Sensor for rotary/angular W axis

[Schtimpy]

I am in the process of setting up a TouchDRO on a vertical mill. I would like to use the W axis for angular measurement/indexing with a rotary fixture.There seems to be very little information about what sensor should be used for an angular sensor or sensor module and how to interface it to the board: all of the scale selection information I can find relates to selection of linear scales. I will be using the preassmbeled adaptor for Igaging scales (the latest version with micro USB connectors).

Any assistance would be greatly appreciated. Would a rotary photo quatrature sensor with the encoder disk similar to those used in every printer be suitable?

 

[pontiac428]

There are two schemes you could use, but I am not sure if it would work at all with TouchDRO. Lucky for you, Yuriy @ycroosh, the developer, is a member of this board and you posted in the right place!

The first thought is a rotary encoder. It's a dumb sensor that only sends AC pulses as it sweeps. It does not know what direction it is going, and it does not know absolute zero. Many systems exist where this works. With multiplexed sensors you can get high resolution. Making it work with software (and making a driver circuit) should be easy, relatively speaking.

The second idea is an angular resolver. Two sensors (just coils and cores) are placed at 90 degrees to each other and a magnet is rotated between them on the axis you want to read. That setup gives you a reading on sin x and cos x (90 degrees) which means you can solve for the position of the north pole of the magnet. So depending on the field strength differential between the two coils, the exact position can be resolved. The conditioning circuit for this sensor (also called a variable reluctor) would be set up to output DC voltage, 0-5v signal for position.

I am a sad hack at electronics, but I think I could pull off making either one of these sensors and driver circuits as a project. It's pretty basic data acquisition. It would take a lot of time to do from scratch, but it is certainly an attainable DIY for the right person.

 

[Weldingrod1]

An optical encoder can do what you want. It will spit out quadrature signals just like linear glass scales. You can get a "z" track that is once per rev and can be used to give you absolute position after you find it. Make sure you get enough pulses per revolution so you can read the increment you want!

You can also buy encoders with more than outputs than quadrature that can give you absolute position without a homing step. However, those require a much different input on your DRO.

Sent from my SM-G892A using Tapatalk

 

[RJSakowski]

T

There are two schemes you could use, but I am not sure if it would work at all with TouchDRO. Lucky for you, Yuriy @ycroosh, the developer, is a member of this board and you posted in the right place!

The first thought is a rotary encoder. It's a dumb sensor that only sends AC pulses as it sweeps. It does not know what direction it is going, and it does not know absolute zero. Many systems exist where this works. With multiplexed sensors you can get high resolution. Making it work with software (and making a driver circuit) should be easy, relatively speaking.

The second idea is an angular resolver. Two sensors (just coils and cores) are placed at 90 degrees to each other and a magnet is rotated between them on the axis you want to read. That setup gives you a reading on sin x and cos x (90 degrees) which means you can solve for the position of the north pole of the magnet. So depending on the field strength differential between the two coils, the exact position can be resolved. The conditioning circuit for this sensor (also called a variable reluctor) would be set up to output DC voltage, 0-5v signal for position.

I am a sad hack at electronics, but I think I could pull off making either one of these sensors and driver circuits as a project. It's pretty basic data acquisition. It would take a lot of time to do from scratch, but it is certainly an attainable DIY for the right person.

The Omron incremental rotary encoder that many of us used in the ELS build has a quadrature detector and provides both steps and direction. It also has a home position for referencing absolute position. More sophisticated rotary encoders provide absolute position. The 1800 p/rev model will resolve to .005º.
https://www.ia.omron.com/support/guide/34/introduction.html

http://www.ia.omron.com/data_pdf/cat/e6b2-c_ds_e_6_1_csm491.pdf

If I recall correctly, the iGaging scales and the like provide an ASCII signal rather than raw step and direction so additional processing hardware would have to be provided to properly interface with the TouchDRO. Yuriy can give a definitive answer to this.

 

[pontiac428]

Arduino is cheap and convenient; would make a good processor to convert rotary position signals to something that iGaging can read.

 

[homebrewed]

I'm not sure how you integrate a rotary encoder into the RT if you want to directly monitor the table rotation. There isn't much spare room inside them.
However, I've seen CNC conversions where the worm gear is driven by a stepper. Putting an encoder on the worm gear shaft might start by taking a look at a few RT/CNC conversions. If going this route, you might be able to get away with an encoder that has lower resolution, since the worm gear (probably) has a 40:1 reduction ratio.

 

[ycroosh]

When I did mine I used an Omron rotary encoder mounted next to the table (coaxially with the table's axis) and used 4:1 toothed belt pulleys underneath. The table and the encoder were bolted to a precision ground plate with a pocket milled out for the pulleys and the belt. It worked very well (I sold it with the last mill; the new table needs this conversion).

Connecting this to the quadrature TouchDRO adapter is easy - the encoder outputs quadrature signal similar to the scales. One thing to keep in mind that the resolution might end up being 4x what the encoder claims. Many encoder specs give the number of pulses per revolution, particularly the fake Omron encoders you can get on Ebay/Aliexpress for around $40.
TouchDRO uses edges, not pulses, to get the position:
When using pulses, the system waits for a positive edge (signal going up) on the "A" line and then depending on whether or not B is high or low determines direction.
TouchDRO (and most other DROs) react to every edge on both lines (this is how glass scales get 5-micron resolution from 20-micron track).
In my case 2000 pulse/rev encoder ended up being 32,000 counts per revolution, or 100 counts per degree.

Connecting this to the iGaging/Shahe firmware will require some sort of translation circuit (that I keep putting off). A friend of mine just made one using an FPGA. I will ask him if he'd be up for sharing his design/code/

Regards
Yuriy