Encoder Basics

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Centroid_Liviu
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Re: Encoder Basics

Post by Centroid_Liviu » Wed Jan 23, 2013 9:13 am

The pictorial tutorial for replacing the encoder on a Bridgeport Servo Motor (SEM motors) can be found here. It details removing the old encoders, installing the new higher resolution encoders, and fashioning a new servo end-cap.

bonestock
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Re: Encoder Basics

Post by bonestock » Tue Oct 08, 2013 11:06 am

I am looking to use linear scales to help improve the accuracy of my axes. Can you please confirm if RS-422A / EIA-422 is the correct signal output I need to interface with my mpu11?

Centroid_Liviu
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Re: Encoder Basics

Post by Centroid_Liviu » Tue Oct 08, 2013 11:16 am

You would need a differential quadrature output the same as the encoders (minus the Z channel). Do you have a manufacture and model number for the scales you wish to use? Please note that each axis will still need an encoder signal.

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Re: Encoder Basics

Post by bonestock » Tue Oct 08, 2013 12:35 pm

Yes I plan to use the scale for position correction. The rotary encoder on the servo will remain as it is.

There are several scales I am looking at and the issue is different manufactures refer to differential quadrature signals differently. I'm trying to distill the differences of terms into actual standards that I can more easily reference.

I see incremental encoder/scales also being referred to as quadrature encoders/scales for the same given model numbers. I also occasionally see ( for example) Mitutoyo flexing their cerebrum with reference to quadrature output as "Two 90 degree phase-shifted sinusoidal signals".

Here are a few scales in my sites.
Acu-rite SENC 150 558115-06, is referred to as both incremental and differential and has a quadrature output. Analog output appears to be compatible.
http://www.acu-rite.com/pdf/manuals/516 ... 0_w-rm.pdf see pages 18 and 19.

Dro Pros scales with RS-422A output. http://www.dropros.com/DRO_PROS_Digital ... _Scale.htm, wiring for RS-422A output appears to compatible.

Centroid_Liviu
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Re: Encoder Basics

Post by Centroid_Liviu » Tue Oct 08, 2013 12:49 pm

Those scales should work. Please see the attached for a connection diagram between the ENC150 and our control.
Attachments
Acu-Rite 200S DB9 to Centroid Encoder DB9 Pinout.pdf
(19.21 KiB) Downloaded 885 times

cnckeith
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Re: Encoder Basics

Post by cnckeith » Wed May 13, 2015 4:38 pm

check this new webpage out..SEM DC Servo Motor Encoder Retrofit Upgrade, preparing the servo for the new CNC control.
Step by step pictorial guide to installing a new Rotary Encoder and direct wire power and encoder cable on a 20 year old DC servo motor for use with a new Centroid CNC control.

http://www.centroidcnc.com/sem_dc_servo ... rofit.html

CNCinTN
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Re: Encoder Basics

Post by CNCinTN » Fri Mar 11, 2016 7:08 pm

Hello,

I would like some clarification concerning encoder specification requirements. But, first, I little bit of a back-story: I bought the Mach3 kit (MPU11 + DC3IOB) a few years ago. (And, boy, am I glad I did, because the price for many of the components have more than doubled over what I paid!) So, even though I've had it for years, I've never installed and used it. The goal is to retrofit a milling machine which originally had an Anilam Crusader M control system. It has taken me much longer that I had originally expected to collect all the necessary components to complete the retrofit. The delay is due mainly to the fact that I'm working on an EXTREMELY limited budget. So, while I've purchased the original kit along with the Ajax power supplies, I still need to acquire a vfd and servo motor encoders. And this is where I'm encountering some difficulty in sourcing parts. Honestly, I do not have the budget to spend $675 ($225 X 3, as sourced from Ajax) on just the encoders. (I do not mention my budget problems to take opportunity to gripe about Ajax/Centroid's prices; that is not my reason for posting!) So, I've been attempting to find a 3rd party source for compatible encoders, and it is in those efforts where I've encountered some confusion. Specifically, in this topic, the encoder requirements have been stated to include "The "low" signal level must be less than 0.5 VDC and the "high" signal level must be at least 3.5 VDC." Well, I've searched high and low and have been unable to find any encoders that meet those standards. For every encoder I've researched that are quadrature (A,-A, B, -B, Z, -Z AKA 6-phase) with "Line Driver Output," either the "low" signal level is stated to be above the 0.5 V or the "high" signal level is stated to be below 3.5 V or both specs are beyond Ajax's stated requirements. This has been true for the ultra-cheap import encoders I've researched as well as for the super-expensive (more expensive than AJax/Centroid's) encoders I've looked at. So, I've been reading up on encoder technology, trying to understand exactly what the "signal level" requirements mean, and I've just found some information that has me even more confused. I downloaded Centroid's "Allin1DC Install Manual," and it states the following:

Encoder Output:
Encoders must have RS422 type (differential) quadrature outputs with A, B, and Z channels to work
with Allin1DC. A low encoder count creates poor performance and accuracy. Centroid recommends 8,000 to 40,000
encoder counts per revolution for most applications. The outputs have additional voltage level requirements described
in the table below: (can't reproduce the table layout, so I've just typed what is shows:)

Encoder channel low level
0.0V-minimum
0.3V-typical
0.5V-maximum

Encoder channel high level
3.0V-minimum
3.5V-typical
5.0V-maximum

So, which is the correct minimum "high" level 3.0V or 3.5V? Incidentally, I've found a couple different economical encoders that would meet the "3.0V" minimum, but not the previously stated "3.5V" minimum. Additionally, I've seen multiple manufacturers that reference the use of a "pull-up" resistor, which one manufacturer defines as the following: "A pull-up resistor is used to pull the logic high voltage level up to the level of the operating voltage. This is useful when the output of the open collector is not reaching the voltage level needed to indicate a logic high signal or when noise is present on the signal line." So, my questions are these: Does anyone with Ajax know what these "pull-up" resistors are/do, and can such a "resistor" be used to "boost" the signal voltage of an encoder that would otherwise not meet the minimum signal-level voltage requirements? (As a side note, I've looked at the Quatum Devices QR12 series of encoders that are those that can be seen in Centroid's "DC Servo Motor Encoder Retrofit upgrade" web-album, but interestingly, none of the documentation I could find on this series of encoder actually publishes the low and high signal level specs! So, what's the real deal with all this signal level stuff?!

Thanks in advance for any assistance you can provide!
Last edited by CNCinTN on Fri Mar 11, 2016 8:12 pm, edited 2 times in total.

cnckeith
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Re: Encoder Basics

Post by cnckeith » Fri Mar 11, 2016 7:21 pm

check out this page..

http://www.centroidcnc.com/sem_dc_servo ... rofit.html

and..fyi...while this page covers a SEM servo motor upgrade, the process of retrofitting an encoder is similar for most any type of dc servo motor.

cnckeith
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Re: Encoder Basics

Post by cnckeith » Fri Mar 11, 2016 7:25 pm


CNCinTN
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Re: Encoder Basics

Post by CNCinTN » Fri Mar 11, 2016 7:33 pm

Thanks, cnckeith. But, I've already seen those pages and referenced in my first post the information found in the retrofit-process web-album. But, neither links contain information that answers my questions about the low and high signal level specs...

P.S. My reply is not meant to be sarcastic, merely matter-of-fact. Thanks.

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