In ClearSCADA, you use a WITS Rate of Change point database item to represent a Rate of Change point in a WITS Field Device. A Rate of Change point is a virtual analog input point.
The Field Device calculates the rate of change of the value of a particular analog point, over a specific calculation period, and then scales this rate of change value to a time base (for example, seconds, minutes or hours). The Field Device then stores the rate of change value in the Rate of Change point. The Rate of Change point is therefore a derived point - that is, it derives its value from the value of another analog point (the parent point) (see Derived Points in the ClearSCADA Guide to Core Point Configuration).
The rate of change calculation is as follows:
((new value - old value) / calculation period) x time base.
where the 'new value' is the current value of the parent point and the 'old value' was the value of the parent point 'calculation period' seconds earlier.
Example:
A device analog point is measuring the level of water in a tank. The current water level is 25cm, the level 30 seconds earlier was 24cm. You want to know the water level's rate of change per minute. The Field Device would perform the following calculation:
((25cm-24cm) / 30 seconds) x 60 seconds = 2cm per minute.
The water level is rising by 2cm per minute. The Field Device would store this value in the Rate of Change point.
The rate of change value will be negative if the analog value is falling, positive if the value is rising, and zero if it is unchanged over the calculation period.
WITS Rate of Change Point database items have a combination of configuration properties that are:
- Common to many types of point
- Common to DNP3 points
- Specific to WITS Rate of Change points.
To configure a WITS Rate of Change Point database item, you use the WITS Rate of Change Point Form.
The Form contains many fields, organized into tabs. Some of these tabs are common to many types of database item (see Tabs on Point Forms in the ClearSCADA Guide to Advanced Drivers).
The Rate of Change tab contains properties that are specific to WITS. Use the fields on the tab to:
- Configure the point's General Properties (see General Point Properties - WITS Rate of Change Points)
- Specify the analog point (the parent point) for which the Field Device should calculate the rate of change value (see Specify a Parent Point )
- Specify the calculation period for the rate of change value (see Calculation Period for the Rate of Change Value)
- Configure the point's Scanning State Properties (see Scanning States for WITS Database Points)
- Configure the point's Alarm Limits (see Alarm Limits for WITS Rate of Change Points)
- Configure any Alarm Limit Profiles (see Profiles for WITS Database Points)
- Configure the point's DNP3 Object Flag Options (see DNP3 Object Flag Actions)
- Configure the point's Formatting Properties (see Define the Appearance of an Analog Point's Values in the ClearSCADA Guide to Core Point Configuration)
- Configure the point's Override Properties (see Define whether an Analog Point can be Overridden in the ClearSCADA Guide to Core Point Configuration)
- Configure the point's Trend Properties (see Trend Settings for WITS Rate of Change Points)
- Configure the point's Logged Data Properties (see Logged Data for WITS Database Points)
- Configure the point's Historic Data Filter Properties (see Historic Data Filter for WITS Database Points).
When you have completed the point's configuration, you need to download the configuration to the Field Device by using the Outstation database item's Download Configuration (WITS Outstation) action. If your Field Device does not support configuration download, you will need to make the same changes to the Field Device's configuration locally.