DC Motor Locking Position?

Question

I'm making a miniature elevator project for a logic design project, and I need a dc motor lifting a box to be the model. The circuit is based on basic logic and 555 timers to run the motor with an H-bridge.

But the problem is when the motor is lifted up and the timers are off, it will go in the other direction because of load. I need some way to KEEP IT LOCKED in the position when it's not running. There was something about "worm gears" that are to give proper torque on low speeds, but will they lock the box in place at any height? And what about reverse polarity? Do they go in the other direction, too?

I'm sorry if it comes off amateurish. I'm an electronics person, and have no idea about mechanical motors and machines. Any help with be appreciated, thanks. :)

Answer 1

In theory a gear train will operate when driven from either end. But worm gears are pitched so that the pinion gear, which has its axis at right angles to the drive gear, is pitched so that it cannot be effectively driven by the large drive gear. Now when you look at the gear train for a worm drive, the smaller diameter pinion, or worm, looks more line a threaded screw, but it can turn the larger drive gear, I don't remember if this is the correct terminology or not. But you can drive in either direction so it can raise or lower without a problem. The advantage is that the reduction ratio of worm drives is high so that the large gear turns slowly and the pinion turns fast. The torque multiplication is high and it is an effective method of doing what you want to do. And when the motor stops the elevator will stop as well. Yes, it will do what you want to do.

Answer 2

OK, I do know the answer to this. The H bridge is fine (necessary). But, you are way off beam using a 555. I would be using a small microprocessor to control the bridge. I happen to use the PICAXE series of micro-processor as anyone can start using them almost immediately - and no special burners needed - just plain old RS-232.



With a controller chip you can do far more - eg timing events, sensing position, measuring speed (of the motor if you so wished) and the list goes on. The smallest guy is an 8 pin which can be clocked at 8MHz Plenty for what you need) but have 14, 18, 28 and 40 pin devices with multiple inputs and outputs and clockable to 64Mhz. I would not even consider a 555 in this application.



The counter-weight idea goes without saying; you place far less load on the bridge (less heat sinking needed).

Answer 3

If this is just a table top working model, then add a counter weight that goes down when the elevator car goes up and vice versa.



Another option would be an electromagnet, just a coil placed on one edge/ corner of the elevator car/ box.

If one of the four guides that the box rides in is a piece of steel, you can wire the electromagnet to be on/ powered, when the elevator motor is "off"

This will hold the box stationary against the guide, unless you remove all the control power.

Cycling the box up or down will turn of the electromagnet and release the box to move.



Wingman

Answer 4

Purchase a motor with a brake.