I'm about to start investigating for a project which will require controlling a motor which will simulate a pendular motion.
Although I've used Arduino to read some simple sensors, I must say I'm almost virgin in electronic stuff so I get lost quite easily in this world populated by solenoids and other strange aliens.
The project requires a pendulum to be maintained in motion forever. I have two options:
completely fake the motion, simulating the gravity effect on velocity;
actually use natural pendular motion but reinforce it constantly so that it never stops.
Since this is to be used in an art instalation, I would like to place all the controlling mechanism in the pendulum axis so that it can be easily hidden and leave the pendulum completely "clean". Also, low noise would be appreciated since this is to be put in an actual living room.
First question: Motor or not?
I'd say attaching a motor to the axis would be the obvious solution. I don't know any other. Electromagnets are out of the question because they'd have to be applied to the bottom of the pendulum. Are there any nice solutions?
Second question: Controller?
(assuming a motor will be used) I've just started browsing and saw a lot of low level stuff like: H-Bridge, Unipolar, Stepper Motors, DC Motors, L297 controllers, etc. For the record: I'm not afraid of these things only because I don't know them. But I'd like to ask for directions so that I don't waste my time looking in the wrong places.
I believe I should start will Arduino + Motor Shield for the controller, right? That includes all the controller stuff, right? Do I need anything else? If so, what and why?
Third question: Motor?
At the tip of the pendulum will be a digital frame which weights 550g. The movement maximum angle doesn't have to be very big. 15º or so. And it should be fairly slow, but realistic. Like a pendulum clock let's say. Considering this weight and the kind of motion needed, what kind of motor should I consider? I realize stepper motors are easy to control, right? Can I use one of these? Which is more silent? How powerful does it have to be to manage to pull the pendulum with this kind of weight?
Sorry for so many questions at once... I guess I got carried away.
This may not be as fun as a full blown microcontroller based project, but you might be able to use something like this if you could tune it precisely enough:
It is a hand wound, iron core coil, a transistor, and a battery. When a magnet swings by, the induced current in the center tapped coil generates a pulse that briefly turns on the transistor, energizing the coil from the battery, and providing a magnetic "kick". When there is no motion, the current drain on the battery is near zero.
If your pendulum arm has a magnet on the end, you should be able to orient the kicker to give it a steady perpetual motion. Best of all, the kicker can be hidden so there is no obvious motor or drive mechanism for the pendulum.
If you are really set on using the Arduino or if tuning the motion seems difficult, you could use sensors (optical or Hall effect come to mind) to provide feedback to the controller and then adjust the kick of the coil.
Note that these "kicking" effects would slightly alter the regular pendulum motion. You could also use an escapement mechanism like those on pendulum clocks. Instead of spring or weight, you could use a motor or solenoid.
Thank you for that great idea!! I really love it. Although playing with controllers is fun, I definitely give priority to simplicity. Using your solution I can have a simple nail as axis and even use the digital frame's power cord to support it. Besides, the magnet kicker can be attached to the wall behind the digital frame hiding it completely.
Some questions arise though:
Can such a thing be bought or would I have to make the coil myself
Being attached to the wall behind the pendulum I am affraid that it would push the pendulum away from the wall, and so the pendulum movement would not be linear anymore.
These things seem to provide power to simple and very light structures. I wonder if they can make my heavy 550g frame bounce. I guess they can, if the pendulum is long enough
I will definitely explore your solution. Any inputs are welcome. Nevertheless I intend to continue investigating the Arduino alternative so that I can learn each solution's advantages and disadvantages.
I'm sorry that this issue became partially off-topic. Do use private message to reply if the answer feels off-topic.
Unless you're very lucky in your scrounging of surplus materials, you'll probably have to wind the coil yourself, but the specs don't seem to need to be precise. Some enamel coated magnet wire and a low speed power drill would probably make it go much faster - just remember to add the center tap. You can Google around for tips on winding electromagnet coils and solenoids and I'm sure there are some good and easy ways.
