Virtual Sensor

by Pedro Perdomo

A Virtual Sensor Implementation using digital twin models .

You need data to train a virtual sensor. This was the main problem to surpass in this project. 
So , how do you pretend training  a neural sensor without data collected?


We started this project with no data in advanced. Starting to collect data and wait for extreme conditions to occur, ie. operating the column and the plant  over limit conditions was not a good idea for operators. We needed to collect data, good amount of data, but with variety.  We tried to produce this by operating at limits , but it is not always possible. In each condition you have to wait for stabilization and in the other hand, you need to collect samples to the lab. These are real issues once you try it out. 

We had a digital model  of the column in a popular simulation software.  This digital model was recently certified by comparing with real data in a very good range of operation conditions. This model was based in first principles.  Wait a minute! . Why we don't just run simulations offline, with no risk of real operations and get rich data?  So, we created a script to feed this offline digital model with rich amount of process conditions, but offline, with  no risk to get the plant down. One we had enough amount of data, which took time to run into the simulator, we were ready to train the neural network ang get the virtual sensor model.
Certainly this model is just for steady state, so you have to take this into consideration when ethane content virtual sensor is shown to  the operator. 

 Implementation is easy  once you use simulation input  variables that already are measured in the field, so you do not have to implement additionals measurements . The math equation you get using a simple two layers neural network is really simple, light to compute, easy to implement in any process controller.

 This is not a conventional way to do a virtual sensor using data, as bias errors could be introduced as the digital model does not consider actual conditions of column. But it works. In fact, once in production, this sensor was a quite trusted reference. 






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