Monday, June 24, 2013

2. Design and selection of parts

This week, we start to shop for the parts we need for out little go karts. We submitted our order last Friday, cause delivery takes time. 

A123 Battery 2S2P 

2S2P battery configuration gives us 24x80=1.92KW, while 3 batteries in series gives us 36x`40=1.44KW. So we'll go with four batteries and pay 75$ for the extra one. Both Jiawei and I agreed that motor is the most important part, so we were looking for those motors that can supply a super large amount of power. I was shopping around hobbyking, then I found some powerful TURNIGY ROTOMAX motors that claims to provide 2KW+ of power. We were very happy and decided to go with that. But Charles pointed out that the power would be a total waste for our go kart cause the maximum power our go kart can provide is only 1.92KW. And although on its data sheet says the maximum current is only 70A, in reality it would suck hundreds of amperes of current and just blow the fuse in the battery. So no ROTOMAX - -. Charles suggests SK3 line, we previously thought that it was not powerful enough, but it turns out that it is actually enough according to our calculation later. For our calculation.....both of us read the dcmotor pdf file so we tried to solve the problem in a systematic mathematical approach. We first wrote a nice EXCEL sheet and tried to fit everything in, and all the mathematical equations as constraints, like solving an optimization problem you know. BUT I think we complicated things too much, all the graphs, analysis, complicated equations...... Two days scratching our head, we decided to take the easy way out, for a rough estimation, we didm't take in the frictions and air resistance. Kv and Kt are negatively proportional  to each other, so lower Kv gives us a  higher acceleration for a fixed gear ratio. Cause gear ratio should be between 3 and 5 to keep things simple, so we set it as 5. With a system voltage of 24 V and a 149rpm/V motor, motor speed with no load =3576 rpm. Our wheel speed is 715.2 rpm with a gear ratio of 5. The wheel we choose has a radius of 0.1 meters, that would give us a vehicle speed of around 7.5m/s < 9.7 m/s. Gear ratio X maximum torque X rotational speed of the wheel = F X speed of the wheel. F=ma and maximum torque= Kt X maximum current. With the 1 minute current 120 A controller, our maximum torque became 7.68, approximate total mass to be 90 kg, we get an acceleration of 4.3 m/s2. Not bad. But with all the friction, we're still not sure how fast it will go, but it's the best we can get within the budget. So...But we 're still not sure about whether to use four batteries or three batteries, cause it seems that 3 batteries in series has higher voltage: 36v, and the max current would be 40A. That way we'll have a higher speed of 11.2 m/s, well that's a much higher speed! But we're still not sure how the power part is playing out > <  For controller we spend 149$ on the 120A KBS line, cause we want to increase our max current, then we still have space to adjust if it's too high and the fuse blows. 

Wheels! To simplify the searching process, we decided to use the same wheel given to us for free for all the three wheels. Razor E150, and it turns pretty smoothly so.... The wheels on Mcmaster are super expensive by the way. We chanced upon this rear wheel assembly, which includes a band brake and a sprocket for chain drive, and it's only 3$ more expensive than a single wheel. PS: we are still sticking to the chain drive plan. 

The batteries and controller would be mounted on a aluminum plate under the seat, with the motor stabilzied behind the controller. We've started doing bits and pieces in solid works. Most of the parts are done already but still need to assemble them. And we still need more aluminum frame to build our chassis, planning on ordering coming Tue. and we'll ahve the 2D drawings of the parts to be cut on aluminum plate ready by tomorrow's class so we can get the stuff back this Friday and start building things next week ;)

Sunday, June 16, 2013

1. Sketching and Idea Generation

For the first week of our EV project, we generate ideas about the rough shape and outline for our go-kart. We read through the websites and the blogs posted by MIT students before, the following are the sketches we did along the way.

From the very beginning, we've decided to use two non-driven wheels in front, one driven wheel at the back. Cause two driven wheels would mean two motors and two controllers. Plus three wheels compose a rather stable structure. When I did this very first sketch, I was thinking more of the aerodynamic shape of a race car or a bullet. So the driver would be kind of lying on his back as he drives the car. This idea was then abandoned during our first discussion cause we feel that the driver would risk a quite high chance of being thrown out when turning, and since the driver is lying down, he cannot really see what's going on in the front, but mostly the sky. 

We then evolved our second idea, in which we lift up the back of the 'seat' so that the driver has a clearer view of what's going on in the front. My teammate Jiawei then proposed that both of the previous ideas make the kart too long, which would inevitably increase the overall weight of the kart. And the gesture of the person still makes it quite possible for the person to be thrown out when turing, cause the driver's center of gravity is quite high.

So we changed the gesture of the driver and decided to make him lean forward, which is kind of like riding a motorcycle. We also tried to ride on the go-karts built by MIT students before and it works like a charm. At first I was thinking more like a motorcycle except the whole body of the kart is lowered down a bit so the center of gravity is low as well. But still, another question would be how to prevent the driver from being thrown out of the kart when turning as we all saw what happened to one of the teams in the Youtube video :p Both my teammate and I liked the kart made by Nick and amwang (I didn't find the kart's name in their blog), which would be kind of secure for the rider to ride on. So we decided to adopt the same gesture. We increased the distance between the two front wheels to increase the stability and also widened the body of the kart a little bit (where the driver sits) so the rear wheel could fit under the kart. We also moved the steering wheel to the very front of the kart so its rotational angle won't be limited when turning. Basically we moved the upper body of their kart backwards to hide the wheel and thus prolonged the part with which the driver would have physical contact. So the driver is still able to lean forward and not reach out of the kart.

Here comes our final sketch for the first week, it looks like those little horses that children would ride on.

We're going with chain drive instead of gear drive so there won't be a part sticking out from one side of the rear wheel thus leaving no space to put the driver's feet. Chain drive is also easier to build, so.... We're probably using the 8 inch wheel provided to us as the rear wheel. As for the design of the steering wheel, we compared the methods of the previous go-karts and finds the chibikart has the best maneuverability, so we are going with that for now. For the curvature part, we are still not sure how to do that. Jiawei pointed out that maybe we could just buy one of those plastic horses for children to ride on, then secure it to the aluminum frame. But will it be kind of heavy? And not very customized for our design. We're also not sure how well plastic and aluminum come together so.... Or maybe stuffed animals? a long pillow? a sofa cushion foam? Or the wood laser-cutting patterns used by the other group before?

The top view and side view are attached below:
*All of the three drawings are in scale. 1'':0.5cm

Top View

Side View 

Note: We haven't decided on the wheel size yet. In the above picture, the rear wheel looks much bigger compared to the two front wheels...which may not be true as we proceed to next week. And we've decided to move the wheel out from the chassis a bit cause it doesn't look very safe intuitively.  We'll look into that later. Btw, we are planning to put all the batteries, controllers etc. on the chassis right in front of the rear wheel.