Remember that the "kick" this device imparts on the pendulum doesn't have to be very large as a massy pendulum loses momentum slowly (a ball bearing on the top would help and you can rip them out of skateboard or roller blade wheels), but the kick does need to be in the proper plane of motion, so I don't think it would work behind the pendulum mass. I was thinking that it should be at the bottom of the swing, but I'm not sure if the effect would be canceled out by the reciprocal motion. Maybe you could use two kickers and angles and set to alternately pause so that each only kicks in one direction.
If you find it difficult to control the kick precisely, remember that you can always replace the transistor with a microcontroller.
That way the microcontroller can time the swings and calculate how big the kick should be.
With two kickers like cmpalmer suggested, you'd be able to detect which way its swinging and fire the electromagnets appropriately.
I was actually thinking that he could have two coils as described by the article and use the transistor's base as a digital input.
Probably the simplest 'sensor' for this situation.
Thank you all. You're ideas are wonderful and you're helping me shape the installation faster than I ever expected. So, I'll try to put everything together here:
By now I can say the motor is history, not needed. Great! Simpler, visually cleaner and probably more silent.
Although you made me understand that a simpler device could also do the trick, I believe using the controller is still a very good bet because it gives me much more flexibility. And I need it because of something I still didn't tell: the pendulum movement will have to be VERY syncronized with the video movement.
The setup as I imagine it, putting all our ideas together, has a Arduino which reads two Hall sensors to detect the pendulum position and which triggers two electromagnets which will kick the pendulum to keep it going. I believe the frame has an USB port which can be used to power the Arduino. Very convenient
I've never used Hall effect sensors but I believe they should work fine. Should I investigate other alternatives? If so, which and why? Ah, and about making my own coils... I'll do my best to avoid it
cmpalmer, you said that I could do with only one kicker. Using only one kicker, how can I get a smooth movement which is completely similar in both directions?
I would like to use the digital frame's power cord as the pendulum rope since it has to go up anyway. Since it is not rigid, I believe it will twist somewhat and the pendulum will not move parallel to the wall. So I'll have to restrain it somehow. I'd like to avoid friction. Do you think I can do it with magnets? If I have two long magnets, on the frame and another on the wall, which always would keep the frame away from the wall, would it work? Or am I being naive?
Thanks again!
Nuno
PS. cmpalmer, I will definitely keep you posted on this project
Thank you all. You're ideas are wonderful and you're helping me shape the installation faster than I ever expected. So, I'll try to put everything together here:
By now I can say the motor is history, not needed. Great! Simpler, visually cleaner and probably more silent.
Although you made me understand that a simpler device could also do the trick, I believe using the controller is still a very good bet because it gives me much more flexibility. And I need it because of something I still didn't tell: the pendulum movement will have to be VERY syncronized with the video movement.
Is there a lot of variation in speed? Or can you tune your pendulum to have a natural frequency equal to the speed of the video?
The setup as I imagine it, putting all our ideas together, has a Arduino which reads two Hall sensors to detect the pendulum position and which triggers two electromagnets which will kick the pendulum to keep it going. I believe the frame has an USB port which can be used to power the Arduino. Very convenient
I've never used Hall effect sensors but I believe they should work fine. Should I investigate other alternatives? If so, which and why? Ah, and about making my own coils... I'll do my best to avoid it
cmpalmer, you said that I could do with only one kicker. Using only one kicker, how can I get a smooth movement which is completely similar in both directions?
If you have a magnet pointing NS parallel to the movement you can switch polarity in the electromagnet. You can also put an coil before and after the electromagnet so you can measure speed and direction. I don't know if this is easier to implement then a hall sensor.
I would like to use the digital frame's power cord as the pendulum rope since it has to go up anyway. Since it is not rigid, I believe it will twist somewhat and the pendulum will not move parallel to the wall. So I'll have to restrain it somehow. I'd like to avoid friction. Do you think I can do it with magnets? If I have two long magnets, on the frame and another on the wall, which always would keep the frame away from the wall, would it work? Or am I being naive?
If it doesn't matter how the pendulum looks like you can make a power cord from two long sheets of metal. This is very rigid perpendicular to the movement. Maybe you can make something from a tape measure.
