3 Terms, definitions and conventions
3.1 Overview
For this specification it is assumed that basic concepts of OPC UA information modelling are understood. This specification will use these concepts to describe the OPC UA for the Weighing Technology Companion Specification Information Model. For the purpose of this document, the terms and definitions given in OPC 10000-1, OPC 10000-3, OPC 10000-4, OPC 10000-5, OPC 10000-7, OPC 10000-100, OPC 10000-200, OPC 30050, OPC 40001-1 as well as in the following apply.
NOTE: OPC UA terms and terms defined in this specification are italicized in the specification.
3.2 OPC UA for Weighing Technology terms
The terminology used in this specification conforms to
The International Vocabulary of Legal Metrology (VIML)
Basic and General Terms in Metrology (VIM)
D 11 General requirements for measuring instruments - Environmental conditions
R 76 Non-automatic weighing instruments,
D 31 General requirements for software-controlled measuring instruments.
In addition, the following definitions apply.
3.2.1 General Terms
| Term | Definition |
| Actual scale interval, d | Value expressed in units of mass of: the difference between the values corresponding to two consecutive scale marks, for analog indication; or the difference between two consecutive indicated values, for digital indication. (OIML R51) |
| Automatic scale | An instrument that weighs without the intervention of an operator and follows a predetermined program of automatic processes, which are characteristic of the instrument. |
| Current weight | Defines the current value that is measured at the sensor at the concurrent timestamp. Might be a highly fluctuating value. |
| Division | Value of the smallest increment. See also (actual scale interval, verification scale interval) |
| Feeder | Devices for conveying the product to or from the weighing module |
| Flow rate | The flow rate is a measured value passes per unit of time. The flow rate can be defined as mass flow rate (flow of mass m through a surface per unit time t) or as volumetric flow rate (flow of volume of fluid V through a surface per unit time t) |
| Gross value | Indication of the weight value of a load on an instrument, with no tare or preset tare device in operation. (see OIML R 76) Weight of an objective or sample (net weight) including its container or packaging (tare weight) |
| Height | Length of an item in direction of global gravity. |
| Item | A physical product in scales that process a set of physical products like catchweigher. In case a “productType” represented a set of physical products the single physical product is called item. |
| Length | In direction of travel. |
| Load | Amount of product that is currently introducing the force on the load receptor. (OIML R61) A general term for any Object when it is intended to convey the meaning that this Object is exerting a weight force |
| Measurement result | Set of quantity values being attributed to a measurand together with any other available relevant information (see VIM 2.9) |
| Minimum Weight | The Minimum Weight is the smallest sample quantity required for a weighment to just achieve a specified relative accuracy of weighing. So it is the smallest load or component of a recipe that may be weighted on the scale. |
| Net value | Indication of the weight value of a load placed on an instrument after operation of a tare device. (see OIML R 76) The weight of a material or sample after deducting the weight of its packaging or of the transport instrument (tare weight) |
| PackML | “PackML stands for Packaging Machine Language and is an interface standard originally used in batch manufacturing in the packaging industry, but which is now used in multiple different types of production and assembly lines.” (see OPC 30050) |
| Preset tare value | Numerical value, representing a weight value, that is introduced into the instrument. It is a predetermined tare value that is used for one or several weighings. (see OIML R 76) |
| Product | For the purposes of this Companion Specification, a product is a physical good that is processed by a weighing system. Depending on the type of scale and type of good, the result may be a single weighing and/or a statistical evaluation of many weighing operations. Additional process steps (e.g., metal detection) can also be performed. A OPC UA Object of type "productType" represented a physical product or a set of physical products and contain additional (meta-)information like a target value or some identifier. |
| Proportional tare | Tare value that is determined by the scale automatically and that is proportional to the gross weight. |
| Recipe | See Annex B |
| RegisteredWeight | Defines the last valid measurement that was recorded and will be used for further processing. |
