advice on DRO scales

[mtraven]

Recently, I fashioned a single axis poor man's dro with a caliper & a few brackets. I really like having the dro, but my makeshift version is a little frustrating (variance in how square it mounts). Anyways, I'd like to get into the digital world..but I'm not real keen on just buying the computer unit. I think I'd rather build an arduino interface & just buy the scales, 1 to start with. I just like the idea of being able to have custom routines & a more graphical interface than a standard 7 segment display. I've done many arduino projects, a couple interfacing with digital dials & calipers. Generally, there is a clock line & a data a line pulsing information, not too tricky. So unless someone has a good reason why thats a bad idea, thats what I'm gonna do. One concern I have is the arduino might be too slow to handle interrupts from 3 separate axis (eventually, Ill have 3)...using a arduino zero with a samd chip would probably eliminate the problem.

Next I need some guidance on installation. First axis will be my cross slide, should the scale itself be mounted to the cross slide table, or the carriage?

here is the lathe:


i think the scale needs to go on the cross slide, but either way, I have a problem: that allen screw you see on the side, thats the table lock. There are also 2 oiler holes that would get covered. I hate the the oil system & have wanted to do a 1 shot system anyways, so that's kinda moot...but what to do about the table lock? I'm open to just about any suggestions. The only think i've come up with is to put the scale on the front where its gonna get a whole lot more chip exposure.

lastly, any recommendations on affordable, decent scales would be appreciated.

thanks all!


ps -- the machine is a lathe-mill combo, hence the eventual desire for 3 axis (x, y, quill)

edit/ add: I also want to make a set of wipers for the carriage--another space issue.

[macona]

No sense in reinventing the wheel, I have used one of this guy's boards and they work good: https://www.yuriystoys.com

He has boards that work with quadrature scales and capacitance scales.

[mtraven]

I'd argue there is sense in reinventing things, from both an educational & explorative standpoint...I'm also a bit of a contrarian😁.

That being said, thanks so much for that reference! Lots of great write ups & his boards look well made. thats certainly on the table. My (initial) hangup is: I'm really pretty set on being able to program is myself..sure he uses a msp430, which is programmable, but I've got no experience with that...and I cringe at the idea of running my software on a another man's hardware.

either way, that resource is going to be of huge value to me.

I'm learning a whole bunch of new terminology today, point of confusion in your post: is "quadtrature" a type of scale? I thought it was a method for cross checking(dual) the measurement?

then there's capacitive, optical & magnetic ? Seems like the caps are the low end, but I'm not sure if optical (same as "glass?) or magnetic are better?

?????????

[mochinist]

then there's capacitive, optical & magnetic ? Seems like the caps are the low end, but I'm not sure if optical (same as "glass?) or magnetic are better?

?????????

no idea on the other stuff you are doing, but in the industry, heidenhain glass scales are usually preferred for high accuracy, Newall type scales are usually the go to by most, as they are definitely more rugged and can handle more abuse and are plenty accurate for most manual machining type work.

[mtraven]

here is my early pick:

https://www.amazon.com/Mophorn-Accuracy-Precision-Aluminum-Accessories/dp/B07SQHRWD4/ref=sr_1_3?crid=1LHF79ETRXFF5&keywords=250mm%2Blinear%2Bscale&qid=1652407850&s=industrial&sprefix=250mm%2Blinear%2Bsca%2Cindustrial%2C277&sr=1-3&th=1

trying to figure out the size, some are listed as total length, others as travel length. I'll figure it out.


they are made by "Vevor" anyone know anything about them?

[Paul Alciatore]

Re: Quadrature

Quadrature is a technique for reading a linear or circular scale which consists of equal width elements of two transparencies or reflectivity (for instance, black and white or clear and opaque). An optical encoder wheel with radial slots is one example of this. Light from a reader will pass through the slots and be blocked by the opaque areas between the slots. The slots and the opaque areas are usually of equal widths.

A single reading element when reading that scale will have a square wave output; in logic terms it will be either high or low representing the clear slots and opaque areas between them. That allows you to read the scale in terms of the number of counts of the scale elements that pass by the reader. And the rising and falling edges of that square wave will provide a resolution of the slot or opaque area width. But the problem with this is that it does not tell you which direction in which the scale is moving or rotating. Worse yet, if the scale were to stop and reverse direction, there is no indication that this has happened. So such a single reading element is not of much use.

In order to solve this problem, TWO readers are used and they are placed so that when one of them is over an edge of a slot, the other one is CENTERED over either a slot or an opaque area. Both of these readers produce square waves, but they are not in sync with each other. They each change polarity (high or low) while the other one is in the middle of a high or low state. This, with the appropriate digital logic circuits, does two things. First and probably MOST IMPORTANT it allows the digital circuits to generate a direction signal. The two square waves will have a fixed sequence when the scale moves in one direction. Something like this where the numbers 1 and 2 represent the two signals from the two readers and the + and - indicate the direction of change:

1+ 2+ 1- 2- 1+ 2+ 1- 2- 1+ 2+ 1- 2- etc.

As long as this sequence if followed, the scale is moving in the same direction. And the sequence tells us WHICH direction that is (left to right or CW etc.) But when the sequence is reversed, then we and the digital logic know that the direction of movement has changed.

2- 1- 2+ 1+ 2- 1- 2+ 1+ 2- 1- 2+ 1+ etc.

This, backwards sequence means that the direction is the opposite as above. Now notice that in the first sequence each 1+ is followed by a 2+ while in the second sequence each 1+ is followed by a 2-.

