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I wish to generate a controllable frequency vibration on a device. The vibration frequency then has to be measured and filtered. For this purpose, I am using a vibrator motor as shown.

enter image description here

Now, i wish to genterate a vibration of frequency 5- 20 Hz but all I am able to get is a frequency greater than 30Hz.

I am controlling the motor by controlling the dc supply.

I tried using PWM and transistor circuit(as shown below), but that doesnt generate a single tone vibration but contains harmonics too, which i observed using labVIEW (may be due to nature of PWM)

enter image description here

I wish to generate a single tone vibration in the range 5-20Hz. How can I do it? Does the sector angle of the CAM shaft have an effect on frequency of vibration?

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    \$\begingroup\$ Inserting a larger offset mass will slow the motor down to the ranges you want, but will wear out the brass bearing rather quickly. \$\endgroup\$ – Sparky256 Jun 7 '16 at 4:54
  • \$\begingroup\$ "I tried using PWM and transistor circuit, ..." Show us the schematic. (There's a button on the editor toolbar.) "... but that doesnt generate a single tone vibration but contains harmonics too." How do you know? How did you measure this? \$\endgroup\$ – Transistor Jun 7 '16 at 5:11
  • \$\begingroup\$ @Sparky256, would it decrease the frequency? if it does, does it depend on the mass only or also on the angle of shaft as well? \$\endgroup\$ – user5089054 Jun 7 '16 at 5:21
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    \$\begingroup\$ Your PWM frequency should be much greater than 20Hz for effective operation. You want to control the average speed of the motor such that it rotates at the speed that will create your desired vibrations (300 to 1200 RPM), so it would be desirable to have a PWM frequency greater than, say, 5kHz. \$\endgroup\$ – user2943160 Jun 7 '16 at 5:28
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    \$\begingroup\$ You may be SOL trying to get such low frequencies from a vibration motor. These guys make them, and there's a lower limit to the vibration frequency that depends on several factors. That link includes some background info (math) as well as info on the specific models that they make, so may be of some use in figuring out what you need to do and if it can be done at all. \$\endgroup\$ – JRE Jun 7 '16 at 9:56
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Before even trying a modulation circuit to control the motor, see if it's possible at all. Connect the motor to a variable supply and see if you can get the frequencies you want. Quite possibly, you can't.

These motors often have significant cogging. That means the magnetic field acts like detents along the way. Such motors can't be run with a steady drive below the level where the toque doesn't overcome the detent force. Once the drive does reach that level, the motor "breaks free" and runs fairly fast. The detents are like little valleys, and once the motor runs they cause little average drag since the push down into the valley is about the same as the drag going back out of the valley.

Start at zero and slowly crank up the variable supply. The motor will probably just sit there, then suddenly start spinning. If that spinning speed is higher than the minimum you want, then you need a different motor.

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  • \$\begingroup\$ Oh. It doesn't. Reaches around 20Hz but not below that. Can any manipulation to the motor shaft shape,size, weight cause the required effect? \$\endgroup\$ – user5089054 Jun 7 '16 at 16:15
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    \$\begingroup\$ @user: Once you get it going, more inertia on the shaft will allow you to keep it going at a lower speed, but won't be of help getting it started. It sounds like this solution won't really work. Step back two levels and explain (in a different question) what you really want to accomplish, and leave supposed solutions, like a vibrating motor, out of it. A voice coil may be a solution, but again, we need to know what you ultimately want to do, not how you imagine solving the problem. \$\endgroup\$ – Olin Lathrop Jun 7 '16 at 17:11

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