside air. Turning the heat up is the wrong response to all three.
• Lid open: a sudden 30-degree drop. That’s not a fault, but the controller should freeze its integral term until the temperature recovers (lesson 6), or it will overshoot when the door closes.

What to take from this

An SSR switches the element fully on or off; time-proportioning (0.7 s on per 2 s window = 35 percent) gives any average power because the smoker’s response is slow. Zero-cross switching keeps noise down; PWM does the same for a blower motor. Organize the controller as states (off, preheat, hold, done, fault), give each an exit, and make every fault end with the element off: open probe, over 300 °F, or no rise at full power.