| Scale | Synonym: weighing instrument Measuring instrument that serves to determine the mass of an amount of material by using the action of gravity on this material. NOTE: In this Recommendation “mass” (or “weight value”) is preferably used in the sense of “conventional mass” or “conventional value of the result of weighing in air” according to OIML R 111 and OIML D 28, whereas “weight” is preferably used for an embodiment (= material measure) of mass that is regulated in regard to its physical and metrological characteristics. The instrument may also be used to determine other quantities, magnitudes, parameters or characteristics related to mass. According to its method of operation, a weighing instrument is classified as automatic or non-automatic. (See OIML R 76) |
| Scale division | See division |
| Scale system | A scale system is the combination of several scales that are addressed by a common interface. A scale system can have additional functions, such as a state machine or a production preset. |
| Setpoint | Time at which an action is triggered |
| Standard deviation | A quantity expressing by how much the members of a group differ from the mean value for the group. |
| Tare value, T | Weight value of a load, determined by a tare weighing device. (see OIML R 76) The mass of packaging or transport container of the material that is being weighed |
| Target value | Requested value of a variable size at a given time and under specified conditions. (see IEC 60050) |
| Tolerable Negative Error (TU1) for prepackages | Only a small number of prepackages may have a content below the nominal quantity minus the maximum permissible error (TU1-limit). These prepackages are known and referred to as ‘defectives’ (see Welmec 6.4) |
| TU2-limit | No prepackage with a quantity of product less than the nominal quantity minus twice the tolerable negative error (TU2-limit) may be ℮-marked. (see Welmec 6.4) |
| Verification scale interval, e | Value, expressed in units of mass, used for the classification and verification of an instrument. (OIML R51) |
| Weighing | Process of determining the mass of a load using the effect of gravity on that load (OIML R61) |
| Weighing bridge | Mechanical component of the scale that carry the load. One or more load cells are integrated into the WeighingBridge as an element of mass determination. |
| Weighing good ID | Defines the identifier of an Object that is to be weighed (load) |
| Weighing instrument | Synonym: scale |
| Weighing module | Part of the weighing instrument that comprises all mechanical and electronic devices (i.e., load receptor, load-transmitting device, load cell, and analog data processing device or digital data processing device) but not having the means to display the weighing result. It may optionally have devices for further processing (digital) data and operating the instrument. (OIML R76) |
| Weighing range | The range of a scale can measure a mass. A scale can have several weighing ranges with different scale divisions |
| Width | In third possible orthogonal direction to height and length |
3.2.2 State machine terms
The terms for state machine defined in OPC 30050.
3.3 Abbreviations and symbols
| value | |
| mean value | |
| DCS | Distributed Control Systems |
| G | Gross |
| HMI | Human Machine Interface |
| HTTP | Hypertext Transfer Protocol |
| IP | Internet Protocol |
| L | Load |
| N | Net |
| OIML | International Organization of Legal Metrology |
| PT | Preset tare |
| T | Tare |
| TCP | Transmission Control Protocol |
| TU1 | lower tolerance limit |
| count | |
| standard deviation |
3.4 Conventions used in this document
3.4.1 Conventions for Node descriptions
Node definitions are specified using tables (see Table 3).
Attributes are defined by providing the Attribute name and a value, or a description of the value.
References are defined by providing the ReferenceType name, the BrowseName of the TargetNode and its NodeClass.
If the TargetNode is a component of the Node being defined in the table, the Attributes of the composed Node are defined in the same row of the table.
The DataType is only specified for Variables; “[<number>]” indicates a single-dimensional array, for multi-dimensional arrays the expression is repeated for each dimension (e.g. [2][3] for a two-dimensional array). For all arrays the ArrayDimensions is set as identified by <number> values. If no <number> is set, the corresponding dimension is set to 0, indicating an unknown size. If no number is provided at all the ArrayDimensions can be omitted. If no brackets are provided, it identifies a scalar DataType and the ValueRank is set to the corresponding value (see OPC 10000-3). In addition, ArrayDimensions is set to null or is omitted. If it can be Any or ScalarOrOneDimension, the value is put into “{<value>}”, so either “{Any}” or “{ScalarOrOneDimension}” and the ValueRank is set to the corresponding value (see OPC 10000-3) and the ArrayDimensions is set to null or is omitted. Examples are given in Table 2.
