Installation manual

Series C Controller Configuration and Operation Manual

1-10  Athena Controls, Inc. 900M050U00

1.5.3.2 Use of Deadband

The controllers also permit you to specify a deadband value for each on/off output. In an

application where the controller’s two outputs are used to control heating and cooling of

the same process, this has the effect of defining a band around the setpoint in which

neither the heating nor the cooling output is on. This avoids having the heating and

cooling devices working against one another.

To allow you to optimize the control of your process, the heating deadband and the

cooling deadband can be different values. When the deadband value is applied to an

output, it has the effect of shifting the setpoint and the hysteresis for that output. In the

diagram below we call the setpoint the controller actually uses the “effective setpoint”.

When a deadband is configured, the value of this effective setpoint is different from the

value of the displayed setpoint.

For a heating (reverse-acting) output, a positive deadband (value greater than zero)

moves the effective setpoint used by the controller below the displayed setpoint. For

cooling (direct-acting outputs), a positive deadband moves the effective setpoint used by

the controller above the setpoint displayed.

In the diagram below, the setpoint is 100, the hysteresis for each output is 10, and a

deadband of 10 has also been configured for each output. (These are probably not

realistic values for most processes, but we’re using these values to demonstrate the

controllers’ behavior when hysteresis and deadband are configured for both heating and

cooling outputs.)

The heating output is on until the process value reaches 95 (setpoint minus heating

deadband plus one half heating hysteresis), but the overshoot does not trigger the

cooling output until the temperature reaches 115 (setpoint plus the cooling deadband

plus half the cooling hysteresis).

The cooling output stays on until the PV reaches 105 (setpoint plus cooling deadband

minus one half cooling hysteresis). The heating output does not come on until the PV

has dropped to 85 (setpoint minus heating deadband minus one half heating hysteresis).

The heating output goes off when the PV gets to 95 (setpoint minus heating deadband

plus one half heating hysteresis).

However, if your application sometimes requires use of heating and cooling simultaneously, you can

specify a negative deadband value. The range of valid deadband values is affected by input type; see

8.2.2 for details.

A negative deadband value has the opposite effect.