Messages

1

General Mach Discussion / Re: Servo Basics

« on: January 22, 2013, 10:48:41 AM »

Hood,

Sorry about the reference to Kernal, I mean sampling rate

Juan

2

General Mach Discussion / Re: Servo Basics

« on: January 21, 2013, 05:40:39 PM »

jvaldes

High resolution encoders is not all about resolution of the machine. Depending on the servo drives and the speed it can react to the encoder then it may well produce a much smoother and cleaner motion if you have a higher count encoder. I posted some pics a while back in another thread showing the difference of a motor I had with a sine/cosine encoder. I could interpolate the encoders counts and by using a higher number and no other changes the drive could keep the motor on track much easier.

Andy,
 Depends, likely your drives have electronic gearing in them, that means you can have less steps per unit in Mach but the drive will multiply that and make the motor move the correct distance. Its not a method I personally like but it can allow you to get more performance from your hardware when using the parallel port. Well when I say performance really I should just say Velocity as your motion may actually suffer slightly, especially at very low speeds.

Hood

Though I agree that the results is as the graphs show the reasoning is incorrect. The larger count encoders at the same kernal speed had different limits for speed and momentum, which results in different error allowance and less deceleration time, which later results in what you have on these graphs. The actual reasoning that should be considered is that if you were to cycle the increased rates at increase sample rates and where to operate them at the same velocity with the same error allowance (not in pulses but in dimensions) the outcome should be the same. But running an axis at high velocities with the expectation of  curve following is a matter of motor torque and inertia not pulse rates.

3

General Mach Discussion / Re: Servo Basics

« on: January 20, 2013, 03:41:55 PM »

A 14 bit encoder has 16384 steps per revolution which provide you 5mm/16384 minimum step size or .0000012 inch per step. This choice of encoder with a 5mm pitch leadscrew will be slow with any computer and unless you have it on a machine that has significant stiffness and flatness, is an overkill. On most home builds or lower level mills like republic, the most you can dream for is .0001" and really better than .0002 is unlikely. .0002 would represent 16 steps on your encoder leads crew combination. you would be better served by a 10 bit encoder which is 16 times faster at the same kernal speed. or a compromise such as a 12bit with a 4 times improvement in max speed.

Good luck