unpowered magnetic brake advice needed

[CheckThePlug]

I'm designing a motion damping system. I can't say what for, but I can tell you where I got the inspiration from. I've noticed that when my force feedback video game steering wheel is unpowered, the DC motor and gear train create a smooth motion with strong resistance when the wheel is rotated. Since I don't really need my rotational mechanism to be powered, I would prefer a similar system comprised of some sort of DC-motor-like magnetic brake and gear set to achieve a similar function, preferably with variable resistance. Furthermore, it is imperative that the magnetic field is contained and doesn't interfere with nearby electronics. Seeing as this was accomplished in the force feedback wheel, I see no reason it couldn't be done in the project I am attempting.

here's a few pictures of a disassembled force feedback wheel to show what I'm talking about:

http://simpleparcel.com/catalog/was/logitech formula force.html

 

[waross]

A spinning aluminum disk will be braked by a magnet in close proximity. A horseshoe magnet on one side with an iron bar on the other side of the disk to provide a low reluctance path for the magnetic flux will work better.
Using a belt drive to speed up the disk will help.
Using a belt drive to spin a fan will also work well.
If you use the motor you are actually making a generator out of it. Experiment with different value loading resistors across the motor terminals.
A permanent magnet doesn't create much interference and a fan even less.
Belts will work much better than gears. Little or no backlash.
Read fast, this sounds like a hobby post and hobby posts are not allowed. This thread may disappear soon.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[CheckThePlug]

This is certainly NOT a hobby post,but I would be disclosing trade secrets if I was to say what it was for. Thanks for the info, though, I'll definitely try using different resistors for different values as well as looking into the fan method.

 

[OperaHouse]

On approach with the DC motor is to use the motor feeding into a full wave bridge. This then feeds into a resistor with a zener in parallel or even one or more diodes in series. This could provide even greater load when the zener voltage is met.

 

[CheckThePlug]

here's a question: could I wire the DC motor directly to a potentiometer (like a radio volume knob-style pot) and then be able to adjust the resistance of the motor? Would I have issues burning up the potentiometer? Sorry if any of these questions seem simple, but most of my experience is in kinematic machine design and using DC motors and circuits to create a damping drag force isn't something I've really delved into before.

 

[OperaHouse]

"experience is in kinematic machine design"........ You should be able to answer this then. Looking it from the other side. Just how many watts would it take to move your hand. A lot of resistance would still be in the motor but just guessing a carbon film pot would have issues long term.

 

[waross]

Use a small rheostat instead of the pot.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[CheckThePlug]

kinematic machine design=metal joints, hinges, etc. and most of that is with pure hydraulic equipment with no electrical elements. I appreciate the help, but why does every response on this forum have to involve some kind of degrading, belittling, or condescending remark tied to it? First I get accused of being a hobbyist, then I'm told "you should know this" when my degree is in Mechanical Engineering and I've never worked with electrical circuits professionally. I am a recent graduate and I don't know everything yet, so please try to leave the extraneous insults out of your posts.

At any rate, I don't need it to be a carbon film potentiometer, I was thinking more along the lines of this type (wire wound):

http://upload.wikimedia.org/wikipedia/commons/b/b5/Potentiometer.jpg

http://upload.wikimedia.org/wikipedia/en/c/c1/Reochord.jpg

I'm sure I would need some sort of heat-dissipating element in line like the load resistor mentioned a few posts up, but there needs to be adjustability in the torque of the DC motor output.

 

[waross]

Well, looks like a hobby post, feels like a hobby post and sounds like a hobby post, but we have given you the benefit of the doubt and taken your word that this is work related. Otherwise the thread would have quickly disappeared.
We don't want to be overwhelmed by hobbyists seeking free information.
We expect posters to be professionals and we expect them to do a little research on their own and not bother us with every little detail. Hence comments such as "You should know this."
But you have been allowed to stay, so far. Welcome to the forum, there is an unbelievable amount of expertise available here.
Back to the question at hand.
The resistor across the motor terminals is not a secondary control device. It is the heat dissipating element that you correctly surmised would be needed. It must be sized to handle the output of the motor.
A small permanent magnet DC motor is also a generator. As a motor the no-load speed is closely proportional to the applied voltage and as a generator the open circuit terminal voltage is closely proportional to the speed. On a larger motor, at a higher voltage (1 HP, 120 V and up) there is often only a few percent difference between Volts and/or RPM going from motoring to generating. The percentage difference will be greater with a small toy sized motor, but you get the idea. When selecting a rheostat I recommend looking at the Wattage rating and calculate the corresponding current. (Watts = current squared times resistance) you may need to use an external resister (check the current capability as above) to avoid burning out your rheostat when only a small percentage of the resistance is in the circuit.
On another track; a strong horse shoe magnet in close proximity to a spinning disk is easy. The drag may be varied by placing an iron bar on the other side of the disk. The bar may be movable, the smaller the air gap in the flux path the greater drag on the spinning disk.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[CheckThePlug]

