Accumulate data with fields

An accumulator field is a field with its property set to Accumulator.The accumulator field keeps a tally of the accumulated total and supplies that value to the reporting point when it triggers. The last recent value in the accumulator when the reporting point triggers is the value that AVEVA™ Production Management stores in the field.

How accumulators count and reset

An accumulator field relies on a constantly increasing tag value. It accumulates value over a defined period and then resets.

Example

An example of this is a tag that counts the number of products that pass it on a production line. The next day the accumulator field resets to zero and begins the count again for the total product for the next day. When the production record is captured, you do not want the actual value for the tag but the difference between the current value of the tag and the value of the tag of the last created record.

Accumulator evaluation process

The accumulator evaluation process involves subtraction of the previous field value from the current field value to obtain the delta, which becomes the captured value.

If the delta is negative and less than the AccumulatorThreshold, AVEVA™ Production Management sets the delta to zero. However, if the delta is negative and its absolute value is greater than the AccumulatorThreshold, it is deemed to have reset. That is, AVEVA™ Production Management treats the accumulator as having reached the Reset Value and then climbed to its current value. In this scenario, the delta value is the field value minus the Reset Value.

This method makes certain that there is a recent and relatively accurate accumulated value available for the reporting point to record when it is triggered by a capture condition.

The accumulator field value that increases each time a positive delta is detected continues to increase until the reporting point triggers and AVEVA™ Production Management records the field value. After the field value is recorded, AVEVA™ Production Management resets the field value to the Reset Value, and then the accumulator increases again in readiness for the next reporting point event.

Example 1

An accumulator field is configured under a Production reporting point.

• An accumulator tag starts at 50.

• You have configured a field, called MyOutput, to capture the accumulator value.

• AVEVA™ Production Management captures a production record at T = 6.

• AVEVA™ Production Management sends the value 125 to the data repository for the MyOutput field.

• The next shift starts at T = 9 and the shift supervisor resets the accumulator tag to 0.

• AVEVA™ Production Management captures a production record at T = 14.

• AVEVA™ Production Management sends the value 250 ((225 - 125) + (150 - 0)) to the data repository for the MyOutput field.

The following graphic depicts this scenario:

Example 2

An accumulator field is configured under a Production reporting point.

• The Reset Value is set to 50 and the AccumulatorThreshold is set to zero.

• The field value starts at 50 and climbs to 280.

• The field value resets to 50 and climbs to 90.

• After 7.5 seconds, the Production reporting point captures a record.

  

This table shows the internal processing of the field value.

The accumulator field value captured in the Production record at 7.5 seconds is therefore 270.

Example 3

An accumulator field is configured under a Production reporting point.

• Both the Reset Value and the AccumulatorThreshold are set to zero.

• The field value starts at zero and climbs to 1800 in approximately five seconds.

• The field value resets to zero and climbs to 1400 before reset.

• After 7.5 seconds, the Production reporting point captures a record.

  

This table shows the internal processing of the field value.

The accumulator field value captured in the Production record at 7.5 seconds is therefore 3100.

In this scenario, the actual accumulated field value was 3200, but the accumulator recorded only 3100 as the total value. This is because the value increased to 1800 before resetting and increasing to 450 in a single poll period. Because of the poll rate, the accumulator missed the increase from 1700 to 1800. It is important to understand the rate of change of a field value and set an appropriate poll rate. A faster poll rate could have avoided the error.

Note: The accumulator field is not a rollover accumulator field. That is, it does not check for a fixed maximum value for the value that it monitors.

Related topics

• Aggregating data with fields

• About fields

• About standard fields