Shopbot PRS 48x96 Acorn / Clearpath Conversion

[paulwall]

Hey all - This will be my first post, and my first ever privately owned CNC and build. In December last year I picked up a router powered Shopbot PRS 4' x 8' in really bad shape - It was covered in rust and needed V-rail replaced on the gantry, and all new guide channels on the Z-axis. Not to mention a sandblast and powdercoat.

The machine operated under power when I bought it which was good because it meant I could sell the controller to a new home, which I did. Since then I've been slowly amassing components and attempting to wrap my head around CNC control system basics.


(here's a photoshopped orange frame for visual reference).



My machine build as of now will include the following:
1. Shopbot PRS 2008 48" x 96" with center legs added.
2. Centroid Acorn Rev4
3. Clearpath CPM-SDSK-3421S-ELN on X, Y, and Z.
4. CNC4PC C86ACCP Board for easier wiring of enable signals.
5. SureGear 5:1 Planetary Gearboxes PGCN34-0550
6. Yaskawa 2.2HP VFD V74X + Spindle

I hope to document the build here, and ask the many questions I'll have along the way. By the time it's done, I'll share all the build schematics and component lists in a new post so it will be easier for others to get through.

If anyone here cares to help me with these first few questions, it would really help me out!

1. It seems many people are running two e-stop driven contactors, one for their VFD/spindle, but also one for drive motor power supplies. My question here is why run a another contactor for your drive power supply if the Acorn when triggered by an e-stop, will disable the drives? By cutting complete power to the motors, you will you not loose entirely where you left off if you fault or hit the e-stop?

[Richards]

Nice looking machine!

When a ClearPath motor faults, you clear the fault by turning off the power to the motor. I use the Power-4 hub from Teknic (about $50) which maintains power to the motors' encoders during the fault.

[Gary Campbell]

Of course there are about a dozen ways to wire servo power. A contactor simply can switch higher loads than a relay, especially when they are inductance loads, like a power supply. I always use them for servo power, not always "thru the estop circuit", sometimes on NoFaultOut

You can assume that if the estop (on a hybrid step & direction system) is hit all position will be lost. Period

E stands for EMERGENCY, not convenience or oops.

[paulwall]

Nice looking machine!

When a ClearPath motor faults, you clear the fault by turning off the power to the motor. I use the Power-4 hub from Teknic (about $50) which maintains power to the motors' encoders during the fault.

Thank you Mike. I've recently been reading about that Power-4 Hub, I found this post from the Teknic crew regarding this very topic. --> https://www.cnczone.com/forums/centroid ... ost2175876

It's a novel concept; kill the power to the 75V supply preventing motion at all, but keep the logic whereabouts alive within the drive in a low voltage state. For anyone interested in how this works, here is the video the Teknic rep references regarding this system. https://youtu.be/mALi3_QqyHk?t=460

Of course there are about a dozen ways to wire servo power. A contactor simply can switch higher loads than a relay, especially when they are inductance loads, like a power supply. I always use them for servo power, not always "thru the estop circuit", sometimes on NoFaultOut

You can assume that if the estop (on a hybrid step & direction system) is hit all position will be lost. Period

E stands for EMERGENCY, not convenience or oops.

I do get the extra layer of safety factor you're describing here Gary. It seems Teknic may have found a suitable loop-hole to the problem for their system alone. I'll likely keep reading into user experiences with this Power-4 Hub before concluding my wiring layout.

edit: Gary, I believe you've also introduced a second layer of safety within your systems by driving the powering up of the drivers through the Acorn relays once and only once the Acorn is booted and powered up, no?

[Richards]

There is a problem when a motor faults. The kinetic energy can keep that axis moving until friction stops all motion. During that time, if the Power-4 hub is used, the position of the axis is known, but not to Centroidcnc. In other words, the Centroidcnc software does not keep an encoder count for each axis. It would have no idea of the absolute axis position.

If the motor faults or power is shut off while an axis is stationary, and if the motor did not "jerk" when re-powered, then the count stored in the power-4 hub would be accurate.

I purchased the hub because it allows me to share two IPC-5 power supplies with up to four motors by using the Teknic cables. It's plug and play.

[RogDC]

There is a problem when a motor faults. The kinetic energy can keep that axis moving until friction stops all motion. During that time, if the Power-4 hub is used, the position of the axis is known, but not to Centroidcnc. In other words, the Centroidcnc software does not keep an encoder count for each axis. It would have no idea of the absolute axis position.

