Robot Arm C

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Re: Robot Arm C

Post by Primate » January 29th, 2012, 6:09 am

starpug wrote:Considering the fact that an electromagnet would probably end up counting as another motor, no I'm not planning on an electromagnet.
Actually, they released an clarification about that:
WHAT IS THE OFFICIAL DEFINITION OF A MOTOR? IS AN ELECTROMAGNET A MOTOR?

A motor is a machine that converts energy into mechanical motion. An electromagnet isn't a motor since it only creates a magnetic field.
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Re: Robot Arm C

Post by sj » January 29th, 2012, 10:56 am

I have a question... Looking at various R/C systems i realized that they are probably all designed so that when the stick is pushed the servo actuates and then when the stick centers the servo recenters. Is there a way to make it so that the stick would proportionally control the speed at which the servo actuates and then when the stick is centered the servo holds its position?

Perhaps someone who has already built a custom R/C arm with servos could explain how they control it?

Thanks!
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2012 Events: Robot Arm, Towers, Gravity Vehicle

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Re: Robot Arm C

Post by chalker » January 29th, 2012, 1:12 pm

sj wrote:I have a question... Looking at various R/C systems i realized that they are probably all designed so that when the stick is pushed the servo actuates and then when the stick centers the servo recenters. Is there a way to make it so that the stick would proportionally control the speed at which the servo actuates and then when the stick is centered the servo holds its position?

Perhaps someone who has already built a custom R/C arm with servos could explain how they control it?

Thanks!

Many systems designed for RC cars / planes already have this feature on at least 1 channel, usually via not having springs that return that stick to center, or with more sophisticated computer code to simulate such things.

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Re: Robot Arm C

Post by ichaelm » January 29th, 2012, 1:58 pm

I completely understand why you'd want to do that! I had to do that before, just not with RC equipment. I used to control my arm with a gamecube controller, and even if I could remove the springs, I didn't want to have to leave the stick in a certain position just to keep the arm poining a certain way. Basiclaly, I wanted the stick to have a spring return to center, but instead of the stick controlling the position of the servo, it should control the speed of the servo, like you want. This is a little like using the RC system to control an ESC, except you still need the servomechanism to counteract the force of gravity acting on the arm. I did it using a PIC microcontroller, since I was already using it for all the other programming (I built my own servos, essentially). You could easily do it with something simpler like an Arduino or a BASIC stamp. I don't know of any other way, unless some radio sets come with that feature, like chalker said.

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Re: Robot Arm C

Post by harryk » January 29th, 2012, 5:26 pm

sj wrote:I have a question... Looking at various R/C systems i realized that they are probably all designed so that when the stick is pushed the servo actuates and then when the stick centers the servo recenters. Is there a way to make it so that the stick would proportionally control the speed at which the servo actuates and then when the stick is centered the servo holds its position?
Now I'm not a hundred percent sure this will work, but I think if you remove the pot from the gearing of the servo it will do what you want. The servo will still think its centered even though it's actually rotating so when you center the joystick it should stop moving, however I don't know if it will continue to hold it there or not
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Re: Robot Arm C

Post by Primate » January 29th, 2012, 7:47 pm

harryk wrote:
sj wrote:I have a question... Looking at various R/C systems i realized that they are probably all designed so that when the stick is pushed the servo actuates and then when the stick centers the servo recenters. Is there a way to make it so that the stick would proportionally control the speed at which the servo actuates and then when the stick is centered the servo holds its position?
Now I'm not a hundred percent sure this will work, but I think if you remove the pot from the gearing of the servo it will do what you want. The servo will still think its centered even though it's actually rotating so when you center the joystick it should stop moving, however I don't know if it will continue to hold it there or not
Center stick sends a PWM length of 1500 microseconds (ish), so that should hold center. I agree that removing the pot should do the trick: it's like you're modifying the servo for continuous rotation, save for the last step where you remove the mechanical stop.
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Re: Robot Arm C

Post by sj » January 30th, 2012, 5:21 am

Thanks for the replies! I ended up finding a decent usb servo control board with 18 I/O so i grabbed that and a few potentiometers and I will probably end up making a master slave system. Any tips about that would be greatly appreciated.
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2012 Events: Robot Arm, Towers, Gravity Vehicle

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Re: Robot Arm C

Post by Primate » January 30th, 2012, 1:43 pm

sj wrote:Thanks for the replies! I ended up finding a decent usb servo control board with 18 I/O so i grabbed that and a few potentiometers and I will probably end up making a master slave system. Any tips about that would be greatly appreciated.
I haven't tried it myself (so those who have, please chime in!), but it seems like a waste of time to me. If you're custom-building your arm with servos and computer-control, why not go almost completely autonomous? The reason a master/slave system works is because we've spent 15 years developing internal kinematics for our arms, and we can map that to the robot nicely. But a computer, correctly calibrated, is even better at simultaneously controlling four or five joints.

I'd let the computer do almost all the work, pausing only briefly before picking up or dropping an object for minor manual position modifications. If you practice, you'll know exactly when to jump in and won't sacrifice any accuracy. It'll also save you a good chunk of work.
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Re: Robot Arm C

Post by illusionist » January 30th, 2012, 3:07 pm

I actually submitted a clarification of whether or not the robot can be completely autonomous, but haven't heard back yet (understandably). But I agree. If you're going to custom build an arm from scratch, why not just make it autonomous?
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Re: Robot Arm C

Post by ichaelm » January 30th, 2012, 4:26 pm

I think the better question to ask here is WHY make it autonomous, not why not. A team that gets maximum score with an autonomous 4-motor robot will not beat one that gets maximum score with 4 motors that's manually controlled. Time is not scored. It will come down to the quality of technical documentation. So why put in all the extra effort with inverse kinematics and making is sturdy enough to be dead accurate with those nails, when it doesn't help your score at all? The only answer I can think of is because it would be fun, of course! But in terms of building a better bot, I can't justify it. If I were you, I'd put more time into making sure the physical bot itself is reliable, and of course some 3D modeling to win the all-important second tiebreaker!

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