EUInformationcontains information about the EngineeringUnits.
The intention of the OPC UA standard is not to define a set of units but a way to expose units based on existing systems. Since there is not a single worldwide set of units used in all industries, the EUInformationstructure includes a separate field (the namespaceUri) to identify the system on which the exposed unit is based.
The default OPC UA mapping is based on UN/CEFACT as defined in clause 5.6.3.3, because it can be programmatically interpreted by generic OPC UA Clients. However, the EUInformationstructure has been defined such that other standards bodies can incorporate their engineering unit definitions into OPC UA. If Serversuse such an approach then they shall identify this standards body by using a proper URI in EUInformation.namespaceUri.
The EUInformationelements are defined in Table 19.
Table 19– EUInformationDataType structure
Name |
Type |
Description |
EUInformation |
structure |
|
namespaceUri |
String |
Identifies the organization (company, standards organization) that defines the EUInformation. |
unitId |
Int32 |
Identifier for programmatic evaluation. −1 is used if a unitIdis not available. |
displayName |
LocalizedText |
The displayNameof the engineering unit is typically the abbreviation of the engineering unit, for example "h" for hour or "m/s" for meter per second. |
description |
LocalizedText |
Contains the full name of the engineering unit such as "hour" or "meter per second". |
Its representation in the AddressSpaceis defined in Table 20.
Table 20– EUInformationdefinition
Attribute |
Value |
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BrowseName |
EUInformation |
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IsAbstract |
False |
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References |
NodeClass |
BrowseName |
DataType |
TypeDefinition |
Other |
|
Subtype of Structure defined in OPC 10000-5. |
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Conformance Units |
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Base Info EUInformation |
This clause specifies how to apply the “Codes for Units of Measurement”published by the “United Nations Centre for Trade Facilitation and Electronic Business” (see https://www.unece.org/cefact/codesfortrade/codes_index.html). This recommendation establishes a single list of code elements to represent units of the International System of Units (SI Units) like units of measure for length, mass (weight), volume and other quantities and in addition covers administration, commerce, transport, science, technology, industry etc. It provides a fixed code that can be used for automated evaluation.
Table 21contains a small excerpt of the relevant columns in the UNECE recommendation:
Table 21– Examples from the UNECE Recommendation
Excerpt from Recommendation N°. 20, Annex 1 |
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Common Code |
Name |
Symbol |
C81 |
radian |
rad |
C25 |
milliradian |
mrad |
MMT |
millimetre |
mm |
HMT |
hectometre |
hm |
KMT |
kilometre |
km |
KMQ |
kilogram per cubic metre |
kg/m3 |
FAH |
degree Fahrenheit |
°F |
The mapping of the UNECE common codes to EUInformation.unitId, EUInformation.displayName, and EUInformation.descriptionis available here:
http://www.opcfoundation.org/UA/EngineeringUnits/UNECE/UNECE_to_OPCUA.csv
This mapping has been generated as follows:
- The namespaceUrishall be http://www.opcfoundation.org/UA/units/un/cefact
- The Common Code(represented as an alphanumeric variable length of 3 characters) has been converted into an 32 Bit Integer and is used for the unitId. The following pseudo code specifies the conversion algorithm:
Int32 unitId = 0; Int32 c; for (i=0; i<=3;i++) {
c = CommonCode[i]; if (c == 0) break; // end of Common Code unitId = unitId << 8; unitId = unitId | c;}
- The Symbolfield is used for displayName. The localeId field of EUInformation.displayNameshall be an empty string.
- The Namefield is used for description. If the name is copied, then the localeId field of EUInformation.descriptionshall be an empty string. If the name is localized then the localeId field shall specify the correct locale.