Optical Encoder

[Ergh]

Hello, I am starting a project that is going to utilise an optical encoder. The optical encoder is being used to measure the speed of the system. However, I am a little confused, as the output of the encoder produces a digital output, how can I turn signal into speed (Mph)? My thoughts are to firstly some how obtain the velocity in rad/s and then do the relevant conversions. But obtaining the value in rad/s from the signal is where I am a confused. I would appreciate if someone could point me in the right direction, thank you!!

 

[ronsimpson]

There are 100s of ways to do this:
Build counter that only counts. No time involved. Find a patch of road that is one mile long. Start counting, drive one mile, stop counting. Now you know how many counts/mile.

If you find that many counts in one minutes then the speed is 60mph.
You could count X counts/1 second.

Are you building a computer to do this.

 

[KeepItSimpleStupid]

Speed depend on units. Is it RPM. That's the speed of a motor.

But, you did say velocity. so, that's related to all of the gear ratios involved including the one to the encoder. Then the length is from the circumference of the tires and lets say fractional revolutions. C=PI*d. You know length traveled and distance, so you can get velocity units distance/time. You might count the number of pulses per some time unit depending on the resolution desired and scale.

I think car sensors are about 1 pulse per foot of travel. Not sure though. Tire diameter does change the indicated speed.

 

[Ergh]

Thanks for such a fast reply. Well basically I am developing a mini cruise control system (closed loop feedback) so I need a way of establishing the speed that the signal represents, then with the use of software I can adjust in proportion to the error. For example if there is a hill the signal will drop hence there will be a rise in the error so acceleration will occur to make sure the cruise control will go back to steady state. Surely there is a way of turning the signal the optical sensor outputs into accurate figures? Thanks.

 

[KeepItSimpleStupid]

If this is a car, then look at the interface and put a meter on it. You should be able to jack up BOTH wheels and put the car in neutral and move the tire and determine the pulse per degree of rotation or whatever. e.g. 1 pulse per 1.5 revolutions. Knowing the circumference of the tires, you can back out speed.

A reed relay was an earlier mechanism for speed. Dunno what you would run into.

 

[ronsimpson]

I think my truck gives one pulse per one turn of the drive shaft.