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Researching - Direct Drive mount


NickK
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So I'm going to start researching options for doing a Direct Drive mount to hold a 4" APO with a couple of cameras.. I'll put my research/thinking/ideas into this thread.

I still have the power point presentation (2007) by Dave Rowe (Starry Ridge), who created a 3 phase direct drive. It doesn't show much and the links I have found in the past have decayed.. so starting at the beginning from scratch may be the better option.

A direct drive motor mount typically consists of a few things:

* mount bearings (or mag flux bearings)

* direct drive flux static magnets attached direct to the shaft 

* direct drive magnetic coils to create flux mounted fixed to the mount casing, these are grouped into phases.

* a mechanism to provide positional feedback as the mount shaft moves - question is.. could a sensing system be good enough to maintain position with power?

* power supply for each phase that varies the power to each phase for rotation but also to maintain position based on the moment from the shaft.

* microcontroller to control the adjustment of phases for each coil.

Now from experience with stepper motors using a chopper driver (that limits the current delivered by chopping the power on and off) would allow more voltage to be used and thus provide better flux control.

With multiple phases comes the difficulty in driving the flux in terms of control (the current needs managing 50K+ times a second) for each phase.

 

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There is a link to a document with constructional details of the motor here:

 http://www.altazinitiative.org/AA Tech & Demo.htm

Just glancing but it seems that only two of the three phases are active at a time, one set of coils repels and another set attracts the magnets. Feedback is via encoder tape. Well, neodymium magnets, magnetic linear tape and “enamelled” copper wire are easily obtained.  I’ll follow this with interest.

 

Edited by Icosahedron
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That article is about an alt-az design. Are you going alt-az (With image rotator) or GEM? I also think that Renishaw Encoders are un-cheap!

Good luck with your project. Even ASA had a lot of trouble getting their DDM mounts to market and working anything like properly. Olly would dispute whether they work properly even now!

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Just been thinking about the encoder - previously I've looked at using optical fiber as a rotational inferometer. Per kindly shot a hole in that idea as it stood because of alignment of the bearing.

However using a prospex/acrylic disk with could be used, and I think I have a couple designs that could make it usable. Either using a direct inferometer style movement (this could be a small disk) or using a laser to vaporise slanted slits in the edge of the perspex/acrylic - once mounted, rotate the shaft at a known speed and then use the laser pulsed at a known frequency.

I like the inferometer version better - I've used optical fibres mounted on a hard drive bearing/platter to show this works really well. The two fibres then emit onto a CCD sensor to give the shifting defraction pattern that could be modelled.

One thing to note - a DVD or CD has too wide pulse widths for encoding, instead a bluray laser would be needed if you wanted to use a small diameter disk.

 

Edited by NickK
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However it could be I'm looking at the wrong.

Without gears, and just the bearing to worry about, the concept of backlash is removed. The issue of location within the flux "bumps" is the same as stepper motors, yet for low speed it seems less of an issue for missing the step.

Do we need an encoder? At the moment focusing doesn't and the mount will always be an approximation for the moment to track the star.

The real question is - if we use a stepper motor style flux and pole system, how much power/torque is required per step to move the load using direct connections when 'micro stepping'. From my experience with the focuser + 100:1 gearbox to a 1:1 connect to the rack and pinion - that's the key calculation.

The next concern is - how difficult is it to create a pole system of this accuracy and still have enough flux from the stator to cope with the load. And I think that would be the difficulty in an accurate step (along with the bearing).

 

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It's my understanding that

58 minutes ago, NickK said:

 how much power/torque is required per step to move the load using direct connections when 'micro stepping'.

I believe that direct drive mounts don't "move" in discrete steps, they accelerate the mount and decelerate it. The forces involved being dependent on the amplitude of the desired movement. Although you won't have backlash to deal with, you'll still have the inertia of the moving mount to overcome and I don't think you can model that to sufficient accuracy to stop the mount exactly where it is supposed to stop - not without some form of closed-loop: encoders or the like.

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I thnk you may be "just making a rod for your own back" :D  My geared mounts work pretty well :)

Edited by Gina
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The way that I see it is that the encoder is everything. Encoders have limited resolution and interpolation of the signals has to be done. The current in the windings has to be adjusted regardless of the load on the motor to the point where the encoder detects the right amount of movement. High resolution conversion and some very clever algorithms are needed to interpret the signals correctly.

It is nothing like a stepper motor. Incremental torque per micro-step decreases as the number of micro-steps per step increases. A stepper motor will remain stationary until the combined incremental torque from a number of micro-steps overcomes the load on the motor.

