Which options exist for transmitting supply voltage and transferring data between the control unit and sensors in a vehicle? What are the advantages and disadvantages of hybrid solutions? and which solution is best for which application?
The automotive future is clearly pointing in the direction of “autonomous driving”.
Autonomous driving requires a large number of sensors such as cameras, radars or LiDAR in the vehicle in order to safely transport the passenger from A to B. This demands a huge amount of data which has to be transmitted, merged and analyzed via highly efficient data cables. Questions that should definitely be asked in this context include: How do I ensure that the right voltage is supplied with enough power from the control unit (ECU) to the sensor, and how can I ensure the safe and correct transfer of data?
It is precisely these interesting topics that we would like to shed light on in this article. Find out here what options exist for ensuring the transmission of supply voltage and the transfer of data between the control unit and sensors in a vehicle. What are the advantages and disadvantages of hybrid plug connectors. And what is MD doing to prepare for future developments?
What options are there for ensuring the transmission of supply voltage and the transfer of data between the control unit and sensors in a vehicle?
As is so often the case, various possible solutions exist which can be used in vehicles:
- Power over Coax (PoC)
- Power over Dataline (PoDL)
- Separate power and data
In this article, we would like to take a closer look at the individual solutions and their advantages and disadvantages.
Power over Coax (PoC)
Three types of signal are sent and received between sensors such as cameras and the Electronic Control Unit or ECU:
- Commands (ECU à camera)
- Image data (camera à ECU)
- Supply voltage (ECU à camera)
Originally, two or three cable pairs were required to transmit these signals. The major disadvantage was the associated heavy weight and the high costs.
With Power over Coax (PoC), it is now possible to transfer all these signals via a single coaxial cable. The light weight, the reduced effort involved in processing and installation, and the reduced costs are some of the indisputable advantages. However, what might seem to be an ideal solution, also has its drawbacks. The coaxial cable has to manage three completely different signal characteristics and two signal directions.
The commands from the ECU to the sensor (e.g. camera) are transmitted in frequencies of up to 4 MHz, while the image data in the range of 700 MHz has to be sent in the opposite direction. In addition, a constant DC supply also needs to be provided.
To ensure signal integrity, passive high-performance filters must be installed on both sides of the transmission (sensor side and ECU side). These filters allow the supply voltage’s DC current through, almost without any losses, while the data signals of the forward and back channels are attenuated in order to filter out any disruptive effects.
The filter on the ECU side merges the DC voltage with the video and communication data. The task of the filters on the sensor side is to separate the DC voltage from the video and communication data in order to enable a stable DC power supply to the sensor.
PoC transmission with filter
Designing the filter is no easy task as many elements such as the electrical properties of the passive components and the filter, the influence of the filter on the performance and signal quality of the serial HF data transmission, voltage drops across the cable, and compliance with EMC regulations must all be taken into account. The small cable cross-sections limit the maximum power load and cable length. When using PoC (Power over Coax), a typical PoC voltage is between 5 volts and 36 volts. The current must be below the rated current of the ferrites or inductors used in the filter (e.g. 150 mA). For this reason, Texas Instruments, for example, recommends a PoC voltage larger than 10 volts in order to enable the highest possible power at a low current. (Power [W] = current [A] * voltage [V])
Power over Dataline (PoDL)
In contrast to PoC (Power over Coax), the differential cable pairs of an Unshielded Twisted Pair (UTP) or a Shielded Twisted Pair (STP) are used for data and power transmission.
The transmission speed determines whether a shielded or an unshielded cable should be used.
10Base-T1 or 100Base-T1 Ethernet connections are usually unshielded while shielded cables are generally used for higher speeds.
The maximum cable lengths depend on the transmission speed:
- 10Base-T1: up to 1000 m
- 2.5, 5, 10 Gbit/s MultiGiG: maximum 15 m
One advantage of Power over Dataline (PoDL) with shielded cables in relation to Power over Coax (PoC) is the EMC (electromagnetic compatibility). With PoC, the shield also functions as a conductor (return cable from the consumer). Due to the flowing current, the screen acts almost like an antenna that “radiates”.
With PoDL, thanks to the twisted pair cables, the shield acts as a “real” screen.
The data and power supply are only transmitted via the two data cables. No currents flow on the screen. Interference such as EMC radiation due to high/low level changes of the data signals remain inside the cable and are not emitted into the environment.
However, similarly to PoC, the current or the maximum useful power that can be made available to the sensor is very limited. According to IEEE 802.3bu, the limit is reached at 50 watts. Voltages of up to 60 volts can be used to keep currents as low as possible.
Separate power and data
4-pole mini-coaxial cable + MQS
One of the main weaknesses of PoC (Power over Coax) and PoDL (Power over Dataline) is the limited power that can be provided to the consumers.
However, sensors such as LiDAR and radar are becoming ever more powerful. Measurement distances are becoming longer and resolutions (number of pixels) are increasing, and this is ultimately reflected in increasing power consumption. PoC and PoDL are reaching their limits here. For this reason, the third solution, where data and supply cables are routed separately, is becoming more popular.
This type of data transfer is mostly implemented with various contacts brought together in the housing, for example HSD (High Speed Data) with two MQS (Micro Quadlok Systems), The signals are transmitted via the HSD cable while the sensors are supplied with power via the MQS strands.
However, these hybrid connections have their disadvantages too.
The advantages and disadvantages of hybrid connectors:
The advantages of hybrid connectors lie in the more flexible choice of cross-section for the supply cables and the higher currents that can be made available to the consumer thanks to the larger cross-sections.
A further advantage is that the data signal does not have to be combined with the power supply by means of expensive filters before being separated again on the other side.
But there are of course also disadvantages here. Manufacturing this type of cable demands considerably greater effort. The fully automated machines must be able to combine the MQS contacts with the different cross-sections of the supply cables. This requires the use of various crimp tools. And the effort involved in checking and validating the different cables also increases dramatically as a result.
Cable manufacturing of the future – what is MD doing to prepare for the future?
MD operates its own production facility at its headquarters near Munich, where its highly complex production systems are developed, designed and built by experienced designers and engineers. This is the optimal basis for achieving maximum flexibility in order to be able to react promptly to new requirements in on-board networks.
With our product portfolio, we are an extremely strong and competent partner who focuses on manufacturing high-quality data cables. As an independent manufacturer, we are a specialist in assembling hybrid data cables and connectors. MD ELEKTRONIK offers the right solution for Coax, UTP (Unshielded Twisted Pair) and STP (Shielded Twisted Pair) products as well as for fiber optic data transmission.
Summary and conclusions:
Three approaches currently exist for transmitting power and data simultaneously between sensors and actuators in vehicles:
- PoC (Power over Coax)
- PoDL (Power over Dataline)
- Separate power and data
PoC (Power over Coax) and PoDL (Power over Dataline) enable transmission of supply voltage via data cables, but are very limited in terms of maximum current due to the narrow thicknesses of the cable.
The third option is the separate routing of power and data. The major advantage of this hybrid solution is that data cables and power supplies are routed separately but then connected together in a connector system. The disadvantage of this solution is the increased manufacturing effort involved.
It is not possible to draw a general conclusion as to which of the three options is the best solution, as this depends very much on the individual application. Every OEM has its own philosophy about how to design the power supply in the on-board network.
In the case of sensors with high power requirements, the hybrid solution is often preferred, as PoC and PoDL are severely limited here by the cable’s cross-section.
However, if electricity consumers such as cameras are to be supplied with a moderate power and there is also a need to simplify assembly and reduce weight, PoC and PoDL technologies will continue to play a major role.