Powering Sensors in Trains
Nov 25, 2022
Many people think of a train as simply a bus on rails! After all, passenger trains have been around since 1825 and the basic concept of a locomotive pulling carriages has not changed much since then.
However, anyone attending the recent Innotrans 2022 exhibition in Berlin, the leading international trade fair for transport technology, will have noticed that trains are becoming increasingly high-tech. One exhibitor even boasted that their high-speed train contained over 7000 sensors. This is far more than would normally be needed in any bus.
One of the reasons for this massive expansion in sensors is the continuous need for improvements in safety, reliability, comfort and performance in modern railway infrastructure. But how can you improve, say, passenger safety in the existing rolling stock that has a working life of decades and is already several years old? The answer to that would be to retrofit improved sensors and monitoring systems to add a layer of control and communication on top of the existing mechanical systems.
A modern train already has more in common with an IT server installation than what meets the eye. An on-board ethernet backbone supports CCTV and PA systems running over an IP network, transmits real-time information for the passenger information systems, has Wi-Fi access points and seat reservation displays and interfaces the distributed processor and gateway units for control, while monitoring and diagnosing with the central train control and management system (TCMS).
Fig. 1: Train Ethernet Network
So, what kind of sensors are needed and why are so many sensors required? On a typical high-speed train, there may be up to 16 different subsystems for control, monitoring and diagnostics – all needing sensors to measure the real-time parameters – but the main systems are:
Sensors that might be required in a railway HVAC system are temperature (internal, external, coolant and evaporator core temperature), relative humidity, pressure (differential, absolute and vacuum), mass-flow, sunlight (used to adjust the heating/cooling rate), filter air restriction and air quality (Carbon Dioxide (CO2), Volatile Organic Compounds (VOC) and particulates).
Sensors that are required in a motor bogie are motor, bearing, brake and gearbox temperature, shock and vibration, air or hydraulic pressure in the brake cylinders, traction control, non-contact magnetic wheel slide, gear and speed sensors and neutral track detectors (to indicate where the overhead power lines switch sections).
Door mechanism sensors that might be required are door opening/closing speed and position indicators, force sensors (to detect obstructions), actuator and linkage angle sensors, capacitive or vandal-proof push-buttons to open or close the door as well as monitoring and diagnostics to detect wear in the door mechanisms. Each door controller will also report back to the TCMS unit to inform the driver that all the doors are properly closed before the train moves off.
Sensors that might be required here are ambient light to adjust the display brightness, ambient noise to adjust the speaker volume and GPS position tracking or touchscreen sensors for human-machine interface (HMI) interactive displays. The CCTV or thermal cameras can also be integrated into the PIS audio/visual systems to determine seat occupancy or to detect fires.
Fig. 2: Train subsystems
Fig. 3: Non fault tolerant sensor system
Although only four out of the 16 subsystems have been looked at, it can be readily seen why the high-speed train mentioned in the introduction needed more than 7000 sensors.
The majority of these sensors will be powered from 24 VDC as this is an industry standard but some may need to be supplied from the primary train power supply, which is usually 110 VDC. While several different sensors connected to a local controller can be powered from a single supply, it is often better to separate the supply rails and isolate them to give the system fault tolerance. This is because if a single sensor is damaged or the cabling is short circuited, it should not bring down the whole system.
Let us take the example of a poor power and daisy-chained data connection scheme. If a single sensor fails, then it could pull down the 24 VDC supply, disabling the controller unit as well as the other sensors. Even if it fails open-circuit, the data daisy chain link could be broken, disabling the data communication from the following sensors after the break.
Fig. 4: Improved sensor system with sensor power supply fail tolerance
In case of an improved sensor and data communication scheme, each component in the system is powered from a separate isolated DC/DC converter, which means the continuous short circuit is protected. Thus, if a single sensor fails, it will not affect the other components in the system or overload the 110V DC supply.
Here, the data communication is via a multi-drop bus network that will continue to function even if the defective sensor does not pull down the data bus. Further, even if the data bus is compromised, the controller will be able to detect the fault and alert the TCMS.
If single fault tolerance is required on all power and data lines, then the following topology can be used (Figure 5):
Fig. 5: Ideal sensor system with power or data line fault tolerance.
The power supply to the controller is redundant such that if one DC/DC converter fails, the other can still supply the required current. Standard DC/DC converters do not have this ‘OR’ function, so special versions are needed. RECOM plug-and-play power solutions are then comparatively better – they offer true current sharing on the outputs so that the power supplies can be paralleled to be used as either a N+1 redundant supply or as an increased current power supply arrangement.
On the low voltage side, each sensor has its own isolated DC/DC power supply and data interface. This has certain advantages: if any one sensor fails, not only it refuses to pull down the power supply and the data bus, but also if the sensor fails short circuit to a higher voltage (say the 110V DC main supply), this external voltage cannot feed back and damage either the controller’s internal power supply or its data bus connection back to the TCMS.
