1988 Fadal 4020 Conversion Thoughts: MESA 6i24 + 7i49 + ?

LinuxCNC Mesa cards = there are sooo many options I am leaning towards the 6i24-25 PCIE 50 pin x3 FPGA board as I need the 7i49 Resolver board. The 7i49 will do the resolvers and analog drives but I have not made up my mind on what other IO boards to use for everything.

The machine requires these connections:
~24 digital outputs (driving mostly active low logic SSR inputs)
~20 digital inputs 0-5v
3x resolver inputs for X, Y, Z and possibly a resolver for the spindle? Or I could put encoder. It has none currently.
4x analog outputs for X,Y,Z,Spindle (can I use one of the 7i49 output for spindle?)
Here is a spreadsheet with the signals necessary, it will be updated over time

After looking at the options more am thinking the 7I64 (RS422 24 output, 24 input isolated I/O card) may be the most robust option because each input / output is isolated from each other so a potential failure on one would not affect the entire board. Does this make sense?

Another option would be 2x of the 7I84D(RS422 32 input, 16 output Isolated - Sinking outputs) which would give me more total I/O(64 input, 32 output) but each board would share common ground which is okay and would probably wiring easier. The trade-off being this would not be quite as robust as the 7i64 solution.

I am also considering adding 1um linear magnetic encoders to the X,Y,Z but not sure it will be worth the effort. Still in the process of testing the ball screws, replacing all thrust bearings + shaft couplings.

The 7i52(6 Channel encoder 6 channel Serial RS-422 interface) would allow me to hook up the RS-422 daughter board(s) + the linear encoders if I do decide to add them down the road, or if I decide to use an encoder for the spindle orientation vs resolver.

The inputs all go through cables that are about 10ft long to the actual sensors.

I believe this is the resolver on all 3 servos 11BRW-300-F1/6 which is a 12V input, 6V output, 2500hz, 0.5 transformation ratio, +/-6 arc minute accuracy
Do you think this resolver will work well with the 7i49? should purchase the HV or normal 7i49?

To run LinuxCNC I have a LGA1200 Intel ITX motherboard with G6400 Pentium dual core 4ghz that will run linuxCNC - I didn't want to use older computer hardware although I know it would run fine: I want the most lifetime out of this setup. Plan is to use VNC to remote into the LinuxCNC machine and run it headless which allows me to control it directly from laptop via WiFi but gives LinuxCNC direct access to the hardware though PCIE. Will have wireless pendant + hardwired emergency stop button.

Two of the brushed DC servos look like they were replaced sometime fairly recently but the Y motor may still be original and needs new bearings in motor / resolver I believe. I checked brushes they are fine.

The DC drives are velocity control with analog generator tachometer feedback from the servos. Two of the drives look like originals and two are newer types (4th axis drive is mounted but I have no 4th axis mechanism currently so this can function as spare).

Everything works as machine is currently under power and moves with the original Fadal CNC88 control. I have thought about replacing with new brushless AC servos but it seems like a waste when the drives and motors still function. If I was to replace them I would go with Panasonic A6 1.5KW AC servos/drives which include 23bit encoders, I have experience tuning these servos.

(I edited this post quite a bit(before any replies) - wanted to avoid lots of little posts not sure if this was preferable vs double posting preferences etc)

This is what I have in my cart right now, will these all work together with the right firmware? Looking for sanity check

6I24-25 FPGA based PCIE Anything I/O card
US$159.00
7I49 6 channel resolver interface + 6 channel analog output
US$184.00
7I52 6 Channel encoder + 6 channel Serial RS-422 interface
US$69.00
7I64 Isolated remote I/O card 24 Input, 24 Output
US$198.00

Sub-Total: US$610.00

I am still wondering if it makes sense to spend the 20-50 hours to program and tune LinuxCNC around these 30 year old DC servos with resolvers and 30 year old DC drives or if I should spend the $$ now and replace all motors with 130 frame AC servos?

I am worried I will go through that process and then something will fail in the drives / motors and the cost of repairs would be such that replacing with new AC servo is less expensive. Also the acceleration and accuracy with 23 bit encoder AC servos would likely be better.

I was thinking of going with 1.5kw Panasonic ones that are $800 each but will consider other options. Reliability is the main thing I am after and would like the system to last at least 10 years with minimum maintenance.

This also changes which Mesa board to buy, seems like swapping the 7i49 for a 7i33 would make sense in this case. I do like the high PWM rate of the 7i33 for faster analog response.

