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Robot Videos / 3D printed Transforming Wheel Robot
« Last post by altapowderdog on November 05, 2014, 12:53:04 PM »
This is a transforming wheel robot that I built. 


The whole thing was made with a Makerbot with the exception of the rack and pinion used to actuate the wheels.  Those were laser cut from acrylic.  An Arduino board controls the motors though an adafruit motor shield, and gets its inputs from a bluetooth module.

I got the bluetooth module for less then ten bucks on eBay, and it was easy to make an Android app using the MIT App Inventor 2 to send the bluetooth module signals through my phone.  It works better then an rc remote fore testing purposes, and is cheap.  Plus itís just cooler then a remote. 

This transforming wheel design will allow robots to navigate indoor environments using encoders, which only work when the wheel is round. Then, when they go outside and can use GPS and compass sensors for position control, the wheels can transform into a spiky design reminiscent of Wheegs.  This will allow the robot to move faster and navigate larger obstacles, which is important for outdoor use.

The spikes are also interchangeable for different surface.  Iíve already made some that work better on sand, and in the future Iíll probably have metal spiked ones for traveling over ice. 
Software / Re: HMC5883L Compass and WLS question?
« Last post by Webbot on November 04, 2014, 05:51:38 PM »
Couple of points here.

I2C addresses are a pain ! They are 7 bit numbers - but in some data sheets they are given as 8 bit numbers but in others are given as 7 bit numbers. Aargh.
The 8 bit number is the same as the 7 bit number x  2.
WebbotLib uses the 8 bit number variant (doesn't matter which I choose as I'll be wrong 50% of the time)!
Even worse then some datasheets give an number but don't say if its 7 or 8 bit.
Converting 7 bit to 8 bit means multiplying it by 2.

ie your datasheet says 0x1E - which multiplied by 2 is 0x3C. Your datasheet gives the 7 bit address.

No easy way around this as there seems to be no standard when quoting i2C addresses.

Next - pullups/IO minV/maxV
I2C uses an open-collector set up. ie you can drive the line low (Gnd) but you can't make it high - you can only go 'open circuit' and hence need pullup resistors to take the line up to the required voltage. The benefit of this is that the cpu and the i2c slave device can be working at different voltages. The IOmin/maxV should be quoted in the datasheet of the underlying chip (HMC5883L) rather than the breakout board. WLS uses its values to  work out if you have a voltage problem  eg what if you have 1 slave with pullups to 5v and another with pullups to 3v3 (as its a 3.3v chip) - the sum of these pullups will probably blow the 3v3 device depending on its IOmaxV.

WLS tries to help by not only showing the basic chip (ie HMC5883L) but also various breakout boards based on this chip (where the decision about IOmax/min and whether the breakout has pullups has been defined for you). Obviously its hard for me to keep up with every breakout board from every supplier based on the same chip - and thats why WLS allows anyone with  a website to publish their own break out boards  (among other stuff).

Electronics / Re: SO.. i might have bitten off more than i can chew
« Last post by mstacho on November 04, 2014, 11:00:37 AM »
Generally speaking people come onto forums like this with very similar problems to yours and they rarely get help :-P The issue is that you aren't asking *specific* questions and haven't really shown us that you're trying to solve the problems that you have.  This is OK (it takes a while to be able to break a large project up into smaller chunks), but you won't get help until you at least try.  Here are some questions for you:

- Can you sense light?  How are you planning on doing that?
- Can you move your motors?  with what?
- What is your plan for turning/driving forward?  How many sensors will you use and how will you read them?

I strongly suggest you work on this one step at a time.  Memory is not feasible for you yet, so don't bother until you get the basic robot working.  Since you are still learning even the basics, some of the steps below might not seem to be relevant until you move on to the next step.  I suggest you do not skip any.  Also, keep in mind that this is a suggestion only - other forum members may disagree!

