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# how to select a DC motor to rotate a platform to compass headings

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#### EHTDesigner

##### New Member
Hello All!

I'm embarking on a project and need to select a motor. As i watch several you tube videos on DC motors....

My task is to rotate a 1/2" 9'x5' 200 lbs (think ping pong table size) thick aluminium platform to a compass heading- and hold there. As a boat maneuvers, the platform must point to the compass heading. I'll have micro controller input to determine where to position the platform. Speed of rotation would be retaliative to the speed of the changing heading of the boat. So, the motor should be quick. The rotation direction would be CW or CCW. Power 24 volts, although could be higher if necessary.

The motor should also be able to hold the platform in position. I've been studying DC Braking motors to see if they'd apply.

The motor would be mounted "vertically" below the center of the platform.

So, what do you think, what references should I review for this application.

thanks.

You have to calculate the rotational inertia of your rotating table. It really matters where on the table the weight is distributed. That is, how far the weight is from the center. Then you need to specify how many degrees per second you want to move the table from a dead stop to a given position. Or, if you expect it to happen, from full CCW speed to full CW speed to reach your desired position.

you'll also need info on friction because, some cheap hard casters on a plywood substrate move differently than steel wheels on a thick steelplate surface.
This can be a very challenging problem.

also, a direct drive as you suggest will require a huge motor - you might want to put the motor near the edge and add a belt to gear it down so you can use a higher speed motor.
Finally, an optical encoder of some sort will let you know where a reference position on the table vs the bow of the boat and you'll need an electronic compass to let you know where your table is oriented vs magnetic North.
Good luck.

That is a big table. I would suggest a "lazy susan" type bearing and a large ring gear that you can drive with a motor. The size of the motor depends on the inertia of the table, the acceleration required and the maximum speed. The simplest way to maintain the same heading is to mount a compass module at the center of the table and somehow remote power it (slip rings, inductive loop) and use something like bluetooth to communicate with your micro. Compass modules are both cheap and small - the little black dot on that board. However, you'll probably need a second micro to read the compass and send the data to the bluetooth module.

Mike.
Edit, for clarity, I'm assuming that the boat heading doesn't matter - only where the table is pointed.

The fact that the table needs to stay on a static heading when the boat heading changes should make the inertia problem a bit less at the table will not be changing it's heading with respect to the earth. It is the boat that is rotating beneath it.

Les.

the table will not be changing it's heading with respect to the earth.
So then the main factor in determining the motor drive arrangement is bearing friction. Ideally, with frictionless bearings, the table wouldn't need to be driven at all .

Maybe just a big gyroscope.

Mike.

If your on a shoe string maybe you could re-purpose something like a 24v tailgate lift motor from a hydraulic tail lift on a vehicle.
Parvalux make geared motors that might work, but you'd have to use an inverter as they are mains powered.
If you use a PID controller maybe an arduino you probably wouldnt need a brake, there are some example son the net about converting a wiper motor to a rc servo with an arduino, you might be able to adapt thi to work.

Maybe just a big gyroscope.

Mike.
If the boat stays flat at all times, it could work. If the boat is pitching/rolling, then the gyro will create a torque and cause the table to oscillate left/right of North Pole with each wave.

If the boat stays flat at all times, it could work. If the boat is pitching/rolling, then the gyro will create a torque and cause the table to oscillate left/right of North Pole with each wave.
A ginbal helps.

there is such a thing, a gyro compass.

By my calculation the weight of the table is 316 lb, not 200 lb. I assume you'll have things mounted on the platform too, so that weight of that needs to be added.

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