Gearing question

A gear train will also tend to hold the mechanism stationary between rotations. A stepper motor turns a precise amount with each pulse, but it's an expensive solution, and would require a pulse generator and counter. This device will be operating in a dirty environment, so you should protect the motor and drive train from crumbs and drool, and develop enough power to overcome friction without turning your feeder into a trap.

Sounds like a good student project, plenty of challenges, and a chance to develop an ingenious solution to the problem. Good luck!

How about a wound spring to turn the food compartment and an escape mechanism that allows the compartment to turn a specified amount. The spring can be as powerful as you need and the escape mechanism can be powered by the motor.

This is correct small electric motors are not designed to be loaded excessively and rely on high speeds to perform. This is fine for fans etc but for your application I would suggest a small geared motor Baldor and other manufacturers make a range of geared motors one of which would be suitable for your application. I suggest you contact them and explain what you are trying to acheive. They are very helpfull

Why not try a Geneva Rotor mechanism if you have room to do so? They are not hard to fabricate and do not take up much room. The gearmotor does not have to be so precise with its stopping position. Look it up with Google and you will see what I am talking about here. Vending machines have used them for years.

Best Regards,

Ing. Robert Forbus

Try a stepper motor spec'd out with enough torque to turn your maximum food container weight. If you find that the motors you find are running at a too high of an rpm than add a gear reduction box to bring down speed to what you think is safe/correct for your application. Motor timing can be controlled by a simple PLC circuit with a timer.

Good Luck, let us know how it works.

It sounds as though the amount of torque you need is low. Can you define it by measuring the inch pounds required to turn the shaft at the speed you want? For simple prototypes you may not need an industrial level motor and drive train. Hobby catalogues for building stuff have things like sub-fractional HP motors with nylon or even plastic gears that are pretty cheap. To get a low speed output for low cost you may consider worm gear boxes. They usually stop quickly when power is taken off the motor and they typically will not back drive. For a little more money Grainger and McMaster Carr have fractional and sub-fractional HP drives with somewhat of a selection. These industrial catalogues usually give accurate torque information. For the ultimate in cheap do you or your classmates have a low speed toy car (or tank or bulldozer since they have a high reduction ratio) that you would want to steal the hardware from? My sons and I have built fun things from this kind of stuff.

you didn't mention budget, a cheap solution would be to make your own pulleys and make a drive belt from an old innertube cut to length and joined with superglue. You could use a face cam operating on microswitches for location, switch on with the timer and heavy or light the face cam stops the dispenser at that location. (You would have to use simple logic or it will never get going again). Only an idea.

Look at these gear motors from All Electronics

The motor, gearing, and built-in position-sensing stop switch from a windshield wiper drive may be all that you need. As long as your "start" signal is short compared to a one-revolution operation at the output shaft [NOT the wiper-arm shaft, which typically rotates through an arc, using levers & links] you should be in good shape. A lot of rear hatch wipers do single-sweep operations almost exclusively, so should be especially suited to this duty.

I'm having a mind blank on what its called but there is an indexing cam used on a lot of CNC milling machinges to switch tools. I had a similar problem in school and used one of those and it worked great. Its some kind of 'yok' I think...

Anyone know what i'm talking about?

Tower hobbies sells these little gearboxes that run on little DC motors. They have many to choose from, and they all are quite cheap. This particular one has two gearing options 41.7:1 and 64.8:1 meaning that every time the DC motor rotates its shaft 64.8 times or 41.7 times (whichever gearing you choose) the output shaft will turn once.

I have put one of these together before and it is very surprising how much torque at low speed you can get through the gearbox with the little DC motors. You may have to figure out the maximum amount of force it will take to dispense the food, and then see how much torque you would need to open the door, or however you have the thing rigged up.

Anyway, here's the link to one of the gearboxes. Tamiya makes them, and they can be found under the science and hobby link on towerhobbies.com

http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXHA28&P=M

David

May you can use a power lock solenoid. It push/pull a fixed distance.

Same idea you can get car wiper/sunroof motor with gearbox for cheap in junkyard. Sunroof one would be best because it has limit switch to stop the motor at certain location.

Pineapple

Thanks for the suggestions everyone.

We decided to try using a very low RPM 6V-12V gearhead motor (Rated around 3-6RPM) since it's cheap and hopefully easy to integrate, though it's going to take painfully long for it to arrive since there were no stores locally with those in stock.

>>It sounds as though the amount of torque you need is low. Can you define it by measuring the inch pounds required to turn the shaft at the speed you want?

We're estimating that the revolving disk that holds/dispenses the feed shouldn't weight more than 2lbs when full, and we're planning to run the motor at about 3RPM. Not sure about the torque of the motor other than that it claims to be "very high torque". We're hoping it's high enough so that the maximum load we'll need to place on it won't have any noticeable impact on the speed, since we'll want the motor to be fairly precise in the distance it turns over a short period of time when activated by the timer, regardless of the weight it's pulling (<2lbs). It'll be a learning experience in any case..

There's something called a "drum timer" which has cams and switches on it. When the switch (typically a microswitch type) actuator falls into the groove, it breaks the circuit to the motor which turns it, and it can either be used in conjunction with a dispenser motor or used directly as the dispenser motor. Another drum timer or electronic timer used as a feeding interval timer can be wired so that it starts the drum timer for the food dispenser. This way, you won't have to rely on precise speed and timing of a gear motor.

This explanation is not clear if you aren't familiar with stuff I have mentioned, but it should give you something to find out about and consider in your design.

Just an additional though. If your "revolving disc" performs some kind of escapement or gating function, the majority of the required input torque could be as a result of this function and not due to turning this light weight device. Maybe some simple friction calculations could be made for "dragging thru the dog chow". Of coarse I'm just guessing since I don't know the configuration of the device.

Mark24:

Maybe you can post a video of your machine during testing. I'm sure we would all like to see your results.

Good Luck and let us know how it goes.