Closed loop control with Mach4

[Roger]

Long long ago, when I first converted the Mill to CNC, the intention was to use a product called 'CNC Brain' by Safeguard Robotics which was supposed to be a Web based control that used both rotary encoders and linear ones to produce an exceptional control system that eliminated backlash. Although I had the product running open loop for a couple of years, it was never completed and I switched to Mach3.

Fast forward to today and I still have the 1micron linear scales on all axes and I'd really like to use them. Most of the time what I have is plenty good enough, but on some 3D machining it would benefit from being more accurate.

Before anyone jumps in to say I should be using Linux, that's a non-starter because the CNC control shares the same computer as the 3D modelling package. I really have no interest in getting involved with a different operating system when Windows 10 is so stable and does everything I need.

So there are at least three different companies that seem to offer products that might fit the bill. The most promising of these is from a company called VitalSystems who appear to have a well thought out product that integrates with Mach4 and can take the linear encoder feedback. I've downloaded the manuals for these which you can find here. There is a bare board version without the breakout board, but I'm not sure I want to delve too much into that since it would be a lot more work.

I'm not entirely sure what's needed, so I've contacted them for more guidance so I have a better idea of what this would all cost.

Watch this space...

[Roger]

I'm still waiting to see if VitalSystems are able to help with this.

In the meantime, Galil have come back to me and it looks like they might have a viable, if expensive, solution. This method of control is not as simple as it might appear because loop stability with a loop within a loop can be an issue when the backlash is significant. The inner loop is to be controlled by the SureServo AC servo amplifier, although they would rather take control of the motors and drive them with their own system, but that would cost £2950.

Galil have a very detailed explanation of all their possible solutions, a list that I don't think is exhaustive. Fuzzy logic would be a candidate for this sort of control system. They have proposed that I use their DMC4153 which is a 5-axis variant in the 5-8 axis enclosure. Although I only have 3 axes at the moment, I plan to build a 4th axis and they have to use an axis for the Spindle Speed control. That's a pity, because the 1-4 axis enclosure would be neater.

I've been told that the SureServo AC servo amplifiers will need to be driven by the +/-10V analogue inputs in Torque mode. This makes up the inner loop while the linear encoders and the the DMC4153 close the outer position loop.

For some reason, they use a 15 way connection for attaching encoders, I don't know of any that don't use a 9 way. That would mean making adaptors for those which is a bit annoying. My last communication with them seems to confirm that there's a Mach4 plugin that they say will work with this, but I need to cross check that with ArtSoft to make sure they support it and will help me if I decided to go down this route.

The price I've been quoted is £1925 ext vat, so it's a big investment and needs to work really well to justify doing it. I'll have a chat with them next week and try to convince myself as to whether this is viable or not. I might just decide to put up with what I've got until I've finished 1501 and then design a fuzzy logic solution for this that might become a commercial product. We'll see.