Application

Sequence of Operation

Venturi Application

Siemens Industry, Inc. Application Note, Apps 2941/2942 140-1319

Restricted 2015-11-04

NOTE:

The calibration table initially contains all zeros by default, that is, it contains no

calibration information. When the application detects all zeros, the application

operates, but runs with only PID control. If PID only control is satisfactory for a given

job, there is no need to populate the table. If necessary, a populated table can later

be edited back to all zeroes to force PID only control.

PID Only (Mode 2)

NOTE:

The default P gain value is intended for PID operation in conjunction with the Venturi

table. The P gain and Venturi table work together to provide an appropriate response

to setpoint changes. When operating without the Venturi table in PID Only mode, the

application is slower to respond. Therefore, you should adjust the P gain as needed

when operating in PID Only mode to ensure acceptable performance.

The Venturi calibration table initially contains all zeros by default, that is, it contains no

calibration information. When the application detects a zero flow for the sixteenth entry

(the table entry with the highest flow), the application does operate, but runs with only

PID control. If PID Only mode control is satisfactory for a given job; there is no need to

populate the Venturi tables.

Open Loop (Mode 3)

This application can operate in open loop mode. In open loop control, there is no

airflow sensor to provide an actual airflow measurement. Instead, operation is based

completely on the values in the Venturi table. For example, if the actuator’s setpoint is

600 cfm, the application will use the Venturi table to derive the voltage setting that

generates a flow of 600 cfm and then set the actuator to that voltage to generate the

desired flow. Assuming the table has accurate entries, the flow (if it were to be

measured) would be at setpoint.

Internally within the application, with open loop control, the flow is always assumed to

be at setpoint As a result there is virtually no difference between flow tracking and

setpoint tracking when the actuator being tracked is operating open loop.

Set OPEN LOOP to YES to indicate that the actuator is to operate open loop. This will

suppress the AVS failure indication that otherwise would occur when no airflow sensor

is connected.