Great deal on a 3 axis gimbal with Alexmos controller

Just had to tell all of you about this $59.95 three axis gimbal deal on EBay:

This seemed like a great deal and looks to be identical to DYS Gimbal on Hobbyking which is $200.00.

I got one and quality appears excellent though I haven't tried to use it yet (I will probably get another.) 

I'm planning on converting to a Storm 32 for a laser rangefinder scanner.

Apparently DSLRPros has moved on and is wholesaling them out.

http://www.ebay.com/itm/DSLRPros-3-Axis-Gimbal-for-the-GoPro-3-3-an...

I get nothing from anybody for this, but if you want a real 3 axis gimbal, this is the best deal you'll ever see.

Heres a link to the manual for the DYS: http://www.getfpv.com/media/wysiwyg/documentation/3AxisbrushlessGim...

Heres a link to the Basecam (Alexmos) Home page: http://www.basecamelectronics.com/

And a link to download the appropriate 8bit Alexmos Gui: http://www.basecamelectronics.com/files/SimpleBGC_GUI_2_40b7.zip

And a link to download the Alexmos manual PDF file: http://www.basecamelectronics.com/files/v10/SimpleBGC_manual_2_4_en...

(This does include the Alexmos 3rd axis expansion board.

Best regards to you all,

Gary

Views: 5864

Comment by Laser Developer on October 9, 2015 at 10:54pm

+1 Gary. From our analysis it seems the biggest limitation of the Alexmos clones is that they are changing the PWM values (the equivalent of setting a position) under software control. In a constantly rotating or accelerating system the timing of the PWM updates is erratic and leads to less than optimum waveform generation. Even a very small timing error, the equivalent of one or two software instructions when entering the interrupt routine, is enough to disrupt the phase relationship of the magnetic fields, dramatically reducing the torque. It's like a petrol engine that is misfiring.

We were able to create a more precise timing system using FPGA hardware and nearly doubled the available torque. Surprisingly, this brings the torque and speed performance of the gimbal motor to the same or better than a stepper motor of the same weight and current consumption. This doesn't make the absolute positional accuracy any better as this is a function of the number of poles, but getting more than 2000 steps per revolution is quite possible.

Comment by Euan Ramsay on October 10, 2015 at 1:23pm

ok...so inherent brushless limitations aside, who has actually managed to get this gimbal working with a go pro, to an even acceptable level?

Comment by Chris on October 10, 2015 at 1:41pm
I had it working pretty decently as a handheld gimbal, the video I posted before was before some tweaks to make it smoother, but I saw the setting posted and changed to them to try to get it working on my Hexacopter and It ended up messing t all up, I swear I took screenshots before I change it but I didn't label which one is which, so now I'm back to where everyone else is again, not working. I mighty try to work on it when I get the time but for now I haven't had the time.
Comment by Gary McCray on October 10, 2015 at 5:12pm

Hi LD,

That makes perfect sense.

I really like your analogy to a petrol engine misfiring, if the PWM isn't updated at the right moment or an edge timing is off, that is exactly what it would be like.

I made my own optical encoders years ago with a decode based on straight flip flop logic and there was always uncertainty during reversing and max speed was set by the characteristics of the TTL logic.

One of the problems with our current Ardu, Android and Linux everything methodology is that everybody is solving hardware problems with software.

As you point out that can be really sub-optimal and at least requires a really strict RTOs to make it work on a heavily time dependent system.

I can easily believe a dedicated FPGA could greatly improve things.

Of course for stabilization positional accuracy isn't so critical because it is a closed feedback loop with the IMU and I would suppose it is still reasonable to produce good positional accuracy based on offsetting the IMU coordinates at least within the limitations of the Gyros on the IMU.

However, I still think the brushless gimbal motors themselves are really a kludge, basically a under designed stepper that is being fooled by the analogue equivalent of microstepping into behaving roughly like an AC or  DC servo motor, using it's dedicated IMU for maintaining absolute position.

These are really just rewound normal hobby grade brushless motors and they make much better brushless motors than they do gimbal control motors.

I favor top down design and this is really one of the most blatant examples of simple adaptive bottom up design I have ever seen.

The fact that they managed to make them work has caused them to propagate everywhere, but the basic concept is heavily flawed and the control motors themselves really need to be seriously rethought.

The method of using them closed loop with the gyros, seems excellent, but even there as you point out, you really need a robust, fast and precise feedback loop to make it work properly and imprecise software timing certainly isn't it.

One of the problems we have in this business is that some guy at home manages to cobble something together that sort of works and immediately everybody promotes it and copies it.

Not to in any way detract from AlexMos, but that should have been the start, not the end.

As for Chris and Euan,

I do think you can get this to work OK with a GoPro, probably more easily in 2 axes than 3 and at least somewhat due to the increasing software timing problems dealing with 3 axes versus 2.

I would strongly suggest, aside from a lot of finicky "tuning" of PIDs and ramped up current during actual moving, that you do as good a job of anti-vibration mounting of the gimbal as possible.

The Storm32 uses a faster more robust controller and ostensibly provides more robust control but I haven't had a chance to hook up mine and try it yet.

From what LD has said, it does seem like it might help however and I will give it a try when I get a chance.

Best Regards,

Gary

Comment

You need to be a member of DIY Drones to add comments!

Join DIY Drones

© 2019   Created by Chris Anderson.   Powered by

Badges  |  Report an Issue  |  Terms of Service