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### Topic: Get precise angle from the center of circle (Read 307 times)previous topic - next topic

#### agaria

##### Oct 18, 2020, 01:01 pmLast Edit: Oct 18, 2020, 01:18 pm by agaria Reason: More Information
Hi

I have a controlled wheel that will move around the circumference of the fixed circle.

I need to find the current angle of the wheel (very precisely with 0.1 degree) with respect to center point of the circle.

What would be the simple way to do it.

Thank you

Edit: Wheel will be connected with center through a shaft

#### Robin2

#1
##### Oct 18, 2020, 01:19 pm
What would be the simple way to do it.
Is your question about the type of sensor that could identify where the wheel is on the the circle?

There are (as you know) 360° in a circle so that an accuracy of 0.1° would require dividing the circle into 3600 parts. I think you can get rotary encoders that have a resolution like that, or better.

A lot may depend on whether the wheel is moving slowly or quickly. It might be difficult to keep up with a rotary encoder if it is producing a huge number of pulses per second.

It will be a lot easier to give useful advice if you describe the project you are trying to create rather than just asking a question about a little part of it.

...R
Two or three hours spent thinking and reading documentation solves most programming problems.

#### 6v6gt

#2
##### Oct 18, 2020, 01:25 pm
Can you draw a diagram and post it. It sounds like a system of 2 wheels in contact with each other.
What is the diameter of the wheel (approximately) and of the circle ?

#### agaria

#3
##### Oct 18, 2020, 02:11 pm
I have attached the diagram for clarification.

Circle diameter can be from 12 to 100 feet.

I'm not sure about the diameter of wheel at this moment.  may be it will be in accordance with the circle diameter.

#### agaria

#4
##### Oct 18, 2020, 02:16 pm
Is your question about the type of sensor that could identify where the wheel is on the the circle?
I'm sorry English is not my first language. but this is my exact question.
You can see my another post with diagram it is like center pivot irrigation like this:
https://en.wikipedia.org/wiki/Center_pivot_irrigation
I have to put water at each 0.1 angle if there are plants beneath the shaft. thats why need accuracy.
hope it helps to understand

#### 6v6gt

#5
##### Oct 18, 2020, 02:41 pm
I'm still not clear about the interaction of the two wheels. Is it something like this, where the small wheel would rotate a number of times as it is runs along the circumference of the large wheel ?

So, by measuring the number of rotations (or part rotations) of the smaller wheel, you can calculate the position of the shaft which joins the two axles ?

#### agaria

#6
##### Oct 18, 2020, 02:55 pm
There is only one wheel, the blue one as per above diagram.
In center is a static point.
Black one just to show circumference, it is not another wheel.

#### hammy

#7
##### Oct 18, 2020, 03:00 pm
It might be better to have some form of sensor on the outside ( circumference) of the wheel , then your accuracy requirement is easier . How about something that hits a micro switch each time the wheel moves to a new position , or an optical reflector sensor ?

#### wildbill

#8
##### Oct 18, 2020, 03:10 pmLast Edit: Oct 18, 2020, 03:11 pm by wildbill
Since it's agricultural and spraying water, putting anything on the wheel would tricky to keep from getting soiled or damaged.

I think an encoder at the center point is the way to go.

Edit: how will you know when there are crops needing water under the sprayer?

#### agaria

#9
##### Oct 18, 2020, 03:13 pm
Is there any sensor which can give angle independent of the wheel movement.

Shouldn't capturing the wheel movement will involve many edge cases? Like motor stopping at some point and/or someone picking it up and placing it somewhere else.

I would lose the precise angle and system will start watering at random places

#### 6v6gt

#10
##### Oct 18, 2020, 03:21 pm
Is is this angle you want to measure (marked A ) ?

#### agaria

#11
##### Oct 18, 2020, 03:46 pm
I think an encoder at the center point is the way to go.
I think this is the best way. Stupid of me to not think this way. I was thinking about attaching encoder with wheel's rotation. Encoder at center would solve most problems. Thank you.
x,y of the plants would be known beforehand. so I already know exactly where the plants are.

@6v6gt thank you for time. I think with encoder at center I could calculate angle precisely

#### Robin2

#12
##### Oct 18, 2020, 03:49 pm
I have to put water at each 0.1 angle if there are plants beneath the shaft. thats why need accuracy.
Rather than measure the position of the wheel precisely I suspect it would be much easier to have some system to detect when it is over the thing that needs water. That would avoid all the problems associated with the machine's position being interfered with.

Somewhat separately, I can see the need to identify the positions of successive plants - in other words to move the correct distance from plant to plant - but I don't see the need to know the actual position of the sprayer.

A more detailed diagram, or maybe a photo of the apparatus would help. It's not just the position on the circle that is relevant but also the construction of the device that moves and does the spraying.

...R
Two or three hours spent thinking and reading documentation solves most programming problems.

#### Paul_KD7HB

#13
##### Oct 18, 2020, 04:29 pm
I think this is the best way. Stupid of me to not think this way. I was thinking about attaching encoder with wheel's rotation. Encoder at center would solve most problems. Thank you.
x,y of the plants would be known beforehand. so I already know exactly where the plants are.

@6v6gt thank you for time. I think with encoder at center I could calculate angle precisely
No, you can't compute the angle. You need to have a common base line that never changes in order to do that.
Paul

#### Robin2

#14
##### Oct 18, 2020, 05:48 pm
Encoder at center would solve most problems
I seem to have missed that bit - perhaps because I assumed all along that it is what you had in mind.

As well as the issue raised by @Paul_KD7HB in Reply #13 it seems to me it would be difficult (or expensive) to create a 50ft radius arm that would be sufficiently rigid to give the precision you want unless it is the radius arm that is causing the sprayer to move. Again, our lack of knowledge of your machine is a problem

Also, most rotary encoder just tell you how far something has moved, not where it is. You can get absolute-position encoders but they are considerably more expensive.

...R
Two or three hours spent thinking and reading documentation solves most programming problems.

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