Help needed with an Engineering project

[Aaron Curran]

I'm not quite sure if this is the correct forum or not and I would like to apologise in advance if it is not, I'm new here!

So basically I'm in my final year at school, I'll be attending University in October all being well, but before I get there I have to sit my final exams, and where I am, in Ireland these exams are called the Leaving Certificate. As a part of my leaving certificate I chose Engineering as a subject in school and as a part of the assessment we are required to make a project to be submitted along with a written examination and a design folio relating to that project. But anyway what I'm here to ask is how do I make this project work. We are given a brief and have to make a project in accordance with that brief and this year's brief was in relation to theme parks and thrill rides etc, so anyway the making of the actual parts and assembling the piece in real life isn't the problem for me its the part concerning the motors where I am stuck. I'll attach a photo with this post of the design I have and if anyone could explain how to get that arm swinging around 360 degrees using motors that would be great, any input is appreciated. Thank you!

 

[Doug Huffman]

Rotate, roll, the red apparently flexible component with a motor secreted in the base.

 

[Aaron Curran]

Rotate, roll, the red apparently flexible component with a motor secreted in the base.

I'm not quite sure what you mean, the arm in the middle is the part which I want to rotate through 360 degrees, the red button looking thing beside the switch was intended to be connected to a potentiometer which would be used to increase or decrease the speed at which the arm rotates, should have cleared that up in the original post!

 

[berkeman]

Welcome to the PF.

Where does the motor go? Is there a straight axle through the middle of that top spindle?

 

[Aaron Curran]

Welcome to the PF.

Where does the motor go? Is there a straight axle through the middle of that top spindle?

I don't really know, I'm completely clueless when it comes to this. Could you maybe tell me how you would go about making it rotate, where you would put the motor etc?

 

[berkeman]

I'd probably build it into that spindle, although the spindle would likely need to get bigger. Use the center shaft of the motor as the axle, and use slip rings to couple DC power into the motor.

If you want speed control, you will need a pulse-width modulated (PWM) circuit to supply the power. You can do some searching and reading on the terms I've used to help you in your design project. Let us know what you find.

 

[Aaron Curran]

I'd probably build it into that spindle, although the spindle would likely need to get bigger. Use the center shaft of the motor as the axle, and use slip rings to couple DC power into the motor.

If you want speed control, you will need a pulse-width modulated (PWM) circuit to supply the power. You can do some searching and reading on the terms I've used to help you in your design project. Let us know what you find.

Could you link me to the type of motor that would be best suited for what I'm trying to do? Thanks so much for your help berkeman, I really appreciate it!

 

[berkeman]

Could you link me to the type of motor that would be best suited for what I'm trying to do? Thanks so much for your help berkeman, I really appreciate it!

Just do a Google search for Hobby DC Motors, and you'll find lots of good information. It's your school project, remember. :-)

 

[jim hardy]

Is that a real photograph or a computer rendering?
Is the "sewing spool" looking piece on top free to rotate on the red tube?
If so, might you stimulate it like a child's swing by a timed magnet in the base ?

 

[Aaron Curran]

Is that a real photograph or a computer rendering?
Is the "sewing spool" looking piece on top free to rotate on the red tube?
If so, might you stimulate it like a child's swing by a timed magnet in the base ?

It's a computer rendered concept so I'm not quite sure yet how it will be when actually built, it's an in an ideal world this is what I would have my project look like type thing but I will change whatever need be to make it actually work! Would magnets be capable of having it rotate through 360 degrees? as that is one of the requirements set out for the project is that it must be able to go through a full 360 degrees, thanks for your help, I like the idea of using magnets though, its different!

 

[jim hardy]

Well you could excite simple harmonic motion with a timed magnetic pull, either electromagnet or permanent magnets with orientation mechanically controlled...

a simple swing can be made to go completely around
though as a child i'd only pump until the chains started to go slack at almost horizontal...

 

[Aaron Curran]

Well you could excite simple harmonic motion with a timed magnetic pull, either electromagnet or permanent magnets with orientation mechanically controlled...

a simple swing can be made to go completely around
though as a child i'd only pump until the chains started to go slack at almost horizontal...

So say I were to use plastic for the base or a non magnetizable metal, I could then put a powerful magnet in there and implement a circuit that would change the polarity of the magnet which would set the swing in motion and then have the circuit timed so that the polarity would switch again when the arm is on the the way down such that the arm will get an extra pull downward rather than just its own weight and then this extra acceleration will help it on its way upward again where the polarity will again be switched to propel the arm away from the base, could this be done without much difficulty? and if so would the magnets in the base have to acting at a certain angle to the arm to ensure that it reaches its full 360 degree rotation?

 

[jim hardy]

could this be done without much difficulty?

That's the basic idea... it's not trivial to do.. What resources have you available? We don't know your skill level..
An electromagnet might be easier to implement.

 

[Aaron Curran]

That's the basic idea... it's not trivial to do.. What resources have you available? We don't know your skill level..
An electromagnet might be easier to implement.

I had an idea whilst waiting for your reply, theoretically I don't see why it couldn't work.
I put the images in spoilers to keep the post tidy.

