Settings for driving Lakeside stepper motors direct

[prusling]

I have Lakeside motor focusers on my 2 imaging scopes and I'm switching from using the standard Lakeside focus controller to a Pegasus eXternal Motor Controller (XMC) driven from a Powerbox.  The standard controller has served me well but I'm moving to drive all my kit from the scope area and they are too bulky.

The Lakeside controllers just worked and there was little in the way of settings to configure, but the XMC offers a choice of full/half/quarter/micro-stepping and max speed limits of 200 or 400. The manual recommends full/half combined with either max speed for the Lakeside stepper.

During my first outing I had some issues which seem to be due to lower stepping units than the native controller which threw out SGP autofocus including its backlash compensation.  The motor was also noisier than before.  As it was a very rare clear night I aborted the switchover and reverted to the standard controller rather than experimenting further.

Has anyone driven a Lakeside stepper motor (McLennan P542-M482U-G21L82) using other means than the standard controller, and if so what settings did you use - ideally to match with standard ones?

[symmetal]

I made an arduino based controller to drive my lakeside focusers for the same reason, that the lakeside controllers are rather large. I used the Moonlite Ascom focuser driver to save having to write a new Ascom driver, so wrote the arduino software to follow the Moonlite protocol. I used the Moonlite DRO driver so I can drive two focusers from one com port for my dual rig setup though it's compatible with the standard Moonlite driver if you only want one focuser.

I made the arduino software mimic the stepping speeds and microstepping selections of the Moonlite. The data sheet for the McLennan states 200 Hz for the maximum stepping rate. The Moonlite offers 250, 125, 63, 32 and 16 Hz stepping rates. At 250 the lakeside was more noisy and vibrated a lot more and ended up missing steps so the focuser position was not reliable. At 125Hz and below it was fine. I could have easily adjusted the arduino so that a Moonlite command of 250 was actually 200 but didn't bother. At 125 it moves slower than the Lakeside driver which probably steps closer to 200 but that's not a problem as the focuser doesn't move very far in normal use.

For microstepping I keep it in full step mode as the Lakeside is heavily geared down and it takes about 5 steps for a noticeable change in focus. Microstepping would slow the movement even more, and also requires the motor to be powered continuously and not just while stepping. For full steps the detent torque of the motor, along with the gearing prevents the focuser slipping when unpowered. The lakeside also just full steps and doesn't power the motor continuously.

Your motor being noisier and throwing autofocus out implies the stepping rate is too high so is missing steps and ends up chattering between steps. The setting of max 200 steps on your new unit should be just slow enough to work with the lakeside. That assumes it really is 200 and not near enough like 210 or so which could cause issues.

The McLennen motors have fewer steps per revolution compared to the direct drive stepper motor focusers commonly used, so travel further with each step, hence the slower maximum stepping rate. The gearing down provides the higher stepping resolution.

Hope that helps prusling. 🙂

Alan

[prusling]

Many thanks for such a detailed reply. I'll throttle back the speed to 200 though I could go lower as there is a custom setting offered. Will report back! 

[symmetal]

Another thing to mention, just for information, is that the Lakeside motors are wired as 5-wire unipolar. Many commonly used stepper driver modules nowadays are bipolar motor drivers as they are more efficient, (though electronically more complicated), than unipolar driver modules. Your Pegasus XMC can be configured to work with either type of stepper motor. You can turn the Lakeside McLennan motor into a bipolar motor, (as I have done in my application), by removing and separating the two wires from pin 5 on the 'D' connector and leaving them unconnected.

I'm not suggesting you do this, but thought it worth mentioning just for completeness. 😀

Alan 

[prusling]

Many thanks, Alan, that's very interesting as I wired it myself using a modular RJ45/8P8C to DB9 connector with a generic pinout I found on the web. I had a challenge soldering the two wires for pin 5 but succeeded in the end, only to discover a pinout in the user manual itself which excludes those two wires completely, which may be preferred for this unit. They are still connected at present (as I assumed they were redundant) but maybe I should disconnect them.  You say bipolar are more efficient, are there any performance/reliability benefits?

[symmetal]

1 hour ago, prusling said:

Many thanks, Alan, that's very interesting as I wired it myself using a modular RJ45/8P8C to DB9 connector with a generic pinout I found on the web. I had a challenge soldering the two wires for pin 5 but succeeded in the end, only to discover a pinout in the user manual itself which excludes those two wires completely, which may be preferred for this unit. They are still connected at present (as I assumed they were redundant) but maybe I should disconnect them.  You say bipolar are more efficient, are there any performance/reliability benefits?

If you look at pages 11 and 12 on this document is shows the different types of stepper motors and their wiring. Bipolar should give more turning force as all the coils present are energized with each step rather than just half of them. Also the two coils on each phase are in series in bipolar mode and so the current required to energize the motor is halved. Halving the current reduces the turning force but this is offset by the previous sentence. The McLennan motors are 6 wire internally but are wired as 5 wire connected to the 'D' connector. A 6 wire motor, (or an 8 wire) can work in bipolar or unipolar mode just by changing the wiring to it.

If your pin 5 is not connected in the XMC wiring, but the two stepper motor wires are still joined together on pin 5, (these are the centre taps on each phase) it will still function as a bipolar motor as long as both the outer wires on the other phase are electrically disconnected when that phase is not driven. This looks like the case in your setup, but I physically disconnected them from each other (and pin 5), so just left them floating with a bit of heatshrink to insulate them, just to be sure.  The XMC can only drive bipolar motors by the looks of it, if it only has 4 wires, and depending on how the unipolar motors are internally wired and externally connected, it can then drive them as well in bipolar mode with the right cable wiring.

Alan 

Edited December 27, 2020 by symmetal
Wrong link

[prusling]

Managed to get out under clear skies last evening and had 4 or 5 perfect autofocus runs with the XMC and Lakeside stepper, so using the unipolar wiring with full steps at 200Hz max, and backlash compensation only in the driver, works well. Many thanks for all your help.

Peter

[symmetal]

Happy to help. 😀 Glad it's all working well now Peter.

Alan