Welcome to LinuxCNC.org

linuxcnc expects positional feedback (and from that can calculate approximate velocity or some of the cards actually calculate the velocity)
I guess to be truly accurate linuxcnc needs both position and velocity - it just calculate velocity from position.
Trying to calculate accurate position from approximate velocity and time seems about impossible.

Well if the drive belt breaks then the axis can't really crash (it can't move) so thats not that bad.
The motor would spin at maximum.
worse is when the feedback device fails - but it doesn't matter what type of feedback device, if it fails the motor is going to try to move faster.

It would seem success of tuning with scales would depend on how much lost motion (backlash) between motor and axis and the resolution of the scales.
Typically you get way more resolution with rotary encoders because of the gearing and leadscrew. your interpolator helps here though I believe the 602 is usually 5x (though it can be 1x - it should say on the box)

you say you are ready to take the necessary measures to make this work you have three practical options that I see.

buy servo drives that use tach feedback and use the scales for positional feedback ( you won't use the 7i29 to drive the motors )
buy some encoders and add them either to the motor or the leadscrew. (this is the probably best option)
use the scales as feedback (probably hard to tune right)

And thats the end of my opinion !
Hopefully I haven't told you any big lies

Chris M

cmorley wrote:
The machine has 32x5mm ballscrews, with 2/1 belting giving 2.5mm/rev at the servos. The HH scales are 20um and interpolate to 4um *hopefully I've go the right 602 units. It doesn't say on the box. The HH documentation states that the 602 is adjustable, but I can find no documentation for the jumper blocks. Maybe someone here knows?





I'll take whats behind door #2 Chris. Say, optical encoders are butt cheap. USDigital has these that could be made to fit:





I would be interested to know of other suppliers as well.

With respect to interfacing the 7i29, the encoder connectors have pinouts for differential IDX,_IDX. I take it that those are optional.

Thanks again
John
.

Note to self: Don't try to post with the micron symbol.

The index on an axis can and should be used to home the axis for a more accurate home position than using just a switch.

Neat machine btw...

John

_jC wrote:
I think that is the only way.

The existing Tachos on the motors connect to the servo drives, and will continue to do so. That allows the drives to work in a closed-loop velocity mode.
ie, 10V from LinuxCNC = 100% servo speed, 1V from LinuxCNC = 10% speed, 9v battery and no controller at all = 90% speed.

Then, inside LinuxCNC you run a position control loop which considers the feedback from the glass scales, and sends out a velocity command to the drives according to how far out of position the carriage is.

I hope that your glass scales have quadrature output though, or it gets a little tricksy.

andypugh wrote:
I see now that you don't have tach-input drives, but intend to use Mesa H-bridges.

In theory this could be done. You would need to read the tachometer signal into LinuxCNC and use that to close a PID velocity loop, and then control position on a separate position PID loop. (Such nested PID loops are quite common outside the LinuxCNC world, and LinuxCNC can happily run dozens of PID components.

The problem, as you have spotted, is getting the Analog voltage into LinuxCNC. This is especially tricky in this case as the voltage is bipolar.

I think you could probably use an Arduino to do this on all three channels. The Arduino has several 0-5V analog inputs. Hopefully the Tachometers have two floating connections. You can hold one terminal at 1/2 Vref and then sample the other, then 0V sampled = -2.5V and 5V = +2.5V. You convert back to signed in the software. As the tachs go rather higher than 2.5V (If I recall correctly, I can't see back that far from this edit window) then you will _also_ need a voltage divider to bring the tach voltage in range.

Quadrature output from the Arduino is trivial, you just take the two lowest-order bits of a binary counter. So, you would measure the voltage (velocity) to create a target value, and then count up or down (in the main loop) a counter and put the two low-order bits on DIO pins. The analogue voltage would need to sample every mS, under interrupt.

As it happens I have nearly all of this ready-made here:
wiki.linuxcnc.org/cgi-bin/wiki.pl?ResolverToQuadratureConverter
That has the bipolar measurement, analogue-to-quadrature and interrupt-scheduled analogue captures all set up. You just need to take out the sine-wave generation parts.

Other options exist.

It seems the exe 602E is either none or 5 fold interpolation. easy to check when you have it all hooked up.
I remember when i got the manual for it - there was no mention of how to change it which to me meant you couldn't even though the docs seem to say you can.
You can buy other versions of the EXE that do other interpolation (ebay is your friend)
Andy the EXE converts the scales 11um sine waves to TTL signals so the scales are easy to connect to the 7i29.