Author Topic: Is there a motor/drive system with intentional slip?  (Read 3341 times)

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Offline appsmanTopic starter

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Is there a motor/drive system with intentional slip?
« on: June 07, 2023, 06:08:50 pm »
I want to replace a clock spring cable retractor with a motorized spool. The retractor has to reel up 50 feet and any mechanical retractor I find is too uneven in force over that distance and also occasionally jams, which in unacceptable since it will be unattended. It seems like this could be done with a motor, and might even be cheaper than coil spring mechanisms which are about $200 for industrial grade versions. The idea is to create 1 pound or so of "drag" force on a spool as the wire is pulled out and let back in. The motor driving the spool would have to allow slip while still trying to reel in the cord. Any ideas what sort of motor/driver can do this?
 

Offline Gregg

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Re: Is there a motor/drive system with intentional slip?
« Reply #1 on: June 07, 2023, 07:40:52 pm »
Shaded pole AC motors can be designed to slip indefinitely; just producing heat.  They are often used in HVAC duct fire dampers where they are used to wind up a spring and stay powered to keep the damper open.  When power is removed, the spring closes the damper. 
 

Offline Benta

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Re: Is there a motor/drive system with intentional slip?
« Reply #2 on: June 07, 2023, 07:47:17 pm »
Seems like an obvious job for a DC motor with brake resistor.
When pulling out the cable, the brake resistor will provide mechanical drag/resistance.
When pulling in, current limiting will limit torque if the cable jams.
 

Offline Infraviolet

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Re: Is there a motor/drive system with intentional slip?
« Reply #3 on: June 08, 2023, 12:26:17 am »
Sounds like any motor type able to cope with long term continuous stalls would be suitable, using a gearbox if necessary to increase torque up to the "pound" (torque = pound * what radius) you want. I am guessing you'll run the motor in the retracting direction at stall "all"* the time, if the force pulling the cable out reduces below that which the stalled motor provides then the cable starts retracting. You could also perhaps supply a higher voltage to the motor at times when you want a higher stall torque, to actively pull the cable in, and a lower voltage when you only want the stall to serve to apply force (for plenty of motor types the stall torque, and stall current when stalls occur, rise with the voltage supplied). Some types of BLDC motor could be suitable, especially with the use of field oriented control electronics to drive them, this site has some exmaples of FOC with BLDCs https://odriverobotics.com/ you might not need something so high performance though. Unless I've misinterpreted your requirement slipping isn't really involved anywhere, just stalling where the motor applies enough force to keep the cable tensioned but not enough to resistwhatever force on the cable end is causing it to extend.

*you might have some logic which tells it not to bother at times when the cable is already fully retracted as determiend by a sensor somewhere
« Last Edit: June 08, 2023, 12:28:43 am by Infraviolet »
 

Offline Circlotron

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Re: Is there a motor/drive system with intentional slip?
« Reply #4 on: June 08, 2023, 01:41:38 am »
 

Offline f4eru

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Re: Is there a motor/drive system with intentional slip?
« Reply #5 on: June 14, 2023, 09:55:01 pm »
Yeah, pretty much any motor with a current limit is good to go.
Direct drive is helpful if shocks and intermittent loads are in the load.
BLDCs have to be overrated if blocked for a long time, because heat is not even between the 3 windings if it does not move but applies torque.

One simpler way to do is to disconnect the spring, and connect directly close to the escapement wheel.
I did this for a clockmaker many years ago, with fine BLDCs, if interested to buy, mail me.

https://www.yumpu.com/fr/document/read/17258053/les-horloges-dedifices-michel-bourreau

Offline Smokey

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Re: Is there a motor/drive system with intentional slip?
« Reply #6 on: June 15, 2023, 02:53:33 am »
Current control IS torque control.  Just pick a controller that has current mode control and you can use any matching motor with appropriate spec, DC or ac.
 

Offline Infraviolet

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Re: Is there a motor/drive system with intentional slip?
« Reply #7 on: June 15, 2023, 04:43:08 am »
"can use any matching motor with appropriate spec"
Probably not healthy in the long term for DC brushed motors though, even if you limit the current to that which they are rated to run at long-term. Simply because brushes/commutator could be damaged if sitting stationary at exactly the wrong relative angle while current is flowing.
 

Offline Berni

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Re: Is there a motor/drive system with intentional slip?
« Reply #8 on: June 15, 2023, 05:27:51 am »
It is possible to use a DC motor for this, but the current has to be limited to way below rater current.

The motors are designed to have cooling from the moving rotor, so in a continuous stall you will be able to use only a fraction of the full current, so you might have to over spec the motor a fair bit. You would also have to limit the voltage so that a bad spot on a commutator won't start glowing hot.

But the proper way to do is are indeed shaded pole AC motors. There the slip happens completely magnetically and they will happily run with a locked rotor all day and night. The only moving part is the rotor, so no commutator to wear out or get caked in dirt or tarnish. Tho the torque they produce is not very high so you will most likely need a gearbox.
 

Offline dietert1

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Re: Is there a motor/drive system with intentional slip?
« Reply #9 on: June 15, 2023, 08:32:05 am »
Or one can use a timeout once the motor blocked. This is what they do in electrical gates. If the gate doesn't arrive at its final position within 30 seconds, they assume there is something wrong and they turn the motor off. They have a mechanical safety clutch as well, so the motor won't burn but the clutch.

Regards, Dieter
 

Offline Smokey

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Re: Is there a motor/drive system with intentional slip?
« Reply #10 on: June 15, 2023, 06:53:59 pm »
Any good motor controller (with current/torque mode control) will have a programmable continuous current limit.  This limit is intended to keep the motor from overheating.  That's it's job.  So you pick a motor that's rated appropriately for the job, and set the continuous current limit to protect it. 
If you get a decent motor, it will also have an internal temperature sensor, which the above mentioned good motor controller will be able to measure and incorporate into the protection of the system.
 

Offline Doctorandus_P

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Re: Is there a motor/drive system with intentional slip?
« Reply #11 on: June 15, 2023, 09:30:12 pm »
Have a look at the video linked below.
It uses a weight on a string to run a clock, and every now and then the weight is rewound, and it uses a differential gear

https://hackaday.com/2023/06/07/billion-year-clock-is-lego-genius-or-madness/
 


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