Difference of analogue channels

Logger example at Dartford https://centrix.org/loggers/manage#!/view/12600?tab=config

We are interested in the imbalance of the voltage of two 6ft long piece of rail to find out about any traction current loss.

The analogue inputs are:

Virtual channels and Rail events are too complicated for the touch-screen UI! These features have to be configured in the XML configuration. Most conveniently using the web config tool.

Create a new virtual channel:

Subtraction is done as operand(1) - operand(2). The order of the operands is shows as an index on the connection.

Virtual channels are meant to be sent in Rail Events, so set up a new acquisition to send data to server:

Once the configuration is applied to the logger (Set Config on Logger), the new channel will be enumerated on Centrix:

Virtual channel configuration in the XML:

<virtualInputs>
	<operator>
		<name>Imbalance</name>
		<type>SUB</type>
		<input>
			<uid>0</uid>
			<type>RealAnalogue</type>
		</input>
		<input>
			<uid>1</uid>
			<type>RealAnalogue</type>
		</input>
		<output>
			<uid>0</uid>
			<type>VirtualAnalogue</type>
		</output>
	</operator>
</virtualInputs>

Notice that the virtual input Subtract operator refers to real analogue inputs uid=0 and uid=1, which are also defined in the XML configuration like:

<anaIn>
	<uid>0</uid>
	<name>AN-A01 Dn Cess</name>
	<calib>
		<type>LIN</type>
		<v4mA>-100</v4mA>
		<v20mA>100</v20mA>
		<units>mV</units>
	</calib>
	<filt>
		<type>DC_RMS</type>
		<freq>30</freq>
		<period>10</period>
	</filt>
</anaIn>

And creates a new virtual input with uid=0 (first virtual analogue channel).

(You could argue uid is not unique at all, only unique within a certain type: real analogue, real digital, virtual analogue, virtual digital.)

The XML configuration of the Rail event:

<railEvents>
	<event>
		<name>Imbalance</name>
		<timeout>5000</timeout>
		<startTrig>
			<type>AOC_ABS</type>
			<chan>
				<uid>0</uid>
				<type>VirtualAnalogue</type>
			</chan>
			<val>3</val>
		</startTrig>
		<capture>
			<logChan>
				<name>Imbalance</name>
				<chan>
					<uid>0</uid>
					<type>VirtualAnalogue</type>
				</chan>
				<mimOffset>40960</mimOffset>
			</logChan>
			<type>AOC_ABS</type>
			<val>1</val>
			<postProc>
				<strip>0</strip>
				<pad>0</pad>
			</postProc>
		</capture>
	</event>
</railEvents>

Rail event is a generalized capture trace: you have to specify start trigger(s), so there can be more than one condition which the capture can start at which are taken into account in an OR logic.

Notice that there is no explicit stop trigger here: the capture will end on a 5 second (5000 ms) timeout. All Rail events must have a timeout! The capture cannot go on indefinitely!

The important bits which may change depending on the output of the acquisition are the thresholds:

start trigger: starts on 3 mV absolute change (AOC_ABS)
capture acqusition threshold: 1 mV absolute change:

You may want to change those bits after setting the configuration without needing to get back to the web config tool.