Storebro Combi Mill Retrofit

Hello!
I bought a Swedish Storebro Combi Mill for some time ago. The machine is controlled by a Heidenhain TNC 131 control system. The controller is very slow and is only allowed to run one servo at a time. The servos are also very slow and in a bad condition. The machine is equipped with Heidenhain linear encoders in all directions.

I have decided to convert the machine to a LinuxCNC system. The only thing I'm keeping from the original control system is the linear encoders.

Here's my thoughts. I will buy three dc servo motors with analog drivers. I will connect these drivers to mesa cards and then into the computer.

These are the parts I've been looking at:

Servo motor: Nema 34 brushed dc motor.
850 oz-in Peak 72V/38A
Constant Torque: 170 oz-in
Peak Torque: 850 oz-in
No load Speed: 4200 R.P.M
Terminal Voltage : 72 VDC
Continuous Current: 7.5 A
Peak Current: 38 A

Servo drivers: AMC 30A8
Peak Current: 30 A
Continuous Current: 15 A
Supply Voltage: 20 - 80 VDC
Input: +- 10 V

Mesa cards:
Mesa 5i23
Mesa 7i33
Mesa 7i37

I will connect the servo drives to the Mesa 7i33 card. The Mesa card is thereby connected to the computer via the Mesa 5i23 card. The Mesa 7i37 card is for e-brake etc and are also connected to the Mesa 5i23 card.

Does this seem to be a good setup?

 
The biggest problem I have now is how I should do with the encoders. I think the best solution (for good accuracy) is to use the Heidenhain scales I already have and connect them to interpolation boxes (to receive a TTL wave to the Mesa card). I've been recommended that I also should use some other encoders or tachometers to make a stable system, if the Heidenhain scales fails. In this recommended system, LinuxCNC is going to compare the position between different encoders/tachometers to make sure everything is fine.

I'm not sure how I should solve this problem with double encoder/tachometer for each axis in the system. The machine have three axis and there will be a total of 6 encoders/tachometers if I choose to create this safe system. Do I need two Mesa 7i33 card if I choose this solution or is there another solution? How do people use to solve these problem?

I would be very grateful if you could give me some tips before I start buying parts!

You might consider a 5I25/7I77 combination instead, as this will give you six encoder inputs
Another optionis a 5I23 card and a 7I48 6 axis analog servo interface instead
of the 7I33, this also has 6 encoder inputs. There are other options as well

That's sounds like a good solution. The 5i25/7i77 seems like a popular combination. The main reason I was thinking about the 5i23/7i33 combination from the beginning is because I have seen it in a lot of retrofits.

Are there any disadvantages if I choose 5i25/7i77 instead of the 5i23/7i33 combination?

The main disadvantage of the 7I77 is that its field I/O is hardwired for active high (sinking) inputs and active high (sourcing) outputs. The 7I37 has individual OPTOs for inputs and individual isolated MOSFETs for outputs so is more flexible since it has no common ground for inputs or outputs. This has disadvantages as well, since there is no input or output common, there is more wiring needed with the 7I37 I/O

Thanks for the answers. This information is above my technology understanding for the moment. I can't see how the disadvantages affects me right now. I will look deeper into this before I make my choice.

I really appreciate all the help!