[SwampDonkey]

So which one of these feedback devices is superior? I have used and understand the good ol optical incremental encoders, but the analog resolvers are pretty mysterious to me. How do they relay position?

Side note: Are the AMT encoders really substandard for "real" cnc machines?

Thanks guys!

[PCW_MESA]

So which one of these feedback devices is superior? I have used and understand the good ol optical incremental encoders, but the analog resolvers are pretty mysterious to me. How do they relay position?

Side note: Are the AMT encoders really substandard for "real" cnc machines?

Thanks guys!

Encoders typically have better absolute accuracy than resolvers
(standard resolvers are typically not better than 500 to 1000 PPM absolute accuracy)
Resolvers with good converters can have around 16 bit resolution (better than most standard encoders)
or higher which helps with smooth velocity feedback at low speeds.

Resolvers function as variable transformers. Common 2 phase resolvers are supplied with a AC signal
that they transform into two variable amplitude AC signals (a sine signal and a cosine signal).
The amplitudes of these AC signals vary with the shaft rotation.

High end encoders with external or built in interpolation can have resolutions (but not accuracy)
of more that 32 million counts per turn, there are no resolvers that can match this resolution

On the other hand, resolvers are inherently absolute and are much more rugged than optical encoders

AMT capacitive encoders suffer from significant lag in their interpolation hardware which can make
tuning of high performance servo systems difficult (resolver converters also have some lag)
Optical encoders have no significant lag

[Jim Dawson]

My prefered encoder is a linear magnetic incremental on the load (table or carriage) rather than a rotary on the motor or leadscrew. Automatically compensates for backlash and leadscrew errors.

[SwampDonkey]

My prefered encoder is a linear magnetic incremental on the load (table or carriage) rather than a rotary on the motor or leadscrew. Automatically compensates for backlash and leadscrew errors.

Jim,

What controller can you use with that? Is it purely in the realm of newer, high-end machines?

[Jim Dawson]

Jim,

What controller can you use with that? Is it purely in the realm of newer, high-end machines?

Any controller that will accept an incremental encoder input can be used. I am using Galil Motion Control products, but there are a lot less expensive controllers out there that will work just fine. They can be installed as a retrofit on almost any machine. The magnetic read heads are about $90 each for 1 micron resolution, and the magnetic tape is about $50/M. Not terribly expensive, less money I think than glass optical scales, and much more compact.

I have these installed on my 48x96 router, my 10x50 mill, and just installed DRO on my 13x40 lathe using mag scales. Have been very happy with the result.

Here is a picture of the X scale on my mill. Sorry for the blurry picture.

http://www.cnczone.com/forums/attach...d=360446&stc=1

[SwampDonkey]

Any controller that will accept an incremental encoder input can be used. I am using Galil Motion Control products, but there are a lot less expensive controllers out there that will work just fine. They can be installed as a retrofit on almost any machine. The magnetic read heads are about $90 each for 1 micron resolution, and the magnetic tape is about $50/M. Not terribly expensive, less money I think than glass optical scales, and much more compact.

I have these installed on my 48x96 router, my 10x50 mill, and just installed DRO on my 13x40 lathe using mag scales. Have been very happy with the result.

Here is a picture of the X scale on my mill. Sorry for the blurry picture.

http://www.cnczone.com/forums/attach...d=360446&stc=1

Thats awesome! Where do you find the magnetic reader head and the tape? Dual encoder feedback is something I'm super interested in. I want to have the first Tormach with not only servos, but both rotary and linear feedback

[Jim Dawson]

I was buying used Renishaw (RLS) LM10 1 micron readers from Ebay vendors, but that source has dried up. So another member here (CountryGuy) contacted Chengdu Ditron Optics-Electronics Equipment Ltd. in China. He put me in touch with them and I asked if they could supply an equivalent to the RLS units. About 4 months later I have almost identical units from Ditron. I tested a sample unit against a unit from RLS, and found them to be functionally equivalent with a few minor differences.

Ditron has a web site, but these products are not listed yet. http://www.cnditron.cn

Contact Jaeger Hou, [email protected]

Read head
DMR200 1um

Tape
MS200

Stainless Steel Belt (covers the tape after install)
DMA ( I think, not sure about the part number)

Just a side note, we installed the RLS units on my friends Shizuoka AN-S and did a test cut of a 4 inch circular pocket, with a 4.5 inch OD. He took it to work and had their metrology guy check it on their 1 micron resolution CMM, max deviation on the OD was 0.00000 on the ID, max deviation was 0.0047 in one spot where there was a tool mark, with the rest of the circle at 0.00000.

[ErnieD]

Hi Swamp,
I am considering changing my Tormach over to servos also. Please document your build. I , and others, will appreciate it.

Ernie D