RECOM manufactures a range of all-in-one DC/DC power supplies that incorporate a railway grade EMC filter, reverse polarity protection and offer paralleling function on the outputs from 40W up to 1000W output power (meaning that the power supplies share the output current equally and the V/I characteristic remains unchanged). The input voltage range can be 4:1 24V, 36V, 110V or ultra-wide 14.4–170V DC to cover all the standard input voltages. Further, they meet temperature class OT4 + ST1 & ST2, with full output power and allow only natural convection cooling.
Fig. 6: RECOM’s RMD and RMSD range of EN 50155 plug and play railway grade DC/DC converters.
RECOM offers board-level DC/DC converters for isolated sensor interfaces including an 8 Watt, EN 50155 certified part in a DIP24 format (32 x 20.3 x 11.2mm) and a 20 or 30 Watt, EN 50155 certified part in a 1” x 1” case.
Fig. 7: RECOM’s RP08, RP12, RPA20 and RPA30 PCB mount railway grade DC/DC converters.
For more demanding railway applications including sensor/actuator combinations, RECOM also offers a range of higher power board-level isolated DC/DC converters up to 200W with either 4:1 or ultra-wide 10:1 input voltage range, with continuous output short circuit protection and EN 50155 certifications.
Fig. 8: RECOM Front-end to Back-end Railway Power Supply Solutions
At present, only RECOM offers such a wide breadth of railway-grade DC/DC converters to enable sensors to be rapidly implemented or retrofitted to the existing rolling stock. Besides being pre-certified to the railway EN50155 standard, they are also compliant to all the major technical railway standards, such as EN50121-3-2 for EMC, EN50124-1 for safety insulation, EN50125-1 for environmental conditions and EN45545-2 for fire safety.
One of the reasons for this massive expansion in sensors is the continuous need for improvements in safety, reliability, comfort and performance in modern railway infrastructure. But how can you improve, say, passenger safety in the existing rolling stock that has a working life of decades and is already several years old? The answer to that would be to retrofit improved sensors and monitoring systems to add a layer of control and communication on top of the existing mechanical systems.
A modern train already has more in common with an IT server installation than what meets the eye. An on-board ethernet backbone supports CCTV and PA systems running over an IP network, transmits real-time information for the passenger information systems, has Wi-Fi access points and seat reservation displays and interfaces the distributed processor and gateway units for control, while monitoring and diagnosing with the central train control and management system (TCMS).
Fig. 1: Train Ethernet Network
So, what kind of sensors are needed and why are so many sensors required? On a typical high-speed train, there may be up to 16 different subsystems for control, monitoring and diagnostics – all needing sensors to measure the real-time parameters – but the main systems are:
1. HVAC
The Heating, Ventilation and Air-Conditioning (HVAC) units are roof-mounted and maintain the passenger environment to keep it warm in winter and cool in summer. In addition, they circulate and filter the air inside the carriages, which was a very important function during the time of Covid infection. Approximately 10–30% of the air is refreshed each minute, so it is also important that the fan-noise is kept low, and that the circulation system is designed to flush out all aerosols in the air without leaving any dead spaces in the footwells, corners or ceiling spaces within the carriage.Sensors that might be required in a railway HVAC system are temperature (internal, external, coolant and evaporator core temperature), relative humidity, pressure (differential, absolute and vacuum), mass-flow, sunlight (used to adjust the heating/cooling rate), filter air restriction and air quality (Carbon Dioxide (CO2), Volatile Organic Compounds (VOC) and particulates).
2. Bogie Assembly
The Bogie assembly consists of the wheels and axles, bearings, primary suspension and brakes; in case of electric trains, the traction motor, coupling and gearbox mechanisms are also incorporated. It is the harshest rolling stock environment requiring particularly robust and reliable sensors.Sensors that are required in a motor bogie are motor, bearing, brake and gearbox temperature, shock and vibration, air or hydraulic pressure in the brake cylinders, traction control, non-contact magnetic wheel slide, gear and speed sensors and neutral track detectors (to indicate where the overhead power lines switch sections).
3. Doors
To avoid carriage doors opening during the journey, either due to faults or to pressure waves caused by two high-speed trains passing each other, the safety requirements for railway door opening and closing are very strict, meaning that door sensing and control are complex systems. Automatic entry or exit doors tend to be electrically operated, while the less-used inter-carriage or toilet doors are typically mechanically operated or pneumatic as the vacuum flush toilets already have a pneumatic connection available (a toilet also needs many sensors such as water and waste tanks levels, air and vacuum pressure, occupancy and passenger alarm detectors).Door mechanism sensors that might be required are door opening/closing speed and position indicators, force sensors (to detect obstructions), actuator and linkage angle sensors, capacitive or vandal-proof push-buttons to open or close the door as well as monitoring and diagnostics to detect wear in the door mechanisms. Each door controller will also report back to the TCMS unit to inform the driver that all the doors are properly closed before the train moves off.
