Monday, July 21, 2008

We have got to use this motor

Thanks to everyone who came on Thursday, and Friday. For those of you who weren't there, here is an update:

On Friday, we found an old treadmill that we took apart to get the bearings. We managed to get 4 heavy duty ball bearings (which I am very excited about, since it saved us a trip to the bearing shop), and some axles that fit nicely into them. There was another surprise in the treadmill as well:

A 230V 2.7hp DC motor.

Now, my first reaction was a negative one. For one thing, the motor weighs about 8kg (although a lot of this weight is from the flywheel). Also, we would need many, many battery cells to run it at 230V. And using our current steering design, we would need a second one to run the other wheel. Overall, it would be too much weight.

Having had some time to think about it, I've changed my stance. We have GOT to use this motor! 2.7hp! That's more than 2kW!! Here are some of our options:

Idea 1: We can use the motor to power our weapon. No messing around with springs or screws - we can run the entire thing electro-mechanically. We can get bursts of high voltage using a bank of capacitors: We can charge them in paralell and then discharge them in series:

This will allow us to get pulses of power at 5x our base voltage. This means we can power the flipper almost directly off the motor, making our build a lot simpler. We will still have to port the heavy motor around, but we don't have to worry about lots of batteries to keep it running constantly.

Idea 2: The above idea is a good way of sticking with our original design, but if we do a bit of a redesign we can make far more efficient use of having such a powerful motor.

First of all, we won't be able to find a matched motor to power the right side of our robot, so we will need to use this big motor to power our rear drive wheels, while using a rack and pinion system to steer. There are a few advantages to this:
  • One motor of this size is more power than we will need to move the robot (pretty much irrespective of weight). A second one to power the right wheel would be total overkill.
  • The remote control that we are getting is designed to operate servo-motors, not continuous motors. Rack and pinion steering is servo-friendly.
  • We already have this motor. Why spend money on two big matched motors, when we could just buy a single small servo to steer?
But what about getting enough power to run this beast? Here comes some maths:
  • First of all, we tested the motor and it will turn with as little as 12V.
  • With 4 x 12V cells in series, we can drum up 48V - about one quarter of the total capacity.
  • 2.7hp = 2.01kW; therefore we would be delivering 0.42 kW just to drive around. This is an enormous amount of energy, especially for such a small robot. We would be able to out push any other robot with this (for reference, our previous plan was 2 x 12V x 2A = 96W).
  • Operating at 0.42kW, for say, 20 minutes means that our batteries will have to have a total capacity of 0.14kWh - 35Wh each if we use 4 cells.
  • Assuming that each cell will operate at 12V, this means we are looking for cells with a mere 2.9 Ah rating.
Of course, we might need to use a different number or type of cell, depending on what we can find. However, these numbers are very reasonable and achievable.

Idea 3: Here is the kicker: What if we combine both of these ideas?

We are running the motor below capacity (about 1/4). If we get 4 capacitors, and charge them up as shown in part 1, we can deliver quick, high voltage bursts of 100% operation.
All we need to do is weld a big metal spike to the front of our bot, and we have a death machine that can stab holes in any robot.

Update: Found these.