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Guest Merlin-Brasil
Posted
Hi, Looking for some direction and there's a *lot* of experience here, so I thought I'd lay it out and see what the geniuses have to say. As I see it, this is a relatively simple problem in motion control. I have two cylinders. The outside cylinder provides support for the inner cylinder and is marginally wider.... approximately 2-3 feet in diameter. The inner cylinder needs to rotate 360 degrees in either direction, in 1/2 degree increments. The weight, firmly attached to the top of the inner cylinder is assumed, for the moment, to be 500-750 pounds. In addition to the weight, there can be a variable and significant torque which would also be transferred to the inner cylinder. I'm figuring to use stainless steel ball bearings under the lower cylinder to reduce the frictional aspect. Similarly, I'm figuring that bearings could be used to slightly separate the inner and outer cylinders for the same purpose. I need to be able to rotate the inner cylinder incrementally in either direction or be able to apply a "go to" command to rotate the cylinder to a specific degree ("go to 94.5 degrees). Speed of rotation isn't critical. I also need to be able to 'order' the system to let the inner cylinder 'float' for long periods of time, allowing it to settle wherever the torque takes it. That's the basic spec for the problem. Stepping up one level from this is the controller. The real-time control would be applied via a control panel, but should also be able to accept commands from a computer. There will be TWO identical cylinder setups as described above. They need to be operated individually and in tandem. In some cases, both will be rotating incrementally in precisely the same "direction" and, in other cases, either both might be "free" or one under precise control while the other is "free". I have a fairly healthy supply of 24VDC with which to accomplish the above. Beyond that, one of the requirements would be that a manual system be in place should the automation fail and the cylinder need to be turned by crank or somesuch. For the sake of efficiency, it might be possible to apply a "brake" to the inner cylinder so that, when it's in a fixed position for a long time, no energy is being used, and the brake handles the torque problem. When a direction change is needed, the brake would release and the motor would take over. First, is this as simple a concept as it seems? Second, given the problem as stated, does it sound like something could be done with off-the-shelf components? Lastly, can you point me in the right direction to bring the above design into reality? (vendors for the different components, recommended controllers, etc.) I'm an experienced programmer, with some exposure to robotics and automation a long long time ago. But this is the first real-world, real-time project in this vein I've undertaken. Best regards, M-B
Posted
How fast do you need to accel and decel? What will the top speed of rotation be? How much torque is this "significant torque" that will occasionally be transferred to the inner cylinder? If you can handle gentle movements, without knowing all the details, it sounds like something that could be done with a standard ac motor with brake, gearbox and inverter drive. For position feedback, a simple encoder setup would do it. 1/2 degree is not too hard to achieve. I've done a few setups with 1/10th degree positioning using some pretty rough components. Jim Rowell
Posted
I've done couple of applications very similar to what you describe but the weight of the payload was in different range (you have to size your axis to handle the load) and sped and precission vere critical. Last two jobs like that I had to use Mitsubishi. They all work great. The speed and accuracy ware way better than requested by customer, you can fully control the speed, position and torque - or let it spin free if you choose so. I was using servo and motion controller (my devices had to do few more things, speed and precision are important and my customers don't shy from spending bucks on servos). But from what I read here I would agree with Jim on using motor, brake and VFD. Be carefull when selecting gearbox because too high ratio will pretty much hold your moving cylinder in position. Also be carefull with the AC motor, if you run it to slow, it will overheat. Maybe you can provide separate motor that runs at full speed to cool your main motor down (this one can be smaller in size)...
Posted
The most critical part of designing a motion control system is to get all of the mechanical data necessary for correctly sizing the motor, drive and gear train. Get that right, and programming will be a breeze. You need to decide how fast the moves need to be and work out a worst-case sequence for the axes. You need to keep the ratio of reflected load inertia to motor rotor inertia as low as possible. If the ratio is too high, you won't be able to tune the system to give the accuracy that you want - either it will oscillate (hunt) around the desired position or it will stop somewhere near but outside your tolerance, depending on loop gains. There are tools to help work these things out - I'm most familiar with Allen-Bradley (Rockwell Automation)'s Motion Book software which you can download from their web-site for free.
Guest Merlin-Brasil
Posted
Hi Jim/Panic/Gerry, Thanks for the responses. Now that I know the data to look for, I'll proceed to educate myself as best I can. Eventually, I'll be able to ask smarter questions as this project spec comes together :) Speed of rotation isn't critical. 360 degrees in a minute or less would be tolerable. It would seem that I need to nail down the weight factor as well as the possible torque factors to arrive at the optimum (or even workable) solution. I'm glad to see that the problem I present seems more or less trivial to you all.... that's encouraging :) Thanks again for the support. Best regards, M-B

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