 

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Interesting - a stepper, when micro stepping is effectively varying the flux as the stator (i.e. the poles) move. I can see what you're saying in terms of flux holding and then accelerating and decelerating - this is close to what a stepper does.

If you look at this: http://planewave.com/technology/mechanical-design/ you will see what is a three phase coil system. A bi-polar stepper is a two phase system where the flux is managed (in realtime) to move the 'step'.

I can see why you're saying it's different - in essence any wind or mass change then creates a dynamic variable between the power and the position. It's an closed loop between the encoder (where it is) and the power delivery (force to get it where it should be). The difference is that the stepper applies X force (I wrote a ardunio chopper micro stepping driver) and that is static (as it assumes the stator moves inline with the step).

 

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And my point about steppers: https://www.pololu.com/file/download/drv8825.pdf?file_id=0J590

Look at the sine waves, the mechanism for the direct drive isn't much different. The closed loop is: Position / desired movement -> three phase power configuration -> movement.

The details about force, acceleration, deceleration etc are all done within that loop. If you connect three or even N phases, the system is still using a wave form to propel the magnets using the flux, still have top cope with the back EMF etc.

 

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So realistically the first task is to calculate the design of the stator/winding pattern to match an encoder. It seems that everyone has done the pancake drive but surely is't possible to create something more ingenious with 3D printing/hobby-CNC. Especially as the last link seems to demonstrate that current single sided drives that DIY folk do are very very inefficient.

It seems metals are important and the temperature has a direct effect on the neodymium magnets commonly used - see the last link above. This is especially important for slow moving drives as the duty cycle for each coil seems far higher and so the current has time to increase the temperature naturally - causing a heating effect on the magnets within close proximity for that time.

What's also nice about that last link is it's demonstration around the flux - how to get the most out of it Although I'm not looking at massive 15kW unit.. the principles are almost identical.

 

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Just reading up around rotational inertia. Interesting as you have two axis to contend with and that integrating becomes a three dimensional affair with the two. Especially with an uneven mass that's spread out in space - i.e. the length of the telescope etc.

However as the system is really a closed loop system, with the effect of wind etc the maximum peaks are what becomes important in the planning. After that everything else is below that because of the close loop control.

You can make the assumption of the largest mass at the furthest point with the maximum force of wind being the peak.

Say for a 35Kg scope system, once you have a look at wind that's say pressing as if it was 20Kg (that's a massive gust and a wide scope), the mass of the mount and the friction - the thing becomes almost as if it was a 100Kg scope for those spit seconds..

There's two ways to control - either make it complex, or make it simple and fast. The complex is to make calculations to cover all eventualities and the simple&fast is to make a looped control system fast enough to control (same occurs with hydraulic control systems as the oil heats up/cools over time making it even more dynamic than a motor).

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Plan of battle for the initial design:

0. Parameters - GEM mount otherwise i'll need to make a rotator. If possible make this fit onto the NEQ6 although that design they have on a polar alignment is attractive.

1. Work out torque required for a specific payload, for a maximum wind gust, etc using the worst case moment of inertia

2. Work out motor design to supply matching torque using 12 or 24V input. I'm guessing that the voltage is require to allow the motor to overcome any EMF etc.

 

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  • 2 weeks later...

Hmm seems my previous post disappeared in the mobile either..

Looking at the planewave design and the MIT paper, there are a couple of points:

1. In the MIT paper it indicates the spacing between flux for the magnets and coils (iron), this shows the reason for using more magnets than coils - the thinner magnets means less flat spots in the waveform that result in dead spots and so more efficient.

2. Magnets and coils shaped as wedges result in better performance due to the flux resulting.

 

It should be possible to use two sets of coils - one set of one side and one set on the other side of the magnets. The result is that you should get higher granularity. the key here is to have the magnets not backed by iron/metal but in slots instead. In fact having the magnets side on may produce a weaker field but more oscillations between North and South poles. However with more coils, the net effect is the same (although the more copper.. and less power results perhaps in less efficiency).

 

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On ‎03‎/‎06‎/‎2016 at 08:21, NickK said:

It seems metals are important and the temperature has a direct effect on the neodymium magnets commonly used -

Yes I've noticed some suppliers give temperature ranges for different grades of magnets.

Some info here (First4Magnets)

I've had some little magnets from the suppliers linked above - I've used mine for clamping thin parts together while gluing, etc. Nothing like the monster you're attempting though! They have some huge (and frankly dangerous) magnets on their list...

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