Which Centroid controller for Bridgeport Interact?

[SimonP]

Hi
I looking for advice as to which route is the best for upgrading my Series 1 Interact mill.
I have the original DC servo motors with the mill so could go with All in One DC, but wondered if it would be better to replace the servo motors with modern AC servos and use Acorn maybe?

Thoughts and advice would be appreciated..!
Thanks

[Muzzer]

Another option would be to use a step/dir driver for your brushed motors and use Acorn controls. This is what I did on my Shizuoka AN-SB. Works fine and avoided having to swap out any of the motors, belts etc. I used the DG4S-16035 servo drive from CNC Drives DG4S-16035 servo drive. I believe there is a US stockist.

[SimonP]

Thanks for your response, I think the thing that's bothering me is building a new system whilst using 40 year old DC servo motors..

(I'm in the UK by the way..)

[tblough]

I would upgrade to AC servos and either Acorn6 or Oak for a true closed-loop system. I don't think you want to keep the old DC technology and go with an Allin1DC but that is also a possibility.

[WesM]

I am kinda going through the same thing on my machine, which is very similar to a BP series 1. I ended up concluding that an acorn + AC servos would be about the same cost, more reliable and perform better then trying to pair a new DC drive with old servos or trying to use an all in one.

I have to admit that I was very intimidated by the idea of tuning DC servos as well, you may want to read up a lot on that process before going that route. One of the perks of a good AC servo system is they will come with a good autotune software.

All in one DC $1856
mill pro for all in one $549
total = $2405

Acorn $329
Mill pro $159
Dmm-Tech 750w AC servos, 3 axis kit $1684
total = $2172

per tblough's comment, my understanding is that there are several types of closed loop systems. the loop can be closed at the control or in the servo drive (or both, with the servo drive providing feedback to the control). In the case of the Acorn it relies on the loop being closed at the servo drive. Oak, Acorn6 and Hickory loops can be closed at the control. closing the loop at the control gives a higher level of fidelity if done right. You would have to decide if the higher level of fidelity is worth the additional cost on an old knee mill retrofit.

I plan to use my machine for prototyping products at home. All of that work will be +/- .005" or loser tollerances, from the impression I have I think the acorn will be fine for that. If I was doing a retrofit on an enclosed VMC that I planned to run lights out, oak or hickory all the way. I just decided to take the plunge and have an acorn being delivered on Monday.

[tblough]

The one benefit from full closed loop control (at the control) I use daily is the ability to move the axis manually.

[WesM]

The one benefit from full closed loop control (at the control) I use daily is the ability to move the axis manually.

That is a pretty cool feature and deffinitly one that would be useful if your machine was originally a manual machine. I plan on simulating manual motion via a pendant. My machine, and I think the BP S1 interact does not have a manual quill or handwheels for the X,Y axis, so the pendant is kinda the only option for "manual" mode... unless we start trying to add handwheels to the axis, which would be interesting on the Z.

[Muzzer]

You really need to use the MPG to make the most of the Acorn in my view, in which case you get full manual control. I often use the MPG to manually square off stock, drill and tap one off holes etc.

As for the "old motors" concern, my Shizuoka is 1983 vintage. Works fine. The std encoders could be replaced with generic Chinesium ones if they ever failed and the brushes would be easy enough to replace. If the worst came to the worst I could always swap out the brushed motors for AC servos but that's not been necessary. Depends if you can be bothered to swap out all the motors, pulleys, belts, mounting plates etc. I've gone for cheap Lichuan AC servos on my last 2 conversions but for this machine it was much simpler just to keep the original motors and mechanicals

I've not been impressed by the "autotune" features of the DMM Tech servos and ended up tuning them myself. TBH, I found DMM servos to be somewhere between hobby and professional grade, not least having dismantled them and looking at their internal construction (speaking as an engineer with many years developing inverters myself). If you go for high end professional drives such as Yaskawa or Delta, they have much better software but that can be as much of a curse as a bonus, as they can be a bit overwhelming. Having said that, even their autotune features still make a poor job of tuning, perhaps because they aren't so well suited to driving axes.

If you go really cheap and choose a generic Chinese servo, you will find that the "software" is little more than tool for downloading and uploading a limited table of parameters, many of which you may struggle to understand (or change). But the Centroid software allows you to do some PID tuning outside of the servo driver.

On my first conversion (Taiwanese Bridgeport 2J clone, manual machine, DMM servos), I thought it would be frightfully clever to retain the original cast iron handwheels. However, the moment of inertia of those beasts far exceeded the combined MoI of the ballscrews, table, saddle, vise etc and really compromised the response. I ended up removing them and use the MPG for manual operations instead.

I'm up near Blackpool (UK), although I work in Leamington. Where are you?

[SimonP]

Some really great and informative replies there, thanks very much!
The thing that started me thinking about alternative servo motors was the need to change out the encoders on all three motors for 2000 line to suit the Centroid requirements. I took the Z axis motor apart to see what size shaft the new encoder would need to fit on and it's 10mm, the motors are SEM MT30 R4-58 and as I understand it, the original encoders aren't high enough resolution. If I keep the DC motors, there is a definite benefit from not having to machine new mounts etc., but would be able to if needed and I went for new AC servos.. interesting comment about the Chinese AC servos not being great quality and challenges in tuning them .

Please keep the comments coming, I welcome your opinions!

I'm near sunny Bournemouth on the south coast..

[Muzzer]

My DC motors are SEM MT30R4-46, so very similar. The 46 runs to 3000rpm; the 58 is limited to 2500rpm otherwise they have the same torque rating.

As for the encoder resolution, IIRC, mine are 1000 lines. With a 5mm pitch, this results in a nominal resolution of almost 1um, yet very few machines are capable of even 10um accuracy. You get 4 increments per "line" with an incremental encoder.

Pretty sure you can specify the encoder resolution in Acorn Wizard. What encoder resolution do you have?

MT30R4-46 Technical_Data_Manual servo motor.pdf

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