No, there is no variation at all. My goal is to make the pendulum move at a fixed frequency similar to the one in the video.
About using one electromagnet: although I believe you, I don't know exactly how who I position the electromagnet so that it would kick the pendulum in both directions. As I see it, I would place two electromagnet one in each end of the pendulum movement and they would kick it back to the center each time it reaches each end. So, using only one and inverting its polarity I'd have to place it in the center, where speed is higher. How would you place it there without interfeering with the movement and how would it kick the pendulum? When cmpalmer suggested using only one I thought he meant kicking the pendulum only on one of the ends. I have almost no experience with electronics and this kind of stuff so forgive my ignorance.
It does matter how the pendulum looks like. No special way, it just have to look nice and slick. I liked your idea. I'll keep it in mind. Maybe I can make it look nice that way!
If you only need one speed then start building a test pendulum. Attach a moveable weight to it so you can tune it to the speed you need. This way you will learn what is needed to keep the pendulum going.
As for the electromagnet, indeed I was thinking about an electromagnet in the center. With this you can have it start by itself too! The speed may seem high to you, but I think it won't be problem for arduino.
Building a test pendulum is indeed my very next step. I am about to order an Arduino board
Ah, I see. Making it start is a nice feature but in this case impossible because of the video sync. The video is not interactive, so it will always have to be started manually, starting the movement in the right moment of the video. Anyway, using one electromagnet would always be better than using two. But, being in the center, how will it not be in the way of the pendulum and still be able to kick it?
I was not worried with the speed but with the quality of the movement, but maybe you're right... and maybe it's even better because the kick will be less visible when the pendulum is already in movement. I'd like the pendulum to look like it is not being kicked, see? Much like a pendulum clock, which is... but doesn't look like it is.
I live in Europe. Where (online) do you advise me to buy both the electromagnet and the Hall effect sensors? And how do I know which electromagnet to buy?
Oh, I forgot about driving the pendulum. If you place a strong, neodymium, magnet in the pendulum, the electromagnetic field will push the magnet. Have a look at how a clock pendulum for the bearing construction, you want to minimize friction.
I know RS had neodymium magnets. Electromagnets, I really don't know. Maybe you can make something from an old relay. for buying stuff you can try farnell, or maybe conrad, I don't know in which country's they sell. But you have to think first before buying it. Sometimes magnets don't work as you would expect.
BTW, did you search around for this kind of pendulum? These things exist, so there is no need to reinvent it again.
I'll look at clock pendulum constructions. Although in this case the forces are applied to the pendulum axis so it's completely different.
Sorry, I got lost: When you refer that neodymium magnet, are you addressing the issue of restricting the pendulum movement in order to try to keep it linear and parallel to the wall? Or are you talking about stimulating the pendulum?
I hope all these magnets and electromagnets don't mess up the digital frame image
I'll look at clock pendulum constructions. Although in this case the forces are applied to the pendulum axis so it's completely different.
Sorry, I got lost: When you refer that neodymium magnet, are you addressing the issue of restricting the pendulum movement in order to try to keep it linear and parallel to the wall? Or are you talking about stimulating the pendulum?
The neodymium magnet is in the pendulum and will be pushed away by the electromagnetic field somewhere hidden away.
I hope all these magnets and electromagnets don't mess up the digital frame image
Nuno
As long as you don't use a CRT monitor it will be ok.
I place a strong magnet in the bottom of the pendulum (behind the frame);
place the electromagnet beneath it, in the center attached to the wall;
place a hall effect sensor attached to the wall, also in the center
Like this:
The black rectangle is the electromagnet, that small red thing is the hall effect and the ball is the strong magnet.
So, when the pendulum reaches the center, the hall effect sensor senses it. After a short delay (giving time for the pendulum to move slightly away from the center) I activate the electromagnet which will kick the pendulum magnet. This works in both directions!
Yes, exactly what I meant. Maybe you can use the electromagnet to measure the incoming pendulum. When you measure something you can switch on the electromagnet. This saves you a hall sensor.