| Notation | DataType | ValueRank | ArrayDimensions | Description |
| Int32 | Int32 | -1 | omitted or null | A scalar Int32. |
| Int32[] | Int32 | 1 | omitted or {0} | Single-dimensional array of Int32 with an unknown size. |
| Int32[][] | Int32 | 2 | omitted or {0,0} | Two-dimensional array of Int32 with unknown sizes for both dimensions. |
| Int32[3][] | Int32 | 2 | {3,0} | Two-dimensional array of Int32 with a size of 3 for the first dimension and an unknown size for the second dimension. |
| Int32[5][3] | Int32 | 2 | {5,3} | Two-dimensional array of Int32 with a size of 5 for the first dimension and a size of 3 for the second dimension. |
| Int32{Any} | Int32 | -2 | omitted or null | An Int32 where it is unknown if it is scalar or array with any number of dimensions. |
| Int32{ScalarOrOneDimension} | Int32 | -3 | omitted or null | An Int32 where it is either a single-dimensional array or a scalar. |
The TypeDefinition is specified for Objects and Variables.
The TypeDefinition column specifies a symbolic name for a NodeId, i.e., the specified Node points with a HasTypeDefinition Reference to the corresponding Node.
The ModellingRule of the referenced component is provided by specifying the symbolic name of the rule in the ModellingRule column. In the AddressSpace, the Node shall use a HasModellingRule Reference to point to the corresponding ModellingRule Object.
If the NodeId of a DataType is provided, the symbolic name of the Node representing the DataType shall be used.
Nodes of all other NodeClasses cannot be defined in the same table; therefore, only the used ReferenceType, their NodeClass and their BrowseName are specified. A reference to another part of this document points to their definition.
Table 3 illustrates the table. If no components are provided, the DataType, TypeDefinition and ModellingRule columns may be omitted and only a Comment column is introduced to point to the Node definition.
| Attribute | Value | ||||
| Attribute name | Attribute value. If it is an optional Attribute that is not set “--” will be used. | ||||
| References | NodeClass | BrowseName | DataType | TypeDefinition | ModellingRule |
| ReferenceType name | NodeClass of the TargetNode. | BrowseName of the target Node. If the Reference is to be instantiated by the server, then the value of the target Node’s BrowseName is “--”. | DataType of the referenced Node, only applicable for Variables. | TypeDefinition of the referenced Node, only applicable for Variables and Objects. | Referenced ModellingRule of the referenced Object. |
| NOTE: Notices referencing footnotes of the table content. | |||||
Components of Nodes can be complex that is containing components by themselves. The TypeDefinition, NodeClass, DataType and ModellingRule can be derived from the type definitions, and the symbolic name can be created as defined in 3.4.3.1. Therefore, those containing components are not explicitly specified; they are implicitly specified by the type definitions.
3.4.2 NodeIds and BrowseNames
3.4.2.1 NodeIds
The NodeIds of all Nodes described in this standard are only symbolic names. Annex A defines the actual NodeIds.
The symbolic name of each Node defined in this specification is its BrowseName, or, when it is part of another Node, the BrowseName of the other Node, a “.”, and the BrowseName of itself. In this case “part of” means that the whole has a 0:HasProperty or 0:HasComponent Reference to its part. Since all Nodes not being part of another Node have a unique name in this specification, the symbolic name is unique.
The namespace for all NodeIds defined in this specification is defined in Annex A The namespace for this NamespaceIndex is Server-specific and depends on the position of the namespace URI in the server namespace table.
NOTE: This specification not only defines concrete Nodes, but also requires that some Nodes shall be generated, for example one for each Session running on the Server. The NodeIds of those Nodes are Server-specific, including the namespace. But the NamespaceIndex of those Nodes cannot be the NamespaceIndex used for the Nodes defined in this specification, because they are not defined by this specification but generated by the Server.
3.4.2.2 BrowseNames
The text part of the BrowseNames for all Nodes defined in this specification is specified in the tables defining the Nodes. The NamespaceIndex for all BrowseNames defined in this specification is defined in Annex A.
If the BrowseName is not defined by this specification, a namespace index prefix like ‘0:EngineeringUnits’ or ‘2:DeviceRevision’ is added to the BrowseName. This is typically necessary if a Property of another specification is overwritten or used in the OPC UA types defined in this specification. Table 18 provides a list of namespaces and their indexes as used in this specification.
