Bridgeport Series II spindle speed control

I am planning a conversion of my '86 Bridgeport Series II (Boss 9 control) mill. I am going to use the Mesa cards (5i20+7I48 or I may just grab the 5i25+7i77) with the existing servo controllers. I believe I have worked out most of the details but I'm not sure how I will do the spindle speed control. The machine is equipped with a fairly standard 2J head (with adjustable belt drive) with the speed control done through an air powered motor. The air motor is controlled through an air solenoid and can move in either direction (increase/decrease speed). The machine normally reads back the current speed setting with a potentiometer on the head (reads back the position of the speed control pulley). Reading this in would require an ADC though.

So it seems I have three options (or feel free to throw better ones in!):

• Interface an ADC with EMC and have it read the speed pulley like the factory controller
• Add a spindle encoder in and have it read the true spindle speed directly
• Use a VFD and replace the existing contactor setup (obviously cooler+higher dollar)

Is there an easy ADC interface option with the MESA cards? Is it easy to do a feedback control like this? If an ADC requires more hardware maybe it is just easier to do the encoder.

Thanks in advance!

If you use a 5I25/7I77 theres are 4 channel of low resolution
A-D available on 4 of the field inputs (8 bits resolution 36V full scale).
There are other Mesa cards that can provide higher resolution
A-D as well (7I87, 7I73. 7I65 etc)
but if 8 bit resolution is OK they are probably overkill

I'd go with adding a spindle encoder , long term it would give you benefits with other tricks with Linuxcnc , such as threading etc

I would agree with the encoder.

It can be something as simple as a rare-earth magnet embedded in the spindle pulley, being read by a hall-effect switch to give an index pulse.
This will provide a good speed reading, I use it on both my mills and costs next to nothing.

regards

I suspect that if you wish to keep the airmotor control on the variable belt drive,
you will need The A-D (or another absolute encoder system for reading back
the current ratio setting).

This is (mostly) unrelated to the spindle encoder (which you very likely will want as well)

That makes a lot of sense. Considering how slow the airmotor changes the speed and latency etc it would likely be pretty difficult to tune the speed adjustment directly from the encoder.

Does the 5i20 also have the ADC inputs? I currently have a 5i20 card but I haven't purchased any of the expansion cards. This may be an argument for selling the 5i20 and getting the 5i25/7i77 combo instead.

Thanks for all the help guys!


James

You can run a 7I77 from a 5I20, just needs a little cable adapter (Our HDDBADPT)

Are the analog inputs on these cards referenced to the field power? Or to a fixed 36V reference? Not quite clear from the datasheet. I am using a 5V field power source... but if it is referenced to 36V with 8 bits of resolution I don't think I want to use 5V as the full scale resolution! Could I run a 24V swing on the ADC with a 5v field power input?

Thanks for all your help guys. I have bought the hardware now (5i25,7i77 and a 7i84 for remote IO) and have started integrating it.


James

The field inputs are always 36.3V full scale so 24V is fine