Closed Loop Stepper Cable Sheild?

[Phil Mayor]

Hello,

Just thinking of the possibility of upgrading to Closed Loop NEMA 34 Stepper Motors for my ongoing build. I’ve done a quick search on the internet and a Closed Loop Stepper Kit popped up, when I looked into it, it states the Motor Wires weren't shielded, but the Encoder wires are? I've only ever had experience with open loop before but these were always suggested to be shielded. I already have a Toroidal Power supply so I don't think I'll go for a kit as it comes with a power suppluy for each drive, but just wondering is it correct you only need to shield the Encoder wires, If so, I could save a lot of money and not get the CY Cable I was planning to buy…

Cheers
Phil

[JohnHaine]

I think the reason for shielding stepper wires is to minimise radiated EMY considering they are carrying fast high current pulses. IF the servo includes its driver in the package the signal levels can be much lower so EMI may not be an issue.

[Phil Mayor]

Thanks John for your reply, I'm going to go for the closed loop option I think, there's probably slightly more work involved in hooking it all up but the benefits are great for what I would want on my build.

[m_c]

Plenty machines run unshielded stepper motor wiring, but those machines will also likely be running 24VDC control circuitry with some differential signals for anything that runs at low voltage.

The primary driver behind shielded wiring on hobby machines, is due to them often using low voltage/high impedance (sub 5V, with only mA of current) control circuity, which is very susceptible to EMI. The EMI from stepper motor wiring can be enough to cause noise problems on those machines.

24V control circuitry is far more resilient, and for most 24V systems you could probably run a HF welder next to the wiring, and not have any issues.

However, the bigger question is why do you need NEMA34 motors?
A suitably geared NEMA 23/24 motor will likely give better performance than NEMA 34, especially if you're not using high voltage drivers.

[Phil Mayor]

Hi m_c, thanks for the detailed info. Although it's a DIY build, it's a bit of a monster, 3000mm x 1650mm Steel frame with 9kw ATC (which weighs 35kg on its own) never mind the Gantry. The current Steppers (Nema34) I have are from my last build I was planning on re-using them, but I've managed to somehow brick my Z-axis Driver �� - Hence why I've started looking for a replacement and looking at Closed Loop set-ups, my power supply is 104Vdc, and the Drives are rated at 110Vdc

[m_c]

At that size of machine, I wouldn't even consider steppers.

You can get 1kw servos for well under £200 an axis, which would give you far better performance, and removes the need for any DC supply.
Have a search for Lichuan Servo on Aliexpress for rough pricing, but contact Lichuan directly via Alibaba, and they'll usually do a better price and customise the order for what you need (i.e.cables whatever length that you need).

[Phil Mayor]

Now that's just opened a whole new can of worms, lol, Servo Motors and Drives!? so from my quick Google they definitely seem like they're the best, but I have even less of a clue about these... you say I don't need a DC PS for them, so are they just hooked up to 240v?? I had a budget of around £150 for each Closed Loop Motor and Drive, so if I can get Servo Motors and Drives for similar I might just go for them, I'd need 4 as my Y-axis is slaved. You say 1kw Servos, are they not sized like normal Steppers, ie Nema23, Nema34, etc..?? I'll get on Youtube and check them out...

[m_c]

Most servos above a few hundred watts are mains powered.


NEMA simply refers to the motor mounting dimensions, and is an older imperial based standard (the 34, means 3.4inches)

You can get NEMA sized servos, but they're not common, instead most modern servos typically use metric frame sizes, 40, 60, 80 etc, with the number referring to the overall size of the mounting square.
An 80 frame motor mount is a bit smaller than a NEMA34 (~85mm), but has far more performance.

I only picked 1000W servo as an example. If maximum performance isn't your goal, 750W would likely be acceptable.

To give an idea of the performance difference, a 750W 3000RPM servo will produce a constant torque of roughly 2.4Nm, and a peak torque of 7Nm. It will deliver that torque from a standstill, all the way to it's rated 3000RPM.
A suitably sized NEMA34 motor will on paper produce over 10Nm, however by a few hundred RPM, that torque will have significantly dropped, and by a 1000RPM will be lucky to produce more than a couple Nm, which will limit the possible acceleration at speed.

The NEMA34 would likely have to be direct drive to maximise it's performance, whereas the servo can be geared (also gives more flexibility with mounting position) to give more torque with less speed. With the example above, at 2:1, you'd get 4.8Nm, and 14Nm, and get that all the way to 1500RPM, which means machine acceleration will be significantly improved.

[Phil Mayor]

This is all great info, thanks so much! I've seen a 1000w 80 Flange from Lichuan on Aliexpress, I've asked the question if they can get custom cable lengths too. A few questions i have, you might be able to help with, what is incremental Encoder and absolute Encoder and what does the 17bit mean, is this good, or are there bigger bits to go for...? Also I have Axbb-e board I assume Servo's are compatible with this...?

Cheers,

Phil

[m_c]

Incremental encoders output a basic Quadrature (A/B usually along with a Z index) signal, which is counted by the drive (aka the drives internal encoder counter increments). When everything powers on, the drive has no idea what the motors rotor position is, or whether it has moved.

Absolute encoders output their rotational position, via serial communication.
Some setups it'll be a once per revolution repeating output, whereas some can keep track of the position over multiple turns, so even if the drives/motors lose position/power, it is theoretically possible to not need to rehome to regain position.

In terms of which is best, for simpler controllers, there's not really any difference.
Both types of drives usually accept some form of step input (be that Step/Dir, Quadrature, or CW/CCW), output a generated quadrature signal should you want to feedback position to a suitable controller, and all the usual inputs/outputs will be fairly similar.

AXBB outputs fairly common Step/Dir, which the majority of servo drives will accept.
The only thing you might need to do, is enable virtual gearing within the drive, so you don't need quite as fast a step signal.
I'm too tired to do the calcs just now, but some of these drives now have a ridicousuly high step count per revolution, so with the AXBB which is capable of 400KHz, you might find the 400KHz step rate is the limiting speed factor, and not the servo speed. So by using the drives internal gearing, the drive will move more than one encoder count per received step.