将预触发值从OPC UA服务器发布到OPC UA客户端

Question

Can anyone help me out with the following use case for OPC UA?: reading triggered measurements from an OPC UA server with additional measurement values that happened in the period just before the trigger condition occurred. This pre-trigger period would be configurable, let's say half a second. This allows seeing what happened just before the trigger of interest occurred.

How would I go about?: As soon as the trigger happens make the results available for the OPC UA clients and then the client should then act on the same trigger to retrieve the preceding historical measurement values of the period before the trigger happened. I hope there is a smarter way so that the client can remain stateless. And there is no requirement that the data arrives a bit later so that the pre-trigger values are sent first before the post-trigger values are sent to the client.

Given that the data must be buffered anyhow to make this possible, could this work?:

• my back end measurement data provider within the OPC UA server could just start returning data values back to the client starting with the values from half a second earlier (the configured pre-trigger period). Ie not returning the current measurement values but starting with the pre-trigger ones.

I have seen in the Milo server example that within the ExampleNameSpace the AttributeValueDelegate construct is used for dynamic nodes. This seems to allow to return data values one at the time including a timestamp. I don't have the proper test tools to see if it works if I start returning relative old values.

The other thing is how this relates to monitored items and sampling intervals. If the client, for example, would request a sampling rate of 10 samples per second, would it then check if the returned monitored items are actually within that range? Ie will older values arriving late be discarded or just pulled in by the client?

Answer 1

No matter what approach you take it's going to require the client to be aware of what you're doing here, so here's an approach that might work:

Create two nodes in the server, a scalar that holds the current measurement value, and an array that holds the last N measurements values.

In the client, create a monitored item for the scalar value with MonitoringMode.Reporting, and create a monitored item for the array value with MonitoringMode.Sampling. Then use the SetTriggering service to create a triggering link between the scalar item and the array item.

What this will result in is you can freely update the array value in the server without the value being reported as changed, but when you update the scalar value and the change is reported, the current value of the array will get reported too.

As a side note, I'd avoid relying on the AttributeDelegate mechanism for new development. It's going to be deprecated and replaced with something else once development on 0.3 starts.