Watlow Electric EZ-ZONE PM On-OffControl, Proportional Control

	Set Point	Actual Temperature
	Temperature		Sensor
			Break
100%		Output Power
40%	Power	2 minutes	Locks in
			Output
0%			Power

Time

Input Error Latching [`i;Er] (Setup Page, Analog Input Menu) determines the controller’s response once a valid input signal returns to the controller. If latching is on, then the controller will continue to indicate an input error until the error is cleared. To clear a latched alarm, press the Advance Key ‰ then the Up Key ¿.

If latching is off, the controller will automatically clear the input error and return to reading the tem- perature. If the controller was in the auto mode when the input error occurred, it will resume closed-loop control. If the controller was in manual mode when the error occurred, the controller will remain in open-loop control.

The Manual Control Indicator Light % is on when the controller is operating in manual mode.

You can easily switch between modes if the Control Mode [`C;M] parameter is selected to appear in the Home Page.

To transfer to manual mode from auto mode, press the Advance Key ‰ until [`C;M] appears in the lower display. The upper display will display [AUto] for auto mode. Use the Up ¿ or Down ¯ keys to select [Man]. The manual set point value will be recalled from the last manual operation.

To transfer to auto mode from manual mode, press the Advance Key ‰ until [`C;M] appears in the lower display. The upper display will display [MAn] for manual mode. Use the Up ¿ or Down ¯ keys to select [AUto]. The automatic set point value will be recalled from the last automatic operation.

Changes take effect after three seconds or immediately upon pressing either the Advance Key ‰ or the Infinity Key ˆ.

On-Off Control

On-off control switches the output either full on or full off, depending on the input, set point and hysteresis values. The hysteresis value indicates the amount the process value must deviate from the set point to turn on the output. Increasing the value decreases the number of times the output will cycle. Decreasing hysteresis improves controllability. With hysteresis set to 0, the process value would stay closer to the set point, but the output would switch on

and off more frequently, and may result in the output “chattering.”

On-off control can be selected with Heat Algorithm [`h;Ag] or Cool Algorithm [`C;Ag] (Setup Page, Loop Menu).

On-off hysteresis can be set with Heat Hysteresis [`h;hY] or Cool Hysteresis [`C;hY] (Operations Page, Loop Menu).

Note: Input Error Failure Mode [faIl] does not function in on- off control mode. The output goes off.

	The heating action switches off when the process
	temperature rises above the set point.
Temperature		Set Point
			Hysteresis
	Process Temperature
		The heating action switches on when the process temperature
	The heating action	drops below the set point minus the hysteresis.
	switches on at startup.
		Time
	The cooling action	The cooling action switches on when
		the process temperature rises above
	switches	the set point plus the hysteresis.
	on at startup.
Temperature	Process Temperature	Set Point	Hysteresis

The cooling action switches off when the process temperature drops below the set point.

Time

Proportional Control

Some processes need to maintain a temperature or process value closer to the set point than on-off control can provide. Proportional control provides closer control by adjusting the output when the temperature or process value is within a proportional band. When the value is in the band, the controller adjusts the output based on how close the process value is to the set point.

The closer the process value is to the set point, the lower the output power. This is similar to backing off on the gas pedal of a car as you approach a stop sign. It keeps the temperature or process value from swinging as widely as it would with simple on-off control. However, when the system settles down, the temperature or process value tends to “droop” short of the set point.

With proportional control the output power level equals (set point minus process value) divided by the proportional band value.

Watlow EZ-ZONE®PM Integrated Controller • 60 • Chapter 9 Features