Greetings, I am stepping into my first self designed, open ended project beyond the ivory walls of school and the grey walls the cubicle, and I have realized I don't know where to begin! If I have found the wrong forum any redirection would be much appreciated.
I am looking to create, for all intents and purposes a low power haptic device. I want to use a PWM controlled DC motor powered from a rechargeable battery to create a wide range of vibrations to indicate information to the user. From this I think I need a motor that has the following features:
1) Relatively small profile
2) Powerful enough to create a wide range of vibrations, even very intense ones
3) High quality, long life, no cheap motors!
4) DC, Single direction is fine
5) As low a power consumption as possible
My issue is I don't know how to go from my list of specifications to a concerted motor search. Any help would be appreciated, from specific recommendations to (perhaps the more helpful over time) help with resources to learn how to conduct this kind of search on my own.
Item 3 points you to Maxon, Faulhauber or Pittmon -- unfortunately the 'not cheap' applies to the cost as well.
If you find those prices too high, Johnson and Mabuchi are the big Japanese motor suppliers, and may have something good enough yet affordable.
For experimenting check the surplus houses -- they all seem to have lots of motors, and many have pager motors with weights already attached.
--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com
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1) What are some good resources for purchasing motors?
2) What specifications for motors define these qualities: a) Can it be controlled via a PWM signal? (I suspect this is any brushed DC motor or a brushless DC with added circuitry, but I am not sure) b) How much weight can it rotate, and at what speed, at the motors peak efficiency? (Alas my memory of dynamics is a bit rusty, how do I go from torque/power to an idea of how much weight the motor can spin at peak efficiency) c) What specifications should I look for to indicate higher efficiency, lower power usage to accomplish the same amount of work as another motor?
Hi, Thanks for your reply, those are some good questions, I hope my answers make enough sense to help better answer my original questions.
Since I am trying to create varying levels of vibration intensity I suspect I will be adding an uneven weight to the end of the motor.
Mass - I intend to experiment with various weights at the end of the motor to see how different masses vary the vibration. Only after more experimentation can I truly settle on an exact mass. Considering that space is an issue, even using more exotic weights, I cant imagine my weight will exceed 100g, around 50g being a more realistic estimate.
Speed - Once again, I dont know what rotational speed is best just yet, though I suspect that to create more intense vibrations I will need rather high speeds. For my purposes now I suspect I will need something that can do a few hundred rpm to 10,000+ rpm to give me a proper range to experiment.
Acceleration - I am not sure how to best describe what I need in this area since I don't really know how to give a specific number or number range for acceleration and my memory of dynamics is rusty at best. A responsive motor is desirable in order to create a more complete range of possible vibration sequences. I don't know how to get acceleration from these numbers but I suspect they are enough for someone who knows more than I do. Lets say, worst case/high end numbers: I want to accelerate a 100g weight from a 0rpm to 10,000rpm in under 0.5 seconds. Is that even reasonable to ask for? How about a 100g weight from 0 to 5,000rpm in under 0.5 seconds and to 10,000rpm in under 1.5 seconds? How can I go from here to a number to look for in a motor spec.
The final thing you asked for is "losses" which, I must apologize, I have been unable to figure out just exactly what that is, and how to take a guess as to what my "losses" will be for my specific application. If you could offer me an explanation or point me to a resource I would very much appreciate it.
I hope that is enough information to offer a better idea at what my requirements are.
For retail, try MPJA associates, All Electronics, Herbach & Rademan, and other surplus outfits.
For the precision motors plan on going to the manufacturer.
Johnson & Mabuchi web sites list distributors -- I have never tried buying these motors, so I couldn't say who to call.
Correct. Just about any DC motor will run off of PWM drive
Many motor data sheets list the peak efficiency point for one or a couple of input voltages. Note, however, that it's in terms of speed and torque; the average torque that your motor will see spinning a weight varies quite a bit with the environment that the thing is in -- running free or gripped hard the motor can't transfer much power to it's environment, and torque is low. With just the right amount of mechanical damping, the power transfer is maximized and so is the torque.
The motor speed constant, armature resistance and free-running current are the basis for the efficiency calculation. You calculate your losses and your power out, do the division -- and there you are.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
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