Analogue Acquisition Changes

The acquire-on-change methodology has been enhanced to allow capture of  "absolute" and "relative" changes in input signals. This was primarily done to accommodate EbiTrack 400 receivers.

Multiple EbiTrack 400 receivers can be configured on a device, with each receiver exhibiting very different levels of "track clear" current between 20 mA and 100 mA. A global absolute Acquire-on-Change setting was unable to capture enough information during the "track occupancy" phase, without gathering too much data in the "track clear" phase or vice-versa. This can be observed in the Centrix graphs below.

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Obtaining quality data for GNF Rx results in too much data for FGC Rx

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Obtaining quality data for FGC Rx results in poor data quality data for HRE Rx

Setting a "relative" change setting of 10% allows good quality data to be obtained during the "track occupied" state, whilst limiting the data collecting during the track clear phase. 10% was chosen, as it lies above the relativistic noise floor of the ITOT channel whilst the track is clear. The Centrix graph below demonstrates the enhanced data quality.

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A new GUI element has been added to the config tool to allow the selection of "Absolute", or "Relative" acquire on change types.

To make the distinction clear between the two types of acquisition, and to provide continuity across the mpec product range:

• Absolute Acquire-on-Change is specified in the engineering units of the sensor / channel
• Relative Acquire-on-Change is specified in "Percent", e.g acquire a sample when the signal has changed by "x%". The engineering units are irrelevant.

The user guide extracts below explains the change in more detail.

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.

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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.

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.

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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.

Relative:

In relative mode, a fresh sample is acquired each time the raw input signal changes by a defined percentage since the previous acquisition. It is defined in percent, for example 5%.

E.g. If the last sample was acquired at 50 mA, the next sample will be acquired at +/- 5% of 50 mA, which is either 52.5 mA, or 47.5 mA. A relative change of 5% is required to trigger the next acquisition.

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

Relative acquisition is a good fit where the minimum and maximum range of the input signal are not well known and where increased resolution is desired for small signals, at the expense of decreased resolution of large signals.

Example Applications for Acquire on Change:

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

See SA380TX-L Applications for more detail.

All unit configuration is performed using this screen.

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Analogue

      Analogue

Change Threshold

If Acq. Method is set to Relative, this number determines the percentage change relative to the last acquired value required to trigger the acquisition of a sample.

If Acq. Method is set to Absolute, this number governs the absolute change in signal level required to trigger acquisition of a sample.

For NR II DC track circuit monitoring a change threshold of 6 mA absolute has been suggested.

EBI Track

      Change Settings

           These settings dictate how sensitive the “Acquire-on-change” settings are for all monitored EbiTracks.

           Parameter / Units - These fields are fixed for EbiTrack monitoring and cannot be changed

           Change Threshold - Change the acquisition sensitivity of each channel in these fields.

• •  If Acq. Method is set to Relative, this number determines the percentage change relative to the last acquired value required to trigger the acquisition of a sample.

• •  If Acq. Method is set to Absolute, this number governs the absolute change in signal level required to trigger acquisition of a sample.

The default settings are suitable for NR II EbiTrack 200 applications.

Where EbiTrack 400 Receivers are to be monitored, it is suggested that IAVE is set to 10% Relative due to the variance in set-up current levels.

See SA380TX-L Unit Configuration for more detail

Server LED Indication Changes

The behaviour of the "Server LED" was incorrect when the SA380TX-L was set to use the "RailDAQ" protocol. The behaviour has been fixed and re-defined. This is explained in the user-guide extract below.

LED’s

The TX-L front panel features 8 LEDs.

The meaning of these LEDs is detailed in the table below:

*It is normal to observe a “Processor Sync Error” during initial boot and during firmware upgrade.

** The table describes behaviour in MIMOSA protocol mode. In RailDAQ mode, the following behaviour is observed:

• The server connection LED will not show fallback, as the timeout is short.
• During Firmware upgrade over RailDAQ, the server LED will not illuminate.
• If the device is not configured within Centrix, the server LED will not illuminate, as it cannot transmit samples.

See SA380TX-L Hardware for more detail

Bug Fixes

• Timeout reductions for RailDAQ connections have been made to improve server communications performance (especially noticeable during firmware upgrade over GPRS).