Spoiler

The above image is the device in its 'off' state, really oversimplified drawing I know, but bare with me. Say we have a magnet mounted in the cart that's attached to the end of the arm, the blue things in the base are the magnets and the red lines represent the imaginary attraction/repulsion between the magnets. So as you can see with both magnets trying to repel the cart it will keep it steady, in a state of equilibrium, however if we flip the switch and allow current to flow into the magnet on the right reversing it's polarity there is no longer a second force present keeping the system in equilibrium and thus it should move off in the direction show in the second image below.

Spoiler

I know that reversing the polarity of said magnet with a current will cause the cart to be attracted to that magnet then instead and this could possibly pose problems, what do you think?
As far as skill level goes its pretty low in relation to circuitry, I have a decent workshop available to me at school and I also study Physics so I know a fair deal about Magnetism and motion etc

 

[BurnIt0017]

Hi, pulse motors are fun. This may give you some ideas. The motor does not work without the flywheel.

 

[Aaron Curran]

Just do a Google search for Hobby DC Motors, and you'll find lots of good information. It's your school project, remember. :)

The maximum power output of the project is limited to 9V, would a 6V Hobby motor be capable of rotating the arm with the cart attached to the end or would it be too heavy for it?

 

[jim hardy]

I'd been thinking of a permanent magnet (or two) that gets rotated or flipped .

There are very powerful hobby motors for RC cars and airplanes
i think they're 7.2 volt and battery packs are commonplace

Your electromagnets might work also

hobbyists are a creative lot, Sparkfun and similar sites have lots of gadgets

an old doorbell solenoid might be interesting... or the motor from an automobile electric door lock, they make plenty of torque for Doug Huffman's flexible shaft idea. .

 

[dragoneyes001]

getting it to spin around is nice but they did make your project about rides at an amusement park right? you might want to be a bit more accurate in the control aspect because humans have some pretty exacting limitations to how much force they can withstand. an electric motor would be much more controlled for speed and forces subjected.
in a small scale item you can use options which would be hard to scale up to actual size like using a flexible cable through the red tube to connect a motor inside the base to the spindle.(this would work for a time till the cable finally ate through the tube with the friction.its already been suggested to use the motor in the spindle. I'd simply suggest you take at least some time to determine just how fast the maximum speed this device can turn before its crushing the passengers into the seat.

 

[Aaron Curran]

getting it to spin around is nice but they did make your project about rides at an amusement park right? you might want to be a bit more accurate in the control aspect because humans have some pretty exacting limitations to how much force they can withstand. an electric motor would be much more controlled for speed and forces subjected.
in a small scale item you can use options which would be hard to scale up to actual size like using a flexible cable through the red tube to connect a motor inside the base to the spindle.(this would work for a time till the cable finally ate through the tube with the friction.its already been suggested to use the motor in the spindle. I'd simply suggest you take at least some time to determine just how fast the maximum speed this device can turn before its crushing the passengers into the seat.

I see where you're coming from here and making it as realistic as possible would be nice but they don't really take this stuff into consideration when marking and grading the project just so long as the actual design fits the brief in terms of theme, maximum dimensions, power output and well functionality of course. They don't give a rat's a** about how realistic it is lol. I had been looking up motors and came across this dc motor with a pwm based circuit that controls the speed and has a nice fancy LED display with it which would be nice and would probably get plenty of marks from the examiner, here's a link to it :

http://www.edgefxkits.com/speed-control-unit-designed-for-a-dc-motor

but I don't think that site ship to where I am and on top of that I'm not one hundred percent sure that it would actually work for what I need it for, what are your thoughts. Also, thanks for your help!

 

[dragoneyes001]

i meant you might want it to turn 4-5 times a minute as opposed to spinning at 50rpm where it would be obviously lethal to a rider. it would give you a benchmark to how strong the motor needs to be and what amount of power you want to use.

I'm concerned a magnet system would be much harder to adjust the acceleration IE 20G's at the start will be obvious even if it settles to a stable rotation

 

[Aaron Curran]

i meant you might want it to turn 4-5 times a minute as opposed to spinning at 50rpm where it would be obviously lethal to a rider. it would give you a benchmark to how strong the motor needs to be and what amount of power you want to use.

I'm concerned a magnet system would be much harder to adjust the acceleration IE 20G's at the start will be obvious even if it settles to a stable rotation

The circuit I linked in the last post has a fairly slow motor and the pwm is capable of slowing it right down to 20% of the original RPM, so say I were to get a 50RPM motor, I could get it down to 10RPMs with that which would only be a full rotation every 6 seconds which I don't think would be too bad as far as realism is concerned.

 

[dragoneyes001]

if its not obviously turning a rider into mashed potatoes in the cab your teacher will appreciate the results much more.

 

[berkeman]

Sorry if I missed it, but are you going to add a counterweight to the opposite side of the spindle from the suspended rod? That will reduce the power required from the motor significantly. Are you familiar with balancing torques/moments? That is how you will calculate the weight and shape of the counterweight...

 

[jim hardy]

Interesting , Greg
more moment of inertia will smooth out speed of rotation and lengthen period, helping relieve the "crush" effect mentioned earlier.