Ladder Units

Hello,

I’m having a difficult time grasping the idea of ladder units. I’ve been working in PicPro recently, and it keeps referencing the term ladder units. When I go and look up the term, it mentions a couple other units. So the three units are,

Ladder Units
Feedback Units
Axis Units


It says that ladder units are simply the units used in the ladder… What exactly does that mean? It then goes on to say feedback units are the units the Servo uses and axis units are the units of the system… It then proceeds to talk about scaling between everything.

So besides the fact that I’m not confident with the real definitions of all of these “units”, the scaling confuses me. Would it be correct to assume that feedback units are the smallest increments of positioning a servo? But then why would you have the ability to make ladder units smaller than feedback units? Isn’t that just adding precision within your ladder that you have no control over? And I have no idea what axis units are. Could that be degrees or radians?
These terms seem to only be referenced by G&L manuals when I google them and can’t find any other PLC developer talking about them.

Ladder units. You can't use floating point math in this version of ladder software. Only whole numbers (integers). For example, you would use 1000 to represent 1 inch.

Feedback units. This is the pulses per inch from the encoder. The example in the manual shows 8000. That means if you tell the machine to move 1000 counts (Ladder Units), it will move 8000 counts (Feedback Units) on the encoder.

Axis units. This is the real world measurement. I used 1000 to represent 1 inch. It could be 1 millimeter, or 1 meter. It's whatever I choose.

The only fixed number is the encoder feedback (Feedback Units). The other units are your choice. You choose whether you want Inch or Metric, and resolution (precision).

Let's say I want Metric. The encoder is 8000 counts per inch, so 8000 per 25.4 mm. Let's round for this discussion. 100 mm would be 32000 counts. I decide I want .1 mm precision.

My Axis Unit is 100.0 mm, Ladder Unit 1000, and Feedback Unit 32000.

That actually makes a lot of sense. I think what confused me was the fact that a piece of code I was looking at had a ratio of ladder units to feedback units being greater than 1. That would make it so your program has more precision than the actual motor capabilities which made no sense to me.

Do you know if it is common practice to have more precision in ladder units than feedback units for future purposes, say for example if higher resolution encoders are released?

Doesn't make sense to me either. I always want feedback to have at least 10x greater than ladder units.
Choice of resolution is based on the accuracy of your design. How much slop in bearings, etc.