Re: [CAD_CAM_EDM_DRO] encoder installation on servo drives

Hi Tom,

One reason for putting an encoder on a servo motor rather than the leadscrew is increased resolution. If a leadscrew has 5 threads per inch then it will rotate 5 times for every inch of linear travel. If the resolution of the encoder is 200 counts per revolution and the encoder is driven directly by the leadscrew then the resolution of the system is the number of revolutions for one linear inch of travel (5 in this example) times the counts per revolution of the encoder (200 in this example) equals 1000 counts per inch. If the ratio of the rotation of the servo motor is 10 turns to 1 turn of the leadscrew and the encoder is directly driven by the servo motor then the system resolution is the number of motor revolutions to accomplish one revolution of the leadscrew (10 in this example) times the number of leadscrew revolutions for one linear inch of travel (5 in this example) times the number of encoder counts per revolution (200 in this example) equals 10,000 counts per inch. A much higher resolution. If closer tolerances are an issue, often it is desirable to have more encoder counts per inch than programming resolution. Example: .001 inch programming resolution and a system with 2000 counts per inch.

Please note: higher resolution produces more accurate parts if and only if the system is able to mechanically and electrically execute and repeat higher resolution commands. The presence of 4 decimal places on the control screen means the cnc computer system can process and send higher resolution commands. The ability of the machine to execute the higher resolution commands is a different issue altogether.

Hope this helps,

Have a good day!

Jim

----- Original Message -----
From: embedded_tom
To: CAD_CAM_EDM_DRO@yahoogroups.com
Sent: Thursday, January 04, 2007 8:04 AM
Subject: [CAD_CAM_EDM_DRO] encoder installation on servo drives


I asked this on cnczone, but no replies. So...

I've noticed that almost all the servo-based setups I've come across:
1. Utilize a belt drive to connect the servo to the leadscrew.
2. Place the encoder on the servo.

I understand the reasoning for the belt drive, but wouldn't it be
better to put the encoder on the leadscrew shaft?

In other words, doesn't the belt drive add some backlash effects with
the encoder on the servo?

Or does the slop help prevent servo hunting when there's no motion?

Thanks.

tom





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