# Chosing the right motor for a vibration platform

The following photo depicts a vibration testbed that uses a motor, springs and some wood. The purpose of this effort is to make a test bed vibrate the attached kinetic energy harvester at the movement frequency controlled by the motor. The vibration plane is connected to a stable surface by means of two springs. On the vibrating place is a motor that has a weight attached to it. This weight is fixed towards a side of the motor. This is similar to the approach utilised in mobile phone vibrators except in this case, I want the motor to drive a larger weight.

As the motor drives the weight (rotate the weight), the vibration surface will start moving up and down at a perticular frequency. This is what I want. Inorder to limit other directions of motion, I will made sure the moving platform is 1 degree of freedom. i.e. it can only move up and down.

By doing this, the attached kinetic harvester will also move up and down at the same frequency of the moving surface. This is the whole purpose of this setup; a testbed for my research.

I want to have a frequency ranging approximately from 0.5Hz - 4Hz. My mass may range from 100g to about 600g.

In that case, can someone please propose me a type of motor that I can utilise for this purpose? I think it should be a DC brushless motor. But I am not sure. Could someone please be kind enough to tell me what type of properties my motor must have? Any additional information will be highly useful. Thank you for your time.

• How much mechanical power do you need to produce? Find out and you'll know how powerful your motor should be. Getting frequencies below 4Hz will most probably require a reductor, but that won't change the power. Mar 31, 2017 at 10:39
• I'm thinking about controlling the speed by means of voltage. Isnt it possible to rotate the type of motor that you would propose at revolution per second? Mar 31, 2017 at 10:41
• Thank you for the idea of reductor. If I cannot go lower than the rated speed limit, I can use a reductor! Mar 31, 2017 at 10:42
• If I were you, I'd consider using a voice coil actuator instead of a motor spinning an imbalanced mass. A voice coil would produce inherently one-dimensional vibration and you would be able to vary the frequency and force independently of each other (by controlling the drive frequency and amplitude, respectively).
– jms
Mar 31, 2017 at 10:45
• im sorry, I cannot come up with a power value. its just that there too many losses to consider in that calculation. My purpose is to rotate a weight of may be maximum 600g in circular manner. I dont mind going for an overkill and at this moment, budget is not a problem. Mar 31, 2017 at 10:48

How much weight you want to rotate is in itself irrelevant. What you need to know is two things:

• what is the vibrating mass, vibration frequency and how fast you need to change it
• what amplitude of vibrations you need
• how much energy you expect your "harvester" to absorb

Vibration amplitude will give you an estimation of torque. For example, a vibration plane of 100 kg vibrating at ±1 cm amplitude will produce a force $F=mr(\frac{2\pi}T)^2≈40N$. That force has to be countered by your rotating weight, e.g. 1 kg weight attached to a 1 m arm. That weight would have the moment of inertia $I=mr^2$, and you will need a torque $\tau=\frac{2\pi I}{T*T_x}≈6.28 N*m$ to get it rotating at $\frac1T$ frequency in $T_x$ time (assuming $T=T_x=1\space s$).

The power of your motor will need to allow that torque to be applied up to 1 RPM plus cover whatever power you expect your harvested to absorb: $P=\frac{\tau*RPM}{9.5488}+P_{harv}\space(kW)$, which is 0.66kW + $P_{harv}$ in my example.

Hope that helps somehow.

• I had a discussion with my supervisor and he seems to like the voice coil actuator idea over my proposed system :-( May be that's better. The weight of my harvester is 1kg max. its got a magnet going up and down inside it. I don't really know if my approach has advantages over the voice coil approach. However, I sincerely thank you for your time and efforts and the elaborative explanation. I am going to go though your answer and pick up every word thats explained. thanks! Mar 31, 2017 at 14:58