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title	Understanding Data Acquisition Concepts

Digital acquisition

Digital event recording allows you to determine the present state of any relay (picked or dropped) and any change in state of any relay.

Front and Back Contacts

You may monitor spare front (normally open) and spare back (normally closed) relay contacts. Where back contacts are monitored the state of the relay will be the inverse of the state of the contacts.

To account for this discrepancy Mpec data loggers allow you to configure a digital input as a front or back contact; the TX-L then automatically ensures the true state of the relay (picked or dropped) is captured.

State Changes

All digital inputs are continuously monitored for any change in state, whenever a change is detected the nature of the change is captured locally (UP to DN, or DN to UP) along with a timestamp accurate to within 10 mS.

Initial States

When an Mpec data logger boots or restarts, it will capture the “initial state” of every digital input, this way you can see the present state of all monitored digital inputs at all times, even if no change in state has taken place on a particular channel.

Initial states are clearly indicated in the historical log, and are marked “UP” or “DN”.

Analogue Acquire-on-Change

All analogue input channels are continuously logged using a process known as “Acquire-on-change”.

A sample is acquired when the measured value changes by more than a certain amount.

If there is no change, there is no sample acquired.

Consider the following waveform. The acquired samples are shown as dots.

The waveform first changes at a fairly leisurely pace, then there is a spike. Each time the input changes significantly, a sample is acquired. It can be seen that more data points are acquired around the spike.

Acquire-on-change is an excellent match for many railway applications. Where there are long periods without much change, very little data is acquired. Where there is more detail in the waveform, more points are acquired.

After the data has been acquired it is possible to go back and just “join the dots” and we have an accurate representation of the entire waveform, with the minimum amount of data logged and transmitted.

Two methods of Acquire-on-Change are supported, however the most common method is “absolute” acquire on change:

Absolute:

In absolute mode, a fresh sample is acquired each time the raw input signal changes by a fixed constant value, for example 5 mA.

E.g. If the last sample was acquired at 50 mA, the next sample will be acquired at +/- 5 mA, which is either 55 mA, or 45 mA. An absolute change of 5 mA is required to trigger the next acquisition.

The chart above shows how samples would be acquired along a straight line slope.

Absolute acquisition is a good fit where the minimum and maximum range of the input signal are well known and an even level of detail is required at all ranges.

Example Applications for Acquire on Change:

Rail Temperature Monitoring
Track Circuit Monitoring
Overhead Line Force and Displacement Monitoring
Insulation Resistance Monitoring
Power Consumption Monitoring

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Triggered Capture

In addition to Acquire-on-Change you may also capture analogue data using a method known as “Triggered Capture”.

Triggered Captures are the method of choice when you want to record intermittent railway events at maximum resolution.

A triggered capture will begin recording when a “start trigger event” is detected.

Analogue data on the selected channel(s) will then be recorded at maximum resolution until a “stop trigger event” is detected.

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A trigger event can be fired by a change in state of a digital input, or by an analogue input transitioning through a pre-determined threshold level.

Sometimes analogue data of interest can lie just outside the time-window defined by the start and end trigger events. To combat this, the data loggers will include analogue data of interest either side of the start and end trigger events in the final triggered capture waveform.

Where you are monitoring assets that may move in two directions, you may also assign a direction (Normal to Reverse or Reverse to Normal) to a triggered capture. In the event that the logger cannot determine direction, the direction will be labelled invalid.

Terms of Reference

Term	Type	Notes
Start Trigger	Trigger Event	A signal to begin the recording of capture data.
End Trigger	Trigger Event	A signal to end the recording of capture data.
Start Pre-trigger	Time (milliseconds)	The amount of extra data acquired before the start trigger.
End Pre-trigger	Time (milliseconds)	The amount of extra data acquired after the end trigger.
End Trigger Debounce	Time (milliseconds)	The end trigger condition must be continuously true for this time to terminate a capture. This prevents early termination due to short “glitches” in the data.
Capture Channel(s)	Analogue Data	The actual analogue data we wish the capture
Zero-Threshold	Absolute Analogue Level	Represents the system offset bias or noise floor. Any data below this level is removed from the capture data once the capture event has ended.
Direction	Data Tag	You may configure each capture such that it associates the data with a direction of movement, e.g Normal to Reverse, or Reverse to Normal
Capture Group	Data Tag	You may configure each capture to associate with other captures. This enables additional logic to spot illegal operations, such as switch machines moving both normal to reverse and reverse to normal simultaneously.
Timeout	Time (seconds)	A capture can not continue indefinitely. Should no end trigger occur, the capture will cease when the timeout is reached.

Example Applications:

Point Condition Monitoring
Powered Mechanical Signal Monitoring
Level Crossing Barrier Monitoring
Boom / Wig-Wag Lamp Monitoring

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