I like the spinning disc idea, but would I be able to get substantial braking from a disc ~2 inches in diameter?

 

[waross]

It depends how fast you spin it. At faster speeds the drag will be greater and then your mechanical advantage will multiply that.
You may cut and twist paddles on the edge of the disk and also use magnetic braking.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[OperaHouse]

Some older audio equipment I worked on had two disks with a viscous fluid like STP to create a slow motion.

 

[CheckThePlug]

@Opera unfortunately, the specific application of this dampening system is to replace existing fluid designs so that it can work in varying climates without needing the fluid changed.

As I've looked into it more, I believe I'll be able to get the torque I need via some timing belts and pulleys to adjust the ratios, the only issue is that I need to be able to vary the resistance on the motor for variable motion dampening.

Is there a variable version of load resistors like this one?

http://i00.i.aliimg.com/photo/v0/343465224/LED_Load_Resistor.jpg

If so, what would it be called when I searched for it? I feel like I'll need something with an integrated heat sink, but perhaps not since motion will occur in short bursts rather than long consistent stints, but I'm not very well versed in electro-thermal properties.

 

[Compositepro]

Check out eddy-current and hysteresis brakes.here is one example:

http://www.magtrol.com/brakesandclutches/permanent_magnet.html

 

[CheckThePlug]

@Compositepro I looked into permanent magnet brakes, but I don't think I'll be able to get enough backlash free ratio reduction to get the RPMs I would need for adequate braking compared to the DC motor. Additionally, the permanent magnet brakes are VERY pricey.

How about an AC motor, could I get more force using an AC electric motor?

 

[MikeHalloran]

You want braking torque? Take a look at the electric drum brakes used on trailers.



Mike Halloran
Pembroke Pines, FL, USA

 

[Skogsgurra]

The DC motor plus variable resistor is the simplest and most flexible way of doing it.

You have not told os what torques and speeds are involved. The reference to game consoles indicates that we are talking low speeds and rather low torques. Similar to what you have in a game kind of thing. If that is correct, you do not need to worry about power and heat.

If that is not correct and if you need to handle more power, then just calculate what kind of speed and torque you expect. Then make a worst case estimate of duty cycle and take it from there. Power in your resistor can never exceed that number.


Re the remarks that you feel are 'degrading, belittling, or condescending'. It is something you can expect when you are putting questions outside of your expertise in a forum where we are used to (and the site rules also indicates that) that the members have done their own research to a degree where elementary technical details have already been dealt with and that the continued work needs clarification of details or selecting between principles or just getting a verification that work is moving in sound directions.

Many of us are long-term members. I, for one have been around for six-seven years and many of the others are members for more than ten years. It is only natural that a 'hobbyish' question is questioned and that most of us also think that you 'ought to know this'. We are a nice bunch. But we are also humans that get irritated by the sometimes blatant laziness and lack of fundamental insight that is reflected in many questions that newcomers put.

Accept that - and you will grow as you learn. We all have done that in one or several ways.



Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[OperaHouse]

So far you have a mystery project with unknown requirements. Lets go back to a previous comment of you should know. More correctly means you are the only one who knows. Is this a cassette door or a 1,000 pound sliding fire door. Its not all unknown unknowns! A simple pot is fine for proof of concept. Adding some small self powered electronics may actually be cheaper than a big wire wound pot. Hardware type things are far more expensive than electronics. This could also add features like making it easier or harder to move as speed increases or be direction dependent. Small stepper motors may be more reliable than the brushes of a motor in environmental applications and higher voltage motors may make things easier. This is where real design comes in.