Robust edge detection

[PIG80085]

Hi
I'm using an optical sensor to detect a home position of a motor. Basically the sensor picks up the grub screw holding the axis to the motor shaft. The shaft is aluminium and the screw is black. The sensor feeds into a basic op-amp comparator circuit ie. if the voltage goes over 2.5V, it outputs a "1". It works just fine for 3 of my 4 motors but the other one gives a weak signal.

I'm thinking of using an integrator to give a positive or negative spike when it reaches a positive or negative edge and then something else to convert the positive or negative spike into a "1" or "0", in order to give more robust detection without having to bodge the sensor (put a shroud on it or whatever).

So pretty much I'm wondering if anyone's played around with this kind of thing and what kind of results they got. Any suggestions?

Thanks,
Pig.

 

[crutschow]

What you are talking about is a differentiator, not an integrator.

What's the signal level out of the weak unit? Could you just lower the threshold below 2.5V to better detect the weaker signal?

 

[Boncuk]

Hi PIG80085,

you didn't post what kind of sensor you use.

Using an IR-reflective barrier, e.g. CNY70 it procuces clear output signals.

Boncuk

 

[PIG80085]

Rather than just changing the threshold for one of the sensors I wanted to use a more general solution, not least because the four sensors I have share the one comparator circuit. The sensor I am using is an SG-2BC, it looks fairly similar to the CNY70. On the axes that work, the output goes from ~<1V for a "0" and ~4.5V for a "1". The problematic one gives around 0.5V up to ~2V. By putting a shroud on the sensor and darkening the are around the grub screw on that particular shaft, I have gotten the output to give a value >2.5V. I was hoping for a more elegant solution. I've already replaced the sensor and I'm satisfied that there are no electrical problems in the system.

 

[crutschow]

Did you darken the area around the grub screw with a matt black type paint? If not look for such a paint with minimum reflection. It will have a flat black, no shine appearance and will look very black even under normal light. That will help improve the amplitude of the signal.

 

[PIG80085]

By putting a shroud on the sensor and darkening the are around the grub screw on that particular shaft, I have gotten the output to give a value >2.5V. I was hoping for a more elegant solution.

By elegant I meant a robust electronic solution.

 

[Roff]

Why don't you change the threshold on your comparator to ≈ 1 - 1.5V?

 

[crutschow]

By elegant I meant a robust electronic solution.

You have a low signal from that one sensor. There's no "elegant" electronic solution, I'm more into simplicity. I was suggesting a possible simple solution to improve the signal level. You could amplify the signal but that adds electronics and doesn't improve the signal to noise ratio which is a basic concern. But you do seem to have a reasonable S/N so why not do as Roff suggests and lower the threshold for all the sensors?