Absolute encoder and distance measurement

HI
what is the formula for distance measured using a absolute encoder coupled to a sprocket. this sprocket is rotated by a motor and a gear box. Thanks

AN Absolute Encoder is just that - Absolute
It gives a value between 0 ~ y000 - (Enter your information here)
It needs to be initially refferenced to a Home / 0 Position.
Then a simple scale program to say that 0 = X Pos.
Y= Max Pos
Bob is your uncle. too easy
You need to give heaps more info

PERSPOLIS said:

HI
what is the formula for distance measured using a absolute encoder coupled to a sprocket. this sprocket is rotated by a motor and a gear box. Thanks

Click to expand...

Distance = Pulses * (Distance Unit / Pulse)

Is the simple answer (see how the units cancel?). Figuring distance per pulse is the tricky part. First you need to know how many pulses per encoder revolution. Is the encoder directly mounted to the shaft where you're wanting to measure distance? If so then:


If the encoder is NOT directly mounted, then you have to figure the gear ratio from the target shaft to the encoder shaft. If this is the case, then modify the formula like so:


Hope this helps.

Cheers,
Dustin

Hi Dustin
Sorry mate can you check your PM inbox

Persepolis,

Sorry I did not read your post very well before replying last night. I apologize if I have misled you with my comments about pulses/rev. This is only applicable on an incremental encoder. An absolute encoder does not give you pulses, but usually a binary or gray code which represents shaft angle or position.

So all you have to do is convert shaft movement into machine travel. The same formula applies. You just need to know the circumference of the driven component, and the gear ratio (if any) between the encodered shaft and the driven component.

So if your absolute encoder is on a motor shaft coupled into a 10:1 gearbox, with a 2:1 sprocket ratio driving a 4" drum, then you have a total gear ratio of 20:1, and a circumference of 4pi = 12.566"
So one motor rev = 12.566/20 = .628" drum travel.

Cheers,
Dustin

In PLC code, I normally have a constant real number representing "final drive factor" to convert units from raw to real, but you still want to verify and work with the absolute value for position control and before converting the absolute to a useful position of rate of the final moving piece.

Use delta D (distance travelled) with a subtract block and a limit check to detect rollover, and then you can accumulate real raw position with the same resolution as the encoder if you are trying to understand how to use an encoder that makes multiple revolutions during a single stroke of the final driven element.

Each sprocket ratio is calculated by hand and written in the rung comment, each gear ratio too, then multiply them all together and type it into the tag value, and save it. You can get fancy and try to make the ratios adjustable, but in theory, this value rarely changes. Use this calculation for a basis for motor rpm commands.

In practice, with things like conveyor belt thicknesses changing and other variables, I often have another scale factor which can be taught or tuned in a setup mode after the machine is commissioned.