Emerson E2 operation manual Output Cell Descriptions

of the Occupancy input. If you wish to follow a schedule for occupancy, this input must be tied to the output of a Time Schedule application.

11.13.2.2 The Setpoint Float Cell

The Setpoint Float cell provides users to raise and lower the control setpoint based on the value of a sensor (called the “float” sensor). The float sensor can be any type of analog sensor, but is typically a temperature sen- sor, since most of the applications for floating setpoints involve HVAC and floating the setpoint based on outdoor air temperature.

To set up the Setpoint Float cell, you must provide three values: a High Float Value, a Low Float Value, and an Output Range.

The Output Range is the maximum amount that the PID Setpoint may vary. An Output Range of 4, for exam- ple, means that the control setpoint may only be increased by a maximum of 2 and decreased by a maximum of 2.

The High Float Value and Low Float Value form a

range of values that determine what portion of the Output Range is applied to the final control setpoint. For example, suppose a Setpoint Float cell is given a High Float Value of 100, a Low Float Value of 0, and an Output Range of 4. When the float sensor value is at the high float value, the setpoint will be increased by its maximum amount (2). When the float sensor value is at the low float value, the setpoint will be decreased by its minimum amount (2). For all float sensor values in between the High Float and Low Float values, the amount added or subtracted to the control setpoint will vary linearly.

If floating setpoint control is not desired, this cell can be effectively disabled by setting the value of the Float Output Range to 0.

11.13.2.3 The PID Control Cell

The PID Control cell uses a PID algorithm to compare the control input value with the control setpoint value. The result of this comparison is a percentage from 0-100%.

The PID Control cell repeats this sequence ever few sec- onds, and the result is a PID percentage that adjusts over time to provide the optimum amount of output to achieve the setpoint.

The 0-100% output from the PID Control cell is passed along to the Filter cell.

Bypassing PID

For users who simply wish to convert an analog output to a pulse width modulation or sequenced (staged) output, a Loop/Sequence Control application can be programmed to bypass PID Control altogether. If this option is selected, the Loop/Sequence Control application completely ignores the Select, Setpoint Float, and PID Control cells,

and passes on the value of the control input directly to the Filter cell.

Note that when bypassing PID, the control input must be an analog percentage from another E2 application or a 0-10VDC analog signal from an input point.

11.13.2.4 The Filter Cell

The Filter cell’s primary function is to slow the rate of change of the PID cell’s output. The filter reads the differ- ence between the current output value and the value x sec- onds ago, where x = a user-specified period of time. The difference between these two values is multiplied by the filter ratio, which is a percentage between 0% and 100%. The result of this multiplication is the output value. Note that if the filter ratio is at 100%, or if the Filter cell is dis- abled, the input is not modified by the Filter cell.

11.13.2.5 The Override Cell

The Override cell’s function is to provide a method of overriding the analog output going to the Sequencer and PWM cells to a user-specified value instead of the value called for by the Filter cell. The Override cell can override the output to any value between 0% and 100%, and may be either fixed or timed. A fixed override remains overrid- den until the user deactivates the override, while a timed override remains in effect until a user-specified time has elapsed.

11.13.3 Output Cell Descriptions

The Loop/Sequence Control application has three con- trol outputs: a standard 0-100% analog output, a set of up to eight staged outputs, and a pulse width modulation (PWM) output. The analog control output comes directly from the Override cell. However, the same control output is fed into the Sequencer and PWM cells for translation into digital staged outputs and pulse width modulation.

11.13.3.1 The Sequencer Cell

The Sequencer cell simply activates a certain percent- age of the Digital State 1-8 outputs based on the percent- age of the control output. For example, if the control output is 50%, the Sequencer cell will activate 50% of the total number of stages. The Sequencer cell will always round DOWN; that is, if there are four stages in a Sequencer cell and the output is 74%, the Sequencer cell will only activate two stages (or 50% of the stages). When the output climbs above 75%, the third stage will activate.

If desired, delays may be specified for stage activation and deactivation. Also, the definitions of OFF and ON may be redefined as either ON, OFF, or NONE.

11.13.3.2 The PWM Cell

The PWM cell converts the control output percentage into a periodic ON pulse. A “pulse” in this cell is a con-

Loop/Sequence Control Software Overview • 11-41