Optical sensor

[mark sheldon]

Hi, I am looking at designing a piece of equipment that will reduce the vibration in a small scale RC Helicopter. For this I am looking to measure the phase of the helicopter and the dc motor I will be using as the shaker. To then ensure the motor is 180 degrees out of phase with the helicopter. The question I have is how will I design a circuit to utilise an optical sensor to collect phase information of the helicopter and then the dc motor? Thank you in advance for your input

 

[dknguyen]

Reflective tachometer at one point to detect the blade and measure the time in between blade detections. Then assume the angular speed remains constant and use a timer to estimate where the blades will be in the intervening time between blade detections.

 

[throbscottle]

I'm new to them but I wonder if a PLL would be the thing. Use a tacho from the rotor blades to provide the frequency you want to track, then the the shaker motor with a matching tacho acts as the VCO. Position your counter-weight relative to the tacho to get the correct phase relationship. You'd need your tachos to give only 1 pulse per revolution, so use dknguyen's reflective tacho but with a reflective spot on only one blade.
(I think I'm allergic to microcontrollers hence the analogue suggestion...)

 

[audioguru]

What causes the vibration?
1) Balance?
2) Motor is jerking with pulses of power?
3) One blade has more thrust than the other blade?
4) All of the above?

 

[Pommie]

Isn't this your third thread on this subject?

Mike.

 

[JonSea]

Answering your question, an optical tach sensor and a piece of reflective tape is everything you need. See the picture below. This sensor will provide a TTL pulse string corresponding to rotor position.

For "real world" size machinery, you can also use an inductive sensor if the shaft has a keyway. Again, this sensor provides a once/rev TTL pulse stream. Picture below.

However, there is a basic flaw in your reasoning. If you are trying to counter mass imbalance, an out of phase force equal to the imbalance could counter it.* But you will not be able to generate this force with an eccentric mass on a motor. The effect of the eccentric mass varies with speed. But you have a fixed speed to counter the force of imbalance. You have no means to vary the force produced by your motor.

* An imbalance typically generates a radial force at 1x rotation rate for a shaft supported on both sides of the rotor. An overhung rotor, meaning it's supported by bearings on only one side of the shaft, will generate radial and/or axial 1x force depending on the exact geometry. A small-scale model will not accurately reflect the real-world situation.

 

[mark sheldon]

Isn't this your third thread on this subject?

Mike.

As I haven’t had an answer to my question yet does it matter?

 

[mark sheldon]

Answering your question, an optical tach sensor and a piece of reflective tape is everything you need. See the picture below. This sensor will provide a TTL pulse string corresponding to rotor position.

For "real world" size machinery, you can also use an inductive sensor if the shaft has a keyway. Again, this sensor provides a once/rev TTL pulse stream. Picture below.

However, there is a basic flaw in your reasoning. If you are trying to counter mass imbalance, an out of phase force equal to the imbalance could counter it.* But you will not be able to generate this force with an eccentric mass on a motor. The effect of the eccentric mass varies with speed. But you have a fixed speed to counter the force of imbalance. You have no means to vary the force produced by your motor.

* An imbalance typically generates a radial force at 1x rotation rate for a shaft supported on both sides of the rotor. An overhung rotor, meaning it's supported by bearings on only one side of the shaft, will generate radial and/or axial 1x force depending on the exact geometry. A small-scale model will not accurately reflect the real-world situation.

View attachment 109876 View attachment 109877

JonSea. Thank you for taking the time to explain, it is very much appreciated. Best regrds

 

[MikeMl]

I have balanced propellers on my own aircraft using a commercial prop balancer. It uses an reflective optical pickup (attached to one prop blade) to determine crankshaft position, and it uses an accelerometer mounted on the front of the aircraft engine to determine the angular position of where to place the balancing weight...