Compact industrial PCs, also called embedded systems or embedded Box PCs, are continuously growing thanks to the progressive reduction of the power consumption and the increase of I/O richness of the modern processors. If we look at the evolution of the process lithography in the recent years, we will note that they have gone from 45nm processes to 14nm processes with enormous improvements in terms of power consumption. Consequently, the tendency to design fanless systems has grown, and at the same time the computing power available with passive dissipation. A further point in favor of the growth of fanless systems is represented by the tendency to use external connections to interface various peripherals. If in the past there was a tendency to add functions via cards on internal expansion slots, today devices with external connectivity like serial or Ethernet interfaces, are capable of offering high performances and are available on a large scale. In fact, the expandability of fanless systems is often limited to the use of Mini PCI Express cards and, in some cases, 1/2 PCI or PCI Express expansion slots. Interfaces like RS-232/422/485, USB, Ethernet, CAN Bus and Digital I/Os are nowadays present on-board most of the systems available in the market. Many projects also require computers to be housed in enclosures, panels and cabinets with reduced space, so compact systems are highly required, for example for DIN-rail installations.
The widespread use of fanless systems on the market has made the costs accessible as the major manufacturers of industrial PCs are able nowadays to scale the production on high volumes.
However, there are extremely different technical features that require a careful analysis in choosing the system to be used for your project. A design method of fanless systems consists in utilizing CPU boards with standard form factors (e.g. Mini-ITX or 3.5 ″) and to create a housing case around them. This solution is the simplest and most immediate but has some disadvantages. First of all, the thermal and dissipation design have to be strictly adapted to the characteristics of the board.
Secondly, it is very likely that to guarantee an adequate set of interfaces it will be necessary to bring back some of them from the board to the case through the use of cables. This solution has disadvantages in terms of reliability since the internal wiring is more sensitive to shocks, vibrations, electromagnetic interference and the deterioration of the connections.
The approach of an ad hoc system developed from scratch, both at the electronic and the mechanical level is different. It is an approach that definitely requires more resources in the development phase but It has for sure greater guarantees in terms of flexibility and reliability. This philosophy makes it possible to optimize the thermal study on the basis of the characteristics that the finished system must have, making the heat dissipation more uniform and avoiding the so-called "hot spots" (points with a higher concentration of heat). The joint development of the board and the enclosure also allows to optimize the mechanical design (e.g. to install HDD/SSD without the need of cables). This approach has a whole series of advantages in terms of reliability and system compactness.
Below we will analyze some points that should be taken into consideration when choosing the fanless system to be used.
1) Fanless architecture: it is always good to check how the system was designed. Is it based on a standard board or has it been designed ad hoc at electronic and enclosure level? It's easy to check this point by consulting the user manual of the system.
2) Cable-free architecture: many manufacturers claim to design cable-free systems, but very often these systems need cables to interface necessary peripherals like HDDs or SSDs (e.g. SATA cables). So are we sure that the system is designed to be completely cable-free?
3) Operating temperature: this is the main discriminating factor between high and low quality systems. Are we sure that the system is able to work at 100% of the CPU performance at the extremes of the declared operating temperature without undergoing clock reductions (CPU Thermal Throttling) ? It is not only meant that the PC has to survives these temperatures but that it has to operate at full speed without experiencing "speed throttling" to reduce the clock. A system capable to work at full speed demonstrates that the thermal design has been done in a proper way and that the CPU can work within its temperature limitations and therefore will also have a higher MTBF (mean time between failures).
4) Power input: the factory environment is often characterized by unstable voltages and currents. Are we sure that the system has good margins of tolerance for input voltages and currents? Even more onerous in these terms are the applications on board vehicles that generate spikes in the ignition phase, so the system must be equipped with the "power ignition" function. This allows the power supply to the system to be delayed until, after switching on the vehicle, the onboard power supply has stabilized.
5) ESD protection: the factory environment is often characterized by high levels of electrostatic discharge. Are we sure that the system has a certain degree of protection and isolation on the interfaces?
6) Easy maintenance: the external accessibility to the peripherals is a function that facilitates maintenance and upgrade operations and It's recommended for devices that are more subject to failures such as HDDs and SSDs.
7) Expandability: very often fanless systems have only internal Mini PCI Express expansion sockets that are typically used to add wireless communication modules that only require the external carryover of the antennas. A window that can be opened on the enclosure can therefore be useful for adding connectors and functionalities for future product upgrades. On some embedded systems, there are also 1-2 PCI or PCI Express expansion slots but the system manufacturer cannot know in advance which type of card will be added by the customer. It is therefore recommended that the manufacturer declare a maximum power consumption for the add-on cards that could be installed. Furthermore, since the cards on slots are subject to shocks and vibrations, the best fanless systems include adjustable fasteners of various measures to ensure the expansion cards to have a robust connection.
6) MTBF and Test Report: it is always a good idea to ask the supplier for documentation regarding the various qualification tests of the machine including shock and vibration tests, thermal tests, MTBF analysis, burn-in reports and the documentation about the various certifications required by law. If the product is validated for railway or vehicle applications, it is advisable to request the relevant certificates such as EN-50155 and E-mark.
7) Longevity: Most of industrial projects require a high product longevity to adequately manage the life cycle of their machinery, both in terms of production, repair management and spare parts. This also allows the customer not to have to manage continuous product approvals due to unstable products. It is therefore advisable to ask the supplier for an official document stating the life cycle of the product, the official end-of-life terms and the related "last-buy order and last-buy shipment" policies.
8) TCO (Total cost of ownership): the evaluation of the costs of an embedded system must take into consideration the entire life cycle of the project and not the simple purchase cost of the machine. The choice of a product that is not suitable for the application just for pure purchasing cost savings, can have dramatic implications for the cost of ownership. First, it generates high costs for hardware replacement and maintenance, and sometimes it makes it impossible to repair PCs except through new replacements. On a production level then, frequently changes of the hardware, force the customer to continuous and onerous product approvals and sometimes also to software redesign if it is no longer possible to find a compatible product.