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Author Topic: Hysteresis
william_b
Newbie
Posts: 10
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Post Hysteresis
on: December 21, 2014, 13:23
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Hello everyone,

I am just starting with my polargraph machine and have some difficulties.
When I draw a line in polargraphcontroller from point 1 to point 2 and back to point 1. I do get verry strange behaviour. The position of point 1 and point 2 on the paper is correct, but the path in between not. It starts with an oblique (straight part), going outwards and at the end it makes a very strong bend going inwards. Going from point 2 to point 1, it is also like this.

Resulting in some kind of hysteresis curve. See image.
http://i.imgur.com/BKV4Nq2.jpg. I went two times over the same curve. Home point is at the top of the page, 120mm underneath the centerline of the sprockets.)

Can someone help me.

ps. my specs:
- arduino uno
- adafruit stepper controller V1
- uploaded the precompiled firmware "polargraph_server_a1_adafruit_v1.cpp", from 5/10/2014
- use 200 steps/rev steppers, in combination with a timing belt

sandy
Administrator
Posts: 1317
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sandy
Post Re: Hysteresis
on: December 21, 2014, 13:42
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Thanks for the complete specs in your report - very helpful.

Actually these hockey-stick shaped lines are very normal behaviour - each motor accelerates, at the same rate from "stopped", and both motors reach the same top speed at the same time, so the line that comes out is the product of these two, synchronised movements. This line is straight-ish, but it's not actually pointing at the target, it's far off to one side.

So each motor has a different target: MotorA is moving 1000 steps, MotorB is moving 1500 steps, so MotorA starts slowing down long before MotorB does, and as MotorA slows down, it pulls the line to one side, actually putting it back on track to hit the target.

The "move to point" function is exactly that and no more, it moves the pen to a particular point, but makes no guarantees about the path the pen would take, and in actual fact, the simplest way to do this his happens to move the pen in an indirect fashion.

To plot a straight path, use "move direct". Good luck!

sn

william_b
Newbie
Posts: 10
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Post Re: Hysteresis
on: December 21, 2014, 15:08
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Thank you, solved.

Greetings,
William

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