ROBOTIC ARM 1ST PROTOTYPE

[SixtyninNaMoose]

Hey everyone, as some of you know I'm a disabled gamer and aspiring modder. I recently spent a lot of time trying to build a circuit to operate a 6v dc motor. It was something very new to me and took my assistant a long difficult time to complete. So we spent 3 weeks or more to figure out something thats not that cool. So my assistant and I decided to " Lets build something cool like a robotic arm".
So I was like that would be sweet, maybe 1 day I can build 1 that can help me work on projects/pick up things like cups or open doors. So i have a policarbonate sheet I haven't used and I ordered screws and servos, and have designed in autocad a small arm. I started the cutting process today but we had no real success. I'm looking 4 a cheap scroll/band saw to make some really nice cuts, any suggestions?

meanwhile check out the designs.

 





here's the parts i'll be using:

Socket head screws, Zinc plated alloy steel, #6-32 x 1/4   
Sheet metal screws, Phillips pan head, Stainless steel 18-8, #2 x 1/4
Sheet metal screws, Hex washer head, Stainless steel 18-8, #6 x 1/2
HITEC HS-422 servo's
HITEC HS-645MG servo's
41G T-Pro SG5010 Servo
2.4G CT6B 6-Channel Transmitter+Receiver
20" x 20" x 0.118" sheet of blue acrylic
24" x 0.394" clear acrylic rod

 


     

[PsychoticWolf]

Very Cool. But can you try to keep your following Project updates in the First post. Remember EDIT is your friend. and also Can't wait to see how this comes along!

[FOOKz™]

Hes got so many pictures it has to be in several posts. Photobucket works great too. Are those Autocad inventor files? they look like models you could download and put the pieces thogether before you build it.

[Rodent]

Nice work!!

[hyper999]

I'm looking 4 a cheap scroll/band saw to make some really nice cuts, any suggestions? 

As my granddad always says "cheap tools are rubbish, and dangerous"

Maybe you could look into getting the parts laser cut by an online company, saves you buying a tool and means that the parts will be cut very accurately, just make sure you spend plenty of time on the CAD drawings so that you don't end up ordering parts that don't work (test designs out using thin card first perhaps).

[SixtyninNaMoose]

Yeah photobucket would have worked out better. yes they are inventor files, how'd you know? I actually made those models myself, I could print out the design and cut pieces of cardboard to test fitting.

hyper, I was considering ordering custom pieces, I actually have a friend who's father owns a local engineering co.
My friend is also handicap, I've actually seen their laser cutter, could call them and see if they'll cut my acrylic.
Maybe free even of charge. I also used a digital caliper to measure one of the servo's, so I'll be able to put those measurements straight into the drawings.

Thnx for your input guys,
I'll be leaving on vacation for a week next sunday, so I'll be waiting until I get back to start the build.
I'll update you as I go.

[FOOKz™]

My buddy gave me a free copy of his Autocad suite plus a few other programs, i thought inventor was one of the easier programs to use =) . They have part catalogs online where you can download models for anything you could think of.

[SixtyninNaMoose]

yeah inventor is deff easier, I got my AUTODESK along with a computer free from the make a wish foundation when I was 18. The representative from local distributor who trained me, sends me all the updates.  I've tried alias before and it's completely foreign to me, but I job shadowed some who used 3ds max my senior year and it actually look relatively simple. what have you used inventor for? I tell you what, it really helped with school projects, mine were always the best.lol   

[FOOKz™]

I used inventor for drawing stuff. Nothing really I just play with it because its good to know how to use. I played halo 3 forge all the time, I used to build remakes of halo maps in inventor for fun. You're lucky that your school offered CAD classes because mine didn't and I was into that stuff plus engineers use it like crazy and thats what im going to do.

[Hazer]

I just looked at this and I see several concerns:

1. Your renderings make it look like the servos will 'be' the axis. A huge problem with this is that even if the parts are laser cut, you cannot guarantee perfect alignment of the assembled part (unless you use a shaft/bearing design) and you are most likely going to wear out the motor shafts due to binding/side-torque.

