Dumb arm robot (modified)

[Art_guy]

Hi, Newbie here who's interested in learning robotics. I'm trying to work out what I need to plan in order to get started.

I'm looking to build a (relatively) simple robot arm with a shoulder, elbow, wrist.

All joints are only going to just move up and down on the one axis - no rotational or other axis's of movement are used. This is so everything moves parallel to a wall. So it's not so much a robot as a animatic.

The length of the robot arm will be long - Shoulder40cm-Elbow40cm-Wrist50cm

The arm wont have any grippers and will only have to move it's own weight. The controller for the arm movement just needs to move the arm so it it makes a shape. Eg an 'N' shape or a 'Z' shape or an 'L' shape.


What type of microcontroller would be used to just run motors to get the arm at the correct shape?

Also - with the arm as long as it it, what problems am I going to run into?

<<edit to simplify the questions>>
Jake

[Eco19R]

Call me stupid but, I don't really know what you are trying to do.

[sdk32285]

I'm also not quite sure i understand what your doing. But depending on the accuracy that you need time based programming is usually easier and cheaper(no need for encoders). Also depending on weight and how much the motor back feeds you might be able to use a rack and pinion discretely.

Another quick thaught is that you can place a magnetic reed switch on the wing tip and a magnet near the buttons for feedback as to when your actualy pushing the button.

[Art_guy]

Thanks for the replies  Sorry about the confusion as I've not learnt the robotic jargon yet. 
I'm simplified my explaination to try and clarify my idea.

The purpose of this 'robot' is simply to make a shape (looking at it from the side) using a simple robot arm.

The shape starts off as an 'N'
rotates at the shoulder to be a 'Z'
extends the top joint to be an 'L'
and then finally extends the elbow to be a _

There are no grippers and the main goal is to automate the above so I can hit a button and the motors 'do their thing' to change the shape of the arm when viewed from the side.

Does this help?

Jake 

[ed1380]

I think what he wants is to have sequences programed into a microcoltroller. and when he pressed a button the sequence does its thing.

[Eco19R]

hmmmm, I think I know what you want to do, stepper or servo motors are going to be needed, along with a MCU to control them.

[Art_guy]

I think what he wants is to have sequences programed into a microcoltroller. and when he pressed a button the sequence does its thing.

Thanks ed1380    - that is exactly what I'm after. Only thing is I'd like 2-3 different sequences. Should most microcontrollers be able to hold enough information in order to move 3 motors in 3 differnet set sequences? 

hmmmm, I think I know what you want to do, stepper or servo motors are going to be needed, along with a MCU to control them.

Hi Eco19R, Just wondering about the stepper / servo motors - Most servo's I've seen have an arm that starts at the 12 o'clock position and depending on the PWM, it can be moved 90 degree either to the 9 or 3 o'clock position to make up the 180 degree of rotation available to the servo...  Will I be able to to modify this some how to start at 3 o'clock position and rotate the full 180 degrees to 9 o'clock. I ask this as I want to be able to hold a position with out having to constantly feed power to the servo (if possible)

Jake

[Eco19R]

Servos would actually be the best motor type IMO - they would be easy to interface.

You can rotate a servo to any exact position you just need to adjust the timing on the output pin of the MCU.

I use PIC MCUs but apparently AVR are also popular, the basic stamp 2 also has a large user base so you have a few MCUs to choose from.

[JonHylands]

Hi Eco19R, Just wondering about the stepper / servo motors - Most servo's I've seen have an arm that starts at the 12 o'clock position and depending on the PWM, it can be moved 90 degree either to the 9 or 3 o'clock position to make up the 180 degree of rotation available to the servo...  Will I be able to to modify this some how to start at 3 o'clock position and rotate the full 180 degrees to 9 o'clock. I ask this as I want to be able to hold a position with out having to constantly feed power to the servo (if possible)

Jake,

Shoulder: 40cm
Elbow: 40cm
Wrist 50cm

With arms as long as what you're talking about, I doubt very highly (without spending some serious bucks) that servos will do the job. They especially won't hold position without supplying a lot of power constantly, if there are large external forces at play.

For this project, if the size is what you specified, I think the only type of actuator you would be able to use is a linear ball-screw type actuator, which will hold position with external forces applying with no power. You won't be able to move the arm by hand, but you have to choose between conflicting goals.

Unfortunately, that type of actuator isn't that easy to find. You might look on ebay, sometimes you can find this sort of thing for a reasonable price.

You might get away with using a servo for the wrist joint, if you use very lightweight materials for the 50cm long piece it is holding up. The other joints have to hold not only the weight of the rest of the arm, but the weight of the outer actuators as well.

- Jon

[Brandon121233]

yeh that is quite long, the only way you might get away with using servos is if you make it really really light, even then you might want to add in some like tension springs or small pneumatic springs to take some of the load off (kinda like what they do for a car trunk lid)

[Admin]

me being a mechanical engineer i should get out my 2 cents on this . . .

first, being an art project, that will make your life so much simpler for making this - it just needs to look good and not be functional.

you probably have little or no experience in calculating mechanical torque, but a good way to know what motors you need is to first calculate required torque based on just the weights and lengths of each joint.
the basics are here:
http://www.societyofrobots.com/mechanics_statics.shtml
i also have a tutorial on robot arms but it is probably much more info then you need:
http://www.societyofrobots.com/robot_arm_tutorial.shtml#joint_force

you will need to calculate the required torque for each joint (unless you plan to use the same exact motor for all joints).

playing with the equations, you will quickly realize that reducing arm length and arm weight that you can significantly reduce required motor torque. doing some quick math, if you reduce the size of all your joints by about half, you could do this project with just servos.

another thought . . . this probably defeats the purpose of an arm, but if your end goal was to make letters only, what if you used motors attached to strings that dangled the sticks into certain positions?

as for the microcontroller question, all that are commercially available should be able to handle what you require.