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

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.

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

Example Applications:

• Point Condition Monitoring

• Powered Mechanical Signal Monitoring

• Level Crossing Barrier Monitoring

• Boom / Wig-Wag Lamp Monitoring

Digital inputs

Four digital inputs are provided as standard to monitor spare contacts of signalling relays.

These inputs are connected as follows:

All of the terminals marked “C” are connected together internally.

This allows easy wiring to signalling relays using twisted pair cable, as specified in railway standards.

Internal resistors limit the sense current to a few milliamps at 24V.

The digital inputs are fully isolated to a minimum of 10M Ohm at 1,000V DC.

This isolation ensures that the inputs are fully separate from earth, the logger’s internal logic and the analogue inputs.

Digital inputs must never be connected into a live circuit (e.g. across a contact that is already in use by the signalling system).

They must only ever be connected to spare relay contacts.

These inputs are for use with volt-free relay contacts only. Do not apply voltages to these inputs.

Analogue inputs

Analogue inputs are designed to accept industry standard 4-20mA sensors.

Many types of external sensor are available with a 4-20mA output, including current clamps, temperature sensors, pressure sensors and voltage transducers.

Each analogue card has four isolated channels, which are capable of powering 4-20mA current clamps. The terminals are:

• 24: 24V out

• S: Signal input

• 0: 0V

The input impedance between S and 0 terminals is 200ohms.

The maximum output power of the 24V sensor power feed is 2W, or 83mA, per analogue input.

The simplest 4-20mA sensors only have two connections and take their power entirely from the loop. Others have three or four wires.

The four wire types use a separate signal and power ground to avoid interference between the power supply and measurement currents.

Example wiring to the different types is shown below:

Most 4-20mA sensors specify the output at 4mA and at 20mA. The sensor output is considered linear between these two points This is the typical usable range of the sensor, however many sensors continue to drive outputs below 4mA and beyond 20mA when excited beyond their normal working range.

Point Machine

Motor Current

LEM PCM20P/SP2

A “uni-directional” “4-wire” current clamp. It is designed to measure “positive current” only between 0 and 20 Amps, AC or DC, however due to the 4-20mA nature, the sensor will read as low as -5 Amps at 0mA output. The split core means it is possible to install without disturbing existing wiring.

Note that when designing and installing these sensors, “conventional current flow” must follow the arrow on the sensor enclosure. E.g. for a positive sensor output, current flow in the measured conductor flows from in the direction of the arrow.

Many electrically driven switch machines can get away with using a single LEM PCM20 sensor. The designer must ensure that all motor current carrying conductors pass through the sensor in the correct direction. This arrangement depending on the machine type and number of feeds.

In some scenarios there may be no spare relay contacts available to trigger a capture and maintain direction of movement information. In such scenarios you may employ two LEM PCM20P sensors, which each sensor capturing switch motor performance for a single direction of movement.

LEM PCM30P/SP2

A “bi-directional” “4-wire” current clamp. It is designed to measure “positive and negative current” between -30 and +30 Amps, AC or DC, however due to the 4-20mA nature, the sensor will read as low as -45 Amps at 0mA output. The split core means it is possible to install without disturbing existing wiring.

Note that when designing and installing these sensors, “positive conventional current flow” must follow the arrow on the sensor enclosure. E.g. for a positive sensor output, current flow in the measured conductor flows from in the direction of the arrow. When current flow in the measured conductor opposes the arrow. negative output is generated.

Many electrically driven switch machines can get away with using a single LEM PCM30 sensor to both trigger and capture switch motor performance in both normal to reverse and reverse directions. The designer must ensure that all motor current carrying conductors pass through the sensor in the correct direction. This arrangement depending on the machine type and number of feeds.

