### Author Topic: Help needed : Simple harmonic motion demo  (Read 1072 times)

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#### lengchai

• Jr. Member
• Posts: 9
##### Help needed : Simple harmonic motion demo
« on: June 03, 2011, 06:23:52 AM »
Hi all,

I am trying to design a "robot" that can sustain the oscillation of a load attached to a spring.

Here is the problem statement :-

1) The spring is attached to a load. The load varies between 5kg to 15kg. However, the load does not change during the oscillation, only before it starts.

2) The load might get some "disturbance" during the course of its oscillation, e.g. some naughty fingers poking at it.

3) The purpose of the "robot" is to :-
a) start the oscillation
b) control the frequency of the oscillation
c) control the excursion (swing distance) of the load
d) prevent any unwanted resonance / instability
e) stop the oscillation

Here is what I have so far :-
- use a DC motor rotating uniformly. Attach a disc to the motor and a magnet which is off centre.
- on the spring-load assembly, atach another magnet.
- put these 2 magnets close together and hopefully, the one on the motor will gently start the swinging of the spring-load and will reach an equilibrium after a while.

Thanks.

#### lengchai

• Jr. Member
• Posts: 9
##### Re: Help needed : Simple harmonic motion demo
« Reply #1 on: June 03, 2011, 06:35:56 AM »
There was an idea to use a motor+cam from "mstacho".

Since posting I managed to see something that might fit the bill ... but do not know how it is done.

If you look at baby swings, e.g.
http://www.amazon.com/Bright-Starts-Comfort-Portable-Biscotti/dp/B002VBXY68

then instead of a forward-back motion, I am sure it can be made to go up/down.

Anyone know what goes to the motor drive unit of these swings ?

Thanks.

#### Soeren

• Supreme Robot
• Posts: 4,672
• Mind Reading: 0.0
##### Re: Help needed : Simple harmonic motion demo
« Reply #2 on: June 03, 2011, 01:04:33 PM »
Hi,

I am trying to design a "robot" that can sustain the oscillation of a load attached to a spring.
How's that a robot?

3) The purpose of the "robot" is to :-
a) start the oscillation
b) control the frequency of the oscillation
c) control the excursion (swing distance) of the load
d) prevent any unwanted resonance / instability
e) stop the oscillation
d) may need to be solved by a large blade swinging around the spring and load - to cut off any interfering fingers

Here is what I have so far :-
- use a DC motor rotating uniformly. Attach a disc to the motor and a magnet which is off centre.
- on the spring-load assembly, atach another magnet.
- put these 2 magnets close together and hopefully, the one on the motor will gently start the swinging of the spring-load and will reach an equilibrium after a while.
If you want to sustain the natural resonance of the spring+load, you shouldn't try matching up a motors revs!
Instead, make use of one of the features of resonance and push it in sync - just like you'd push a child on a swing.

Detect when eg. the load bearing plate is near its maximum (or minimum), either optically or by a Hall switch (detecting the magnet on the plate and fire a short pulse from an electro-magnet to dispell the permanent magnet, to build and then sustain the oscillations - if you use a linear Hall device (instead of a switch), you can use the output sense the magnitude of the swing and adjust the amount of power to the electro-magnet to keep it within bounds.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Engineering is based on numbers - not adjectives

#### lengchai

• Jr. Member
• Posts: 9
##### Re: Help needed : Simple harmonic motion demo
« Reply #3 on: June 06, 2011, 01:51:13 AM »
Hi Soeren,

Thanks for posting.

- Robot Definition : Well, this is kinda grey ... it has a task, there are certain "disturbances" that it needs to deal with and it needs to do this without human intervention. In my books, it is the simplest form of a robot.

- I would love to have swings and blades to stop interfering fingers, but unfortunately it won't pass health and safety !

I'll look into the Hall switch idea ... but it sounds a bit complicated ? e.g. I need to make sure there are no false detection, and how to handle those interfering fingers ...