Encoder Basics

All things related to the Centroid Acorn CNC Controller

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cnckeith
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Re: Encoder Basics

Post by cnckeith » Wed Oct 04, 2017 12:44 pm

DICKEYBIRD wrote:
Sun Oct 01, 2017 11:48 am
Good morning folks; is the TTL type encoder unusable with Acorn? A friend has a Heidenhain 2000 line encoder he'll let me have very reasonable but an earlier post mentioned that TTL is a no-go with the All in One & others. Same rule apply to Acorn?
1. Encoder Type:
Centroid MPU11 based CNC hardware (Oak, Allin1DC, Acorn) requires quadrature, differential encoders which operate on 5VDC. This is the most common type of encoder. Encoders labeled "TTL type" (single ended, non differential) are unsuitable, and fortunately are quite rare. converter boxes are not recommended.

buy a quadrature, differential encoder and it works.

macona
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Re: Encoder Basics

Post by macona » Thu Oct 05, 2017 4:54 am

Just jumping in here, TTL is a signal level, not how the output is driven. There are basically 4 signal levels for quadrature encoders, 1vPP, 11uAPP, TTL, and HTL. The first two are usually sine wave output quadrature and have to go through an interpolator box to be converter to standard quad output. HTL can vary but it is often 12-24v so we are not interested in it for most cnc controllers.

TTL is 5v based and it come in two flavors, single ended and differential. With TTL single ended you just have 5 wires out for a ABZ encoder that pulse 5v out on each line. TTL Differential encoders have a built in 26LS31 IC that internally converts the single ended output into a complementary output for each of the three channels according to the RS422 standard. This helps with noise rejection and run length of encoder cabling.

So, what you should be saying is you want a TTL encoder with differential output. You can use a single ended encoder as long as you use a converter that changes the output from single ended to differential, USDigital and a few others sell them. They are great for retrofitting old systems that use and encoder that is hard to replace.

DICKEYBIRD
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Re: Encoder Basics

Post by DICKEYBIRD » Fri Oct 06, 2017 4:47 pm

When looking at the Acorn P10 encoder pin-out on the .pdf schematics, are the pin labels shown as if the DB9 male connector is in your hand "looking up" at you? Also, does the Acorn itself provide the 5V at pin 9 or do I need to provide that power? I know, I know; rudimentary questions but man I have never worked around encoders want to do this right the 1st time.

Does anyone have a schematic of a typical Acorn encoder installation including power, ground, shield and the plus & minus A's B's & Z's?
Any enlightenment would be much appreciated. :)
Milton in Collierville, TN

"Accuracy is the sum total of your compensating mistakes."

cnckeith
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Re: Encoder Basics

Post by cnckeith » Fri Oct 06, 2017 6:51 pm

get a magnifying glass out and take a look at the encoder connector on Acorn board, the DB9 pins are actually numbered in the corner and those numbers match the pinout on the Acorn schematic. I think that'll answer your questions as to what pin is what.
and yes the acorn board is supplying the 5 volts and the ground connection just like the schematic shows so you just solder it and wire it up just like the schematic shows, plug it in and it works. 😁 Go to PID menu to see encoder counts count up or down as the encoder rotated. I'll post a video of this in my next post.

DICKEYBIRD
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Re: Encoder Basics

Post by DICKEYBIRD » Fri Oct 06, 2017 8:06 pm

Thanks, that'll help. The drawing for the the Allin1DC makes more sense to me. It does say "Encoder Cable Male End" and "Cable Side" Hopefully the one in the Acorn drawing means the same thing even though it's drawn differently. Thanks for the patience, I need it!
Milton in Collierville, TN

"Accuracy is the sum total of your compensating mistakes."

ScotY
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Re: Encoder Basics

Post by ScotY » Sat Oct 07, 2017 2:49 pm

admin wrote:
Mon Mar 15, 2010 2:56 pm
3. Encoder Resolution:
Typical Machine tool applications (milling machines, lathes and routers) require at least 1000 line (4000 counts per rev) encoders, We recommend and sell 2000 line (8000 counts per rev) encoders. To achieve reasonable accuracy and smooth motion, the encoder/motor/ballscrew configuration must achieve a minimum resolution of 20,000 encoder counts per inch of travel (787 counts/mm). A typical knee mill which uses a 5tpi (turns per inch) ballscrew and has the motors belted at a 1:1 ratio, a 1000 line encoder just meets the minimum counts per inch at 20,000 for smooth motion, this will work however a 2000 line encoder (8000 counts per rev) would be a better choice. Older 250 and 500 line encoders will not work to satisfaction.

some examples

5 turns per inch ballscrew belted at 1:1 with the servo motor = 5 turns of the servo motor per inch of axis travel

5 turns per inch x 8000 counts per rev = 40,000 counts per inch of axis travel.
I have a question. I have cut a bunch of stuff out of the original quoted post. My mill setup physical setup is similar to this example...5 TPI ballscrews, 1:1 motor/ballscrew...using Clearpath 800 count motors. At 4000 counts/inch, this is nowhere near to the recommended minimum of 20,000 counts/inch. I am still within Teknik's 90 day guarantee window. Should I exchange my motors for their 6400 count motors? That would bring it up to 32,000 counts/inch.

Changing the subject, for lathe or mill SPINDLE (not axis movement motors), what is the minimum counts/rev encoder that can be used with good results? I would imagine the requirements would be less stringent?

Thanks, Scot

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Re: Encoder Basics

Post by cnckeith » Mon Oct 09, 2017 2:35 pm

6400 is better than 800 on a 5tpi machine.

spindle encoder counts = 2000 line (8000 counts) works good.

ScotY
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Re: Encoder Basics

Post by ScotY » Mon Oct 09, 2017 6:39 pm

Hi Keith,

So I'm still confused about this. On my mill setup, one step of the motor is 0.00025". It's a dirt cheap hobby grade mill but I'm obviously not keeping that in mind when purchasing parts for it. :lol:

I can't imagine achieving accuracy anywhere close to 0.00025" but there must be a reason for the high count encoder requirements you guys are wanting. Even on a better machine, I think that kind of resolution sounds more than adequate. Can you explain why? There must be something more beyond what I'm thinking.

cnckeith
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Re: Encoder Basics

Post by cnckeith » Wed Oct 11, 2017 12:58 pm

what's important is: encoder counts per unit of linear movement of the axis not just encoder counts per rev of the motor.

for industrial low inertia machine tools we recommend at least 20,000 encoder counts per inch of axis travel. (in high speed, high inertia machine tools 40,000 in the recommended counts per inch)

so for example: if the axis motor had a 500 Line encoder that produces 2000 counts per rev of the axis motor, that motor would need to turn 10 times for 1 inch of travel on the axis to get 20,000 counts in 1 inch which would yield good results. so if the axis had a 5TPI screw the motor would have to be belted 2:1 to the screw to get 10 turns of the motor to yield 1 inch of travel with 20,000 counts per inch.

anything less than 20,000 counts per inch will still work, and for a wood cutting hobby machine is certainly overkill but running anything less than that is your prerogative and experimental in nature.

cnckeith
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Re: Encoder Basics

Post by cnckeith » Wed Oct 11, 2017 1:00 pm

Acorn Spindle Encoder Overview video


youtu.be/Gk1T_9-7_4g

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