absolute linear magnetic encoders

[raman00084]

i am doing a project on absolute linear encoders for wood engraving machine. the output of the encoder is digital quadrature in rs 422 format ( A A+ BB+ ZZ+) my doubt is how to get the absolute values. that is when power fails and comes back how to recall the position .



regards
kalyan

 

[doraemon]

Hello!

As usual, without further information, it's quite difficult to reply. For example, if you are sure there will be no
movement when your device is not powered, it's simpler as if there can be movements without power.

A few methods:
1. When you power up the tool, one solution is to go back to the 0 position. At one point, you will
have a switch which tells you you have reached the 0.
Drawback: you have to set the 0 position everytime you power the engraver. But many low-cost
machines work that way. One swich for each axis and you're done.

2. Linear magnetic scale
The output of the scale itself is a pair of sine cosine signals that are interpolated usually with a dedicated
chip although you could also make it with a DSP or even a microcontroller if it's fast enough.
Method: read sine, cosine, calculate tangent and get the angle. 2π corresponds to one pitch of your
magnetic scale. In this case, at power up, you know your exact position within one pitch, but you don't
know which period it is.
-> If you are sure that your machine never moves more than half a pitch when not powered, then
you can store the rough position at every movement and restore it when powering up.

3. Vernier method
Using 2 scales with different pitches.
If you use 2 different pitches (for example one at 2.56 mm and the other at 2.52 mm, then both scales
will display a different phase. The difference of these 2 phases gives you the absolute raw position, and the
phase in the master track gives you the accurate position data. Inconvenience: the linear magnetic scale is
a bit costly. On top of that, the above figures suggests that you cannot move more than 2.56 x 64 = 160mm.

As the Vernier method allows to get an accurate value in a 160 mm segment, you can combine this with
other methods to detect in which 160mm segment you currently are. This gives you the possibility to extend
the working range to a few meters.

Dora.