Industrial Automation - Max-i Fieldbus

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Industrial Automation
Supplement to Ethernet
Ethernet is growing fast in industrial process control. This puts traditional 5-V based fieldbus systems under pressure. Because they need a power supply to convert the supply voltage to 5 V, a timing crystal and some microprocessor assistance to handle the communication stack, they are too big and expensive to be included in the smallest and most price sensitive devices like push buttons, lamps, micro switches, motor contactors etc. It is therefore only possible to use these bus systems for distributed I/O and for connection to more complicated devices, but Ethernet can do that too at very much the same price. There may even be Ethernet in the building already, which may save some money for cabling. Ethernet will however never get out to individual actuators and sensors - even if the price and size is not taken into consideration. Because Ethernet is today a point-to-point communication, the signals in a big plant would have to pass so many routers and with that so many cable connectors and so much electronics that the reliability would fall to a totally unacceptable level with maybe up to one failure per week.

In the future, there will therefore probably only be room for two types of bus systems - Ethernet with an added layer to make it deterministic like Ethernet PowerLink, EtherCAD, ProfiNet, Ethernet/IP, Time Sensitive Networking (TSN) or Sercos, and an ultra-low-cost, but still high performance and deterministic bus system for connection to individual actuators, sensors and lamps. This bus system must:

  • Be a multiple master bus so that it is not necessary with an extra layer (distributed device) between fieldbus and actuators and sensors.
  • Be deterministic.
  • Be cheap and small enough for even the smallest and most price sensitive devices as these devices by far make up the majority of devices in most plants. A vendor of a complex device may find it easy to add for example a CANOpen or Profibus Interface if the device already contains a power supply and a sufficient powerful microprocessor with bus controller, and the necessary software stack is available, but selecting a bus system, which is too complex for simple devices on the same bus, will not lead to an optimized total solution.
  • Be powerful enough for even very complex and demanding devices like mass flow gauges and motion control. On a 35 m trunk line, Max-i is able to synchronize 7 servo motor axes with 32-bit precision to an accuracy of 0.1 μs at an industrial state-of-the-art communication cycle of 400 μs. This is much faster than CAN and close to or even better than the performance of a much more expensive and complex Ethernet solution, which also has much lower reliability due to the necessary routers and other complicated electronics.
  • Be able to supply enough power for even big actuators where more amperes may be needed and do it in a cost efficient way. This demand excludes all fieldbus systems, which use special communication cables. A lot of fieldbus systems are really only sensor busses due to the low power or the use of communication methods, which do not allow switching of heavy loads, but in a typical process plant there are only approximately 1.5-2 times more inputs than outputs, so a bus, which does not take the actuator side seriously, will not lead to an optimized solution.

With its outstanding versatility, Max-i is so far the only fieldbus, which fulfills all these demands! With Max-i, it is even possible and very easy to utilize the gamma correction and 3rd order smoothing filters of the advanced LED controller to limit the inrush current of for example filament lamps, heaters and DC-motors. This only requires a fast switch, a diode and small coil if the device is not already inductive.

Plant Numbering System (PNS)
In a process plant, it is very important to be able to interpret all process values without corresponding cross-reference tables or else devices from different manufacturers are not able to communicate and it is not possible to do service if you stand in the middle of nowhere and receive an error message from the plant on your mobile phone. This problem may be handled by means of for example OPC UA, which has become quite popular, but it is an extremely complicated standard, which requires months to study and a microprocessor to implement and therefore reduces the reliability considerably compared to a pure hardware solution. Instead, Max-i uses well specified data types, which may be scaled to SI-units, and a new numbering system (PNS) based on alternating characters and digits. PNS utilizes the 31-bit identifier of Max-i to its maximum, but does not require further bytes, which keeps the efficiency very high compared to other fieldbus systems. The idea is:

  • To create a numbering system where the machine numbers themselves contain enough information to enable communication with the various devices by means of fieldbus systems and especially Max-i. It shall not be necessary with separate fieldbus addresses, cross-reference tables etc.
  • To be able to number the various measurement types like temperature, pressure, level, flow etc. as properties of the equipment to with they belong like tank level and temperature, pump pressure, pipe flow etc. instead of using gauge names. This fits with the publisher-subscriber model of Max-i and XML (Extensible Markup Language) and makes it much easier to overlook the process control system. For example, production line PP2 may contain a tank - T9 (PP2T9) - with two level switches, which controls pump PP2P1. In a traditional instrumentation drawing, the two level switches will be named as level switches like for example PP2LG1 (level gauge 1) and PP2LG2, but in this way, it is only possible to tell that two level switches are controlling a pump - not where these level switches are located. However, with PNS it is also possible to use function codes like for example PP2T9L1 and PP2T9L2. Now it is obvious that two levels - L1 and L2 - of tank PP2T9 are controlling the pump. In the same way, the various gauges on a pipe or a tank may also be named as properties of the pipe or the tank like for example PP2WL3F2 (water pipe 3 flow 2) and PP2WL3T1 (water pipe 3 temperature 1).
  • To be able to number all parts of the plant including spare parts with the same numbering system. It shall not be necessary with separate numbering systems for machinery, instrumentation, pipes etc.
  • To be able to exchange data with all other systems.
  • To make it possible to get a record of all components for example for flow gauge A1FG2 just by searching the documentation for this name (without the function code), so that it is not necessary to specify the use of a gauge. In for example numbering systems like DEP, which uses the ISO 3511 standard, the use of a gauge is specified by means of letters following for example F for flow, P for pressure, L for level, T for temperature and Q for any quality parameter like pH, density, power etc. (not specified). For example, the ISO 3511 number #FITBRQCSZA# means a flow indicator (I) and transmitter (T) with a status display (B), a recorder (R) and a totalizer (Q), which is used for control (C), switching (S), trip initiation (Z) and for an alarm (A). It is obvious that this number requires quite a number of bits and a great symbol on the drawing, and if just a small thing is changed like removing the alarm, all documentation, which contains this part, must be rewritten and redrawn. With the ISO 3511 standard, it is hopeless (with a normal search program) to search the documentation for equipment and components if the use is not known because there are so many possibilities.

Note that PNS is only a first draft. Everybody is very welcome to send us any comments and suggestions. The specification is primary based on experiences from feed mills and heating and power stations so there are without doubt many equipment types missing for other types of plants.

The specification of PNS may be downloaded here: www.max-i.org/plantnumbering.pdf

This page is created with WebSite X5 and updated April 30th 2020

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