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non-continuous torque transmission

hi all I'm having a problem in a device. its a centrifuge in fact it has to work in two deferent speeds : 10000 rpm to perform its main task and a speed of that small that can simply be controlable to locate the rotor in desired angular locations. for first speed I am going to use a high speed DC motor but because of the accuracy of the angular positioning in the second step I have to use a stepper motor to turn the rotor. so stepper motor will be connected to the shaft of the rotor by some kind of for example timming belt. whenever we need the accuracy we will turn the rotor by stepper and the main high speed motor will be off but when we run the high speed motor the stepper has to turn because they are in fact connected through timming belt . the problem is that the stepper and timming belt would not tolerate this high speed of about

10000 rpm. the idea is to cut the torque transmission to the stepper when main high speed motor is running . but clatch is not acceptable because of the slipage in clatch that will destroy the accuracy we need. anybody has any practical Idea or some similar experiences? thanks in advance all replays would be appreciated regards , Shantia
Reply to
shantia
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"shantia" wrote in news:1148341542.587579.79500@

38g2000cwa.googlegroups.com:

Some additional details (size, hp, positional accuracy needed etc.) would make it easier to visualize, but here are a couple things you can think about.

Since you have the DC motor, if you put an angular position feed back sensor (absolute optical encoder, hall effect sensor, laser reflection targets) on the centrifuge, you can use a different motor controller rather than a different motor to jog to position. Simple proportional feedback control should be plenty.

Feedback control also eliminates the problem with the clutch in your stepper motor solution. In that case, you don't need full position feedback, just a registration mark. If you know 1 fixed point on the centrifuge, you know how many steps to your final position.

For that matter it just occured to me that if your timming belt is a 1:1 drive, you can make a clutch that stays in register. Think of the clutch being two disks, 1 drive, 1 driven. The drive disk has 2 pins, 1 pin on each of two center distances. The driven disk has matching holes. As the drive disk pins slide into the driven disk, it always registers. That means it is also registered with the centrifuge.

Reply to
Charly Coughran

One option is the type of mechanism used to connect a starter motor to the flywheel in a car, but instead of using a spring to engage the gear, you can use a solenoid. That way the stepper is geared to the main shaft when it's running, but isn't connected when the solenoid is off and you can let the high-speed motor do its thing.

There are also very good one-way clutches used in automatic transmissions, the backlash is functionally zero, so unless you're positioning accuracy needs to be extremely high I don't see why you can't just use that. When the main motor is off and the stepper turns the one-way clutch, it will rotate the shaft. When the main motor is on the one-way will slip and the stepper won't turn.

Tom.

Reply to
Tom Sanderson

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