1) Read the intensity of light coming off of a photoresistor - there are MANY ways of doing this, but the simplest might be a "voltage divider" circuit.
2) Dim/brighten an LED - this is the fundamental skill needed for making motors move at arbitrary speeds, but it's simpler because you don't need to worry about making sure you can give your motors enough current.
3) Dim/brighten TWO LEDs with a positive and negative voltage (this means: put the LEDs in a circuit such that their biases oppose each other.  This allows you to experiment with how you would make your motors turn forwards and backwards, again without worrying about powering the motors.
4) Do the above 2 projects with a motor (take the old circuits and interface them to something that can power your motors)
5) Do the same thing for the other motor (you need at least two motors to make the robot move forward and turn)
6) Coordinate the motors using the arduino - get them to move the robot forwards, backwards, and to turn left and right.
7) Mount two light sensor circuits on opposite sides of the robot and make those sensors control how the robot turns.  Voila, a photovore!

Mechanics and Construction / Working Transformer Costume
« Last post by scott111000 on November 04, 2014, 10:18:26 AM »
Hello,  New member here and first posting. It may be a stretch to call this a robotics project but here goes.  It's a working transformer costume, similar to this:


I need help sizing the motor. 

Wheel diameter is expected to be 10" so I estimate 50 to 90 rpm to achieve walking speed.  Total weight with costume between 170 and 190 lb. 

I'm looking to direct drive a single front wheel with a battery powered DC motor to accelerate from stopped position to walking speed on level asphalt.   How much torque would I need?  Can anyone recommend a vendor or even a specific motor? 
Mechanics and Construction / Difference Between A Cylindrical and Screw Joint
« Last post by arafatasghar on November 04, 2014, 09:54:34 AM »
Hi Everyone,

I am a new entrant into the field of robotics and familiarizing myself with some basic concepts about space configuration. While reading about joints and their associated degrees of freedom, I came across cylindrical and screw joints. Now both have rotational and translational motion yet the cylindrical joint has 2 DOF while screw joint is classed as having only 1 DOF. Why is this?

P.S I would really appreciate if someone referred some online educational resources that would help clarify the whole concept.
Mechanics and Construction / Re: Omni Wheel/Ball Transfer/Caster?
« Last post by bdeuell on November 04, 2014, 08:54:10 AM »
Your first robot looks great.

If you are looking for heavier duty ball transfers look here:http://www.mcmaster.com/#standard-caster-ball-transfers/=ug5l7x

I can think of a few advantages for both the omni wheel and the ball  transfer:

One advantage of the ball transfer is the wheel radius is the same regardless of which direction i.e. the ball will be able to roll over the same size object regardless of the direction it is traveling.

the omni wheel also has the disadvantage of being an uneven rolling surface. this can cause vibrations in the platform as the contact point of the wheel changes.

An advantage of a omni wheel is the internal bearing surfaces are separate from the ground contact surface. in a ball transfer when the surface of the pall gets damaged (from abrasive surfaces such as concrete) it will degrade the bearing surfaces inside the ball transfer.

The omni wheel is probably more optimized for forward movement given a certain mass. i.e. an omni wheel with the same mass as a ball caster would probably have a larger primary diameter making it able to roll over larger obstacles in forward movement. however the rollers would probably have a smaller diameter the the advantage would be lost during turning.
Electronics / Re: Filtering power supplies
« Last post by mstacho on November 04, 2014, 08:46:10 AM »
Hm...interesting.  I guess the question is: what happens if the cap is too large or small?  Is there a universal behaviour you'll see or is it just up to the circuit itself?
Electronics / Re: need brushless ESC/driver > 65V > 10A
« Last post by jkerns on November 03, 2014, 06:30:44 PM »
One of my students got one that apparently is for a washing machine - I think about 100 volts, and I think from E-Bay. I'll ask next time I see him.
Electronics / Re: need brushless ESC/driver > 65V > 10A
« Last post by sdk32285 on November 03, 2014, 06:04:06 PM »
Is this for a sensorless (back emf style) motor or one with hall sensors?
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