I was wondering when you would show your support for resolvers Andy I've been reading tons of posts on this forum and have seen your praise for them.

Do you know of any quantitative analysis comparing their specs (update rate, accuracy) vs encoders? I have googled this but not come up with any good resources mostly just industry stuff saying they each have their place etc etc with no real hard numbers. Ideally comparison would be between resolvers with 7i49 vs a high count encoder used with the 7i52 or similar? but I am not sure this exists.

None of this likely matters since if I keep the current servos I believe the brushed DC motors + ancient DC drives will be my limiting factors in system performance, not the resolvers themselves.

I am fully convinced of resolvers advantages in longevity and robustness given their simplicity and continued use in harsh environments like on the Tesla Motors rotor location sensor so in that regard have no problem with the resolvers themselves.

The thing I am most worried about failing is the brushed DC motors(4 brushes per motor), brushed DC tachometers(4 brushes per tachometer) and the 30 year old DC motor drives that rely on those tachometers for speed feedback.

Besides them failing I am worried I will not be happy with the following error or response speed of this system with these older design components, but that may be misplaced and I wont really know until I buy the conversion boards and rig it all up to try and spend the time to tune them.

I just got in some new(used from ebay!) motor couplings that should be way better than the original rubber jaw type that weren't even attached to the ball screw properly with loose set screw, they just slid off the ball screw shaft when I unbolted the motor haha! These couplings are rated for 60Nm torque and the current DC servos put out ~20Nm peak and 5Nm continuous so should be okay.

Thanks for the help making these decisions - trying to work out these details now before I spend even more time going down rabbit holes actually buying parts and wiring stuff up. But that may be the only way to really know..

Oh and I did also just remember that I could use these DC drives in torque mode by disconnecting the tachometers. Someone did this with a Kflop conversion of the same mill with the same drives - by cutting a resistor lead and soldering a wire onto said resistor past the velocity feedback loop on the drives.

Does anyone have insights on if I would be better off bypassing the tachometers like this?

With LinuxCNC + 7i49 it seems like the resolvers would provide accurate speed feedback? Then I have one less failure point without the tachometers in the mix.

Thanks for the insight Andy! That was a great explanation and basically answers all the questions I had about resolvers.

The Fadal machines actually have no homing or end stop switches at all! They instead have alignment pointers on all 3 axis - and as long as you shut down the machine at home position or jogged it close to those markers and then issued the CS command it would use the absolute nature of the resolvers to find its home position without any other sensors - neat and dead simple system.

Judging by what you've done with your lathe sounds like I could duplicate this and not need to add any new homing switches which was something I wondered about.

Is there an easy way for me to do this same setup that Fadal used with the alignment markers on LinuxCNC? Maybe I would have to write a python routine to find some absolute reference value from the resolver? I believe the resolver position must be less than +/- 90 degrees of the resolver reference position for this to work. By "reference" I mean some arbitrarily chosen position on the resolver as it is by nature absolute, not an actual reference like on an encoder.

hilo90mhz wrote:

Do you know of any quantitative analysis comparing their specs (update rate, accuracy) vs encoders? I have googled this but not come up with any good resources mostly just industry stuff saying they each have their place etc etc with no real hard numbers. Ideally comparison would be between resolvers with 7i49 vs a high count encoder used with the 7i52 or similar? but I am not sure this exists.

The update rate is set entirely by the device that excites and reads back the position. The resolver is basically a transformer with a magnetic coupling that changes with angle of rotation.

The converter I build uses the Analog Devices 2S1200 converter chip, and the default excitation is 10 KHz.
So, 20,000 times a second it makes a reading of the sine and cosine signals coming back from the resolver and fits them to an angle. A tracking circuit approximates the velocity and generates counts that are converted to quadrature. It can go as fast as 4 million counts/second (but that would require the resolver to be spinning at 60,000 RPM.)

The thing I am most worried about failing is the brushed DC motors(4 brushes per motor), brushed DC tachometers(4 brushes per tachometer) and the 30 year old DC motor drives that rely on those tachometers for speed feedback.

Besides them failing I am worried I will not be happy with the following error or response speed of this system with these older design components, but that may be misplaced and I wont really know until I buy the conversion boards and rig it all up to try and spend the time to tune them.

Well, in a hobby environment, the motors should last a lifetime. Since the tachs don't carry current, they should last even longer. If you are putting this machine into full-scale production, then replacing them with modern brushless motors makes sense.

Jon