How to avoid MxActivitySensor false alarms

MxActivitySensor is a really powerful tool to reliably detect motion but it’s not false-alarm free.
Under specific circumstances, changes in the illumination can trigger events. For example the headlight of a car passing by or the shadow projected by the sun on moving clouds.
Fortunately we can mitigate this issue by using other tools such as VM and, most importantly, VM reference windows.

Below we explain the configuration principle with an example.

To reduce false alarms on Activity Sensor you can sequentially put in place the suggestions below until you get the desired result:

  • Increase the threshold of AS from 33% (default value) to 80-90%. Before you make the change I suggest you to plot the progress of AS to see what values are actually reached in the case of change of lighting due to car’s headlights. The scroll chart can be enabled from the page Setup Menu > Text and Display Settings. If you see that the chart never exceed 80% then you can raise the threshold to 90% to get a reliable detection. When a person walks in front of the camera, the chart goes usually up to full scale (100%).

  • Reduce the sensitivity of AS by enabling the parameter “Detect Fast Motion” (Setup Menu > Event Overview> Video Analysis). By doing this the camera will focus on fast movements (many pixels/frame) and this leads to a reduction of the sensitivity in case of standard movement of at least 20%. If you apply this change it may be helpful to review the detection threshold; 80-90% may be too high.

  • Use the combination of AS and VM as Event Logic to trigger the event. You can create a kind of minefield as shown in the picture below. The AS area should be centered at the point of interest and within it you can place many small VM windows. Even better if the windows are proportioned to the size of the object to be detected as shown in the picture below.

  • Create reference VM windows placed at points of the image where you do not expect a movement. If one of the reference windows is triggered, it is only because there was a change in the lighting and not due to the movement of a person. Once you have configured the reference VM windows you can set the “VM Suppression Time” which is the inhibition time (in ms) of the VM when a reference window triggers an event.

Final result: