What is a Enocder sensor ?

Technical Technical Discussion
1

What is a Enocder sensor ?

all i know basically is that it counts the teethes on the gear which helps accurately judge how far (distance) it goes per the rotation.

is their a site were i can find more info on it ?

or can anyone tell me?

thanks alot

appreciate it :wink:

2

Do you want encoder or gear tooth sensor? A gear tooth sensor counts gear teeth outside of it (say on the sprocket half an inch away) using a magnetic field, but an encoder, as far as I know, counts how many times its own shaft rotates (oh wait, that’s a potentiometer…or is it?) using something similar.

3

An encoder typically refers to a optical sensor that reflects a infrared beam off a disk with an alternating black and white pattern. The nice advantage over a gear tooth sensor is that by changing the pattern. You can determine the direction the wheel is rotating (though on second thought you can do this by recording direction through the speed controller) by monitoring the pattern. They’re both good but I like the optical encoder more. Here’s an example of a disk.
http://www.netrino.com/Publications/Glossary/quadrature_encoder.gif
I would like to end this by saying that often these type have questions can easily be answered by googling (especially google image) with the info you have.

4

There is a white paper on quad encoders that will give you some background.

http://www.chiefdelphi.com/forums/papers.php?s=&action=single&paperid=283

5

I’m not sure you can use the direction of the Victor on such a thing. Suppose I’ve got both of my drive motors set to 140 (forward, but really slow). Then another robot comes up and pushes me backwards at a rate of 12.93 feet per second. If I don’t change my motor’s direction, the program would think I’m going forward, when I am without a doubt going backwards. Then your software thinks you’re somewhere you’re not.

6

Encoders and potentiometers can come in similar form factors, but their internals are quite different.

In general, potentiometers output a DC signal that is proportional to the absolute position of the internal wiper connected to its shaft. If you apply a 5 Volt differential to the outer pins of the pot, the wiper pin will have an analog range of 0-5Volts. Pots are a good match for your application if you have a fixed range of motion.

As for encoders, they are used to measure relative position. They output a pulse as the shaft rotates. It is up to whatever is connected to the encoder to count these pulses and make sense of the data.

On a very basic level, think of the spokes on a bicycle wheel. Imagine holding a straw between the spokes and rotating the wheel. You can determine how many rotations that wheel has made by counting the number of times the straw hits a spoke.

In reality, there are components within the encoder housing that detect the rotation of the shaft. You only need to be concerned with counting the pulses.

Encoders are a good match for your application if there are no fixed ends to your range of motion, such as wheel rotations. You can also use it for something like our (111) lift last year, however, you will probably need to incorporate some sort of home-switch to know your absolute position (since the encoder only gives you relative position).

7

thanks everbodr …

and Dave Shecks thanks you analogys really helps… :cool:

8

While it is true that you can be pushed backwards while trying to go forewards, using the encoders in a quadrature state requires more processing power (interrupts). If you are only using the encoders during autonomous, it is not overly likely that another robot will be on your end of the field, and the extra processing power may come in handy.

Furthermore, even in other situations, if you try to implement some kind of PID control, the processing required may mean that you will simply need to risk getting pushed backwards.

9

there is more to it than that. Usually you use an encoder as the feedback sensor in a PID loop.

With PID loops, you compair what the controller is telling the robot to do, and you use the sensors (encoders) to see what the robot is actually doing. The difference is the error signal that you use to control the voltage to the motors.

For example, if your auton mode tells the bot to go forward, turn right, backup… at some points the controller will be saying (its time to turn) but the bot will still be going forwards. You dont want to mix these signals up - your bot will go open loop (digital insanity!)

Which is why encoders that sense speed and direction were invented. Both are required from the feedback sensors.