How to improve the accuracy of a drawing machine

[Shenal]

According to post, How to build a drawing Machine. Now we are in final steps.But the problem is we couldn't make a method to draw smoothly.According to our method it will be drawn by small lines.So is there any other way to do this

[SemiconductorCat]

it depends on what kind of actuator that you are using in the final step.
are they linear ?
And then the answer is yes, you can ,

simply I have a idea for that , suppose you have a function to draw, you just differentiate it, you know
how to do it using a simple Analog circuit, something like
http://www.art-sci.udel.edu/ghw/phys245 ... ass36.html
[don't ask how to because I'm not a experienced analog circuit designer, or show me the simulator results,I'm
sorry just idea, I don't have,Since I only got the big idea rest is up to you ]

and you can give that difference to the actuator. It's up to you to design the stabilized circuit , I'm sorry
I can't help more than that.Somebody who is good in Design and Simulation Analog circuits will help you
for rest of that matter.

[Neo]

Sandun, It is good to see you join the discussion. Please read the mentioned topic first to get a clear idea on their project. They use stepper motors so no PID mechanism is required since motor's position can be controlled precisely without any feedback mechanism. It is good to see people try to answer posts but please make extra measures to give accurate answers. Otherwise the damage to the reputation of ROBOT.LK is far too higher than that.

Shenal, Since you are using stepper motors, you don't need to consider PID (Peripheral-Integral-Derivative) method which is used to smoothly control DC/AC servo motors in motor drives.

However, when you use stepper motors, the accuracy is limited to a single rotation (called a step). You can't get accuracy more than that. Anyway, there are stepper motors with lots of teeth (which means each step is a very small movement) which you can use to precise moving.

Since the rod is connected to the motor, the width of two threads in rod really can't matter as long as the stepper motor can precisely turn.

Have a look at this animation to understand how it works.

StepperMotor.gif (77.23 KiB) Viewed 15491 times

Do these two things to find what precision you can get at the moment.
1. Mark a starting point on the rotor and count how many steps you have in a complete rotation.
2. Mark a point on the moving part connected to the rod and measure the distance it travels on one complete rotation.

Do the above to both motors.

With these measures, we can calculate the precision of both direction.

If you read your previous topic, you might see that I mentioned about using another motor or solenoid to control the drawing pen. If you use this, you can precisely mark dots, not lines.

My feeling on the free wheel (bearing) is also not good. You might face practical problems with it on real environment. I suggest you to use a similar motor/rod on that side too.

I know time is against big modifications, but it is good to have a good system rather than a model.

[PraveenAlexis]

My feeling on the free wheel (bearing) is also not good. You might face practical problems with it on real environment. I suggest you to use a similar motor/rod on that side too.

I know time is against big modifications, but it is good to have a good system rather than a model.

if we mount it vertically, we dont need another motor right?? because the "x" axis is fixed to the board it should move!! like this in the picture

Untitled.png (14.15 KiB) Viewed 15488 times

Do these two things to find what precision you can get at the moment.
1. Mark a starting point on the rotor and count how many steps you have in a complete rotation.
2. Mark a point on the moving part connected to the rod and measure the distance it travels on one complete rotation.

i think we took the distance that rod travels on one complete rotation its 1mm
btw i think its 48 steps per rotation (im not sure, thats how i heard when shenal is telling me about it)

[Neo]

if we mount it vertically, we dont need another motor right??

I already understood the design. My concern is whether the bearing can hold the friction and vibration of motors? If you are going to plot 1mm dots, it really matters. When X axis moves, if Y bar can travel the same distance and stop at the same point as the other end, it is okay. But in practical terms, my feeling is, it will stop 1 or 2 mm behind the target (due to friction, vibration of motors and little lose space in the T joint). If that error is > 1mm, then you will see it on the drawing. If you have used a similar setup on the other end, you command the motors to exactly move the same distance, so it is guaranteed that both ends will stop exactly at the same location. Okay, let's do some experiments to see whether the error is > 1mm. If it is less, then it is hardly noticeable on the drawing.

i think we took the distance that rod travels on one complete rotation its 1mm

I guess you refer 1 step = 1mm. This is not bad (not very good.. if it was 0.5 mm I would be happy). So all depends on the way you handle the pen now? What's the diameter of the tip you are going to use. I think better to use 1mm tip since you have 1mm accuracy.

btw i think its 48 steps per rotation

Again I guess you are referring the motor has 48 steps per one revolution.

Okay so a complete revolution means 48mm displacement. Is that correct?

[Shenal]

In our stepper motors there are 7.5 degrees gap between every step.So it needs 48 steps to finish a full round.(360/7.5=48)
There is a 1mm gap between two threads of threaded rods.When we rotate it one round,total displacement is 1mm.So we can mark 48 different positions within a one millimeter.We don't need a such accuracy.But the problem is according to our method the drawing will be drawn by small lines.Not by a single line.So it will be like a printer.That is the problem now we are facing.

Machine.png (43.04 KiB) Viewed 15471 times

[Neo]

I still don't understand the problem. Do you need to know a method to move the pen up and down as required? So you can mark dots (or small lines) instead of a single line? Can you elaborate a bit more please?

[Shenal]

In our method we have to put the image into a 1mm grid and we have to number every cell and we have to feed it to pic manually. So its difficult. So we hope to make only letters and smileys.is there any other way to do this ?
i think you got this

[Neo]

I think I understand the idea. You are lacking space to store the image? If that is the case, just make a little protocol as below.

1. Read a BMP file in PC
2. Convert it to monochrome
3. Send each line to PIC using serial port (Use MAX232 chip we supplied).
4. After drawing a line, feedback PC that the machine is done with processing
5. Send the next line until PC says DONE!

In this way, you can limit the requirement of memory.

If you have enough time, you might also interface a SD card (or MMC card) with an image and connect it to PIC using SPI bus. You need to have a simple file system to read the file and decode as you need. With the limited amount of memory, I guess this could be little tricky.