Physical dimensions and units

[doganulus]

Hello @COVESA community,

I have recently encountered this project as we are working on verification and validation tooling for connected and autonomous systems. Having a well-defined specification for everything is very important for our goals. Therefore, we have the issue of physical quantities that frequently come up in our tooling when representing signals. So, we have thought about the issue quite a bit and wondered if there is a similar issue for the VSS project as well.

In the following, I would like to share some ideas to discuss whether this discussion is in the scope of VSS. I have to say that there are several approaches when it comes to representing physical quantities but here we take an SI-based approach to represent dimensions, units, and quantities. The SI defines 7 base dimensions such as length, mass, and time and other domains are derived from these dimensions such as frequency and speed. Assume we have domains.yml as follows:

# domains.yml
time: 
  time: 1

frequency: 
  time: -1

length:
  length: 1

speed:
  length: 1
  time: -1

...

Then we can define units with a domain and a ratio. You may guess that the ratio has been inspired by the std::chrono library to represent different time units. I think this works well for the general case. Here is an example snippet of units.yml under this approach:

# units.yml
meter:
  domain: length
  ratio_num: 1
  ratio_den: 1

millimeter:
  domain: length
  ratio_num: 1
  ratio_den: 1000

meters_per_second:
  domain: speed

kilometers_per_hour:
  domain: speed
  ratio_num: 5
  ratio_den: 18

miles_per_hour:
  domain: speed
  ratio_num: 1397
  ratio_den: 3025

...

These ideas have been implemented as our prototype C++ library of scalr here. However, note that there are a few other issues in this discussion, not adequately discussed or handled such as

• angular quantities
• affine quantities (e.g. temperature quantities)
• non-physical quantities
• vector quantities

I found out this is related to #669

[erikbosch]

Thanks for your input. I like the idea, so I hope to be able (with my PR) to find a format where we later could add dimensions/ratios like above if needed/wanted. At the moment there seems to be some mixed opinions on whether it is useful or not.