2. At first I thought these servos may be too low-powered. I checked the specs on them, and the HI TECHs look like they are higher-torque so you may be OK. The T-Pros may be under powered though. I was also having problems finding a decent source for those, they may be hard to replace in the future?

3. What was of more concern: These servos are not continuous, and one of them is only 90 degree. Your design has these paired up in opposing movement (one motor would have to move clockwise while the other would be moving counter-clockwise). There is a whole slew of problems here:
-Controlling two motors like this would be a nightmare, even for me with an Allen Bradley Kinetix system.
-Even if the contorls were done correctly, the two motors would not respond at the EXACT same time, which means they would be binding when not in synch and fighting each other the whole time.
-If the motors only have 90 degrees of movement and are opposing, you have to install each pair where each they were mechanically in thier opposing travel limits on each axis. You would have to move them on the test bench before installing them.
-You will get less than 90 degrees travel as you will not want one motor burning out because the other motor was forcing it into its travel limit.
-You will need to install mechanical homing and travel switches to get each axis like this to synch the feedback for every power-on.


I am by no means a mechanical expert, but I do work with machine automation and controls. The only time I have seen paired-direct motor coupling done like this is with stepper motors. If those 'slip' they do not keep fighting each other, but servos dont slip and work on the basis that they have an isolated feedback so servos keep applying more and more torque until the end of the shaft gets to where you commanded it (the feedback reachs its destination) or it overheats. I have seen dual servo designs, but they have always been coupled to the system using timing belts.

I am also slightly concerned about the motors that are closest to the base of the design (or the shoulder). Most 6-axis robotic arms I have seen, these motors are usually geared instead of direct.

On top of this, the controls will need to be precise. You cannot simply have one controller driving both motors in each axis pair, they are moving in opposite directions. And you cannot command each motor in turn since if they dont move at exactly the same time they will be binding/torquing against each other. So your master control will have to command your servo drives at the exact same time.

[Falkenheart]

Cool robotic arm Moose.

[Hazer]

So, recently I have been reading some info about this particular subject relating to robotic arms using hobby servo motors. After some research, I need to retract some of my previous statements.

Most of what I said earlier is based upon common/industrial/typical servo design. Everything I stated is accurate if you were to try to build a robotic arm using real servo motors and metal materials.

But in this type of project, that is not the case and here is why:

These hobby 'servos' are not typical servos. They barely fit the term servo in that they are a motor controlled by position feedback. This is true, and they actually have thier 'servo drive' also built in. Normal servo design allows the designer to set minimal parameters for at least basic positioning movements (indexing, continuous velocity, etc) upto more advanced control (CAM profiling, sequence positioning, and kinematics (this is real robot arm control)). These hobby servo motors are much more simple in that they take a pulse input and use the pulse-width to set a position (from positive 45 degrees to negative 45 degrees). These motors are usually used for steering in RC vehicles. As such, they do not need as stringent control as bigger servo motors.

You can gain two together, but you should orient them the same way. You should also make the mounting in a way to adjust the angle that the arm is attached to avoid the motors not being aligned with thier control positioning. Lastly, you should send the control pulse to both motors and bench test them to make sure they move the same amount with the same control pulse (make sure they both move 45 degrees instead of one moving 45 and the other 55).

It is also common for people to use the motor shaft as the axle for the robot joints (they take the full load of the arm). Since others have done so, you can keep things simple and repeat this method. It would be better to have some other part take the load of the weight, and the servo is attached just to move the arm but the design of the servo itself does not provide any easy method to do so (you could solve this problem using offset gears, belts, or offset mounting but it is way too much effort when you can simply do what others have already done).

I would suggest using all plastic hardware, including screws. I would also drill extra holes in the long pieces just to reduce weight.

Just know that using the same methods as described above will at best come out with a movable arm that will last for only so long. As long as this is a project for fun and coolness effect, thats all you need. Most of my previous suggestions is based on larger hardware/motors/controls for the purpose of performing repetitive work for extended periods of time in an industrial environment.