Emerson E2 Loop/Sequence Control Cell Descriptions, Output Cells, Diagram, Select Cell

change drastically. The filtered PID percentage is sent to the final control cell, the Override cell.

Step 5: Override - Once the PID percentage is figured, the Override cell provides for overrides of the PID per- centage. The Override cell, when activated by a user-initi- ated manual override, will block the PID percentage from being sent to the outputs, replacing it with a fixed value specified by the user.

The value from the Override cells is then sent to the Loop/Sequence Control’s PID output, and also to the two output cells for conversion to stages and PWM.

11.13.1.2 Output Cells

The two output cells make no alteration of the PID per- centage determined by the control cells, but re-interpret them in a way that can be used for systems with multiple

stages and/or with pulse width modulation.

Sequencer - The Sequencer cell will use the percent- age to activate an equivalent percentage of its defined stage outputs. The Sequencer supports up to eight cells, and also provides for minimum on/off times and first-on/ last-off sequencing.

PWM - The PWM cell drives a digital output whose ON time per pulse width modulation period is equal to the PID percentage. In other words, if the PWM cell is set with a pulse width of 10 seconds, a PID percentage of 60% turns the output ON for 6 seconds, OFF for 4 seconds, ON for 6 seconds, etc.

11.13.1.3 Diagram

An abstract diagram of the control and output cells is shown in Figure 11-24.

Occ SP	SELECT CELL			FILTER			OVERRIDE		Analog PID/PWM
AV	In 1						In	Out	Loop Output
Unoc SP	In 2		AV	In	Out	AV			AV
AV
Occupied	Occup
DV
							AV
	SETPT FLOAT				SEQUENCER				Digital Stage
AV							Stage 1	DV	1-8 Output
	SP In				Input		Stage 2	DV
Float		SP Out					Stage 3	DV
	Float						Stage 4	DV
AV
							Stage 5	DV
							Stage 6	DV
	PID CONTROL						Stage 7	DV
AV							Stage 8	DV
	Setpoint
Control Value	Input	PID Out	AV
AV
				AV	PWM
									Digital PWM
					In	Out		DV	Output
	AV	AV = Analog Value		DV	DV = Digital Value
Figure 11-24- Loop/Sequence Control Application Diagram

11.13.2Loop/Sequence Control Cell Descriptions

11.13.2.1 The Select Cell

The Select cell’s function is to provide the control set- point to the PID Control cell, and to switch between occu- pied and unoccupied setpoints based on the current state of

occupancy.

To determine whether to use the occupied setpoint or the unoccupied setpoint, the Select cell reads a digital input. If this input is ON (OCC), the occupied setpoint is used. If this input is OFF (UNO), the unoccupied setpoint is used.

If the occupancy input is NONE, the Select cell will

Loop/Sequence Control Software Overview • 11-41