# Frequency control of vibrator motor

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.

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)

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?

• Inserting a larger offset mass will slow the motor down to the ranges you want, but will wear out the brass bearing rather quickly. – Sparky256 Jun 7 '16 at 4:54
• "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? – Transistor Jun 7 '16 at 5:11
• @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? – user5089054 Jun 7 '16 at 5:21
• 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. – user2943160 Jun 7 '16 at 5:28
• 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. – JRE Jun 7 '16 at 9:56