The Digital Filter Constant is a configurable setting available on the analog input channels of the OCS that helps reduce electrical noise and signal fluctuations which may be unavoidable in certain installations. In industrial environments, factors such as long cable runs, nearby motors, variable frequency drives, or electrical interference can introduce instability into analog signals. The Digital Filter Constant helps smooth out these fluctuations to provide more stable and reliable readings.
The filter value can be adjusted from 0 to 7 (Model 0 to 5) and from 0 to 15 (Model 6), with the selected value applying across all analog input channels (Model 0 to 5) and individually (Model 6). The lower the filter value, the faster the system responds to changes in the input signal. Conversely, increasing the filter value produces a steadier reading, but with a slower response time to sudden changes in the actual input.
At a setting of 0, no filtering is applied, meaning the input reacts immediately to changes but may display noticeable noise or fluctuations. At the maximum setting of 7 (Model 0 to 5) or 15 (Model 6), the signal becomes significantly more stable, although rapid input changes will take longer to appear in the controller. This creates a balance between responsiveness and signal stability, allowing the filter to be tailored to the specific application.
The filtering method itself uses a running average approach. With a filter value of 1, the OCS firmware gives equal weighting to the current analog conversion and the existing running average, combining both values to produce the filtered result. As the filter constant increases, the running average carries more influence over the final value.
For example, when the filter value is set to 3, the current reading contributes only one quarter of the weighting compared to the running average. This means sudden changes in the input are introduced gradually, resulting in smoother transitions and a more stable displayed value. As higher filter values are selected, the response becomes progressively slower, but the overall signal quality improves.
This behaviour is particularly useful in applications where small fluctuations in analog signals could otherwise cause instability, nuisance alarms, or unnecessary control actions. In contrast, applications requiring rapid reaction to changing inputs may benefit from lower filter settings.
The Digital Filter Constant can be configured directly through the Filter Constant field or dynamically by using a Register Configure Register(Micro Series Only). When a register is assigned, predefined values can be written to the register during operation to adjust the filter level as required. Any value written to the register overrides the fixed value entered in the Filter Constant field, allowing filtering behaviour to be modified during runtime.
For a more detailed explanation and practical examples of analog filtering techniques, watch the following webinar from 3:38 to 9:10:
