The TDM data model is derived from an international standard called ASAM ODS. ASAM is the “Association for the Standardization of Automation and Measuring Systems” and ODS stands for “Open Data Services”. National Instruments, as one of the co-founders of ASAM and an active member of the ASAM ODS working group, is bringing the ASAM ODS standard to the measurement industry. ASAM ODS defines the storage of measurement data, for instance, in an Oracle or mixed-mode server and a Corba-based API for data access. The major benefit is the definition of a so-called base data model, which allows you to add semantic (meta-) information to the data stored in the database.
The ASAM ODS data model provides templates, or base objects, which can be used to define a user specific data model: the application model. The templates are grouped by the different aspects of data: Environment, Administration, Measurements, Descriptive Data, Dimensions and Units, Security and Other.
Figure 1. An overview of the ASAM ODS data model.
The major objective of ASAM ODS is to store and organize measurement data, making Measurements the center of the ASAM ODS base model.
Working with ASAM ODS requires the definition of a so called application model derived from the ASAM ODS base model. There are two major design alternatives: the “object approach” and the “property approach”.
In the “object approach” meta data is grouped into several objects, in most cases picked from the Descriptive Data group combined with several AoAny derived objects. The advantage is that information can be composed into logical objects to avoid redundancies. Otherwise this often requires requesting several objects (tables) to generate a report. An example of the “object approach” is the openMDM data model.
The alternative to the object approach is the “property approach”. In the “property approach” additional information (properties, name-value pairs) is directly added to the measurement objects and the surrounding test and channel objects. The advantage of the “property approach” is that the necessary information is stored close to the measurement and can be retrieved easily. The drawback is the added redundancy, possibly resulting in a bigger database size. An example of the “property approach” is the TDM data model.
While both approaches are obviously different, the result as presented to the user is comparable. For instance, the name of a UnitUnderTest (UUT) can be retrieved as measurement.uut.name (name is stored on a uut object referenced by the measurement object) in the “object approach” and as measurement.uut_name (uut name is directly attached to the measurement object). In both cases the information may presented to the user as UUTName.