FIGURE 1. More representative of common process heating applications, this water bath has thermal lags and is susceptible to temperature overshoot if on/off control is used.


In my last column, I reviewed the elements of a controller that can throttle back the power well ahead of the temperature reaching setpoint and provide a way to defeat temperature overshoot and cycling: a PID (proportional + integral + derivative) controller. Among the parameters that you adjust to optimize (or tune) your controller are the proportional band, integral time and derivative time. I'll pick up with tuning your PID controller.

FIGURE 2.The controller delivers long pulses for high power and short pulses for low power while keeping the same time (typically 10 sec) between the start of successive pulses.

Startup Overshoot

Suppose that a 10 oF (5.5oC) proportional band gives you the tight, stable control that you want after the process has settled down, but that power throttles back too late to avoid overshoot on startup. You could increase the proportional band -- and that would work -- but then you would lose some of that tight control.

Low Cutback.
You could design the controller to introduce a pause at, say, 20oF (11oC) before setpoint; that is, move the proportional band temporarily downwards 20oF. The controller now throttles back the power that much earlier, then slowly takes the proportional band upwards to its normal position. This gives you a new adjustable parameter called low cutback, which is set to 20oF in this example.

Analogy.
When you drive a vehicle to a junction, you ease up on the gas some 200' away (low cutback), then brake for the last 100' (proportional band).
If you notice a startup overshoot, try a low cutback setting equal to the amount of overshoot. Note that this parameter name and means of implementation will vary with manufacturer.



Self-Tuning

Many controllers have a feature calledself-tuneorautomatic tune. Upon initiation of self-tune, the controller typically gives a controlled dose of heat to the process. Then, from the temperature response back from the process, it calculates optimum settings for the PID parameters and sometimes other parameters. You should record these after a successful self-tune because they could be different for different zones on the same process.
Self-tuning can take anything from a minute or two, in the case of fast heating processes such as a fast tungsten lamp applying heat to a low mass material, to one hour or more, with a slow, large mass process.
It is advisable to start self-tune when the process is at or a bit lower than its normal working temperature. Watch the temperature at this stage and be ready to intervene if the initial heat dose threatens to overheat your material or equipment. You must make your own judgement of optimum control. For example, if your processed material takes no harm from a certain overshoot, you might accept that in exchange for a faster time to settle at set temperature.

How the Controller Varies Power to the Heater

Earlier in this series (Ocober 2005), I referred to the controller's amplifier putting out power. The final stage of this sometimes uses silicon-controlled rectifiers (SCRs), which deliver heater power by smooth variation of AC voltage (like a lamp dimmer). This is done in some processes but is not usually necessary or economical on extruder heaters.
In figure 1, the power curve looks like a continuous modulation of power. But, the usual method is topulse the heat contactortypically once every 10 sec (figure 2). This is calledcycle-timeand should not to be confused with the temperature cycling of an unstable control loop.
The controller delivers long pulses for high power and short pulses for low power while keeping the same time (typically 10 sec) between the start of successive pulses. The same idea works on an electric stove ring, or burner. If you turn the knob half way around, the contact closes for 5 sec and then opens for 5 sec continually. This makes your 2 kW ring deliver an average power of 1 kW.
The temperature controller determines the duration of the pulses. Although it can only turn the poweronoroff, it is called atime-proportioning-- not anon/off-- controller. It turns the power progressively lower by ever shorter “on” pulses as the temperature gets closer to the setpoint. The pulses then have just the right duration to hold the temperature at setpoint.
More on tuning the controller next month.

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