Basic linear sclae reader head question

Basic linear scale reader head question

I understand the basics of how a dro reads notches on a glass or magnetic scale and counts the notches going by to measure how far it has moved. I looked and found no information on how it determines which way it is moving. AFAIK the notches are symmetrical so no help that way. Does it read in like three spots at a time and they are spaced at a logarithmic distance apart so it sees one before the other in each direction. Some what like a viener scale.
The cheap dro scales sold on ebay are they mostly magnetic these days or glass?
Bill D.

Check but I was told the direction of the scale is built in, RH to LH the lines increase in length a little to determine direction, this is verbal and to be honest I wouldn’t have a clue if correct, but it is feasible
Can’t think of another way but it’s not my field to say the least
Mark

Google quadrature counting or incremental encoder.
There are two sensors of the lines or magnetic fields. These out of phase with each. Normally called A and B.
As one transitions from high to low the other has a state that tells you if it a up or down count.
One can do it with a few old TTL gate chips and a counter chip. Very simple circuit.
A tad fancier is detecting the illegal states and making the readout blink an error code.
If aligned correctly there is no length difference. Ideally spacing should be equal.
In the old style light bulbs ones it was my job as field tech to attach A and B to the scope H/V inputs and "tweak" the bulb/lens position to get a circle.

Absolute encoders are different and there are pulse and direction encoders.

In incremental there are sine wave and square wave and sin/cos dividers not to mention voltage or current. And then some float the sin/cos wave all above zero.
At the base it is all sin/cos at initial signal but the reader head may square up or even do the division.
The deep inside counter input likes digital or square waves because it needs counts.

Basically two signals out of phase solve the direction count problem.
Or maybe little elves inside and they tell the world moving right or left and flip switches...
Bob

Well that made more sense, thank you
Mark

Like CarbideBob said. The read head has two sensors that detect the scale lines. They are spaced so that when one is exactly on a line, the other is off by 1/4 (90 degrees) of the line spacing. One provides the A signal and the other the B. Direction is determined by which signal leads the other by 90 degrees, B leads A or A leads B.

Jon

Makes sense. I knew it had to be a simple geometry thing with at least two sensors. Thanks for all the info.
Bill D

Fun fact, they don't count the little lines

that would be tedious and probably jittery

there are grates on both sides such that when all the lines line up. light passes, then they don't dark
Or so Heidenhain does it as explained to me...........

gustafson said:

Fun fact, they don't count the little lines

that would be tedious and probably jittery

there are grates on both sides such that when all the lines line up. light passes, then they don't dark
Or so Heidenhain does it as explained to me...........

Click to expand...

Confused, I thought that was how the lines were counted on optical.
More confused on magnetic "reader gates" in such type scales.
Bob

I’ve been reading a bit, seems there’s a second “grating” slightly skewed to the scale, as the two pass it causes “ moire” fringes, like when you pass railings with another vertical line, I think it amplifies the count pulse as far as I understand, or at least gets a better pulse, the more you find out the more is left to find out, fascinating world,
Mark

There are a lot of different types of scale and they all work differently. Here is a good explanation on the magnetic scale principle.