Belt drive for large diameter lazy susan platform

If you have an encoder on the platform (final load) then I would think that it hardly maters what your drive looks like because the control computer will adjust as needed based on the actual position. Assuming you can get the accuracy you need from you encoder.
 
Makes sense about the encoder tied to the platform. I'll need to tightly couple that to the platform. Sure would be easier if I had a center axle but I will work out a good encoder coupling near the perimeter
 
Wheel encoders are available if you want to drive the encoder at the perimeter.
https://www.ttco.com/encoders/wheel.html

Might also pick up an ebay shaft encoder and fit a hard rubber/urethane tire to it and have it ride on the belt OD. At your speeds it shouldn't slip, even with low pressure (to keep loads off the encoder shaft) and with the huge table OD you'll get a very large effective PPR even with a low-PPR encoder.

You could use a servo, but that seems expensive for this application - especially if you have a load-coupled encoder. I'm sure someone could suggest a controller to drive a simple DC motor and use the load encoder for feedback & PID to hit your target angle.

As for the inside out belt glued on, you can purchase open-length belting online in just about any profile and pitch/width. I was about to suggest HTD or GT2 profile, but the inventor of the Bell-Everson drive system indicated that trapezoidal teeth meshed better when run tooth-to-tooth (vs. tooth to pulley) than did the curvilinear profiles. I don't know if this holds true for tooth profiles bent back on themselves, but it's worth a look.

I do see that B-E is using an actual toothed pulley for their rotary tables and not attaching an inside out belt to a blank hub. I'd speculate that an inside-out belt profile didn't work out for them, but your precision requirements may be less stringent.
 
If Reddinr is planing on maintaining +/- 0.5 degree over 2000 revolutions he will need a mechanically locked system. Friction coupling between the drive and platform (like mine) or the friction encoders you linked to will not work. Maintaining +/- 0.1 degree over one rotation is even unlikely.

Ken
 
KMoffett said:
If Reddinr is planing on maintaining +/- 0.5 degree over 2000 revolutions he will need a mechanically locked system. Friction coupling between the drive and platform (like mine) or the friction encoders you linked to will not work. Maintaining +/- 0.1 degree over one rotation is even unlikely.

Ken
Click to expand...
You're absolutely right. I missed his post about the accuracy requirement. Ixnay on the friction drive.

Back to low-backlash belts (AT, GT2) or a direct encoder mount on a shaft.
 
Add a self calibrating feature.

Simply place a "switch" of some type at a zero position.

Type depends on your accuracy needs but the operation is simple.

At any time the table is assigned to the zero position it must trip the switch.

Code can be written to hunt for it on approach meaning it is looking for position from wherever it is and as it approaches where it thinks zero is it now looks for the switch and modifies a "calibration factor" which is how the controller compensates for any slop or stretch.

Secondary can be "in passing compare" meaning the controller should know where the table is at all times and whenever the switch activates the controller compares its position.

Easy theory code not so much but a challenge.

Sent from my SAMSUNG-SM-G930A using Tapatalk
 
Forgot a comment...

Without a positive feedback the system is "open loop" meaning the controller assumes table is someplace and moves it a given amount to get to someplace else.

An encoder directly on center of turntable would be "closed loop" in that the controller knows exactly where the table is.

Having the encoder any place else allows errors thus opening the loop by factor of errors.

The zero position switch above closes that opening a bit in that it allows the controller to test position with positive action.

Sent from my SAMSUNG-SM-G930A using Tapatalk
 
Thanks everyone. I think my drive problem is in hand now. I will be using a toothed belt and toothed pulleys. I now need to figure out a good coupling for the encoder. I had a friction wheel in mind but I agree that might lose registration fairly quickly. I'm now thinking of coupling the encoder to the belt drive loop and just making the belt a bit longer. I think that would achieve the accuracy I'm looking for. For sure I'll have an index switch.
 
Easy to add an encoder with a tooth pulley to the drive belt. But, what encoder? For 0.1 degree accuracy means you are going to need 3600 counts per revolution of the platform. Tooth ratio between the platform pulley and the encoder pulley becomes critical. TQ60's suggestion of a " zero position" switch will help in long term accuracy. I would suggest an optical beam-break switch. What type of motor do you intend to drive the system with? Stepper? Permanent magnet DC?

Ken
 
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