These our working drawings. Due to miscommunication between myself and Chris there will be many changes made, mostly to due with the arm and claw.




The arm will be attached directly to the arm, instead of the mechanism with the wooden wheel/dowel and the string for lifting the arm as depicted above. (directly as in x radians of rotation in the servo wheel equals an equal angular displacement of the arm). We lose a significant amount of torque the other way because we're not applying the force perpendicular to the lever arm (remember you have to multiply F * d * sin theta). Again, this was just from poor communication, we need to work on that.

A lot of changes we are really only able to see the use of making once we are actually in the shop with the actual materials in hand. Like this styrofoam stuff we had no idea was back there. It's pretty strong, so we made the arm out of that instead and reinforced it with pine wood on the sides. This will reduce the weight significantly without affecting strength too much.

The claw mechanism, while we're not 100% sure on it, is probably going to end up looking how I described it before - rubber binder attached to a claw, and a string on a pulley to pull the claw back. The servo will have to be right on the arm or very close to it to avoid the string from moving, or we'll have to work something to let the string can give a little slack when necessary. Another change made after actually seeing what we're working with was the addition of a thin rubber material we found. We're going to wrap this around the claw to increase friction. We think we're going to  make the claw out of masonite, which is lightweight, and we can get it a lot thinner than the pine wood, so it's easier to build with. We're going to have 2 masonite fingers/claws, one on each side of the arm, and a masonite board at an angle to the end of the arm so that when the claw hits the bottom, the flat piece of masonite willl be perpendicular to the floor. We'll have to calculate this angle based on the length of the arm and the length of the masonite piece.

The drive mechanism remains unchanged. We went with thinner, larger wheels to get more speed. Measurements are in the drawing. We still need to do actual power and torque calculations, but we don't know the actual final weight of our vehicle, and the motors and servos are so overpowered we're not too concerned. This has been discussed with respect to the servos in an eariler post.

We will try and post a more detailed drawing of the claw design tonight, as well as a more detailed drawing of the exact design of the structure that connects the arm to to the servo.

Day 5, I think

So the revised calculations, once the errors were fixed, came out to 19 oz/in of torque needed. Enough to lift the heaviest object at only 4.8V, and with plenty to spare.

Stall Torque (4.8V): 44 oz/in. (3.2kg.cm, 3.66lb/in) VS 19 oz/in


At this rate we could basically lift them 2 at a time. More on the redonkulousness of this number: with two of them we could spin a half gallon milk jug around.

Also, Gearboxes are in. Calculations for those to come later today....or tomorrow. Going to figure out the minimum amount of power we need to move our vehicle, given liberal assumptions of it's weight (4 lbs). Final sketches being drawn up tomorrow. Pretty confident in our design.

Day 3

Our Top, side and Back Views of our Preliminary Sketches.

Good news! We finally got the specs for our servos:

Control System: +Pulse Width Control 1520usec Neutral

Required Pulse: 3-5 Volt Peak to Peak Square Wave

Operating Voltage: 4.8-6.0 Volts

Operating Temperature Range: -20 to +60 Degree C

Operating Speed (4.8V): 0.23sec/60 degrees at no load

Operating Speed (6.0V): 0.19sec/60 degrees at no load

Stall Torque (6.0V): 56.8 oz/in. (4.1kg.cm)

Operating Angle: 45 Deg. one side pulse traveling 400usec

360 Modifiable: Yes

Direction: Counter Clockwise/Pulse Traveling 1520-1900usec

Current Drain (4.8V): 7.2mA/idle

Current Drain (6.0V): 8mA/idle

Motor Type: 3 Pole Ferrite

Potentiometer Drive: Indirect Drive 

Bearing Type: Single Ball Bearing

Gear Type: All Nylon Gears

Connector Wire Length: 12"

Dimensions: 1.6" x 0.8"x 1.4" (41 x 20 x 36mm)

Weight: 

1.3oz. (37.2g)

Will we have enough torque to lift the object? This has concerned me since we started designing it, but Brandon assured me we will have more than enough power. Time to find out:

Assumptions: The wood weighs .23oz/cubic inch, the arm has dimensions .5" by .5" by 10" (I will explain how we arrived at this approximate length later); 2.5 cubic inches, it weighs .575 oz. The object we're lifting weighs, at most, 1.15 oz.

First, we calculate the center of gravity using this equation:

WRONG: UPLOAD NEW PIC
DENOMINATOR SHOULD BE M1 + M2

m1 = .575oz

m2 = 1.15oz

x1 = 5"

x2 = 10" + Radius (assuming to be 4"; Brandon has the data at the moment)

Xc = 28cm
Total Mass = 1.725oz
           

Now calculate the torque needed to lift the object: 

T = F * r * sin(θ)

T = Torque

F = M * a = total weight (in this case 1.726 ounces)
r = distance to center of gravity, because this is the point on the object gravity can actually be thought of as acting on. In our case 28cm, or 11 inches

θ = angle between the lever arm and the force (in this case, gravity) being applied to it. The torque from gravity will be strongest θ = 90 degrees (the or the horizontal. Sin(90) = 1.

T = 1.726oz * 11 = 19 oz * in of torque

Day 2

A few concept sketches for our robot above. Starting at the top and working clockwise: 

The drive system: We decided on a differential drive system, which means one motor spins the right wheel, the other spins the left, and a caster wheel on the front. This design is simple, sturdy, and allows the robot to pivot in place, hopefully giving us greater mobility. 

A mockup for a crane, later scrapped. The idea was to have a servo at the back which operates the crane arm, and one at the front which somehow moves the claw. We decided the added weight of a servo on the end would be too much, and this idea was scrapped.

A sketch of a hydraulic crane system, using hydraulic cables and 2 syringes operated by servo motors. We eventually discarded this too, in favor of a servo motor at the front of the robot (back of the crane arm), near the other servo, which operates a pulley connected to a claw. This will press against a flat surface attached to the crane arm, and allow us to grab the pipe. 

A very rough sketch of the pulley system:

We have yet to decide how long the arm will be or how high off the base of the robot it will extend. We know that Base Height + Length * Sin(θ) has to be 12-13" to allow us to place it on the hook

The distance the arm extends horizontally will be equal to: Length * Cos(θ)

Finally, we decided to attach a scoop to the back of the robot to move the pieces into the goal. This is illustrated in the bottom right panel of the first picture.

I'll post equations for calculating how much torque needs to come from the servos to lift the heaviest goal objects later. Hopefully tomorrow we'll get the specs for the servos.