Let's Make Robots!
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After seeing RatBots cool drifting moves I wanted to try caster wheels. I had a Ducted fan lying about from an experiment gone wrong (prior to joining LMR) and thought that it might make for an interesting combination with the casters. I was going to remote control this but have had trouble catching up with my brother to borrow some radio gear. If I get a chance I'll post a video of it being radio controlled.

 

The first video is just an engine test. Alternating between 0 and 100% thrust. It definately has enough thrust to pick up a respectable speed.

 

 

 

The second video shows the trike under radio control. This is no longer a robot since it has proven too uncontrollable to be autonomous.

 

 

 

 

 

 

 

 

 

 

 

The third video isn't as exciting, just a different camera angle.

 

 

 

 

 

 

 

 

 

 

This is what I've got so far: A meccano chassis although it could easily have been paint sticks, lego etc.

Under_construction__small_.jpg

The electronic speed controller (yellow) is mount at the base close to the fan. The battery fits neatly on top.

Fet_switch.jpgBecause the motor / controller can draw in excess of 20A I'm using a pair of FETs in parallel to switch main power. This is a bit of overkill as one FET would do. The switch was originally buit for BoozeBot. When I turn on the low power circuit for the 28X1 a signal then turns on the FETs. In effect they are a solid state relay.

 

 

 

 

 

 

 

 

 

 

 

 

 

The ducted fan did not come with any specifications but when run off a car battery at less than full power, the blades streched enough due to centrifugal force to gouge out the ducting. At full power it probably would have destroyed itself. The 7.4V battery seems about right.

Ducted_Fan1__small_.jpg

My caster wheels are a bit heavy being solid rubber. I will probably change them later to a lighter plastic caster wheel as the reduced momentum will improve responsiveness.

I've attached the speed control instruction sheet. It can be a little bit confusing at first as it has many programable options. I've had problems reprogramming it via the 28X1 and get many error codes. Last time I used it with a dedicated circuit to imitate radio control gear with no problems. It seems the twitchy servo outputs of the picaxe processors confuse it as the most common error message is "throttle signal is irregular".

 

 

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echorobotics's picture
dude i totally see this being the next big toy that every kid wants for christmas :)
shaked87's picture

first of all the originality of this machine is amazing, it has inspired me deeply.

i have began searching for ducted fans and found this one:

http://www.dealextreme.com/details.dx/sku.15230

I have ordered from this site before,it has low prices+ free worldwide shipping.

compared to other ducted fans for 100$ or 50$ this price is truly low.

as i can see it has 3 wires, i actually thought it will have only 2 - ground and voltage (lets say 12 dc)

so i guess the 3rd one is pwm please correct me if im wrong.

 ______

if that is not true then in addition to the ducted fan's motor ill need this:

http://www.dealextreme.com/details.dx/sku.6120

this is an  electronic speed controller for brushless motors.

i tried to understand whats the deal with it but couldnt really understand it, ive noticed you have one as well

so if you can explain a little about it that would be great.

 

oh and another question:

lets say im trying to get a desent thrust from it, do you think that the current can actually get to 40 A as it is stated in the first link?

that would creat a need for a huge amper-hour battery in order to get decent time of work without the need to recharge.

 

thanks for reading.

and once again , i am very inspiraed by your work here!

OddBot's picture

Thanx for the compliment, as I said at the start, I was inspired in turn by Guibot's "RatBot" so in all honesty, it's not that original.

The ducted fan, it's a 3 phase motor which is why it has three wires and a special programable speed controller. This allows it to be "brushless" as the magnetic field rotates.

As for thrust, theTurbo Trike weights about 1Kg which is why it's a bit slow. I'll make the next one with a balsa wood frame and lightweight casters which should give it about 4x the accelleration.

Current draw from my 2.2AHr 7.4V LiPo is well in excess of 10A at full thrust. The speed controller is rated at 25A. The battery lasted about 12 minutes. This is one reason I used FETs to make a solid state relay so a processor could switch that much current on or off.

Looking at the specs of the ducted fan you linked to, it's bigger in diameter and twice as powerful!  The speed controller you linked to will not handle that motor for long befor it frys since that max current draw of the motor is 7A more than the speed controller is rated for. You need to get a speed controller rated for at least 50A preferably more to allow for hot days, limited ventilation and heat from the battery.

 

Good luck with your build :D

 

 

shaked87's picture

first, thanks for the detailed comment.

1. so if i understood it right, what you are saying is that the electronic speed controller transforms dc voltage to 3 phase ac voltage

2. can you explain more about how exactly the electronic speed controller is connected to the picaxe (what outputs, and what method of data sending)

3. it would be great if you could also explain how the FET is affecting the circuit and making the processor handle with that amount of current.

4. lets say im connecting two of these motors to the same electronic speed controller ( i want them to act the same always, and trying to cut off the costs of another electronic speed controller) it needs to be rated at ~80A ?(the sum of the 2 motors currents)

 

thanks for the guidance. 

OddBot's picture

The Speed controller connects to the picaxe just like any other servo. As a bonus it includes a battery eliminator that allows supplies power to your other servos. Make sure you check the instructions on the speed controller. Mine took the 7.4V of the LiPo and regulated it down to 5V so that it doubled as my regulator.

If your battery eliminator puts out 6V then you'll need an LM2940 low dropout 5V regulator to run your processor and sensors.

You control it with the servo and servopos commands the same as a normal servo.

I'm not sure it's a good idea to connect two motors to the one speed controller as the controllers often use feed back from the motor to syncronise. If you do then you need a speed controller rated for at least 100A since the peak current of those motors was 47A. Another problem you'll have is the battery. If you try to draw more power from the LiPo than it's designed for then it can explode. You may need one battery and one speed controller for each motor.

Talk to the hobby shops as they know more than I do.

The Fet is effectively just a relay so that a little power switch can turn on all that current without melting.

ndupont's picture

What about steering the rear wheels with some servo's...   and add a brake to the front ?

It could get steerable and keep lots of freedom to drift  --> lots more easy to be autonomous

 

OddBot's picture
I agree although too much control will take the fun out of it. Once I've rebuilt it to make it lighter I'm going to make a simple gyro style sensor and try and make a wall racer for carparks.
guibot's picture
If you put some bright lights on the bot it would be cooler to drive in the night :)
OddBot's picture
True, first I need to cut the weight down. If possible I'll make the next one transparent and add lights.
MaltiK's picture
How are you able to make so many robots in such a short amount of time???