Failure Sensor Detection by Pattern Comparison on Time Series

Purpose

Find the interesting patterns in the time series data in order to detect the failure sensors.

Process

  1. Time series data source. We leveraged Databricks and delta table to store our source data which includes sensor_id, timestamp, feature_id, feature_value. To make it run quicker, you may aggregate the time to hour or day level.
  2. In order to vectorize the features, the features in the rows have to be converted into columns.
  3. Clean outlier. Even the data source is spotless on the aggregation level. There may still have some outliers, like extremely high or low value. We set 5 STDs to identify and clean these outliers by per sensor ID.
  4. Normalization. Use max_min function to normalize selected columns. You can either use max_min normalization for the features based on all sensors or by each sensor, depends on how variety between these sensor data.
  5. Pattern comparison. We use an existing reference pattern to compare the time series from very left to very right time point. For example, list [1,1,1,1,0,0,1,1,1,1] (visualization: ) has 10 points, it slides on 30 points’ time series, you will get 20 comparison results. There are two ways to compare two time series.
    • Pearson correlation.
    • Euclidean distance.
  6. Filters. The result of comparison will give you how similarity of all-time points. You can set the threshold for this similarity value to filter non-interesting points. Also, you can set how frequency of pattern happened as threshold to further filter. Here is an example of threshold.
    • corr_ep_thredhold= 0.8 (similarity greater than 0.8)
    • freq_days_thredhold = 20 (the pattern happened more than 20 times in all time series)
    • freq_thredhold = 40% (from this pattern starts to ends, more than 40% days it happened)
  7. The result you can persist into a database or table.

Pros

  • Since the feature is normalized, the comparison is based on the sharp rather than absolute value.
  • Statistic comparison is quick, special running on the spark.
  • The result is easy to understand and apply to configured filters.

Cons

  • Right now, the Pearson correlation and Euclidean distance only apply to one feature in a time. We need to figure out a way to calculate the similarity on the higher dimensions in order to run all features in a matrix.
  • Pattern discovery. To find the existing pattern, we based on experience. However, there is an automatic way to discover pattern. Stumpy is a python library to do so. It can also run on GPU.

Future work

  • How to leverage steaming to find the pattern immediately once it happens, rather than the batch process.
  • High dimension matrixes comparison.
  • Associate Stumpy to find interesting pattern automatically, then apply the new patterns into configured filters.

Read More

Leave a Reply

Your email address will not be published. Required fields are marked *