Designing a Circuit to Measure Phase with Optical Sensor for RC Helicopter Vibration Control

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

My first question on reading this was why use an optical sensor for this and not an accelerometer?
On further thought, I'm assuming that you will be using optical sensors similar to the one in this data sheet.  You will also need some sort of interrupter disk on the motor shaft.
http://omronfs.omron.com/en_US/ecb/products/pdf/en-ee_sx398_498.pdfFor the sake of an example, I'll also assume that you have a 5V supply.There are two part to the circuit required. First, you need to provide a current to the LED. this is done by attaching a resistor from the 5V to the A  pin and attach ground to the K pin. For 20mA the resistor should be about  (5-1.2)V/20mA = 190 Ohm (not standard so use 180 or 220 Ohm)
Then for the output side, you  connect 5V to the V pin, Ground to G pin and a resistor between the O pin and 5V (I'd use 10 K or so for this).  The O pin is your output and should give a series of pulses as the motor spins.These component values are for the part in the datasheet so they will vary according to what part you have. Most parts of this type will be similar.

Mark, I haven't much experience here, but wouldn't it be better to attack the source of the problem rather than the symptom?Lets assume the main rotor is the cause of the vibration.  It would not be easy to identify the out-of-balance part but there's a trick you could try.  A stroboscope with variable frequency flashes could "stop" or slow  the motion of the rotor and (if you made a small mark on one of the blades) identify which side is heavier. (If you get the strobe frequency near the rotation frequency then you would see which side it is biased towards).   Then either trim some off that side or put a weight (a drop of epoxy glue maybe) on the other side, and try again.You could just remove the rotor and hang it on a thread and that may also tell you if one side is heavier.Either of these techniques would be more tricky on a 3-or 4-bladed rotor, but should still work.A tricky problem - good luck with it. // David

Hi Elizabeth,My reason for not using an accelerometer is due to the fact that the model helicopter I am using is a military grade model worth in excess of £100,000 so I am restricted as to how I achieve my results. I am unable to attach anything to the main rotor head that may cause damage hence why i was looking at using an optical sensor and a reflective piece of tape. Thank you for taking the time to reply 

Hi David,The reason for me needing to achieve the phase of the helicopter and shaker is so I can ensure both are 180 degrees out of phase to each other then by doing this I can counter the vibration produced by the helicopter with the shaker unit. It is the overall structure vibration of the helicopter that is my aim to reduce. Thank you very much for taking the time to reply to my question 

So you will be using a reflective sensor rather than interrupting. Those are a bit trickier to get working because they can be affected by stray light and are sensitive to dust etc. on the reflective tape. On the other hand, the circuit is going to be similar for just about any optical sensor. Provide current to the emitter side  and usually a pull-up for the output transistor. Just take a good look at the datasheet. In your case with a reflective sensor, pay close attention to the distance allowed between the sensor and the reflective tape.

Have you thought of a Hall effect sensor and a small magnet on the rotating part. weights are not much different to leds and sensors and less likely to be upset by dust and extraneous light. If using Leds then use a frequency pulsed  led and use the reflected frequency only as the signal to evaluate. This is common technology for beam counters and optical alarms. 

If you insist on using an optical system, then a modulated low power laser would do the job, together with your reflective tape. A detector at the bottom of a tube, with a narrow band optical filter, together with ac coupling, is usually enough  to get good detection results in the presence of daylight, The laser gives a good signal, so detector gain can be kept low enough to avoid DC saturation. I have used such systems in the past with a good success rate,cheers,Richard

Richard, thank you for your input. You mention you have successfully achieved this in the past. Are the components required easily obtainable and is the system easy to build? Thank you again for your reply 

Hi,you will have to shop around.Element14 have laser diodes, as do other companies of there ilk. You only want a few mW, eye safe.iCHause have laser diode driver chips, and small demo laser driver boards as well.Places like Thor Labs and Edmund Optics have laser diode band pass filters, you have to match the band pass to the laser wavelength, particularly if you use interference filters, rather than Wratten(dye).The idea of putting the detector at the bottom of a longish tube is to help reject stray light by reducing the acceptance angle, and the filter takes care of the broad spectrum ambient light. You may not need the filter if you limit the acceptance angle of the detector sufficiently, and the detector gain is low, which should be the case if using a laser.There are plenty of detectors around, Element14 and there ilk will have them. 
I was using very high front end gain and phase detection, since  I often need the DC component of the signal, but in your case this is getting unnecessarily complicated.How much signal you get back from your reflector will be an unknown, since I don't know how much curvature you have, and the blades will flex as well, so you may need a wide area detector, or a lens. It's something you will have to play around with. You may find you don't need to modulate, and won't even need an amplifier stage. If you do modulate, you will need a number of cycles over the time the signal arrives, so you need to do some maths around rotation rates and  reflector width etc, to figure that out. A tarnished reflector will result in scattering and absorbtion, so allowance needs to be made for that as well.If you need to modulate the laser, you keep the dc gain of the transimpedance amplifier following the detector  low enough that under high ambient light conditions it won't saturate. You can then follow it with AC gain and a detector/comparator.
My applications were in experimental physics, a little different, but the principals are the same.Hope that is of some help.