4. PIS
Passenger information systems (PIS) are needed to help passengers prepare to leave the train before it arrives at the next station and to inform them of the train’s progress or of any possible delays. Some systems also include local weather reports and many incorporate news, entertainment or advertisements. On older trains, this might be done via a public address (PA) announcement by the driver or guard but on many modern intercity and regional trains, the PIS takes the form of bulkhead or ceiling-mounted TFT or LED dot-matrix displays. Data is transmitted and updated to the displays by 100Mbps ethernet or by RS485 serial buses that are also linked to the seat reservation displays.Sensors that might be required here are ambient light to adjust the display brightness, ambient noise to adjust the speaker volume and GPS position tracking or touchscreen sensors for human-machine interface (HMI) interactive displays. The CCTV or thermal cameras can also be integrated into the PIS audio/visual systems to determine seat occupancy or to detect fires.
Fig. 2: Train subsystems
Fig. 3: Non fault tolerant sensor system
Although only four out of the 16 subsystems have been looked at, it can be readily seen why the high-speed train mentioned in the introduction needed more than 7000 sensors.
The majority of these sensors will be powered from 24 VDC as this is an industry standard but some may need to be supplied from the primary train power supply, which is usually 110 VDC. While several different sensors connected to a local controller can be powered from a single supply, it is often better to separate the supply rails and isolate them to give the system fault tolerance. This is because if a single sensor is damaged or the cabling is short circuited, it should not bring down the whole system.
Let us take the example of a poor power and daisy-chained data connection scheme. If a single sensor fails, then it could pull down the 24 VDC supply, disabling the controller unit as well as the other sensors. Even if it fails open-circuit, the data daisy chain link could be broken, disabling the data communication from the following sensors after the break.
Fig. 4: Improved sensor system with sensor power supply fail tolerance
In case of an improved sensor and data communication scheme, each component in the system is powered from a separate isolated DC/DC converter, which means the continuous short circuit is protected. Thus, if a single sensor fails, it will not affect the other components in the system or overload the 110V DC supply.
Here, the data communication is via a multi-drop bus network that will continue to function even if the defective sensor does not pull down the data bus. Further, even if the data bus is compromised, the controller will be able to detect the fault and alert the TCMS.
If single fault tolerance is required on all power and data lines, then the following topology can be used (Figure 5):
Fig. 5: Ideal sensor system with power or data line fault tolerance.
On the low voltage side, each sensor has its own isolated DC/DC power supply and data interface. This has certain advantages: if any one sensor fails, not only it refuses to pull down the power supply and the data bus, but also if the sensor fails short circuit to a higher voltage (say the 110V DC main supply), this external voltage cannot feed back and damage either the controller’s internal power supply or its data bus connection back to the TCMS.
RECOM manufactures a range of all-in-one DC/DC power supplies that incorporate a railway grade EMC filter, reverse polarity protection and offer paralleling function on the outputs from 40W up to 1000W output power (meaning that the power supplies share the output current equally and the V/I characteristic remains unchanged). The input voltage range can be 4:1 24V, 36V, 110V or ultra-wide 14.4–170V DC to cover all the standard input voltages. Further, they meet temperature class OT4 + ST1 & ST2, with full output power and allow only natural convection cooling.
Fig. 6: RECOM’s RMD and RMSD range of EN 50155 plug and play railway grade DC/DC converters.
RECOM offers board-level DC/DC converters for isolated sensor interfaces including an 8 Watt, EN 50155 certified part in a DIP24 format (32 x 20.3 x 11.2mm) and a 20 or 30 Watt, EN 50155 certified part in a 1” x 1” case.
Fig. 7: RECOM’s RP08, RP12, RPA20 and RPA30 PCB mount railway grade DC/DC converters.
For more demanding railway applications including sensor/actuator combinations, RECOM also offers a range of higher power board-level isolated DC/DC converters up to 200W with either 4:1 or ultra-wide 10:1 input voltage range, with continuous output short circuit protection and EN 50155 certifications.
Fig. 8: RECOM Front-end to Back-end Railway Power Supply Solutions
At present, only RECOM offers such a wide breadth of railway-grade DC/DC converters to enable sensors to be rapidly implemented or retrofitted to the existing rolling stock. Besides being pre-certified to the railway EN50155 standard, they are also compliant to all the major technical railway standards, such as EN50121-3-2 for EMC, EN50124-1 for safety insulation, EN50125-1 for environmental conditions and EN45545-2 for fire safety.
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