Hydraulic Pressure

The range of clamp-lock style switch machines manufactured by SPX typically feature two in-built pressure transducers installed at the manifold outlet of the “normal” and “reverse” drive hoses. One of these sensors will output signal during “normal” movement of the switch, whilst the other sensor will output a signal during the “reverse” movement of the switch. This is useful, as it means that direction information triggers may be obtained from these sensors without the need to monitor interface relays or valve feed circuits. Pressure is reported on a scale of 0 to 120 bar. Both transducers require monitoring for each switch machine.

Note the sensor is “loop powered” only requiring two-wire operation.

Valve / Relay Feeds

NIC-RI-361BB

Sometimes, there may be no mean obtaining direction of motion of a switch machine from motor current sensors alone

• Motor always turns in the same direction, and no relays are available

• LEM PCM30P sensors are unavailable, and there are no accessible relay contacts.

In such instances it is possible to use the NIC-RI361BB sensor to provide a “fake” relay input to a data logger.

This current sensor features a volt-free-contact output that operates at 60 mA. This can be used to generate a digital trigger signal from an otherwise analogue reading. The signal could be from a relay coil, where no spare contacts are available, or from the solenoid valve feeds of hydraulic switch machine equipment. The sensor has no split core meaning, that existing wiring must be disconnected and rerouted thorough the sensor aperture.

DC Track Circuits

Track Circuit Current

NIC-RI-361BD

The 4-20 mA range of this 4-wire sensor is 0 to +600 mA. This makes it ideal for monitoring DC track circuit current in most applications. They are typically fitted at the relay end, but some times at the feed end also. The sensor has no split core meaning, that existing wiring must be disconnected and rerouted thorough the sensor aperture.

Note that when designing and installing these sensors, “positive conventional current flow” must follow the arrow on the sensor enclosure. E.g. for a positive sensor output, current flow in the measured conductor flows from in the direction of the arrow. When current flow in the measured conductor opposes the arrow. negative output is generated.

Every data logger that is to be connected to the Network Rail RADAR system must:

• Follow a defined naming convention

• Have a unique device ID

Logger Name

The naming convention is of the form Engineers Line Reference (ELR), followed by an underscore, followed by the route name, followed by an underscore, followed by a 2 digit number

The 2 digit number is used to identify an individual data logger if more than one data logger shares the same ELR.

Device ID

Every Mpec data logger connected to the Network Rail RADAR system must be assigned a unique device ID by the Network Rail RADAR team. The number will be between 1 and 65,534. No other RADAR logger must share this number.

Working out how many data loggers you require for a given installation is relatively simple.

1. Count how many analogue and digital inputs are required

2. Select the lowest cost data logger combination that satisfies these numbers of inputs:

SA380TX Hardware Variants

The SA380TX is more expensive than the SA380TX-L, it does however have advanced features, such as the touchscreen, battery back-up, advanced data processing options and master/slave capability.

SA380TX-L Hardware Variants

Master / Slave Devices

As stand-alone devices, each data logger will require an active SIM and GSM antenna in order to transmit data to the RADAR system. This can become troublesome for large installations installed in tight spaces.

Using a “Master / Slave” arrangement permits up to 7 SA380TX-L devices to connected over RS485.

All data is marshalled through the master SA380TX device. Consequently all configuration, data collection and transmission, is controlled from the master SA380TX. In theory the maximum number analogue channels become 78, and digital channels becomes 166.

SA380TX

The power supply is internally isolated from earth and the rest of the SA380TX.

Power can therefore be taken directly from the signalling 110V supply and no additional isolating transformer is required.

The earth pin of the IEC C6 socket is not connected internally.  The unit must be earthed through its connection to the equipment racking.

The unit requires earthing for functional purposes (EMC ground). The unit does not require a protective earth connection in a rail environment.

SA380TX-L

The power supply is internally isolated from earth and the rest of the SA380TX-L

Power can therefore be taken directly from the signalling 110V supply and no additional isolating transformer is required.

The earth pin of the mini-fit socket is connected internally.  The unit may be earthed through its connection to the equipment racking or through the power cable.

The unit requires earthing for functional purposes (EMC ground). The unit does not require a protective earth connection in a rail environment.