If the motor faults or power is shut off while an axis is stationary, and if the motor did not "jerk" when re-powered, then the count stored in the power-4 hub would be accurate.

I purchased the hub because it allows me to share two IPC-5 power supplies with up to four motors by using the Teknic cables. It's plug and play.

Isn't the encoder position known to the clearpath motor? I don't believe there is feed back to the Acorn from the SDSK series if I am reading the schematics correctly.

[Richards]

The power-4 hub retains the position but the Acorn does not receive that information and the operator can't read that information. It seems to me that the best practice would be to re-zero the machine and then to jog to a known position in the program and resume. The Acorn board is amazing in what it does and so are ClearPath servos but the Acorn only has one encoder input and ClearPath servos retain the encoder count internally except for +/- indication possible with the HLFB signal. I tried those settings right after I received my first ClearPath motors and then decided to use the HLFB signal as a motor fault indicator instead.

On my personal journey, I started with a version 3 Acorn, added various models of AutomationDirect PLCs to enhance the Acorn, installed a 1616 expansion board and removed the external PLCs, and finally bought two more version 4 Acorns which I'm using without external PLCs or the 1616 expansion board. I had to pare down the number of I/O points that I monitor, but everything that I need to monitor is being monitored and everything that I need to control with 8 outputs is being controlled. It works for me.

[Gary Campbell]

Paul...
I see no advantage on a hybrid system (no encoder feedback to the controller)in most normal situations, but I can see a couple that would turn out bad if the axes were powered back up and the servos moved to their "last known" position on their own. In any case until the controller is made aware of that exact position, there is no advantage.

[gerald martin]

Sounds like you will have a great machine! It will be similar to the 60x96 that Gary retrofitted for me:

https://www.centroidcnc.com/shopbot_gcnc.html

One consideration on your motors - you might be as well served with the 3411 servos like Gary installed on mine. When we compared the speed and torque chart, we found that at higher speeds the 3411 outperformed the 3421, and at lower speeds the 3411 already has more power than the machine requires with that 5:1 gear reduction. We're running 20T pinions, I believe.

Here's the comparison of 3411 vs 3421 motors:

Looking forward to watching your progress!

Gerald

[paulwall]

The power-4 hub retains the position but the Acorn does not receive that information and the operator can't read that information. It seems to me that the best practice would be to re-zero the machine and then to jog to a known position in the program and resume. The Acorn board is amazing in what it does and so are ClearPath servos but the Acorn only has one encoder input and ClearPath servos retain the encoder count internally except for +/- indication possible with the HLFB signal. I tried those settings right after I received my first ClearPath motors and then decided to use the HLFB signal as a motor fault indicator instead.

On my personal journey, I started with a version 3 Acorn, added various models of AutomationDirect PLCs to enhance the Acorn, installed a 1616 expansion board and removed the external PLCs, and finally bought two more version 4 Acorns which I'm using without external PLCs or the 1616 expansion board. I had to pare down the number of I/O points that I monitor, but everything that I need to monitor is being monitored and everything that I need to control with 8 outputs is being controlled. It works for me.

All good information, thanks Mike. It looks like I'll be following what you and others have done using the HLFB to signal fault states.

Paul...
I see no advantage on a hybrid system (no encoder feedback to the controller)in most normal situations, but I can see a couple that would turn out bad if the axes were powered back up and the servos moved to their "last known" position on their own. In any case until the controller is made aware of that exact position, there is no advantage.

Well noted. One can only hope Centroid will continue to improve on the Acorn peripherals to include additional encoders. I'll keep my fingers crossed for that.

Sounds like you will have a great machine! It will be similar to the 60x96 that Gary retrofitted for me:

https://www.centroidcnc.com/shopbot_gcnc.html

One consideration on your motors - you might be as well served with the 3411 servos like Gary installed on mine. When we compared the speed and torque chart, we found that at higher speeds the 3411 outperformed the 3421, and at lower speeds the 3411 already has more power than the machine requires with that 5:1 gear reduction. We're running 20T pinions, I believe.

Here's the comparison of 3411 vs 3421 motors:

clearpath comparison.JPG

Looking forward to watching your progress!

Gerald

Gary had the same recommendation for me. I opted for the 3421S ELN model, and am going to pair them with 25t pinions to squeeze just a bit more speed out of them. We shall see how that goes. How has the adaptation to the new control software been for you? I assume Gary set the machine up to make the transition nice and easy.