Brushless Motors and encoders

[Lotusjohn]

I am in the process of CNC converting a bench top mill. My plan has been to use a BLDC motor with internal hall devices. This will drive the axis through a Gilmer belt (1:4). I am planning on mounting the differential encoder directly to the ballscrew to maintain accuracy. After reading some of the other threads, I realize this may not be a good choice. Is this a stable setup ?? Would a brushed motor be a better choice ?? Is there an accuracy issue when the encoder is mounted to the motor but the motion is transmitted through a Gilmer belt ??

John

[H.O]

Hi,
It's possible to mount the encoder on the screw but you must not have ANY backlash what so ever between the motor and screw as that will upset the servo-loop causing vibrations and/or need to reduce the gain in order to prevent oscillation. In other words the belt drive may not introduce any backlash what so ever. The type of motor does not matter in this context.

Mounting the encoder on the screw would account for any errors caused by the belt-drive, (concentricity in of the pulleys etc) but apart from that I can't see much benefit (?).

On the flip side, if you mount the encoder on the motor it gives the servo-loop 4 times as many encoder counts per unit to work with which will help making the system "stiffer".

/Henrik.