Predicting time to failure (TTF) or remaining useful life (RUL) is a common need in IIOT world.
We are looking here at some ways to implement it. We are going to use one of the Nasa dataset publicly available that simulates the Turbofan engine degradation (https://c3.nasa.gov/dashlink/resources/139/) . The original dataset has got 26 features as below
Column 1 – asset id Column 2 – cycle/time of sensor data collection Column 3- 5 – operational setting Column 6-26 – sensor measurement
In the training dataset the sensor measurement ends when the failure occurs.
Data Collection
Since the prediction model is based on historic data, the data collection is a critical point. In some cases the data would have been already collected form the past and you need to make the best out of it. See the Data preparation chapter below. In situation where you are collecting data, a few points are good to keep in mind, some may or may not apply depending on the type of data to be collected.
More frequent (higher frequency) collection is usually better, especially for electronic measure.
In situation where one or more specific sensor values are known to impact the TTF, it is good to take measure at different values of this sensor until the failure without artificially modifying the values. For example, for a light bulb with normal working voltage of 1.5V, it is good to take some measure at let’s say 1V, 1.5V , 2V , 3V and 4V. But each time run till the failure. Do not start at 1.5V and switch to 4V after 1h. This would compromise what the model can learn.
More variation is better as it helps the prediction model to generalize. In the same example of voltage it is best to collect data for 1V, 1.5V , 2V , 3V and 4V rather than just 1.5V which would be the normal running condition. This also depends on the use case, for example if we know for sure that voltage will always be between 1.45V and 1.55V, then we could focus only on data collection in this range.
Once the failure is reached, stop collecting data. We are indeed not interested in what happens after the failure. Collecting data after the failure will also lead to lower prediction model accuracy.
Each failure run should be a separate cycle in the dataset. In other word from a metadata stand point, each failure run should be represented by a different ENTITY_ID.
TTF business need
Before going into data preparation and model creation we need to understand what information is important in term of TTF prediction for our business need.
There are several ways to conceive the TTF, for example:
Exact time value when failure might occur
This is probably going to be the most challenging to predict
However one should consider if it is really necessary. Indeed do we need to know that a failure might occur in 12 min as opposed to 14 min ? Very often knowing that the time to failures is less than X min, is what is important, not the exact time. So the following options are often more appropriate.
Threshold
For some application knowing that a critical threshold is reached is all that is needed.
In this case a Boolean goal, for example lessThan30min or healthy with yes/no values, can be used
This is usually much easier than the exact value above.
Range
For other applications we may need to have a bit more insight and try to predict some ranges, for example: lessThan30min, 30to60min, 60to90min and moreThan90min
In this case we will define an ordinal goal
The caveat here is that currently ThingWorx Analytics Builder does not support ordinal goal, though ThingWorx Analytics Server does support it. So it only means that the model creation needs to be done through the API. This is the option we will take with the NASA dataset.
The picture below shows the 3 different types of TTF listed above
Data Preparation
General Feature engineering Data Preparation is always a very important step for any machine learning work. It is important to present the data in the best suitable way for the algorithms to give the best results. There are a lot of practices that can be used but beyond the scope of this post. The Feature Engineering post gives some starting point on this. There are also a lot of resources available on the Internet to get started, though the use of a data scientist may be necessary. As an example, in the original NASA dataset we can see that a few features have a constant value therefore there are unlikely to impact the prediction and will be removed. This will allow to free computational resources and prevent confusion in the model.
Sensor data resampling The data sampling across the different sensor should be uniform. In a real case scenario we may though have sensors data collected at different time interval. Data transformation/extrapolation should be done so that all sensor values are at the same frequency in the uploaded dataset.
TTF feature Since we want to predict the time to failure, we do need a column in the dataset that represent this values for the data we have. In a real case scenario we obviously cannot measure the time to failure, but we usually have sensor data up to the point of failure, which we can use to derived the TTF values. This is what happens in the NASA dataset, the last cycle corresponds to the time when the failure occurred. We can therefore derive a new feature TTF in this dataset. This will start at 0 for the last cycle when failure occurred, and will be incremented by 1 up to the very first measurement, as shown below:
Once this TTF column is defined, we may need to transform it further depending on the path we choose for TTF prediction, as described in the TTF business need chapter.
In the case of the NASA dataset we are choosing a range TTF with values of more100, 50to100, 10to50 and less10 to represent the number of remaining cycles till the predicted failure. This is the information we need to predict in order to plan a suitable maintenance action. Our transformed TTF column look as below:
Once the data in csv is ready, we need to create the json file to represent the metadata. In the case of range TTF this will be defined as an ordinal goal as below (see attachment for the full matadata json file)
{
"fieldName": "TTF",
"values": ["less10",
"10to50",
"50to100",
"more100"],
"range": null,
"dataType": "STRING",
"opType": "ORDINAL",
"timeSamplingInterval": null,
"isStatic": false
}
Model creation
Once the data is ready it can be uploaded into ThingWorx Analytics and work on the prediction model can start. ThingWorx Analytics is designed to make machine learning easy and accessible to non data scientists, so this steps will be easier than when using other solutions. However some trial and error are needed to refine the model which may also involve reworking the dataset.
Important considerations:
When dealing with Time to failure prediction, it is usually needed to unset the Use Goal History in the Advanced parameters of the model creation wizard. If using API, the equivalent is to set the virtualSensor parameter to true.
Tests with Redundancy Filter enabled should be done as this has shown to give better results.
In a first attempt it is a good idea to keep lookback Size to 0. This indicates to ThingWorx Analytics to find the best lookback size between 2, 4, 8 and 16. If you need a different value or know that a different value is better suited, you can change this value accordingly. However bear in mind the following:
Larger lookback size will lead to less data being available to train, since more data are needed to predict one goal.
Larger lookback do lead to significant memory increase – See https://www.ptc.com/en/support/article?n=CS294545
In the case of the NASA dataset, since we are using an ordinal goal, we need to execute it through API. This can be done through mashup and services (see How to work with ordinal and categorical data in ThingWorx Analytics ? for an example) for a more productive way. As a test the TrainingThing.CreateJob service can be called from the Composer directly, as shown below:
Once the model is created we can check some performance statistics in ThingWorx Analytics Builder or, in the case of ordinal goal, via the ValidationThing.RetrieveResults service. The parameter most relevant in the case of ordinal goal will be the confusion matrix.
Here is the confusion matrix I get
Another validation is to compute some PVA (Predicted Vs Actual) results for some validation data. ThingWorx Analytics does validation automatically when using ThingWorx Analytics Builder and present some useful performance metrics and graph. In the case of ordinal goal, we can still get this automatic validation run (hence the above confusion matrix), but no PVA graph or data is available. This can be done manually if some data are kept aside and not passed to the training microservice. Once the model is completed, we can then score (using PredictionThing.RealTimeScore or BatchScore for ordinal goal, or Builder UI for other goal) this validation dataset and compare the prediction result with the actual value. here is one example:
Depending on the business case this model can be deemed acceptable or may need rework, such as change the range values, change learners’ parameters, modify dataset …
There is certainly a fair amount of experimentation before creating the optimal model but hopefully this post does give some good starting points.
Resources:
Original Dataset attached as train_FD001-original.csv Transformed dataset attached as train_FD001-TTF-transformed.csv
json metadata file for transformed dataset attached as train_FD001-ttford.json
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