3.4.3 Common Attributes
3.4.3.1 General
The Attributes of Nodes, their DataTypes and descriptions are defined in OPC 10000-3. Attributes not marked as optional are mandatory and shall be provided by a Server. The following tables define if the Attribute value is defined by this specification or if it is server-specific.
For all Nodes specified in this specification, the Attributes named in Table 4 shall be set as specified in the table.
| Attribute | Value |
| DisplayName | The DisplayName is a 0:LocalizedText. Each server shall provide the DisplayName identical to the BrowseName of the Node for the LocaleId “en”. Whether the server provides translated names for other LocaleIds is server-specific. |
| Description | Optionally a server-specific description is provided. |
| NodeClass | Shall reflect the NodeClass of the Node. |
| NodeId | The NodeId is described by BrowseNames as defined in 3.4.2.1. |
| WriteMask | Optionally the WriteMask Attribute can be provided. If the WriteMask Attribute is provided, it shall set all non-server-specific Attributes to not writeable. For example, the Description Attribute may be set to writeable since a Server may provide a server-specific description for the Node. The NodeId shall not be writeable, because it is defined for each Node in this specification. |
| UserWriteMask | Optionally the UserWriteMask Attribute can be provided. The same rules as for the WriteMask Attribute apply. |
| RolePermissions | Optionally server-specific role permissions can be provided. |
| UserRolePermissions | Optionally the role permissions of the current Session can be provided. The value is server-specific and depend on the RolePermissions Attribute (if provided) and the current Session. |
| AccessRestrictions | Optionally server-specific access restrictions can be provided. |
3.4.3.2 Objects
For all Objects specified in this specification, the Attributes named in Table 5 shall be set as specified in the table. The definitions for the Attributes can be found in OPC 10000-3.
| Attribute | Value |
| EventNotifier | Whether the Node can be used to subscribe to Events or not is server-specific. |
3.4.3.3 Variables
For all Variables specified in this specification, the Attributes named in Table 6 shall be set as specified in the table. The definitions for the Attributes can be found in OPC 10000-3.
| Attribute | Value |
| MinimumSamplingInterval | Optionally, a server-specific minimum sampling interval is provided. |
| AccessLevel | The access level for Variables used for type definitions is server-specific, for all other Variables defined in this specification, the access level shall allow reading; other settings are server-specific. |
| UserAccessLevel | The value for the UserAccessLevel Attribute is server-specific. It is assumed that all Variables can be accessed by at least one user. |
| Value | For Variables used as InstanceDeclarations, the value is server-specific; otherwise it shall represent the value described in the text. |
| ArrayDimensions | If the ValueRank does not identify an array of a specific dimension (i.e., ValueRank <= 0) the ArrayDimensions can either be set to null or the Attribute is missing. This behavior is server-specific. If the ValueRank specifies an array of a specific dimension (i.e., ValueRank > 0) then the ArrayDimensions Attribute shall be specified in the table defining the Variable. |
| Historizing | The value for the Historizing Attribute is server-specific. |
| AccessLevelEx | If the AccessLevelEx Attribute is provided, it shall have the bits 8, 9, and 10 set to 0, meaning that read and write operations on an individual Variable are atomic, and arrays can be partly written. |
3.4.3.4 VariableTypes
For all VariableTypes specified in this specification, the Attributes named in Table 7 shall be set as specified in the table. The definitions for the Attributes can be found in
| Attributes | Value |
| Value | Optionally a server-specific default value can be provided. |
| ArrayDimensions | If the ValueRank does not identify an array of a specific dimension (i.e., ValueRank <= 0) the ArrayDimensions can either be set to null or the Attribute is missing. This behavior is server-specific. If the ValueRank specifies an array of a specific dimension (i.e., ValueRank > 0) then the ArrayDimensions Attribute shall be specified in the table defining the VariableType. |
3.4.3.5 Methods
For all Methods specified in this specification, the Attributes named in Table 8 shall be set as specified in the table. The definitions for the Attributes can be found in OPC 10000-3.
| Attributes | Value |
| Executable | All Methods defined in this specification shall be executable (Executable Attribute set to “True”) unless it is defined differently in the Method definition. |
| UserExecutable | The value of the UserExecutable Attribute is server-specific. It is assumed that all Methods can be executed by at least one user. |