So, if the scale is moving in the first direction (1+ then 2+) and then suddenly reverses direction while in the 1+ state, then the next change will be 1- instead of 2+ as was expected. This tells the logic that in the interval between 1+ and the expected 2+, the scale reversed direction. Similar logic applies to each of the other three changes. So the direction of travel can be known for each and every movement of the scale that produces a change in either of the two square wave signals. This is what a reversal looks like:

1+ 2+ 1- 2- 1+ 2+ 1- 2- 1+ 1- 2+ 1+ 2- 1- 2+ 1+ 2- etc. Notice the 1+ followed by a 1- in the middle where the direction changes.

The two readers are said to be "in quadrature" because they produce timing edges on the square waves that occur at FOUR times the frequency of the square waves themselves. Or perhaps more to the point, if you look at the two waveforms you will see that there are four different states for the two waveforms: 0,0, 0,1, 1,0, and 1,1.

The second, and less important result of using two readers is that it doubles the resolution available when a single reader is employed. A single square wave has only two timing edges for each cycle while a quadrature reader had four.

This article in Wikipedia on incremental encoders has some graphics that may make this clearer. The section on quadrature outputs is the most informative in this respect. It also discusses some ways that errors can creep in.

https://en.wikipedia.org/wiki/Incremental_encoder

In a way, quadrature is a type of scale. But only in the way in which the scale is read with two readers.

I'd argue there is sense in reinventing things, from both an educational & explorative standpoint...I'm also a bit of a contrarian😁.

That being said, thanks so much for that reference! Lots of great write ups & his boards look well made. thats certainly on the table. My (initial) hangup is: I'm really pretty set on being able to program is myself..sure he uses a msp430, which is programmable, but I've got no experience with that...and I cringe at the idea of running my software on a another man's hardware.

either way, that resource is going to be of huge value to me.

I'm learning a whole bunch of new terminology today, point of confusion in your post: is "quadtrature" a type of scale? I thought it was a method for cross checking(dual) the measurement?

then there's capacitive, optical & magnetic ? Seems like the caps are the low end, but I'm not sure if optical (same as "glass?) or magnetic are better?

?????????

[JRouche]

FYI. When you see that "i"I think I'd rather build an arduino interface & just buy the scales," In a way ,small way. I think, yes please include me, for fun. The small packages these days ( not that stupid name I said). What we can fit n a small package is surprising. Electronics have moved so far forward. It has always been the case.

Lasers in the Gulf? Look it up. JR

[Tobias-B]

I started down the cheap digital caliper path- then discovered quickly that it'd been already done, and packaged pretty well, too.

So for the 'doesn't warrant a real DRO' applications, I've been using the little inexpensive scales with a remote wired readout.
Yep, I've thought they could be hooked to something bigger- but 98% of the time, just having a relatively accurate number's good enough.

fwiw.
t

[mtraven]

I started down the cheap digital caliper path- then discovered quickly that it'd been already done, and packaged pretty well, too.

So for the 'doesn't warrant a real DRO' applications, I've been using the little inexpensive scales with a remote wired readout.
Yep, I've thought they could be hooked to something bigger- but 98% of the time, just having a relatively accurate number's good enough.

fwiw.
t

I already did the caliper thing, that was the proof of concept I needed to warrant buying proper scales. I just want to build the brain for several reasons:
1) more bang for my buck
2) my own customizable interface
3) I just want to, I like coding...writing some little routines for bolt patterns & such sounds like fun.
4) I generally seek a pretty deep understanding of how things I used work, building the system builds thats understanding (for me)


@Paul Alciatore

an information packed reply, per usual. I have worked with rotary quad encoders before, so I have some basis understanding of how they work. though I've never used both the rise & fall edges to improve resolution....at the time of my post, I didn't realize most of these scales use quad encoding rather than the data & clock system most digital calipers use. If anything, actually seems easier to make. I dunno, I'm still in the early stages of planning this one out.

Paul-- do you have an opinion on glass vs magnetic?

[Paul Alciatore]

Paul-- do you have an opinion on glass vs magnetic?

In terms of a DRO, I do not have much experience with either type. So I will not comment. Much has been said and written on the internet. A quick search will locate more than you care to read.

[elf]

3 axis glass scales with readout are too inexpensive to bother with Arduino or other mcu's. If your accuracy and repeatability are low, go ahead with the Arduino😀

[mtraven]

3 axis glass scales with readout are too inexpensive to bother with Arduino or other mcu's. If your accuracy and repeatability are low, go ahead with the Arduino😀

cost is fairly low on the list of why I'd like to do this (see my other posts)

curious what makes you think an arduino based dro brain would be inherently less accurate than a commercial one? I've not been inside one of the inexpensive commercial units, but there's got to be microprocessor running it, probably one less capable than the samd21 I intend to use.

[Tobias-B]

The 'fun' of the DIY approach would be in combining quill and knee on a mill, adding a rotational input for a rotary table, etc.

Or similar with the lathe cross slide and compound travel and angle.

Probably no cheaper, and certainly a time soak, but neat to be able to customize it so that it could interpret multiple axes in
varied ways.

'not quite CNC'

t

[mtraven]

The 'fun' of the DIY approach would be in combining quill and knee on a mill, adding a rotational input for a rotary table, etc.

Or similar with the lathe cross slide and compound travel and angle.

Probably no cheaper, and certainly a time soak, but neat to be able to customize it so that it could interpret multiple axes in
varied ways.

you get me! the one I had in mind was coordinating the spindle speed with the cutting diameter when doing long facing cuts. Your ideas are interesting too, probably a bunch more possibilities too...which is a big part of my I like building myself. I also have no one to yell at but myself when software glitches 😁