This Weighing Technology Companion Specification describes how a scale system is addressed via OPC UA. For this purpose, the following primary use cases for the different scale types from section 5.1.2 were considered. Other use cases can also be covered or specified in other standards (vendor or Companion Specification).
In all defined use cases the scale system usually has the role of an OPC UA server. Therefore, this specification describes and defines the way an OPC UA client can read or write data and control the scale system via OPC UA. A scale system may also have a different interface (e.g., an HMI) that can control the scale as well. Some peripheral devices (e.g., printer, feeder) are also described in this specification but the communication between the peripheral devices and the scale is not part of the description.
NOTE: The OPC UA Companion Specification for Weighing Technology does not consider any use cases that are affected by legal requirements (like official verification). For example, further parameters may be necessary for external data access or existing parameters may not be read out. A country-specific extension of the Companion Specification would be conceivable.
An important application of a scale is the data access of individual measurement results. In many cases, the measurement result is a weight value with a unit derived from the weight (e.g., flow rate, volume, ...). A distinction can be made between the continuous display of the measured value (called “current Weight”) and a measurement (internal or external) that has been registered (called “registered Weight”). A registration could be triggered from an OPC UA-Client. The use case also includes the needed meta-data (e.g., product ID, unit) for identifying the result in further processing.
In some cases, the individual measurement result is not relevant or only of minor relevance. In some cases, a statistic (mean value, standard deviation) over several measured values or over a period of time is required.
A defined state machine enables the user to obtain a uniform overview of different devices from different manufacturers. This is useful in applications with a large number of machines. The interaction between a client system and the scale system depends on the current state of the device. Transition between two states can be triggered by method calls from an OPC UA client, by another interface or internally. Each state change creates an event in the OPC UA server. The PackML state machine is used for the scale system, as it is already in use in Weighing Technology, e.g., for checkweighers, and can be adapted accordingly for other scales. A more detailed description of the state machine can be found in the Companion Specification PackML for OPC UA. NOTE: Scales do not have to implement the SubStateMachine within the state "Execute".
Part of the access of a higher-level system to a weighing system is the management of products. This standard describes a way of creating, editing and selecting products on the scale. The entire information flow via OPC UA can also run completely or in parts via different interfaces. The schematic structure of production management is described in Section 5.1.3.
NOTE: For recipe scales, this use case also includes the creation, editing and administration of recipes. Recipe management is based on the OPC UA Companion Specification for machine vision. The structure and concept for recipes are described in section 5.1.4 and Annex B.