[Thomas Utley]

All,

I'm setting out to build my second gantry router and working up the nerve to graduate from steppers to servos. Seems like there should be an ideal ppr range for encoders, but just want to check my assumption that mechanical gear reduction from the servo motor to the linear drive mechanism will bump up the encoder count by an inverse fraction. For example, a gearhead with 10:1 reduction will make a 500 ppr encoder act effectively like a 5000 ppr unit while still keeping the quadrature count seen by the electronics down to something manageable at high RPM.

Am I missing anything obvious for a rack and pinion axis?

Thanks,

Tom

[Al_The_Man]

Pretty much governed by the maximum frequency capability of the controlling device, (drive/controller).
Al.

[keebler303]

Generally you want the highest encoder count that will work for your application. You need to keep in mind the limits of the encoder itself, as well as the controlling device, as Al mentioned.

Assuming you plan to use DC servos and gecko servo drives or similar, each step pulse is one encoder count so your maximum step frequency would be your maximum counts/sec for your servo. For example if you were using mach3 and the parallel port you would probably max out around 35kHz step frequency. That would give you a max encoder speed of 35,000 counts per second. Assuming a 250 line encoder (1000 quadrature counts) you could get 35 revolutions per second or 2100 RPM. With a 10:1 rive ratio your pinion would turn a max of 210 RPM which would be something like 200 IPM for a normal hobby sized rack and pinion. Many drives (including the G320X) allow you to move multiple encoder counts per step pulse so you could speed the machine up by changing that setting.

Matt

[mactec54]

Thomas Utley
How many encoder counts/rev is too many?

More the better, There are many boards that can help with high frequency output For Step/Dir use (Smooth Stepper Etc ) These type of boards give you an overall better running machine as well, as they don't rely on the computer frequency output to get you up to speed

Dmm have there Encoders at 14Bit 16384 CPR Absolute, & this is about the minim you want to go for a smooth running machine, Yaskawa Sigma5 are standard at 20 Bit 1,048,576 CPR Plus quadrature, There Sigma II Yaskawa are 17 Bit 65,536 CPR + quadrature

So what ever you can afford to get, more is better

[Al_The_Man]

Although if the frequency of the pulses, (rpm and/or pulse count) exceeds the frequency response of the controller you will lose pulses etc.
BTW the correct terminology is 'Quadrature X 4'.
http://www.cnczone.com/forums/dynomo...sing_loop.html
Al.

[mactec54]

Al_The _Man
Although if the frequency of the pulses, (rpm and/or pulse count) exceeds the frequency response of the controller you will lose pulses etc.

Yes that could happen, if you have a system that has been poorly designed,

But when the servo loop is closed in the servo drive you don't have to worry what the output of the control is, all that happens is the motors won't reach there max RPM if the control has less output than needed, This is when you use something like the Smooth Stepper to get the max output needed

[mike_Kilroy]

Yes that could happen, if you have a system that has been poorly designed,

But when the servo loop is closed in the servo drive you don't have to worry what the output of the control is, all that happens is the motors won't reach there max RPM if the control has less output than needed, This is when you use something like the Smooth Stepper to get the max output needed

Tom,

Please do not ignore what Al has told you so far.

ALL controls have a max encoder input limit, some higher than others: Some of the highest performance servos have what appear to be low encoder max freq limits but will outperform many others with higher max freq inputs..

Please do not assume that if "servo loop is closed in the servo drive" is a magic bullet; these are just words that have no meaning as written, without much more description. In a decent servo system there are three 'servo loops,' all of which required the encoder for various reasons, not just top speed limit.

[mactec54]

mike_Kilroy
In a decent servo system there are three 'servo loops,' all of which required the encoder for various reasons, not just top speed

We are not talking about a industrial system, try Hobby level to get on the same page

We are not talking about a control that is closing the loop, When someone is using a control with a low frequency/ step/dir output, it does not matter at all this will have no affect what's so ever, to the servo motor, if it has a high count encoder which has closed the loop in the drive, if the Drive is smart enough it will take care of the encoder counts in a ratio to suit the input, But if you use a Smooth Stepper type Board,( 4Mhz output ) then you never have a problem with not enough output as you are not relying on the control output anymore, the only thing that can happen if the frequency output from the control is to low, & you are not using Smooth Stepper type board, is the motor will not reach it's max RPM

I have done may hour's of testing with different controls, High End, closing the loop in the control, & very basic controls, so I know what the end results are, & what works & what does not

I guess you have never done this, in true testing to see what the out come really is, or you would not be saying what you have

You should buy a Smooth Stepper, run it from Mach , use one of your best drives & motors, I have tested up to 20Bit encoders with this board, 4 (Yaskawa ) motors at once, you will then see the results for yourself

[Al_The_Man]

In a decent servo system there are three 'servo loops,' all of which required the encoder for various reasons, not just top speed

I guess Galil et al, have been doing it all wrong for 30+ years?
Al.

[mactec54]

Al_The_Man
I did not post that, mike_Kilroy did, In a decent servo system there are three 'servo loops,' all of which required the encoder for various reasons, not just top speed


There are not to many in the Hobby world using Galil

Galil is very good, I did all there install training courses,I have used many of them, there is a place for everything, But they don't always work well with some servo drives, they don't compare to, Delta Tau, but as I said there is a place for everything, there are many more advanced controls out there today but it is not much use bringing them here, This is Hobby world, Were Simple is best, Easy to Configure, get the Hobby guys going with incredible results